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BATCo document for Province of British Columbia 5
November 1999


INTRODUCTION
Tobacco has been in common use by man for many
hundreds of
years, the exact origin of tobacco use being lost
in the
myths of time. Currently there is a wide range and
diversity
of tobacco products in general use throughout the
world, and
covers such activities as chewinq tobaccos, snuff,
pipe
smoking, cigars and cigarettes. It is of interest
and relevant
to our study of nicotine to examine the particular
style of
product used and the manner in which it is
consumed. Chewing
tobaccos as the name implies are consumed in the
month, snuff
(ground tobacco) is essentially drawn into the
nasal cavity.
Pipe tobacco and cigars are consumed by combustion
of the
tobacco, the generated smoke being essentially
taken into the
mouth for a short duration before it is expelled
again.
Cigarettes are also consumed by combustion, with
cigarette
smoke being drawn into the mouth during a distinct
puffing
action which is usually followed by the inhalation
of cigarette
smoke into the lungs. Apart from these broad
distinctions
between the uses of tobacco there are many
identifiable
differences within a specific product ranqe which
include
different tobacco types, frequency and duration of
use, dose
of smoke generated or tobacco consumed.
C-igarette smoking is of particular interest in
that in
different reqions of the world distinct,
d.ifferent tobacco
blend preferences can be identified which can be
divided into
three categories. In broad terms these are U.S.
blended
products. European blended products and the U.K.
Virginia
blended products. Within these broad divisions
there is
considerable microheterogeneity within these
respective market
divisions highlightinq consumer preferences.
There is little doubt that tobacco products
provide the user
with a degree of pleasure and a measure of
satisfaction which
is difficult to quantify in objective terms. From
the wide
range of product use. it is little wonder that
many investigations (-.T1
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BATCo document for Province of British Columbia 5
November 1999


-2-
have attempted to look for a unifying factor
across the
range of tobacco product uses that is capable of
providing
or could account for the satisfaction derived from
tobacco.
It is the diversity of product use that has
prompted many
researchers to conclude that the only conceivable
tobacco or
smoke component that could provide the link
between them is
nicotine. These observations are also responsible
in general
for the belief that nicotine intake by the various
routes of
administration, mouth nasal cavity and lung, is
the primary
and major motivating factor involved in the
maintenance of
tobacco product use.
Nicotine has a lonq and interestinq scientific
history which
effectively originates from the studies of Possett
and Reiman
who first isolated nicotine from the leaves of
Nicotiana
tobacum, the tobacco plant, in 1828. From this
time the
interest in nicotine and its actions in the body
has steadily
increased and developed to the present day where
an enormous
body of scientific knowledge exists. Nicotine has
had an
important role to play in the development of the
science of
pharmacology, with Orfila initiatinq the first
pharmacological
studies involving nicotine in 1843. These studies
were
followed in 1889 with Langley and Dickinson
identifying the
prime site of nicotine interaction within the
nervous system
of animals. It is the demonstrable ability of
nicotine to
interact within the body and to subsequently
elicit a host of
events that has continued to focus attention on
the role of
nicotine both within the smoking process and the
suggested
involvement in the etiology of a number of disease
processes.
Nicotine, one of the few naturally occurring
li-quid alkaloids
has some intriguing and interesting chemical
properties. It
is extremely soluable in water and readily forms
aqueous salts,
the free nicotine material is also very soluable
in organic
media. These interesting physical and chemical
properties
which are pH dependant, have a bearing on the type
and mode C=>
of use of the tobacco products and are intimately
involved in
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BATCo document for Province of British Columbia 5
November 1999


-3-
the process of eliciting its biological
properties. The
reported biological properties of nicotine are
extensive, the
material being attributed with the ability to
elicit
psychological, pharmacological, physiological,
biochemical
and toxicological effects within the body.
Superficially at least, nicotine would appear to
be of
considerable if not vital interest to the tobacco
industry.
However it is of primary importance to critically
examine
the role of nicotine in maintaining, influencing
and modifying
the whole process of smoking behaviour and thereby
attempt
to identify the contribution of nicotine to
product acceptability
and smoker satisfaction. As part of this process
it is
necessary to critically review the available
scientific
information to examine the premise that smokers do
indeed
smoke at least in part for nicotine, and to
establish whether
this is a direct or indirect requirement. The
terms smoker
satisfaction and product acceptability are
nebulus, ill-defined
concepts that are difficult to quantify which are
frequently
arrived at empirically or by a combination of
subjective
factors. In quantitative terms the role of
nicotine in this
process is poorly understood.
It is of obvious and fundamental advantage in
terms of current
and future cigarette development to be able to
quantify the
extent to which nicotine can contribute to product
acceptability
and consumer satisfaction within the context of
product use.
If this is to be achieved it is of vital
importance to identify
the elements within the smoking process that can
be influenced
by a modified nicotine. In terms of cigarette
smoking the
areas for consideration and detailed study can-be
divided
into several broad categories:
(i) Product attributes; to what extent and under
what
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circumstances can the generation and transfer of
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nicotine from the product to the smoker be
modified C-71
in terms of product design, and can these changes
be
perceived by the smoker.
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November 1999


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(ii) Consumer variables; to identify the
conditions
under which the consumer modifies and regulates
the
quantity of nicotine (and other components) taken
or derived from the product and the motivating
factors for this action.
(iii) Wholebody responses; to identify how the
smoker
determines his need for nicotine and senses the
fulfilment of that requirement and to what extent
these needs are satisfied by specific products.
The relationship between these events effectively
constitutes
the complete smoking event. Consideration of the
individual
elements of this process provides the basis of the
physical
description of the interaction of the product with
the
consumer, however an integrated assessment of
these factors
may begin to provide an understanding of smoker
motivation
and satisfaction. In terms of tobacco smoke use
the ability
of the smoker to estimate product strength or the
quantity of
nicotine delivered from the product during
puffing, inhaling
or following adsorption of nicotine (and the
significance of
these events for product satisfaction) have not
been adequately
quantified despite considerable research effort.
The underlyinq factors involved in smoker
satisfaction are
almost certainly derived at least in part from the
interaction
of nicotine within the body. However the temporal
relationship
between smoke generation, nicotine absorption and
wholebody
response is yet to be determined. The wholebody
interaction
of primary importance to the consumer arises from
the
pharmacological properties of nicotine, the
effects elicited
by nicotine interacting within the central and
peripheral
nervous system. The development of our
understanding of the
relationship between the physical factors involved
in the
smoking event e.g. product type, puffing and
inhaling
characteristics, and the pharmacological
properties of nicotine
is of fundamental importance in determining the
probable
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BATCo document for Province of BritiSh Columbia 5
November 1999


-5-
basis of smoker motivation, product acceptability
and
consumer satisfaction.
The ability of nicotine to interact within the
body not only
provides the probable pharmacological basis of
smoker motivation
and contributes to product acceptability and
smoker satisfaction
it has also been implicated as the basis for the
involvement
of nicotine in the development of a number of
disease
conditions. The partition of the wholebody
interaction of
nicotine into beneficial on the one hand and
detrimental on
the other at this stage of our knowledge is
unrealistic,
artificial and scientifically invalid. The more
reasonable
and scientific approach is to continue to develop
our
understanding of the total process of the
interaction of
nicotine (and other components) within the body to
determine
the fundamental nature of the interaction within
the integrated,
whole system.
It is not the purpose of the report to simply
generate a
catalogue of reported observations relating to a
series of
discrete aspects of nicotine interaction with the
consumer.
The report attempts to examine the available
scientific
information derived either from the published
scientific
literature or from information available within
the industry
and as far as possible to develop an integrated
assessment of
the involvement of nicotine in all aspects of the
smoking
process. However, because of the complexity of the
subject
it has been necessary to exercise a degree of
judgement in
the interpretation and integration of reported
observation.
In addition within the field of nicotine research
there are
many areas in which there is a lack of scientific
knowledge
or indeed conflicting and contradictory evidence,
and under
these circumstances the interpretation inevitably
reflects
an individual view. However. every effort has been
made to
remain objective in the evaluation of data, this
is particularly
important in attempting to identify areas of
controversy and
future research direction.
BATCo document for Province of British Columbia 5
November 1999


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The report has been arranged in a sequence of
logical sections,
L/ designed to systematically identify and
integrate our under-
standing of all aspects of the role of nicotine in
the smoking
process. Initially the report investigates as far
as possible
the aspects of the product that can influence the
amount of
nicotine available to the smoker and to review the
way the
product is smoked in an effort to identify the
factors that
influence or control these events. This forms the
basis of
the smoking behaviour section which is used
subsequently to
develop the relationship between the way in which
products
are smoked and the influence of this process on
the amount
of nicotine absorbed and distributed in the body,
the so-called
pharmacokinetics. The manner in which nicotine is
obtained
and distributed in the body is an influencing if
not a
controlling factor for the extent to which
nicotine can
elicit wholebody pharmacological (and other)
responses. The
mode of action and the pharmacological properties
of nicotine
as currently understood are reviewed in detail in
a specific
section. These initial sections form the basis of
current
knowledge r.elatinq to the role of nicotine in
providing and
maintaining smoker satisfaction and product
acceptability.
The section on the pharmacological properties of
nicotine
also represent the basis for extending our
understanding of
the role of nicotine in the process of wholebody
responses
generally and the possible involvement of nicotine
in the
development of disease processes or disturbances
in norinal
body function. The possible role of nicotine in
the development
of cardiovascular disease, the relationship
between nicotine
and pulmonary carcinogenicity and the reported
involvement of
nicotine in terms of maternal smoking are
considered within
the context of the smoking process and are
presented in
separate sections. In terms of the reported
involvement of
nicotine in the development various diseases an
attempt is
made to assess the significance of the reported
observations
within the context of smoking and health.
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November 1999


-7-
An additional area of consideration follows
naturally from
any investigation of the role of nicotine in the
smoking
process. If it is established either directly or
empirically
that nicotine is indeed the primary motivating
factor in the
maintenance of tobacco product use, to what extent
is it
feasible or desirable to develop alternative
products based
on nicotine alone. A further consideration would
be to
evaluate to what extent such products might
substitute for,
or replace conventional tobacco products. The
extent to
which this is feasible and the current status of
this approach
is outlined in this report.
Finally the report attempts to highlight the role
of nicotine
in the smoking process and to identify future
areas of research
that might serve to improve product design and
satisfy consumer
needs of the future.
BATCo document for Province of British Columbia 5
November 1999


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SECTION 1 - NICOTINE AND SMOKING BEHAVIOUR
SUMMARY
The section initially attempts to identify the
origin of
nicotine within the tobacco plant and proceeds to
outline in
general terms the factors of cigarette design and
the combustion
process that influence the transfer of nicotine
from the
cigarette to the smoker.
The major and primary purpose of this section is
to identify
as far as possible the role of nicotine in
modifying and
influencing the smoking process.
Numerous smoking behaviour studies have been
conducted by
many researchers in an effort to identify the
factors involved
in the smoking process that are capable of
influencing how
the product is smoked. A major difficulty in the
field of
smoking behaviour research is that smoking
behaviour is a
collective term for a range of complex activities
that
constitute the complete smoking process. This fact
is
frequently not appreciated or to some extent
ignored during
the interpretation of reported observations. A
series of
reported smoking behaviour studies have been
reviewed in an
effort to establish the experimental and
methodological basis
for the range of diverse observations and the
frequent
contradictory and conflicting findings. It would
seem highly
relevant, that despite considerable research, no
clear
unequivocal role for nicotine has been
established.
Based on these findings an attempt is made to
critically
examine the available evidence to begin to
identify the
specific aspects of the smoking process that might
reasonably
be influenced by nicotine. Using this approach,
the ability
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of nicotine to directly elicit subjective
responses within CZ),
the respiratory system and its contribution to the
assessment
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November 1999


-9-
of product strength is examined. In addition, the
premise
that i~~nq' cigarette smokers smoke primarily for
nicotine
is examined as far as possible from a smoking
behaviour
standpoint.
If it is established that a smoker does indeed
smoke at least
in part for nicotine, it is important in terms of
product
development to identify how this can be achieved
by the smoker
in satisfying his requirements. In addition to
examining how
this might be achieved through modifying smoking
behaviour
parameters, the basis on which a smoker can assess
the dose
of nicotine obtained from the product is an
essential part of
the process and equally if not more important in
terms of
product development. To this end the means by
which smokers
might manipulate their nicotine dose within the
elements of
the smoking behaviour process are investigated and
the
mechanism and motivation for modifying the
nicotine dose is
discussed.
Finally, an.attempt is made to identify specific
areas of
research that might be used to further clarify the
role of
nicotine in the smoking behaviour process.
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BATCo document for Province of British Columbia 5
November 1999


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NICOTINE AND SMOKING BEHAVIOUR
Nicotine; The Tobacco Plant
The concentration of nicotine in cigarette smoke
is essentially,
although not entirely, a function of the cigarette
blend nicotine
content. In general terms arising from widespread
custom and
practice this is of the order of 1.8-2.5% by
weight of the
tobacco blend. Using conventional tobacco types
and blending
on an economic basis (i.e. utilising essentially
the whole
plant), the level of blend nicotine could be
increased to an
upper level of approximately 4.5% if considered
desirable.
There are, however, cultivars of tobacco that have
considerably
higher natural levels of tobacco containing about
10-12". by
weight of the blend. The feasibility and
practicality for
widespread use and the agricultural economics of
using these
cultivars is difficult to assess. The smoking
characteristics
of these tobaccos in future, novel product
situations may
provide exciting opportunities.
The role and functional significance of nicotine
in the
development of the nicotiana species is
essentially unknown,
although it has been suggested to be a plant
growth regulator
by some and a possible natural insecticide by
others. Our
knowledge of the specific synthesis, distribution
and
availability of nicotine within the tobacco plant
and the
means whereby this might be modified is limited
and understudied.
The bulk of the plant nicotine is synthesised in
the roots of
the tobacco plant although there is potential for
synthesis
in other regions of the plant. Nicotine is
translocated to
the other parts of the plant in the plant
circulation system
(in the phloem). The rate of nicotine synthesis,
translocation
and accumulation varies for different tobacco
types and is
influenced by the staqe of the plant growth cycle
and nutritional
status.
It is interesting to note that the radiochemical
industry
produces radioactive nicotine and many other
derivatives by
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BATCo document for Province of BritiSh Columbia 5
November 1999


_11-
incubating freshly harvested tobacco leaves in the
presence
of radiolabelled carbon dioxide. The yields of
radioactive
intermediates are maximised under conditions for
optimum
photosynthesis. Although the quantitative
significance of
this process is unknown, it does indicate at least
in
qualitative terms that detatched tobacco leaves
are able to
synthesise nicotine (either de-novo or via
residual inter-
mediates). The ability and extent to which this
proces
might proceed in the harvested crop and during
curing, and
the conditions which might favour these events,
have not
been fully explored and their manipulation may be
valuable
in modifying blend yields and properties.
Nicotine; Transfer to Smoke, Processing Combustion
and
Filtration Effects
When non-ventilated products are smoked under
standard smoking
conditions
approximately 10% of the nicotine present in a
cigarette is
transferred from the tobacco to the mainstream
smoke. For
ventilated products the proportion of nicotine
(and other
components) is essentially diluted in simple
proportion to
the level of ventilation. The mechanism of
nicotine transfer
arises from the decomposition of the tobacco
during the
combustion process, the nicotine arising
essentially from two
zones of formation. The first zone of formation is
close to
the burn line (at the rear of the coal) and the
second arises
from distillation of nicotine previously condensed
onto the
tobacco at a location remote from the combustion
zone. The
distribution and mass balance of nicotine within
the smoked
cigarette (at least for non-ventilated products)
can be
summarised as follows: 10-15% of the blend
nicotine is
transferred to the mainstream smoke, 10-15% of the
blend
nicotine is trapped in the filter, approximately
20-30%
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of the blend nicotine is transferred to the
sidestream smoke.
A further 10-15% may be accounted for by the
cigarette butt
BATCO document for Province of BritiSh Columbia 6
November 1999


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tobacco including condensed nicotine remaininq
with the
uncombusted tobacco. The remaining amount of
nicotine, up
to M of the blend, is lost possibly as combustion
products
and through thermal decomposition (values in the
range of
7-27% for decomposition have been reported (1)).
In view of the fact that nicotine is volatilised
at around
150*C, considerably below the combustion zone
temperature, it
would seem reasonable to suggest that certain
factors may
lead to nicotine 'trapping' actually within the
tobacco.
This may be a consequence of physical and/or
chemical phenomena
associated with the distribution of nicotine
within the
tobacco or indeed the possible association of
nicotine with
other plant materials. It would seem both feasible
and
plausible to suggest that pretreatment of tobacco
and to
some extent the processing procedures adopted may
provide an
opportunity to enhance the levels of nicotine that
can be
obtained from conventional tobacco blends.
The precise distribution of nicotine between the
gas phase
and particulate phase both during smoke aerosol
formation and
in mainstream smoke is essentially unknown. The
partitioning
is likely to be influenced by the pH (acidity or
alkalinity)
of smoke and hence the ionised and un-ionised form
of nicotine.
The more alkaline the 'smoke' pH, the higher the
proportion
of un-ionised nicotine and the greater the
readiness for
nicotine to exist in equilibrium with the vapour
phase.
As stated earlier, approximately 10-15% of the
blend nicotine
is retained in the filter of unventilated products
during
smoking. The absolute level of nicotine retention
and hence
the nicotine filtration efficiency is essentially
(filter
type and) flowrate dependent. The higher the
velocity with
which smoke containing nicotine is drawn through
the filter,
the lower the level of nicotine filtration
efficiency. The
smoke particulate material and nicotine have
different
filtration efficiencies, the filtration efficiency
beinq (-.n
BATCo document for Province of BritiSh Columbia 5
November 1999


-13-
generally lower for nicotine than particulate
material.
Naturally, the absolute levels of nicotine and TPM
filtration
will be filter type dependent but, as a general
rule (i.e.
for cellulose acetate and polypropylene
conventionally
constructed filters) the filtration efficiencies
will be of
the or der of 40 and 50% respectively. This gives
rise to
yields of proportionately more nicotine than
particulate
material when compared to unfiltered smoke. Use of
filter
materials which tend to be alkaline
(e-g.rz~jck%%--( ) give
proportionately higher yields of nicotine, whilst
the converse
is true for acidic filter materials (e.g.6L'-C'~#
In general terms, under machine smoking conditions
successive
puffs along a cigarette gives rise to higher per
puff smoke
deliveries. It is possible for different filter
types and
materials to exhibit characteristic filtration
properties
under these circumstances. It is possible for some
filter
types for the filtration properties and
efficiencies to
remain unchanged as the per puff delivery
increases. However,
in the case of cellulose acetate filters, as the
puff number
increases the efficiency of the filter for
nicotine retention
is increased. This observation arises from the
phenomenon of
hot filtration; as the combustion coal approaches
the cellulose
acetate filter, its temperature increases (and
that of the
smoke), elevating the distribution of nicotine
within the
filter, leading to specific retention of nicotine
from later
puffs. This has the effect of modifying the T/N
ratio on a
puff by puff basis, the ratio increasing (more tar
relative
to nicotine) in the later puffs for cigarettes
containing
cellulose acetate filters. So far we have outlined
tar and
nicotine filtration in relation to machine smoking
conditions.
In general terms, the predominant shapes of human
puff profiles
conform to what is termed an early triangle and
are frequently
of shorter puff durations; this gives rise to
higher smoke
velocities for a given puff volume of smoke
passing through C-D
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the filter compared with machine smoking
conditions.
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November 1999


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Human smoking behaviour patterns give rise to
distortions in
the machine-derived tar:nicotine ratios in the
direction of
a decreased ratio, by virtue of the fact that the
higher
smoke velocities preferentially retain particulate
material.
The filter properties and their specific
characteristics may
have a significant role to play in terms of
modifying or
contributing to the product satisfaction for the
smoker.
Nicotine; Influence on Smoking Behaviour - A
Review of
Smoking Behaviour Studies
Numerous smokinq behaviour studies have been
conducted to
characterise how cigarettes are smoked and to
identify the
factors relating to either the smoker or product
that influence
the process. A series of smoking behaviour studies
has been
reviewed from the early 1970s to the present day
to reflect
the general experimental approach and current
status of
nciotine as a controlling or regulating factor in
human
smoking behaviour. These studies will be used to
provide an
opportunity to develop and examine a number of
hypotheses to
.explain' smoking behaviour and attempt to evolve
an approach
to characterise and manipulate the complete
smoking event.
For convenience of discussion, the smoking
behaviour studies
have been divided into three categories:
(i) Studies essentially concerned with the numbers
of
cigarettes smoked in relation to their nicotine
deliveries.
(ii) Changes in the physical characteristics of
puffing
in response to brand switching or reduced nicotine
availability.
(iii) Brand switching studies that involve
biological
fluid measures as estimates of nicotine uptake.
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(i) Changes in Cigarette Consumption in Relation
to the
Nicotine 'Delivery' of the Product
Goldfarb (2) conducted a study in which the
proportion of a
smoker's own brand available for smoking was
restricted either
by marking the product along its length or by
physically
cutting the cigarette to a pre-determined length.
The smokers
smoking the restricted length of product increased
their
cigarette consumption compared to normal smoking
frequency.
Russell (3) switched essentially middle tar
smokers to both
lower and higher nicotine-containing products.
Switching to
lower nicotine containing products (0.3 mg
nicotine/cigarette)
caused an increase in numbers of cigarettes smoked
and smoking
the high nciotine-containing products caused a
reduction in
the numbers of cigarettes smoked compared to own
brand smoking
frequency. Schacter (4) studied 11 subjects when
switched to
higher or lower nicotine-containinq products. He
concluded
that the heavy smokers (those smoking more than 20
plus
cigarettes per day) smoked more of the lower
nicotine delivery
product (0.3 mg nic; 14 mg tar) than the higher
delivery product
(1.3 mg nic; 19 mg tar). Stepney (5) reviewed a
number of
smoking behaviour studies and concluded that where
a 50%
reduction in the nicotine delivery of the product
occurred,
a corresponding increase of approximately 10% in
consumption
(cigarette numbers) was likely to result. This
conclusion
was based on switching studies from middle tar
products to
low tar products (although in practics most low
tar products
were in the 8-10 mg tar range).
(ii) Changes in Puffing Characteristics Following
Brand
Switching or Reduced Nicotine Availability
Ashton (6) studied the smoking habits of 36
smokers whilst
performinq two tasks of differinq complexity. In
this study
it was observed that subjects smoking lower
tar/nicotine
cigarettes (1.0 mg nicotine) took more frequent
puffs than CD
BATCo document for Province of British Columbia 5
November 1999


-16-
the higher nicotine product smokers. During the
period of
study, the respiratory characteristics of the
subjects were
monitored and no change in rate or depth of
inhalation was
observed in these smokers. Based on the smokers'
cigarette
butt analysis and machine smoked values (to
estimate
deliveries based on filtration efficiencies of the
products),
they concluded that the smokers smoking the lower
nicotine
content cigarettes were compensating (oversmoking)
compared
to the smokers of higher delivery products. When
smokers
were switched acutely (I day/cigarette type) to
three products
with differing nicotine and tar levels - (i) f.02
: 14.6;
Oi) 1.37 : 17.1; (iii) 2.11 : 30.8 - the numbers
of cigarettes
smoked viere linearly related to nicotine (and
tar) delivery
(7). The hiqher nicotine-containing cigarettes
remained
alight the longest and hence had the greatest
intervals
between puffs and hence few puffs were taken
(assuming constant
smoulder rates). Curiously, product (ii) was
smoked to the
highest puff volume. It was concluded from these
studies
that the smoking behaviour was modified according
to the
nicotine level of the cigarettes. In a study by
Ague (8)
smokers smoked four products with differing levels
of nicotine:
0 (a lettuce leaf cigarette), 0.75, 1.02 and 2.11
mg of
nicotine. Based on butt analysis as an indicator
of smoke
nicotine delivery, the low (0 and 0.75 mg nic )
and high
(2.11 mg nic) delivery products were undersmoked;
the middle
nicotine-containing product was smoked according
to the
machine smoked deliveries. In a study by Turner
(9), nine
subjects smoked middle, middle to low and low tar
products
for a period of one week per cigarette. Cigarette
consumption
increased with reducing nicotine deliveries. From
butt
analysis the estimated nicotine deliveries to the
smokers
indicated that the middle tar products were
undersmoked
whereas the low and low to middle tar products
were oversmoked
relative to machine smoked figures. Forbes (10)
switched 24
own brand smokers to two experimental brands
before returning
to their own brands; the subjects were monitored
for one
week smoking each cigarette type. Based on butt
analysis it
BATCo document for Province of British Columbia 5
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was estimated that changing to lower delivery
products gave
rise to reduced mouth-level intakes. Freedman (11)
conducted
a long-term switching study over 20 months with
approximately
60 smokers. The subjects smoked a control product
(1.4 mg
nicotine) and a new smoking material (NSM)
cigarette (1.0 mg
nicotine). Both groups of smokers smoked similar
numbers of
cigarettes. The butt analyses indicated that the
lower
nicotine delivery cigarettes had yielded lower
deliveries of
nicotine to the smokers. However, the yield of
nicotine
under human smoking conditions compared to machine
smoking
yields had indicated oversmoking (compensation).
Adams (12) conducted a switching study involvinq
124 smokers
over a six-month period in which subjects smoked
eight
cigarettes, with individual cigarettes smoked for
approximately
2 weeks each. The cigarette deliveries were in the
range
0.7-1.6 mg nicotine and 10-19 mg tar. Overall,
there was
little response to change in nicotine yield,
although there
was a tendency for puff volume to increase as
cigarette
nicotine (and tar) decreased. When cigarette
nicotine was
controlled for, increasing cigarette tar yield
resulted in
longer times for the cigarette to be alight,
suggesting a
reduced number of puffs taken and/or lonqer
intervals between
puffs. Increases in both tar and nicotine
suggested a
reduction in puff volume under these conditions.
Creighton
(13) monitored 16 subjects on a cigarette yielding
1.4 mg
nicotine and then switched half onto a lower
yielding product
(1.0 mg) and the others onto a higher yielding
product (1.8 mg)
for a period of one month. The subjects were then
subsequently
monitored smoking their own brands. The nicotine
delivery
of the product had little effect on numbers of
-cigarettes
smoked. Compared to their own brands, the smoking
intensity
decreased with increasing nicotine yield and
increased with
decreasing nicotine yield. The experimental high
and middle
tar products resulted in similar mouth level
intakes whereas
the low tar product resulted in a mouth level
intake lower
CZ)
than that obtained from the other two brands.
Sutton (14)
(JI
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November 1999


_18-
conducted a study in which 18 subjects smoked a
product in
holders with either high or low levels of
ventilation.
Cigarettes smoked with the high ventilation holder
were smoked
more intensively (based on butt analyses) than the
low level
ventilation holder. Ashton (15) studied the
smoking habits
of 14 subjects on successive days, smoking a fixed
number of
full cigarettes and switched to identical products
of 2/3
normal lenqth. The nicotine deliveries of the
products smoked
indicated that, compared to the machine smoked
deliveries,
the full-length cigarettes had been smoked
normally whereas
the 2/3 cigarettes had been oversmoked. Adams (16)
reported
that subjects compensated for reduced tar delivery
as well as
reductions in nicotine delivery by taking more
puffs of longer
durations. Rawbone (17) studied own brand middle
tar smokers
and switched them to low tar products for four
weeks. The
low tar products were oversmoked by taking larger
puff volumes;
however, the compensation (the extent to which the
lower
delivery product was smoked to equal the delivery
of the own
brand middle tar product) was not total.
In a brand switching study by Jaffe (18) it was
indicated
that the changes in butt length represented a
major mechanism
by which compensation occurs, suggesting that the
shorter tile
butt length the more puffs (or higher intensity
puffing) are
taken from the cigarette. Jarvik (19) conducted
two studies.
The first involved successively reducing the
lengths of
subjects' own brand products. Under these
circumstances he
reported increased numbers of cigarettes smoked
and increased
puffing frequency. In the second study, 28
subjects smoked
two products with similar tar yield but with
varying levels
of nicotine (2 mg and 0.2 mg respectively).
Subjects smoking
the low nicotine delivery product showed an
increase in
the numbers of cigarettes smoked and increased
numbers of
puffs taken.
Herning (20) studied a qroup of smokers smoking
high, medium
and low nicotine yield cigarettes - (i) 32 mg tar
: 0.4 mg
BATCo document for Province of British Columbia 5
November 1999


_19-
nicotine; (ii) 25 mg tar : 1.2 mg nicotine; (iii)
29.6 mg
tar : 2.5 mg nicotine. These products were claimed
to have
similar taste properties with similar CO yields.
It was
concluded from the study that puff volume could be
used to
discriminate between the nicotine yield of the
products
(i.e. -larger puff volumes from lower
nicotine-containing
products), and that nicotine is controlled by the
smoker
from the beginning of the smoking event. The
authors presumably
mean at the puffing level, which would necessitate
a rapid
and efficient feedback mechanism during this
event.
Tobin (21) reported than when subjects smoking
higher tar
products (26 mg tar : 1.8 mg nicotine) were
switched to low
tar products (4 mg tar : 0.4 mg nicotine), the
puff volumes
increased very significantly.
(iii) Influence of Nicotine on Smoking Behaviour
Based
on Biological Fluid Measurement
Goldfarb (22) indicated from his studies that the
yield of
nicotine from a product influences the number of
cigarettes
smoked. In addition, he reported that the urinary
level of
nicotine was positively correlated with the yields
of nicotine
from 6 cigarette types studied. Gritz (23) studied
12 subjects
smoking fixed numbers of standard and modified
cigarettes,
each for a period of 7 days. The standard
cigarettes were
modified by cutting to half normal length or by
restricting
the amount of the full product available for
smoking. Based
on urinary nicotine figures, it was concluded that
the
reduction in the amount of the product available
for smoking
results in an increase in smoking intensity.
Guillerm (24)
switched 75 Gauloises smokers (1.7 mg nicotine) to
a lower
delivery product (0.7 mg nicotine). He observed a
slight
increase in cigarette consumption but noted that
the level of CD
carboxyhaemoglobin in these smokers was increased
dramatically
even though the lower delivery product had a 30%
reduction in CD
BATCo document for Province of British Columbia 5
November 1999


-20-
carbon monoxide delivery compared with machine
smoked values
for Gauloises. These results implied a
considerable oversmoking
of the lower delivery product because of the
reduced availa-
bility of nicotine. Russell (25) conducted a study
in which
own brand smokers were monitored smoking the full
cigarette
or cigarettes reduced to 75 or 50% of their
original length
(resulting in a machine smoke reduction in
nicotine of 25 and
40% respectively). He concluded that reducing the
cigarette
length available for smoking resulted in a shorter
butt
length, an increase in the numbers of cigarettes
smoked and
an increase in puffing intensity. Based on plasma
nicotine
measurements (and carboxyhaemoglobin levels), it
was concluded
that the deliveries to the smokers had remained
constant
through compensation (over-smoking).
Ashton (26) conducted a cross-over study in which
subjects
were monitored on their own brands and then
switched to three
alternative products - high (1.8 mg), medium (1.4
mg) and low
(0.6 mg) levels of nicotine. It was concluded from
the study
that, compared to the medium nicotine containing
and own brand
cigarettes, the high delivery product was
under-smoked whilst
the low delivery product was over-smoked, based on
plasma
urinary and butt nicotine levels.
Hill (27) monitored a limited number of subjects
smoking
individual cigarettes from 10 brands and also
followed subjects
switched to single brands for a further two weeks.
He
concluded that the rise in plasma nicotine and
cotinine was
related to the nicotine delivery of the cigarette
and that
subjects adjust their smoking styles to maintain a
constant
cotinine level. Benowitz (28) monitored 12
subjects who
smoked more than 30 cigarettes/day, and then
switched to two
experimental brands containing high (2.5 mg) and
low (0.4 mg)
levels of nicotine. Based on plasma nicotine
estimates, he
concluded that the high nciotine containing
cigarettes were
smoked less intensively; however, the low nicotine
containing <=)
cigarettes were not smoked more intensively. The
authors
CC)
BATCO document for Province of BritiSh Columbia 5
November 1999


-21-
suggest that neither the numbers of cigarettes
smoked nor the
machine smoking deliveries are predictors of
nicotine dose to
the smoker.
Sutton (29) monitored 55 own brand smokers and
concluded that
the total volume of smoke puffed from the
cigarette was a
better predictor of peak blood levels of nicotine
than machine
smoked nicotine and tar yields. butt length or
reported numbers
of cigarettes smoked. In this study, where
nicotine was
controlled for, smokers of low tar cigarettes
puffed more
(greater voluff-es) than the high tar smokers and
had higher
blood nicotine values. When tar was controlled for
low
nicotine smokers puffed less smoke than the high
nicotine
cigarette smokers.
Fagerstrom (30) monitored 12 subjects smoking
three cigarettes;
each cigarette was smoked for four weeks. Subjects
smoked
their own brands and two experimental brands - (i)
4.8 mg tar
4.0 mg CO : 0.5 mg nicotine and (ii) 5.8 mg tar :
4.1 CO
1.1 nig nicotine. In this study there were no
changes in
cigarette consumption and no observed differences
in plasma
nicotine or cotinine across the three cigarette
types.
Examination of these reported studies gives a
clear indication
of the range of different approaches to
determining and
characterising human smoking behaviour and indeed
the diverse
interpretation of the reported data to explain
smoking
behaviour patterns. An attempt has been made in
the subsequent
part of this section to identify and/or develop
plausible and
tangible links between the specific aspects of the
smoking
behaviour process.
Subjective Assessment of Nicotine and Cigarette
Strength
It is generally accepted (although not fully
substantiated)
that the presence of nicotine in whole cigarette
smoke is
BATCo document for Province of BritiSh Columbia 6
November 1999


-22-
primarily responsible for the subjective sensory
response of
impact, experienced on inhalation. Nicotine may
also contribute
to other sensory attributes, although this has not
been
established. The smoke impact response can be
modified
either by changing the nicotine content of the
blend or by
changing the smoke pH of products with equivalent
nicotine
contents (31, 32). Increasing smoke pH (i.e. more
alkaline)
has the effect of increasing the perceived impact
rating of
the product. These observations are important as
impact is
intimately involved in the assessment of product
strength for
inhaling ciQarette smokers and is probably the
most important
contributor to the 'estimation' of strength.
Several important
questions arise as a consequence of this
observation:
0 ) Where are the sites and what is the mechanism
for
impact assessment?
(ii) Is nicotine the only component capable of
eliciting
an impact response on inhalation?
(iii) What is the relevance of smoke pH to impact?
Impact assessment is achieved in the upper
respiratory system,
i.e. within the larynx and upper trachea. This
region of the
respiratory system is well supplied with nerves to
assess the
presence and deposition of chemicals and irritant
stimuli as
part of the normal protection mechanisms for the
lung. The
upper respiratory tract stimulation is elicited
through the
excitation of receptors, and these are of
essentially two
types - chemical /i rri tant and cough receptors.
This
observation is important in terms of how smokers
adapt or
accommodate to smoke inhalation without
difficulty. It is
suggested that smokers might adapt to smoke
inhalation either
by adaptation of the receptor or by increasing its
threshold
to stimulation, or an alternative explanation is
that nicotine C=)
(or other components) might suppress couqh and or
irritation CD
by desensitisation of the receptor. The
identification of CD
(-.M
CX:)
CD
BATCo document for Province of British Columbia 5
November 1999


-23-
the mechanism and level of adaptation within the
upper
respiratory tract receptors following smoke
exposure is of
considerable importance in terms of improving
smoke quality
and enhancement of the impact of smoke. A series
of external
studies has been initiated (specifically Dr. P.
Richardson,
St. George's Hospital, London), involving animal
studies, to
characterise the nature of the upper respiratory
tract response.
A number of empirical studies have been conducted
to identify
the component(s) in cigarette smoke that can
elicit an impact
response. It has been determined, albeit not
exclusively,
that nicotine is responsible for a smoker's
impact. A correla-
tion has been established to demonstrate a
relationship
between the level of 'available' nicotine and
impact (31).
The level of 'available' nicotine refers to the
amount of
nicotine that can be extracted from condensate
using organic
solvents compared to the total level of nicotine
present.
The proportion of nicotine that can be extracted
from condensate
is related to the condensate pH (frequently
referred to as
smoke pH); the more alkaline the smoke, the
greater the
percentaqe of extractable nicotine. This effect
can be
clearly demonstrated in subjective product panel
testing,
e.9. by comparing US air-cured tobacco products
(relatively
alkaline smoke) and Virginia flue-cured products
(relatively
acidic smoke) nominally matched for total nicotine
content.
This effect can also be demonstrated within
tobacco blends of
different condensate pH. The studies of Backhurst
(31)
indicated that small shifts in smoke pH can have
dramatic
effects on 'available' nicotine levels and hence
radically
alter the quality of smoke. This knowledge has not
been
fully exploited to identify the ability to modify
a control
smoke pH as a mechanism for modifying impact and
other sensory
characteristics of smoke.
It is perhaps worthy of note that the upper
respiratory tract CD
responses are also responsible for other
subjective responses
CX:)
BATCo document for Province of BritiSh Columbia 5
November 1999


-24-
associated with smoke inhalation, the most
important probably
being irritation. The subjective report of
irritation
following smoke inhalation is probably although
not exclusively
of pharyngeal origin. Identification of the sites
of response
and the causal mechanism would have obvious
advantages for
improving product quality.
A Smoker's Requirement for Nicotine
The hypothesis that smokers smoke for an absolute
amount of
nicotine has been advanced by Russell and others.
This so-
called nicotine titration hypothesis predicts that
a smoker,
when presented with a cigarette of lower nicotine
content
than his own brand, will adjust his smoking
behaviour to
attempt to increase the level of nicotine intake.
This can
be achieved by increasing the number of cigarettes
consumes
(the hypothesis does not discriminate necessarily
between
peak nicotine intake per puff, per cigarette and
overall
daily intake), a more intensive style of smoking
including
an increase in puff volume, frequency and changes
in the
proportion of smoke inhaled and retention in the
lung. This
hypothesis was advanced for middle tar smokers
smoking products
in the 16-18 mg tar : 1.3-1.5 mg nicotine range
and it was
assumed that this category of smokers (derived
from previous
high tar smokers) would continue to 'demand' from
the product
these levels of nicotine and modify their smoking
behaviour
accordingly when presented with products of lower
nicotine
deliveries. Russell has presented 'evidence' to
support his
original proposal from two types of experiment.
The rise in plasma nicotine levels obtained by
smGkers smoking
products from their own brands across a range of
product
deliveries and from smokers when switched to lower
delivery C7-5
nicotine products have been determined. The
essential lack of
difference between these values have prompted the
conclusion
leading to the development of the nicotine
titration hypothesis. U I
CO
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November 1999


-25-
These studies, however, are experimentally and
scientifically
weak. From the point of view of the nicotine
pharmacokinetics
it can be argued that plasma nicotine levels
change extremely
rapidly following uptake and distribution within
the available
tissue water space. Hence the nicotine is rapidly
and exten-
sively diluted., which introduces analytical
errors, the plasma
nicotine level being extremely time dependent
post-smoking.
A further complication with interpretation of
these findings
is that subjects within a brand switching exercise
are frequently
not fully acclimatised to alternative products,
often no account
of the effect of the time of day (i.e. previous
smoking events)
and the relationship between the pre-smoking level
and the
plasma increase in nicotine level (i.e. does the
plasma
nicotine level increase linearly with nicotine
dose). In
addition, single measures of estimating plasma
nicotine do
not take into account the contribution of nicotine
intake
from other cigarettes.
Other workers, however, have independently
demonstrated that
the nicotine dose in brand switching studies is
essentially
independent of machine smoked deliveries and for
an individual
smoker is related to his own brand deliveries of
nicotine.
These findings are subject to the limitations of
estimating
nicotine dose from biological fluid analysis (see
Section 2).
In studies where a subject's smoking behaviour has
been
monitored, nicotine (and/or smoke) intake can be
estimated
in several ways; from butt nicotine analyses,
total puff
volume taken from a product, and from human
smoking record
duplication. These measures can be used to
estimate the
maximum level of nicotine (and smoke) presented to
the smoker.
These measures have been used in brand switching
studies to
determine intake. These findings have indicated
that, when
smokers are switched to a product with a reduced
nicotine
delivery, these measures are usually increased,
giving rise C=
to (nicotine) compensation. As a general
observation, although
Un
there is considerable inter-subject variability,
middle tar
BATCO document for Province of British Columbia 5
November 1999


-26-
smokers, when moved to lower tar products, tend to
compensate
by changing their style of smoking to achieve a
nicotine
dose close to mid-way between the deliveries of
the two
product types.
As indicated, the measures used to assess smoking
behaviour
essentially only estimate the amount of material
taken into
the smoker's mouth, the so-called mouth-level
intake.
Therefore, the smoking behaviour data is not
necessarily at
odds with the nicotine titration hypothesis
provided that the
proportion of the smoke taken from the mouth into
the
respiratory system is inversely related to the
product yields.
This can only be estimated by simultaneously
monitoring the
physical characteristics of smoking (that
determine the mouth
level intake) in relation to monitoring a smoker's
respiratory
characteristics. Although this knowledge would be
an advance
on estimating mouth-level intake alone, our
understanding of
nicotine adsorption during inhalation is
inadequately studied.
The ability to monitor puffing characteristics in
relation to
inhalation characteristics exist within GP&OC (33)
and experiments
are continuing, to determine the relationship
between these
events. It is worthy of note that other U.K.
tobacco companies
are actively pursuing this line of research.
Additional information exists which can give an
insight into
how cigarettes are smoked in terms of nicotine
yields. Using
brand switching studies, the amount of nicotine
absorbed by
the smoker over a 24 hour period can be estimated
from urinary
nicotine netabolites (see Section 2). A comparison
of the
daily intake of nicotine with the amount of
nicotine available
(estimated from the frequency of smoking and the
machine
smoke yield) indicates on a relative basis how
cigarettes are
being snaked. These studies have indicated that,
above
10-12 mg nicotine available per day, a relative
constant
percentage of nicotine is absorbed from the
product. However,
when the daily available dose was below 10-12 Pig
of nicotine,
a disproportionate increased amount of nicotine
was absorbed
00
BATCo document for Province of BritiSh Columbia 5
November 1999


-27-
from the product (34). These results can be
interpreted to
indicate that there may indeed be a threshold
requirement
for nicotine below which the smoker is not
prepared to go.
An alternative approach to explain a smoker's
smoking behaviour
is being advanced by Higenbottam, and suggests
that, rather
than an absolute requirement for nicotine, the
ratio of tar
to nicotine is important for the smoker in terms
of smoke
inhalability. Higenbottam (35) has advanced the
hypothesis
that nicotine facilitates the intake of tobacco
smoke into
the lungs and indeed the ratio of tar : nicotine
yield of
ciqarettes may be an important determinant of puff
patterns.
He has proposed that the puffing pattern of a
smoker is
determined by the need to maintain a constant
ratio of the
irritant components (particulate material) and
nicotine in
the inhaled smoke. The rationale behind this
hypothesis is
that nicotine specifically and actively suppresses
the upper
airway irritant and cough receptor response.
The hypothesis is based on three lines of
evidence:
(i) Changes in puffing patterns during smoking a
single
cigarette. Under machine smoking conditions, puff
volume and interval remain constant with the puff
by puff tar to nicotine ratios tending to
increase.
Human smokers tend to reduce puff volume and
increase
the interval between successive puffs, a style of
smoking likely to result in a decrease in the tar
to nicotine ratio. However, an alternative
explana-
tion to this ovservation is that, if tar and
nicotine
yields are increasing as the cigarette is smoked,
the smoker may be sensitive to the changing
delivery
and hence reduces smoking behaviour to maintain a
'constant' dose of smoke. The tar to nicotine
ratio would then tend to be a consequence rather
than the cause of the subject's smoking style. CD
QJI
CO
BATCo document for Province of British Columbia 5
November 1999


-28-
(ii) Puffing patterns of own brand smokers smoking
products with different deliveries. The study of
Sutton (36) is used as the basis for providing
evidence that, when tar delivery was controlled
for, smokers of products with lower nicotine
content
(i.e. increased tar : nicotine ratios) took
smaller
puff volumes and less frequent puffs compared to
the smokers of higher nicotine-containing
products.
These findings are presented as evidence against
the nicotine titration hypothesis. Caution should
be exercised in interpreting the Sutton data as
virtually all the smokers in this study were
middle
tar smokers smoking relatively high numbers of
cigarettes and if the nicotine threshold proposal
has any basis of fact, then an adequate quantity
of
nicotine should have been readily available from
all products smoked.
(iii) changes in puffing patterns when switching
to
cigarette brands with different cigarette yields.
Higenbottam has reviewed nine brand switchinq
studies in which puff volume was recorded. He has
attempted to relate changes in puff volume to the
tar and nicotine yields or the ratio of tar
n1 cotine. Within the nine switching studies,
twelve test brands were smoked. From the investi-
gation no clear relationship emerged for puff
volume
changes when tar and nicotine yield alone were
studied. However, for 10 of the 12 brands studied,
puff volume varied inversely to the tar to
nicotine
ratio - i.e. as the tar to nicotine ratio
increased
the puff volume decreased, whilst the converse was
also observed. From these observations he
concluded
that tar : nicotine ratio is important in terms of
facilitating smoke inhalation.
CD
CD
CO
ON
BATCo document for Province of British Columbia 5
November 1999


-29-
Russell, in recent years, has moved away from the
nicotine
titration hypothesis and has further suggested,
based on his
recent research findings, that smokers may in fact
smoke for
tar. Whether or not nicotine is the primary
motivating
factor for cigarette smokers, an inevitable
consequence of
smoke inhalation is the absorption of nicotine
which will
elicit a pharmacological response. It would seem
logical
to conclude that the pharmacological response is
pleasurable
and probably the major reinforcing factor
contributing to
the overall product acceptability and
satisfaction. If we
examine how the smoker might manipulate the dose
of nicotine
he can obtain from the product, a clearer picture
can be
presented in an attempt to clarify the
relationship between
product design/acceptability and smoking
behaviour.
Interestingly, the simplest and most obvious way a
smoker
could increase or achieve a dose of nicotine if
this were the
only criterion would be to smoke higher delivery
products and
in terms of economics the smoker would smoke fewer
higher
delivery cigarettes. However, this has not been
the case,
which suggests that smoking is a much more complex
process.
Nicotine Dose Manipulation and its Relation to
Smoking
Behaviour
Workinq from the premise that nicotine is a
pre-requisite for
the inhaling cigarette smoker, how can the
quantity of nicotine
be assessed and/or modulated by the smoker. There
are
essentially three levels at which this could
occur:
(i) Mouth-level assessment
It is conceivable that the nicotine content of
cigarette smoke can be assessed directly within
the
mouth and, if this were a rapid instantaneous
C:)
process, it can be envisaged that the puff volume
C)
could be modified accordingly. Based on the use of
ILn
CO
BATCo document for Province of British Columbia 5
November 1999


-30-
nicotine chewing gum, chewing tobacco and nasal
application of nicotine gel, there is little or
no direct evidence that nicotine can be readily
assessed directly within the mouth (or nasal
cavity),
or indeed that it elicits significant sensory or
subjective responses. This is an important
observation, particularly since significant
quantities
can be absorbed via these routes. It is reasonable
to suggest that an estimate of smoke dose within
the mouth can be assessed in terms of its
mouthful/
mouthfeel characteristics (and other sensory
attributes). This assessment is likely to be a
function of the quantity of tar (particulate)
present within the mouth, albeit modified by the
tar quality. In this manner it is reasonable to
develop the concept of a relationship between the
tar and nicotine delivery of a product where the
tar in a sensory sense is used to 'estimate' the
anticipated nicotine content within the smoke. For
commercial products, the ratio of tar : nicotine
is
within the range 10:1-14:1 and therefore, within
the
realm of a smoker's general experience. it can be
envisaged that tar could be used as a sensory cue
for estimating nicotine content across the range
of
product delvieries (i.e. puff volume can be
increased
on this basis). There is some indirect evidence to
support this concept; where experimental products
have been manufactured with tar : nicotine ratios
of 7:1 and 5:1, smokers have rejected them on the
basis of too much impact (see below) and wholebody
responses (i.e. CNS effects, see Section 3). These
findings suggest that, once the puff volum (and
smoke dose) has been taken, other physiological/
pharmacologocal events occur and may thereafter be
to some extent ordained, without being used as
controlling factors.
CO
BATCo document for Province of British Columbia 5
November 1999


_31-
There is an additional possible controlling factor
for nicotine (smoke) dose manipulation after the
puff has been taken throuqh smoke exhalation (or
mouth spill) prior to inhalation and hence the
residue is proportional to the mouth level taken
for inhalation. The quantitative significance of
mouth spill is essentially unknown and may be an
idiosyncratic event.
(ii) Assessment during inhalation
Unlike mouth level assessment that could give rise
to within puff modification from either direct or
indirect assessment, any assessment based on
inhalation or wholebody response will only
influence
subsequent puffinq characteristics. However, the
assessment that occurs within the upper
respiratory
tract (i.e. pharynx, larynx or trachea) could be
used to modify smoking behaviour through
respiratory
pattern change and hence smoke dose retention.
Using
this level of assessment, the rate and volume of
inhalation (and therefore the smoke dilution)
could
be changed to facilitate inhalation or modify the
nicotine dose or response for a given individual.
A reduction in the rate and volume of inhalation
could conceivably increase the upper respiratory
tract responses. The depth and length of smoke
hold period within the respiratory system could be
important determinants for total nicotine dose to
the individual smoker.
Preliminary results from a comparison of own brand
middle and low tar smokers puffing and inhalation
characteristics have indicated particularly for
the
CD
inhalation measures a high degree of within
subject
consistency and has not revealed any striking
differences between subjects in terms of depth,
rate
or volume of inhalation post-puffing (37). Indeed,
BATCo document for Province of British Columbia 5
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-32-
even when subjects have been switched from low tar
to high tar products, and vice versa, their
individual
inhalation characteristics have remained
reasonably
constant. These findings would suggest that
nicotine
dose (smoke dose) is probably not modulated per se
by use of inhalation pattern and, more
importantly,
indicates a particular smoking (inhalation) style
which may be related to a smoker's acceptability
criteria. It must be stressed, however, that these
smokers have not been switched dramatically
outside
their range of product experience (or over
extended
time periods) and inhalation characteristics may
be modified at substantially higher or lower
product
deliveries and over time.
There are two additional areas in which the role
of
nicotine and the upper respiratory tract
stimualtion
is important. The stimulation of the nerve
reflexes
within the upper respiratory tract has been
suggested
by some researchers to be inherently pleasurable
in
itself and, as such, is an important element of
the
smoking satisfaction equation. This phenomenon
has been likened to eating hot spices and peppers
in which it has been suggested (although not
demonstrated) that the CNS can be stimulated to
release endogenous endorphins and enkephalins as a
conditioned response to mouth stimulation. The
upper respiratory tract stimulation (e.g. impact)
is, or can potentially be, used as a sensory cue
by the smoker to evaluate the strength of a
product
based on previous experience and to 'Judge'
product
acceptance in terms of the expected
pharmacological
response or requirements.
(iii) Assessment based on a wholebody response
Objective and even subjective measures of the
wholebody pharmacological responses are difficult
CD
CD
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-33-
to assess in established smokers (see section on
nicotine pharmacology). In addition. most smokers
do not exhibit significant physiological changes
following smoking their own brand product, except
following periods of smoking abstinence. The
major basis on which wholebody nicotine effects
are
judged tend to be based on psychological measures
of performance and arousal. However, it is
interesting to note that, where subjects smoked
products with modified, decreased tar : nicotine
ratios (disproportionately high relative nicotine
levels), they reported symptoms usually associated
with novice smokers, i.e. dizziness, headache,
nausea and head and heart 'pounding'. These
indirect
observations suggest that, under normal
circumstances,
the nicotine dose is usually 'controlled' to give
a
CNS pharmacological response with a minimum of
physiological events. The rate of nicotine
absorption
in animals has been shown to be extremely rapid
and the time for transfer of nicotine from the
lung to the brain is estimated to be in the order
of seconds. The time taken for nicotine to elicit
and maintain a CNS effect is unknown. It may be
that the time scale for the pharmacological
response
is outside the time taken for the puffing and
inhalation events and hence has to be assessed on
other sensory cues in anticipation of the eventual
CNS effect.
Summary and Proposals for Further Smoking
Behaviour Research
Based on the currently available evidence, the
role of nicotine
in influencing the smoking process is being
identified and
characterised. However, it must be stressed that
our knowledge
in this area is far from complete. An attempt has
been made
CD
to interpret current findings and summarise the
role of
BATCo document for Province of BritiSh Columbia 5
November 1999


-34-
nicotine within the smoking process and to
identify areas of
research relevant to current and future product
design to
help meet the needs of the industry.
It would seem reasonable to conclude that,
inhaling cigarette
smokers, smoke at least in part for nicotine. In
this sense,
smokers have an absolute requirement for nicotine
in terms of
a satisfying product; however, the minimum
acceptable dose
is not known with any degree of certainty. There
would appear
to be a requirement in terms of product
acceptability for an
intimate balance between the ratio of tar and
nicotine that
can substantially influence how the product is
smoked. The
need for a tar : nicotine balance probably arises
from the
sensory attributes of tar acting as a cue for the
associated
nicotine level. Indeed, this assessment is likely
to occur
within the mouth and be a major factor in
influencing puff
volume and puffing characteristics generally. In
terms of
product acceptability the sensory cues used during
cigarette
puffing must provide a product expectation that
staisfies a
tolerance band-width for pleasure/sensory
reinforcement
during inhalation. The pharmacological wholebody
response
that arises from nicotine (smoke component)
absorption must,
for an acceptable product, fall within a tolerance
band-width
of normal experience associated with the
appropriate sensory
cues derived at the mouth and as part of the
sensory appraisal/
satisfaction subsequently occurring within the
upper respiratory
tract.
To extend our understanding of the role of
nicotine in
influencing and modifying smoking behaviour, there
is a need
to establish the following factors:
(i) The role of a constant tar : nicotine ratio in
influencing smoking behaviour over a range of tar
C=)
delivery categories. CD
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November 1999


-35-
(ii) The effect of modifying the tar to nicotine
ratio
in the direction of enhanced relative amount of
nicotine (i.e. reduced tar : nicotine) over a
range
of tar delivery categories, either by product
blending or by blend enhancement through nicotine
addition.
(iii) Following the identification of the most
acceptable
tar : nicotine ratio at the appropriate delivery
level, an attempt to systematically improve the
mouth-level characteristics of the product by
blend
modification or through cigarette design.
(iv) To direct research towards experimentally
producing
nicotine enriched cigarette smoke, and the
development
of synthetic aerosols containing nicotine to
determine
the contribution of nicotine to the sensory and
subjective elements of the smoking process.
BATCo document for Province of British Columbia 5
November 1999


-36-
SECTION 2 - PHARMACOKINETICS OF NICOTINE; NICOTINE
DOSE
ESTIMATES
SUM14ARY
The study of nicotine (and cotinine)
pharmacokinetic parameters
provide an important if not vital link between
observed
smoking behaviour, the means by which nicotine is
obtained
and the wholebody pharmacological response, the
probable
motivating and driving force for nicotine intake.
Within this section an attempt is made to outline
in broad
terms what is meant by pharmacokinetics and to
identify the
elements within this process and to examine them
in relation
to the absorption of nicotine dry smoking.
The localisation and extent of nicotine uptake
during smoke
inhalation has important implications and
consequences for
its subsequent wholebody effects and the potential
significance
of these events are examined. The ability of
nicotine to
rapidly distribute within the body is considered
in relation
to estimating nicotine dose from plasma nicotine
estimates.
It has been established that nicotine is
metabolised extensively
to cotinine a metabolite that has a lonq residence
time within
the body relative to nicotine. The potential value
of using
this metabolite for estimating the dose of
nicotine adsorbed
following smoke inhalation is examined in relation
to actual
and expected plasma values. The significance of
these measures
in relation to smoking behaviour observation is
discussed.
The clearance of nicotine and other metabolites~
in the urine
have potential importance in terms of estimating
daily nicotine
intake, and again these observations have
significance in
terms of evaluating a smokers requirement for
nicotine.
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November 1999


-37-
PHARMACOKINETICS OF NICOTINE; NICOTINE DOSE
ESTIMATES
Background to Pharmacokinetics
Pharmacokinetics is the study of the uptake,
distribution,
metabolism and ultimate clearance of materials
from the body.
The pharmacokinetic properties of a specific
compound can be
highly complex depending on its physical and
chemical properties
and the manner in which it interacts with the
body. It will
be apparent that to accurately determine the
pharmacokinetic
properties of a compound demands a detailed
knowledge of the
rate of uptake, distribution, metabolism, and
clearance rate
of the material from within the body. In practice
this is
rarely the case, and hence a series of
approximations have
to be adopted according to the complexity of the
problem.
To make valid criticism of the published and
reported data
it is necessary to treat the various
pharmacokinetic elements
separately and to identify the strengths and
weaknesses of
that information and its significance for
quantifying the
level and extent of nicotine interaction within
the body.
Nicotine Uptake
Quantifying the nicotine dose derived from
cigarette smoking
during inhalation is a complex problem. However,
based on
human smoke recording and simulated smoke
duplication the
quantity of nicotine delivered to the mouth
(intake) can be
estimated. The extent to which nicotine is
absorbed i.e. the
level of nicotine uptake achieved during
inhalation is
difficult to quantify. Estimates based on inh'aled
exhaled
differences measurements (38) would indicate that
nicotine,
present in cigarette smoke is quantitatively
absorbed during
inhalation. These studies are difficult to conduct
and are
fraught with analytical problems. In an effort to
determine
the absolute level of nicotine retention from
different
products under varying conditions of inhalation,
techniques
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-38-
for collecting exhalate and trapping nicotine have
been
developed and are currently under evaluation (39,
40).
Even after quantifying the level of nicotine
retained
following smoke inhalation will still leave
several important
areas for consideration. The most important are
(i) what is
the site(s) of uptake of nicotine and (ii) is it
effectively
adsorbed as a bolus. These two parameters can have
significant
effects on the nature and interpretation of the
pharmacokinetic
data. Immediately it can be appreciated that in
the case of
nicotine uptake during smoke inhalation.
difficulties exist
in quantifying the dose, determining the time
scale of uptake
and identifying the sites of uptake. The latter is
an important
question in terms of identifying the vasculature
(blood
vessels) in which the absorbed dose is carried and
distributed
within the body. Lack of knowledge and research in
these
areas limit the potential for improving the
quality of the
smoke product and perpetuates the problems
associated with
the interpretation and evaluation of selected
aspects of
pharmacokinetic data reported in the literature.
A number of studies involving the administration
of nicotine
by various routes to mimic nicotine uptake by
smoke inhalation
have been reported (75. 77, 41, 43). When nicotine
is absorbed
via the lungs, it is returned to the heart
(essentially
independent of vascular drainage) with the
oxygenated blood
and thereafter is pumped to the tissues in the
arterial
blood flow, the nature of the blood flow making
nicotine
immediately available for the brain. In the case
of cigarette
smoking, this process occurs repeatedly for a
single smoking
event correspondinq to the number of inhaled puffs
taken
from the cigarette. This factor is an important
point of
comparison between cigarette smoking as a means of
nicotine
dosing and other studies involving alternative
means of
nicotine dosing. There are no detailed published
reports of
studies in which nicotine alone is taken directly
via the
lungs, however Russell has indicated (42) that he
has been
unable to achieve measurable increases in plasma
nicotine
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-39-
followinq nicotine aerosol inhalation. This
finding would
suggest that in this study, the nicotine aerosol
was not
taken into the respiratory system probably because
of the
formation of aerosols with incorrect aerodynamic
particle
sizea diameter (such nebuliser systems generate
large
particle aerosols and hence deposit in the mouth
and pharynx).
As previously mentioned studies have been reported
in which
nicotine has been injected or infused directly
into the
blood stream of subjects usually via a vein in the
arm.
Nicotine injected or infused in this manner will
follow a
differential route of body distribution,
metabolism and
localised concentration, compared to nicotine
absorbed from
smoke. Consequently the pharmacokinetic parameters
together
with the reported physiological and
pharmacological observations
(first pass effect) may well differ from those
reported for
smoke inhalation. A study to attempt to resolve
the differential
effects of nicotine on physiological and
pharmacological
responses following administration by alternative
routes
including infusion, bolus injection, smoke
inhalation and
nicotine aerosol intake via the respiratory system
is in
progress (44).
Nicotine; Whole Body Distribution and Metabolism
Durinq smoVe inhalation the dose of nicotine
reaches the
brain rapidly for the reasons outlined above and
indeed it
has been reported that nicotine will reach the
brain within a
few seconds of inhalation. This observation has
led to the
statement that smoke inhalation is like injecting
nicotine
directly into the carotid artery (the vessel
supplying blood
to the brain). The pulsed, changing concentration
wave of
nicotine within the blood then circulates the
whole body. In
human studies where smokers blood is sampled
immediately pre-
and post-smoking, the plasma (blood) values of
nicotine
increase in the range of 0-50 ng (1 ng =_ 10-9
9)/m1. Based U-1
on this observation it is possible to make some
assumptions
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-40-
to estimate the fate and distribution of the
absorbed nicotine:
assuming an average bodyweight of 70 kg a
corresponding
blood volume will be approximately 4L (4000 ml),
with an
average rise of 25 ng/ml following smoking a I mg
nicotine
delivery cigarette, based on simple dilution a
plasma value
of 250 ng/ml would have been expected. This
apparent anomaly
can arise for several possible reasons:
M the absorbed nicotine is not uniformly
distributed
in the blood, however because local, high
concentrations
of nicotine are not encountered this is unlikely,
(although rapid uptake of nicotine into specific
tissues is a possibility),
Oi) absorbed nicotine is rapidly converted (via
metabolic
processes) to other materials (metabolites), again
because no (or very low levels of metabolites) are
observed within this timescale this is unlikely,
(iii) the most reasonable explanation is that
nicotine is
not confined to the blood volume (4L) but rather a
larger volume of the order of 60L plus based on
simple dilution.
This volume is equivalent or approaching the total
tissue
water space of the body (and would include the
retained urine
volume in the bladder). This is a reasonable
assumption of
the fate of nicotine and fits with the known
properties of
nicotine in being able to rapidly pass across
biological membranes.
Increase in plasma levels of nicotine have
been'achieved
through administration of nicotine as nicotine
chewinq gum
(buccal absorption), snuff and gelatenous
preparations of
CD
nicotine taken via the nose and following
intravenous infusion/
pulsed injections that are similar to those
achieved following CD
CD
smoke inhalation. However, having developed this
general
Ul
picture of an extremely efficient uptake and
distribution 1.0
cc
BATCo document for Province of British Columbia 5
November 1999


-41-
process for nicotine within the body it is
important to
re-examine the consequences of these properties
for (i)
estimating the nicotine dose from the circulating
levels of
nicotine, and (ii) the comparative physiological
/pharmacological
effects of nicotine when administered in smoke and
via other
routes.
In smoking behaviour studies where blood samples
have been
taken and analysed for nicotine content as a
measure of
nicotine dose, the validity of that measurement is
critically
dependant on the timing of blood sampling. In
studies when
blood sampling is carefully controlled this can
only reasonably
be used as a crude measure of nicotine dose for an
individual,
and it is doubtful whether sufficient information
exists for
individual subjects (pharmacokinetic parameters
for nicotine
clearance and metabolic profile) to enable an
absolute estimate
of nicotine dose to be calculated.
Nicotine uptake following smoke inhalation can
potentially
give rise to local, relatively high concentrations
of nicotine
to the tissues (particularly the heart and brain)
consequently
the physiological and pharmacological consequences
of this
mode of nicotine absorption is likely to differ
from effects
observed when nicotine is infused intravenously or
arterially.
These specific differential effects are being
investigated in
a human nicotine pharmacokinetic study at the
Dept. of Clinical
Pharmacology University of Southampton.
Following the whole body distribution of nicotine,
it is
readily metabolised to a wide variety of
intermediates (45) as
part of normal or adaptive detoxification
processes. The
primary site of metabolism is the liver although
many tissues
have the ability to metabolise nicotine. The
primary
metabolites formed from nicotine are cotinine and
nicotine
N-l-oxide. The metabolic conversion of nicotine to
cotinine
is a relatively slow process compared with the
whole body
distribution of nicotine. The rate of conversion
of nicotine
BATCo document for Province of British Columbia 5
November 1999


-42-
to cotinine has been investigated in a limited
number of
smokers (46). In this study the cigarettes were
labelled
with radioactive nicotine and the time scale for
the uptake.
distribution and loss of nicotine from the blood
was determined.
Simultaneously the formation of cotinine was
similarly
determined and an estimate of the rate of
conversion of
nicotine to cotinine established. In this and
other nicotine
dosing studies, nicotine dosing is restricted to
relatively
short time periods and similarly the time course
for following
the conversion of nicotine to cotinine is
relatively short.
Under natural smoking conditions each smoking
event elevates
the whole body nicotine dose and hence contributes
to the
.pool' of cotinine (and other nicotine
metabolites). In
general terms it can be appreciated that as this
process
approaches a steady-state or equilibrium condition
where a
given rate of nicotine dosinq will establish a
relative
constant level of cotinine. The absolute level of
cotinine
(and other metabolites) will be a combination of
many factors
(generally individual dependant) these include the
metabolising
capacity, the concentration of nicotine absorbed,
dose
frequency (numbers of cigarettes smoked and
frequency) and
the equilibrium between nicotine and cotinine (a
function
of distribution and clearance rates of these
respective
metabolites). It has been demonstrated by many
workers that
the ratio of plasma nicotine to cotinine is in the
order of
1 :10 reflecting different pharmacokinetic
parameters for
these metabolites. The rate at which cotinine is
cleared
from the body (expressed in terms of half-life,
the time
taken to effectively clear 50% of the administered
material
from the body) is in the order of 20 hours (47).
This lonq
residence time of cotinine within the body
(rel'ative to
nicotine) would suggest that this metabolite may
be a relatively
stable marker for the level of nicotine absorbed
by the
smoker. This assumption has not been fully
validated for
practical reasons, the approach would require
information on CD
the nicotine dose (from each cigarette), time at
which cigarettes
were smoked in relation to the sampling time and
the rate at
ON
Cz:)
CD
BATCo document for Province of British Columbia 5
November 1999


-43-
which an individual metabolised nicotine to
cotinine and the
specific clearance parameters. In practice this
would be an
extremely difficult (if not impossible) task and
therefore
it is necessary to derive pharmacokinetic
equations derived*
from a series of studies that can be used to
approximate to
the specific situation. Such kinetic parameters
have been
derived by Curry (48) and applied by other workers
(49).
The Curry equation can be used to calculate the
expected
plasma cotinine level based on a nominal nicotine
dose to
the body. This calculated value can be compared to
actual
plasma cotinine values as a means of estimating
the level of
nicotine derived from a given cigarette product.
This
calculation provides the basis for the development
of a
hypothesis for studying how smokers might respond
in smoking
behavioural terms to products of differing
nicotine deliveries.
The equation can be expressed as:
Expected plasma cotinine (ng/ml)
machine smoked rate of conversion
nicotine deliveries x of nicotine to continine x
(1-0.5a/b)
The estimated distribution volume (1 kg bw =_ 1000
ml)
where a = frequency of smoking (average time
interval between
cigarettes)
b = the half-life clearance rate for cotinine.
This approach has been used by Gori (49). in which
he compared
actual and expected plasma cotinine values for
smokers smoking
ultra low delivery products (sub 5 mg tar; 0.5 mg
nicotine/
cigarette). In these studies the actual plasma
cotinine
levels were considerably higher than the predicted
values.
This approach has been extended here to calculate
the expected
values for 'smokers' smoking a range of nicotine
delivery C:~
products of 0-2 mg, with varying numbers of
cigarettes smoked,
ON
BATCo document for Province of British Columbia 5
November 1999


-44-
10, 15, 20 and 30/day and the results expressed in
Figure 1.
From the figure it can be seen that values in
excess of 750
ng/ml would be expected for many smokers.
Interestingly, for
the majority of inhaling cigarette smokers the
plasma cotinine
values are found in the range 200-400 ng/ml rarely
exceeding
this upper value. Based on the observation of a
trend of
increasing plasma cotinine level with increasing
nicotine
intake (calculated from the numbers of cigarettes
x machine
smoked delivery), actual smokers plasma cotinine
levels have
been sub-divided by several workers into 3 broad
categories:
sub 200-250 ng/ml equivalent to low smoking
activity
250-300 ng/ml equivalent to medium smoking
activity
and 300-350 ng/ml equivalent to high smoking
activity.
These figures are based on available data.
Figure 1 outlines the 'window' which prescribes
the delivery
and consumption conditions which would correspond
to the
observed plasma cotinine levels. It is apparent
from the
figure that smokers of low-middle and middle tar
products
consuming between 10 and 30 cigs/day would have to
undersmoke
these products (relative to machine smoked
conditions) to the
extent of 30 to 60%. Similarly, ultra low delivery
smokers
would have to oversmoke these products by several
hundred
percent relative to machine smoking deliveries in
many
circumstances.
The comparison of observed and actual plasma
cotinine values
and their interpretation has numerous limitations.
The major
factor is the assumption that plasma cotinine
values increase
linearly with nicotine dose, it is probably more
reasonable
to assume that a ranqe of curvilinear
relationships exist
between plasma cotinine, per cigarette nicotine
intake and
numbers of cigarettes smoked. However, it is
unlikely to
have a major effect on the lower nicotine intake
levels and
it would seem reasonable to expect that relative
differences C=:)
between plasma cotinine and nicotine uptake would
exist even
under these conditions. As a consequence of the
difficulties
CZ)
N.)
BATCo document for Province of British Columbia 5
November 1999


-45-
associated with practical measurements of cotinine
pharmaco-
kinetics including nicotine quantification (via
smoke inhalation)
and medical ethical problems it is unlikely that
the actual
relationship will be fully elucidated.
The observation that plasma cotinine values for
smokers tend
to be similar, together with this proposed
interpretation of
the data provides some indirect evidence for
smokers smoking
to a specific requirement (not necessarily
nicotine) resulting
in broadly similar nicotine intake levels.
The plasma cotinine levels reported (50) for
subjects involved
in a smoking behaviour study in which products
with varying
levels of nicotine were each smoked for a period
of 1 week
were in broad agreement with values observed in
own brand
smokers. The experimental cigarettes were derived
from a
common product to which was added varying levels
of nicotine
to produce products with nicotine deliveries in
the range
0.06-1.3 mg/cigarette. In this study it was
apparent that
smokers of products containing 0.5 mg or greater
nicotine/
cigarette had relatively constant plasma cotinine
values
again indicating under smoking of the higher
delivery products.
When these subjects smoked the products containing
less than
0.5 mg cigarette lower values of plasma cotinine
were recorded
in association with a reported lack of smoking
satisfaction.
This may be additional indirect evidence of a
possible threshold
requirement for nicotine (see section on smoking
behaviour).
Nicotine Clearance; Urinary Nicotine and Cotinine
Levels
In simple terms an equilibrium situation exists
between
nicotine uptake and its clearance in the urine
either as
nicotine or as other metabolites.
A number of studies have been conducted in which
smokers'
urine has been analysed for nicotine and cotinine
content in
BATCo document for Province of BritiSh Columbia 5
November 1999


-46-
a limited number of studies the analysis has
included nicotine
N-1-oxide (51). These measures have been used as
an estimate
of nicotine uptake. The specific predictive value
of these
measures requires further consideration. In view
of the
varying level of urinary output throughout the day
the most
appropriate and relevant nicotine and cotinine
figures are
based on 24 hour urinary outputs. The level of
nicotine and
cotinine outputs under these circumstances is
likely to
reflect the previous 24 hour nicotine intake.
There is
debate regarding the most appropriate metabolite
to use to
estimate nicotine intake. Because of the potential
for
differential excretion of nicotine or cotinine
(and other
metabolites) depending on urinary pH and urinary
flow (52)
it is advantageous to determine as many nicotine
metabolites
as practical. In reality this is usually nicotine
and cotinine,
these metabolites collectively accounting for
approximately
30% of the nicotine uptake (up to 50% if nicotir:e
N-1-oxide
is included (53)). In an analogous manner to
plasma cotinine,
it is unrealistic to estimate absolute levels of
uptake of
nicotine from these measures but rather to obtain
a relative
estimate of nicotine uptake. There is some
evidence, which
supports the plasma cotinine finding, based on
urinary analysis
for nicotine and cotinine that smokers tend to
smoke in a
manner which results in a maintenance of at least
a minimum
dose of nicotine uptake (see section on smoking
behaviour).
C:D
CD
C=)
C7 \
C=)
BATCo document for Province of British Columbia 5
November 1999


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BATCo document for Province of British Columbia 5
November 1999


-47-
SECTION 3 - NICOTINE; PHARMACOLOGICAL AND
PHYSIOLOGICAL
CONSIDERATIONS
SUMMARY
The science of pharmacology is complex, and it has
been
considered necessary to provide some general
background
information on this subject before considering the
detailed
aspects of specific nicotine pharmacology.
An initial outline description of how the nervous
and regulatory
systems within the body are organised, and the
mechanisms and
need for communication between these different
body functions
and activities, the so-called maintenance of
homeostasis is
presented. This is followed by an attempt to
indicate in
general terms how these pathways are controlled
and to identify
the role of regulatory substances termed
transmitters, in
this process. In an effort to set these
observations in the
context of pharmacological events, some
consideration is
given to the general principle whereby neuronal
regulatory
processes have been elucidated using
pharmacological techniques.
This background information has been used as the
basis for
examining the specific pharmacological properties
of nicotine.
The relative importance of the ultimate tissue
concentrations
of nicotine in terms of subsequent pharmacological
action are
considered and discussed in relation to current
knowledge of
the uptake and distribution of nicotine following
smoke
inhalation. The mechanisms and specific sites
whereby nicotine
can elicit pharmacological responses within the
body are
presented and discussed. These sites of nicoti.ne
interaction,
the nicotinic cholinergic receptors and the
functional
significance of receptors generally are discussed
in relation
to pharmacological events.
The relationship between nicotine, the nicotinic
chlinergic
receptor and the elicited pharmacological response
and their
C=)
C:)
BATCo document for Province of British Columbia 5
November 1999


-48-
relevance for subsequent mediation of biochemical
and
physiological events is of considerable importance
in terms
of the overall wholebody effects of nicotine. The
significance
of these events for the smoker are considered.
Some of the major limitations to progress in the
area nicotine
pharmacology is the essential inability to study
the direct
effects of nicotine action in the brain. These
practical
limitations and possible application of novel
techniqueg for
future research use are examined.
The section is concluded by examining the
relationship in
pharmacological terms between adaptation,
tolerance and
possible dependence following chronic use of
nicotine.
c:::>
--J
BATCo docUMOnt for Province of British Columbia 6
November 1999


-49-
NICOTINE; PHARMACOLOGICAL AND PHYSIOCHEMICAL
CONSIDERATIONS
General Pharmacology
To enable a detailed description and critical
evaluation of
the pharmacological interaction of nicotine with
the whole
body, it is necessary to outline the essential
functioning of
the nervous and hormonal (usually abbreviated to
neurohumeral)
system at least as far as it is generally and
currently
understood. The neurohumeral system serves to
interconnect
the activities of different tissues and organs of
the body
allowing a highly integrated, co-ordinated and
interactive
level of control to be exercised to maintain a
system of
balance within the organism. This system of
balance and its
maintenance is termed homeostasis. As well as
maintaining
the organisms internal status quo, the
neurohumeral system
interacts with the external environment to
perceive and
respond to numerous stimuli.
Communication Within the Neurohumeral System;
Organisation of the Nervous System
Sensory (or afferent) nerves, as the name implies
are concerned
with receiving stimuli including those from the
external
environment and transmitting information to the
central
nervous system (CNS; brain) for co-ordination,
assessment and
response. The sensory stimuli arise from
activation of
sensory receptors associated with nerve fibre
endings. The
CNS acts as the highest order of nervous control,
processed
information being corimunicated out from the CNS
as a nerve
impulse through efferent (or motor) nerve fibres.
The efferent
nerves are essentially the major elements of the
autonomic CD
nervous system or alternatively named the
involuntary system.
The autonomic nervous system can be further
divided anatomically
into the sympathetic and parasympathetic nerve
pathways. The
ON
pathways usually (but not exclusively) work in
opposition CD
co
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-50-
i.e. if one pathway results in the elevation of
blood pressure
then the other is likely to lower blood pressure.
The
mechanism of communication along a nerve fibre is
via electrical
conduction termed a nerve impulse. The means
whereby an
impulse can be elicited or transfered and
communicated between,
and to successive nerve pathways is through
neurohumeral
transmitter materials. Obviously if information is
communicated
between nerve pathways by chemical transmitter
release, then
it follows that a system for receiving the
transmitters must
exist. These molecular information processing
sites are
termed receptors.
Locations for Nerve Pathway Control
To maintain a highly sensitive and responsive
mechanism for
controlling homeostasis, a system for mediating
and modulating
nerve impulses has evolved within the nervicus
system. This
process occurs at essentially for locations within
the nervous
system:
(i) at the level of the sensory receptor for
afferent/
sensory nerves (involved in control of information
flow to the CNS),
(ii) at locations termed ganglia, these are highly
organised and specialised cellular structures
within
the nerve pathway. Ganglia occur external to the
CNS and are associated with pathways conducting
information out from the CNS via the autonomic
nervious system,
(iii) within the CNS itself, this extremely
complex level
of control is involved in information processing
and response,
(ZD
ON
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-51-
(iv) at the level of the interface between the
efferent
or motor nerve ending and the tissue served by the
nerve fibre (e.g. conversion of a nerve fibre
impulse and muscular contraction).
Mechanisms of Nerve Pathway Regulation
The process of normal function and control of the
nervous
system is mediated essentially at these
organisational control
centres (i-iv). As previously mentioned the
mechanism of
communicating between nerve fibres is via
neurotransmitter
release followed by its subsequent contact with a
receptor
which can then initiate a further nerve impulse.
These
centres serve as areas of discontinuity which
permit or allow
an opportunity for interruption or normal
neurotransmitter
receptor interaction. The precise nature of the
modulation
depends on the nature and mode of action of the
regulatory
agent (which might well be a secondary
neurcitransmitter or
hormone) and its piatential to perturb the normal
system of
communication (Figure 2).
FIGURE 2
C) 64 S") GL~, k cw,-,:
CD
CD
011
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-52-
Regulation of the nerve impulse retains a high
level of
integrity and specificity by utilising a range of
neurohumeral
transmitter substances each capable of interacting
with highly
specialised receptors which in turn can modifying
their
response according to the presence of specific
neurohumeral
transmitter modifiers.
Within the autonomic nervous system (the system
controlling
the involuntary body functions) there are
essentially two
primary neurohumeral transmitter substances,
acetyl choline
and noradrenalin (norepinephrine). Where a
particular pathway
utilises a particular neurotransmitter this is
used to classify
the pathway and hence these pathways are termed as
cholinergic
and adrenergic respectively.
Within the CNS a number of neurotransmitter
substances have
been identified and as yet unidentified
neurotransmitters are
certain to exist. The most widely studied CNS
neurotransmitters
are acetyl choline, dopamine, noradrenalin, GABA
(gamma
aminobutyric acid), substances P, endorphins and
enkephalins.
As with the peripheral nervous system specific
neurotransmitters
combine and interact with receptors capable of
eliciting a
specific resonse which may however be modified by
the presence
of secondary neurotransmitters.
The sensory nerve neurotransmitters arising from
practical
considerations have been poorly studied, however
it would
appear that a range of transmitters are involved
including
noradrenalin and substance P and many others.
The nature of the interaction of the
neurotransmitters with
their respective receptors and the fundamental
nature of the
regulatory mechanism of neuronal control is a
crucial element
in understanding the pharmacological properties of
nicotine
and will be discussed in greater detail.
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Specific Neurotransmitter-Receptor Regulation of
Nervous Conduction
In the ground or resting state the membrane
associated with
the receptor has a charged potential across the
membrane,
maintained as a consequence of an ionm gradient
generated
through an active, energy dependant process. The
nerve
impulse is initiated by de-polarisation of the
membrane. The
sequence of events leading to the induction of the
depolarisation
process is highly complex and will be presented in
simplified
form. As outlined earlier the initial part of the
process
arises from the stimulated release of the
neurotransmitter
substance. The neurotransmitter diffuses across
the region
of discontinuity, the rate of diffusion is a
function of
concentration. The liberated neurotransmitter is
then able
to interact with its specific receptor. The
neurotransmitter-
receptor interaction is analogous to an anzymic
reaction in
that the receptor involves a protein complex and
is highly
specific for its 'substrate'. The binding of the
neurotransmitter-
receptor complex results in a secondary response
in initiating
a change in the membrane organisation. The region
of the
membrane that is modified is termed an ion
channel. The
neurotransmitter-receptor complex is in intimate
communication
with the ion channel. The change in the membrane
organisation
causes a transient opening of the ion channel. The
open ion
channel allows an efflux of charged ions resulting
in the de-
polarisation of the membrane and thus the
initiation of the
nerve impulse. For the channel to re-open and
initiate a
second nerve impulse it is necessary for the
neurotransmitter
to leave the receptor. The dissociation of the
neurotransmitter-
receptor complex should occur for a sufficiently
long period
of time to allow the receptor to recover such that
a subsequent
binding of a neurotransmitter results in a
productive binding CD
capable of re-opening an ion channel. This so
called on-off
time is an important regulatory process within the
nervous
system. If there is a high concentration of
neurotransmitter
e.g. arising from continuous or repeated
stimulation of the
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-54-
nerve, this process results in an acute
desensitisation of
the response because there is insufficient time
for receptor
recovery. This phenomenon is frequently observed
and described
as tachyphalaxis, a fall-off in nerve response
following
repeated stimulation of a nerve.
Under normal conditions of nerve stimulation the
neurotransmitter
substance, once released is in a process of
deactivation.
This event occurs at a high rate and is
responsible for the
rapid return to normality following termination of
the initial
stimulation. Should continuous nerve stimulation
be required,
the response is maintained by continual release of
the
neurotransmitter. The de-activation of the
transmitter
substance is usually through an enzymic
modification (leading
to a loss in receptor recognition which is highly
specific
for the neurotransmitter). The modified,
deactivated
transmitter is essentially re-absorbed into the
storage
vesicle and reconverted into its active form.
The presence of excess neurotransmitter in the
region of
discontinuity is also capable of inhibiting its
own release
from the stimulated storage vesicle by a process
of feedback
inhibition. So far we have attempted to build a
picture of
normal neural control of the nerve pathways and
this can be
summarised:
CD
CD
ON
(__j
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-55-
FIGURE 3
Sra (AM
V
UKcEsg N
I
Sr6V-AC,IE VVZ;1CX---
Over and above this normal control mechanism,
there are
additional systems for integrating and modifying
nervous
acti vi ty. These can be summarised by the
following:
(i) An extremely important element of control that
has
not been fully presented here is that receptors do
not work in isolation and in vivo nerve pathways
frequently involve the use of a range of
additional
neurotransmitter/regulatory substances. Other
systems of 'communication and regulation' exist in
neural control /regulatory centres particularly
hormones. The hormones of specific interest are
the catecholamines (adrenalin and noradrenalin).
These biochemical modifiers may arise or be
released
locally termed local endogencus substances or be
released at a remote site (oxygenous material) and
CD
0-N
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-56-
transported to the tissues in the blood. In the
former situation the endogenous substances
frequently
have a greater potency in terms of response
(through
local concentration effects) and of a faster time
scale, the exogenous substances being subject to
metabolic deactivation en route. The recognition
of regulatory process involves the utilisation of
a
micro-population of heterogeneous receptors that
exist to co-ordinate and regulate a particular
response in accordance with the functional
significance
of the nerve and the effector organ/tissue. The
overall response is the net effect of these
interactive control mechanisms.
(ii) In addition to the biochemical modulation of
the
neurotransmitter release, these chemical modifiers
can interact with the receptor complex at
locations
remote from the normal receptor-neurotransmitter
binding sites but in doing so modify the affinity
(ur binding characteristics) of the receptor-
neurotransmitter interaction. This may have the
specific effe ct of reducing or enhancing the
affinity
of the receptor-neurotransmitter interaction, the
net effect being a subtle balance between the
amount
of neurotransmitter present and the change in the
on-off time (the period required for receptor
recovery).
(iii) A further level of control is to perterb the
relationship between the receptor and the
essential
link in the subsequent chain of events, the ion
channel and at other inter-connecting processes
not
fully understood between these events and the
actual
initiation of the nerve impulse.
(iv) Another frequently confusing and confounding
observation is that nerve pathways nominally
utilise
C-D
CD
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-57-
identical neurotransmitters to elicit differential
responses. It must be remembered that the nervous
system transmits regulated stimuli it is the
particular specialisation of the stimulated
organ/tissue that denotes the response arising
from spatial distribution of the nerve pathways.
Elucidation of the Mechanisms of Neurohumeral
Contro;
Pharmacological Considerations
The major advances in the elucidation of these
complex
processes has involved the use of specific,
pharmacologically
active chemicals and drugs. In simple terms the
mechanism of
neurohumeral control and regulation can be
identified by
interrupting or modifying the normal processes in
a specific
manner by the addition of a specific drug or
chemical to the
system. Where the drug or chemical elicits or
prevents a
specific response and the precise location at
which the
chemical interacts with the system can be
determined it
follows that a clear insight into the
communication and
regulatory process including the structural
activity relation
of the event can be established. However, in
practice the
process of elucidating this communication and
control system
is much more complex and some of the limitations
of this
approach can be outlined:
(i) For practical considerations it is frequently
necessary if not the only mechanism for study, to
prepare isolated tissue samples for
experimentation.
Where this is the case there is a loss of
wholebody
involvement and modulation, an important
consideration
in atempting to identify and characterise the
functional and physiological significance of the
findings in vivo.
cr*\
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-58-
(ii) It is extremely difficult to identify and
characterise
the in vivo physiological conditions that prevail
in the intact, conscious animal. Lack of knowledge
of the specific tissue concentrations of
interacting
chemicals, blood flow, the ability of material to
cross blood/membrane barriers, secondary or
mediating
effects and the specific 'location' of the site of
action reduce the ability to interpret the
significance
of findings.
(iii) It is frequently not possible to study the
specific
interaction of a chemical within the normal
process
directly but rather to follow the response some
way
downstream of the interaction, again ?????? the
power of the interpretation.
(iv) Within this area of active research there is
a
continual need to test the 'established'
hypothesis
in the light of new information and to develop the
thinking to account or explain new research
findings.
(v) Pharmacological responses are frequently
assessed
usinq acute studies (i.e. studies involving only
short time scales) rather than chronic (long-term)
studies. The chronic observations are extremely
important in pharmacological studies in terms of
the adaptation of the neurohumeral system to
continuous administration of active agents.
Research has been advanced through the use of
essentially 2
classes of pharmacologically active
substances,'agonists and
antagonists.
In broad terms an agonist can be considered as an
agent that
is capable of initiating or potentiating the
'communication'
C=)
system and eliciting a nerve impulse and a
response. Agonists,
although giving rise to a similar end result can
elicit the 0_~
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-59-
response by different mechanisms. An agonist may
stimulate
the release (or change the half-life) of the
neurotransmitter
and stimulate a response. It may in fact bind
directly with
the receptor at the neurotransmitter site or at a
regulatory
site, the agonist exhibiting structural
similarities to the
natural neurotransmitter. Again, in general terms
where an
agonist binds with the specific
neurotransmitter-receptor
site, it will compete with the natural transmitter
for the
site. Knowledge of this competition process (the
so-called
receptor binding kinetics) provides a valuable
insight into
the functional significance of the receptor in
vivo.
Additionally the agonist may act by having a
direct stimulatory
effect on the opening of the ion channel.
Characterisation
of the type and nature of the interactions of
agonists can
be used to great effect in building our knowledge
of the
types of receptors involved in specific pathways
or events
and as such is an essential research tool.
Again in general antagonists function by
effectively blocking
neural pathway transmission by interrupting the
normal process.
the blockage can arise from different locations
and at several
levels the overall effect being similar. The major
effect
however, is Usually through the antagonist
effectively blocking
the neurotransmitter-receptor interaction. The
antagonist may
bind directly with the natural neuroreceptor site
and physically
block the normal neurotransmitter-receptor
interaction.
Contrary to agonist action this binding does not
cause a
productive stimulation of the nerve. The strength
of the
antagonist-receptor binding has an important and
fundamental
effect on the duration of the blocking process.
and like
agonists they are an important research tool.
*Antagonists
may also inhibit neurotransmitter release or
affect the
opening of ion channels. Antagonists like agonists
have been
instrumental in identifying the mechanism and
control of
neural transmission. In addition antagonists have
had an
enormous impact on the treatment of a number of
clinical
disorders particularly of the cardiovascular
system (e.g.
hype rtensi on).
CO
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Specific Pharmacological Properties of Nicotine
It has been known for a considerable period that
nicotine is
capable of stimulating the mammalian nervous
system, indeed
this was one of the earliest pharmacological
observations.
Indeed it is equally well established that through
nicotine's
structural similarity to acetylcholine a
ubiquitous neuro-
transmitter, it is capable of eliciting a range of
responses
through interacting with the acetylcholine
receptors. There
are essentially two classes of acetylcholine
receptors that
can be differentiated on the basis of specific
binding
characteristics. These are the so called nicotinic
cholinergic
receptors and the muscarinic cholinergic
receptors. The
nicotinic cholinergic receptor nChR can be
stimulated by
nicotine whereas nicotine does not bind to the
muscarinic
cholinergic receptor to any significant degree.
Although
there are the two broad classes of cholinergic
receptors
there are likely to be further specialisation of
the receptors
depending on location and regulatory function.
Although
nicotine interacts predominantly with the nicotine
cholinergic
receptor as a cholinergic agonist the response
elicited is
not restricted to nChR pathways alone. Stimulation
of a
nChR upstream of a neural pathway (and through
interconnecting
pathways) that subsequently utilise alternative
neurotransmitters
can cause neural transmission (subject to
regulatory control)
along the pathway. This phenomenon arises from the
ability
of the primary nicotine-cholinergic, receptor
depolarisation
event giving rise to an action potential that is
conducted
along the nerve capable of triggering and firing
subsequent
neurotransmitter release.
The involvement and distribution of cholinerqic
receptors in
the peripheral nervous system are reasonably well
documented
and are summarised in Figure 4. Nicotine can
elicit a
direct pharmacological response mediated through
nicotinic
cholinergic receptors at 4 major locations within
the body: 0~
BATCo document for Province of British Columbia 5
November 1999


Tict. Lt OLMI~M or RtAyf-
TrIMS I-MANTMITV,
Suftmw= .
RULTONOfAic Nrzvt
NitM141L
&,4
NFd2M WI dN
4 ~M -S4'JMWnC f6l0i
NOWNVC~Ic,
~z
CD
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-61-
(1) 05. The brain utilises multiple
neurotransmitter
substances, however, acetylcholine and
cholinergic.
receptors are important in mediating many brain
activities and function. Particular regions of
the brain have relatively high concentrations of
P
cholinergic receptors (e.g. cortical and
hipocampal
membranes). 11
(ii) The autonomic nervous system particularly at
the
level of autonomic ganglia. There are high
concentrations of nicotinic cholinergic receptors
on both the sympathetic and parasympathetic
ganglia.
Tfiese regulatory areas are concerned primarily
with
the control and mediation of involuntary activity/
maintenance of homeostasis.
(iii) The neuromuscular junctions (NMJ). nChR are
involved
in the process of communication between the nerve
terminal and the muscle response, the stimulation
of the nChR inducing muscular contraction (tone).
Ov) The Adrenal Medulla. The interaction of
nicotine
with the nChR associated with the control of the
adrenal medulla stimulates the release of the
adrenal hormones, adrenalin and noradrenalin (the
sympathins) into the bloodstream.
Nicotine is also capable of interacting and
stimulating
sensory nerve endings and chemoreceptors within
the body.
However, these responses are unlikely to be
through nicotinic
cholinergic receptor interacts but rather of a
more non-
specific chemical 'irritant' interaction. In
addition to
these primary levels of nicotine interaction there
are numerous
secondary mediated responses (neurotransmitter
relay effects)
C=)
associated with and attributed to nicotine.
CD
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-62-
The concentration dependance of nicotine action on
neural
pathway stimulation adds a further level of debate
and
confusion into the field of nicotine pharmacology.
In terms
of classical neural pathway control, at or above
critical
concentrations of neurotransmitter substances or
transmitter-
like substances (such as nicotine) several levels
of effect
can be observed; stimulation, tachyphalaxis and
ultimate
inactivation (nerve pathway inactivation/nerve
paralysis).
Some or all of these phenomena have been
attributed to nicotine.
Stimulation of the autonomic and CNS pathways,
stimulation
at different locations may give rise to opposing
effects.
In the case of the autonomic pathway these may
manifest
themselves as different physiological events and
differential
stimulation of different brain centres could be
responsible
for the so called stimulant and depressnt effects
of nicotine
attributed to different psychological states.
Repeated and prolonged administration of a
pharmacologically
active agent at physiological doses of agonists
frequently
results in a change in the receptor - ligand (e.g.
nicotine)
binding characteristics or its regulation. In
addition the
physical number of receptors can decrease. These
events
constitute a state of adaptation and tolerance,
the degree
of tolerance can be dependant on dose and specific
tissue
and agonist responses. It is of importance that
the observed
nicotine pharmacological responses are viewed in
relation to
duration of exposure.
From the foregoing it is easy to appreciate whey
nicotine,
from a research point of view appears to be
all-things to all
men. Taking a global view of the 'actions' of
nicotine on
biological systems serves to confuse rather than
clarify our
understanding of the role of nicotine in the
smoking process.
To make coherent sense of the reported properties
of nicotine, CZ)
an attempt will be made to outline and identify
areas of C=)
research that will provide a greater comprehension
of the C=>
(DIN
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-63-
interaction of nicotine in the body. The primary
objective
must be to understand the pharmacological effects
of nicotine
elicited under human smoking conditions. Because
of practical
limitations and the complexity of the problem,
such a research
programme must be broad based but ultimately
referenced
against human smoking behaviour.
The areas of research to determine the
pharmacological
properties of nicotine can be summarised:
(i) the assessment of nicotine intake,
distribution and
resulting tissue concentrations,
(ii) identification and kinetic characterisation
of the
primary nicotinic receptors,
(iii) functional significance of the interaction
of
nicotine with the nicotinic receptor and other
receptors and their manipulation,
(iv) determination of the biochemical and
physiological
consequences of the pharmacological properties of
nicotine,
(y) identify the specific interaction of nicotine
with
the CNS,
(vi) adaption, tolerance and possible nicotine
dependance,
(vii) pharmacological basis of smoking behaviour.
Assessment of Nicotine Uptake, Distribution and
Resulting Tissue Concentrations
Little attention generally has been given to the
quantitative
CZ)
estimation of nicotine retention from cigarette
smoke inhalation C=:)
including the effect of smoke properties and
inhalation depth 0-1
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-64-
on this parameter. The best available evidence
would suggest
that nicotine is absorbed quantatively during
inhalation. In
addition'little or no information exists on the
rate of
absorption of nicotine in human smokers and its
site of uptake
within the respiratory system. Either factor may
give rise
to local differential concentrations of nicotine
within the
pulmonary vasculature which could influence the
'instantaneous'
nicotine concentration within the CNS. The
distribution of
nicotine within the whole body including the
pharmacokinetic
properties of nicotine have been covered in
Section 2.
Insufficient consideration has been given to the
estimation
and assessment of the potential differential
tissue effects
arising from variations in tissue nicotine
concentrations.
The high lipid solubility of unionised nicotine
will give
rise to significant differential tissue effects
depending on
their composition. Naturally the high lipid
content of the
CNS and nervous tissue qenerally are likely to
result in
specific higher nicotine concentrations than many
other
tissues. -Obviously it is not practical to
determine the
actual tissue concentrations of nicotine in
smokers, although
the studies of Turner (54) have provided some
insight into
nicotine distribution in animals using whole body
autoradiography.
It may be appropriate to re-investigate the
distribution of
nicotine with a range of tissues in animals at
physiological
nicotine dosing regimes following improvement in
the analytical
procedures for nicotine and its metabolites. This
knowledge,
in combination with estimates of the absorbed
nicotine dose
derived from smoke inhalation together with the
increase in
our understanding of nicotine pharmacokinetics can
be used
to provide an imformed insight into the
significance and
upper limits for tissue concentrations of
nicotine.
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Identification. Kinetic Characterisation, and
Localisation
of the Primary Nicotinic Receptors
The in vitro preparation and isolation of neural
membranes
containing associated receptors can be utilised to
characterise
the binding properties of the receptors using
radiolabelled
ligands (any receptor binding substrate). This
technique is
used to determine the ability of neurotransmitters
or any
other chemical agent (a ligand) to bind to the
receptor. In
these studies a suitable radiolabelled ligand is
used to
determine the level and extent of ligand-receptor
interaction
(binding). If the concentration of the ligand is
known, it
is possible to determine the affinity of the
ligand for the
receptor. In practice. the dissociation constant
(KD) of the
ligand-receptor complex is calculated which is
inversely
proportional to the affinity of the ligand for the
receptor,
i.e. the lower the KD the higher the affinity. In
addition
to the determination of the affinity of the ligand
for the
receptor, it is possible to estimate from the
binding
characteristics the maximum number of ligand
binding sites
(Bmax, amount of bond ligand/unit amount of
membrane). The
B'ax value is an indication of the population of
receptors
associated with the membranes.
These studies can be extended to include the
effect of
competing.ligands to displace the bound
radiolabelled ligands
from the receptor sites. Using the technique of
ligand
competition, with chemicals of known
pharmacological actions,
the receptor can be specially identified and
characterised in
relation to its binding properties.
Using the radiolabelled ligand (-)-[3H]-nicotine,
the natural
isomer of nicotine, the location and
characterisation of the
nicotinic cholinergic receptor in the neuronal
tissue has
been achieved. A number of reports have been
recently
published indicating the binding kinetics of
(-)-[3H]-nicotine
for CNS nicotinic cholinergic receptors. The
reported findings CN
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of 2 receptor sites, a high and low binding sites
for nicotine
in the US of the nouse (55) are in broad agreement
with those
reported for rat brain in studies conducted at
GR&DC (56) as
are the values for the number of binding sites
(Bmax values).
The ability of the (+)-[3H]-nicotine ligand to
bind to the
nicotinic cholinergic receptor has been reported,
these
results indicate that the affinity of this
unnatural isomer
of nicotine is reduced (by a factor of 3) which
highlights
the highly stereo specific nature of receptor
binding and
supports the observed reduced pharmacological
potency of this
material compared to the natural isomer.
The binding studies carried out at GR&DC have
characterised
the ability of nicotine metabolites including
cotinine
to displace nicotine from the receptor. These
studies would
indicate that these materials have markedly
reduced capabilities
to bind to the nicotinic cholinergic receptor and
would
suggest a low potential to elicit a
pharmacological (and
probably physiological) response in vivo. This
finding is
in general agreement, at least for cotinine with
the recent
report of who assessed its pharmacoligical and
physiological properties in vivo (57).
Although receptor binding studies have provided
fundamental
information regarding the identification and
characterisation
of receptors the information has limited
application. The
kinetics of the receptor binding are conducted
under steady-
state (equilibrium) conditions and it is unlikely
that such
conditions prevail in vivo. In addition the tissue
concentrations
of the ligands/neurotransmitters are difficult 'to
assess and
hence the relative significance of the binding
properties of
receptors for ligands is somewhat limited. The
major weakness
of these studies is that they are unable to
differentiate
between agonist and antagonist action in only
detecting the
ability of a ligand to combine with a receptor not
its ability
to stimulate or initiate a specific response. It
therefore NJ
011
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-67-
follows that binding studies are also unable to
identify the
relationship between the receptor and its
ionophore and hence
the potential mediation of the receptor regulation
is
unidentified. Binding studies do however provide a
useful
and valuable vehicle for studying the adaptation
of the
receptor in terms of its binding characteristics
following
chronic exposure to nicotine (see section on
adaption,
tolerance and dependence).
Functional Significance of the Interaction of
Nicotine with
the Nicotine Receptor and Other Receptors and
Their Manipulation
The study of receptor binding has provided
valuable information
regarding the kinetics of the interaction of the
ligand with
the receptor and in addition have identified
ligands capable
of combining with the receptor. However as stated
previously
they are unable to differentiate between agonist
and antagonist
action and hence the inability to discriminate
between
productive binding, inhibition of receptor
activity or the
general regulatory function of ligands. Where
ligands bind
to receptors, the functional significance of this
act is of
fundamental importance to understanding the
regulation and
control of neurotransmission, and is highly
relevant to
understanding the interaction of nicotine in
neural pathways.
An experimental model is being developed to assess
the ability
of nicotine to bind to nicotinic cholinergic
receptors and to
demonstrate the functional significance of these
effects up
to and including membrane depolarisation. The
model will
provide the opportunity to identify the ineans or
mechanism
of modifying or mediating these responses either
directly
(naturally occuring neurotransmitters) or
indirectly ('chemical
modifiers). It is also envisaged that this model
will be
used to assess chronic changes that occur in vivo
following
nicotine administration. The model involves the
use of
tissue slices derived from the striatum (a
specific region
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-68-
of the brain) which are pre-incubated with
radiolabelled
dopamine (following removal of the free
radiolabel) any
subsequent stimulation of the nChR with a
cholinergic agonist
will cause a release of the radiolabel, this can
be surnmarised:
'Functional' Unit
This systeff. is being evaluated for use in the
determination
of how nicotine can perturb a nChR and how this
interaction
can be potentiated or diminished by agonist
antagonist action
(58). This approach has the attraction that the
receptor
and ionophore relationship is integrated into the
system,
although it has some limitations. It suffers from
some of
the same problems as the kinetic binding studies
in that the
in vivo concentrations of the ligands are unknown
and that CD
CD
the system is studied in an isolated rather than
in an
ON
r11-3
CC)
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-69-
integrated system. These limitations tenbd towards
providing
information of a generalised nature rather than
that relating
to a specific neural pathway.
There is little or no information relating to the
specific
nature of the interaction of nicotine at sensory
or chemo-
receptor sites within the body. These interactions
are
believed to be through non-cholinergic interaction
and arise
from general chemical 'irritant' properties. The
interaction
of nicotine (and other smoke components) at
sensory nerve
endings within the respiratory system are of
particular
relevance to the assessment of smoke (strength)
during
inhalation. This is a much under studied and
important
subject in terms of product acceptability (59).
The possibility
of developing a productive research programme
involving
external researchers to assess the pharmacological
properties
of sensory nerve endings within the respiratory
tract is
being reviewed at GROC.
Determination of the Biochemical and Physiological
Consequences
of the Pharmacoloqical Properties of Nicotine
The majority of biochemical and physiological
events associated
with nicotine intake appear to be mediated or
expressed in
the peripheral tissues. The primary peripheral
response to
nicotine is probably the stimulation of the
autonomic nervous
system through the interaction of nicotine at the
nicotinic
cholinergic receptors of the autonomic ganglia.
This
interaction has the potential to stimulate both
the sympathetic
and parasympathetic pathways of the autonomic
nervous system.
The peripheral responses are essentially mediated
through the
release of catecholamines. The catecholamines are
released
from these primary locations as a consequence of
nicotine
mediated stimulation:
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BATCo