The Journal of Clinical
J. Clin. Pharmacol. 2002; 42; 71
DR Tashkin, GC Baldwin, T Sarafian, S Dubinett and MD Roth
Respiratory and immunologic consequences of marijuana smoking
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RTNAEOSSVPHEIKRMAINBTEOERTRSYAUALPNPLDEIMMEMNUTNOLOGIC CONSEQUENCES OF MARIJUANA
Respiratory and Immunologic
Consequences of Marijuana Smoking
Donald P. Tashkin, MD, Gayle C. Baldwin, PhD, Theodore Sarafian, PhD,
Steven Dubinett, MD, and Michael D. Roth, MD
Marijuana is usually taken by inhaling smoke
formed by the combustion of plant material in a
cigarette or pipe. Smoke inhaled in this manner comes
into direct contact with the airway mucosa and the
gas-exchanging surface of the distal lung before individual
constituents are absorbed into the circulation.
As such, the lung is exposed to the highest concentration
of all smoke constituents, including the volatile,
particulate, and biologically active components. The
composition of marijuana smoke is qualitatively similar
to that of tobacco, except for the presence of
cannabinoids in marijuana and nicotine in tobacco.1,2
As a result, some of the pulmonary consequences of habitual
marijuana smoking appear to be similar to those
of regular tobacco use.3-7 However, differences in the
daily quantity of each substance that is generally
smoked8 and differences in the profile and pattern of
smoking one substance versus the other result in
somewhat different patterns of exposure to their particulate
and gaseous ingredients.9,10 These variables
could contribute to differences in respiratory consequences
of the two plant substances. Moreover, the
cannabinoids contained in marijuana, particularly
Δ9-tetrahydrocannabinol (Δ9-THC), appear to have
pharmacological, immunologic, and toxic effects that
contribute to or modify the respiratory effects of the
smoke exposure. This paper briefly reviews the evidence
accumulated over the past two to three decades
concerning the respiratory consequences of marijuana;
effects of the whole smoke will be reviewed separately
from those of Δ9-THC.
J Clin Pharmacol 2002;42:71S-81S 71S
From the Department of Medicine, UCLA School of Medicine, Los Angeles.
Supported by the National Institute on Drug Abuse, grant no. R37
DA03018. Address for reprints: Donald P. Tashkin, MD, Department of
Medicine, UCLA School of Medicine, 10833 Le Conte Avenue, Los Angeles,
CA 90095-1690.
DOI: 10.1177/0091270002238797
Habitual smoking of marijuana has a number of effects on the
respiratory and immune systems that may be clinically relevant.
These include alterations in lung function ranging from
no to mild airflow obstruction without evidence of diffusion
impairment, an increased prevalence of acute and chronic
bronchitis, striking endoscopic findings of airway injury (erythema,
edema, and increased secretions) that correlate with
histopathological alterations in bronchial biopsies, and
dysregulated growth of the bronchial epithelium associated
with altered expression of nuclear and cytoplasmic proteins
involved in the pathogenesis of bronchogenic carcinoma.
Other consequences of regular marijuana use include
ultrastructual abnormalities inhumanalveolar macrophages
along with impairment of their cytokine production,
antimicrobial activity, and tumoricidal function. Cannabinoid
receptor expression is altered in leukocytes collected from the
blood of chronic smokers, and experimental models support
a role for Δ9-tetrahydrocannabinol in suppressing T cell
function and cell-mediated immunity. The potential for marijuana
smoking to predispose to the development of respiratory
malignancy is suggested by several lines of evidence, including
the presence of potent carcinogens in marijuana
smoke and their resulting deposition in the lung, the occurrence
of premalignant changes in bronchial biopsies obtained
from smokers of marijuana in the absence of tobacco,
impairment of antitumor immune defenses by
Δ9-tetrahydrocannabinol, and several clinical case series in
which marijuana smokers were disproportionately
overrepresented among young individuals who developed
upper or lower respiratory tract cancer. Additional welldesigned
epidemiological and immune monitoring studies
are required to determine the potential causal relationship
between marijuana use and the development of respiratory
infection and/or cancer.
Journal of Clinical Pharmacology, 2002;42:71S-81S
LONG-TERM RESPIRATORY
CONSEQUENCES OF WHOLE
MARIJUANA SMOKE
Aside from the short-term and relatively mild
bronchodilator effect of Δ9-THC (vide infra), smoking
marijuana does not appear to produce acute respiratory
effects of any clinical relevance. The important respiratory
consequences produced from marijuana smoking
develop primarily following long-term use.
Effects on Respiratory
Symptoms
Three independent, controlled, population-based
studies have reported that habitual use of marijuana is
associated with symptoms of chronic bronchitis, defined
as cough and sputum production on most days
for ≥ 50 days (or ≥ 3 months) per year for ≥ 2 years.3-5
Wheezing is also frequently reported. In a UCLA study,
a convenience sample of young adults (mean age = 33
years) that included 144 daily smokers of marijuana
only (MS), 135 smokers of both marijuana and tobacco
(MTS), 70 smokers of tobacco only (TS), and 97 nonsmokers
(NS) recruited from the greater Los Angeles
area was followed over time for the prevalence of respiratory
symptoms.3 MS, compared to NS, had a significantly
higher prevalence (p < 0.05; χ2) of chronic cough
(18% vs. 0%, respectively), chronic sputum production
(20% vs. 0%), and wheeze (25% vs. 3.5%). MS
also reported a higher incidence of acute bronchitis,
defined as one or more episodes of increased cough and
sputum lasting for ≥ 3 weeks (13% vs. 2%).3 Interestingly,
the prevalence of chronic bronchitis and the incidence
of acute bronchitis were not significantly different
between MS and TS, despite a marked disparity in
the daily amount of plant substance smoked (3-4 marijuana
cigarettes vs. 22 tobacco cigarettes). No additive
effects of marijuana and tobacco on chronic respiratory
symptoms were noted. In a second study involving
young MS (n = 54), MTS (n = 56), TS (n = 20), and NS
(n = 502) recruited from a random stratified cluster of
households in Tucson, Arizona, more MS than NS reported
cough, sputum, wheeze, and shortness of
breath, with the differences between MS and NS being
significant for sputum and wheeze (p ≤ 0.05).4 Moreover,
an additive effect of marijuana and tobacco on
chronic respiratory symptoms was noted in that study,
in contrast to findings from the Los Angeles group.3 In a
third study, 943 young adults, all 21 years of age, were
evaluated from a birth cohort born in Dunedin, New
Zealand;5 91 (9.7%) members of this cohort were cannabis
dependent. Morning sputum production, wheezing
apart from colds, shortness of breath on exertion,
and nocturnal awakenings with chest tightness were
significantly more common in the cannabis-dependent
subjects compared to nonsmokers, after controlling for
tobacco use.5 Respiratory symptoms were as common
in cannabis-dependent subjects as in smokers of up to
one-half pack of tobacco cigarettes per day. In summary,
findings from three separate controlled studies
indicate that habitual use of marijuana is associated
with frequent respiratory symptoms, including
chronic bronchitis, acute bronchitis, wheeze, and/or
exertional dyspnea.
Long-Term Effects
on Lung Function
In the Tucson study, the ratio of the forced expired volume
in 1 second (FEV1) to the forced vital capacity
(FVC), a functional measure of airflow obstruction, was
significantly lower on average in MS than NS. The
FEV1/FVC ratio for MS was even numerically lower
than that measured in TS, although the mean values for
this measure were still within statistically normal limits.
4 In a 6-year follow-up to this study, both the FEV1
and the ratio of FEV1 to FVC were significantly reduced
in relation to reported previous use of marijuana, suggesting
that continuing marijuana smoking may lead to
a progressive decline in lung function and the development
of obstructive lung disease later in life.11 In the
New Zealand study, more than one-third of 21-year-old
cannabis-dependent subjects, most of whom had only
developed cannabis dependence since age 18, had a reduced
FEV1/FVC ratio, compared to only one-fifth of
nonsmokers from the same birth cohort (p = 0.04).
These findings suggest that only a few years of heavy
cannabis use could lead to early obstructive ventilatory
impairment in young individuals, although it is unclear
from these data whether potential confounding
by concomitant tobacco use was accounted for.
On the other hand, conflicting results were reported
from the Los Angeles study, in which no association
could be found between heavy, habitual use of marijuana
(mean of > 3 joints/day for an average of > 15
years) and abnormalities in any measure of lung function,
including forced expiratory flow rates at low lung
volumes and indices derived from single-breath nitrogen
washout.3 The latter tests are among the most sensitive
measures of early obstructive lung disease involving
small airways, which are the major site of
obstruction in tobacco-related chronic obstructive pulmonary
disease (COPD). Furthermore, no association
was noted between regular marijuana use and an abnormal
single-breath diffusing capacity for carbon
72S J Clin Pharmacol 2002;42:71S-81S
TASHKIN ET AL
monoxide (DLCO), a sensitive physiologic indicator of
emphysema. In contrast, a significant association was
noted between regular tobacco smoking and abnormalities
in measures of small airways obstruction, as well
as in DLCO. However, no adverse interaction between
heavy, habitual marijuana use and regular tobacco
smoking was found. Since the development of COPD is
characterized by an accelerated rate of decline in FEV1
with age, the Los Angeles investigators measured FEV1
serially at intervals of ≥ 1 year for up to 8 years in 87
MS, 42 TS, 63 MTS, and 63 NS to determine whether
regular marijuana use might lead to progressive declines
in lung function over time that might not have
been evident from the cross-sectional analysis of their
data.12 In contrast to the effect of tobacco smoking,
which was associated with a significant age-related decline
inFEV1 compared to nonsmoking, no effect of any
amount of marijuana smoking (up to 3 joints per day)
on the annual decline in FEV1 was noted, nor was an
additive effect of marijuana and tobacco observed.
These results do not support an association between
regular marijuana smoking and COPD. Further support
for this conclusion comes from studies in rats exposed
to progressively increasing doses of marijuana or tobacco
smoke for 6 months,13 in which the lungs of the
tobacco-exposed rats, but not the marijuana-exposed or
unexposed rats, exhibited loss of alveolar surface area
and decrease in lung elasticity, which represent, respectively,
morphological and physiological evidence
of emphysema.
Effects on Airway Pathology
Visual Evidence
of Airway Injury
Videobronchoscopy was performed in a subset of the
Los Angeles cohort, consisting of 10 NS, 10 MS, 10 TS,
and 10 MTS, to determine whether regular marijuana
smoking is associated with visual and microscopic evidence
of airway injury and inflammation, even in the
absence of clinically significant respiratory symptoms
or lung function abnormality.14 Visual grading of the
presence and extent of airway erythema, edema, and
mucus hypersecretion was used as a semiquantitative
index of mucosal inflammation (bronchitis index).
Endobronchial biopsies were also performed to correlate
histopathological evidence of airway injury and inflammation
with bronchitis index scores. In addition,
bronchial lavage was performed to assess markers of inflammation
in airway lining fluid, including
neutrophil numbers and levels of interleukin-8 (IL-
, a
neutrophil chemoattractant and activator. All smoking
groups, including smokers of marijuana only, had significantly
higher bronchitis index scores than
nonsmokers. Similarly, nearly all smokers, including
MS, had mucosal biopsies that revealed the presence of
at least two of the histopathological findings characteristic
of bronchial inflammation and remodeling in response
to smoking-related airway injurysubmucosal
vascular hyperplasia, submucosal edema, inflammatory
cell infiltrates, and goblet cell hyperplasia
whereas none of the biopsies from nonsmokers exhibited
more than one positive finding. Analysis of bronchial
lavage fluid showed markedly elevated
neutrophil counts (that correlated with IL-8 levels) in
some of the subjects in each of the smoking groups, especially
combined smokers of marijuana and tobacco,
but in none of the nonsmokers. These findings suggest
that habitual smoking of marijuana and/or tobacco by
young adults frequently leads to clinically silent,
endoscopically apparent central airway injury, which
parallels microscopic evidence of airway inflammation,
even in the absence of respiratory symptoms or
lung function abnormality.
Tracheobronchial
Histopathology and
Immunohistopathology
Bronchoscopic biopsies of the tracheobronchial mucosa
were performed in a large subset of participants in
the Los Angeles cohortwho were selected only by
their willingness to undergo the procedurefor the
purpose of systematically examining the effect of regular
use of marijuana on microscopic evidence of
mucosal injury of the central airways. Volunteers included
40 MS, 31 TS, 44 MTS, and 53 NS, most of
whom denied significant respiratory symptoms and
had no significant abnormalities in pulmonary function.
Histopathological characteristics that were assessed
included basal (reserve) cell hyperplasia, stratification,
squamous metaplasia, goblet cell hyperplasia,
cellular disorganization, nuclear variation, mitotic figures,
increased nuclear-to-cytoplasmic ratio, inflammation,
and basement membrane thickening; these features
were interpreted according to the criteria of
Auerbach et al.15 Biopsies revealed that regular smoking
of marijuana only (mean of 3-4 joints per day) was
associated with a greater frequency and severity of abnormalities
for most of the histopathological characteristics
examined compared to the findings noted in the
nonsmokers. Moreover, the alterations noted in the
marijuana-only smokers were at least as extensive as
those observed in the smokers of tobacco alone (mean
of 22 cigarettes per day), despite the marked disparity
between the daily number of marijuana versus tobacco
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RESPIRATORY AND IMMUNOLOGIC CONSEQUENCES OF MARIJUANA
cigarettes consumed. Furthermore, for nearly all
histopathological features examined, abnormalities
were noted more frequently in the combined smokers
of marijuana plus tobacco than in smokers of either
substance alone.
Bronchial biopsies obtained in 12 MS, 14 TS, 9 MTS,
and 28 MTS from the UCLA cohort were examined by
immunohistology to assess expression of the protein
products of some of the genes known to be involved in
the transformation of lung cells from normal to malignant,
including Ki-67 (a marker of cell proliferation),
epidermal growth factor receptor (EGFR), and p53,
one of the most common suppressor genes altered in
human cancer.16 Findings indicated marked
overexpression of Ki-67 and EGFR in the bronchial epithelium
of marijuana-only smokers compared to nonsmokers,
as well as more frequent expression of these
molecular markers in the marijuana-only smokers than
was noted in the biopsies from tobacco-only smokers.
Moreover, p53 was expressed in 11% of subjects who
smoked marijuana together with tobacco.
The clinical implications of these findings are as follows:
(1) regular marijuana smoking can lead to potentially
serious airway injury at a young age, even in the
absence of symptomatic or functional evidence of respiratory
disease. (2) Habitual marijuana use is as injurious
to the epithelium of the large airways as regular
tobacco smoking, despite a much smaller daily number
of marijuana than tobacco cigarettes smoked, consistent
with greater injury from marijuana than tobacco
per cigarette smoked. Reasons for this disparity could
be due to differences in cigarette filtration and smoking
topography for marijuana versus tobacco cigarettes: the
former do not have filter tips, are more loosely packed,
and are smoked with, on average, a fourfold longer
breath-holding time than the latter, thus affording more
opportunity for respiratory deposition of ultra-fine
smoke particles and absorption of toxic volatile-phase
constituents in the smoke from marijuana.9,10 (3) Effects
of marijuana and tobacco on bronchial mucosal
histopathology appear to be additive, a particularly
worrisome observation in view of the fact that the prevalence
of tobacco smoking is substantially higher
among regular marijuana smokers (~ 50% in the UCLA
cohort) than among nonsmokers of marijuana. (4) The
frequently noted hyperplasia of mucus-secreting surface
epithelial (goblet) cells and of nonciliated reserve
(basal) cells (in addition to the metaplasia of squamous
cells) that replace the normal ciliated columnar epithelium
of marijuana-only smokers probably accounts for
the high frequency of symptoms of chronic bronchitis
in smokers of marijuana alone. Replacement of the normal
ciliated cells by nonciliated cells on the mucosal
surface, in addition to increased mucus production by
hyperplastic goblet cells, impairs mucociliary clearance,
leaving cough as an alternative mechanism for
clearing retained mucous secretions. Moreover, the impairment
in normal lung clearance mechanisms compromises
the lungs normal defense against inhaled
microorganisms, potentially predisposing to lower respiratory
tract infections. (5) The squamous
metaplasia, cellular disorganization, nuclear variation,
mitotic figures, and increased nuclear-to-cytoplasmic
ratio that are found with substantially increased frequency
in the bronchial mucosa of smokers of marijuana
alone or with tobacco are believed to be potential
precursors of subsequent bronchogenic carcinoma.15
(6) The immunohistological findings in the bronchial
epithelium of marijuana smokers suggest that smoking
marijuana, like tobacco smoking, causes dysregulated
growth of bronchial epithelial cells that may result in
an increased risk for the subsequent development of
lung cancer. Taken together with the alterations noted
on light microscopy, the immunohistochemical findings
are consistent with the concept that habitual
smoking of marijuana may be an important risk factor
for the later development of respiratory malignancy.
Oxidant Stress
The aforementioned effects of marijuana on
tracheobronchial mucosal inflammation, edema, and
cell injury could be mediated, in part, by oxidant stress
from exposure to the smoke of marijuana. To examine
this possibility, an endothelial cell line (ECV 304) was
exposed to smoke from marijuana containing different
concentrations of THC.17 Findings from these studies
indicated that whole marijuana smoke stimulated formation
of reactive oxygen species (ROS) in ECV 304
cells in proportion to the concentration of THC in the
smoke. On the other hand, exposure to vapor-phase
smoke, which does not contain THC, led to the highest
production of ROS, suggesting that most of the oxidative
effects on cell injury are produced by toxic gases in
the smoke, although THC could be a contributing
factor.
Effects on Alveolar Macrophages
and Regional Immune Function
The lungs are protected from the inhalation of toxic
and infectious agents by a combination of mechanical
factors (cilia, mucus, and cough), innate effector cells
(alveolar macrophages and neutrophils), and adaptive
immune responses (mediated by lymphocytes and
cytokines). Alveolar macrophages (AMs) are the most
numerous cells residing in the distal air spaces of the
74S J Clin Pharmacol 2002;42:71S-81S
TASHKIN ET AL
lung and play a key role in the lungs immune defense
through their ability to phagocytose and destroy pathogenic
organisms and particulate matter, as well as their
production of protective proinflammatory cytokines.
Increased numbers of AMs are recovered when the
lungs of MS, TS, and MTS are washed out by a procedure
called bronchoalveolar lavage (BAL). When compared
to the cell recovery in healthy NS, roughly twofold,
threefold, and fourfold more AM are recovered
from subjects in these different smoking groups, respectively.
18 This presumably results from an increased
chemotaxis and/or in situ replication stimulated by
components within the smoke.19Transmission electron
microscopy reveals striking ultrastructural alterations
in the AMs recovered from marijuana and/or tobacco
smokers, consisting of larger and more complex cytoplasmic
inclusions than observed in the AMs from
NS.20 In addition, ultrastructural differences were
noted between the AMs of MS and TS, implying that
exposure to the different types of smoke might lead to
differences in the functional activity of these immuneeffector
cells.
Functional studies demonstrate that AMs from both
MS and TS are impaired in their ability to kill Candida
albicans21 and Candida pseudotropicalis22 compared
to AMs from NS. More recently, AMs from MS were
found to be significantly impaired in their ability to
phagocytose and kill Staphylococcus aureus, whereas
AMs from TS showed no impairment in either
phagocytic or killing activity against these pathogenic
bacteria. It is very likely that cannabinoids, which are
present only in marijuana smoke, account for this differential
effect of marijuana and tobacco smoking on
AM function. THC has been shown to alter specific
cytoskeletal components involved in phagocytosis
(tubulin and actin) and to inhibit macrophagemediated
phagocytosis in vitro.23
Defects in the bactericidal activity of AMs from marijuana
smokers appear to be due to a marijuana-related
impairment in production of nitric oxide (NO), a reactive
nitrogen intermediate that serves as an important
effector molecule in bacterial killing. This conclusion
is supported by the finding that inhibition of nitric oxide
synthase impairs killing of S. aureus by AMs from
NS and TS but not by AMs from MS.22 More recent preliminary
data directly demonstrate that expression of
the inducible nitric oxide synthase (iNOS) gene and
production of NO are deficient when AMs recovered
from the lungs of MS are tested in the S. aureus killing
assay.24 Resting macrophages must be primed with inflammatory
cytokines, such as tumor necrosis factoralpha
(TNF-α
, interferon-gamma (IFN-γ
, or
granulocyte, macrophage-colony stimulating factor
(GM-CSF), to up-regulate expression of iNOS. Interestingly,
production of NO by AMs recovered from MS
was restored by the addition of these proinflammatory
cytokines (IFN-γ or GM-CSF) to the S. aureus killing assay.
In contrast, the addition of cytokines had no effect
on the expression of iNOS or bacterial killing when
added to cells from NS. These preliminary findings
suggest that impairment in the bactericidal activity of
AMs from MS is due to a marijuana-related inhibition
of key proinflammatory cytokines that are needed, in
turn, to induce iNOS. Testing this hypothesis, we
found that lipopolysaccharide, a bacterial wall component
involved in macrophage activation, failed to stimulate
normal release of TNF-α, IL-6, or GM-CSF from
AMs recovered from the lungs of MS.22 AMs recovered
from TS produced normal levels of these cytokines
and, as reported above, exhibited normal antibacterial
activity. The clinical implications of these findings
are that regular marijuana smoking may compromise
the lungs defense against infection by impairing the
antimicrobial function of AMs and the production of
proinflammatory cytokines that are required for immune
activation.
Reactive oxygen species are produced by AMs under
conditions that stimulate a respiratory burst and
also serve as important effector molecules for microbial
killing. It is of interest, therefore, that while AMs from
TS release higher than normal levels of superoxide anion
(O2
) under both basal and stimulated conditions,
the respiratory burst of AMs from MS was unaltered or
even reduced,21 possibly contributing to their impaired
microbicidal activity.On the other hand, since reactive
oxygen species released from AMs can also cause lung
tissue damage, marijuana-related impairment in the respiratory
burst activity of AMs might serve a protective
role against injury to the distal lung, possibly accounting
for the absence of abnormalities in small airways
function and alveolar diffusing capacity in marijuanaonly
smokers, in contrast to the presence of such findings
in smokers of tobacco.3,21,25 Matrix metalloproteinases
(MMPs) are a group of proteinases with an
ability to degrade most components of the extracellular
matrix. These enzymes are believed to play a role in the
inflammatory process and connective tissue destruction
and remodeling that occur in tobacco-related
COPD. The primary sources of MMPs in lung inflammation
are neutrophils, AMs, and lymphocytes, as well
as activated type II pneumocytes and mesenchymal
cells. Preliminary data from subjects in the Los Angeles
cohort who underwent bronchoscopy and BAL have
revealed that TS, but not MS or NS, express high levels
of MMP-2 (72 KD gelatinase) and MMP-9 (92 KD
gelatinase).26 Again, these findings may help explain
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RESPIRATORY AND IMMUNOLOGIC CONSEQUENCES OF MARIJUANA
why smokers of tobacco develop small airways disease
and emphysema, while smokers of marijuana do not.
Carcinogenic Effects
Several lines of evidence point to a possible role of marijuana
smoking in respiratory carcinogenesis. These
are briefly reviewed below.
Carcinogens and
Procarcinogens
in Marijuana Smoke
The smoke from combustion of marijuana contains
several known carcinogens and co-carcinogens: vinyl
chlorides, phenols, nitrosamines, reactive oxygen species,
and various polycyclic aromatic hydrocarbons
(PAHs),1,2 including the highly procarcingenic PAH,
benzo[α]pyrene. Benzo[α]pyrene is present in approximately
50% higher concentration in marijuana than tobacco
tar.1,27 Its active metabolite binds to the human
p53 tumor suppressor gene at mutational hotspots associated
with human respiratory tract cancer.28 Approximately
four times as much tar (insoluble
particulates) from marijuana smoke is deposited in the
lung compared to the tar from the same quantity of tobacco
due to differences in cigarette filtration and
smoking technique,9,10 thus increasing exposure of the
respiratory tissues to the carcinogens in the tar phase. It
is also noteworthy that THC, which accounts for a large
fraction of the tar phase of marijuana smoke, has recently
been shown to activate transcription of the
cytochrome P4501A1 (CYP1A1) gene to increase production
of CYP1A1, the enzyme primarily responsible
for converting PAHs to carcinogens in the lung.29 While
the induction of CYP1A1 by PAHs is well known and
believed to be an important step in the development of
tobacco-related cancers, the up-regulation of CYP1A1
by THC is additive to that of PAHs in the tar. On the
other hand, in addition to being an inducer of CYP1A1,
THC also appears to serve as a substrate for the enzyme,
29 thus competing with procarcinogenic PAHs for
metabolism by CYP1A1. Therefore, the resultant effect
of THC on the enzymatic conversion of PAHs to carcinogens
is unclear, and further studies are needed to determine
its exact role in the activation of smoke-related
carcinogens.
Experimental and
Observational Studies
In vitro studies using the Ames salmonella/microsome
test have shown marijuana smoke to be as mutagenic as
tobacco tar,30,31 thus potentially predisposing to malignancy.
Studies in which tar from marijuana cigarettes
was painted onto the skin of mice have revealed the development
of metaplastic and neoplastic lesions,32 similar
to results obtained with tobacco tar. Hamster lung
explants repeatedly exposed over 2 years to either marijuana
or tobacco smoke developed alterations in cell
morphology, proliferation, and genetic equilibrium associated
with accelerated malignant transformation;
marijuana-exposed explants developed even more
striking alterations than those exposed to tobacco.33 In
observational human studies noted above, biopsies of
tracheobronchial mucosa from habitual smokers of
marijuana alone showed extensive hyperplastic,
metaplastic, and dysplastic alterations compared to
findings in nonsmokers; the microscopic changes
noted in the marijuana smokers are known to be precursors
of bronchogenic carcinoma.6,7,15 The bronchial
histopathology in the marijuana smokers was comparable
to that of tobacco-only smokers, and the microscopic
abnormalities in combined smokers of marijuana
and tobacco suggested additive effects of the two
substances. Moreover, immunohistochemical examination
of bronchial biopsies from 52 of these subjects
showed marked overexpression of EGFR and Ki-67 in
the biopsies from the marijuana smokers compared to
the nonsmoking control subjects.16 EGFR and Ki-67 are
molecular markers known to be linked to an increased
risk of lung cancer. Interestingly, expression of these
markers was even more common in the marijuana-only
than tobacco-only smokers. In addition, p53, a growth
suppressor gene that is commonly altered in human
cancers,28 was expressed in 11% of the subjects who
smoked marijuana along with tobacco. These
immunohistochemical findings suggest that habitual
marijuana smoking, like regular tobacco smoking,
leads to dysregulated growth of bronchial epithelial
cells. Taken together with the light microscopic
changes, these findings support the notion that marijuana
smokers are at increased risk of developing respiratory
cancer and that this risk may be additive to that
of tobacco in combined smokers of both substances.
Effects of 9-THC on
Antitumor Immune Defenses
The development of antitumor immunity is critical to
the hosts ability to defend itself against the growth of
malignant tumors. THC has been found to be a potent
immune modulator, with a predominantly immunosuppressive
effect on various immune cells, including
macrophages, natural killer cells, and T lymphocytes.34
These effects are most likely mediated by cannabinoid
receptorspredominantly CB2 receptorsthat have
76S J Clin Pharmacol 2002;42:71S-81S
TASHKIN ET AL
been found to be expressed on leukocytes.35 THC has
been shown to inhibit T lymphocyte production of
immunostimulatory Th-1 cytokines (e.g., interleukin-2
[IL-2] and IFN-γ
while promoting production of
immunoinhibitory Th-2 cytokines, such as interleukin-
10 (IL-10) and interleukin-4 (IL-4).36-38 That this
immunosuppressive effect of THC could impair the
hosts ability to develop antitumor immunity and thus
augment tumor cell growth has been demonstrated in
murine models of lung cancer.38 Compared to mice
subcutaneously implanted with non-small-cell lung
cancer cell lines and treated with a vehicle control,
tumor-bearing mice intermittently treated with
intraperitoneal injections of THC exhibited a markedly
accelerated rate of tumor growth. This THC-induced
augmentation in tumor growth was associated with
production of decreased amounts of Th-1 cytokines
(IFN-γ
and increased amounts of Th-2 cytokines (IL-10
and transforming growth factor-beta [TGF-β]) both at
the tumor site and by splenocytes. Moreover, the enhancement
of tumor growth by THC was blocked by either
anti-IL-10 or anti-TGF-β monoclonal antibodies,
suggesting that the regulation of these cytokines by
THC played a central role in its immunosuppressive
effects. Administration of a selective CB2 receptor antagonist
blocked the biological impact of THC on tumor
growth, confirming that this was a cannabinoid
receptor-mediated effect. These findings suggest that
THC might augment tumor growth by suppressing
antitumor immune defenses via a cytokine-dependent,
cannabinoid receptor-mediated mechanism.
That human immune defenses against lung cancer
might be impaired by marijuana smoking, as in animal
models, is suggested by several lines of evidence. As
noted above,AMsharvested by bronchoalveolar lavage
from the lungs of habitual smokers of marijuana (who
were thus chronically exposed in vivo to marijuana
smoke components, including THC) are impaired in
their ability to kill tumor cell targets, including
non-small-cell lung cancer cells, compared to
macrophages obtained from nonsmokers and even
from tobacco-only smokers.22 The decrease in
tumoricidal activity of human AMs from MS was associated
with significant impairment in their production
of proinflammatory cytokines when stimulated by
lipopolysaccharide in comparison with the responses
of macrophages from NS and TS (p < 0.05).22 Furthermore,
when purified human T cells and allogeneic
antigen-presenting dendritic cells were incubated in
the presence and absence of THC, a dose-dependent
suppression ofTcell proliferation byTHCwas noted in
parallel with an inhibition of IFN-γ release.37 Because of
the important role of dendritic cells in processing tumor
antigens and stimulating the proliferation of
cytolytic T cells that selectively target tumor cells, the
impairment of the function of antigen-presenting cells
and T cells by THC could undermine antitumor immune
defenses and predispose to human cancer. To
evaluate whether cannabinoid receptors are activated
in marijuana smokers, peripheral blood leukocytes
were collected from habitual marijuana smokers and
NS, and the expression of CB1 and CB2 receptormRNA
was compared using quantitative and semiquantitative
RT-PCR.39 Steady-state expression of mRNA encoding
for both the CB1 and CB2 receptors was significantly
increased (1.5- to 2-fold) in peripheral blood immune
cells obtained from the MS, consistent with an effect of
marijuana smoking and systemic THC exposure on the
turnover rate of these cell surface receptors.
Clinical Case Series and
Epidemiological Studies of Cancer
Risk Associated with Marijuana
An unusually high proportion of marijuana smokers
among young individuals (younger than ages 40-45
years) diagnosed with either upper respiratory (head
and neck) cancer40,41 or lung cancer42 has been noted in
several small case series. In one series, only 10 out of
887 patients with respiratory cancer (8 head and neck
cancers and 2 lung cancers) diagnosed over a 4-year period
at a single medical center were younger than age
40 years, underscoring the relative rarity of respiratory
cancer in young adults.40 Interestingly, of these 10
young patients with respiratory cancer, 7 were selfreported
heavy or regular users of marijuana, and an
additional patient was believed probably to have used
marijuana. Other identified risk factors included tobacco
smoking and alcohol use, although these were
not present in all of the marijuana smokers. In a second
series of young patients (ages 17-41 years) with head
and neck cancer from a single clinical practice, an exceptionally
high proportion, 21 of 23 (92%), admitted
to using marijuana,43 although other risk factors were
not assessed. In a third case series, only 11 of more than
2500 cases (< 0.5%) of head and neck cancer were
younger than age 40 years,41 of whom 9 (86%) reported
smoking marijuana while only 4 were tobacco smokers.
In another small series of 3 young marijuana smokers
(31-37 years of age) with cancer of the lung,
nasopharynx, or tongue, no history of tobacco smoking
or other significant risk factors was noted.44 In a fifth series
of 13 patients younger than age 45 years with lung
cancer seen at a single center, all reported smoking
marijuana, whereas 1 of the 13 denied ever having
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smoked tobacco.42 Findings from these combined case
series, while representing uncontrolled observations,
nonetheless provide suggestive evidence that habitual
use of marijuana poses a significant risk for the unusual
development of respiratory cancer in young individuals.
It is also possible that the carcinogenic effects of
marijuana in combination with tobacco in this population
could be either additive or synergistic.
While case series may provide suggestive evidence
of a causal association between marijuana smoking and
respiratory cancer, epidemiological studies that control
for the effects of tobacco and other concomitant risk
factors are needed to provide a true estimate of the link
between marijuana smoking and cancer. Two epidemiological
studies have examined the possible association
between marijuana use and cancer. In one of these
studies, a cohort of nearly 65,000 Northern California
Kaiser Permanente health plan members 15 to 49 years
of age who completed a health screening questionnaire,
which included a few questions regarding marijuana
use, were followed for up to 14 years.45 No relationship
between lifetime or current use of marijuana
and the development of new tobacco-related cancers
was noted. Limitations of this cohort study include an
inadequate number of heavy or long-term marijuana
users to observe an effect and the relatively young age
of the cohort at follow-up (mean = 43 years), which is
considerably younger than the average age at which
most tobacco-related cancers are detected. The second
study was a case control study of 173 patients with
newly diagnosed squamous cell head and neck cancer
and 176 noncancer age- and gender-matched controls
evaluated at a single clinical center between 1992 and
1994.46 Marijuana use was associated with a significantly
increased risk of head and neck cancer (odds ratio
[OR] for ever vs. never users = 2.6; 95% CI = 1.1-6.6)
after adjusting for age, gender, race, education, alcohol
use, and cigarette smoking.46 Dose-response relationships
were also found for frequency and duration of
marijuana use (p < 0.05). In addition, the effects of marijuana
and tobacco smoking were more than multiplicative
(OR for dual smokers of marijuana plus tobacco =
36.1), suggesting possible synergism between the effects
of marijuana and tobacco on the development of
upper respiratory tract cancer. It is noteworthy that associations
between marijuana use and head and neck
cancer were stronger (OR = 3.1; 95% CI = 1.0-9.7) for
younger subjects (≤ 55 years) who were more likely to
have smoked marijuana. Further well-designed, adequately
powered epidemiological studies are required
to evaluate the potential causal relationship between
marijuana use and the development of respiratory
cancer.
RESPIRATORY
EFFECTS OF Δ9-THC
Short-Term
Physiological Effects
Smoked marijuana containing 1.0% to 2.6% THC
(3.23-7 mg THC/kg body weight) results in significant
acute decreases in airway resistance (Raw) and increases
in specific airway conductance (SGaw), both in habitual
healthy smokers of marijuana and in marijuana-naive
asthmatic subjects, indicating an acute bronchodilator
response.47-50 These bronchodilator effects are apparent
within minutes after smoking, peak at 15 to 20 minutes,
and persist for at least 60 minutes. A marijuana cigarette
from which the cannabinoids had been extracted
produced no changes in Raw or SGaw, with the
bronchodilator response restored by adding synthetic
Δ9-THC to the extracted cigarette, indicating that the
bronchodilator effect was due mainly toTHCand not to
other ingredients in marijuana. This conclusion was
supported by the finding of a dose-dependent
bronchodilator response to oral synthetic THC (10-20
mg).49,50 In stable asthmatic subjects, smoked marijuana
also caused rapid reversal of bronchoconstriction provoked
by methacholine inhalation or exercise.51 While
these findings suggested that THC might be therapeutically
useful as a bronchodilator in asthma, the smoked
route of delivery ofTHCto asthmatic patients is contraindicated
because of the noxious gases and particulates
in the smoke, the potential long-term effects of which
include chronic airway irritation and/or malignant
change (vide supra). Moreover, oral THC is not suitable
for therapeutic use in asthma because of its unwanted
central nervous system intoxicant and cardioaccelerator
effects in relation to its only modest bronchodilator
properties.52,53 Moreover, tolerance to the bronchodilator
effect of THC occurs after several weeks of use.54
The mechanism of THC-induced bronchodilation
has recently been elucidated.55 In earlier studies,
THC-induced bronchodilation was found not to be mediated
by stimulation of beta-adrenergic receptors or
blockade of muscarinic receptors on airway smooth
muscle.56,57 However, that THC might still have a
parasympatholytic effect at a cholinergic site proximal
to the muscarinic receptor on airway smooth muscle
was suggested by evidence that it inhibited the release
of acetylcholine from postganglionic nerve endings following
vagal and chorda tympani stimulation in dogs58
and the myenteric plexus in guinea pigs.59 THC is now
known to act through specific seven-transmembrane
inhibitory G-protein-coupled cannabinoid receptors:
78S J Clin Pharmacol 2002;42:71S-81S
TASHKIN ET AL
CB1 receptors that are primarily expressed in the nervous
system60 and CB2 receptors that are mainly expressed
peripherally in the immune system.61 CB1 receptors
have recently been identified on axon
terminals of airway nerves in rat lungs in close proximity
to airway smooth muscle cells.55 Stimulation of
these receptors by the endogenous THC-like
cannabinoid, anandamide, led to inhibition of
capsaicin-induced bronchospasm and cough. This effect
was blocked by a selective CB1 receptor antagonist,
55 suggesting that the inhibitory effect of
anandamide on capsaicin-induced bronchospasm was
due to CB1 receptor-mediated inhibition of the
prejunctional release of excitatory neurotransmitters
such as acetylcholine. On the other hand, when airway
smooth muscle was completely relaxed by vagotomy,
resulting in abolition of cholinergic tone, anandamide
caused bronchoconstriction rather than bronchodilation,
and this effect was also inhibited by the cannabinoid
receptor blockade.55 The authors concluded that endogenous
cannabinoids may play a bidirectional role
in regulating airway smooth muscle tone, causing
bronchodilation when the smooth muscle is constricted
and bronchospasm when the muscle is
relaxed.
Other Effects of THC Relevant
to the Respiratory System
Other effects of THC that could have respiratory consequences
of potential clinical significance have already
been noted above. These include immunosuppressive
effects on immune cells, including alveolar
macrophages, T lymphocytes, and antigen-presenting
cells that could critically impair antimicrobial and
antitumor defenses, thereby predisposing to lower respiratory
infections and tumor growth. These immunologic
effects of THC appear to be cannabinoid receptor
mediated. THC is also a powerful inducer of gene transcription
for CYP1A1,29 an effect that probably is not
cannabinoid receptor mediated but rather is related to
binding of THC directly to the cytoplasmic aryl hydrocarbon
receptor. The increased production of CYP1A1
byTHCis additive to the induction of CYP1A1 production
by PAHs in marijuana and tobacco smoke. On the
other hand, since THC also serves as a substrate for the
CYP1A1 enzyme, its net effect on enzymatic conversion
of PAHs in the inhaled smoke to carcinogenic
formand thus its possible contribution to the carcinogenic
potential of marijuanaremains to be defined.
THC also appears to contribute to the oxidant stress of
marijuana smoke in isolated cells,17 thus possibly contributing
to the inflammation, edema, and cell injury
observed in the tracheobronchial mucosa of marijuana
smokers. In recent experiments,THChas been found to
have a toxic effect on mitochondrial electron transport
in isolated A549 lung tumor cells, as indicated by reducedATP
levels and impaired cell energetics.62 One of
the potential consequences of the apparent mitochondrial
toxicity of THC is an inhibition of apoptosis. THC
has also been shown to inhibit fas-mediated caspase-3
activation in A549 lung tumor cells with disruption of
the apoptotic pathway,63 thus possibly contributing to
lung cell injury and tumor promotion. Further studies
are required to determine the relevance of these toxic
cellular effects of THC to in vivo carcinogenesis.
CONCLUSIONS AND
FUTURE RESEARCH NEEDS
In summary, habitual marijuana smoking has been
shown to have a number of respiratory and immunerelated
effects that have potentially important clinical
consequences. These include symptoms of chronic
bronchitis, an increased incidence of acute bronchitic
episodes, and microscopic and immunohistochemical
alterations in tracheobronchial biopsies that are indicative
of airway injury and dysregulated growth. These
changes are likely to compromise the lungs
mucociliary clearance function and could predispose
to lower respiratory infection and malignancy. In addition,
regular marijuana use results in ultrastructural
and functional changes in pulmonary alveolar
macrophages, the major resident immune-effector cells
in the distal lung. Marijuana-related impairment in alveolar
macrophage fungicidal and bactericidal activity,
as well as the demonstrated suppressive effects of
Δ9-THC on T cell function and cell-mediated immunity,
could predispose to pneumonia, especially in otherwise
immunocompromised patients, such as those
with AIDS, those who have had organ transplantation
requiring immunosuppressive therapy, or those receiving
chemotherapy for cancer. A link between regular
marijuana use and the development of respiratory cancer
is suggested by several lines of evidence. These include
the presence of relatively high concentrations of
potent carcinogens in marijuana smoke, the exceptionally
high proportion of the inhaled tar from marijuana
smoke that is deposited in the lungs, findings of
premalignant changes in bronchial biopsies from marijuana
smokers, THC-related impairment in antitumor
immune defenses, and several case series of an unusually
high proportion of marijuana smokers among
young individuals with diagnosed upper and lower respiratory
malignancy. Additional studies employing
both cell-based systems and animal models, in addi-
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tion to well-designed, adequately powered epidemiological
studies, are required to elucidate the impact of
THC and marijuana on carcinogenesis, immune function,
and infection, including the risk of HIV infection
and progression of AIDS.
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