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CHEMISTRY AND PHARMACOLOGY OF MARIJUANA The cannabis plant (Cannabis sativa) thrives under a variety of growing conditions. It has been cultivated for centuries, mainly for hemp fiber, but also for its psychoactive and putative medicinal properties (Abel, l980; Turner et al., l980). Although the behavioral and psychological effects were well described in literature of the nineteenth century (Kalant and Kalant, l968), the complex chemistry and pharmacology of the cannabis plant discouraged extensive investigation until about l5 years ago. The most prominent effects of cannabis are on psychological phenomena and behavior. Psychopharmacology and behavioral pharmacology have developed as divisions of scientific inquiry only over the past 25 years; therefore, the older cannabis literature, no matter how valuable for observations on other matters, does not provide a basis for quantitative pharmacological analysis and evaluation. Early pharmacologists could work only with crude extracts of the plant. Although the general structure of the cannabinoids (Figure l) was known by the turn of the century, the particular cannabinoids that were identified early and were available as pure substances were largely devoid of the characteristic psychoactive and other pharmacological effects of cannabis. Synthetic cannabinoids with cannabislike activity became available in the l930s. It was not until l964 that an active ingredient of cannabis was identified as A-9-tetrahydrocannabinol (THC) and synthesized (Figure l) (Gaoni and Mechoulam, l964; Mechoulam and Gaoni, l965, l967). In the mid-l960s, the isolation and synthesis of the main psychoactive component of cannabis and related cannabinoids, together with a rapid increase in the use of marijuana by middle class North American students, stimulated scientific activity (Waller et al., l976; Waller et al., in press). This chapter, an overview of cannabis chemistry and pharmacology, emphasizes difficulties in the study of this drug (explored further in subsequent chapters) and in evaluating the literature. l2
l3 OH Cannabinol CH2OH OH Cannabidiol FIGURE l Cannabinoid structures. CsHn 1 l-hydroxy-A-9-THC CANNABIS CHEMISTRY Chemistry of the Plant Cannabis, the crude material from the plant Cannabis sativa, contains hundreds of chemicals. Most of these are found in other plants, but 6l, termed cannabinoids, are unique to the cannabis plant (Table l). Natural and most synthetic cannabinoids are relatively insoluble in water, but dissolve in fats and fat solvents and are therefore called lipid soluble. A single cannabinoid, A-9-THC, produces almost all the char- acteristic specific pharmacological effects of the complex, crude cannabis mixtures. A number of synthetic cannabinoids have pharmaco- logical effects similar to A-9-THC. Other cannabinoids in the plant, for example, cannabinol (Figure l), are almost inactive pharmacologically or may interact with A-9-THC to modify its actions. One cannabinoid, cannabidiol (CBD), can influence the metabolism of another, A-9-THC (Siemens et al., l976). A few cannabinoids have effects quite different from A-9-THC. For example, cannabidiol (Figure l) has relatively little psychoactive and cardiovascular effect but is an active anticonvulsant (Karler and Turkanis, l98l). Investigators have chemically altered the A-9-THC molecule in an attempt to determine which of its structural elements are required to produce behavioral or other effects (Mechoulam et al., l980). Studies of structure-activity relationships indicate that, to produce
l4 TABLE l Chemical Constituents of Cannabis Preparations l. Cannabinoids: 6l known a. Cannabigerol (CBG) type: 6 known b. Cannabichromene (CBC) type: 4 known c. Cannabidiol (CBD) type: 7 known d. A-9-Tetrahydrocannabinol (A-9-THC) type: 9 known e. A-8-Tetrahydrocannabinol (A-8-THC) type: 2 known f. Cannabicyclol (CBL) type: 3 known g. Cannabielsoin (CBE) type: 3 known h. Cannabinol (CBN) type: 6 known i. Cannabinodiol (CBND) type: 2 known j. Cannabitriol (CBT) type: 6 known k. Miscellaneous types: 9 known l. Other cannabinoids: 4 known 2. Nitrogenous compounds: 20 known 3. Amino acids: l8 known 4. Proteins, glycoproteins, and enzymes: 9 known 5. Sugars and related compounds: 34 known 6. Hydrocarbons: 50 known 7. Simple alcohols: 7 known 8. Simple aldehydes: l2 known 9. Simple ketones: l3 known l0. Simple acids: 20 known ll. Fatty acids: l2 known l2. Simple esters and lactones: l3 known 13. Steroids: ll known l4. Terpenes: l03 known l5. Noncannabinoid phenols: l6 known l6. Flavanoid glycosides: l9 known l7. Vitamins: l known l8. Pigments: 2 known SOURCE: Adapted from Turner, l980.
l5 effects on behavior, a pyran ring must be part of the three-ring system, a free phenolic hydroxyl on the aromatic ring at C-l, and a lipophilic side chain (C5Hl:L) at C-3 (Figure l). Understanding chemical structure-effect relationships is important to guide the synthesis of cannabinoids with differing pharmacological effects. Different effects of A-9-THC activity by chemical design will require further syntheses and pharmacological study of a large number of cannabinoids. Chemistry of the Smoke It is impossible to understand the effects of cannabis without quantitative control of the composition and the amount of the active substances, that is, control over the dose. Systematic pharmacology must therefore be performed using pure compounds. In the United States, cannabis usually is smoked, which complicates the pharmacology. The smoke from any burning plant contains hundreds of chemicals that may have biological effects. This poses a dilemma for researchers, because consequences of smoking cannabis cannot be fully determined by studies only of the pure cannabinoids. Studies also are needed with doses of A-9-THC delivered, however imperfectly, by smoking. The dose of A-9-THC obtained from smoking cannabis varies greatly, depending on many factors (Table 2). First, the content of A-9-THC depends on the genetic background or phenotype of the plant, the sex of the plant, conditions of growth and storage, and the plant preparation smoked. Second, much of the A-9-THC in fresh leaves that can be detected by gas-liquid chromatography (GLC) is in inactive carboxylated form. Decarboxylation to the active A-9-THC occurs slowly during storage and rapidly during heating, such as occurs in smoking or GLC analysis. Third, the way in which a cigarette is smoked can greatly affect how much of the A-9-THC content is absorbed by the smoker. Cannabis smoke is similar to tobacco smoke in that it is a mixture of very small particles and a gas-vapor phase. Both the particulate and vapor phases contain many identified and probably some still unidentified constituents that, based on clinical experience with tobacco smoke, must be assumed to be potentially harmful (Leuchtenberger and Leuchtenberger, l976). The amounts of some materials in tobacco cigarette and marijuana cigarette smoke are compared in Table 3. Toxic substances, such as carbon monoxide, hydrogen cyanide, and nitrosamines occur in similar concentrations in tobacco and marijuana smoke; so do the amounts of the particulate material known collectively as "tars." It is not easy to compare the toxicity of a given number of marijuana cigarettes to a given number of tobacco cigarettes. There are general similarities in the composition of the smoke, but the variations in composition of both tobacco and marijuana cigarettes and differences in smoking techniques make simple extrapolations of risks of tobacco versus marijuana smoking not valid.
l6 TABLE 2 Concentrations of A-9-THC in Different Varieties of Marijuana Type Percent A-9-THC (Percent by Weight) Normalized Averages6. NepalE 2.8l Mexico^ l.68 l.00 Pakistan0. l.30 Colombia6. 3.00-3.50 India! 0.46 Jamaica (Ganja)Jl United StatesS. Sinsemilla (fiber)^ Sinsemilla (intermediate)^ Sinsemilla (drug)^ Hashish (U.N. standard)^ NIDA (cigarette l)Â£ NIDA (cigarette 2)Â£ Crude marijuana extractÂ£ Illicit hashish oilS. Research harvests! (grown above 2000 m) l.39 (grown below 2000 m) 2.80 (mean) 0.35 0.2l 3.58 6.28 2.22 (7.40)Â£ 0.84 l.86 (2.8)3 20.00 l0.00-30.00 (up to 60)5. 0.90-2.80 3.00-ll.00 l.90 20.00 SOURCES: (a) Jones, l980; (b) Braenden, l972; (c) Turner, l974; (d) Turner, l980; (e-) Turner, l98l; (f-) Turner et al., l979; (3.) Rosenkrantz, l98l; (h) Marshman et al., l976.
l7 TABLE 3 Marijuana and Tobacco Reference Cigarette Analysis of Mainstream Smoke A. Cigarettes Marijuana Cigarette (85 mm) Tobacco Cigarette (85 mm) Average weight, mg 1115 1110 Moisture, percent 10.3 11.1 Pressure drop, cm 14.7 7.2 Static burning rate, mg/s 0.88 0.80 Puff number 10.7 11.1 B. Mainstream smoke I . Gas phase Carbon monoxide, vol. percent 3.99 4.58 mg 17.6 20.2 Carbon dioxide, vol. percent 8.27 9.38 mg 57.3 65.0 Ammonia, Mg 228 199 HCN, Mg 532 498 Cyanogen <CNI_,i Mg 19 20 Isoprene, Mg 83 310 Acetaldehyde, Mg 1200 980 Acetone, Mg 443 578 Ac role in , vq 92 85 Acetonitrile, Mg 132 123 Benzene, Mg 76 67 Toluene, Mg 112 108 Vinyl chloride, ng3- 5.4 12.4 Dimethylnitrosamine, ng3- 75 84 Methylethylnitrosamine, ng- 27 30 pH, third puff 6.56 .14 fifth puff 6.57 .15 seventh puff 6.58 .14 ninth puff 6.56 .10 tenth puff 6.58 .02 II. Particulate phase Total particulate matter, dry, mg 22.7 39.0 Phenol, pg 76.8 138.5 o-Cresol, Mg 17.9 24 m- and Â£-Cresol, Mg 54.4 65 Dimethylphenol, Mg 6.8 14.4 Catechol, Mg 188 328 Cannabidiol, Mg 190 â A9-Tetrahydrocannabinol, Mg 820 - Cannabinol, Mg 400 - Nicotine, Mg - 2850 N-Nitrosonornicotine, ng"- - 390 Naphthalene, Mg 3.0 1.2 1-Methylnaphthalene, Mg 6.1 3.65 2-Methylnaphthalene 3.6 1.4 Benz (a)anthracene, ng8- 75 43 Benzo(a)pyrene, ngi 31 21.1 VIndicates known carcinogens. SoURCES: Hoffmann et al., 1975, 1976; Brunnemann et al., 1976, 1977.
l8 Other Preparations Besides the crude plant leaf material for smoking, usually called marijuana, resinous material from the plant, called hashish, and solvent extracts of the plant, termed hashish oil, sometimes appear on the illicit market. In many parts of the world, hashish is more commonly used than marijuana. As with all cannabis preparations, the A-9-THC content of hashish varies enormously, but the upper limits of A-9-THC content are usually much higher than for marijuana: 7 percent or higher and even higher for hashish oil (Table 2). However, even these generally more potent forms of cannabis may occasionally contain much less A-9-THC. The mere designation of the nature of a cannabis preparation is an unreliable predictor of its A-9-THC content. The practical consequence of this for the clinical researcher is that the exposure to cannabis users is not known. What Potency of Marijuana Is Available From Street Samples? Because of the many confounding variables mentioned above, it is difficult to know what potency of psychoactive drug is in marijuana sold illicitly. The concentration of A-9-THC in a given sample will vary (Ritzlin et al., l979). The content of A-9-THC from various street samples has been assayed. Marijuana from Drug Enforcement Administration confiscated samples; samples received through psychiatrists, police departments; and state crime laboratories, and fugitive* samples were quantitatively analyzed for A-9-THC and other cannabinoids. A physical description of the sample was madeâe.g., buds, sinsemilla. The plants were also categorized by originâwhere they were cultivated. The analysis showed that tremendous variability exists in the potency of A-9-THC on the street; normalized samples ranged from zero to ll percent A-9-THC (Turner, l98l). Analytic Methods Detection and measurement of cannabinoids and their metabolites in body fluids is far more difficult than with such drugs as alcohol. The blood and tissue levels resulting from use of ordinary cannabis are very lowânanogramst per milliliter or lower. In addition, compounds like steroids, occurring normally in body fluids interfere with the measurement of cannabinoids in blood and can make the test much less sensitive than if pure cannabinoids in an uncontaminated * Samples received, when no arrests were made, tone billionth of a gram.
19 solution are being analyzed (Harvey et al., l980; Harvey and Paton, l980). A combination of gas-liquid chromatography and mass spectrometry is the most sensitive direct method of measuring cannabinoids. That, however, requires skilled technicians and expensive equipment not readily available. Using modifications of this experimental technique, one can measure as little as 5 picograms* of A-9-THC in a milliliter of plasma (Harvey et al., l980; Harvey and Paton, l980) . Radioimmunoassay and enzyme immunoassay techniques also are available, the lower limits of sensitivity of these methods now are not adequate for reliable measurements of A-9-THC in human blood more than a few hours after drug administration. A readily available enzyme immunoassay will detect cannabis metabolites in the urine for as long as a week after the smoking of a single marijuana cigarette. Thus, a positive urine test by this method is not necessarily indicative of use within the previous few hours and does not provide evidence of recent intoxication as a breath test does for alcohol. Assays for cannabinoids are likely to remain far more complicated than for alcohol and many other drugs. PHARMACOLOGY OF CANNABIS Implicit in a discussion of the effects of any drug is some determination of dose. The intensity and duration of effects in relation to drug dose must be determined or inferred from adequate pharmacologic study. The intensity and duration of a drug effect depends on at least three major factors: l. The concentration of the drug at the sites of action in the body. This is determined by the dose, what the drug is dissolved in or mixed with, the route of administration, and the pharmacokinetics of the drug. 2. The sensitivity of the cells the drug acts upon. 3. The physiological state of the bodily systems being affected. This, in turn, depends on interactions with other systems and, especially for drugs with behavioral and psychological effects, as well as environmental and experiential factors, including the presence of other drugs. With cannabis, many or even most of these factors are not always measurable or under the control of an investigator. *l pg = l0~l2 grams.
20 Potency and Pharmacokinetic Considerations Pharmacokinetic studies of the absorption, distribution, metabolism, and elimination of A-9-THC determine how long A-9-THC and its metabolites remain in the body. Pharmacokinetics vary with the route of drug administration and such factors as lipid solubility; A-9-THC tends to remain for long periods of time in fatty tissue. When smoked, A-9-THC is rapidly absorbed by the blood in the lung. If taken orally, A-9-THC is not absorbed into the blood as rapidly. The rate of disappearance of A-9-THC from the blood varies with time (Lemberger et al., l97la,b, l972; Ohlsson et al., l980). High blood levels fall rapidly for the first 30 minutes, as the A-9-THC distributes to tissues with high blood flow. After the initial distribution, the blood level falls much more slowly with a half-life* of l9 hours or more (Hunt and Jones, l980). Metabolitest of A-9-THC have their own independent rates of elimination. Typically, metabolites are eliminated more slowly, having a half-life of approximately 50 hours (Hunt and Jones, l980). After an injection of a single dose of A-9-THC, approximately 25-30 percent of the compound and its metabolites remain in the body at l week (Lemberger et al., l97lb; Hunt and Jones, l980). Essentially complete elimination of a single dose may take 30 days or longer (Jones, l980). Thus, repeated administration of even small doses may lead to an accumulation of drug higher than levels reached at any time after a single dose. Absorption Inhaling smoke from a cannabis cigarette or pipe is pharmaco- kinetically different from ingesting cannabis. Smoking is a far more efficient way of delivering cannabinoids to the brain than ingestion because of the large surface area of the lungs. Inhaled, the cannabinoids in the smoke go rapidly from the lungs into the blood to the left side of the heart and are carried in seconds to the brain and other organs before passing through the liver. When smoked, a drug reaches the brain with relatively little time for metabolism or dilution. Many substances with high lipid solubility such as cannabinoids go quickly from blood into tissues, including brain tissues. Psychological and cardiovascular effects of cannabis are *The half-life is a measure of how rapidly a drug is eliminated. It is the time required for the level of a drug to be reduced by one-half. If starting levels are ten units and the half-life is 24 hours, then l day after administration, the level will be 5 units, 2 days after administration 2.5 units, etc. tThere are more than 45 metabolites of major cannabinoids identified in different species, at least one of which, ll-OH-A-9-THC, is psychoactive.
2l evident within a few seconds of inhalation. Peak effects occur about the time smoking is completed. When taken by mouth, cannabinoids usually are in solutions or suspensions. The material they are mixed with affects the rate of absorption. For example, blood levels of A-9-THC were higher and lasted longer when given in an oily solution than in an ethyl alcohol solution (Perez-Reyes et al., l973). This suggests that cannabis eaten in food mixtures containing fat is better absorbed. An important difference between smoking and ingestion is that when cannabinoids are absorbed from the gut, the blood containing them first goes directly through the liver. The liver rapidly clears the A-9-THC from the blood and enzymatically changes much of the A-9-THC to other metabolites before it reaches the brain (Hunt and Jones, l980). A large amount is metabolized to ll-hydroxy-A-9-THC (Figure l). It is unknown if the spectrum of effects of this metabolite is identical to that of A-9-THC. when taken by mouth, in contrast to when smoked, two or three times more A-9-THC is required to obtain equivalent acute psychological and physiological effects. After oral doses the effects develop more slowly, last longer, are more variable, and cannot be controlled by the recipient once the cannabis has been swallowed. In contrast, the smoker feels the effects quickly and can modify inhalation at any time, although overdosage is still possible. Unpleasant reactions to overdose are more common following ingestion than inhalation. A variety of other routes of administration have been used experimentally in humans and in animals, including intravenous, intraperitoneal, subcutaneous, intramuscular, topical (on the skin), and into the conjunctival sac (eye). These various routes influence the time to onset of effect, duration and peak intensity, and the rate with which the effect disappears. Direct comparison of findings in studies using differing administration routes is difficult and must take these factors into consideration. Human users of cannabis vary in their preferred routes of use. In some countries and cultures cannabis is mainly taken by ingestion (for example, India) and in others by inhalation (for example, the United States). Because of the effects of route of administration on pharmacology, it is reasonable to expect different health consequences of the different routes of administration; therefore, comparisons of health statistics among countries must be made with care. Although smoking avoids many of the absorption problems discussed above, a host of other variables affecting dose are introduced, such as the size and packing of the cannabis cigarettes, the way the smoke is inhaled, the number of puffs and the interval between puffs, the temperature produced in the burning cigarette, and whether a cigarette is shared. Because of the progressive concentration of cannabis constituents in the cigarette butt, the last few puffs yield con- siderably more A-9-THC and particulate matter than do the earlier puffs. All these and other factors affect the dose received, and only rarely have they been measured. Only some of these factors are under the conscious control of the cannabis smoker. About half of the A-9-THC originally in a cannabis cigarette is lost by
22 combustion, by butt entrapment, in smoke not inhaled, and in smoke exhaled (Fehr and Kalant, l972; Rosenkrantz, l98l). It has been reported that, like nonsmokers of tobacco, individuals in a poorly ventilated room where cannabis is smoked may passively inhale active components (Zeidenberg et al., l977). Because only trace amounts of cannabinoid metabolites are present in urine of these passive inhalers, it is unlikely that the low levels of the absorbed cannabinoids from the ambient air account for the so-called "contact high." Experiencing subjective cannabis effects in the presence of cannabis smokers could be explained by psychologic factors in addition to any pharmacologic ones. But, because studies have shown that children of parents who smoke tobacco are more likely to have respiratory infections during the first year of lifeâwhich may be due to their being exposed to cigarette smoke in the atmosphere (U.S. Department of Health, Education, and Welfare, l979)âthe issue of passive inhalation of marijuana smoke is worth further study. Distribution The lipid solubility of A-9-THC and other cannabinoids, including those with highest pharmacologic activity, facilitates distribution readily into tissues and cells throughout the body so blood levels drop rapidly. Initially, cannabinoid concentrations are highest in such tissues as lung, liver, and kidney that have a high blood flow (Agurell et al., l969, l970; Klausner and Dingell, l97l). Delta-9-THC crosses the placenta and enters the fetus of experimental animals (Kennedy and Waddell, l972) . Cannabinoid levels in the human fetus have not been studied. Small amounts are also found in the milk of experimental animals and can be transferred to progeny (Jakubovic et al., l973; Chao et al., l976). After initial distribution, concentrations of cannabinoids in tissues, cells, and subcellular compartments are highly nonuniform, determined no doubt by solubility and other physicochemical characteristics. Therefore, blood concentrations do not reflect concentrations at pharmacologically active sites, as they do with alcohol. Metabolism and Elimination Elimination of drugs and their metabolites is mostly through excretion by the kidney into the urine or by the gall bladder via the bile into the intestine and out with the feces. Cannabinoids do not pass out of the blood into the lungs and do not appear in breath in appreciable quantities. Some cannabinoids going into the intestine with bile are reabsorbed. Some also diffuse back through the kidney tubules during the process of urine formation, so the amounts finally excreted per unit of time are small. The net result of this recycling is that the cannabinoids are only slowly eliminated from the body.
23 Studies of the disappearance of A-9-THC from human plasma have led to reports of values of half-lives that ranged from 19 hours in experienced users (Hunt and Jones, l980) to 57 hours in naive users (Lemberger et al., l97lb). Whether this difference in half-life is due to the experience of the user has not been established. Because of their high lipid/water partition coefficients, A-9-THC and some of its metabolites can be sequestered in fatty tissues. Following the intravenous administration of radioactive A-9-THC to human volunteers, however, 67 percent of the radioactivity was excreted in l week, 22 percent in the urine and 45 percent in feces (Lemberger et al., l97la). Almost no A-9-THC itself was excreted in the urine. There may be fairly rapid and complete metabolism of free A-9-THC followed by slow release and metabolism of sequestered A-9-THC and retained metabolites. Because no direct measurements of cannabinoid levels have been made in tissue samples from human cannabis users and the data are limited in experimental animals, one can only infer from blood levels what metabolites are accumulating and where. In rats, after inhalation or intravenous administration of radioactive A-9-THC, radioactivity persisted in the brain for at least 7 days, mostly as metabolites (Ho et al., l970). When given subcutaneously in rats, even at intervals as great as a day or two apart, A-9-THC will accumulate as metabolites (Kreuz and Axelrod, l973). Accumulation of some cannabinoids with even less frequent intake appears likely. Although most metabolites are concentrated in fatty tissues, they will slowly pass into plasma and circulate though all parts of the body, particularly including such organs as the brain, and generally all membranes. The health consequences of the continued presence of such foreign molecules are not known. The marked persistence of the cannabinoids is quite unlike other widely consumed agents, such as alcohol, nicotine, and caffeine, that are rapidly metabolized and leave no trace a few hours after moderate intake. WHAT IS A LARGE OR SMALL CANNABIS DOSE? Large and frequent doses of any drug are more likely to produce adverse health effects than small infrequent doses of the drug. Thus, judgments of health consequences of the use of cannabis can only be made with implicit or explicit knowledge about dose. For the reasons discussed above, the range of cannabinoid doses consumed varies widely. Investigators usually report dose in terms of marijuana cigarettes per unit of time, or they give some estimate of the concentration of A-9-THC used for oral application. This is not an adequate way to quantify the amount of cannabinoids actually entering the body. Only one epidemiologic study provides a breakdown of varying dose levels in excess of one cannabis cigarette daily (Bachman et al., l98l). Epidemiologic surveys have not quantified A-9-THC levels. When reporting less frequent use patterns than one cigarette per day, investigators use measures that make it difficult to compare studies. In this report, any general or average dose estimates are approximations.
24 It is generally agreed that smoking five or six l-gram cannabis cigarettes daily is a large dose (Dornbush et al., l97l; Rosenkrantz, l98l). Because of the variability of A-9-THC content of cannabis available from street samples, it would be more appropriate to consider this heavy use. The definition of a low dose is more controversial. Some consider one marijuana cigarette a day to be a large dose. Others think even one cigarette a week is regular, frequent, and a high dose. With tobacco and alcohol, for which dose is easier to quantify, it took many years to establish what a small or large dose might be in terms of specifying doses that significantly increased the risk of various behavioral and health consequences. Even with those drugs, there is still disagreement as to precisely what a small and "safe" dose might be. There will be even more problems in specifying typical cannabis doses and predicting their likely health consequences. In controlled laboratory conditions, ingested doses of more than 20 mg of A-9-THC generally are considered by both investigators and cannabis users to be large doses. Doses of less than l0 mg are considered small. Marijuana cigarettes containing more than 20 mg of A-9-THC seem to be a large dose, and those with l0 mg produce effects generally considered the result of a small dose. When volunteers were allowed to select their own self-determined smoked doses in controlled experiments, some smoked only one or two 20-mg cigarettes daily, while other similar volunteers smoked six to ten or more cigarettes per day. Variability in smoking patterns is great and not easily quantified; only broad range estimates of dose are possible. GENERAL TOXICOLOGY Delta-9-THC and related cannabinoids have very low lethal toxicity. That is, a very high single acute dose of A-9-THC is required to kill half of a population of experimental animals. This lethal dose for 50 percent of the animals is called the LD5g. The lack of well-authenticated cases of human deaths from acute A-9-THC or cannabis overdose is consistent with the experimental animal data. The lethal dose increases as the phylogenetic tree is ascended. The rat has an LD50 of 40 mg/kg intravenously, in contrast to a l25 mg/kg in the monkey (Rosenkrantz, l98l). Death is usually due to cardiac dysfunction. Delta-9-THC appears to be the most toxic of the cannabinoids. Studies of chronic cannabis administration to animals have demonstrated delayed lethality. Animals die after several days of a repeated high dose (Rosenkrantz, l98l). The reason for this pattern is unclear. It could be related to accumulation of A-9-THC or metabolites in tissues. A l-year chronic treatment of rats with lower doses of cannabinoids produced a pattern of toxicity consisting of weight loss, pulmonary pathology when the drug is inhaled, and slowly
25 developing behavioral toxicity characterized by hyperactivity, vertical jumping, fighting, and seizures (Rosenkrantz, l98l). RELEVANCE OF NONHUMAN ANIMAL MODELS Much of what is known about cannabis comes from experiments in animals. Some aspects of the pharmacology of any drug can only be studied in animals other than human beings. Findings from animal experiments have been criticized because of what were thought to be unreasonably high doses of cannabis given to the animals as compared with doses commonly used by human beings. Although extrapolation of human effects from animal data must be done with caution because of species differences in metabolic pathways and differing sensitivity and physiology, a blanket criticism of animal studies because of high doses is inappropriate. When an effect of a drug occurs consistently in several species, it is likely to occur in human beings. Compari- sons of A-9-THC blood levels in human beings and in several species suggest roughly similar intensity of effects at similar blood levels in the various species (Rosenkrantz and Fleishman, l979). CANNABIS CONTAMINANTS On occasion cannabis has been reported not only to contain the herbicide paraquat, but also salmonella bacteria and aspergillus fungus. Deliberate addition of such drugs as lysergic acid diethylamide (LSD), heroin, and phencyclidine (PCP) has been claimed. A plant material such as cannabis is not always handled in the most sanitary way, and a variety of contaminants are possible. Paraquat There is no question that large doses of paraquat by mouth or by aerosol can cause pulmonary fibrosis, but no cases in human beings have yet been proved to result from paraquat-contaminated cannabis. Few cannabis smokers are expected to be exposed to the large amounts of paraquat known to cause severe lung damage. This is not to say that no lung damage will occur from such exposure. A more extensive discussion of paraquat is in Appendix D. Bacteria and Fungi A few outbreaks of salmonellosis epidemiologically linked to marijuana use were reported from Ohio and Michigan (Schrader et al. , l98l). Marijuana was found to be contaminated with the same type of salmonella that was obtained from the 62 patients experiencing diarrhea, fever, and abdominal pain.
26 Aspergillus, a fungus, is a common contaminant of some cannabis (Llewellyn and O'Rear, l977; Llamas et al., l978). The spores pass easily through contaminated marijuana cigarettes and when smoked are presumed to enter the body. CELLULAR TOXICITY A variety of effects on cellular processes have been reported, usually based on studies of in vitro systems. The low water solubility of the cannabinoids and the need to add solvents and emulsifiers, along with a common tendency to use higher in vitro concentrations than occurs in living animals, makes interpretation of such experiments difficult. In related studies, A-9-THC alters the actions of a number of intracellular enzyme systems. The biological relevance of these drug/enzyme interactions is still unclear at this time, but, together with the cytotoxicity, it suggests that A-9-THC is producing marked effects on cell membranes and intracellular processes. Almost nothing is known of the molecular mechanisms by which cannabinoids produce their effects in cells. TOLERANCE AND DEPENDENCE Repeated administration of most psychoactive drugs leads to the development of tolerance. This state of increased drug resistance results from two general mechanisms (Kalant et al., l97l): * Dispositional tolerance resulting from lower drug concentrations at sites of action, usually because of increased rates of drug metabolism or elimination * Functional tolerance arising from decreased sensitivity of the target cells. Tolerance to most cannabinoid effects has been demonstrated both in animals and human beings (Jones, l98l). Tolerance can develop rapidly after only a few small doses. It disappears at an equally rapid rate for many effects, although after large doses in experimental animals some tolerance may persist for long periods (Jones, l98l). Systematic studies of tolerance loss have rarely been done. Many characteristics of tolerance to A-9-THC, particularly its pattern of rapid acquisition and loss, are similar to that occurring with opiates, nicotine, and cocaine (Jones, l98l). Most evidence suggests functional rather than dispositional means of acquiring tolerance. The development of such tolerance to cannabis does not necessarily have health implications. However, if tolerance should lead to higher or more frequent doses, adverse consequences, e.g., respiratory effects, associated with higher usage could result.
27 Physical dependence, manifested by withdrawal signs and symptoms, can develop rapidly in animals and in human beings (Jones, l98l). The withdrawal syndrome is not life threatening. It is similar in many respects to the mild dependence produced by low doses of other sedatives. Withdrawal symptoms can include restlessness, irritability, mild agitation, insomnia, and sleep EEG disturbance. Cannabis dependence does not mean the same thing as cannabis addiction. Dependence means only that a withdrawal syndrome can occur when drug taking is stopped. Addiction implies compulsive behavior to acquire the drug. The relationship between dependence and increased drug seeking or drug using is more theoretical than well documented, particularly in experiments with human beings. Given the appearance of tolerance and dependence with almost any psychoactive drug, it would be unusual not to find tolerance and dependence with the right dose and dosage schedule of cannabis. Good studies of the relationship of dependence, if any, to persistent drug use are important. DRUG INTERACTIONS Because cannabis often is consumed with other drugs, interactions can be expected. Other illicit drugs, tobacco, caffeine, alcohol, and over-the-counter or prescribed medications should be studied in combination with cannabis, because A-9-THC and its first metabolite are strongly bound to proteins in the plasma (Garrett and Hunt, l974) and may interact with other drugs similarly bound. Cannabis and many other drugs share disposition by the hepatic metabolic enzyme systems, and there are possible interactions at the drug metabolism level. For example, drugs such as alcohol or pentobarbital can inhibit metabolism of A-9-THC by enzyme substrate competition. Or, if after a period of inhibition one drug is removed, the enzyme activity can increase so that faster than expected metabolism follows. If given simultaneously with other drugs, A-9-THC can slow metabolism of drugs such as theophyllin, antipyrine, ethanol, and pentobarbital (Benowitz and Jones, l977; Jusko, l979). Cannabidiol can also inhibit the metabolism of a variety of drugs normally metabolized by the shared hepatic enzyme systems. Drug interactions also can occur by means of functional mechanisms. These can be additive, resulting in enhancement or prolongation of behavioral and psychological effects by cannabis when combined with other central nervous system depressant drugs, such as alcohol and barbiturates. Animals less tolerant to cannabis will also be less sensitive to other central nervous system depressants. This phenomenon is known as cross-tolerance. Drug interactions will be mentioned in subsequent chapters.
28 SUMMARY AND CONCLUSIONS Cannabis is not a single drug, but a complex preparation containing many biologically active chemicals. The psychological and physiological effects produced by A-9-THC probably result from actions at sites within the central nervous system and elsewhere in the body, leading to the likelihood of complicated effects depending on dose, duration of use, and many other considerations. The intensity of effect an individual experiences varies considerably according to the cannabis preparation and the amount taken, route of administration, frequency of use, and probably other not-well-recognized biological considerations. Dose variability must be considered both in conducting and in interpreting any studies of the effects of cannabis, particularly when trying to predict health consequences. In research the use of pure A-9-THC avoids some problems of dose control but cannot provide a complete picture of cannabis effects, because the effects of A-9-THC in crude preparations of the plant may be influenced by other components. Other consequences of cannabis use, for example, exposure to harmful components in its smoke, will have deleterious health consequences in addition to anything produced by the A-9-THC. The long persistence of cannabinoid metabolites in the body may have delayed effects or health implications not yet recognized, because, even with relatively infrequent use, there is chronic exposure to biologically unknown materials. In this respect, cannabis differs fundamentally from such drugs as alcohol, nicotine, and caffeine, which are rapidly metabolized and eliminated from the entire body. Cannabinoid effects can be modified by many events, including interaction with other drugs and the development of tolerance. Both tolerance and dependence develop to many effects of the drug. The health significance of tolerance and dependence, particularly their importance in drug-seeking and drug-using behavior, has not been studied properly. It is unlikely that adequate epidemiologic data will be available (soon) to enable good estimation of the health consequences of various usage levels. A prerequisite is that adequate chemical analytical methods be applied on a large-scale basis to monitor actual exposures. Continued studies in experimental animals will play an essential role in the assessment of the health risks of cannabis. For example, the biological activities of A-9-THC metabolites can be assessed in experimental animals, but these tests are technically more difficult to do in human beings. RECOMMENDATIONS FOR RESEARCH Several research priorities are identified by the preceding discussion:
29 ' Cannabinoids and their metabolites persist for relatively long periods in the body. More information is needed on the biological significance of that persistence in human beings. As a first step, the toxicological effects of the various metabolites need to be determined. * Drug interactions alter the actions of cannabis. Cannabis use alters other drug effects. More information is necessary to make the combined effects of cannabis and other licit and illicit drugs more predictable, especially with respect to behavioral impairment and toxicity to lungs, liver, and other organs. * Studies of the mechanism of action of cannabis should continue. Knowledge of mechanism is likely to provide powerful insights into the potential health effects. * Improved chemical analytical methods are necessary. Epidemiologic appraisal of the health effects of cannabinoids requires methods suitable for wide-scale assays of exposures. Pharmacological verification of the self-reported extent of use will make experimental and clinical results much easier to interpret. A chemical "marker" of the frequent user would be useful. Screening techniques for the purpose of identifying and discouraging cannabis-impaired driving would also be valuable. ' Characterization of the toxicological significance of common cannabis contaminants such as paraquat and other chemicals, fungi, and bacteria should be continued. * The development of tolerance is a factor that potentially modifies the expression of all psychoactive drug effects. Additional studies on the rates of acquisition and loss of tolerance and the relationship of these phenomena to dependence are necessary. The biological significance of the changes that underlie the development of tolerance should be established. The relationship, if any, between tolerance and dependence and drug-seeking behavior should be established. * Cannabis products are variable and complex. More information on the amount, nature, and potency of the various preparations used around the world would facilitate calculations of exposures to its constituents. For example, what is the biological and toxicological significance of the minor components of cannabis smoke? REFERENCES Abel, E.L. Marihuana: The First Twelve Thousand Years. New York: Plenum Press, l980. Agurell, S., Nilsson, I.M., Ohlsson, A., and Sandberg, F. Elimination of tritium-labelled cannabinols in the rat with special reference to the development of tests for the identification of cannabis users. Biochem. Pharmacol. l8:ll95-l20l, l969. Agurell, S., Nilsson, I.M., Ohlsson, A., and Sandberg, F. On the metabolism of tritium-labelled delta-l-tetrahydrocannabinol in the rabbit. Biochem. Pharmacol. l9:l333-l339, l970.
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