Synthetic Cannabinoids: Pharmacology, Behavioral Effects, and Abuse Potential
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Cannabis has been used throughout the world for centuries. The psychoactive effects of cannabis are largely attributable to ∆9-tetrahydrocannabinol (∆9-THC), the prototypical cannabinoid that occurs naturally in the plant. More recently, chemically and pharmacologic-distinct synthetic cannabinoids (SCBs) have emerged as drugs of abuse. Compared with ∆9-THC, the distinct structures of these compounds allow them to avoid legal restrictions (at least initially) and detection in standard drug screens. This has contributed to the popularity of SCBs among drug users who seek to avoid positive drug screens. Importantly, the distinct structures of the SCBs also typically result in increased affinity for and efficacy at cannabinoid CB1 receptors, which are thought to be responsible for the psychoactive effects of ∆9-THC and its analogues. Accordingly, it seems likely that these more powerful cannabimimetic effects could result in increased adverse reactions and toxicities not elicited by ∆9-THC in cannabis. Animal models useful for the study of emerging SCBs include the cannabinoid tetrad, drug discrimination, and assays of tolerance, dependence, and withdrawal. However, these in vivo procedures have not been particularly informative with regards to drug efficacy, where the majority of SCB effects are comparable with those of ∆9-THC. In contrast, essentially all in vitro measures of drug efficacy confirm ∆9-THC as a relatively weak CB1 partial agonist, whereas the majority of the SCBs detected in commercial preparations are full agonists at the CB1 receptor. As use of these emerging SCBs continues to rise, there is an urgent need to better understand the pharmacology and toxicology of these novel compounds.
KeywordsCannabis Synthetic cannabinoids SCBs THC Psychoactive effects CB1 receptors
Cannabis is the most commonly used recreational drug, especially among teens and young adults . Cannabis is primarily abused for its psychoactive effects, (eg, subjective euphoria, relaxation, and elevated mood), attributed to its main psychoactive constituent ∆9-tetrahydrocannabinol (∆9-THC). Since the discovery of ∆9-THC, hundreds of novel analogues have been synthesized and used as therapeutic agents, as pharmacologic tools to enhance our understanding of the endocannabinoid system (see below), and most recently, as recreational drugs of abuse. Across the United States, commercial preparations of synthetic cannabinoids (SCBs) (eg, labeled “K2” or “Spice”) have gained much attention among drug users and lawmakers. According to a 2012 survey, SCBs are the second-most commonly used illegal drug among young adults, with only cannabis use occurring at a higher rate . In a 3-year product surveillance study in our state, 26 individual SCBs were detected in commercial products [3••]. The most common SCBs detected in Arkansas were JWH-018, AM2201, JWH-122, JWH-210, and XLR11, and it was not uncommon to find 2 or 3 SCBs in combination in commercial products sold in the state [3••]. Use of SCBs produces psychoactive effects similar to those of cannabis, they are easily accessible and difficult to detect in standard urine drug screens , which all contribute to their high rate of use. Despite local, state, and federal regulations of the more prevalent synthetic cannabinoids, SCB products can still be purchased with ease from the Internet, head shops, and convenience stores. Legislators are concerned with finding effective measures to curtail SCB availability, however, this has been particularly challenging given the constantly changing composition of commercial SCB products [3••]. This has also frustrated efforts at developing a standardized drug test that screens for SCB use.
Although commercial SCB products are typically sold as a mixture of plant materials (touted as “herbal incense” or “potpourri”), they have also been documented to exist as tablets, capsules, and as powders [3••]. According to the Drug Enforcement Administration (DEA), SCBs commonly found in commercial preparations include the aminoalkylindoles JWH-018, JWH-073, and JWH-200, and the cyclohexylphenols CP-47497 and CP-47497 C8, all of which are currently listed as Schedule I controlled substances . Despite strict regulations on these particular SCBs, other analogs are still emerging in commercial products, including AKB48, AM-2201, JWH-081, JWH-122, UR-144, and XLR11 [6, 7]. Commercial preparations of SCBs often contain multiple combinations and varying concentrations of SCBs, even within products marketed with the same name and packaging, leading to marked dose/drug inconsistency from batch to batch [8, 9].
A strong motivation for controlling SCBs, in addition to their cannabis-like intoxicating effects, is the relatively high incidence of adverse effects associated with their use, including acute psychosis, confusion, anxiety, tachycardia, drowsiness, dizziness, agitation, hypertension, seizures, convulsions, vomiting, nausea, high blood pressure and chest pain [10, 11, 12, 13], acute central nervous system (CNS) and cardiovascular toxicity [14•], and long-term abuse-related effects of dependence and withdrawal [15, 16]. According to the American Association of Poison Control Centers, there were 2906 reported cases of synthetic cannabinoid exposure in 2010, which more than doubled in 2011 to 6968 . Amidst a flurry of media attention and strict legal regulations, exposures moderately decreased to 5228 in 2012 and 2639 in 2013 . The implications of acute SCB use on human health remains poorly understood, and even less is known about the long-term effects of these drugs. Currently, there are no direct treatments for complications arising from SCB use, and only supportive care is provided in these cases.
Much of our understanding of commercial SCB products has primarily been achieved through studying the endocannabinoid system. Below, we briefly introduce the endocannabinoids to provide a background against which to compare the newly emerging SCB drugs of abuse. The remainder of this review will focus on current findings on the in vivo pharmacologic effects of synthetic cannabinoids, with a particular emphasis on their abuse potential.
The 1964 identification of the highly lipophilic ∆9-THC set the stage for the eventual discovery of the first cannabinoid (CB1) receptor in 1990, followed 3 years later by the characterization of the CB2-receptor [18, 19]. Both CB1 and CB2 receptors are members of the 7 transmembrane G protein coupled receptor (GPCR) superfamily . CB1 receptors are among the most abundant GPCRs in the mammalian CNS. Outside the brain, CB1 receptors are expressed at lower levels in a variety of peripheral tissues including fat, heart, intestine, liver, endocrine, pancreas, and uterus . CB2 receptors are primarily expressed in peripheral tissues, such as liver, lung, and kidney, and are closely associated with the immune and hematopoietic system [22, 23]. More recently, increasing evidence supports the existence of CB2 receptors in the brain, strengthening the idea of a functional significance of CB2 receptors in the CNS [24, 25]. In this review, we will primarily focus on the effects of SCBs at CB1 receptors since they are primarily responsible for the abuse-related psychoactive effects of these drugs. The study of endocannabinoids is presently flourishing, and detailed reviews of endocannabinoid synthesis, signaling and metabolism are available [26, 27, 28].
Preclinical In Vivo Pharmacology of Synthetic Cannabinoids
Administration of cannabinoid agonists from multiple structural classes elicits a characteristic cluster of effects in laboratory animals. This cluster of the 4 classical endpoints of hypothermia, analgesia, catalepsy, and locomotor suppression has been termed the cannabinoid tetrad [30, 31]. Qualitatively consistent data are reliably observed for cannabinoid agonists across tetrad measures, characterized by dose-dependent decreases in measures of body temperature and motor activity, and dose-dependent increases in measures of analgesia and catalepsy. The advantage of using these tetrad measures to evaluate cannabinoids is that none of these assays require any particular training of the animals. Thus, data can be generated rapidly. Another advantage of the tetrad is that all of these endpoints can be assessed in quick succession in the same subject, decreasing total animal usage.
The cannabinoid tetrad has been extremely useful in the characterization of the biological activity of cannabinoid ligands. Interestingly, ∆9-THC tends to elicit tetrad effects of similar magnitude to higher efficacy cannabinoids such as WIN 55-212 and CP-55,940 . The SCBs JWH-018 and JWH-073 also elicit characteristic tetrad effects in the mouse after intraperitoneal injection . CB1-receptor mediated tetrad effects after exposure to smoke produced from combustion of an herbal incense product containing 5.4 % JWH-018 have also been reported [34•]. Importantly, a Phase I hydroxylated JWH-018 metabolite retained marked cannabimimetic effects in vivo, eliciting profound hypothermic and locomotor depressant effects in mice, which were blocked by pretreatment with AM251, a CB1-receptor selective antagonist [35••]. Similarly, the analogous metabolite of JWH-073 also decreased core temperature and locomotor activity in the mouse . However, not all biologically active Phase I hydroxylated metabolites of these compounds are cannabinoid agonists, as pretreatment with a different metabolite of JWH-073 significantly blunted hypothermic effects elicited by JWH-018 in the mouse without altering core temperature on its own . Interestingly, this metabolite of JWH-073 did not alter the analgesic, cataleptic, or locomotor effects of JWH-018 in the mouse, perhaps suggesting that it did not penetrate the CNS at the dose tested .
Overall, the tetrad assay reveals that commercially available SCBs elicit effects similar to those of the prototypical cannabinoid ∆9-THC. Furthermore, these effects are believed to be CB1-receptor mediated since pretreatment with CB1-receptor antagonists can block these effects. Finally, the activity of Phase I metabolites of some common SCBs raises the intriguing possibility that genetic polymorphisms in cytochrome P450 enzymes may play an important role in determining an individual’s response to these drugs. For example, an individual with a liver enzyme profile leading to biased production of antagonist metabolites might experience greatly attenuated drug effects, while an individual with a liver enzyme profile favoring formation of agonist metabolites might experience potentiated or longer-lasting drug effects.
Discriminative Stimulus Effects
Cannabinoids exert numerous effects on perception and other unobservable variables in humans, thus, drug discrimination is useful as a preclinical and clinical model of these subjective effects. The drug discrimination assay can be thought of as an in vivo procedure to identify drugs with similar subjective effects to a well-characterized training drug. Thus, when studying emerging SCB drugs of abuse, a dose of ∆9-THC is typically used as the standard training stimulus. Centrally-active cannabinoid agonists reliably induce ∆9-THC-like effects in animals trained to discriminate ∆9-THC. For example, full substitution for ∆9-THC was observed with WIN 55212 and 1-butyl-2-methyl-3-(1-naphthoyl) indole in monkeys , and with R(-)-methanandamide in rats . Thus, drug discrimination may be particularly useful in the study of cannabinoid in vivo pharmacology, both due to its high degree of pharmacologic specificity, and because results from preclinical studies are predictive of the subjective effects of cannabis in humans .
Synthetic compounds found in commercial products
CB1 affinity Ki (nM)
CB1 affinity (x THC)
CB1 efficacy EC50 (nM)
CB1 efficacy (% CP)
Assessment of the reinforcing effects of drugs is accomplished using the self-administration technique, in which an operant response results in the immediate administration of the drug. Despite recreational use and abuse of cannabinoids throughout human history, the reinforcing effects of cannabinoids have not been widely investigated in laboratory animals. Intravenous self-administration of low ∆9-THC doses were demonstrated in squirrel monkeys [44, 45]. Furthermore, reports have shown that high efficacy cannabinoids WIN 55,512 and HU-210, both found in some SCB commercial products, maintain intravenous self-administered behavior in mice and rats [46, 47, 48]. These data may suggest that other high efficacy cannabinoids, such as those SCBs present in commercial products, might also display reinforcing effects in self-administration procedures, but thus far, no published reports bolster this supposition. Importantly, the reports in squirrel monkeys and rats demonstrate that the reinforcing effects of intravenous cannabinoids are significantly attenuated by pretreatment with CB1-receptor antagonists, strongly suggesting that the abuse-related effects of these substances are indeed mediated by central cannabinoid systems.
Conditioned Place Preference
Another way to indirectly assess abuse-related effects of cannabinoids in experimental animals is to study their capacity to elicit a conditioned place preference. Studies investigating the capacity of cannabinoids to induce conditioned place preference present a complex and contradictory picture, which is discussed in detail elsewhere . Nevertheless, robust preferences for Δ9-THC-paired contexts have sometimes been reported in rats  and mice [51, 52, 53] under carefully controlled experimental conditions. Recently, it was demonstrated that the SCB JWH-018 induced a dose-dependent place aversion in previously drug-naïve mice, but elicited place preference in mice with a history of ∆9-THC administration . This finding, coupled with the fact that place preference induced by either Δ9-THC  or CP 55,940  is attenuated by pretreatment of CB1-receptor antagonists, again implies that augmentation of cannabinoid signaling can lead to conditioned rewarding effects in rodents.
Tolerance, Dependence, and Withdrawal
Cannabinoid administration may lead to rapid tolerance to antinociceptive effects, anticonvulsant activity, cataleptic effects, suppression of locomotor activity hypothermia, hypotension, release of corticosteroids, and other effects in multiple species . However, tolerance does not develop to all cannabinoid effects, eg, ∆9-THC-elicited adrenocorticotropic hormone (ACTH) secretion remained remarkably stable across 5 daily administrations in the rat . Receptor theory states that tolerance to drug effects produced by treatment with a low efficacy ligand can be at least partially surmounted by administration of a high efficacy agonist, while tolerance to effects induced by repeated treatment with a high efficacy compound will elicit profound cross-tolerance when low efficacy ligands are tested. However, applying this theory of intrinsic efficacy in tolerance and cross-tolerance to cannabinoids is unreliable. In other words, a similar degree of tolerance to the hypothermic effects of ∆9-THC, a low efficacy cannabinoid agonist, CP-55,940 and WIN 55,212, both full efficacy cannabinoid agonists, was reported following chronic ∆9-THC administration, despite the substantial efficacy differences between these drugs . However, a different regimen of ∆9-THC resulted in dramatic tolerance to ∆9-THC effects on locomotor activity, hypothermia, and antinociception in mice. Nevertheless, only moderate cross-tolerance was apparent when high efficacy agonists CP-55,940 or WIN 55,212 were tested, and only to some of these effects . Furthermore, ∆9-THC treatment decreased sensitivity to the discriminative stimulus effects of ∆9-THC in rhesus monkeys, but did not alter the ∆9-THC-like interoceptive effects of high efficacy cannabinoids CP-55,940, JWH-073, or JWH-018 . These apparent differences in tolerance to some behavioral and physiological effects are also reflected at the receptor level, as chronic administration of ∆9-THC or WIN 55212-2 resulted in similar levels of tolerance to drug-elicited hypoactivity, hypothermia, and antinociception, but a greater degree of CB1-receptor desensitization was quantified in the ∆9-THC-treated mice in some brain areas (eg, cerebellum amygdala, nucleus accumbens, and hippocampus) despite the substantially lower CB1-receptor efficacy . A better understanding of the relationship between tolerance to in vivo effects and regulation of expression and function of CB1 receptors after chronic administration of cannabinoid agonists with varying efficacies is needed.
Drug dependence is not directly observable in vivo, rather it is assumed to be present when either the discontinuation of drug administration (spontaneous withdrawal) or the administration of an antagonist (precipitated withdrawal) elicits a withdrawal syndrome. The lack of a reliable and readily observed spontaneous withdrawal syndrome after discontinuation of ∆9-THC has caused much debate on the clinical relevance of cannabis withdrawal in humans. However, the terminology “cannabis withdrawal syndrome” has gained acceptance as described by the Diagnostic and Statistical Manual of Mental Disorders , and clinical reports of sleep disturbances, strange dreaming, decreased appetite, irritability, anxiety, depressed mood, physical discomfort, and drug cravings are said to occur as a result of discontinuation of cannabis use [63, 64]. WIN55,212-2, an aminoaklylindole SCB, which acts as a full agonist at CB1 receptors, elicited measurable signs of spontaneous withdrawal in rats . In addition to the higher intrinsic efficacy, WIN55,212-2 is also eliminated more readily from adipose tissue due to its low lipophilicity, in comparison to ∆9-THC, which is stored and eliminated at a slower rate. One might speculate that the structurally related aminoalkylindole SCBs present in commercial preparations (JWH-018, AM2201, etc) might also elicit a cannabinoid withdrawal syndrome after abrupt discontinuation. To date, there are no preclinical data to directly support this notion, but clinical reports suggest a relatively high incidence of withdrawal in frequent/daily users of K2 and Spice products. The withdrawal syndrome elicited after abrupt discontinuation of SCBs is similar to reports with ∆9-THC, and consists of chills, drug cravings, headaches, insomnia, anorexia, inner unrest, nausea, and nocturnal nightmares, but also includes potentially more severe symptoms such as seizures and hallucinations [6, 15, 16]. Interestingly, Gunderson and colleagues recently reported that the psychoactive effects of commercial SCB preparations substituted for those of cannabis in cannabis-dependent patients, and also blocked or attenuated cannabis withdrawal syndrome in these same patients [14•].
Much of our understanding of cannabinoid tolerance, dependence, and withdrawal has been based on studies involving ∆9-THC, a relatively weak partial agonist at CB1 and CB2 receptors. However, the SCBs commonly found in quasi-legal commercial products, such as K2 and Spice, are typically full cannabinoid receptor agonists. Importantly, a drug’s efficacy determines how “powerful” its maximal effects may be in biological systems. A low efficacy cannabinoid like ∆9-THC will have a less pronounced maximal effect than a higher efficacy cannabinoid, such as the SCBs present in commercial products, and this difference in maximal effects cannot be overcome simply by increasing the dose of ∆9-THC. In other words, no amount of ∆9-THC can stimulate cannabinoid receptors to the same degree as the SCBs currently emerging as drugs of abuse. This has left researchers working with these high efficacy SCBs in the unusual position of having to determine whether their effects are related to the unprecedented degree of cannabinoid receptor stimulation elicited by these compounds, or whether they are produced by interactions with other, noncannabinoid receptor systems.
In light of the growing popularity of commercial SCB products, it has become critically important to re-evaluate our understanding of cannabinoid abuse. Increasing evidence suggests that there is a strong abuse potential for the high efficacy SCBs, at least comparable to that of cannabis itself. Furthermore, these SCB products are readily accessible and can be purchased easily from the comfort of home through the Internet. As long as there is a market for SCBs, competitive pricings and attractive gimmicks will be used to increase sales. Given the prevalence of SCB consumption, there is an urgent need to better understand the pharmacology and toxicology of synthetic cannabinoids. In particular, the role of intrinsic efficacy in abuse-related effects and adverse effects should be targeted in future studies. Finally, commercial SCB products can no longer be viewed as innocuous alternatives to cannabis. Instead, the profound psychoactive effects of these preparations must be recognized and acknowledged to result from the combined actions of a complex mixture of different SCBs present in commercial preparations, almost all of which are more efficacious than ∆9-THC.
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Conflict of Interest
S. Tai and W. E. Fantegrossi declare that they have no conflict of interest.
Human and Animal Rights and Informed Consent
This article does not contain any studies with human or animal subjects performed by any of the authors.
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