Brian F. Thomas, Ph.D., Principal Scientist, RTI International

Jenny L. Wiley, Ph.D., Senior Fellow, RTI International

Gregory W. Endres, Ph.D., VP of Chemistry, Cayman Chemical

Synthetic cannabinoids were originally designed and synthesized as molecular probes to characterize structure-activity relationships and to elucidate the biological basis and pharmacological properties of the chemical constituents in Cannabis.¹ These efforts culminated in identification of the chemical structure of the plant’s primary psychoactive compound, Δ⁹-THC, and its later synthesis.^2,3 Marinol^® and Cesamet^®, formulations of synthetic cannabinoids based upon the THC structural scaffold, eventually became licensed and marketed for their therapeutic utility, but they were limited in their market acceptability and use due to their central nervous system (CNS) effects, abuse liability, and rigorous scheduling as controlled substances. Hence, considerable interest remained in deriving compounds with increased therapeutic potential and decreased adverse side effects in the CNS.

In the 1980s, Central Pfizer Pharmaceuticals conducted a medicinal chemistry campaign and synthesized an extensive series of potent synthetic cannabinoids, including CP 55,940, a compound that was more than 100-fold more potent than Δ⁹-THC in producing cannabinoid (CB) receptor effects in laboratory animals.^4-7 Radiolabeling of this synthetic compound facilitated the demonstration of saturable, high-affinity binding sites (CB₁ and CB₂ receptors) whose localization, activation, and signal transduction could be correlated with the production of cannabimimetic effects.^8-11 However, the clinical development of compounds in this series was terminated due to the continuing presence of cannabimimetic CNS effects. Nevertheless, the series opened a new era in cannabinoid research that served to dramatically increase the understanding of the pharmacological effects and structure-activity relationships of synthetic cannabinoids. Pfizer’s early efforts in cannabinoid research were followed by others in which the resulting molecular structures of the compounds deviated further from the template of Δ⁹-THC. As a result, novel chemical classes of CB receptor agonists with high affinity and efficacy for binding and activation of CB receptors were discovered and were shown to produce pharmacological effects of considerable therapeutic interest.^12-16 Unfortunately, the new agonists also typically possessed cannabimimetic activity that could not be dissociated from the more clinically desirable effects.

While the intoxicating effects and abuse liability of CB receptor agonists were considered to be untoward effects for therapeutic agents, individuals with less noble goals recognized that synthetic cannabinoids based upon non-THC structural scaffolds could be distributed and used recreationally without criminal prosecution and penalty under international scheduling and control laws. Thus, in the early 21^st century, compounds originally derived from both Central Pfizer (CP 47,497) and Sterling-Winthrop (WIN 48,098), as well as independent researchers (JWH 018 and AM2201), were detected in herbal designer drug formulations (Figure 1).^17-20 These formulations were often labeled as “incense” and “not for human use,” and in many instances, were found to contain more than one synthetic cannabinoid.²¹ In response to the rapidly increasing prevalence and use of synthetic cannabinoids, drug and law enforcement agencies across the globe began to control the most commonly occurring versions and to prosecute their manufacturers, distributors, and users. As is often the case with designer drugs, however, new replacement chemical entities would be synthesized as quickly as the older compounds were detected, identified, and banned.

Figure 1. Compounds detected in herbal designer drugs labeled as “incense.”

Because of their association with the effects produced by phytocannabinoids in Cannabis, synthetic cannabinoids are often mistakenly assumed to be “safer” than other designer drugs. However, the incidence of individuals who have used synthetic cannabinoids and suffered significant adverse effects (including death), called poison control centers, or were admitted into emergency medical care has seen a dramatic increase over time.^22-32 In addition to the adverse effects of the primary chemical contained in these products, the uncontrolled nature of the manufacture, formulation, and use of synthetic cannabinoids may affect identity, purity, strength, or chemical stability of the compounds, resulting in creation of and exposure to other chemical entities. Hence, individuals are often exposed to a variety of chemicals, including chemical degradants and thermolysis products. For example, after JWH 018 and AM2201 were banned by the DEA, similar compounds containing their pentylindole core, but linked via a ketone to a tetramethylcyclopropyl ring substituent instead of the prototypical naphthalene ring system, were increasingly encountered in new designer drug formulations. When stored at room temperature for long periods of time, the tetramethylcyclopropyl ring system in UR-144, XLR11, and other tetramethylcyclopropyl ring-containing analogs is prone to ring-opening (i.e., degradation).³³ Studies conducted in RTI International laboratories, performed in collaboration with Cayman Chemical and supported in part by the National Institute of Justice, demonstrated that heating or combusting these compounds speeds up this process and leads to rapid and complete conversion of XLR11 and UR-144 to their ring-open forms (Figure 2). Furthermore, in vitro studies revealed that the ring-open degradants possess high affinity and increased efficacy at the CB₁ receptor as well as greater potency over their non-degraded forms in laboratory animal assays selective for cannabimimetics (manuscript in preparation). Exposure to extremely potent synthetic cannabinoid compounds such as these may produce more pronounced dependence and withdrawal, or may be responsible for the continuing incidence of panic attacks, adverse effects, and fatalities that have been reported recently.

Figure 2. Aerobic thermolysis studies of JWH 018, AM2201, XLR11, and NNEI at 800°C.

Note the differing degrees of degradation that occur at 800°C due to differences in chemical structure.

“Between January and May 2015, U.S. poison centers in 48 states reported receiving 3,572 calls related to synthetic cannabinoid use, a 229 percent increase from the 1,085 calls received during the same January through May period in 2014. The 2015 figures included a spike of 1,501 calls in April, and 15 reported deaths, a three-fold increase over the five deaths that were reported in 2014.”³⁴

Despite increasing recognition of the harm associated with the use of synthetic cannabinoid formulations, their abuse continues to occur, particularly in athletes, military personnel, employees who undergo frequent drug testing, and other individuals seeking intoxication while hoping to evade detection ^.30,35-38 Yet, while the acute health risks associated with synthetic cannabinoids are scientifically documented and widely disseminated by the press and media, the chronic and long-term toxic effects of these compounds remain relatively unknown and may not become apparent for years or even generations (as occurred with diethylstilbestrol and thalidomide).^35-37 Further collaborative research efforts have demonstrated that some of the newer synthetic cannabinoids found in seized materials, such as NNEI, MN-18, -38, and -39, contain amide-linked naphthylamine substituents that are suspected or known bladder carcinogens in humans and are liberated as thermolysis products during heating or combustion of herbal products for inhalation. These studies suggest that degradation or heating and thermolysis of the bulk chemicals or herbal products may result in inhalation of a completely different chemical or a mixture of chemicals. Furthermore, the data suggest that the volatility and thermolytic stability of the synthetic cannabinoids identified to date can vary dramatically, even with relatively modest changes in chemical structure, such that they are often difficult to predict with certainty. However, under typical conditions of use involving elevated temperatures, anticipated chemical exposures include lung irritants and known or suspected mutagens and carcinogens (e.g., naphthalene, isocyanatonaphthalene, 8-OH-quinoline, 1-naphthylamine, and 2-naphthylamine) as well as chemicals of unknown health impact.

In summary, identification and detection of the chemicals present in a bulk drug substance or formulation is clearly important for law enforcement efforts. However, as described above, the chemicals present in the substance or formulation may not be the same as those that are actually being inhaled and absorbed into the bloodstream. Understanding the chemical fate of synthetic cannabinoids during use, with emphasis on the actual chemicals of exposure and their metabolic products formed after absorption, is crucial in determining the health effects. Exposure to even minor chemical constituents may have profound implications, as demonstrated by the “frozen addicts” resulting from inadvertent exposure to MPPP, a trace impurity formed from overheating during the synthesis of the designer opiate meperidine.⁴¹

These drug users suffered extensive destruction of dopaminergic neurons in the substantia nigra, producing immediate and irreversible Parkinson’s disease symptoms. Determination of the chemical responsible for the Parkinson-like neuronal degradation was understood fully only after careful analytical and pharmacological assessment in multiple laboratory animal species.

In order to avoid similar painful lessons, research scientists need to continue careful assessment of abused substances, including their chemical exposure profiles and associated pharmacological and toxicological endpoints. Given the increasing number of synthetic cannabinoid variants available and their varying potency and toxicity, this evaluative process is critically necessary for effective prevention and treatment.^41-45

Acknowledgements

Research reported in this article was supported in part by Award No. 2012-R2-CX-K001, awarded by the National Institute of Justice, Office of Justice Programs, U.S. Department of Justice. The opinions, findings, and conclusions expressed in this publication are those of the authors and do not necessarily reflect those of the Department of Justice.

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