Synthetic cannabinoid receptor agonists: classification and nomenclature.

Introduction: The emergence of novel psychoactive substances has changed the epidemiology of drugs used recreationally throughout Europe and have posed significant challenges for clinicians, researchers and regulators. Synthetic cannabinoid receptor agonists have made up a large proportion of these novel psychoactive substances. Developed for legitimate scientific research, synthetic cannabinoid receptor agonists are potent agonists at CB₁ and CB₂ receptors and there have been many case reports of severe or fatal toxicity following their recreational use. At least 180 analytically confirmed compounds belonging to this group of drugs have been reported in Europe as of January 2019. Synthetic cannabinoid receptor agonists have a complex molecular structure, consisting of four pharmacophore components termed the 'core', 'tail', 'linker' and 'linked' groups. This structural complexity offers multiple opportunities for chemical modification to evade drug control legislation based on chemical structure, and this explains the large numbers of individual products that have been detected. Objectives: To discuss the chemical structure of synthetic cannabinoid receptor agonists and to describe the different nomenclature used to identify individual compounds thereby increasing understanding of their chemical heterogenicity and the potential relevance of their molecular structure to the risk of toxicity. Methods: The European Database on New Drugs (EDND) and EMCDDA-Europol annual implementation reports (2010-2017) was searched for compounds with known agonist activity at CB₁ and/or CB₂ receptors. Information on the different names and chemical structures of each compound was extracted and analysed for patterns. PUBMED, Google Scholar and MEDLINE databases were searched, in addition to non-peer reviewed sources, for data on structure, structure-activity relationships and nomenclatures for each compound. Nomenclature of synthetic cannabinoid receptor agonists: The structural complexity of synthetic cannabinoid receptor agonists presents challenges for nomenclature. There are several nomenclature systems in use. Colloquial and clandestine names: Non-scientific names (e.g. AKB-48, 2NEI, XLR-11) have been used to refer to specific compounds and most have probably been invented by vendors, presumably for the purpose of successful marketing of recreational products, however such names do not convey useful information about structure. Systematic chemical names: Each compound has a systematic chemical name that describes its exact structure; however, it is complex, unwieldy, inaccessible to non-chemists and not suitable for routine communication or clinical use. Serial names: Represent iterative designations assigned to compounds produced as a series in a laboratory (e.g. 'WIN-', 'HU-', 'CP-', 'JWH-' and 'AM-'). This nomenclature does not provide structural information or reflect structural similarities between compounds. Systematic abbreviated names: Succinctly describe each compound utilising structural pharmacophores. The chemical motif in each pharmacophore group is assigned a unique code-letter and assembled into a name with the format of 'Linked Group - TailCoreLinker'. Frequently encountered groups include indole and indazole cores, amino-acid-like like groups, most notably methyl-3,3-dimethylbutanoate (MDMB), methanone linker groups and pentyl, 5-fluoropentyl and 4-fluorobenzyl tails. There has been inconsistent usage of this nomenclature, likely due to a lack of consensus and identification of code-letters for several chemical motifs. Emerging compounds and practices: Tricyclic carbazole and γ-carbolinone core analogues have been identified and may represent the next significant structural analogues to emerge onto the recreational market. There is a need to establish basic pharmacological and toxicological data for these analogues. Conclusions: There is a need for international consensus on the nomenclature used to name synthetic cannabinoid receptor agonists to ensure precise and effective communication between professional groups in the clinic and for the purposes of research and regulation, especially with the emergence of analogues of existing compounds and novel structural motifs. A well-defined nomenclature system also supports quick and accurate communication of the structure-activity of these compounds, potentially highlighting compounds that carry a significant risk of toxicity.