Synthetic CB1 Modulators in Clinical Trials: A Medicinal Chemistry Review

The endocannabinoid system, comprising CB1 and CB2 receptors, endogenous ligands, and metabolic enzymes, regulates appetite, energy balance, nausea, immune responses, and neuroprotection. Despite its therapeutic potential, clinical translation has been limited, with only dronabinol and nabilone approved since the 1980s, largely due to central adverse effects associated with CB1 modulation. This review examines synthetic small-molecule CB1 modulators that have advanced to clinical trials from a medicinal chemistry perspective. Nine compounds (five agonists and four antagonists) were identified for indications including autism spectrum disorder, neuropathic pain, osteoarthritis, obesity, acute cannabinoid intoxication, and cognitive impairment in Down syndrome. A comparative structure-activity relationship analysis encompassing 189 molecules revealed recurrent design strategies across different chemical classes, particularly modifications in homologous regions such as lateral chains and tail domains. While agonists generally exhibited limited selectivity between CB1 and CB2 receptors, antagonists more readily achieved CB1 selectivity. Central nervous system adverse effects remain the principal challenge, driving the development of peripherally restricted ligands, biased modulators, and compounds with improved metabolic profiles. Collectively, the insights discussed herein highlight how the integration of structural, pharmacological, and clinical data can guide the rational design of safer and more effective CB1 receptor modulators.