Chemotype-dependent dissociation of binding affinity and functional potency in synthetic cannabinoids.

Synthetic cannabinoids are a structurally diverse class of cannabinoid receptor 1 agonists that frequently exhibit ultrapotency and are associated with severe toxicity and mortality. To define the structural determinants underlying this ultrapotency, we performed in vitro pharmacological characterization of 74 synthetic cannabinoid receptor agonists spanning multiple chemotypes, with wide variation in headgroup, core, and N-alkyl tail chemistry, including PINACAs, BUTINACAs, PICAs, PRINACAs, and related scaffolds. Using radioligand binding and functional assays, we quantified equilibrium affinity (Ki), functional potency (EC50), and efficacy (Emax) across compounds. We observed marked dissociation between affinity and potency for many compounds, with many displaying subnanomolar EC50 values despite moderate nanomolar binding affinity. Comparative analysis revealed that indazole-based cores and optimal N-pentyl or 5-fluoropentyl tails primarily enhance binding stability, whereas headgroup chemistry, while also contributing to affinity, emerges as a key determinant of coupling efficiency and ultrapotent functional responses. Notably, matched headgroups across PICA and PINACA scaffolds showed similar affinity-potency dissociation but uniformly lower absolute values in PICAs, supporting a division of labor in which the core governs binding whereas the headgroup governs activation efficiency. Together, these findings provide a unifying framework for understanding the extraordinary potency of contemporary synthetic cannabinoid receptor agonists and underscore why equilibrium affinity alone is insufficient to predict their biological and toxicological impact. SIGNIFICANCE STATEMENT: Synthetic cannabinoids frequently exhibit extreme cannabinoid receptor 1 potency and are responsible for severe toxicity and fatalities. By systematically comparing 74 compounds across multiple chemotypes, we show that ultrapotency arises from dissociation between binding affinity and functional activation, with the core and tail primarily determining affinity while headgroup chemistry governs binding and activation efficiency. These findings provide a mechanistic framework for predicting the potency and toxicological risk of emerging synthetic cannabinoids.