The cannabinoid CB₂ receptor positive allosteric modulator EC21a exhibits complicated pharmacology in vitro

Schizophrenia is a complex disease involving the dysregulation of numerous brain circuits and patients exhibit positive symptoms (hallucinations, delusions), negative symptoms (anhedonia), and cognitive impairments. We have shown that the antipsychotic efficacy of positive allosteric modulators (PAMs) of both the M₄ muscarinic receptor and metabotropic glutamate receptor 1 (mGlu₁) involve the retrograde activation of the presynaptic cannabinoid type-2 (CB₂) receptor, indicating that CB₂ activation or potentiation could result in a novel therapeutic strategy for schizophrenia. We used two complementary assays, receptor-mediated phosphoinositide hydrolysis and GIRK channel activation, to characterize a CB₂ PAM scaffold, represented by the compound EC21a, to explore its potential as a starting point to optimize therapeutics for schizophrenia. These studies revealed that EC21a acts as an allosteric inverse agonist at CB₂ in both assays and exhibits a mixed allosteric agonist/negative allosteric modulator profile at CB₁ depending upon the assay used for profiling. A series of compounds related to EC21a also functioned as CB₂ inverse agonists. Overall, these results suggest that EC21a exhibits complicated and potentially assay-dependent pharmacology, which may impact interpretation of in vivo studies.