Discovery and characterization of GPR6 inverse agonists (1144.11)

GPR6 is a Class A G protein‐coupled receptor and a member of a highly related subset that includes GPR3 and GPR12. Although GPR6 is a constitutively‐active orphan GPCR, it has been reported that some signaling activity may be enhanced by lipids. Following transient transfection of the rat GPR6 ortholog into HEK293s cells, surface expression of GPR6 was confirmed using Fluorescence Activated Cell Sorting. A corresponding increase in intracellular cAMP was also detected, consistent with constitutive activity and G α,s coupling. A high‐throughput, random screen of approximately 130,000 compounds was then performed. Compounds from two chemical series (benzimidamides and 2‐naphthimidamides) were identified as putative inverse agonists of GPR6. Representative compounds demonstrated concentration‐dependent inhibition of cAMP production. The inverse agonism of cellular cAMP production correlated with inhibition of constitutive GTPγS activity in membranes derived from cells expressing GPR6. Anadamide‐stimulation of rGPR6 elevated cellular cAMP levels and was also inhibited by the inverse agonists. Several exemplar compounds were further characterized and found to be selective against other members of the subclass (i.e., GPR3 and GPR12) and other fatty acid GPCRs (i.e., CB1, CB2, and GPR55). Three compounds from the larger chemical class (2‐naphthimidamides) were evaluated for the ability to reverse neurite outgrowth in a native preparation and found to block GPR6‐induced sprouting. The identification and characterization of inverse agonists of GPR6 should enable further investigation into the role of this brain specific GPCR as a potential drug target for psychiatric or neurologic conditions.