John R. Hadcock

Orphan G protein-coupled receptors (oGPCRs) are a class of integral membrane proteins for which endogenous ligands or transmitters have not yet been discovered. Transgenic animal technologies have uncovered potential roles for many of these oGPCRs, providing new targets for the treatment of various diseases. Understanding signaling pathways of oGPCRs and validating these receptors as potential drug targets requires the identification of chemical probe compounds to be used in place of endogenous ligands to interrogate these receptors. A novel chemical probe identification platform was created in which GPCR-focused libraries were screened against sets of oGPCR targets, with a goal of discovering fit-for-purpose chemical probes for the more druggable members of the set. Application of the platform to a set of oGPCRs resulted in the discovery of the first reported small molecule agonists for GPR39, a receptor implicated in the regulation of insulin secretion and preservation of beta cells in the pancreas. Compound 1 stimulated intracellular calcium mobilization in recombinant and native cells in a GPR39-specific manner but did not potentiate glucose-stimulated insulin secretion in human islet preparations.

Takeda G-protein-coupled receptor 5 (TGR5) represents an exciting biological target for the potential treatment of diabetes and metabolic syndrome. A new class of high-throughput screening (HTS)-derived tetrahydropyrido[4,3-d]pyrimidine amide TGR5 agonists is disclosed. We describe our effort to identify an orally available agonist suitable for assessment of systemic TGR5 agonism. This effort resulted in identification of 16, which had acceptable potency and pharmacokinetic properties to allow for in vivo assessment in dog. A key aspect of this work was the calibration of human and dog in vitro assay systems that could be linked with data from a human ex vivo peripheral blood monocyte assay that expresses receptor at endogenous levels. Potency from the human in vitro assay was also found to correlate with data from an ex vivo human whole blood assay. This calibration exercise provided confidence that 16 could be used to drive plasma exposures sufficient to test the effects of systemic activation of TGR5.

Reported herein is a medicinal chemistry effort towards the identification of orally available TGR5 agonist 12, which served as a dog tool compound for studies to increase confidence in this mechanism. With the challenge of striking the balance of TGR5 potency and desired clearance profile, the screening strategy as well as medicinal chemistry strategy are discussed in this article.

Obesity-induced inflammation is a key component of systemic insulin resistance, which is a hallmark of type 2 diabetes. A major driver of this inflammation/insulin resistance syndrome is the accumulation of proinflammatory macrophages in adipose tissue and liver. We found that the orphan GPCR Gpr21 was highly expressed in the hypothalamus and macrophages of mice and that whole-body KO of this receptor led to a robust improvement in glucose tolerance and systemic insulin sensitivity and a modest lean phenotype. The improvement in insulin sensitivity in the high-fat diet–fed (HFD-fed) Gpr21 KO mouse was traced to a marked reduction in tissue inflammation caused by decreased chemotaxis of Gpr21 KO macrophages into adipose tissue and liver. Furthermore, mice lacking macrophage expression of Gpr21 were protected from HFD-induced inflammation and displayed improved insulin sensitivity. Results of in vitro chemotaxis studies in human monocytes suggested that the defect in chemotaxis observed ex vivo and in vivo in mice is also translatable to humans. Cumulatively, our data indicate that GPR21 has a critical function in coordinating macrophage proinflammatory activity in the context of obesity-induced insulin resistance.

Antagonism of cannabinoid-1 (CB₁) receptor signaling has been demonstrated to inhibit feeding behaviors in humans, but CB₁-mediated central nervous system (CNS) side effects have halted the marketing and further development of the lead drugs against this target. However, peripherally restricted CB₁ receptor antagonists may hold potential for providing the desired efficacy with reduced CNS side effect profiles. In this report we detail the discovery and structure–activity-relationship analysis of a novel bicyclic scaffold (3) that exhibits potent CB₁ receptor antagonism and oral activity in preclinical feeding models. Optimization of physical properties has led to the identification of analogues which are predicted to have reduced CNS exposure and could serve as a starting point for the design of peripherally targeted CB₁ receptor antagonists.