pH dependence of a GPR4 selective antagonist hampers its therapeutic potential
Inflammatory bowel disease (IBD) is characterized by chronic mucosal inflammation of the gastrointestinal tract and is associated with extracellular acidification of mucosal tissue. Several extracellular pH-sensing receptors, including G protein-coupled receptor 4 (GPR4), play an important role in the regulation of inflammatory and immune responses and GPR4 deficiency has been shown to be protective in IBD animal models. To confirm the therapeutic potential of GPR4 antagonism in IBD, we tested Compound 13, a selective GPR4 antagonist, in the IL10-/- mouse model of colitis. Despite reasonable bioavailability, Compound 13 treatment did not improve colitis in this model and there were no signs of target engagement. Interestingly, Compound 13 behaved as an “orthosteric” antagonist, i.e., its potency was pH-dependent and mostly inactive at pH levels lower than 6.8 with preferential binding to the inactive conformation of GPR4. Mutagenesis studies confirmed Compound 13 likely binds to the conserved orthosteric binding site in G protein-coupled receptors, where a histidine sits in GPR4 likely preventing Compound 13 binding when protonated in acidic conditions. While the exact mucosal pH in the human disease and relevant IBD mice models is unknown, it is well established that the degree of acidosis is positively correlated with the degree of inflammation, suggesting Compound 13 is not an ideal tool to study the role of GPR4 in moderate-to-severe inflammatory conditions. Overall design: Gene expression profiling analysis of RNAseq from distal colons of wildtype and IL10 knockout mice treated with Compound 13 and anti-p40