GLP1R and type 2 diabetes mellitus: However, when it was reported that GLP-1 was a substrate for DPP-4 in pharmacological in vitro kinetic studies (18), this was quickly followed by the demonstration that the metabolite generated by DPP-4 cleavage was the major circulating component of GLP-1-like immunoreactivity in healthy individuals (19) and that the same metabolite formed rapidly following exogenous administration of GLP-1 in both healthy subjects and those with T2DM (20).