GLP1R and diabetes mellitus: Dipeptidyl peptidase-4 (DPP-4) inhibitors are oral diabetes medications that inhibit the enzyme DPP-4, a ubiquitous enzyme that is expressed on the surface of most cell types and deactivates a variety of other bioactive peptides, including glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1).