DPP4 and type 2 diabetes mellitus: Although three new classes of T2D agents were introduced in the 1990s (α-glucosidase inhibitors, meglitinides and thiazolidinediones), it was not until the turn of the Twenty-First Century that the so-called “newer T2D agents” were introduced: dipeptidyl peptidase-4 (DPP-4) inhibitors, glucagon-like peptide-1 (GLP-1) receptor agonists and sodium–glucose transporter 2 (SGLT2) inhibitors [13].