DPP4 and diabetes mellitus: The pathophysiology of DR is driven by prolonged hyperglycemic episodes (elevated blood glucose concentrations) arising from suboptimal glycemic control in patients with either type I or II diabetes mellitus (DM) using dietary modifications, oral drug therapy (metformin, sulphonylurea, dipeptidyl-peptidase-4 inhibitors, sodium-glucose cotransporter-2 inhibitors), or injected insulin formulations.