DPP4 and type 2 diabetes mellitus: The current SARS-CoV-2 crisis has revealed that patients with pre-existing conditions such as type-2 diabetes have a greater risk of mortality due to SARS-CoV-2 infection and that the known DPP4 inhibitor sitagliptin was shown to be effective in treating COVID-19 infections in diabetic patients, although its exact mechanisms are not clearly understood [18,20,22,36].