INS and diabetes mellitus: In addition, another study related to DPP4 inhibitors found that diabetes patients are more likely to be infected with SARS-CoV-2 and develop severe cases of COVID-19; moreover, SARS-CoV-2 can change the expression of DPP4 in diabetes patients through interaction with insulin, leptin and interleukin-6 (IL-6) and trigger uncontrolled glucose metabolism disorder and inflammation, thereby increasing the mortality of COVID-19 (Rakhmat et al., 2021).