DPP4 and chronic obstructive pulmonary disease: ACE: angiotensin-converting enzyme; ARBs: angiotensin II receptor blockers; CCB: calcium channel blockers; CKD: chronic kidney disease; COPD: chronic obstructive pulmonary disease; COVID-19: coronavirus disease 2019; DPP-4: dipeptidyl peptidase 4; NA: not available; IQR: interquartile range; SD: standard deviation; SGLT2: sodium-glucose cotransporter-2; T2D: type 2 diabetes