DPP4 and chronic obstructive pulmonary disease: DM: diabetes mellitus; COPD: chronic obstructive pulmonary disorder; ACEi: angiotensin-converting enzyme inhibitors; ARB: angiotensin-2 receptor blockers; CCB: calcium channel blocker; GLP1R: glucagon-like peptide-1 receptor; SGLT2: sodium-glucose cotransporter-2; DPP4: dipeptidyl-peptidase 4; SSRI: selective serotonin reuptake inhibitors