DPP4 and diabetes mellitus: Common strategies to improve peptide stabilityinclude sequence modifications such as N-methylation,d-amino acid substitution,−, , , ,  cyclization,−,  and conjugation with polyethylene glycol (PEG) or lipids.−,  A well-known recentexample is glucagon-like peptide-1 (GLP-1), a peptide hormone used for diabetes treatment, which has been engineeredwith fatty acid conjugation (e.g., liraglutide) or backbone modifications (e.g., semaglutide) to resist degradation by dipeptidyl peptidase-4(DPP-4).