Peptide Half-Life

Definition and Importance

Peptide half-life (t½) refers to the time required for the concentration of a peptide to decrease by 50% in a biological system or experimental setting. In pharmacokinetics, this encompasses distribution, metabolism, and elimination. Understanding half-life is critical for experimental design, dosing regimen planning, and interpreting research results. Native peptides often have very short half-lives (minutes to hours), while modified analogs can achieve half-lives of hours to days or even weeks.

Determinants of Biological Half-Life

Peptide half-life in biological systems is determined by multiple factors: (1) Proteolytic degradation—peptidases in plasma, tissues, and cells rapidly cleave unprotected peptides; (2) Renal clearance—small peptides (<5-10 kDa) are filtered by the kidneys and excreted or metabolized; (3) Receptor-mediated uptake—some peptides are internalized via receptor binding; (4) Non-specific binding—plasma protein binding can extend half-life by protecting from degradation; (5) Volume of distribution—affects relationship between dose and plasma concentration.

Proteolytic Degradation

Proteases responsible for peptide degradation include: (1) Aminopeptidases—cleave from N-terminus; (2) Carboxypeptidases—cleave from C-terminus; (3) Endopeptidases—cleave internal peptide bonds (e.g., neprilysin, dipeptidyl peptidase IV/DPP-4, angiotensin-converting enzyme); (4) Intracellular proteases—lysosomal and cytoplasmic enzymes. Specific cleavage sites depend on peptide sequence and enzyme specificity. DPP-4 is particularly important for incretin hormones (GLP-1, GIP), leading to development of DPP-4-resistant analogs.

Half-Life Extension Strategies

Research has developed numerous strategies to extend peptide half-lives: (1) D-amino acid substitution—resistant to most proteases; (2) N-terminal acetylation and C-terminal amidation—block exopeptidases; (3) Cyclization—reduces conformational flexibility and protease access; (4) PEGylation—polyethylene glycol attachment increases size and reduces renal clearance; (5) Lipidation—fatty acid attachment enables albumin binding; (6) Fc fusion—attachment to antibody Fc region extends half-life via FcRn recycling; (7) Non-natural amino acids—modified structures resistant to cleavage.

Examples of Half-Life Modification

Native GLP-1 has a half-life of ~2 minutes due to DPP-4 cleavage. Modifications have extended this dramatically: exenatide (DPP-4 resistant, ~2.4 hours), liraglutide (lipidated, ~13 hours), glp1-s (lipidated + modified backbone, ~1 week), and glp2-t (~5 days). Similarly, native insulin has a short half-life (~5 minutes), while analogs like insulin glargine and insulin degludec achieve 24+ hour duration through various modifications that slow absorption or increase albumin binding.

Research Considerations

When designing experiments with peptides, consider: (1) Native peptides may require frequent dosing or continuous infusion; (2) Modified analogs may have altered receptor pharmacology; (3) In vitro half-life differs from in vivo due to lack of proteases; (4) Species differences exist in protease expression and activity; (5) Degradation products may be biologically active; (6) Half-life measurements depend on assay method—immunoassays may detect inactive fragments. Understanding these factors helps interpret results and design appropriate experimental protocols.

Key Takeaways