How peptides are engineered to last longer in the body

Native GLP-1, for example, is degraded by dipeptidyl peptidase 4 (DPP-4) and neutral endopeptidases and has a half-life of 1.5 to 2 minutes according to the liraglutide label ¹. DPP-4 removes a dipeptide from the N-terminus of peptides with a proline or alanine in the second position, a pattern found in GLP-1, GIP and growth-hormone-releasing factor (1-29) ². Peptide drugs also face rapid kidney clearance, proteolysis and, in some cases, immune recognition ³.

Blocking the cleavage site

One approach is substitution at the vulnerable position: semaglutide is modified at position 8 to stabilise it against DPP-4, with aminoisobutyric acid (Aib) used in that position ^4,5. Tirzepatide likewise contains Aib at positions 2 and 13 ⁶. Changes can carry a cost: in the 2000 GLP-1 derivative series, simultaneous N-terminal modification for metabolic stability interfered with fatty-acid derivatisation and led to loss of potency ⁷.

Fatty-acid acylation and albumin binding

Serum albumin is a multifunctional transport protein that binds a wide variety of endogenous substances and drugs ⁸. In 1995, researchers acylated insulin with fatty acids of 10 to 16 carbons, showed binding to the long-chain fatty-acid sites on human albumin, and proposed that albumin-binding derivatisation could be applied generally to prolong peptide drugs ⁸. In 2000, a series of GLP-1 derivatives followed: those with fatty acids of 12 carbons or more were markedly more protracted than GLP-1, although longer fatty acids lost more potency and a second fatty acid caused considerable potency loss ⁷.

The semaglutide label identifies albumin binding as the main protraction mechanism, via a hydrophilic spacer and a C18 fatty di-acid on lysine 26, reports more than 99% albumin binding, and gives an elimination half-life of approximately one week, with the drug present in circulation for about 5 to 7 weeks after the last injectable dose ⁴. The design paper reports that the fatty-acid moiety and linker chemistry were the key features securing albumin affinity and receptor potency, and that semaglutide bound the GLP-1 receptor with roughly three-fold lower affinity than liraglutide while its albumin affinity was higher ⁵. Tirzepatide uses a C20 fatty diacid for the same purpose and has a half-life of approximately five days ⁶. For comparison, liraglutide has a plasma half-life of 13 hours ¹.

Albumin conjugation and Fc fusion

CJC-1295, a long-acting analogue of growth-hormone-releasing factor, was built for covalent bonding to albumin by adding a maleimide-containing lysine derivative that reacts with the free thiol on Cys34 of albumin; in rats it remained in plasma beyond 72 hours, and albumin conjugates of the parent peptide showed enhanced stability against DPP-IV in vitro ⁹. In a human study in healthy adults, the estimated half-life of CJC-1295 was 5.8 to 8.1 days ¹⁰. The reactive group is referred to as a drug affinity complex (DAC), and analytical papers distinguish CJC-1295 from CJC-1295 with DAC ^11,12. A longer exposure also changes physiology: after a single CJC-1295 injection, pulsatile growth-hormone secretion was preserved, but trough growth-hormone levels rose markedly ¹³.

Dulaglutide, a protein-engineering alternative, links a GLP-1 analogue to a modified IgG4 Fc fragment and has an elimination half-life of approximately five days ¹⁴.

PEGylation

Pegylation is the attachment of polyethylene glycol chains to protein and peptide drugs ³. A widely cited review explained that it increases the molecular mass of proteins and peptides and shields them from proteolytic enzymes, thereby improving pharmacokinetics ³. A 2013 study of GLP-1/GIP co-agonists reported that duration of action was refined through site-specific lipidation or PEGylation to support less frequent administration ¹⁵.

D-amino acids and other substitutions

Replacing selected residues with their D-forms is another way to improve enzymatic stability ¹⁶. In a model epitope peptide, partial D-amino acid substitution in the flanking regions retained antibody recognition and conferred high resistance to proteolysis in diluted human serum and lysosomal preparations ¹⁶. That study used a vaccine-epitope model rather than a hormone, so it illustrates the principle rather than predicting behaviour for any given peptide ¹⁶.

What longer half-life does and does not mean

The semaglutide label notes that treatment for overdose symptoms may need to take the long half-life into account ⁴. Half-life values also depend on species and assay: semaglutide had a plasma half-life of 46.1 hours in mini-pigs ⁵, versus about one week in people ⁴. For research peptides without regulatory labels, half-life figures may come from animal or small early studies, as with the rat pharmacokinetics first reported for CJC-1295, so the study model matters ⁹.

Key points

• Native GLP-1 has a half-life of 1.5 to 2 minutes, largely because of DPP-4 ^1,2.
• Fatty-acid acylation promotes albumin binding, and semaglutide's label reports an elimination half-life of about one week ^4,8.
• CJC-1295 achieves a multi-day half-life through covalent albumin binding, with an estimated 5.8 to 8.1 days in healthy adults ^9,10.
• PEGylation and D-amino acid substitution shield peptides from clearance and proteolysis, but each change must be balanced against potency ^3,7,16.