Incretins are hormones that are secreted from the gastrointestinal tract into the circulatory system in response to nutrient ingestion, enhancing glucose-stimulated insulin secretion. Incretins are estimated to account for approximately 50–70% of the total insulin secretion after oral glucose administration, and this has been dubbed the “incretin effect”. To date, two incretins, glucagon like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), have been identified.
GLP-1 receptor agonists are broadly applied in type 2 diabetes mellitus (T2DM) therapy. Exenatide is a non-human peptide analogue originally isolated from the saliva of the Gila monster (Heloderma suspectum). Compared to GLP-1, the second residue of exenatide is Gly rather than Ala; thus, it can circumvent the degradation of dipeptidyl peptidase-4 (DPP-4) and has a prolonged intravenous half-life of 30 min and a 2–3 h half-life after subcutaneous administration.
Liraglutide is designed to bind to human albumin via a C16 fatty acid and a spacer covalently attached to Lys26. A residue substitution of Lys34 to Arg34 occurs to avoid the fatty acid being installed in a wrong place. Liraglutide has an intravenous half-life of 8–10 h and 13–15 h half-life after subcutaneous administration.
Semaglutide converges these principles: the second residue is replaced by α-aminoisobutyric acid (Aib) to avoid DPP-4 degradation, Lys34 is replaced by Arg34, and a C18 fatty acid is linked to Lys26 via a γGlu-2xOEG spacer providing higher affinity to albumin. The half-life of semaglutide after subcutaneous administration is up to 183 h.
Tirzepatide is a unimolecular, bifunctional peptide invented by Eli Lilly and Company (Indianapolis, IN, USA), which simultaneously activates GLP-1R and GIPR. Tirzepatide is composed of 39 amino acids, is amidated at the C-terminal, conjugates a C20 fatty diacid moiety via a spacer connected to Lys20, and has a half-life of 116.7 h. Tirzepatide has a higher affinity with GLP-1R than GLP-1 does (pIC50: 8.90 vs 8.62), and its cAMP activation activity is significantly lower (1–2 decrease in pEC50). Tirzepatide has an obviously lower affinity with GIPR than GIP does (pIC50: 6.70 vs 7.87), and its cAMP activity is approximately equal to that of GIP.
Human serum albumin (HSA) and IgG have plasma half-lives of around 21 days, owing to HSA and the Fc domain of IgG interacting with the neonatal Fc receptor expressed in blood vessel endothelial cells, which rescues the proteins in the endosomes after pinocytosis, preventing them from entering the lysosomal pathway for destruction. By binding with HSA or IgG-Fc, the half-life of protein drugs could be extended. HSA and IgG-Fc can bind proteins not only through covalent attachments, but also through non-covalent labyrinthine interactions between endogenous albumin and long fatty acid chains, which have been added to GLP-1/GIP in some previous studies. The crucial elements of this strategy are the length of the fatty acid chain and the position of lipidation. Generally, the longer the fatty acid chain, the more readily it binds to HSA. However, excessive length may impede ligand to bind and activate receptor.