Lipidation and Fatty Acid Acylation

Lipidation is the covalent attachment of a fatty acid to a peptide, in current drug design almost always to the epsilon-amino group of a lysine side chain and usually through a spacer. The point is not lipid solubility as such but reversible binding to serum albumin: the acyl chain docks into albumin's fatty-acid binding sites, so most circulating drug travels bound, shielded from glomerular filtration and from proteases, and only the small free fraction can act or be cleared.

The GLP-1 series shows the effect quantitatively. Native GLP-1 survives roughly two minutes. Liraglutide, bearing a C16 palmitic acid on lysine 26 through a glutamate spacer, lasts about 13 hours and is given daily. Semaglutide replaces that with a C18 diacid on a longer spacer and adds an alpha-aminoisobutyric acid at position 8 to block DPP-4, reaching roughly a week. Insulin degludec uses a C16 diacid and tirzepatide a C20 diacid. The backbone changes little; the duration changes by orders of magnitude.

Against the alternatives, lipidation is small, chemically defined and free of the clearance and immunogenicity burden a PEG chain or an antibody fusion adds. It also slows absorption from the subcutaneous depot, so the flat profile of a weekly agent comes partly from slow input, not only slow elimination. Because so much drug is protein-bound, total plasma concentration overstates the free active fraction.

The abuse of the term is worth watching for. Lipidation is a covalent bond formed during synthesis, so a formulation merely combining a peptide with a lipid or an oil is not a lipidated peptide and inherits none of the albumin-binding kinetics. Listings offering a compound under an acylated drug's name while supplying the unmodified backbone exploit this gap, and the difference is straightforward to see by mass, since the acyl group and spacer add several hundred daltons.