Source of Albumin binding half-life extension

{{Infobox method | name = Albumin binding half-life extension | subtitle = Peptide engineering strategy | image = peptide-chain.svg | caption = A fatty diacid attached through a spacer to a lysine side chain, distal to the receptor pharmacophore. | Also called = Lipidation, acylation | Carrier = Human serum albumin (≈600 μM in plasma) | Typical extension = 10- to 100-fold }} '''Albumin binding half-life extension''' is a peptide-engineering strategy in which a fatty acid or fatty diacid is covalently attached to a peptide so that the conjugate binds reversibly to circulating serum albumin. The bound fraction is protected from renal filtration and from proteolysis, and acts as a depot from which free peptide is released continuously.{{r|knudsen2019}} The strategy is the basis of the long dosing intervals achieved by [[Liraglutide|liraglutide]] and [[Semaglutide|semaglutide]], and it is used in insulin analogues and in several unrelated peptide drugs. Its principal advantage over covalent fusion to a large carrier is reversibility: the free and bound pools remain in equilibrium, so the peptide retains full receptor activity when released, and the peak-to-trough ratio flattens without the pharmacophore being encumbered.{{r|lau2015}} Design involves three choices — where to attach, what spacer to use, and which fatty acid. All three are constrained by the requirement that the modification lie distal to the receptor-binding region of the peptide, which for the incretin analogues means the C-terminal half.{{r|knudsen2019}} == Physical basis == Human serum albumin circulates at roughly 600 μM and carries several hydrophobic binding sites, principally the Sudlow sites, which in physiology transport long-chain fatty acids, bilirubin and a wide range of drugs. A peptide bearing a fatty-acid tail occupies one of these sites with an affinity that can be tuned across several orders of magnitude by chain length and by the presence of a terminal carboxylate.{{r|lau2015}} Albumin itself has a plasma half-life of about nineteen days, maintained by FcRn-mediated recycling that rescues it from lysosomal degradation. A peptide that spends most of its time bound inherits part of this protection: it is too large as a complex to be filtered at the glomerulus, and its proteolytic exposure is reduced. The equilibrium is what makes the approach work. If binding were irreversible the peptide would never reach its receptor; if it were too weak the depot effect would be negligible. Reported bound fractions for the marketed incretin analogues exceed 99%, meaning free drug is under 1% of total at any moment and the receptor sees a small, steady concentration rather than a peak.{{r|knudsen2019}} == Design variables == | Variable | Liraglutide | Semaglutide | Effect of the change | |---|---|---|---| | Fatty acid | C-16 monoacid | C-18 diacid | Diacid binds more tightly | | Spacer | γ-Glu | γ-Glu plus two OEG units | Distance and flexibility | | Attachment | Lys26 | Lys26, with Arg34 substitution | Prevents acylation at the wrong lysine | | Protease resistance | None added | Aib at position 8 | DPP-4 resistance | | Resulting half-life | ≈13 h | ≈165 h | Daily to weekly dosing | The progression from the first to the second is instructive: three changes acting together produced roughly a twelvefold extension, and no single one of them would have sufficed. The terminal carboxylate of the diacid increases albumin affinity substantially; the OEG spacer holds the peptide away from the albumin surface so that receptor binding is not sterically impeded; the Aib substitution removes the [[Dipeptidyl peptidase-4|DPP-4]] cleavage site that would otherwise have become rate-limiting once renal clearance was slowed.{{r|lau2015}} The lysine substitution is a manufacturing consideration rather than a pharmacological one. A peptide with two available lysines acylates at both, and separating the mono- and di-acylated species is a costly [[Preparative HPLC purification|preparative chromatography]] problem; substituting the unwanted lysine removes the impurity at source.{{r|knudsen2019}} == Analytical consequences == Acylated peptides behave differently from their unmodified parents in almost every assay. They are markedly more hydrophobic and retain much longer on [[Reverse-phase HPLC|reverse-phase]] columns, requiring higher organic content and often an elevated column temperature to elute with acceptable peak shape.{{r|usp1503}} They also self-associate. Acylated incretin analogues form oligomers in concentrated solution, which is part of what stabilises the marketed formulations but which complicates [[Peptide aggregation|aggregation]] assessment: an apparent high-molecular-weight species may be a reversible oligomer rather than an irreversible aggregate, and size-exclusion chromatography under dissociating and non-dissociating conditions will disagree.{{r|ich_q6b}} For anyone comparing a [[Certificate of analysis|certificate]] on an acylated peptide against one on an unmodified peptide, the practical point is that the chromatographic conditions are not transferable and a method that resolves one will not necessarily resolve the other. A certificate that does not name its gradient and column cannot be checked for adequacy.{{r|usp1503}} == References == {{reflist}} <ref name="knudsen2019">Knudsen LB, Lau J. "The discovery and development of liraglutide and semaglutide." ''Frontiers in Endocrinology'' 10:155 (2019). PMID 31031702.</ref> <ref name="lau2015">Lau J, Bloch P, Schäffer L, et al. "Discovery of the once-weekly glucagon-like peptide-1 analog semaglutide." ''Journal of Medicinal Chemistry'' 58(18):7370–7380 (2015). DOI:10.1021/acs.jmedchem.5b00726. PMID 26308095.</ref> <ref name="usp1503">United States Pharmacopeia, General Chapter <1503>, ''Quality Attributes of Synthetic Peptide Drug Substances''.</ref> <ref name="ich_q6b">International Council for Harmonisation, ''Q6B: Specifications — Test Procedures and Acceptance Criteria for Biotechnological/Biological Products'' (1999).</ref> == See also == * [[Semaglutide]] * [[Liraglutide]] * [[GLP-1 receptor agonist]] * [[Dipeptidyl peptidase-4]] * [[Peptide aggregation]] {{DEFAULTSORT:Albumin binding half-life extension}} [[Category:Pharmacokinetics]] [[Category:Peptide synthesis]] [[Category:Compounds and pharmacology]]