Table 2.

Examples of half-life-extension strategies employing polypeptide fusions to small proteins and peptides to generate biobetters

Strategy	Specific approach	Construct	Mechanism for half-life extension
Fusion to human protein with inherently long serum half-life	Fusion to human IgG Fc domain	Genetic fusion to C-terminus or N terminus of human IgG Fc, which has a half-life of about 14 days in human serum	Recycling via FcRn [43–46]
	Fusion to HSA	Genetic fusion to C-terminus or N terminus of HSA, which has ~19-day half-life in human serum	Recycling via FcRn [45, 47, 48]
	Fusion to human transferrin	Genetic fusion to C terminus or N terminus of human transferrin, which has a ~12-day half-life in human serum	Recycling via transferrin receptor [49]
Fusion to non-structured polypeptide to increase overall size and hydrodynamic radius	XTENylation (also known as rPEG)	Genetic fusion of non-exact repeat peptide sequence (Amunix, Versartis) to therapeutic peptide	Increase in size and hydrodynamic radius [17]
	PASylation	Genetic fusion of polypeptide sequences composed of PAS (XL-Protein GmbH) forms uncharged random coil structures with large hydrodynamic volume	Increase in size and hydrodynamic radius [28]
	ELPylation	Genetic fusion to ELP repeat sequence (PhaseBio) can extend half-life	Increase in size and hydrodynamic radius [29, 30, 50]
	HAPylation	HAP (e.g., homopolymer of glycine residues)	Increase in size and hydrodynamic radius [27]
	GLK fusion	Fusion with artificial GLK	Increase in size and hydrodynamic radius [51]
Fusion to highly anionic polypeptide to increase negative charge	CTP fusion	Genetic fusion of CTP peptide from human CG β-subunit to antibody fragment (Prolor Biotech)	Increase in negative charge via sialylation of CTP [34, 35]

CG chorionic gonadotropin, CTP carboxy-terminal peptide, ELP elastin-like peptide, Fc constant fragment, FcRn neonatal Fc receptor, GLK gelatin-like protein, HAP homo-amino acid polymer, HSA human serum albumin, Ig immunoglobulin, PAS proline-alanine-serine polymer, rPEG recombinant poly-ethylene glycol, XTEN genetic fusion of non-exact repeat peptide sequence