A novel combined strategy for the physical PEGylation of polypeptides

Abstract

Poly(ethylene glycol) (PEG) may be covalently conjugated to peptide drugs to overcome their rapid clearance but in doing so their potency can be lost. Here, a non-covalent approach was used to conjugate PEG bearing a terminal cholanic moiety (mPEG5kDa-cholane) to a 28 amino acid peptide, vasoactive intestinal peptide (VIP). Palmitoylation of the peptide was essential to facilitate physical interaction via a single binding site involving two mPEG5kDa-cholane molecules with an affinity constant of ~3·10(4)M(-1); these calorimetry data corroborating Scatchard analysis of dissolution data. The peptide/polymer complex (below 10-12nm diameter) provided for up to 5000-fold greater solubility of the peptide at pH7.4 (4μg/mL) and markedly increased peptide solution stability at 25°C over 30days. Mannitol enabled the complex to be lyophilized to yield a freeze-dried formulation which was efficiently reconstituted albeit with an ~10% decrease in solubility. The predominantly α-helical conformation of the peptide alone at pH5-6.5 was lost at pH7.4 but fully recovered with 2 molar equivalents of mPEG5kDa-cholane. After lyophilization and reconstitution an ~10% loss of α-helical conformation was observed, which may reflect the equivalent decrease in solubility. Pharmacokinetic studies following subcutaneous administration of the peptide (0.1mg/Kg) alone and with 2 molar equivalents of polymer showed that mPEG5kDa-cholane dramatically increased peptide concentration in the systemic circulation. This is the first demonstration of non-covalent PEGylation of acylated peptides, an important biologic class, which improves in vitro and in vivo properties, and thereby may prove an alternative to covalent PEGylation strategies.

Keywords: Acylation; Bioavailability; Formulation; Self-assembly; Stability.