Ghrelin-liposome interactions: characterization of liposomal formulations of an acylated 28-amino acid peptide using CE.

Ghrelin is a pharmacologically interesting peptide hormone due to its effects on appetite and metabolism. The cationic, octanoylated 28 amino acid peptide has a short biological half-life; thus, prolonged release formulations are of interest. Acylated peptides have been suggested to bind to or be incorporated into liposomes. Formulations based on neutral dipalmitoylphosphatidylcholine (DPPC) liposomes and phosphatidylcholine:cholesterol (70:30 mol%) liposomes, and negatively charged dipalmitoylphosphatidylcholine:dipalmitoylphosphatidylserine (DPPC:DPPS) (70:30 mol%) liposomes (2 mM total lipid concentration) were characterized using ACE. Pre-equilibrium CZE and frontal analysis CE methods circumventing capillary wall adsorption of the peptide and the liposomes and suitable for characterizing ghrelin-liposome interactions were developed. The cationic peptide exhibited low affinity (<10% bound) for DPPC and phosphatidylcholine:cholesterol (70:30 mol%) liposomes whereas electrostatic interactions caused a higher affinity for DPPC:DPPS (70:30 mol%) liposomes. Studies on desacyl ghrelin instead of ghrelin demonstrated the significance of the n-octanoyl side chain as an affinity providing moiety towards DPPC:DPPS liposomes (48 and 73% bound peptide, respectively). CE experiments showed that the binding was characterized by rapid dissociation kinetics.