Encapsulation into PEG-Liposomes Does Not Improve the Bioavailability of Pulmonary Delivered Salmon Calcitonin

BACKGROUND: Salmon calcitonin (sCT) is approved for the short-term treatment of Paget's disease and hypercalcemia. As pulmonary delivery might improve the drug's efficacy, a variety of liposomal sCT formulations for inhalation were prepared and characterized with the intention of developing a controlled release formulation. METHODS: The influence of pH of the loading buffer, charge of the vesicular surface, and membrane rigidity on particle size, ζ-potential, and sCT encapsulation efficiency of formulations was studied. The most promising systems were investigated for their ability to withstand nebulization stresses using an Aeroneb(®) vibrating mesh device. In vitro studies were carried out to determine sCT release from the vesicles and the bioactivity of the peptide post nebulization. Lastly, pharmacokinetics of sCT liposomes following intratracheal aerosolization in an experimental rat model were investigated and compared with intravenous injection. RESULTS: Liposomes prepared with acidic loading buffer and comprising rigid lipid membranes showed an optimal compromise between small particle size, high encapsulation efficiency, and sCT stability. Polyethylene glycol (PEG) liposomes showed the highest encapsulation efficiency overall, regardless of the ζ-potential of the vesicles. Positive surface charge, however, yielded higher entrapment in non-PEGylated liposomes. All liposomes tested were stable during nebulization. The bioactivity of sCT after formulation into liposomes was 52-55%. Intratracheal nebulization in rats revealed that the bioavailability and other pharmacokinetic parameters were not enhanced by liposomes, when compared with sCT solution. Following intravenous administration, however, liposomes showed significantly higher bioavailability and AUCinf (area under the curve to the infinity time point) than controls. CONCLUSIONS: The developed liposomal formulations were not optimal carriers for pulmonary delivery of sCT. Due to the low amounts of peptide released from the vesicles, enzymatic digestion by peptidases in the airspace reduced the bioavailability significantly. Liposomal encapsulation of sCT, nevertheless, resulted in improved pharmacokinetics following injection.