Calcitonin-loaded liposomes: stability under acidic conditions and bile salts-induced disruption resulting in calcitonin-phospholipid complex formation.

Calcitonin-loading in liposomes composed of phosphatidylcholine, cholesterol and stearylamine or dipalmitoyl phosphatidylglycerol was studied at low pH values and in the presence of bile salts to check whether liposomal entrapment could be a possible means of protecting the peptide against the aggressive conditions present in the gastrointestinal tract. The association of calcitonin with the lipidic vesicles was monitored using radioactive labelling of the peptide and gel-filtration separation of the free and liposome-associated fractions. The results show that for all phospholipid compositions tested, loading was preserved in light acidic or basic buffers, and that only a slight disruption was observed at pH 2.5. Cholate caused a significant but only partial release of calcitonin even when the cholate-to-phospholipid ratio was increased. To understand the mode of calcitonin entrapment in the vesicles, the release of liposome-entrapped calcein was monitored concomitantly and taken as a stability criterion. Liposome integrity appears to be resistant at low pHs but to be totally destroyed by 4 mM cholate in a manner quasi-independent of the phospholipid concentration. These results strongly suggest that bile salts induce a disruption of the liposomes which results in the formation of new lipidic structures involving calcitonin and probably cholate.