CHOLATE-INDUCED DISRUPTION OF CALCITONIN-LOADED LIPOSOMES - FORMATIONOF TRYPSIN-RESISTANT LIPID-CALCITONIN-CHOLATE COMPLEXES

Purpose. The work was performed to obtain a better understanding why t he oral administration of calcitonin (CT)-loaded liposomes to rats res ults in a hypocalcemia, while liposomes are normally disrupted in the gastro-intestinal tract and cannot protect the hormone from enzymatic digestion. Methods. In vitro comparisons between the stability of calc ein and CT-loaded liposomes in the presence of cholate solutions led t o an interpretation of the results observed. By means of gel filtratio n, turbidimetry, and fluorescence measurements, the interactions betwe en CT and lipids were studied after sonicated liposomes had been broke n down by cholate. Results. Experiments showed that CT in the external medium of a liposome suspension had no effect on the vesicles. Gel fi ltration of cholate-treated liposomes loaded with calcein and CT resul ted in a total separation of calcein from the lipid fraction for deter gent concentrations higher than 4 mM. However, 50% of the CT was reenc apsulated even when the cholate-to-phospholipid molar ratio was increa sed up to 100. Incubation of cholate-solubilized liposomes with 1% try psin resulted in a partial CT-breakdown. Conclusions. These results st rongly suggest that during membrane solubilization by cholate, lipid-C T complexes are formed which retain most of the CT initially embedded in the liposomal membrane, and which offer some protection to CT under the action of trypsin. The existence of these complexes could be one of the reasons for the reported hypocalcemia in rats after oral admini stration of CT-loaded liposomes.