Intestinal absorption of carboxyfluorescein entrapped in liposomes in comparison with its administration with lipid-surfactant mixed micelles.

The absorption of carboxyfluorescein (CF) entrapped in liposomes from the small intestine was investigated in vivo using bile fistula rats. Within the intestine CF was not metabolized, but 32% of the entrapped CF was released for 3 h from the liposomes when incubated with luminal incubation solution. The data of CF disappearance from the intestinal lumen and of CF plasma concentration showed that no apparent difference was observed between administration of the liposomal CF and that of free dye. Especially in initial 60 min after administration of liposomal CF, the plasma concentration was obviously lower than that in free CF administration. On the contrary, the coadministration of 40 mM lipid-surfactant mixed micelles induced a remarkable promotive effect on the absorption of free CF already in the early periods of the initial 60 min. When liposomal CF was administered into the intestinal lumen of thoracic duct-cannulated rats, the concentration in lymph was lower than that in plasma. Intramural injection of liposomal CF into intestinal wall, however, showed a much higher concentration in lymph than that in plasma. These findings suggest that liposomal entrapment of CF has rather difficulty to be absorbed through the intestinal mucosa, while lipid-surfactant mixed micelles exert a remarkable promotive effect on the absorption of CF.