Liposomal sustained-release delivery systems for intravenous injection. II. Design of liposome carriers and blood disposition of lipophilic mitomycin C prodrug-bearing liposomes.

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Abstract
Various types of unilamellar liposomes possessing a narrow size distribution were prepared by controlled dialysis and their blood clearance was studied in mice and rats to assess their suitability as drug carriers for intravenously injectable liposomal sustained-release delivery systems. Also, the utility of these liposomal carrier systems combined with lipophilic prodrugs of mitomycin C (MMC) was evaluated. The fate of the liposomes was monitored using N-4-nitrobenzo-2-oxa-1,3-diazole-dipalmitoylphosphatidylethanolamine (NBD-PE), a liposomal membrane marker. The effect of vesicle size on the blood clearance was investigated for neutral liposomes. Small-sized (S-) liposomes (90 .+-. 15 nm) were cleared slowly compared with medium-sized (M-) liposomes (181 .+-. 31) nm) and larged-sized liposomes (281 .+-. 38 nm). Surface charge was also an important determinant of the disposition of small-sized liposomes. S-Liposomes were retained in the circulation longer than positively (S+-) and negatively (S--) charged liposomes. The integrity of S-liposomes in the circulation was examined by simultaneous determination of NBD-PE and carboxyfluorescein (CF) entrapped in the liposomal membrane and liposomal aqueous phase, respectively. CF administered in the liposome-encapsulated form was cleared slowly in a fashion similar to NBD-PE, while free CF, administered as an aqueous solution, was rapidly removed from the circulation. These results reveal that S-liposomes show the best pharmacokinetic properties as a carrier vehicle for intravenously injectable sustained-release delivery systems. S-Liposomes loaded with the lipophilic MMC prodrug, N(cholesteryloxycarbonyl)glycyl MMC or cholesteryloxyacetyl MMC, successfully gave sustained blood levels of the parent drug following intravenous injection. Thus, the potential utility of MMC prodrug-bearing S-liposomes as an intravenously injectable MMC sustained-release dosage form was demonstrated.