Liposomal Sustained-Release Delivery Systems for Intravenous Injection. I

1a-N-Stearoyl mitomycin C (MMC) and six 1a-N-substituted derivatives of MMC possessing the cholesteryl moiety with different spacers were synthesized, and their biopharmaceutical properties were studied to assess the feasibility of such derivatives as prodrugs for intravenously injectable liposomal sustained-release carrier systems. All compounds showed increased lipophilic indices (logk'0) in high performance liquid chromatography. It was found that all the derivatives could be almost completely entrapped in liposomes, although MMC itself was hardly encapsulated. The derivatives with the exception of cholesteryloxycarbonyl MMC (II) and N-(cholesteryloxy-carbonyl)-4-aminophenylacetyl MMC (VI) were converted to the parent drug in rat serum. The suceptibility of the compounds to hydrolysis was strongly affected by the spacer structure between MMC and the cholesteryl moiety. Cholesteryloxyacetyl MMC (IV) was converted to MMC mainly by chemical hydrolysis. N-(Cholesteryloxycarbonyl)glycyl MMC (III) was also hydrolyzed to MMC chemically but in this case hydrolysis was accelerated in the presence of mouse, rat and human serum. No species differences were observed in these bioactivation phenomena. Entrapment of derivatives III and IV in liposomes resulted in enhancement of the stability against both chemical and enzymatic hydrolysis. The derivatives possessing the cholesteryl moiety were firmly associated with liposomes in the circulation, while stearoyl MMC (VIII) was rapidly recoved. These results suggest that derivatives III and IV have the potential to be utilized as lipophilic prodrugs for liposomal sustained-release carrier systems to be delivered by intravenous injection.