Effective Prodrug Liposome and Conversion to Active Metabolite

Some antitumor agents encapsulated in liposomes have been used clinically. However, the usefulness of liposomes is limited to the liposomalization of active compounds. Irinotecan hydrochloride (CPT-11) is a prodrug of closed lactone ring form of SN-38, which is an active metabolite with antitumor activity and side toxicity. The plasma concentrations of closed CPT-11 and SN-38 increased with the liposomalization, and their blood circulation was prolonged by the polyethyleneglycol (PEG) modification. The antitumor activity of CPT-11 increased due to the elevated tumor distribution of closed CPT-11 and SN-38 levels by the PEG-modified liposomes. In the tumor, CPT-11 was converted to SN-38. Thus, it is considered that passive targeting to the tumor by liposomalization elevated the SN-38 level in the tumor especially and increased the antitumor activity of CPT-11. The closed/total ratio of SN-38 in the tumors of the liposomes group was greater than that of the CPT-11 solution group. Namely, SN-38 was thought to be generated in intact liposomes containing CPT-11. The generation of SN-38 in the liposomal membrane was shown after the incubation of liposome containing CPT-11 with carboxylesterase. It is therefore considered that part of CPT-11 is converted to SN-38 in intact liposomes. Furthermore, intestinal disorder, a side toxicity of CPT-11, decreased to depend on the closed SN-38 concentrations in the bile by liposomalization. Although the liposomes induce the improved tissue distribution of the prodrug, the tissue distribution of active metabolites do not always improve. However, CPT-11 entrapped liposome was useful.