Development and characterization of a novel liposome-based formulation of SN-38

SN-38, 7-ethyl-10-hydroxycamptothecin, is the active metabolite of Irinotecan (CPT-11), a topoisomerase I inhibitor commercially available as Camptosar(R). SN-38 is approximately 200-2000-fold more cytotoxic than CPT-11. Despite its promising anticancer potential, SN-38 thus far has not been used as an anticancer drug due to its poor solubility in any pharmaceutically acceptable solvents. In addition, SN-38 has low affinity to lipid membranes; it tends to precipitate in aqueous phase resulting in a very low drug-to-liposome entrapment. SN-38 also reversibly converts to an inactive open lactone ring structure at physiological pH. We have developed a novel, liposome-based SN-38 formulation (LE-SN-38). The formulation contains liposomes of uniform size distribution (<200 nm), and it is easy-to-use. Drug entrapment efficiency of the formulation is >95 percent. Long-term stability studies indicate that the lyophilized LE-SN-38 is physically and chemically stable for at least 6 months at 2-8 degrees C. In preclinical studies, LE-SN38 has shown promising results in terms of increased cytotoxicity against various tumor cell lines and better therapeutic efficacy towards xenograft mouse models compared to CPT-11.