NK012, SN-38-incorporating micellar nanoparticle, improves tumor disposition of SN-38 and elicits superior antitumor activity to irinotecan.

5627 Introduction: NK012 is a novel micelle-forming macromolecular prodrug of SN-38, an active metabolite of irinotecan (CPT-11), which is gradually released from the molecule in an enzyme-independent manner at physiological pH. We previously reported that NK012 continued to circulate in the blood stream and accumulate at high concentration in tumor tissue for more than 7 days after an intravenous injection, and exhibited superior antitumor activity to CPT-11 in a xenograft model subcutaneously transplanted with human colon cancer HT-29 (Koizumi F et al, Cancer Research, 2006; 66: 10048-). Here, we evaluated tumor extracellular fluid (ECF) disposition of released SN-38 in an HT-29 xenograft model, and further we microscopically examined the distribution of NK012 in tumor tissue.
 Method: NK012 and CPT-11 were given intravenously to nude mice bearing HT-29. Microdialysis probes were placed in tumors to determine free SN-38 concentration in tumor ECF. NK012 distribution in tumor tissue was also investigated by observing fluorescence of the drug using a fluorescence microscope.
 Results: Release of SN-38 in tumor ECF was confirmed after NK012 administration (30mg/kg as SN-38 equivalent) in nude mice bearing HT-29. Free SN-38 in tumor ECF was slowly eliminated with the terminal half-life (T1/2z) of about 90 hours. In contrast, when CPT-11 (66.7 mg/kg, i.e. 39 mg/kg as SN-38 equivalent) was administered, free SN-38 disappeared rapidly from tumor ECF with the T1/2z of about 6 hours. Exposure to free SN-38 released from NK012 in tumor ECF was 3-fold greater than that produced from CPT-11. The IC50 value of SN-38 against HT-29 cells in vitro corrected with protein-unbound ratio of SN-38 (0.65) in medium containing 10% FBS was 1.9 ng/mL. Free SN-38 concentration in tumor ECF sustained over the IC50 value and lasted for 3-7 days after NK012 administration, while free SN-38 level continued less than 2 days after CPT-11 administration. NK012 gradually extravasated from tumor blood vessels to tumor tissue and widely distributed over the tumor tissue and remained until 3rd day. On the contrary, CPT-11 leaked immediately out of capillary, but was not observed on the following day of the administration. Conclusion: These results suggest that the improved antitumor efficacy of NK012 was exerted by the sustained exposure time and concentration of free SN-38 released from the micellar NK012 after specific accumulation in tumor tissue due to leakiness of tumor blood vessel for such nanosize molecules as NK012.