Abstract 5522: Comparison of pharmacokinetics/pharmacodynamics of NK012, a micelle-forming prodrug of SN-38, and CPT-11 in human breast cancer xenograft model

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 have demonstrated that antitumor activity of NK012 at 3.8 mg/kg/day by q4d×3 (1/8 MTD) and above was superior to that of CPT-11 at its MTD (67 mg/kg by q4d×3) against human breast cancer MC-05-JCK, and a phase II clinical trial of NK012 is ongoing against triple-negative breast cancer in the US. In the present study, we compared pharmacokinetics and pharmacodynamics of NK012 with those of CPT-11 in human breast cancer MC-05-JCK-bearing nude mice. Methods: NK012 (3.8 and 30 mg/kg) and CPT-11 (67 mg/kg) were given intravenously to nude mice bearing MC-05-JCK, and microdialysis probe was placed in the tumor tissue to determine free SN-38 concentration in tumor extracellular fluid (ECF). The drug concentrations in the plasma and tumor tissue were determined by HPLC system. Intratumoral distribution of NK012 was also investigated by observing auto-fluorescence of the drug with a fluorescence microscope. Cytotoxic effects of the drugs were examined immunohistochemically using anti-phosph γH2AX and anti-phospho histone H3 antibodies to detect cleaved DNA and M-phase cells, respectively. Results: After a single NK012 administration, polymer-bound SN-38 and polymer-unbound SN-38 (released SN-38) showed remarkably prolonged blood circulation. On the other hand, CPT-11 and its active metabolite SN-38 were promptly removed from the circulation. NK012 also retained in the tumor tissue for a long period as compared with CPT-11. Microdialysate experiment revealed that the free SN-38 from NK012 in the tumor ECF was eliminated more slowly than that converted from CPT-11. NK012-originated fluorescence in the tumor tissue was mainly observed in the stroma, necrotic area and perivascular region. The phosphorylation of γH2AX due to DNA breakage and consequent reduction in M-phase cells were observed for a longer period in NK012-treated tumor tissue than that treated with CPT-11. HE staining of MC-05-JCK revealed that apoptotic tumor cells appeared 3 days after administration of both NK012 and CPT-11 injection, but fibrosis in the tumor stroma was only observed with NK012 treatment (day 10). These histopathological changes caused by the drugs were associated with the elimination rate of SN-38 in the tumor tissue. Conclusion: The superior antitumor efficacy of NK012 to CPT-11 was associated with the sustained release of free SN-38 from the NK012 accumulated in tumor tissue as well as the prolonged circulation of NK012 in the bloodstream. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5522.