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Antitumor Effect of SN-38–Releasing Polymeric Micelles, NK012, on Spontaneous Peritoneal Metastases from Orthotopic Gastric Cancer in Mice Compared with Irinotecan

¹ Investigative Treatment Division, Research Center for Innovative Oncology, National Cancer Center Hospital East, Kashiwa, Chiba, Japan; ² Gastrointestinal Oncology Division, National Cancer Center Hospital, ³ Central Animal Laboratory, and ⁴ Section for Studies on Metastasis, National Cancer Center Research Institute, Tokyo, Japan; and ⁵ Hematology and Oncology Department, Clinical and Experimental Oncology Division, Research Institute for Radiation Biology and Medicine, Hiroshima University, Hiroshima, Japan

Requests for reprints: Yasuhiro Matsumura, Investigative Treatment Division, Research Center for Innovative Oncology, National Cancer Center Hospital East, 6-5-1 Kashiwanoha, Kashiwa, Chiba 277-8577, Japan. Phone: 81-4-7134-6857; Fax: 81-4-7134-6866; E-mail: yhmatsum{at}east.ncc.go.jp.

Key Words: NK012 • SN-38 • drug delivery system • orthotopic gastric cancer model • peritoneal metastasis

7-Ethyl-10-hydroxy-camptothecin (SN-38), an active metabolite of irinotecan hydrochloride (CPT-11), has potent antitumor activity. Moreover, we have reported the strong antitumor activity of NK012 (i.e., SN-38–releasing polymeric micelles) against human cancer xenografts compared with CPT-11. Here, we investigated the advantages of NK012 over CPT-11 treatment in mouse models of gastric cancer with peritoneal dissemination. NK012 or CPT-11 was i.v. administered thrice every 4 days at their respective maximum tolerable doses (NK012, 30 mg/kg/day; CPT-11, 67 mg/kg/day) to mice receiving orthotopic transplants of gastric cancer cell lines (44As3Luc and 58As1mLuc) transfected with the luciferase gene (n = 5). Antitumor effect was evaluated using the photon counting technique. SN-38 concentration in gastric tumors and peritoneal nodules was examined by high-performance liquid chromatography (HPLC) 1, 24, and 72 hours after each drug injection. NK012 or CPT-11 distribution in these tumors was evaluated using a fluorescence microscope on the same schedule. In both models, the antitumor activity of NK012 was superior to that of CPT-11. High concentrations of SN-38 released from NK012 were detected in gastric tumors and peritoneal nodules up to 72 hours by HPLC. Only a slight conversion from CPT-11 to SN-38 was observed from 1 to 24 hours. Fluorescence originating from NK012 was detected up to 72 hours, whereas that from CPT-11 disappeared until 24 hours. NK012 also showed antitumor activity against peritoneal nodules. Thus, NK012 showing enhanced distribution with prolonged SN-38 release may be ideal for cancer treatment because the antitumor activity of SN-38 is time dependent. [Cancer Res 2008;68(22):9318–22]

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