PURPOSE: To investigate the advantages of treatment with the SN-38-incorporating polymeric micelles NK012 over CPT-11 in combination with cisplatin [cis-dichlorodiammineplatinum (II) (CDDP)] in mice bearing a small cell lung cancer xenograft in terms of antitumor activity and toxicity, particularly intestinal toxicity. EXPERIMENTAL DESIGN: Cytotoxic effects were evaluated in human small cell lung cancer cell lines [H69, H82, and vascular endothelial growth factor (VEGF)-secreting cells (SBC-3/VEGF and its mock transfectant SBC-3/Neo)]. In vivo antitumor effects were evaluated in SBC-3/Neo-bearing and SBC-3/VEGF-bearing mice after NK012/CDDP or CPT-11/CDDP administration on days 0, 7, and 14. Drug distribution was analyzed by high-performance liquid chromatography or fluorescence microscopy, and the small intestine was pathologically examined. RESULTS: The in vitro growth-inhibitory effects of NK012 were 198- to 532-fold more potent than those of CPT-11. A significant difference in the relative tumor volume on day 30 was found between NK012/CDDP and CPT-11/CDDP treatments (P = 0.0058). Inflammatory changes in the small intestinal mucosa were rare in all NK012-treated mice but were commonly observed in CPT-11-treated mice. Moreover, a large amount of CPT-11 was excreted into the feces and high CPT-11 concentration was detected in the small intestinal epithelium. On the other hand, a small amount of NK012 was found in the feces and NK012 was weakly and uniformly distributed in the mucosal interstitium. CONCLUSIONS: NK012/CDDP combination may be a promising candidate regimen against lung cancer without severe diarrhea toxicity and therefore warrants further clinical evaluation.