The intraperitoneal administration of paclitaxel has been shown to be a promising treatment strategy for peritoneal malignancy. The present study evaluated the effects of intraperitoneal administration of NK105, a paclitaxel-incorporating micellar nanoparticle, which has been shown to have a remarkable effect in a mouse model of gastric cancer. Intraperitoneal NK105 significantly reduced peritoneal tumors in vivo compared with the conventional paclitaxel formulation of paclitaxel solubilized in Cremophor EL and ethanol (PTX-Cre). Moreover, intraperitoneal NK105 significantly reduced the size of subcutaneously inoculated tumors, whereas no such effect was seen with PTX-Cre. Similar systemic toxic effects were observed following the intraperitoneal administration of both NK105 and PTX-Cre. Although NK105 disappeared rapidly almost within a day from the peritoneal cavity, the paclitaxel concentration in peritoneal nodules 4 h after intraperitoneal administration was significantly higher in the NK105 group than in the PTX-Cre group (P < 0.05), whereas there were no significant differences in liver paclitaxel concentrations between the two groups. We also evaluated the pharmacokinetics following intraperitoneal administration of NK105 and PTX-Cre. Serum paclitaxel concentrations 6, 12, 24, and 48 h after the intraperitoneal administration of the drugs were significantly higher in the NK105 than the PTX-Cre group. Furthermore, the peak serum concentration was higher in the NK105 than PTX-Cre group (24 100 ± 3560 vs 108 ± 25 ng/mL, respectively; P < 0.001), as was the area under the concentration-time curve from 0 to 48 h (191 000 ± 32 100 vs 1500 ± 108 ngÁh/mL, respectively; P < 0.001). Therefore, intraperitoneal chemotherapy with nanoparticulate paclitaxel NK105 may offer a novel treatment strategy for improving drug delivery in gastric cancer with peritoneal dissemination because of enhanced drug penetration into peritoneal nodules and its prolonged presence in the systemic circulation. (Cancer Sci 2012; 103: 1304-1310 G astric cancer is the fourth leading cause of cancer-related deaths worldwide, and peritoneal dissemination is the most frequent and life-threatening form of metastasis and recurrence in patients with gastric cancer.(1,2) The current systemic chemotherapy regimens for gastric cancer have not achieved satisfactory results, particularly in the treatment of peritoneal dissemination.(3) One of the problems with this type of therapy is the limited delivery of systemically administered drugs to the peritoneal cavity.(4) Intraperitoneal chemotherapy has been shown to be safe and effective in ovarian cancer (5)(6)(7) and is expected to provide a breakthrough in the treatment of gastric cancer with peritoneal dissemination.(8-10) However, in addition to the intraperitoneal administration of drugs, it is necessary to develop new strategies for the treatment of gastric cancer to achieve better results. Research into drug delivery systems (DDS) is ongoing and is mostly aimed at improving the penetration of drugs...