Photodynamic therapy (PDT) is a potential treatment for the peritoneal dissemination of gastric cancer, because its cytotoxicity is limited to superficial lesions. We examined the accumulation of talaporfin in peritoneal metastatic nodules and determined the optimal laser condition for these nodules. We also evaluated the pathological response after therapy. We created a peritoneal metastasis model in nude mice using the MKN-45 EGFP cell line. We evaluated the accumulation of talaporfin in peritoneal metastatic nodules and normal organs by spectrophotometric analysis 2-8 h after i.p. talaporfin. To determine optimal PDT conditions, we treated metastatic nodules and the small intestine using multiple laser doses (2, 5, and 10 J/cm2, respectively). Accumulation of talaporfin was detected in metastatic nodules in higher intensities than in the small intestine. The fluorescent intensity of the peritoneal metastatic nodules gradually decreased dependent on the time interval between the laser treatment and talaporfin administration. Fluorescent intensity in the small intestine decreased more than in the metastatic nodules. The pathological response rates by dose were 52.5% at 2 J/cm2, 43.2% at 5 J/cm2, and 64.4% at 10 J/cm2, respectively, when the laser treatment was used 2 h after talaporfin administration, whereas at 4 h, they were 20.8, 25.5, and 26.2%, respectively. Finally, the recommended treatment conditions were considered to be a 2 J/cm2 laser dose and a 4-h interval in terms of toxicity. Talaporfin-mediated PDT may be an effective treatment modality for patients with advanced gastric adenocarcinoma and metastatic peritoneal nodules.

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