Purpose: Photodynamic therapy (PDT) is a new approach to cancer treatment that utilizes photochemical reactions induced by a combination of an oncophilic photosensitizing agent and laser light. With an aim to apply PDT for intraperitoneal disseminated foci of advanced or recurrent ovarian cancers, the present study was conducted to evaluate the antitumor effect of PDT using a methyl ester of 5-aminolevulinic acid (Methyl-ALA) on various types of human ovarian cancer in a subcutaneous xenograft model in nude mice and to elucidate the mechanism of its antitumor effect.
Methods: HTOA, MCAS, and TOV21G cell lines derived from human ovarian serous, mucinous, and clear cell adenocarcinoma, respectively, were used in this study. The mice in the treatment group and in the control group received an intraperitoneal injection of 250 mg/kg of Methyl-ALA and PBS alone, respectively. PDT was administered by 10 min irradiation using a 150 W halogen light, 3 h after Methyl-ALA or PBS injection. Each mouse received PDT twice a week for 3 weeks.
Results: Methyl-ALA-PDT significantly suppressed the growth of HTOA tumors as compared to control, whereas there was no significant effect on the growth of MCAS or TOV21G tumors. Methyl-ALA-PDT significantly increased apoptosis in implanted HTOA tumors as well as cultured cells. Western blot analysis showed that amount of expression of milk fat globule-EGF-factor 8, which binds to apoptotic cells and thereby facilitates their phagocytosis, significantly increased in HTOA tumors receiving Methyl-ALA-PDT, compared with untreated HTOA tumors. In addition, reduced vascular endothelial growth factor and CD34-positive microvessel density were found in solid HTOA tumors treated by Methyl-ALA-PDT, suggesting that the antitumor effect of Methyl-ALA-PDT is due to induction of apoptosis and reduction of angiogenesis. In comparison with HTOA cells, HPLC analysis demonstrated a significantly smaller intracellular amount of protoporphyrin IX (PpIX) in MCAS and TOV21G cells. PpIX is readily converted from Methyl-ALA and elicts photocytotoxicity.
Conclusion: We conclude that Methyl-ALA-PDT could be an effective treatment in ovarian cancer and should be tested to apply intraperitoneally disseminated micro-foci during surgery.