Nitric oxide-mediated activity in anti-cancer photodynamic therapy.

Cell recurrence in cancer photodynamic therapy (PDT) is an important issue that is poorly understood. It is becoming clear that nitric oxide (NO) is a modulator of PDT. By acting on the NF-κB/Snail/RKIP survival/anti-apoptotic loop, NO can either stimulate or inhibit apoptosis. We found that pheophorbide a/PDT (Pba/PDT) induces the release of NO in B78-H1 murine amelanotic melanoma cells in a concentration-dependent manner. Low-dose PDT induces low NO levels by stimulating the anti-apoptotic nature of the above loop, whereas high-dose PDT stimulates high NO levels inhibiting the loop and activating apoptosis. When B78-H1 cells are treated with low-dose Pba/PDT and DETA/NO, an NO-donor, intracellular NO increases and cell growth is inhibited according to scratch-wound and clonogenic assays. Western blot analyses showed that the combined treatment reduces the expression of the anti-apoptotic NF-κB and Snail gene products and increases the expression of the pro-apoptotic RKIP gene product. The combined effect of Pba and DETA/NO was also tested in C57BL/6 mice bearing a syngeneic B78-H1 melanoma. We used pegylated Pba (mPEG-Pba) due to its better pharmacokinetics compared to free Pba. mPEG-Pba (30 mg/Kg) and DETA/NO (0.4 mg/Kg) were i.p. injected either as a single molecule or in combination. After photoactivation at 660 nM (fluence of 193 J/cm(2)), the combined treatment delays tumor growth more efficiently than each individual treatment (p<0.05). Taken together, our results showed that the efficacy of PDT is strengthened when the photosensitizer is used in combination with an NO donor.