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Recent studies have revealed that several cancer cell types can upregulate inducible nitric oxide synthase (iNOS) and iNOS-derived nitric oxide (NO) after a moderate photodynamic challenge sensitized by 5-aminolevulinic acid (ALA)-induced protoporphyrin-IX. The NO signaled for cell resistance to photokilling as well as greater growth and migration/invasion aggressiveness of surviving cells. On this basis, it was predicted that diffusible NO from PDT-targeted cells in a tumor might enhance growth, migration, and invasiveness of non- or poorly PDT-targeted bystander cells. This was tested using a novel approach in which ALA-PDT targeted cancer cells on a culture dish were initially segregated from non-targeted bystander cells of the same type via impermeable silicone-rimmed rings. Several hours after LED irradiation, the rings were removed, and both cell populations analyzed in the dark for various responses. After a moderate extent of targeted cell killing (~25%), bystander proliferation and migration were evaluated, and both were found to be significantly enhanced. Enhancement corelated with iNOS/NO upregulation in surviving PDT-targeted cancer cells in the following cell type order: PC3 > MDA-MB-231 >U87 > BLM. If occurring in an actual PDT-challenged tumor, such bystander effects might compromise treatment efficacy by stimulating tumor growth and/or metastatic dissemination. Possible mitigation of these negative effects by using pharmacologic inhibitors of iNOS expression or activity as PDT adjuvants will be discussed.

Low level nitric oxide (NO) produced by inducible NO synthase (iNOS) in many malignant tumors is known to play a key role in the survival and proliferation of tumor cells. NO can also induce or augment resistance to anti-tumor treatments such as platinum-based chemotherapy (CT), ionizing radiotherapy (RT), and non-ionizing photodynamic therapy (PDT). In each of these treatments, tumor cells that survive the challenge may exhibit a striking increase in NO-dependent proliferative, migratory, and invasive aggressiveness compared with non-challenged controls. Moreover, NO from cells directly targeted by PDT can often stimulate aggressiveness in non- or poorly targeted bystander cells. Although NO-mediated resistance to many of these therapies is fairly-well recognized by now, the hyper-aggressiveness of surviving cells and bystander counterparts is not. We will focus on these negative aspects in this review, citing examples from the PDT, CT, and RT publications. Increased aggressiveness of cells that escape therapeutic elimination is a concern because it could enhance tumor progression and metastatic dissemination. Pharmacologic approaches for suppressing these negative responses will also be discussed, e.g., administering inhibitors of iNOS activity or iNOS expression as therapeutic adjuvants.