Pro-tumor effects of nitric oxide from stress-upregulated inducible nitric oxide synthase in anti-tumor photodynamic therapy.

Non-invasive photodynamic therapy (PDT) is an increasingly applied modality for solid tumors. Many malignant cells that can withstand PDT upregulate nitric oxide, which not only acts cytoprotectively, but stimulates growth and migration of surviving cells. These anti-PDT responses are still not well-recognized by PDT researchers and practitioners. We provide here a background on these negative effects and offer perspectives on how they can be suppressed for a therapeutic benefit, e.g. by inhibiting transcription of inducible nitric oxide synthase.