The Role of Nitric Oxide in Cancer Treatment: Ally or Foe?

Nitric oxide (NO), the first gaseous molecule identified as a signaling mediator, plays a pivotal role in numerous physiological processes including cardiovascular regulation, immune response, and neurotransmission. Synthesized from L-arginine by nitric oxide synthase (NOS), NO exerts both protective and cytotoxic effects depending on its local concentration. At low levels, NO supports tumor growth by mitigating oxidative stress, while at high concentrations, it induces apoptosis through mechanisms such as p53 activation, cytochrome c release, and peroxynitrite formation. These dual properties position NO as a complex but promising agent in cancer therapy. Recent studies have highlighted the potential of NO in enhancing the efficacy of photodynamic therapy (PDT), where it synergizes with reactive oxygen species (ROS) to induce cytotoxic effects in tumor cells. Despite its promise, challenges such as rapid diffusion and limited tumor accumulation hinder NO's therapeutic utility. This has spurred the development of NO donors and nanotechnology-based delivery systems to enable controlled, site-specific release. Moreover, NO has been shown to counteract multidrug resistance, improve tumor perfusion by dilating vasculature, and potentiate ROS-based therapies like PDT and radiotherapy. However, an emerging concern is NO's role in promoting proliferation and migration of non-targeted "bystander" tumor cells following PDT-induced stress, primarily through iNOS upregulation. This feedback loop can contribute to tumor aggressiveness and metastasis, underscoring the need for a deeper understanding of NO's molecular actions. While iNOS inhibitors show preclinical promise in various inflammatory and neoplastic conditions, no such agents have reached clinical approval, due to the complexity and context-dependent effects of NO. Future research should focus on refining NO delivery systems, developing selective iNOS inhibitors, and elucidating NO's dual role in cancer biology to fully harness its therapeutic potential in PDT and beyond.