Can irradiated tumors take NO for an answer?

Role of nitric oxide in hyper-aggressiveness of tumor cells that survive various anti-cancer therapies

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.

Supramolecular Nitric Oxide Depot for Hypoxic Tumor Vessel Normalization and Radiosensitization

In cancer radiotherapy, the lack of fixed DNA damage by oxygen in hypoxic microenvironment of solid tumors often leads to severe radioresistance. Nitric oxide (NO) is a potent radiosensitizer that acts in two ways. It can directly react with the radical DNA thus fixing the damage. It also normalizes the abnormal tumor vessels, thereby increasing blood perfusion and oxygen supply. To achieve these functions, the dosage and duration of NO treatment need to be carefully controlled, otherwise it will lead to the exact opposite outcomes. However, a delivery method that fulfills both requirements is still lacking. A NO depot is designed for the control of NO releasing both over quantity and duration for hypoxic tumor vessel normalization and radiosensitization. In B16-tumor-bearing mice, the depot can provide low dosage NO continuously and release large amount of NO immediately before irradiation for a short period of time. These two modes of treatment work in synergy to reverse the radioresistance of B16 tumors more efficiently than releasing at single dosage.