RSR13 is a synthetic allosteric modifier of hemoglobin. It decreases the oxygen affinity of hemoglobin and augments oxygen unloading the microvasculature which increases the oxygen diffusion gradient to the tissues. By increasing tissue oxygenation, RSR13 may reduce tumor hypoxia and enhance the cytotoxic effects of radiation therapy used to treat cancer patients with solid tumors. The goal of readioenhancement therapy is to achieve maximal physiological concentrations of oxygen in the tumor tissue during the radiation treatment, thus decreasing the hypoxic fraction of cells and increasing the radioresponsiveness of malignant tumors.