A synthetic small molecule with radiosensitizing activity. Efaproxiral increases oxygen levels in hypoxic tumor tissues by binding non-covalently to the hemoglobin tetramer and decreasing hemoglobin-oxygen binding affinity. Increasing tumor oxygenation reduces tumor radioresistance. OR A substance being studied in the treatment of brain tumors and some other types of cancer. It increases the amount of oxygen in tumor tissues, which may make the tumor cells easier to kill with radiation therapy. Efaproxiral is a type of radiosensitizing agent. Also called RSR13.