Efaproxiral Sodium (RSR13 sodium), a synthetic allosteric modifier of hemoglobin, is utilized for brain metastases originating from breast cancer.

Efaproxiral, a synthetic allosteric modifier of hemoglobinoxygen binding affinity, has been shown to bind reversibly to hemoglobin, stabilizing the deoxyhemoglobin tetramer conformation to reduce its affinity for oxygen. [1]

Efaproxiral plus oxygen breathing reduces the radiobiological hypoxic fraction of EMT6 tumors from the value of 24% found in both air-breathing and oxygen-breathing mice to 9% and improves the response of the tumors to radiation. Carboplatin (100 mg/kg) slowes tumor growth in air-breathing mice, producing a growth delay of 3.3 days. Efaproxiral plus oxygen increases the growth delay to 5.7 days; this is 2.4 days (71%) greater than that for carboplatin alone and 2.1 days (57%) greater than that for carboplatin plus oxygen breathing. Efaproxiral plus oxygen breathing, therefore, improves the tumor growth delay obtained with 100 mg/kg carboplatin to or beyond that obtained with the highly toxic dose of 150 mg/kg carboplatin, but does so without increasing the toxicity beyond that seen with 100 mg/kg carboplatin in air-breathing mice.[1] Efaproxiral significantly increases tumor oxygenation by 8.4 to 43.4 mmHg within 5 days in C3H mice with RIF-1 tumors, with maximum increases at 22-31 min after treatment. Efaproxiral plus oxygen plus Radiation produces tumor growth inhibition throughout the treatment duration in C3H mice with RIF-1 tumors, and inhibition is significantly different from radiation plus oxygen from day 3 to day 5. [2]