Development of New Small Molecule Bone Marrow Radioprotectors.

New ways to protect the bone marrow from irradiation damage are required. We are currently developing new small molecules that can be given orally to protect the marrow from irradiation damage. One small molecule is EUK-134, a manganese superoxide dismutase (MnSOD) mimetic. A nitric oxide synthase inhibitor, and a nitroxide homologue which have been both conjugated to a mitochondrial localization peptide signal, delivers the NOS inhibitor (another radioprotector) and nitroxide to the mitochondria. In addition, we tested ethyl pyruvate (EP), which has been demonstrated to protect tissues from ischemia-reperfusion injury at the level of the mitochondria. Incubation of 32D cl 3 hematopoietic cells 1 hour before irradiation in the presence of 20 μM EUK-134 or 100 μM of NOS inhibitor demonstrated an increased shoulder on the irradiation survival curve in one case (n = 5.55 ± 0.17 for EUK-134 compared to 1.60 ± 0.38 for control irradiated, respectively, p = 0.0044) and increase in the D₀ for the mitochondrial targeted NOS inhibitor compared to irradiated control 32D cl 3 cells (D₀ = 3.04 ± 0.27 compared to 1.29 ± 0.01 Gy, p = 0.0236). In experiments with 32D cl 3 cells in EP before irradiation or following irradiation, there was an increase in the D₀ to 2.2 ± 0.25 and 2.21 ± 0.15, respectively, compared to irradiated controls 1.42 ± 0.09 Gy for the 32D cl 3 cells (p = 0.0447 and 0.0119, respectively). Furthermore, incubation of 32D cl 3 cells in EP both before and after irradiation significantly increased the shoulder on the survival curve compared to irradiated control 32D cl 3 cells (n = 4.14 ± 1.59 compared to 1.70 ± 0.6 for the control irradiated cells, respectively, p=0.0485). To determine whether EP protected the bone marrow from irradiation in vivo, C57BL/6J mice were injected with EP intraperitoneally (70 mg/kg) 30 minutes before irradiation, daily for 5 days after irradiation, or both before and following irradiation to the LD 50/30 TBI dose of 900 cGy. Mice injected with EP either after irradiation or both before and after irradiation had prolonged survival compared to irradiated control mice or those injected with EP only before irradiation. (Percent survival at 50 days 100% and 100%, respectively, compared to 60%, p<0.05). These results demonstrate the potential effectiveness of new small molecule bone marrow radioprotectors.