Thrombopoietin-Mimetic Romiplostim Confers the Complete Survival Rate to Mice Exposed to Lethal Ionizing Radiation

Radiation-related casualties following exposure to a lethal dose of ionizing radiation show severe acute radiation syndromes (ARS) involving bone marrow death and gastrointestinal death. ARS cause decreases in red blood cell count, white blood cell count, platelet count and gastrointestinal dysfunction, finally leading to death caused by systemic bleeding. Therefore, reconstitution and restoration of hematopoiesis is a top priority. Although bone marrow transplantation (BMT) is also available for recovery from radiation-induced bone marrow damage, BMT for victims in radiation accidents has many limitations, including histocompatibility, age constraints, HLA type and the fact that immunosuppression would be required to reduce the risk of graft versus host rejection. In contrast, pharmacological approaches can accommodate a large number of victims with few limitations. Our previous study showed that the combined administration of erythropoietin, granulocytecolony stimulating factor and nandrolone decanoate after lethal ionizing irradiation resulted in the survival of approximately 50% of irradiated mice at day 30. When a c-Mpl agonist (Romiplostim: RP) was added to this protocol, 100% survival was obtained. Finally, we found that RP play a key role in the survival of irradiated mice. In the present study, we examined the effects of RP alone on mice exposed to lethal radiation.

RP was administered at a dosage of 50 μg/kg of body weight/day to 8-weekold female C57BL/6JJcl mice for 1, 3, or 5 days immediately following exposure to a lethal 7 Gy dose of ¹³⁷Cs γ-rays. The condition of each animal was analyzed via morphological evaluations of the small intestine and various parameters such as the numbers of peripheral blood cells, bone marrow cells, and hematopoietic progenitor cells along with cell surface antigen expression. By day 30, all untreated irradiated control mice died, whereas RP administration for 3 or 5 consecutive days after irradiation led to a 100% survival rate among the irradiated mice. At this time, the numbers of peripheral blood cells, bone marrow cells and hematopoietic progenitor cells were not significantly different between RP-untreated non-irradiated and RP-treated irradiated mice. In addition, the expression of macrophages, granulocytes and erythroid progenitors-related cell surface antigens on the bone marrow cells was significantly recovered in RP-treated irradiated mice compared to RP-untreated irradiated mice until day 20 after γ-irradiation. And, to estimate the effects of RP on gastrointestinal tissues in each individual, morphological evaluation H&E stain of the small intestine was performed until day 20 after γ-irradiation. As a result, RP promoted the recovery of gastrointestinal tissues damages in RP-treated irradiated mice compared to RP-untreated irradiated mice.

Regarding cell death, radiation-induced gamma-H2AX expression in the nuclear of bone marrow cell was significantly decreased in RP-treated irradiated mice compared to RP-untreated irradiated mice immediately and after a period of 24 hours following a lethal 7 Gy dose of X-irradiation, indicating that the rate of apoptotic bone marrow cells was significantly decreased by RP-treatment. Meanwhile, 53BP1, which is well known as non-homologous end joining (NHEJ) factor, was significantly increased, showing that RP promoted NHEJ DNA repair in bone marrow cells treated with RP. These results demonstrate that c-Mpl agonist RP promotes the recovery of serious damages caused by lethal irradiation to the hematopoietic and gastrointestinal systems, and RP might be a useful radiomitigator in the case of ARS.