THE EFFECT OF RADIATION DOSE AND VARIATION IN NEUPOGEN® INITIATION SCHEDULE ON THE MITIGATION OF MYELOSUPPRESSION DURING THE CONCOMITANT GI-ARS AND H-ARS IN A NONHUMAN PRIMATE MODEL OF HIGH-DOSE EXPOSURE WITH MARROW SPARING

A nonhuman primate (NHP) model of acute high-dose, partial-body irradiation with 5% bone marrow (PBI/BM5) sparing was used to assess the effect of Neupogen (R) [granulocyte colony stimulating factor (G-CSF)] to mitigate the associated myelosuppression when administered at an increasing interval between exposure and initiation of treatment. A secondary objective was to assess the effect of Neupogen (R) on themortality or morbidity of the hematopoietic (H)-acute radiation syndrome (ARS) and concurrent acute gastrointestinal radiation syndrome (GI-ARS). NHP were exposed to 10.0 or 11.0 Gy with 6 MV LINAC-derived photons at approximately 0.80 Gy min(-1). All NHP received medical management. NHP were dosed daily with control article (5% dextrose in water) initiated on day 1 post-exposure or Neupogen (R) (10 mu g kg(-1)) initiated on day 1, day 3, or day 5 until recovery [absolute neutrophil count (ANC) >= 1,000 cells mu L-1 for three consecutive days]. Mortality in both the 10.0 Gy and 11.0 Gy cohorts suggested that early administration of Neupogen (R) at day 1 post exposure may affect acute GI-ARS mortality, while Neupogen (R) appeared to mitigate mortality due to the H-ARS. However, the study was not powered to detect statistically significant differences in survival. The ability of Neupogen (R) to stimulate granulopoiesis was assessed by evaluating key parameters for ANC recovery: the depth of nadir, duration of neutropenia (ANC < 500 cells mu L-1) and recovery time to ANC >= 1,000 cells mu L-1. Following 10.0 Gy PBI/BM5, the mean duration of neutropenia was 11.6 d in the control cohort vs. 3.5 d and 4.6 d in the day 1 and day 3 Neupogen (R) cohorts, respectively. The respective ANC nadirs were 94 cells mu L-1, 220 cells mu L-1, and 243 cells mu L-1 for the control and day 1 and day 3 Neupogen (R) cohorts. Following 11.0 Gy PBI/BM5, the duration of neutropenia was 10.9 d in the control cohort vs. 2.8 d, 3.8 d, and 4.5 d in the day 1, day 3, and day 5 Neupogen (R) cohorts, respectively. The respective ANC nadirs for the control and day 1, day 3, and day 5 Neupogen (R) cohorts were 131 cells mu L-1, 292 cells mu L-1, 236 cells mu L-1, and 217 cells mL(-1), respectively. Therefore, the acceleration of granulopoiesis by Neupogen (R) in this model is independent of the time interval between radiation exposure and treatment initiation up to 5 d post-exposure. The PBI/BM5 model can be used to assess medical countermeasure efficacy in the context of the concurrent GI- and H-ARS.