The New Zealand White Rabbit Animal Model of Acute Radiation Syndrome: Hematopoietic and Coagulation-based Parameters by Radiation Dose Following Supportive Care

Purpose: Animal models that accurately reflect human responses to radiation injury are needed for advanced mechanistic investigation and development of effective therapeutics. The rabbit is an established animal model accepted by the FDA for studies of cardiovascular disease, lipid metabolism, the development of anticoagulants, testing of bone implants, and the development of treatments for infectious diseases such as HIV. The purpose of this study was to investigate the New Zealand White (NZW) Rabbit model as a model of acute radiation exposure because of its established similarity to human vascular, immune, and coagulation responses.Materials & Methods: Two sequential studies were performed in a total of 81 male NZW rabbits, 16-20 weeks of age. All animals underwent clinical observations and peripheral blood analyses following a single dose of 0, 6,7,8,8.5, 9, or 10 Gy of total body irradiation via a 6 MV Linear accelerator photon source on Day 0. Animals were treated with timed release fentanyl patch (Day 0-30), subcutaneous hydration (Day 1, Study 2 only), and oral sulfamethoxazole/trimethoprim 30mg/kg once daily (Days 3-30) and were followed for 30 days or to time of mortality.Results: Study 1 revealed the estimated LD30, -50, -70, and -90 with 95% confidence intervals (CI) at 30 days to be 6.7 (CI:5.9-7.4) , 7.3 (CI:6.7-7.8), 7.9 (CI: 7.3-8.4), and 8.8 (CI: 7.9-9.7) Gy respectively. In study 2, a survey of blood coagulation and biochemical parameters were performed over time and necropsy. Complete blood counts taken from animals exposed to 7, 8, or 10 Gy, demonstrated dose-dependent depletion of lymphocytes, neutrophils, and platelets. Platelet counts recovered to baseline levels in survivors by Day 30, whereas lymphocyte and neutrophil counts did not. Decedent animals demonstrated Grade 3 or 4 neutropenia and lymphopenia at time of death; 64% of the decedents experienced a 30% or greater drop in hematocrit. Decedent animals demonstrated more than 100% increases from serum baseline levels of blood urea nitrogen, creatinine, aspartate aminotransferase, and triglyceride levels at the time of death whereas survivors on average demonstrated modest or no elevation.Conclusion: This NZW rabbit model demonstrates dose-dependent depletion of hematopoietic parameters. The LD_50/30 of 7.8 Gy (95% CI: 6.6-8.4) with supportive care appears to be close to the ranges reported for rhesus macaques (5.25-7.44 Gy) and humans (6-8 Gy) with supportive care. These findings support the utility of the NZW rabbit model for further mechanistic investigation of acute radiation exposure and medical countermeasure testing.

Keywords: acute radiation syndrome; dose response; hematopoietic response; rabbit; radiation injury; total body irradiation.