Given an ever-increasing risk of nuclear and radiological emergencies, there is a critical need for development of medical radiation countermeasures (MRC) that are safe, easily administered and effective in preventing and/or mitigating the potentially lethal tissue damage caused by acute high-dose radiation exposure. Since the efficacy of MRC for this indication cannot be ethically tested in humans, development of such drugs is guided by the FDA's Animal Rule. According to this rule, human efficacious doses can be projected from experimentally established animal efficacious doses based on equivalence of the drug's effects on efficacy biomarkers in the respective species. Therefore, identification of efficacy biomarkers is critically important for drug development under the Animal Rule. CBLB502 is a truncated derivative of the Salmonella flagellin protein that acts by triggering Toll-like receptor 5 (TLR5) signaling and is currently under development as a MRC. Here we report identification of two cytokines, granulocyte colony-stimulating factor (G-CSF) and interleukin-6 (IL-6), as candidate biomarkers of CBLB502's radioprotective/mitigative efficacy. Induction of both G-CSF and IL-6 by CBLB502 (i) is strictly TLR5-dependent; (ii) occurs in a CBLB502 dose-dependent manner within its efficacious dose range in both non-irradiated and irradiated mammals, including non-human primates; and (iii) is critically important for the ability of CBLB502 to rescue irradiated animals from death. Following evaluation of CBLB502 effects on G-CSF and IL-6 levels in humans, these biomarkers will be useful for accurate prediction of human efficacious CBLB502 doses, a key step in development of this prospective radiation countermeasure.
- Received May 2, 2012.
- Revision received July 24, 2012.
- Accepted July 26, 2012.
- The American Society for Pharmacology and Experimental Therapeutics