Development of targeted nuclear receptor ligands as medical radiation countermeasures - Abstract Injuries to bone marrow, GI, respiratory and cardiovascular systems are major determinants of lethality after total-body irradiation (TBI). Although progress is being made in management of systemic radiation injury, development of additional effective and safe countermeasures against structural injury and dysfunction are urgently needed, especially in view of increasing risks of nuclear/radiological accidents or attacks. To be useful in a mass casualty situation, a radiation mitigator must be able to retain its therapeutic efficacy when administered at 24 hours or more after exposure as per NIH Center for Medical Countermeasures Against Countermeasures Against Radation. Activators of a second, more recently-discovered estrogen receptor, termed ER-beta, may address this need by providing a new class of medical radiation mitigators. Importantly, ER-beta is known to be expressed and active in diverse tissues that are very susceptible to radiation injury. Ongoing studies show that subcutaneous administration of the ER-beta agonist DPN (2,3-bis[4-hydroxyphenyl]- propionitrile) in mouse models in vivo exhibited potent radiation mitigating properties, with up to 100% post- irradiation survival when treatment was started 24 hours after TBI in mouse models. Findings from our group indicate that DPN helps to promote the recovery of hematopoietic cells including stem/progenitor cells and other ERb-expressing tissues after TBI. Importantly, DPN efficacy as a radiation mitigator is lost when using ER-beta knockout mouse models in vitro, thereby helping to confirm an underlying mechanism of action for DPN. We now propose to synthesize and test a series of new DPN analogues with better oral bioavailability to provide improved access for civilians and potentially patients post-radiation exposure. A promising set of derivatives of DPN are currently available. We plan exploratory study of the radiation mitigator properties of these improved DPN analogues in vivo to aid in selecting an effective drug-like compound at an optimal oral dose. Male and female C3H and C57BI/6 mice will be exposed to total body irradaiation; and DPN analogue or control treatments will be given daily for varying numbers of days, beginning 24–72 hrs after irradiation. Animal survival will be recorded as an index of radiation mitigator activity. The contribution of selected systemic cytokine levels will be estimated by analyzing plasma samples, and tissue specimens will be assessed postmortem for histologic/IHC indices of molecular responses to radiation injury. These experiments will provide further pathophysiological data on mechanisms for radiation mitigation by analogues of DPN by use of well-characterized animal models that are expected to predict responses in humans and use an endpoint- namely animal survival- as an index for prevention of major mortality. Overall, we hope this work will lead us on a pathway toward development of a previously-unsuspected new class of oral radiation mitigators for use in the event of a nuclear attack or accident and potentially for managing side-effects of clinical radiotherapy.
