The objective of this project is to find small molecule inhibitors of the anthrax protein Edema Factor (EF). This toxin is secreted by Bacillus anthracis in a catalytically inactive state. When the toxin is transported into the cellular cytoplasm of anthrax victims by the anthrax-derived transporter, Protective Antigen, it forms a complex with calmodulin (CAM) - the key intracellular calcium-binding protein in vertebrates. This association activates EF, and converts it into an adenylyl cyclase with 1000 times greater catalytic activity than the victim's own cyclic AMP-producing enzymes. Conventional anthrax therapies (which target the anthrax bacterium, not the anthrax toxins) are highly effective, but are insufficient to save all anthrax victims. EF inhibitors represent a completely distinct, and wholly complementary approach to combating anthrax. Such inhibitors will, by treating the downstream effects of the disease, help restore cellular equilibrium, and are likely to slow the course disease. By allowing more time for conventional treatments and the immune system to work, these inhibitors may significantly increase anthrax survival rates. We have solved crystal structures of EF (the inactive state) and the EF/CaM complex (the active state). Using these structures as a guide, and in close consultation with pharmaceutical researchers, we will use structure-based, computational drug discovery methods to identify small molecules representing two distinct types of EF inhibitors; Type A, inhibitors that occlude the active site, and Type B, those that maintain EF in its CAM-free, catalytically inactive state. We already have assays for inhibitors of Type A. We will complete the development of a simple assay for inhibitors of Type B, and then apply these assays to the top approximately 1000 compounds suggested by our computational drug screens. We will also determine the crystal structures of EF in complex with those small molecules identified by this screening procedure, so that the pharmaceutical properties of these inhibitors may be improved through rational drug design methods.
