Slow-Onset Inhibition of the Fabl Enoyl Reductase from Francisella tularensis: Residence Time and in Vivo Activity

Francisella tularensis is a highly virulent and contagious Gram- negative intracellular bacterium that causes the disease tularemia in mammals. The high infectivity and the ability of the bacterium to survive for weeks in a cool, moist environment have raised the possibility that this organism could be exploited deliberately as a potential biological weapon. Fatty acid biosynthesis (FAS-II) is essential for bacterial viability and has been validated as a target or the discovery of novel antibacterials. The FAS-II enoyl reductase ftuFabl has been cloned and expressed, and a series of diphenyl ethers have been identified that are subnanomolar inhibitors of the enzyme with MIC90 values as low as 0.00018 mu g mL(-1). The existence of a linear correlation between the K-l and MIC values strongly suggests that the antibacterial activity of the diphenyl ethers results from direct inhibition of ftuFabl within the cell. The compounds are slow-onset inhibitors of ftuFabl, and the residence time of the inhibitors on the enzyme correlates with their in vivo activity in a mouse model of tularemia infection. Significantly, the rate of breakdown of the enzyme-inhibitor complex is a better predictor of in vivo activity than the overall thermodynamic stability of the complex, a concept that has important implications for the discovery of novel chemotherapeutics that normally rely on equilibrium measurements of potency.