Indole naphthyridinones as inhibitors of bacterial enoyl-ACP reductases FabI and FabK.

Bacterial enoyl-ACP reductase (FabI) is responsible for catalyzing the final step of bacterial fatty acid biosynthesis and is an attractive target for the development of novel antibacterial agents. Previously we reported the development of FabI inhibitor 4 with narrow spectrum antimicrobial activity and in vivo efficacy against Staphylococcus aureus via intraperitoneal (ip) administration. Through iterative medicinal chemistry aided by X-ray crystal structure analysis, a new series of inhibitors has been developed with greatly increased potency against FabI-containing organisms. Several of these new inhibitors have potent antibacterial activity against multidrug resistant strains of S. aureus, and compound 30 demonstrates exceptional oral (po) in vivo efficacy in a S. aureus infection model in rats. While optimizing FabI inhibitory activity, compounds 29 and 30 were identified as having low micromolar FabK inhibitory activity, thereby increasing the antimicrobial spectrum of these compounds to include the FabK-containing pathogens Streptococcus pneumoniae and Enterococcus faecalis. The results described herein support the hypothesis that bacterial enoyl-ACP reductases are valid targets for antibacterial agents.

Defining and combating the mechanisms of triclosan resistance in clinical isolates of Staphylococcus aureus.

The MICs of triclosan for 31 clinical isolates of Staphylococcus aureus were 0.016 micro g/ml (24 strains), 1 to 2 micro g/ml (6 strains), and 0.25 micro g/ml (1 strain). All the strains for which triclosan MICs were elevated (>0.016 micro g/ml) showed three- to fivefold increases in their levels of enoyl-acyl carrier protein (ACP) reductase (FabI) production. Furthermore, strains for which triclosan MICs were 1 to 2 micro g/ml overexpressed FabI with an F204C alteration. Binding studies with radiolabeled NAD(+) demonstrated that this change prevents the formation of the stable triclosan-NAD(+)-FabI complex, and both this alteration and its overexpression contributed to achieving MICs of 1 to 2 micro g/ml for these strains. Three novel, potent inhibitors of FabI (50% inhibitory concentrations, < or =64 nM) demonstrated up to 1,000-fold better activity than triclosan against the strains for which triclosan MICs were elevated. None of the compounds tested from this series formed a stable complex with NAD(+)-FabI. Consequently, although the overexpression of wild-type FabI gave rise to an increase in the MICs, as expected, overexpression of FabI with an F204C alteration did not cause an additional increase in resistance. Therefore, this work identifies the mechanisms of triclosan resistance in S. aureus, and we present three compounds from a novel chemical series of FabI inhibitors which have excellent activities against both triclosan-resistant and -sensitive clinical isolates of S. aureus.