1,2-dithiole-3-ones as potent inhibitors of the bacterial 3-ketoacyl acyl carrier protein synthase III (FabH)

Abstract

The enzyme FabH catalyzes the initial step of fatty acid biosynthesis via a type II dissociated fatty acid synthase. The pivotal role of this essential enzyme, combined with its unique structural features and ubiquitous occurrence in bacteria, has made it an attractive new target for the development of antibacterial and antiparasitic compounds. We have searched the National Cancer Institute database for compounds bearing structural similarities to thiolactomycin, a natural product which exhibits a weak activity against FabH. This search has yielded several substituted 1,2-dithiole-3-ones that are potent inhibitors of FabH from both Escherichia coli (ecFabH) and Staphylococcus aureus (saFabH). The most potent inhibitor was 4,5-dichloro-1,2-dithiole-3-one, which had 50% inhibitory concentration (IC50) values of 2 microM (ecFabH) and 0.16 microM (saFabH). The corresponding 3-thione analog exhibited comparable activities. Analogs in which the 4-chloro substituent was replaced with a phenyl group were also potent inhibitors, albeit somewhat less effectively (IC50 values of 5.7 and 0.98 microM for ecFabH and saFabH, respectively). All of the 5-chlorinated inhibitors were most effective when they were preincubated with FabH in the absence of substrates. The resulting enzyme-inhibitor complex did not readily regain activity after excess inhibitor was removed, suggesting that a slow dissociation occurs. In stark contrast, a series of inhibitors in which the 5-chloro substituent was replaced with the isosteric and isoelectronic trifluoromethyl group were poorer inhibitors (IC50 values typically ranging from 25 to >100 microM for both ecFabH and saFabH), did not require a preincubation period for maximal activity, and generated an enzyme-inhibitor complex which readily dissociated. Possible modes of binding of 5-chloro-1,2-dithiole-3-ones and 5-chloro-1,2-dithiole-3-thiones with FabH which account for the role of the 5-chloro substituent were considered.

FIG. 1.

Structure of TLM (A) and the corresponding 3D Unity constraints (B) used to search for related compounds in the NCI database.

FIG. 2.

General synthesis of 1,2-dithiole-3-thiones and 1,2-dithiole-3-ones. R₄ and R₅ are shown in Table 1. R was generally either a methyl or ethyl group. LDA, lithium diisopropylamide.

FIG. 3.

Effect of preincubation time with compound 5 (squares) or compound 3 (circles) on residual activity of saFabH.

FIG. 4.

Effect of added substrate on residual activity of saFabH inhibited with compounds 3 and 5.

FIG. 5.

Decrease in intrinsic fluorescence of ecFabH with addition of compounds 3 (A) and 11 (B) and restoration of fluorescence of ecFabH after removal of unbound inhibitor (C).