Fluoroquinolone and Quinazolinedione Activities against Wild-Type and Gyrase Mutant Strains of Mycobacterium smegmatis

Abstract

Quinazolinediones (diones) are fluoroquinolone-like inhibitors of bacterial gyrase and DNA topoisomerase IV. To assess activity against mycobacteria, C-8-methoxy dione derivatives were compared with cognate fluoroquinolones by using cultured Mycobacterium smegmatis. Diones exhibited higher MIC values than fluoroquinolones; however, MICs for fluoroquinolone-resistant gyrA mutants, normalized to the MIC for wild-type cells, were lower. Addition of a 3-amino group to the 2,4-dione core increased relative activity against mutants, while alteration of the 8-methoxy group to a methyl or of the 2,4-dione core to a 1,3-dione core lowered activity against mutants. A GyrA G89C bacterial variant was strikingly susceptible to most of the diones tested; in contrast, low susceptibility to fluoroquinolones was observed. Many of the bacteriostatic differences between diones and fluoroquinolones were explained by interactions at the N terminus of GyrA helix IV revealed by recently published X-ray structures of drug-topoisomerase-DNA complexes. When lethal activity was normalized to the MIC in order to minimize the effects of drug uptake, efflux, and ternary complex formation, a 3-amino-2,4-dione exhibited killing activity comparable to that of a cognate fluoroquinolone. Surprisingly, the lethal activity of the dione was inhibited less by chloramphenicol than that of the cognate fluoroquinolone. This observation adds the 2,4-dione structural motif to the list of structural features known to impart lethality to fluoroquinolone-like compounds in the absence of protein synthesis, a phenomenon that is not explained by X-ray structures of drug-enzyme-DNA complexes.Fluoroquinolones are lethal antibacterial agents that are widely used to control many bacterial infections (23); for some diseases, such as multidrug-resistant tuberculosis, fluoroquinolones are key to successful treatment (4). As with other antimicrobials, fluoroquinolone use is threatened by an increasing prevalence of resistance (1). One way to address the resistance problem is to identify new derivatives that are particularly active with resistant mutants. For example, with some bacterial species, addition of an 8-methoxy substituent increases fluoroquinolone activity against mutants (8,45,46). Other examples include quinazolinediones, which have shown good activity with gyrase resistance mutants selected by fluoroquinolone treatment of Escherichia coli, Staphylococcus aureus, and Streptococcus pneumoniae (11,16,34). We found with E. coli that a quinazoline-2,4-dione was almost as active with gyrase mutants as with wild-type cells (12). How quinazolinediones behave with mycobacteria is unknown.To compare quinolone-like compounds with regard to the likely effects of fluoroquinolone resistance mutations, sets of gyrase mutants are prepared in which the strains are otherwise isogenic. The MIC for the mutant is measured and related to the MIC for the wild type to correct for differences in uptake and efflux. The ratio of the MIC for the mutant to the MIC for the wild type de...

Fluoroquinolone-Gyrase-DNA Complexes

“…Previous work with this organism indicated that substitution of Cys for Gly at position 89, the equivalent of position 81 in E. coli, increases ciprofloxacin and moxifloxacin MIC by 32-fold (19,32). In the present work, the MIC was 16 times higher with the mutant for ciprofloxacin and the non-crosslinkable derivative Cip-Ac (Fig.…”

Fluoroquinolone interactions with Mycobacterium tuberculosis gyrase: Enhancing drug activity against wild-type and resistant gyrase

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Bactericidal Activity of ACH-702 against Nondividing and Biofilm Staphylococci

“…The lethality of moxifloxacin against M. tuberculosis cells in the absence of protein synthesis has been reported, whereas gatifloxacin and two additional investigational quinolones in the same study required protein synthesis for their bactericidal action (26). More recently, a fluoroquinolone-like quinazolinedione was found to retain greater bactericidal activity against M. smegmatis than its cognate quinolone when tested in the presence of protein synthesis inhibitors (29). The mechanism for the bactericidal action of these compounds in the absence of protein synthesis is unknown.…”