Benzoxazoles, Phthalazinones, and Arylurea-Based Compounds with IMP Dehydrogenase-Independent Antibacterial Activity against Francisella tularensis

Abstract

<i>Francisella tularensis</i> is the causative agent of tularemia and a potential biowarfare agent. The virulence of <i>F. tularensis</i> is decreased by deletion of <i>guaB</i>, the gene encoding IMP dehydrogenase (IMPDH), suggesting that this enzyme is a target for antibacterial design. Here we report that <i>F. tularensis</i> growth is blocked by inhibitors of bacterial IMPDHs. Seventeen compounds from two different frameworks, designated the D and Q series, display antibacterial activities with MICs of <1 μM. These compounds are also active against intracellular infections. Surprisingly, antibacterial activity does not correlate with IMPDH inhibition. In addition, the presence of guanine does not affect the antibacterial activity of most compounds, nor does the deletion of <i>guaB</i> These observations suggest that antibacterial activity derives from inhibition of another target(s). Moreover, D compounds display antibacterial activity only against <i>F. tularensis</i>, suggesting the presence of a unique target or uptake mechanism. A Δ<i>guaB</i> mutant resistant to compound D73 contained a missense mutation (Gly45Cys) in <i>nuoB</i>, which encodes a subunit of bacterial complex I. Overexpression of the <i>nuoB</i> mutant conferred resistance to D73 in both wild-type and Δ<i>guaB</i> strains. This strain was not resistant to Q compounds, suggesting that a different off-target mechanism operates for these compounds. Several Q compounds are also effective against <i>Mycobacterium tuberculosis</i>, in which a second target has also been implicated, in addition to IMPDH. The fortuitous presence of multiple targets with overlapping structure-activity relationships presents an intriguing opportunity for the development of robust antibiotics that may avoid the emergence of resistance.