Novel MenA Inhibitors Are Bactericidal against Mycobacterium tuberculosis and Synergize with Electron Transport Chain Inhibitors

Abstract

Mycobacterium tuberculosis is the leading cause of morbidity and death resulting from infectious disease worldwide. The incredible disease burden, combined with the long course of drug treatment and an increasing incidence of antimicrobial resistance among M. tuberculosis isolates, necessitates novel drugs and drug targets for treatment of this deadly pathogen. Recent work has produced several promising clinical candidates targeting components of the electron transport chain (ETC) of M. tuberculosis, highlighting this pathway's potential as a drug target. Menaquinone is an essential component of the M. tuberculosis ETC, as it functions to shuttle electrons through the ETC to produce the electrochemical gradient required for ATP production for the cell. We show that inhibitors of MenA, a component of the menaquinone biosynthetic pathway, are highly active against M. tuberculosis MenA inhibitors are bactericidal against M. tuberculosis under both replicating and nonreplicating conditions, with 10-fold higher bactericidal activity against nutrient-starved bacteria than against replicating cultures. MenA inhibitors have enhanced activity in combination with bedaquiline, clofazimine, and inhibitors of QcrB, a component of the cytochrome bc₁ oxidase. Together, these data support MenA as a viable target for drug treatment against M. tuberculosis MenA inhibitors not only kill M. tuberculosis in a variety of physiological states but also show enhanced activity in combination with ETC inhibitors in various stages of clinical trial testing.

Keywords: Mycobacterium tuberculosis; antitubercular; bactericidal; electron transport chain; menaquinone; respiration; synergy; tuberculosis.

FIG 1

Structures of MenA inhibitors used in this study.

FIG 2

MenA inhibitors are bactericidal against M. tuberculosis. M. tuberculosis H37RvLP was cultured in the presence of the indicated concentration of NM-4 under aerobic (individual replicates in A and C) or starvation (individual replicates in B and D) conditions. Samples were taken at the indicated times. The dotted lines represent the upper and lower limits of detection.

FIG 3

NM-4 causes synergistic killing with inhibitors of the ETC. Killing kinetics of NM-4 at approximately 1× MIC in combination with BDQ (A), CLO (B), or IMP (C) at subbactericidal concentrations were assessed under replicating conditions. Combinations were tested against H37RvLP. Data are the mean ± standard deviation of two independent experiments. The dotted lines represent the upper and lower limits of detection.