Indoleamides are active against drug-resistant Mycobacterium tuberculosis

Abstract

Responsible for nearly two million deaths each year, the infectious disease tuberculosis remains a serious global health challenge. The emergence of multidrug- and extensively drug-resistant strains of Mycobacterium tuberculosis confounds control efforts, and new drugs with novel molecular targets are desperately needed. Here we describe lead compounds, the indoleamides, with potent activity against both drug-susceptible and drug-resistant strains of M. tuberculosis by targeting the mycolic acid transporter MmpL3. We identify a single mutation in mmpL3, which confers high resistance to the indoleamide class while remaining susceptible to currently used first- and second-line tuberculosis drugs, indicating a lack of cross-resistance. Importantly, an indoleamide derivative exhibits dose-dependent antimycobacterial activity when orally administered to M. tuberculosis-infected mice. The bioavailability of the indoleamides, combined with their ability to kill tubercle bacilli, indicates great potential for translational developments of this structure class for the treatment of drug-resistant tuberculosis.

Figure 1. Indoleamide compounds are active in vitro against Mycobacterium tuberculosis

(a) Structure of compound 1, the initial hit indoleamide. (b) Structures of compounds 2 and 3, derivatives of compound 1. (c) In vitro kill curve of M. tuberculosis exposed to 4X and 16X MIC of the indoleamide derivative compounds 2 and 3. Data are presented as mean±S.E.M. (n=3). nNH, number of hydrogen bond donors; nON, number of hydrogen bond acceptors; MW, molecular weight; TPSA, topological polar surface area; nRot. bond, number of rotatable bonds; MIC, minimum inhibitory concentration.^aCalculated using molinspiration online service;^bCalculated using ChemDraw Ultra 13.0, CambridgeSoft.

Figure 2. MmpL3 is a validated target in Mycobacterium tuberculosis

(a) Illustration of the topology of the MmpL3 mycolic acid transporter protein in the M. tuberculosis inner membrane. Colored circles represent the locations of amino acid changes associated with resistance to compounds known to target this protein: the diamide SQ109, the pyrrole derivative BM212, and the urea derivative AU1235. (b) Structures of BM212, AU1235 and SQ109.

Figure 3. The indoleamide compound 3 is active against Mycobacterium tuberculosis in a dose-dependent manner during in vivo infection of BALB/c mice

Lung CFU counts were assessed 4 weeks after starting daily oral administration of compound 3. Each dot represents CFUs from the lungs of an individual mouse, and the bars indicate mean±S.D. CFU counts in each group (n=5 for treated groups and n=4 for untreated control because of one accidental death prematurely). Statistical significance was assessed using the one-way ANOVA with Tukey’s multiple comparison test. CFU, colony forming unit.

Figure 4. Pharmacokinetic analysis of compound 3 in female BALB/c mice

(a) Concentration in plasma and (b) concentration in lung following a single 100 mg/kg dose administered by oral gavage. Data are presented as mean±S.E.M. (n=3).