Novel Inhibitors of InhA Efficiently Kill Mycobacterium tuberculosis under Aerobic and Anaerobic Conditions

Abstract

Drug resistance in Mycobacterium tuberculosis has become a serious global health threat, which is now complicated by the emergence of extensively drug-resistant strains. New drugs that are active against drugresistant tuberculosis (TB) are needed. We chose to search for new inhibitors of the enoyl-acyl carrier protein (ACP) reductase InhA, the target of the first-line TB drug isoniazid (also known as isonicotinoic acid hydrazide [INH]). A subset of a chemical library, composed of 300 compounds inhibiting Plasmodium falciparum enoyl reductase, was tested against M. tuberculosis. Four compounds were found to inhibit M. tuberculosis growth with MICs ranging from 1 M to 10 M. Testing of these compounds against M. tuberculosis in vitro revealed that only two compounds (CD39 and CD117) were bactericidal against drug-susceptible and drug-resistant M. tuberculosis. These two compounds were also bactericidal against M. tuberculosis incubated under anaerobic conditions. Furthermore, CD39 and CD117 exhibited increased bactericidal activity when used in combination with INH or rifampin, but CD39 was shown to be toxic to eukaryotic cells. The compounds inhibit InhA as well the fatty acid synthase type I, and CD117 was found to also inhibit tuberculostearic acid synthesis. This study provides the TB drug development community with two chemical scaffolds that are suitable for structureactivity relationship study to improve on their cytotoxicities and bactericidal activities in vitro and in vivo.The fight against tuberculosis (TB), a disease caused by the bacillus Mycobacterium tuberculosis, is facing new challenges with the surge of multidrug-resistant (MDR) TB and the recent emergence of extensively drug-resistant (XDR) TB (8). TB strains resistant to the first-line anti-TB drugs (FLDs) isoniazid (also known as isonicotinoic acid hydrazide [INH]) and rifampin (RIF) are classified as MDR-TB, while XDR-TB is defined as MDR-TB resistant to any fluoroquinolone and one or more of the three injectable drugs (6). The need for novel TB drugs has spurred a new interest in TB drug development, and several new TB drugs are currently being tested in clinical trials (18). Nevertheless, expanding the pharmacopeia of active TB drugs remains an important goal, as M. tuberculosis has proven to be more than adept at acquiring drug resistance. One strategy to develop new drugs effective against MDR-and XDR-TB is to target essential functions that are not aims of the current anti-TB drug regimen. An alternative is to develop new drugs with novel requirements for inhibition of a bona fide target, with the goal of circumventing extant drug resistance.TB is resistant to most commonly used antibacterial agents due in part to its unusual cell wall structure. The mycobacterial cell wall contains unique long-chain (C 70 to C 90 ), ␣-alkyl, ␤-hydroxy fatty acids called mycolic acids. The synthesis of these fatty acids requires the coenzyme A (CoA)-dependent fatty acid synthase type I (FASI) and the acyl carrier protein (ACP)-dependent multienzyme fatty ...

The Mycobacterial Cell Envelope--Lipids

“…This type of approach has been used extensively to rationally optimize inhibitors of the FAS-II elongation cycle-in particular, analogs of thiolactomycin as inhibitors of KasA/ KasB and triclosan derivatives as inhibitors of InhA (Pan and Tonge 2012). The screening of inhibitors showing activity against the Plasmodium falciparum enoyl reductase and bhydroxyacyl-ACP dehydratase, FabZ, identified compounds that appear to target the corresponding FAS-II enzymes in Mtb and display an MIC against whole mycobacterial cells (Bhowruth et al 2008;Gratraud et al 2008;Vilchèze et al 2011). Pyridomycin, a cyclic depsipeptide isolated from Streptomyces with an MIC value of 0.4 mg/mL against Mtb, is now known to also target InhA (Haatkoorn et al 2012), as are novel structural classes of inhibitors in the lead-optimization phase reported by the Working Group on New TB Drugs (Table 1).…”

Diarylcoumarins inhibit mycolic acid biosynthesis and kill Mycobacterium tuberculosis by targeting FadD32

“…Because resistance to INH is on the rise worldwide, with an estimated 13% of tuberculosis cases exhibiting resistance to this important drug, its long-term utility may be limited. As a result, significant effort has been directed toward identifying novel inhibitors of InhA (12)(13)(14), including an effort by Glaxo-Smith Kline and the TB Alliance. Identification of a drug that targets the critical pathway of mycolic acid biosynthesis at a step that is distinct from InhA, thereby bypassing INH resistance, would have a major impact on treatment of MDR and XDR tuberculosis.…”

Boronic-aurone Derivatives as Anti-Tubercular Agents: Design, Synthesis and Biological Evaluation

“…Indeed, studies seem to bear out this rationale [20,21]. Therefore, to minimize the probability of a single mutation leading to resistance, we have argued elsewhere [4] that it may be beneficial to target multiple enzymes representing metabolic "hubs" which are essential for multiple biosynthetic pathways involved in mycobacterium cell wall synthesis.…”