Identification of 2-aminothiazole-4-carboxylate derivatives active against Mycobacterium tuberculosis H37Rv and the beta-ketoacyl-ACP synthase mtFabH

Abstract

Background: Tuberculosis (TB) is a disease which kills two million people every year and infects approximately over one-third of the world's population. The difficulty in managing tuberculosis is the prolonged treatment duration, the emergence of drug resistance and co-infection with HIV/AIDS. Tuberculosis control requires new drugs that act at novel drug targets to help combat resistant forms of Mycobacterium tuberculosis and reduce treatment duration.

Methodology/principal findings: Our approach was to modify the naturally occurring and synthetically challenging antibiotic thiolactomycin (TLM) to the more tractable 2-aminothiazole-4-carboxylate scaffold to generate compounds that mimic TLM's novel mode of action. We report here the identification of a series of compounds possessing excellent activity against M. tuberculosis H(37)R(v) and, dissociatively, against the beta-ketoacyl synthase enzyme mtFabH which is targeted by TLM. Specifically, methyl 2-amino-5-benzylthiazole-4-carboxylate was found to inhibit M. tuberculosis H(37)R(v) with an MIC of 0.06 microg/ml (240 nM), but showed no activity against mtFabH, whereas methyl 2-(2-bromoacetamido)-5-(3-chlorophenyl)thiazole-4-carboxylate inhibited mtFabH with an IC(50) of 0.95+/-0.05 microg/ml (2.43+/-0.13 microM) but was not active against the whole cell organism.

Conclusions/significance: These findings clearly identify the 2-aminothiazole-4-carboxylate scaffold as a promising new template towards the discovery of a new class of anti-tubercular agents.

Figure 1. The chemical structures of thiolactomycin 1, its analogue 4, and inhibitors 2 and 3.

Figure 2. The modeling studies of the 2-aminothiazole-4-carboxylate analogues with mtFabH.

(A) The hypothetical template of the 2-aminothiazole-4-carboxylates for mtFabH inhibitor development. This illustrates the key H-bonding interactions with the catalytic triad amino acid residues. (B) The binding pose of methyl 2-amino-5-methylthiazole-4-carboxylate in the active site of mtFabH showing the NH₂ group proximal to His 244 and directed towards the lateral channel, with the 5-methyl group directed towards the longitudinal channel. (C) The binding pose of methyl 2-(2-bromoacetamido)-5-(3-chlorophenyl)thiazole-4-carboxylate with the bromomethylene portion in the vicinity of the Cys112 thiol group.

Figure 3. Synthesis of the 2-aminothiazole-4-carboxylate analogues.

Figure 4. The in vitro activity and molecular properties of the 2-aminothiazole-4-carboxylates.

^{a, b} Compounds regarded as not active (N/A) if no inhibition is observed at 200 µg/ml. ^c FAS-I/II assay conducted at 200 µg/ml and compounds regarded as not active is <50% inhibition observed. ^d AlogP and logD calculated using Pipeline Pilot (SciTegic) software. ^eFrom reference . ^fFrom reference .

Figure 5. The cytotoxic effects of the compounds against HS-27 human fibroblast cells.

The data indicates that only the carboxyl esters of the bromoacetamido analogues were toxic.