Pyrazole and Triazole Derivatives as Mycobacterium tuberculosis UDP-Galactopyranose Inhibitors

Abstract

UDP-galactopyranose mutase (UGM) is an essential enzyme involved in the bacterial cell wall synthesis, and is not present in mammalian cells. Thus, UGM from Mycobacterium tuberculosis (Mtb) represents a novel and attractive drug target for developing antituberculosis agents. A pyrazole-based compound, MS208, was previously identified as a mixed inhibitor of MtbUGM which targets an allosteric site. To understand more about the structure activity relationship around the MS208 scaffold as a MtbUGM inhibitor, thirteen pyrazoles and triazole analogues were synthesized and tested against both MtbUGM and Mycobacterium tuberculosis in vitro. While the introduced structural modifications to MS208 did not improve the antituberculosis activity, most of the compounds showed MtbUGM inhibitory activity. Interestingly, the pyrazole derivative DA10 showed a competitive model for MtbUGM inhibition with improved Ki value of 51 ± 4 µM. However, the same compound did not inhibit the growth of Mycobacterium tuberculosis.

Keywords: UDP-galactopyranose mutase; antituberculosis; enzyme kinetics; inhibitors; pyrazoles; triazoles.

Figure 1

Reaction catalyzed by UGM by forming UDP-Galf from UDP-Galp, and the chemical structure of MS208.

Figure 2

Chemical structures of targeted compounds DA1–DA13. C-acylated pyrazole series included DA1–DA5, O-acylated pyrazole series included DA6–DA10 and triazolo series DA11–DA13.

Scheme 1

Pyrazolo analogues synthesis. Reactants and conditions: (a) absolute EtOH, reflux 3 h, (b) dry Ca(OH)₂, anhydrous 1,4-dioxane 80 °C, RCOCl (1.1 equivalent), 3M HCl, (c) TEA, chloroform, 0 °C then RCOCl, reflux, (d) DA8, anhydrous K₂CO₃, anhydrous 1,4-dioxane, reflux, 1M HCl, (e) DA3, K₂CO₃, methanol, water, reflux.

Scheme 2

Triazolo analogues synthesis. Reactants and conditions: (a) CH₂Cl₂, TEA, RT, (b) NaNO₂/conc. HCl, H₂O then NaN₃, (c) CH₂Cl₂, CuSO₄·H₂O, Na ascorbate, RT, (d) anhydrous THF(e) t-BuOH:H₂0 1:1, CuSO₄·H₂O, Na ascorbate, RT, (f) CH₂Cl₂, MnO₂, RT.

Figure 3

% Inhibition of MtbUGM at 15 µM of UDP-Galf using 60 µM of the test compounds. % Inhibition bar of MS208 in black, C-acyl pyrazole derivatives (DA1–DA5) in red, O-acyl pyrazole derivatives (DA6–DA10) in blue and triazolo derivatives (DA11–DA13) in green.

Figure 4

Lineweaver Burk plot of MtbUGM activity in presence of DA10 at three different concentrations (0, 200, and 400 µM).

Figure 5

Diagnostic plots areshowing competitive model of inhibition of MtbUGM by DA10: (a) Dixon plot has crossing lines in the fourth quadrant; (b) Cornish-Bowden plot is showing parallel lines.