Tuberculosis is an infectious disease which is caused by Mycobacterium tuberculosis, and remains the leading cause of death in undeveloped countries. The first-line drug for antituberculotic treatment is isoniazid. It disrupts the biosynthesis of mycolic acids by inhibiting the enzyme InhA. Inhibition of this enzyme causes mycobacterium cell death. Isoniazid is a prodrug that needs oxidative activation with mycobacterium catalase-peroxidase KatG. Mono-resistance against isoniazid is the most common resistance against first-line antituberculotic agent. Thus, the direct inhibitors of InhA, that do not need activation by KatG, are very promising to combat resistant M. tuberculosis strains.
We tried to synthesize four new potential direct inhibitors of InhA enzyme (compounds 8, 12, 17 in 26). Our synthesis was based on tetrahydropyran inhibitor molecule structure (compound I), in which we substituted the tetrahydropyran part with the more rigid 1,2,3,4-tetrahydropyrrolo[1,2-a]pyrazine. With the introduction of different substituents on to 1,2,3,4-tetrahydropyrrolo[1,2-a]pyrazine we researched the chemical space of InhA active site.
The IC50 values of synthesized compounds were determined on the Faculty of Pharmacy, University of Ljubljana, on an isolated InhA enzyme. These values are 70 µM and 20 µM for compounds 8 and 26, respectively. Compound 17 degraded before we managed to test its activity. HRMS analysis of synthesized compound confirmed that unsubstituted pyrrole is unstable and is oxidized when in contact with oxygen. Electron-withdrawing substituents should be introduced to the pyrrole ring in order to increase chemical stability. The attempt to introduce bromine to the pyrrole ring was however not successful. We presume the product of the bromination would not be active, however it should be more chemically stable.