Synthesis, oxidation potential and anti-mycobacterial activity of isoniazid and analogues: insights into the molecular isoniazid activation mechanism

Tuberculosis (TB) is one of the leading causes of death due to infectious diseases. Among the specific drugs currently employed to treat tuberculosis, isoniazid (INH), a pro-drug, attracts a great interest. However, Mycobacterium tuberculosis (MTB) clinical isolates resistant to INH are significantly increasing thus compromising the efficiency of TB-treatment. Concerning the mechanism of resistance to INH, it is nowadays well established that it mainly results from mutations in the katG genes encoding for INH enzymatic activator. Recently, it was proposed that mutation in katG induces differences in heme or side chain redox potential so that the enzyme loses its ability to oxidize INH to radical species. In this work, we synthesized and selected a series of INH analogues for a study of their oxidation potential and evaluation of their anti-TB efficiency toward MTB wild-type and drug resistant strains. On the contrary to what was postulated, no correlation exists between the easier oxidation of a molecule and its anti-MTB activity toward resistant strains. Based on experimental data and theoretical calculations, we proposed an activation mechanism for INH and analogues based on a one-electron oxidation step of the hydrazyl function at the proximal nitrogen followed by a radical transposition to the distal nitrogen, which then induces a β-homolytic cleavage of the C(=O)-N bond to afford diazene and the isonicotinoyl radical species.