Isoniazid metal complex reactivity and insights for a novel anti-tuberculosis drug design

For over a decade, tuberculosis (TB) has been the leading cause of death among infectious diseases. Since the 1950s, isoniazid has been used as a front-line drug in the treatment of TB; however, resistant TB strains have limited its use. The major route of isoniazid resistance relies on KatG enzyme disruption, which does not promote an electron transfer reaction. Here, we investigated the reactivity of isoniazid metal complexes as prototypes for novel self-activating metallodrugs against TB with the aim to overcome resistance. Reactivity studies were conducted with hydrogen peroxide, hexacyanoferrate(III), and aquopentacyanoferrate( III). The latter species showed a preference for the inner-sphere electron transfer reaction pathway. Additionally, electron transfer reaction performed with either free isoniazid or (isoniazid) pentacyanoferrate(II) complex resulted in similar oxidized isoniazid derivatives as observed when the KatG enzyme was used. However, upon metal coordination, a significant enhancement in the formation of isonicotinic acid was observed compared with that of isonicotinamide. These results suggest that the pathway of a carbonyl-centered radical might be favored upon coordination to the Fe(II) owing to the p-back-bonding effect promoted by this metal center; therefore, the isoniazid metal complex could serve as a potential metallodrug. Enzymatic inhibition assays conducted with InhA showed that the cyanoferrate moiety is not the major player involved in this inhibition but the presence of isoniazid is required in this process. Other isoniazid metal complexes, [Ru(CN)(5)(izd)](3-) and [Ru(NH3)(5)(izd)](2+) (where izd is isoniazid), were also unable to inhibit InhA, supporting our proposed self-activating mechanism of action. We propose that isoniazid reactivity can be rationally modulated by metal coordination chemistry, leading to the development of novel anti-TB metallodrugs.