[Fe(CN)5(isoniazid)]3 −: An iron isoniazid complex with redox behavior implicated in tuberculosis therapy

Tuberculosis has re-emerged as a worldwide threat, which has motivated the development of new drugs. The antituberculosis complex Na3[Fe(CN)5(isoniazid)] (IQG607) in particular is of interest on account of its ability to overcome resistance. IQG607 has the potential for redox-mediated-activation, in which an acylpyridine (isonicotinoyl) radical could be generated without assistance from the mycobacterial KatG enzyme. Here, we have investigated the reactivity of IQG607 toward hydrogen peroxide and superoxide, well-known intracellular oxidizing agents that could play a key role in the redox-mediated-activation of this compound. HPLC, NMR and electronic spectroscopy studies showed a very fast oxidation rate for bound isoniazid, over 460-fold faster than free isoniazid oxidation. A series of EPR spin traps were used for detection of isonicotinoyl and derived radicals bound to iron. This is the first report for an isonicotinoyl radical bound to a metal complex, supported by 14N and 1H hyperfine splittings for the POBN and PBN trapped radicals. POBN and PBN exhibited average hyperfine coupling constants of aN = 15.6, aH = 2.8 and aN = 15.4, aH = 4.7, respectively, which are in close agreement to the isonicotinoyl radical. Radical generation is thought to play a major role in the mechanism of action of isoniazid and this work provides strong evidence for its production within IQG607, which, along with biological and chemical oxidation data, support a redox-mediated activation mechanism. More generally the concept of redox activation of metallo prodrugs could be applied more widely for the design of therapeutic agents with novel mechanisms of action.

Graphic abstractNa3[Fe(CN)5(isoniazid)] is an iron(II) isoniazid complex that shows promising biological activity against Mycobacterium tuberculosis. Here, we show that it can be oxidized rapidly and efficiently by biological oxidizing agents, while generating metal-bound isoniazid-based radicals. These data support a redox-mediated mechanism of activation and open up new opportunities for therapy.Figure optionsDownload full-size imageDownload as PowerPoint slide