Isoniazid Scavenges Hypochlorous Acid and Impairs Myeloperoxidase- and Neutrophil-Mediated Mycobactericidal Activity

Isoniazid (INH) is a cornerstone antimicrobial for tuberculosis treatment, yet its potential immunomodulatory interactions with host innate immune effectors remain poorly characterized. Myeloperoxidase (MPO), an abundant neutrophil heme peroxidase involved in pathogen clearance, oxidizes INH into radical intermediates. This study investigated the biochemical and immunopharmacological effects of INH on MPO-dependent oxidative bacterial killing using Mycobacterium smegmatis mc²155 as a model. Using competitive kinetics with taurine, we demonstrated that INH reacts rapidly with hypochlorous acid (HOCl), with a second-order rate constant of 3.7 x 10⁷ M^-1 s^-1. This reaction markedly impaired HOCl-mediated bactericidal activity, while INH retained most of its antimicrobial activity. In purified enzymatic assays, INH significantly reduced MPO/H₂O₂-mediated bacterial killing, restoring bacterial survival by up to approximately 70%. Furthermore, in human neutrophil-mediated killing assays, INH increased mycobacterial survival, with effects comparable to those observed with inhibitors of MPO and NADPH oxidase. INH did not significantly inhibit superoxide (O₂•^-) generation by activated neutrophils, indicating that its effects occur downstream of the primary NADPH oxidase-mediated oxidative burst. Together, these findings demonstrate that INH interferes with the MPO/HOCl antimicrobial pathway through both MPO inhibition and HOCl scavenging, revealing a previously underrecognized interaction between this first-line antimycobacterial drug and neutrophil oxidative antimicrobial mechanisms.