Pyrazinamide inhibits the host enzyme PARP1 to reduce inflammation and accelerate bacterial clearance Stefanie Krug1, Pankaj Kumar1,2, Bong Gu Kang1, Ted M. Dawson1, Valina L. Dawson1, & William R. Bishai1* 1The Johns Hopkins School of Medicine, Baltimore, MD, USA, 2Jamia Hamdard University, New Delhi, New Delhi, India Background: Pyrazinamide (PZA) is an enigma: Despite low bactericidal activity and brief administration, PZA is an irreplaceable sterilizing component of most TB regimens. Interestingly, PZA also has immune-modulatory activity but the relevance and targets of PZA’s host effects are unclear. We hypothesized that PZA inhibits the host enzyme Poly(ADP-ribose) Polymerase 1 (PARP1) based on its structural similarity to the PARP inhibitor nicotinamide. PARP1 drives inflammation, and we recently discovered that PARP1 is robustly activated and enhances cytokine production in TB. Since destructive inflammation can antagonize mycobacterial killing, we further hypothesized that PZA might reduce lung damage and accelerate bacterial clearance by inhibiting PARP1. Methods: We evaluated PZA-PARP1 interactions by structural alignment and DSF; PZA’s effects on PARP activity in cells and mice; PZA’s efficacy in PARP1-/- mice; and pharmacologic PARP inhibition as a novel adjunct TB therapy by comparing disease progression, cytokines and inflammation in mice treated with PZA or the FDA-approved PARP inhibitor talazoparib. Results: Here, we identify PARP1 as the first host target of PZA and show that standard PZA treatment reverses PARP activity in TB-infected mice to uninfected levels. Like PZA, adjunctive PARP inhibition with talazoparib reduces IFNγ, IL-1ß, TNFα, IL-6 and MCP-1 levels and lung pathology in a CFU-independent manner. Remarkably, PZA-treated PARP1-/- mice have 0.5 log10 more bacteria and higher cytokine levels than WT mice, indicating that PARP inhibition contributes to PZA’s activity. Conclusions: We identified PZA as a novel PARP inhibitor and demonstrate for the first time that host effects are integral to PZA’s mode of action. PARP inhibition presents an exciting new avenue for TB therapy with the potential of greatly improving patient outcome.