Polymerase Theta Inhibition Kills Homologous Recombination Deficient Tumors

PARP inhibitors (PARPi) have become a new line of therapy for Homologous Recombination (HR)-deficient cancers. However, resistance to PARPi has emerged as a major clinical problem. DNA polymerase theta (POL{theta}) is synthetic lethal with HR and a druggable target in HR-deficient cancers. Here, we identified the antibiotic Novobiocin (NVB) as a specific POL{theta} inhibitor that selectively kills HR-deficient tumor cells in vitro and in vivo. NVB directly binds to the POL{theta} ATPase domain, inhibits its ATPase activity, and phenocopies POL{theta} depletion. BRCA-deficient tumor cells and those with acquired PARPi resistance are sensitive to NVB in vitro and in vivo. Increased POL{theta} expression levels predict NVB sensitivity. The mechanism of NVB-mediated cell death in PARPi resistant cells is the accumulation of toxic RAD51 foci, which also provides a pharmacodynamic biomarker for NVB response. Our results demonstrate that NVB may be useful alone or in combination with PARPi in treating HR-deficient tumors, including those with acquired PARPi resistance. One Sentence SummaryWe identified Novobiocin as a specific POL{theta} inhibitor that selectively kills naive and PARPi resistance HR-deficient tumors in vitro and in vivo.