Abstract ED12-1: Pol-theta inhibitors in HRD tumors
Authors: d’Andrea Alan 
Published: March 2023

Abstract
Deficiency of homologous recombination (HR)-mediated DNA repair occurs through genetic or epigenetic inactivation of the BRCA1 and BRCA2 (BRCA1/2) genes, and it plays a role in the initiation and progression of many tumor types. HR-deficiency also provides unique opportunities for targeted therapy, as exemplified by the extreme sensitivity of BRCA1/2-mutated tumors to poly (ADP-ribose) polymerase inhibitors (PARPi). In the last decade, several PARP inhibitors have been approved for clinical use. At first, PARPi were solely used in combination with chemotherapeutics, and platinum sensitivity was used as a surrogate marker of HR-deficiency for enrolling patients for treatment. The promising clinical response of patients with germline BRCA1/2-mutations prompted the use of PARPi for patients with somatic BRCA1/2 mutations as well. In addition, it also opened the possibility for extended use of PARPi for the treatment of various types of ovarian, breast, pancreatic and prostate tumors with HR defects. PARPi efficacy is currently being evaluated in different clinical settings such as first line chemotherapy, neoadjuvant therapy, and combination therapy with chemo- or immuno-therapies.
Despite initial sensitivity to PARPi, resistance to these drugs is emerging as the major obstacle to its clinical effectiveness in patients with HR-deficient tumors. PARPi resistance can result from several independent mechanisms, often leading to the restoration of Homologous Recombination and/or Replication Fork stabilization. In addition, resistance to PARPi often correlates with platinum resistance, which remains the backbone therapy for most BRCA1/2-mutated tumors. The absence of alternative therapeutic options for patients with tumors with innate or acquired resistance underlines the urgency to develop additional therapeutics. While several mechanisms of PARPi resistance have been described, an effective method for overcoming such resistance is still lacking.
DNA polymerase theta (POLθ or POLQ) recently emerged as a new promising drug target for the treatment of HR-deficient tumors. POLθ expression is particularly high in subtypes of breast and ovarian tumors with defects in HR, where it mediates backup DNA double-strand breaks (DSBs) repair, thus compensating for the loss of HR. As a result, POLθ is synthetic lethal with HR, and POLθ inhibition in HR-deficient tumors induces cell death. In addition, POLθ inhibition synergizes with PARPi in the killing of HR-deficient tumors.
Synthetic lethality between HR-deficiency and POLθ inhibition hinges on several functions of POLθ. The enzyme maintains genomic stability and prevents tumorigenesis. It is a crucial enzyme in the mutagenic microhomology-mediated end joining (MMEJ) repair of DSBs, a pathway that plays critical role in genomic stability. Inhibiting PARP1, a key enzyme in MMEJ, prevents POLθ recruitment to sites of laser micro-irradiation. Since POLθ inhibition and PARPi have additive effects on HR-deficient cells, these data suggest that POLθ also functions outside the PARP-mediated MMEJ pathway and in pathways that are critical to the survival of HR-deficient cells.
POLθ is a large protein that contains 3 domains and is structurally and functionally distinct from other polymerases. The N-term contains a helicase-like ATPase domain that can unwind several types of DNA structures, while the central domain binds RAD51, displaces RPA proteins from resected DSBs, and antagonizes HR repair in an ATP-hydrolysis dependent manner. Finally, the C-term contains a nuclease domain which trims DNA ends and a polymerase domain that fills in nucleotides during MMEJ. Both the ATPase and polymerase domains are required for MMEJ repair.
POLθ has several functions that preserve genomic stability, and POLθ-mediated MMEJ repair can compensate for the loss of HR. It remains unclear which of the many functions of POLθ underlies the synthetic lethality with HR. Nevertheless, POLθ exhibits unique features of druggability, providing a strong rationale for developing POLθ inhibitors.
We have recently performed a small-molecule screen for inhibitors of POLθ ATPase activity and identified the antibiotic novobiocin (NVB) as a specific and potent inhibitor of human POLθ. NVB binds to purified POLθ protein, prevents its recruitment to DNA damage, and inhibits MMEJ repair. Importantly, we have shown that NVB selectively kills HR-deficient (BRCA1- and BRCA2-deficient) cells over wild-type cells and potentiates the cytotoxic effect of PARPi in HR-deficient tumor cells in vitro and in vivo. Moreover, NVB kills HR-deficient, PARPi-resistant tumor cells. These results suggest that NVB can be used alone or in combination with PARPi for treating HR-deficient tumors, even after they have acquired PARPi resistance. Accordingly, clinical trials have now been initiated for the use of NVB in the management of these tumors.
Citation Format: Alan d’Andrea. Pol-theta inhibitors in HRD tumors [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr ED12-1.