PARP Inhibitors Research Articles

18F]F-Poly(ADP-Ribose) Polymerase Inhibitor Radiotracers for Imaging PARP Expression and Their Potential Clinical Applications in Oncology.

Including poly(ADP-ribose) polymerase (PARP) inhibitors in managing patients with inoperable tumors has significantly improved outcomes. The PARP inhibitors hamper single-strand deoxyribonucleic acid (DNA) repair by trapping poly(ADP-ribose)polymerase (PARP) at sites of DNA damage, forming a non-functional "PARP enzyme-inhibitor complex" leading to cell cytotoxicity. The effect is more pronounced in the presence of PARP upregulation and homologous recombination (HR) deficiencies such as breast cancer-associated gene (BRCA1/2). Hence, identifying HR-deficiencies by genomic analysis-for instance, BRCA1/2 used in triple-negative breast cancer-should be a part of the selection process for PARP inhibitor therapy. Published data suggest BRCA1/2 germline mutations do not consistently predict favorable responses to PARP inhibitors, suggesting that other factors beyond tumor mutation status may be at play. A variety of factors, including tumor heterogeneity in PARP expression and intrinsic and/or acquired resistance to PARP inhibitors, may be contributing factors. This justifies the use of an additional tool for appropriate patient selection, which is noninvasive, and capable of assessing whole-body in vivo PARP expression and evaluating PARP inhibitor pharmacokinetics as complementary to the currently available BRCA1/2 analysis. In this review, we discuss [18F]Fluorine PARP inhibitor radiotracers and their potential in the imaging of PARP expression and PARP inhibitor pharmacokinetics. To provide context we also briefly discuss possible causes of PARP inhibitor resistance or ineffectiveness. The discussion focuses on TNBC, which is a tumor type where PARP inhibitors are used as part of the standard-of-care treatment strategy.

Polymerase theta is a synthetic lethal target for killing Epstein-Barr virus lymphomas.

Treatment options for Epstein-Barr virus (EBV)-cancers are limited, underscoring the need for new therapeutic approaches. We have previously shown that EBV-transformed cells and cancers lack homologous recombination (HR) repair, a prominent error-free pathway that repairs double-stranded DNA breaks; instead, EBV-transformed cells demonstrate genome-wide scars of the error-prone microhomology-mediated end joining (MMEJ) repair pathway. This suggests that EBV-cancers are vulnerable to synthetic lethal therapeutic approaches that target MMEJ repair. Indeed, we have previously found that targeting PARP, an enzyme that contributes to MMEJ, results in the death of EBV-lymphoma cells. With the emergence of clinical resistance to PARP inhibitors and the recent discovery of inhibitors of Polymerase theta (POLθ), the polymerase essential for MMEJ, we investigated the role of POLθ in EBV-lymphoma cells. We report that EBV-transformed cell lines, EBV-lymphoma cell lines, and EBV-lymphomas in AIDS patients demonstrate greater abundance of POLθ, driven by the EBV protein EBNA1, compared to EBV-uninfected primary lymphocytes and EBV-negative lymphomas from AIDS patients (a group that also abundantly expresses POLθ). We also find POLθ enriched at cellular DNA replication forks and exposure to the POLθ inhibitor Novobiocin impedes replication fork progress, impairs MMEJ-mediated repair of DNA double-stranded breaks, and kills EBV-lymphoma cells. Notably, cell killing is not due to Novobiocin-induced activation of the lytic/replicative phase of EBV. These findings support a role for POLθ not just in DNA repair but also DNA replication and as a therapeutic target in EBV-lymphomas and potentially other EBV-cancers as EBNA1 is expressed in all EBV-cancers.IMPORTANCEEpstein-Barr virus (EBV) contributes to ~2% of the global cancer burden. With a recent estimate of >200,000 deaths a year, identifying molecular vulnerabilities will be key to the management of these frequently aggressive and treatment-resistant cancers. Building on our earlier work demonstrating reliance of EBV-cancers on microhomology-mediated end-joining repair, we now report that EBV lymphomas and transformed B cell lines abundantly express the MMEJ enzyme POLθ that likely protects cellular replication forks and repairs replication-related cellular DNA breaks. Importantly also, we show that a newly identified POLθ inhibitor kills EBV-cancer cells, revealing a novel strategy to block DNA replication and repair of these aggressive cancers.

CHK1 inhibitor induced PARylation by targeting PARG causes excessive replication and metabolic stress and overcomes chemoresistance in ovarian cancer

Chemoresistance contributes to the majority of deaths in women with ovarian cancer (OC). Altered DNA repair and metabolic signaling is implicated in mediating therapeutic resistance. DNA damage checkpoint kinase 1 (CHK1) integrates cell cycle and DNA repair in replicating cells, and its inhibition causes replication stress, repair deficiency and cell cycle dysregulation. We observed elevated Poly-ADP-ribosylation (PAR) of proteins (PARylation) and subsequent decrease in cellular NAD+ levels in OC cells treated with the CHK1 inhibitor prexasertib, indicating activation of NAD+ dependent DNA repair enzymes poly-ADP-ribose polymerases (PARP1/2). While multiple PARP inhibitors are in clinical use in treating OC, tumor resistance to these drugs is highly imminent. We reasoned that inhibition of dePARylation by targeting Poly (ADP-ribose) glycohydrolase (PARG) would disrupt metabolic and DNA repair crosstalk to overcome chemoresistance. Although PARG inhibition (PARGi) trapped PARylation of the proteins and activated CHK1, it did not cause any significant OC cell death. However, OC cells deficient in CHK1 were hypersensitive to PARGi, suggesting a role for metabolic and DNA repair crosstalk in protection of OC cells. Correspondingly, OC cells treated with a combination of CHK1 and PARG inhibitors exhibited excessive replication stress-mediated DNA lesions, cell cycle dysregulation, and mitotic catastrophe compared to individual drugs. Interestingly, increased PARylation observed in combination treatment resulted in depletion of NAD+ levels. These decreased NAD+ levels were also paralleled with reduced aldehyde dehydrogenase (ALDH) activity, which requires NAD+ to maintain cancer stem cells. Furthermore, prexasertib and PARGi combinations exhibited synergistic cell death in OC cells, including an isogenic chemoresistant cell line and 3D organoid models of primary patient-derived OC cell lines. Collectively, our data highlight a novel crosstalk between metabolism and DNA repair involving replication stress and NAD+-dependent PARylation, and suggest a novel combination therapy of CHK1 and PARG inhibitors to overcome chemoresistance in OC.

Abstract B005: Regulation of replication-induced PARP1/PARP2 activation by base excision repair: Implications for PARP and PARG inhibitor resistance

Abstract Acquired poly (ADP-ribose) polymerases (PARP)-inhibitor (PARPi) and/or poly (ADP-ribose) glycohydrolase (PARG)-inhibitor (PARGi) resistance in BRCA1/2-defective tumors is common, emphasizing the need for identifying new targets. Protein poly ADP-ribosylation (PARylation) is a posttranslational modification of proteins catalyzed by PARP to form poly-(APD-ribose) (PAR) chains on itself and chromatin-associated proteins in response to DNA damage. Our study reveals that PARylation occurs in a replication-, PARP1-, and PARP2-dependent manner even in undamaged cells. Using proximity labeling, specifically Split-TurboID, we found that replication-dependent PARylation recruits the base excision repair (BER) factors XRCC1, POLB, APTX, and LIG3. These factors, in turn, suppress further PARylation. The BER factors XRCC1, POLB, APTX, or LIG3 promote PARPi and PARGi resistance as loss of these replication-associated BER (R-BER) factors re-sensitizes cancer cells to these inhibitors. Of interest, BRCA1 and BRCA2 localize with R-BER factors in undamaged cells. Lastly, XRCC1, POLB, APTX, and LIG3 depletion activate the S-phase checkpoint kinase CHK1, and targeting either this kinase or ATR under these conditions overcomes PARGi resistance in glioblastoma and ovarian cancer cells. In conclusion, our data highlight the crucial role of R-BER in protecting the cell during S-phase and propose a potential intervention strategy for resistant tumors. Citation Format: Md Ibrahim, Md Maruf Khan, Wynand P. Roos, Rasha Q. Al-Rahaleh, Faisal Hayat, Marie E. Migaud, Robert W. Sobol. Regulation of replication-induced PARP1/PARP2 activation by base excision repair: Implications for PARP and PARG inhibitor resistance [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Expanding and Translating Cancer Synthetic Vulnerabilities; 2024 Jun 10-13; Montreal, Quebec, Canada. Philadelphia (PA): AACR; Mol Cancer Ther 2024;23(6 Suppl):Abstract nr B005.

Abstract PR002: Nimbolide targets RNF114 to induce the trapping of PARP1 and synthetic lethality in BRCA-mutated cancer

Abstract Despite the tremendous success of PARP1 inhibitors (PARPi) for the treatment of BRCA mut tumors, intrinsic and acquired resistance to PARPi remains a critical challenge in the clinic. This is, at least in part, due to the incomplete understanding of the mechanism of action (MoA) of PARPi. Recent studies from both our lab and others have pointed to PARP1 trapping as a key determinant of the anticancer effects of PARP1 inhibitors (PARPi). However, the molecular underpinnings of PARP1 trapping are poorly understood. Here we performed a proteomics-based chromatin relocalization screen, and identified RNF114, as a PARylation-dependent, E3 ubiquitin ligase involved in DNA damage response. Upon sensing genotoxicity, RNF114 was recruited, in a PAR-dependent manner, to DNA lesions, where it targeted PARP1 for degradation. The blockade of this pathway interfered with the removal of PARP1 from DNA lesions, leading to profound PARP1 trapping. A unique feature of this pathway is its druggability. We showed that a natural product, nimbolide, inhibited the E3 ligase activity of RNF114 and thus caused PARP1 trapping. However, unlike conventional PARPi, nimbolide treatment induced the trapping of both PARP1 and PARylation-dependent DNA repair factors. The unique mechaism of action of nimbolide translated into its superior cytotoxicity against BRCA mut cancer cells, compared to regular PARPi. It is also able to potentially overcome intrinsic and acquired resistance to PARPi, both in vitro and in vivo. In collaboration with Dr. Tian Qin, we recently performed the first total synthesis of nimbolide via a pharmacophore-directed, late-stage coupling strategy. Using this method, we have obtained a large panel of novel nimbolide analogs with improved activity and drug-like properties. We have further garnered key SAR (structure-activity relationship) insights into nimbolide. These results point to the exciting possibility of translating nimbolide analogs into novel PARP1-targeting agents for the treatment of BRCA mut cancers. Citation Format: Peng Li, Yuanli Zhen, Chiho Kim, Zhengshuai Liu, Jianwei Hao, Heping Deng, Hejun Deng, Min Zhou, Xu-Dong Wang, Tian Qin, Yonghao Yu. Nimbolide targets RNF114 to induce the trapping of PARP1 and synthetic lethality in BRCA-mutated cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Expanding and Translating Cancer Synthetic Vulnerabilities; 2024 Jun 10-13; Montreal, Quebec, Canada. Philadelphia (PA): AACR; Mol Cancer Ther 2024;23(6 Suppl):Abstract nr PR002.

Abstract PR005: CX-5461 sensitizes DNA damage repair-proficient castrate-resistant prostate cancer to PARP inhibition

Abstract Monotherapy with PARP inhibitors is effective for the subset of castrate-resistant prostate cancer (CRPC) with defects in homologous recombination (HR) DNA repair. New treatments are required for the remaining tumors, and an emerging strategy is to combine PARP inhibitors with other therapies that induce DNA damage. Here we tested whether PARP inhibitors are effective for HR-proficient CRPC, including androgen receptor (AR)-null tumors, when used in combination with CX-5461, a small molecule that inhibits RNA polymerase I transcription and activates the DNA damage response, and has antitumor activity in early phase I trials. The combination of CX-5461 and talazoparib significantly decreased in vivo growth of patient-derived xenografts of HR-proficient CRPC, including AR-positive, AR-null, and neuroendocrine tumors. CX-5461 and talazoparib synergistically inhibited the growth of organoids and cell lines, and significantly increased the levels of DNA damage. Decreased tumor growth after combination therapy was maintained for 2 weeks without treatment, significantly increasing host survival. Therefore, combination treatment with CX-5461 and talazoparib is effective for HR-proficient tumors that are not suitable for monotherapy with PARP inhibitors, including AR-null CRPC. This expands the spectrum of CRPC that is sensitive to PARP inhibition. We have recently initiated a clinical trial combining CX-5461 with talazoparib in mCRPC patients (REPAIR trial, NCT05425862) to explore this combination in both HR proficient and deficient settings. Citation Format: Mitchell G. Lawrence, Laura H. Porter, Nicholas Choo, Elaine Sanij, Renea Taylor, Richard Pearson, Kaylene J. Simpson, Ross D. Hannan, Shahneen Sandhu, Luc Furic. CX-5461 sensitizes DNA damage repair-proficient castrate-resistant prostate cancer to PARP inhibition [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Expanding and Translating Cancer Synthetic Vulnerabilities; 2024 Jun 10-13; Montreal, Quebec, Canada. Philadelphia (PA): AACR; Mol Cancer Ther 2024;23(6 Suppl):Abstract nr PR005.

Abstract IA009: Single strand DNA GAP accumulation as a functional biomarker for DNA Repair Inhibitors

Abstract Deficiency of homologous recombination (HR)-mediated DNA repair occurs through genetic or epigenetic inactivation of the BRCA1 and BRCA2 (BRCA1/2) genes. HR-deficiency also provides unique opportunities for targeted therapy, through the mechanism of synthetic lethality, as exemplified by the hypersensitivity of BRCA1/2-mutated tumors to PARP inhibitors (PARPi). To date, several PARP inhibitors have been approved for clinical use. 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, these results extended the use of PARPi for various types of ovarian, breast, pancreatic, and prostate tumors with HR defects. PARPi resistance is emerging as the major obstacle to clinical effectiveness in patients with HR-deficient tumors. PARPi resistance results from several independent mechanisms, leading to the restoration of Homologous Recombination and/or Replication Fork stabilization. The absence of alternative options for patients with tumors with innate or acquired resistance underlines the urgency to develop additional therapeutics. More recent studies have identified new synthetic lethal opportunities for BRCA1/2 deficient tumors. Knockout of the genes for POLQ (DNA polymerase theta) or for USP1 (Ubiquitin Specific Protease 1) results in killing of these tumors, and inhibitors of these enzymes are emerging as promising agents for HR-deficient tumors. POLQ and USP1 expression is particularly high in subtypes of breast and ovarian tumors with defects in HR. As a result, POLQi or USP1i in HR-deficient tumors induces cell death. POLQi or USP1i can synergize with PARPi in killing HR-deficient tumors or PARPi-resistant tumors. BRCA1/2 deficient tumors have an increase in single strand DNA gaps (ssGAPs) near replication forks. Treatment with PARPi results in an increase in the size and number for these ssGAPs, ultimately leading to DSBs and cell death. Interestingly, PARPi resistance results, at least in part, from the ability of resistant cells to close the ssGAPs through enhanced POLQ and USP1 activity, thereby providing a rationale for the combination of PARP1, POLQi, and USP1 in HR-deficient cancers. USP1 inhibition was synergistic with PARP and POLQ inhibition in BRCA1-mutant cells, with enhanced ssDNA gap accumulation. Moreover, this synergy was observed in a set of patient-derived ovarian cancer organoids (PDOs), thus confirming the sensitivity of BRCA1-deficient cells to inhibition by these agents. The accumulation of ssDNA gaps after treatment with a USP1i, PARP, or POLQi correlated with the sensitivity to these drugs in all models tested. Ovarian cancer PDOs provide a powerful tool for detecting drug synergy and for rapid in vitro sensitivity testing. The detection of ssDNA gap accumulation may be a useful predictive biomarker for response to DNA Repair inhibitors as monotherapy or in combination in ongoing clinical trials. Citation Format: Alan D. D'Andrea. Single strand DNA GAP accumulation as a functional biomarker for DNA Repair Inhibitors [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Expanding and Translating Cancer Synthetic Vulnerabilities; 2024 Jun 10-13; Montreal, Quebec, Canada. Philadelphia (PA): AACR; Mol Cancer Ther 2024;23(6 Suppl):Abstract nr IA009.

Abstract IA008: Exploiting tumor selective vulnerabilities with structure based drug design

Abstract It is well appreciated that the integrity of both pluripotent and terminally differentiated cellular genomes is under constant pressure from damage provoked by cell-intrinsic and environmental factors. As such, defects in the cellular machinery preserving genomic integrity lead to the accumulation of variants that can promote cancer, but they also confer tumor-cell specific vulnerabilities that present synthetic-lethal therapeutic opportunities. The deepening mechanistic appreciation of tumor-selective pressure on DNA replication-fork fidelity, namely oncogene-induced replication stress, has provoked great interest in the prosecution of drug targets within the cellular machinery that otherwise supports replication fork stability. Poly (ADP-Ribose) glycohydrolase (PARG) is an enzyme with a unique role in the resolution of DNA damage repair and DNA replication fork restart through hydrolysis of Poly (ADP-ribose) (PAR) chains. IDE161 is a potent, selective small molecule inhibitor of PARG that is being developed as an anti-cancer therapeutic for patients with advanced or metastatic cancer harboring defects in homologous recombination repair. In culture, the accumulation of PAR chains upon PARG inhibition causes delayed repair of DNA breaks, replication stress and mitotic catastrophe in settings with collateral liabilities in DNA damage repair and/or replication fork stability. Genome-wide CRISPR-mediated fitness screens across multiple genetic backgrounds revealed strong synthetic lethal interactions of IDE161 with otherwise non-essential components of cellular machinery supporting base excision repair, homologous recombination repair and replication fork stability. In addition, evaluation of IDE161 antiproliferative activity across a panel of 350 molecularly characterized cancer cell lines returned a response profile that was only partially overlapping with that observed with PARPi and was enriched for models harboring vulnerabilities associated with replication stress and/or defects in DNA repair. Notably, many homologous recombination deficient (HRD) cell models display differential dependency on PARG activity vs PARP1/2 activity, suggesting PARG inhibition as a potential therapeutic strategy distinct from PARP inhibitors. In vivo assessment of IDE161 anti-tumor activity in CDX and PDX models mirrored these mechanistic relationships. A number of HRD PDX models displayed a superior response to IDE161 versus PARPi, including robust regressions selectively observed with IDE161 versus niraparib in BRCA2 altered ER+, HER2-breast cancers. Thus, IDE161 has potential as novel targeted therapy that exploits the synthetic lethal relationship between PARG and genomic instability, leading to selective anti-proliferative activity in tumors harboring defects in DNA repair and replication fidelity. Citation Format: Reeja Maskey, Diana Munoz, Megan Conway, Arjun Rao, Vidhya Nagarajan, Ivan Shabalin, Kelly Trego, Jonathan Ryan, Peter Teriete, Christian Frey, Josh Taygerly, Paul Barsanti, Claire Neilan, Jasgit Sachdev, Michael White. Exploiting tumor selective vulnerabilities with structure based drug design [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Expanding and Translating Cancer Synthetic Vulnerabilities; 2024 Jun 10-13; Montreal, Quebec, Canada. Philadelphia (PA): AACR; Mol Cancer Ther 2024;23(6 Suppl):Abstract nr IA008.

A phase III randomized, double-blinded, placebo-controlled study of suvemcitug combined with chemotherapy for platinum-resistant ovarian cancer (SCORES).

LBA5516 Background: Suvemcitug is a new-generation recombinant humanized anti-VEGF rabbit monoclonal antibody. In the previous P1b study, Suvemcitug demonstrated its favorable safety profile and efficacy signals when combo with chemotherapy in platinum-resistant ovarian cancer (PROC). SCORES is the first randomized, double-blind, placebo-controlled phase III clinical trial to confirm the efficacy of Suvemcitug combo with chemo in PROC, whether or not previously received antiangiogenic agents or PARP inhibitors. Methods: Eligible patients had progressed during platinum-based therapy or within 6 months after ≥4 cycles of platinum-based therapy with at least one measurable lesion. After the investigators chose chemotherapy (CT) (weekly paclitaxel 80 mg/ m2 d1, 8, 15 & 22 q4w, pegylated liposomal doxorubicin 40 mg/m2 d1 q4w or topotecan 4 mg/m2 d1, 8 & 15 q4w), patients were randomly assigned (2:1) to either Suvemcitug (1.5 mg/kg q2w) or placebo combine with CT until progression or unacceptable toxicity. The primary endpoint was progression-free survival (PFS) by blinded independent review committee (BIRC) according to the RECIST 1.1. Key secondary end point is the overall survival (OS). Results: A total of 421 patients were enrolled between June 2021 and June 2023 in China. As the data cut off of the primary efficacy analysis (8 December 2023), 197 PFS events (70.1%) in Suvemcitug arm and 111 PFS events (79.3%) in placebo arm had occurred with the median follow-up of 14.36 and 14.26 months, respectively. The median PFS by BIRC was 5.49 months in Suvemcitug arm versus 2.73 months in placebo arm (hazard ratio[HR] 0.46, p<0.0001). OS data are immature. 42.7% and 50.7% of patients are deceased in Suvemcitug arm or placebo arm respectively, and there is a trend of OS benefit trend: median OS 16.07 months vs. 14.88 months, HR 0.79, p=0.1244. Efficacy results are summarized below. Treatment-emergent adverse events (TRAEs) occurred in all patients in Suvemcitug arm, with the most common being neutrophil count decreased, white blood cell count decreased and platelet count decreased. No Suvemcitug-related grade 5 TEAE occurred. Conclusions: To the best of our knowledge, this is the first double-blinded phase III study demonstrated promising antitumor activity of anti-angiogenic agent in patients with PROC. The improvement in PFS, ORR and DCR gained by adding Suvemcitug to single-agent CT was observed with no new safety concern. Clinical trial information: NCT04908787 . [Table: see text]

Abstract IA005: Therapeutic vulnerabilities of cohesin-mutant myeloid malignancies

Abstract Splicing modulation is a promising treatment strategy pursued to date only in splicing-factor mutant cancers; however, its therapeutic potential is poorly understood outside of this context. Like splicing factors, genes encoding components of the cohesin complex are frequently mutated in myeloid malignancies, including 15-20% of myelodysplastic syndromes (MDS) and secondary acute myeloid leukemia (AML), where they are associated with poor outcomes. I will discuss our recent findings identifying cohesin mutations as biomarkers of sensitivity to drugs targeting splicing-factor SF3B1 (H3B-8800 and E-7107) and describe the mechanism by which drug-induced alterations in splicing of DNA repair genes, such as BRCA1 and BRCA2, underlie this sensitivity. We have demonstrated that treatment of cohesin-mutant cells with SF3B1 modulators results in impaired DNA damage response, accumulation of DNA damage, and increased sensitivity to PARP inhibitors and a panel of chemotherapeutic agents in vitro and in vivo, using AML cell lines and patient-derived xenograft models. Furthermore, we identified RAD51 foci formation as a predictive biomarker of sensitivity to SF3B1 splicing modulation alone or followed by sequential treatment with PARP inhibition and chemotherapy, and have identified additional non-cohesin mutant subtypes of MDS/AML and ovarian and breast cancer which are sensitive to this therapeutic approach. Our findings expand the potential therapeutic benefits of SF3B1 splicing modulators to include cohesin-mutant MDS/AML and we propose this as broader strategy for therapeutic targeting of other DNA damage-repair deficient cancers. Citation Format: Zuzana Tothova. Therapeutic vulnerabilities of cohesin-mutant myeloid malignancies [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Expanding and Translating Cancer Synthetic Vulnerabilities; 2024 Jun 10-13; Montreal, Quebec, Canada. Philadelphia (PA): AACR; Mol Cancer Ther 2024;23(6 Suppl):Abstract nr IA005.

Abstract A019: Inhibition of nicotinamide adenine dinucleotide (NAD) production is a potent therapeutic strategy to inactivate homologous recombination in cancer cells

Abstract Homologous recombination (HR) is the most faithful DNA double-strand breaks (DSBs) repairing pathway. HR deficiency (HRD) can result from mutations in BRCA1, BRCA2, and PALB2, which are associated with genomic instability and cancer. HR deficient cells can survive through the use of alternative DNA repair pathways. Therefore, inhibiting these repair pathways in HRD cells trigger cancer cell death. This phenomenon is called synthetic lethality. The best-known example of synthetic lethality is that involving BRCA1/BRCA2/PALB2 deficiency with PARP-1 inhibition by Olaparib. However, ∼40% of patients will develop resistance to PARP inhibition because of several mechanisms, including a second mutation in BRCA1/2, restoring their full-length expression or RAD51 overexpression. RAD51 is one of the most important mediators of HR as it promotes the invasion of a sister chromatid allowing faithful DSB repair. There are many pieces of evidence which show that RAD51 overexpression is correlated with resistance and worse overall survival. Our hypothesis is that targeting RAD51 can trigger HR deficiency, particularly in cancer cells resistant to standard treatments. We thus sought to find inhibitors of the RAD51 recombinase. We have developed an in cellulo screening technique to test the direct effect of chemical compounds on RAD51 foci formation following irradiation. Using this technique, we identified , among 1381 chemical molecules, FK866 (Daporinad) which is a Nicotinamide Phosphoribosyltransferase Inhibitor (NAMPTi). NAMPT is involved in producing cellular Nicotinamide Adenine Dinucleotide (NAD+), which plays a crucial role in various metabolic processes within the cell, primarily in energy production and DNA repair. Remarkably, treatment of Daporinad reduced RAD51 foci formation and triggered a destabilization of the RAD51 protein by the proteasome. This destabilization seemed to be specific for RAD51, as NHEJ factors were not affected by NAMPTi. We will present data related to our objectives: i) Monitoring the effect of Daporinad on different cancer cell lines originated from cancer types which has shown resistance and worse overall survival because of RAD51 overexpression. Western blot and qPCR techniques will be used to investigate the effect of Daporinad on RAD51 protein and mRNA level and using Zeiss Celldiscoverer 7 system and Incucyte live cell imaging microscopy to observe the effect of Daporinad on cell survival; ii) To confirm the specific targeting of HR by Daporinad, we will monitor the effect of Daporinad on HR and other DNA DSB repair pathways using in cellulo reporter systems; iii) In the long term, we aim to evaluate the effect of Daporinad in vivo on tumor cell growth in pre-clinical models, such as mouse xenografts. In conclusion, small molecules affecting HR provide a source of new cancer therapies targeting RAD51 overexpression or using synthetic lethality. Citation Format: Sadaf Valeh Sheida, Thibaut Peterlini, Mélissa Thomas, Guy Poirier, Jean-Yves Masson. Inhibition of nicotinamide adenine dinucleotide (NAD) production is a potent therapeutic strategy to inactivate homologous recombination in cancer cells [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Expanding and Translating Cancer Synthetic Vulnerabilities; 2024 Jun 10-13; Montreal, Quebec, Canada. Philadelphia (PA): AACR; Mol Cancer Ther 2024;23(6 Suppl):Abstract nr A019.

Abstract A020: TP53 mutation and prediction of platinum response in BRCA-mutated ovarian cancer: A prospective case-series analysis

Abstract Background and aim of the study: Currently, platinum sensitivity (PS) is a prerequisite for first-line PARP inhibitors (PARPi) in locally advanced and relapsed high grade serous ovarian cancer (HGS-OC). BRCA mutations are recognized as predictive of PS and, therefore, of response to PARPi . Notably, platinum and PARPi cytotoxic action is mainly related by their ability of inducing p53-mediated apoptosis. Therefore, the integrity of p53 machinery is crucial for platinum-related activity whereas the presence of p53 mutation is a fairly frequent event in ovarian cancer, particularly in HGS-OC and in BRCA mutated ones. Patients and methods: We prospectively analyzed 208 women with primary ovarian cancer undergoing surgery at the Department of Gynecologic Oncology, ARNAS G. Brotzu, Cagliari, Italy, between 2019 and 2023. Somatic NGS analysis was performed to detect BRCA and HRD mutations. TP53 mutations were classified according to according to hotspot, structural (missense or nonsense) and functional classification as “gain of function” (GOF) or “loss of function” (LOF), based on the IARC TP53 database. Comparative testing with Fisher's exact test was used to examine TP53 mutation distribution and associations with clinicopathologic factors and PS. The BRCA mutation status was further used to stratify the analysis. Results: Globally, we included 127 adult HGS-OC pts. 84.2% had stage III-IV disease. TP53 mutation was found in 83.4 % of the entire cohort. Somatic BRCA mutations were found in 28.3% pts. Overall, HGS-OC with somatic BRCA mutations had higher TP53 mutation frequency (88.8%) when compared to BRCA WT (81.3%, p=0.1510). Employing the structural classification scheme, most cancers harbored a missense TP53 mutation (76.5%). LOF TP53 mutations were found in 59.4% while GOF in 31.2%. No significant disparity was observed in the distribution of specific TP53 mutations within each classification scheme between cases with BRCA mutations and those without. As for BRCA mutated pts, TP53 WT were all PS. Among those p53 mutated, GOF mutations were associated with PS in 7 pts and platinum resistance in 3 pts; LOF mutations were associated with PS in 7 pts and platinum resistance in 12 pts. The difference in distribution of PS between functional categories of p53 mutations was significant (p=0.0291). As for BRCA WT pts, TP53 WT were all PS. Among TP53 mutated, GOF mutations were associated with PS in 14 pts and platinum resistance in 10 pts; viceversa, LOF mutations were associated with PS in 19 pts and platinum resistance in 25 pts, even if these findings were not statistically significant (p=0.2357). Of relevance, in 5 cases where LOF mutations of p53 was associated with null expression of HIC p53, patients were refractory to platinum-based chemotherapy. Conclusions: Even if preliminary, our data show that HGS-OC harboring p53 null mutations are the poorest prognostic subgroup, especially in terms of PS. Further studies are needed to confirm our findings and the role of TP53 mutation as a biomarker of inherent or acquired platinum resistance. Citation Format: Eleonora Lai, Manuela Neri, Elisabetta Sanna, Sonia Nemolato, Fabio Bardanzellu, Mario Scartozzi, Sabrina Giglio, Antonio Macciò, Clelia Madeddu. TP53 mutation and prediction of platinum response in BRCA-mutated ovarian cancer: A prospective case-series analysis [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Expanding and Translating Cancer Synthetic Vulnerabilities; 2024 Jun 10-13; Montreal, Quebec, Canada. Philadelphia (PA): AACR; Mol Cancer Ther 2024;23(6 Suppl):Abstract nr A020.

Identification of CXCL13 as a Promising Biomarker for Immune Checkpoint Blockade Therapy and PARP Inhibitor Therapy in Ovarian Cancer

Identification of CXCL13 as a Promising Biomarker for Immune Checkpoint Blockade Therapy and PARP Inhibitor Therapy in Ovarian Cancer

The prognostic and predictive value of homologous recombination deficiency status in patients with advanced stage epithelial ovarian carcinoma after first-line platinum-based chemotherapy.

Homologous recombination (HR) comprises series of interrelated pathways that repair double-stranded DNA breaks and inter-strand crosslinks. It provides support for DNA replication to recover stalled or broken replication forks. Compared with homologous recombination proficiency (HRP), cancers with homologous recombination deficiency (HRD) are more likely to undergo cell death when treated with DNA-damaging agents, such as platinum agents, and have better disease control. Patients diagnosed with stage III/IV ovarian cancer, early stages with recurrence, who received adjuvant chemotherapy after debulking surgery, and who also had known HR status were eligible. Forty-four patients were included, with 21 in the HRD group (including 8 with germline mutations) and 23 in the HRP group. The HRD group was composed predominantly of serous carcinoma (95.2%), while mucinous (n=3) and clear cell (n=1) cases were all found in the HRP group. Stage III/IV disease was 66.7% and 91.3% in HRD and HRP groups, respectively (p=0.064). Patients who were optimally debulked to no residual disease was 90.0% and 72.7% (p=0.243), respectively. Late line use of PARP inhibitors was 33.3% and 17.4% (p=0.303). Median PFS was 22.5 months (95% CI, 18.5 - 66.6) and 21.5 months (95% CI, 18.3-39.5) (p=0.49) in HRD and HRP respectively. Median platinum free interval (PFI) was 15.8 months (95% CI 12.4-60.4) and 15.9 months (95% CI 8.3-34.1) (p=0.24), respectively. Median OS was 88.2 months (95% CI 71.2-NA) and 49.7 months (95% CI 35.1-NA) (p=0.21). The PFS of the patients with germline BRCA mutations (n=5) was 54.3 months (95% CI 23.1-NA) and 21.5 months (95% CI 18.3-39.5) in the HRP group (p=0.095); the PFI difference was 47.7 months (95% CI 17.6-NA) in the BRCA mutation group, and 15.9 months (95% CI 12.4-60.4) in HRP, showing statistical significance (p=0.039); while the median OS was NA and 49.7 months (95% CI 35.1-NA) respectively (p=0.051). When adding two additional patients with somatic BRCA mutations to the germline BRCA mutation carriers, the median OS is NA (95% CI 73, NA) versus 49.7 months (95% CI 35.1, NA) for HRP (p=0.045). HRD status was not associated with longer PFS or PFI in advanced ovarian cancer who received first line adjuvant platinum-based chemotherapy. Its role as a prognostic marker for overall survival is suggested, particularly in the subgroup with germline and somatic BRCA mutations.

Abstract B002: POLB knockout is synthetic lethal with PARP inhibition leading to complete and durable responses in BRCA-mutant tumor xenografts

Abstract Despite the clinical benefit of PARP1/2 inhibitors (PARPi), which are FDA-approved for the treatment of certain BRCA-mutant cancers, many patients achieve incomplete disease control and develop progressive disease. Motivated by this clinical need, we utilized our CRISPR target discovery screening platform to identify novel targets that synergize with PARP inhibitor treatment. By conducting parallel screens in both BRCA-mutant and wildtype cells, we identified DNA polymerase beta (POLB) as a novel target that - when combined with PARPi - selectively kills BRCA-mutant lines while sparing normal cells. POLB knockout and cDNA rescue experiments using both BRCA1 and BRCA2-mutant isogenic cell lines further demonstrated that the catalytic activity of POLB is required for synthetic lethality with PARPi. Most strikingly, POLB knockout combined with sub-therapeutic doses of PARPi, led to profound tumor regression and prevented in vivo tumor regrowth, even after cessation of drug treatment. Mechanistically, POLB knockout is associated with increased single and double strand DNA breaks, accumulation of poly-ADP-ribose polymers, cell cycle arrest and apoptosis. Together, these results suggest that POLB inhibitors in combination with PARPi have the potential to drive deep and durable responses providing a novel therapeutic option for cancer patients with BRCA1/2-mutations. Citation Format: Madhavi Bandi. POLB knockout is synthetic lethal with PARP inhibition leading to complete and durable responses in BRCA-mutant tumor xenografts [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Expanding and Translating Cancer Synthetic Vulnerabilities; 2024 Jun 10-13; Montreal, Quebec, Canada. Philadelphia (PA): AACR; Mol Cancer Ther 2024;23(6 Suppl):Abstract nr B002.

Abstract IA015: Identifying and targeting synthetic lethalities of aneuploid (cancer) cells

Abstract Aneuploidy, an imbalanced number of chromosomes or chromosome arms, is a genetic hallmark of cancer cells, yet aneuploidy remains a biological enigma and a missed opportunity for cancer therapy. Aneuploid cells must cope with several types of cellular stresses, potentially creating synthetic lethalities that can be used to target aneuploid cancer cells. Here, I will describe our efforts to identify synthetic lethalities of the aneuploid state (in contrast to synthetic lethalities of specific recurrent aneuploidies. Specifically, I will focus on three recent unpublished studies, where we report that: (1) Aneuploid cells are preferentially sensitive to perturbation of the spindle assembly checkpoint (SAC) and its regulator KIF18A; the expression and activity of CDC20 determine the sensitivity to SAC inhibition. (2) Aneuploid cells depend on MAPK signaling for overcoming aneuploidy-induced DNA damage; targeting MAPK signaling can sensitize aneuploid cells to DNA damage inducing agents and to PARP inhibitors. (3) Aneuploid cells depend on RNA and protein degradation mechanisms to attenuate the cellular consequence of extra chromosomes; this renders aneuploid cells more sensitive to inhibition of nonsense-mediate decay, miRNA-mediated gene silencing, and the proteasome complex. Citation Format: Uri Ben-David. Identifying and targeting synthetic lethalities of aneuploid (cancer) cells [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Expanding and Translating Cancer Synthetic Vulnerabilities; 2024 Jun 10-13; Montreal, Quebec, Canada. Philadelphia (PA): AACR; Mol Cancer Ther 2024;23(6 Suppl):Abstract nr IA015.

Abstract PR009: Cytidine diphosphate diacylglycerol synthase 2 is a synthetic lethal target in mesenchymal cancers

Abstract Synthetic lethal interactions (SLIs) can provide a therapeutic index, as illustrated by PARP inhibition of BRCA-deficient cancers. This clinical success, and other examples, have spurred efforts to identify additional cancer-associated SLIs. Whereas additional SLIs based on genomic alterations in cancer have been identified, we set out to explore the SLI space as a function of differential RNA expression profiles in cancer and normal tissue, covering all ∼3.4e8 gene pairs. In our bioinformatic pipeline, synthetic lethality is scored by correlating DepMap dependency and expression data (n = 913 cancer cell lines), while cancer specificity is scored by comparing TCGA expression data for 9264 tumors and 741 healthy samples. Cancer specificity is confirmed by comparing also calibrated GTEx data for 17382 healthy tissue samples with calibrated DepMap expression data. With this pipeline we uncovered a frequent cancer-specific SLI between the paralogs cytidine diphosphate diacylglycerol synthase 1 (CDS1) and CDS2 (p<0.001 in all three analyses). Essentiality of CDS2 is observed in one third of cultured cancers (DepMap). Using CRISPR-Cas9 we confirm the CDS1-dependency for CDS2 essentiality in a panel of 12 cultured cancer cell lines, with lethality by CDS2 ablation reaching up to 98% in CDS1-negative cell lines. We also confirm the SLI using admixing experiments in tumor-bearing mice for two cell lines (6 mice per group, up to 95% synthetic lethality, p<0.001). The essentiality of CDS2 for cell survival is observed for mesenchymal-like cancers, which commonly express low levels of CDS1. To biochemically define the effects of CDS2 perturbation in CDS1-negative cancer cells, we performed multi-omic analyses in a panel of CDS1-negative cancer cell lines. The results show that mechanistically, the CDS1-2 SLI is accompanied by disruption of lipid homeostasis, including extensive accumulation of cholesterol esters and triglycerides. Additionally, quantitative western blotting for cleaved caspase-3 showed induction of apoptotic cell death (p<0.001 in 2 cell lines). To challenge the robustness of the SLI, we performed genome-wide CRISPR-Cas9 knockout screens in a panel of four CDS1-negative cancer cell lines. This failed to identify a common escape mechanism of death caused by CDS2 ablation. These findings suggest that no common escape to the combined loss of CDS1 and CDS2 is possible through loss-of-function. In conclusion, by computational, genetic, biochemical and functional analyses we demonstrate that CDS2 may serve as a target in mesenchymal cancers, meriting therapeutic exploration. Citation Format: Tim Arnoldus, Alex van Vliet, Adriaan F.H. de Groot, Niek Blomberg, Onno B. Bleijerveld, Susan E. van Hal-van Veen, Anita E. Grootemaat, Rolf Harkes, Nicole N. van der Wel, Maarten Altelaar, Martin Giera, Daniel S. Peeper. Cytidine diphosphate diacylglycerol synthase 2 is a synthetic lethal target in mesenchymal cancers [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Expanding and Translating Cancer Synthetic Vulnerabilities; 2024 Jun 10-13; Montreal, Quebec, Canada. Philadelphia (PA): AACR; Mol Cancer Ther 2024;23(6 Suppl):Abstract nr PR009.

Epigenetic and Genomic Hallmarks of PARP-Inhibitor Resistance in Ovarian Cancer Patients.

Patients with advanced-stage epithelial ovarian cancer (EOC) receive treatment with a poly-ADP ribose-polymerase (PARP) inhibitor (PARPi) as maintenance therapy after surgery and chemotherapy. Unfortunately, many patients experience disease progression because of acquired therapy resistance. This study aims to characterize epigenetic and genomic changes in cell-free DNA (cfDNA) associated with PARPi resistance. Blood was taken from 31 EOC patients receiving PARPi therapy before treatment and at disease progression during/after treatment. Resistance was defined as disease progression within 6 months after starting PARPi and was seen in fifteen patients, while sixteen patients responded for 6 to 42 months. Blood cfDNA was evaluated via Modified Fast Aneuploidy Screening Test-Sequencing System (mFast-SeqS to detect aneuploidy, via Methylated DNA Sequencing (MeD-seq) to find differentially methylated regions (DMRs), and via shallow whole-genome and -exome sequencing (shWGS, exome-seq) to define tumor fractions and mutational signatures. Aneuploid cfDNA was undetectable pre-treatment but observed in six patients post-treatment, in five resistant and one responding patient. Post-treatment ichorCNA analyses demonstrated in shWGS and exome-seq higher median tumor fractions in resistant (7% and 9%) than in sensitive patients (7% and 5%). SigMiner analyses detected predominantly mutational signatures linked to mismatch repair and chemotherapy. DeSeq2 analyses of MeD-seq data revealed three methylation signatures and more tumor-specific DMRs in resistant than in responding patients in both pre- and post-treatment samples (274 vs. 30 DMRs, 190 vs. 57 DMRs, Χ2-test p < 0.001). Our genome-wide Next-Generation Sequencing (NGS) analyses in PARPi-resistant patients identified epigenetic differences in blood before treatment, whereas genomic alterations were more frequently observed after progression. The epigenetic differences at baseline are especially interesting for further exploration as putative predictive biomarkers for PARPi resistance.

The OVAREX study: Establishment of ex vivo ovarian cancer models to validate innovative therapies and to identify predictive biomarkers

BackgroundOvarian cancer is the first cause of death from gynecological malignancies mainly due to development of chemoresistance. Despite the emergence of PARP inhibitors, which have revolutionized the therapeutic management of some of these ovarian cancers, the 5-year overall survival rate remains around 45%. Therefore, it is crucial to develop new therapeutic strategies, to identify predictive biomarkers and to predict the response to treatments. In this context, functional assays based on patient-derived tumor models could constitute helpful and relevant tools for identifying efficient therapies or to guide clinical decision making.MethodThe OVAREX study is a single-center non-interventional study which aims at investigating the feasibility of establishing in vivo and ex vivo models and testing ex vivo models to predict clinical response of ovarian cancer patients. Patient-Derived Xenografts (PDX) will be established from tumor fragments engrafted subcutaneously into immunocompromised mice. Explants will be generated by slicing tumor tissues and Ascites-Derived Spheroids (ADS) will be isolated following filtration of ascites. Patient-derived tumor organoids (PDTO) will be established after dissociation of tumor tissues or ADS, cell embedding into extracellular matrix and culture in specific medium. Molecular and histological characterizations will be performed to compare tumor of origin and paired models. Response of ex vivo tumor-derived models to conventional chemotherapy and PARP inhibitors will be assessed and compared to results of companion diagnostic test and/or to the patient’s response to evaluate their predictive value.DiscussionThis clinical study aims at generating PDX and ex vivo models (PDTO, ADS, and explants) from tumors or ascites of ovarian cancer patients who will undergo surgical procedure or paracentesis. We aim at demonstrating the predictive value of ex vivo models for their potential use in routine clinical practice as part of precision medicine, as well as establishing a collection of relevant ovarian cancer models that will be useful for the evaluation of future innovative therapies.Trial registrationThe clinical trial has been validated by local research ethic committee on January 25th 2019 and registered at ClinicalTrials.gov with the identifier NCT03831230 on January 28th 2019, last amendment v4 accepted on July 18, 2023.

The genomic landscape of breast and non-breast cancers from individuals with germline CHEK2 deficiency.

CHEK2 is considered to be involved in homologous recombination repair (HRR). Individuals who have germline pathogenic variants (gPVs) in CHEK2 are at increased risk to develop breast cancer and likely other primary cancers. PARP inhibitors (PARPi) have been shown to be effective in the treatment of cancers that present with HRR deficiency-for example, caused by inactivation of BRCA1/2. However, clinical trials have shown little to no efficacy of PARPi in patients with CHEK2 gPVs. Here, we show that both breast and non-breast cancers from individuals who have biallelic gPVs in CHEK2 (germline CHEK2 deficiency) do not present with molecular profiles that fit with HRR deficiency. This finding provides a likely explanation why PARPi therapy is not successful in the treatment of CHEK2-deficient cancers.