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Poly (ADP-ribose) glycohydrolase (PARG) is a dePARylating enzyme which promotes DNA repair by removal of poly (ADP-ribose) (PAR) from PARylated proteins. Loss or inhibition of PARG results in replication stress and sensitizes cancer cells to DNA-damaging agents. PARG inhibitors are now undergoing clinical development for patients having tumors with homologous recombination deficiency (HRD), such as cancer patients with germline or somatic BRCA1/2 -mutations. PARP inhibitors kill BRCA-deficient cancer cells by increasing single-stranded DNA gaps (ssGAPs) during replication. Here, we report that, like PARP inhibitor (PARPi), PARG inhibitor (PARGi) treatment also causes an accumulation of ssGAPs in sensitive cells. PARGi exposure increased accumulation of S-phase-specific PAR, a marker for Okazaki fragment processing (OFP) defects on lagging strands and induced ssGAPs, in sensitive cells but not in resistant cells. PARGi also caused accumulation of PAR at the replication forks and at the ssDNA sites in sensitive cells. Additionally, PARGi exhibited monotherapy activity in specific HR-deficient, as well as HR-proficient, patient-derived, or patient-derived xenograft (PDX)-derived organoids of ovarian cancer, and drug sensitivity directly correlated with the accumulation of ssGAPs. Taken together, PARGi treatment results in toxic accumulation of PAR at replication forks resulting in ssGAPs due to OFP defects during replication. Regardless of the BRCA/ HRD - status, the induction of ssGAPs in preclinical models of ovarian cancer cells correlates with PARGi sensitivity. Patient-derived organoids (PDOs) may be a useful model system for testing PARGi sensitivity and functional biomarkers., Competing Interests: Competing interests statement:A.D.D. reports consulting for AbbVie, Deerfield Management Company L.P., Impact Therapeutics, PrimeFour Therapeutics, Schrödinger Inc., and Servier Bio-Innovation LLC; is an Advisory Board member for Covant Therapeutics, and Impact Therapeutics; stockholder in Impact Therapeutics, and PrimeFour Therapeutics; and reports receiving commercial research grants from Bristol Myers Squibb, EMD Serono, Moderna, and Tango Therapeutics. G.I.S. reports grant support from Merck KGaA/EMD-Serono, Tango Therapeutics, Bristol Myers Squibb, and Merck & Co., all related to DNA repair inhibitors, as well as from Eli Lilly and Pfizer. He has served on advisory boards for Merck KGaA/EMD-Serono, Circle Pharmaceuticals, Schrödinger, Janssen and Xinthera. He holds patents entitled, “Dosage regimen for sapacitabine and seliciclib,” also issued to Cyclacel Pharmaceuticals, and “Compositions and methods for predicting response and resistance to CDK4/6 inhibition.” J. Liu reports personal fees from AstraZeneca, Bristol Myers Squibb, Clovis Oncology, Daiichi Sankyo, Eisai, Genentech/Roche, GlaxoSmithKline, Regeneron Therapeutics, Revolution Medicine, Zentalis Pharmaceuticals, and Deciphera Pharmaceuticals outside the submitted work and institutional funding for clinical trials from 2X Oncology, Aravive, Arch Oncology, AstraZeneca, Bristol Myers Squibb, Clovis Oncology, GlaxoSmithKline, Impact Therapeutics, Pfizer, Regeneron, Seagen, SystImmune, Vigeo Therapeutics, and Zentalis Pharmaceuticals.

Purpose: The multicenter, open-label, randomized phase 2 NCI-9944 study (NCT02595892) demonstrated that addition of ATR inhibitor (ATRi) berzosertib to gemcitabine increased progression-free survival (PFS) compared to gemcitabine alone (hazard ratio [HR]=0.57, one-sided log-rank P = .044, which met the one-sided significance level of 0.1 used for sample size calculation)., Methods: We report here the final overall survival (OS) analysis and biomarker correlations (ATM expression by immunohistochemistry, mutational signature 3 and a genomic biomarker of replication stress) along with post-hoc exploratory analyses to adjust for crossover from gemcitabine to gemcitabine/berzosertib., Results: At the data cutoff of January 27, 2023 (>30 months of additional follow-up from the primary analysis), median OS was 59.4 weeks with gemcitabine/berzosertib versus 43.0 weeks with gemcitabine alone (HR 0.79, 90% CI 0.52 to 1.2, one-sided log-rank P = .18). An OS benefit with addition of berzosertib to gemcitabine was suggested in patients stratified into the platinum-free interval ≤3 months (N = 26) subgroup (HR, 0.48, 90% CI 0.22 to 1.01, one-sided log-rank P =.04) and in patients with ATM-negative/low (N = 24) tumors (HR, 0.50, 90% CI 0.23 to 1.08, one-sided log-rank P = .06)., Conclusion: The results of this follow-up analysis continue to support the promise of combined gemcitabine/ATRi therapy in platinum resistant ovarian cancer, an active area of investigation with several ongoing clinical trials.

In a trial of patients with high grade serous ovarian cancer (HGSOC), addition of the ATR inhibitor berzosertib to gemcitabine improved progression free survival (PFS) compared to gemcitabine alone but biomarkers predictive of treatment are lacking. Here we report a candidate biomarker of response to gemcitabine versus combined gemcitabine and ATR inhibitor therapy in HGSOC ovarian cancer. Patients with replication stress (RS)-high tumors (n = 27), defined as harboring at least one genomic RS alteration related to loss of RB pathway regulation and/or oncogene-induced replication stress achieve significantly prolonged PFS (HR = 0.38, 90% CI, 0.17-0.86) on gemcitabine monotherapy compared to those with tumors without such alterations (defined as RS-low, n = 30). However, addition of berzosertib to gemcitabine benefits only patients with RS-low tumors (gemcitabine/berzosertib HR 0.34, 90% CI, 0.13-0.86) and not patients with RS-high tumors (HR 1.11, 90% CI, 0.47-2.62). Our findings support the notion that the exacerbation of RS by gemcitabine monotherapy is adequate for lethality in RS-high tumors. Conversely, for RS-low tumors addition of berzosertib-mediated ATR inhibition to gemcitabine is necessary for lethality to occur. Independent prospective validation of this biomarker is required., (© 2021. The Author(s).)

DNA polymerase theta (POLθ) is synthetic lethal with Homologous Recombination (HR) deficiency and thus a candidate target for HR-deficient cancers. Through high-throughput small molecule screens we identified the antibiotic Novobiocin (NVB) as a specific POLθ inhibitor that selectively kills HR-deficient tumor cells in vitro and in vivo . NVB directly binds to the POLθ ATPase domain, inhibits its ATPase activity, and phenocopies POLθ depletion. NVB kills HR-deficient breast and ovarian tumors in GEMM, xenograft and PDX models. Increased POLθ levels predict NVB sensitivity, and BRCA-deficient tumor cells with acquired resistance to PARP inhibitors (PARPi) are sensitive to NVB in vitro and in vivo. Mechanistically, NVB-mediated cell death in PARPi-resistant cells arises from increased double-strand break end resection, leading to accumulation of single-strand DNA intermediates and non-functional RAD51 foci. 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. (151/150).

Combined PARP and immune checkpoint inhibition has yielded encouraging results in ovarian cancer, but predictive biomarkers are lacking. We performed immunogenomic profiling and highly multiplexed single-cell imaging on tumor samples from patients enrolled in a Phase I/II trial of niraparib and pembrolizumab in ovarian cancer (NCT02657889). We identify two determinants of response; mutational signature 3 reflecting defective homologous recombination DNA repair, and positive immune score as a surrogate of interferon-primed exhausted CD8 + T-cells in the tumor microenvironment. Presence of one or both features associates with an improved outcome while concurrent absence yields no responses. Single-cell spatial analysis reveals prominent interactions of exhausted CD8 + T-cells and PD-L1 + macrophages and PD-L1 + tumor cells as mechanistic determinants of response. Furthermore, spatial analysis of two extreme responders shows differential clustering of exhausted CD8 + T-cells with PD-L1 + macrophages in the first, and exhausted CD8 + T-cells with cancer cells harboring genomic PD-L1 and PD-L2 amplification in the second.

Purpose: PARP inhibitors are approved for the treatment of high-grade serous ovarian cancers (HGSOC). Therapeutic resistance, resulting from restoration of homologous recombination (HR) repair or replication fork stabilization, is a pressing clinical problem. We assessed the activity of prexasertib, a checkpoint kinase 1 (CHK1) inhibitor known to cause replication catastrophe, as monotherapy and in combination with the PARP inhibitor olaparib in preclinical models of HGSOC, including those with acquired PARP inhibitor resistance., Experimental Design: Prexasertib was tested as a single agent or in combination with olaparib in 14 clinically annotated and molecularly characterized luciferized HGSOC patient-derived xenograft (PDX) models and in a panel of ovarian cancer cell lines. The ability of prexasertib to impair HR repair and replication fork stability was also assessed., Results: Prexasertib monotherapy demonstrated antitumor activity across the 14 PDX models. Thirteen models were resistant to olaparib monotherapy, including 4 carrying BRCA1 mutation. The combination of olaparib with prexasertib was synergistic and produced significant tumor growth inhibition in an olaparib-resistant model and further augmented the degree and durability of response in the olaparib-sensitive model. HGSOC cell lines, including those with acquired PARP inhibitor resistance, were also sensitive to prexasertib, associated with induction of DNA damage and replication stress. Prexasertib also sensitized these cell lines to PARP inhibition and compromised both HR repair and replication fork stability., Conclusions: Prexasertib exhibits monotherapy activity in PARP inhibitor-resistant HGSOC PDX and cell line models, reverses restored HR and replication fork stability, and synergizes with PARP inhibition., (©2019 American Association for Cancer Research.)

Background: Based on preclinical work, we found that combination of poly (ADP-ribose) polymerase (PARP) inhibitors with drugs that inhibit the homologous recombination repair (HRR) pathway (such as PI3K inhibitors) might sensitise HRR-proficient epithelial ovarian cancers to PARP inhibitors. We aimed to assess the safety and identify the recommended phase 2 dose of the PARP inhibitor olaparib in combination with the PI3K inhibitor alpelisib in patients with epithelial ovarian cancer and in patients with breast cancer., Methods: In this multicentre, open-label, phase 1b trial following a 3 + 3 dose-escalation design, we recruited patients aged 18 years or older with the following key eligibility criteria: confirmed diagnosis of either recurrent ovarian, fallopian tube, or primary peritoneal cancer of high-grade serous histology; confirmed diagnosis of either recurrent ovarian, fallopian tube, or primary peritoneal cancer of any histology with known germline BRCA mutations; confirmed diagnosis of recurrent breast cancer of triple-negative histology; or confirmed diagnosis of recurrent breast cancer of any histology with known germline BRCA mutations. Additional patients with epithelial ovarian cancer were enrolled in a dose-expansion cohort. Four dose levels were planned: the starting dose level of alpelisib 250 mg once a day plus olaparib 100 mg twice a day (dose level 0); alpelisib 250 mg once a day plus olaparib 200 mg twice a day (dose level 1); alpelisib 300 mg once a day plus olaparib 200 mg twice a day (dose level 2); and alpelisib 200 mg once a day plus olaparib 200 mg twice a day (dose level 3). Both drugs were administered orally, in tablet formulation. The primary objective was to identify the maximum tolerated dose and the recommended phase 2 dose of the combination of alpelisib and olaparib for patients with epithelial ovarian cancer and patients with breast cancer. Analyses included all patients who received at least one dose of the study drugs. The trial is active, but closed to enrolment; follow-up for patients who completed treatment is ongoing. This trial is registered with ClinicalTrials.gov, number NCT01623349., Findings: Between Oct 3, 2014, and Dec 21, 2016, we enrolled 34 patients (28 in the dose-escalation cohort and six in the dose-expansion cohort); two in the dose-escalation cohort were ineligible at the day of scheduled study initiation. Maximum tolerated dose and recommended phase 2 dose were identified as alpelisib 200 mg once a day plus olaparib 200 mg twice a day (dose level 3). Considering all dose levels, the most common treatment-related grade 3-4 adverse events were hyperglycaemia (five [16%] of 32 patients), nausea (three [9%]), and increased alanine aminotransferase concentrations (three [9%]). No treatment-related deaths occurred. Dose-limiting toxic effects included hyperglycaemia and fever with decreased neutrophil count. Of the 28 patients with epithelial ovarian cancer, ten (36%) achieved a partial response and 14 (50%) had stable disease according to Response Evaluation Criteria in Solid Tumors 1.1., Interpretation: Combining alpelisib and olaparib is feasible with no unexpected toxic effects. The observed activity provides preliminary clinical evidence of synergism between olaparib and alpelisib, particularly in epithelial ovarian cancer, and warrants further investigation., Funding: Ovarian Cancer Dream Team (Stand Up To Cancer, Ovarian Cancer Research Alliance, National Ovarian Cancer Coalition), Breast Cancer Research Foundation, Novartis., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Based on genomic analysis, 50% of high-grade serous ovarian cancers (HGSC) are predicted to have DNA repair defects. Whether this substantial subset of HGSCs actually have functional repair defects remains unknown. Here, we devise a platform for functional profiling of DNA repair in short-term patient-derived HGSC organoids. We tested 33 organoid cultures derived from 22 patients with HGSC for defects in homologous recombination (HR) and replication fork protection. Regardless of DNA repair gene mutational status, a functional defect in HR in the organoids correlated with PARP inhibitor sensitivity. A functional defect in replication fork protection correlated with carboplatin and CHK1 and ATR inhibitor sensitivity. Our results indicate that a combination of genomic analysis and functional testing of organoids allows for the identification of targetable DNA damage repair defects. Larger numbers of patient-derived organoids must be analyzed to determine whether these assays can reproducibly predict patient response in the clinic. Significance: Patient-derived ovarian tumor organoids grow rapidly and match the tumors from which they are derived, both genetically and functionally. These organoids can be used for DNA repair profiling and therapeutic sensitivity testing and provide a rapid means of assessing targetable defects in the parent tumor, offering more suitable treatment options. Cancer Discov; 8(11); 1404-21. ©2018 AACR. This article is highlighted in the In This Issue feature, p. 1333 ., (©2018 American Association for Cancer Research.)

Background: Increased number of single nucleotide substitutions is seen in breast and ovarian cancer genomes carrying disease-associated mutations in BRCA1 or BRCA2. The significance of these genome-wide mutations is unknown. We hypothesize genome-wide mutation burden mirrors deficiencies in DNA repair and is associated with treatment outcome in ovarian cancer., Methods and Results: The total number of synonymous and non-synonymous exome mutations (Nmut), and the presence of germline or somatic mutation in BRCA1 or BRCA2 (mBRCA) were extracted from whole-exome sequences of high-grade serous ovarian cancers from The Cancer Genome Atlas (TCGA). Cox regression and Kaplan-Meier methods were used to correlate Nmut with chemotherapy response and outcome. Higher Nmut correlated with a better response to chemotherapy after surgery. In patients with mBRCA-associated cancer, low Nmut was associated with shorter progression-free survival (PFS) and overall survival (OS), independent of other prognostic factors in multivariate analysis. Patients with mBRCA-associated cancers and a high Nmut had remarkably favorable PFS and OS. The association with survival was similar in cancers with either BRCA1 or BRCA2 mutations. In cancers with wild-type BRCA, tumor Nmut was associated with treatment response in patients with no residual disease after surgery., Conclusions: Tumor Nmut was associated with treatment response and with both PFS and OS in patients with high-grade serous ovarian cancer carrying BRCA1 or BRCA2 mutations. In the TCGA cohort, low Nmut predicted resistance to chemotherapy, and for shorter PFS and OS, while high Nmut forecasts a remarkably favorable outcome in mBRCA-associated ovarian cancer. Our observations suggest that the total mutation burden coupled with BRCA1 or BRCA2 mutations in ovarian cancer is a genomic marker of prognosis and predictor of treatment response. This marker may reflect the degree of deficiency in BRCA-mediated pathways, or the extent of compensation for the deficiency by alternative mechanisms.