Safety assessments and clinical features of PARP inhibitors from real-world data of Japanese patients with ovarian cancer.

Poly (ADP-ribose) polymerase inhibitors have been increasingly used in ovarian cancer treatment. However, the real-world safety data of these drugs in Japanese patients are limited. This retrospective study included 181 patients with ovarian cancer who received olaparib or niraparib at two independent hospitals in Japan between May 2018 and December 2022. Clinical information and blood sampling data were collected. Regarding patient backgrounds, the olaparib group had higher proportions of patients with serous carcinoma, BRCA positivity, homologous recombination deficiency, and those receiving maintenance therapy after recurrence treatment than the niraparib group. Regarding toxicity properties, the most common reasons for discontinuation in the olaparib group were anemia, fatigue, and nausea, while the reason in the niraparib was thrombocytopenia. Thrombocytopenia caused by niraparib treatment occurred earlier than anemia caused by olaparib treatment. Patients with a low body mass index or who had undergone several previous treatment regimens were more likely to discontinue treatment within the first 3 months. Although we analyzed blood collection data, predicting treatment interruptions due to blood toxicity was challenging. In this study, we revealed the characteristics of patients and the timing of interruptions for each drug, highlighting the importance of carefully managing adverse effects.

Paris saponin VII reverses resistance to PARP inhibitors by regulating ovarian cancer tumor angiogenesis and glycolysis through the RORα/ECM1/VEGFR2 signaling axis.

The emergence of Poly (ADP-ribose) polymerase inhibitors (PARPi) has marked the beginning of a precise targeted therapy era for ovarian cancer. However, an increasing number of patients are experiencing primary or acquired resistance to PARPi, severely limiting its clinical application. Deciphering the underlying mechanisms of PARPi resistance and discovering new therapeutic targets is an urgent and critical issue to address. In this study, we observed a close correlation between glycolysis, tumor angiogenesis, and PARPi resistance in ovarian cancer. Furthermore, we discovered that the natural compound Paris saponin VII (PS VII) partially reversed PARPi resistance in ovarian cancer and demonstrated synergistic therapeutic effects when combined with PARPi. Additionally, we found that PS VII potentially hindered glycolysis and angiogenesis in PARPi-resistant ovarian cancer cells by binding and stabilizing the expression of RORα, thus further inhibiting ECM1 and interfering with the VEGFR2/FAK/AKT/GSK3ß signaling pathway. Our research provides new targeted treatment for clinical ovarian cancer therapy and brings new hope to patients with PARPi-resistant ovarian cancer, effectively expanding the application of PARPi in clinical treatment.

Feasibility and cost-effectiveness of genetic counselling for all patients with newly diagnosed ovarian cancer: a single-centre retrospective study.

BACKGROUND AND AIMS OF THE STUDY: Due to its importance for treatment and potential prevention in family members, germline testing for BRCA1/2 in patients with newly diagnosed ovarian cancer is decisive and considered a standard of care. Maintenance therapy with poly(ADP-ribose) polymerase (PARP) inhibitors substantially improves progression-free survival in patients with BRCA mutations and homologous recombination-deficient tumours by inducing synthetic lethality. In Switzerland, they are licensed only for these patients. Therefore, it is crucial to test patients early while they are receiving adjuvant chemotherapy. This study aimed to determine whether genetic counselling followed by homologous recombination deficiency testing is feasible for initialising maintenance therapy within eight weeks and cost-effective in daily practice in Switzerland compared to somatic tumour analysis of all patients at diagnosis. METHODS: This single-centre retrospective study included 44 patients with newly diagnosed high-grade serous ovarian cancer of a Federation of Gynaecology and Obstetrics (FIGO) stage of IIIA-IVB diagnosed between 12/2020 and 12/2022. It collected the outcomes of genetic counselling, germline testing, and somatic Geneva test for homologous recombination deficiency. Delays in initiating maintenance therapy, total testing costs per patient, and progression-free survival were examined to assess feasibility and cost-effectiveness in clinical practice. RESULTS: Thirty-seven of 44 patients (84%) with newly diagnosed ovarian cancer received counselling, of which 34 (77%) were tested for germline BRCA and other homologous recombination repair gene mutations. Five (15%) BRCA and three (9%) other homologous recombination deficiency mutations were identified. Eleven of the remaining 26 patients (42%) had tumours with somatic homologous recombination deficiency. The mean time to the initiation of maintenance therapy of 5.2 weeks was not longer than in studies for market authorisation (SOLO1, PAOLA, and PRIMA). The mean testing costs per patient were 3880 Swiss Franks (CHF), compared to 5624 CHF if all patients were tested at diagnosis with the myChoice CDx test (p <0.0001). CONCLUSION: Using genetic counselling to consent patients with newly diagnosed ovarian cancer for germline testing fulfils the international gold standard. Subsequent somatic homologous recombination deficiency analysis complements testing and identifies more patients who will benefit from PARP inhibitor maintenance therapy. Contrary to previous health cost model studies, the procedure does not increase testing costs in the Swiss population and does not delay maintenance therapy. Therefore, all patients should be offered a primary germline analysis. The challenge for the future will be to ensure sufficient resources for prompt genetic counselling and germline testing.

A PARP1-TIMELESS alliance in cancer therapy.

In a recent paper in Nature, Petropoulos et al.1 report that PARP1 acts together with the replisome components TIMELESS and TIPIN to protect the genome from transcription-replication conflicts, which has important implications for the clinical use of PARP inhibitors.

Comparing niraparib versus platinum-taxane doublet chemotherapy as neoadjuvant treatment in patients with newly diagnosed homologous recombination-deficient stage III/IV ovarian cancer: study protocol for cohort C of the open-label, phase 2, randomized controlled multicenter OPAL trial.

BACKGROUND: Maintenance therapy with niraparib, a poly(ADP-ribose) polymerase inhibitor, has been shown to extend progression-free survival in patients with newly diagnosed advanced ovarian cancer who responded to first-line platinum-based chemotherapy, regardless of biomarker status. However, there are limited data on niraparib's efficacy and safety in the neoadjuvant setting. The objective of Cohort C of the OPAL trial (OPAL-C) is to evaluate the efficacy, safety, and tolerability of neoadjuvant niraparib treatment compared with neoadjuvant platinum-taxane doublet chemotherapy in patients with newly diagnosed stage III/IV ovarian cancer with confirmed homologous recombination-deficient tumors. METHODS: OPAL is an ongoing global, multicenter, randomized, open-label, phase 2 trial. In OPAL-C, patients will be randomized 1:1 to receive three 21-day cycles of either neoadjuvant niraparib or platinum-taxane doublet neoadjuvant chemotherapy per standard of care. Patients with a complete or partial response per Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST v1.1) will then undergo interval debulking surgery; patients with stable disease may proceed to interval debulking surgery or alternative therapy at the investigator's discretion. Patients with disease progression will exit the study treatment and proceed to alternative therapy at the investigator's discretion. After interval debulking surgery, all patients will receive up to three 21-day cycles of platinum-taxane doublet chemotherapy followed by niraparib maintenance therapy for up to 36 months. Adult patients with newly diagnosed stage III/IV ovarian cancer eligible to receive neoadjuvant platinum-taxane doublet chemotherapy followed by interval debulking surgery may be enrolled. Patients must have tumors that are homologous recombination-deficient. The primary endpoint is the pre-interval debulking surgery unconfirmed overall response rate, defined as the investigator-assessed percentage of patients with unconfirmed complete or partial response on study treatment before interval debulking surgery per RECIST v1.1. DISCUSSION: OPAL-C explores the use of niraparib in the neoadjuvant setting as an alternative to neoadjuvant platinum-taxane doublet chemotherapy to improve postsurgical residual disease outcomes for patients with ovarian cancer with homologous recombination-deficient tumors. Positive findings from this approach could significantly impact preoperative ovarian cancer therapy, particularly for patients who are ineligible for primary debulking surgery. TRIAL REGISTRATION: ClinicalTrials.gov NCT03574779. Registered on February 28, 2022.

Exploration of poly (ADP-ribose) polymerase inhibitor resistance in the treatment of BRCA1/2-mutated cancer.

Breast cancer susceptibility 1/2 (BRCA1/2) genes play a crucial role in DNA damage repair, yet mutations in these genes increase the susceptibility to tumorigenesis. Exploiting the synthetic lethality mechanism between BRCA1/2 mutations and poly(ADP-ribose) polymerase (PARP) inhibition has led to the development and clinical approval of PARP inhibitor (PARPi), representing a milestone in targeted therapy for BRCA1/2 mutant tumors. This approach has paved the way for leveraging synthetic lethality in tumor treatment strategies. Despite the initial success of PARPis, resistance to these agents diminishes their efficacy in BRCA1/2-mutant tumors. Investigations into PARPi resistance have identified replication fork stability and homologous recombination repair as key factors sensitive to PARPis. Additionally, studies suggest that replication gaps may also confer sensitivity to PARPis. Moreover, emerging evidence indicates a correlation between PARPi resistance and cisplatin resistance, suggesting a potential overlap in the mechanisms underlying resistance to both agents. Given these findings, it is imperative to explore the interplay between replication gaps and PARPi resistance, particularly in the context of platinum resistance. Understanding the impact of replication gaps on PARPi resistance may offer insights into novel therapeutic strategies to overcome resistance mechanisms and enhance the efficacy of targeted therapies in BRCA1/2-mutant tumors.

Updates in the Use of Targeted Therapies for Gynecologic Cancers.

Targeted therapies have changed the treatment landscape in gynecologic cancer. Studies released over the past year have led to the incorporation of immunotherapy (IO) into the treatment for all patients with endometrial and cervical cancers at some point during their disease course. Poly(ADP-ribose) polymerase (PARP) inhibitors continue to play a role in women with ovarian carcinoma, particularly in homologous repair deficient tumors. Furthermore, the benefit of PARP inhibitors in challenging subgroups continues to be elucidated. Biomarker identification has led to the approval or compendium listing of several antibody-drug conjugates (ADCs). This review will update on IO, ADCs, and PARP inhibition for the treatment of gynecologic cancers.

Novel frontiers in urogenital cancers: from molecular bases to preclinical models to tailor personalized treatments in ovarian and prostate cancer patients.

Over the last few decades, the incidence of urogenital cancers has exhibited diverse trends influenced by screening programs and geographical variations. Among women, there has been a consistent or even increased occurrence of endometrial and ovarian cancers; conversely, prostate cancer remains one of the most diagnosed malignancies, with a rise in reported cases, partly due to enhanced and improved screening efforts.Simultaneously, the landscape of cancer therapeutics has undergone a remarkable evolution, encompassing the introduction of targeted therapies and significant advancements in traditional chemotherapy. Modern targeted treatments aim to selectively address the molecular aberrations driving cancer, minimizing adverse effects on normal cells. However, traditional chemotherapy retains its crucial role, offering a broad-spectrum approach that, despite its wider range of side effects, remains indispensable in the treatment of various cancers, often working synergistically with targeted therapies to enhance overall efficacy.For urogenital cancers, especially ovarian and prostate cancers, DNA damage response inhibitors, such as PARP inhibitors, have emerged as promising therapeutic avenues. In BRCA-mutated ovarian cancer, PARP inhibitors like olaparib and niraparib have demonstrated efficacy, leading to their approval for specific indications. Similarly, patients with DNA damage response mutations have shown sensitivity to these agents in prostate cancer, heralding a new frontier in disease management. Furthermore, the progression of ovarian and prostate cancer is intricately linked to hormonal regulation. Ovarian cancer development has also been associated with prolonged exposure to estrogen, while testosterone and its metabolite dihydrotestosterone, can fuel the growth of prostate cancer cells. Thus, understanding the interplay between hormones, DNA damage and repair mechanisms can hold promise for exploring novel targeted therapies for ovarian and prostate tumors.In addition, it is of primary importance the use of preclinical models that mirror as close as possible the biological and genetic features of patients' tumors in order to effectively translate novel therapeutic findings "from the bench to the bedside".In summary, the complex landscape of urogenital cancers underscores the need for innovative approaches. Targeted therapy tailored to DNA repair mechanisms and hormone regulation might offer promising avenues for improving the management and outcomes for patients affected by ovarian and prostate cancers.

Identification of restrictive molecules involved in oncolytic virotherapy using genome-wide CRISPR screening.

Oncolytic viruses (OVs) offer a novel approach to treat solid tumors; however, their efficacy is frequently suboptimal due to various limiting factors. To address this challenge, we engineered an OV containing targets for neuron-specific microRNA-124 and Granulocyte-macrophage colony-stimulating factor (GM-CSF), significantly enhancing its neuronal safety while minimally compromising its replication capacity. Moreover, we identified PARP1 as an HSV-1 replication restriction factor using genome-wide CRISPR screening. In models of glioblastoma (GBM) and triple-negative breast cancer (TNBC), we showed that the combination of OV and a PARP inhibitor (PARPi) exhibited superior efficacy compared to either monotherapy. Additionally, single-cell RNA sequencing (scRNA-seq) revealed that this combination therapy sensitized TNBC to immune checkpoint blockade, and the incorporation of an immune checkpoint inhibitor (ICI) further increased the survival rate of tumor-bearing mice. The combination of PARPi and ICI synergistically enhanced the ability of OV to establish durable tumor-specific immune responses. Our study effectively overcomes the inherent limitations of OV therapy, providing valuable insights for the clinical treatment of TNBC, GBM, and other malignancies.

CA-125 KELIM as an Alternative Predictive Tool to Identify Which Patients Can Benefit from PARPi in High-Grade Serous Advanced Ovarian Cancer: A Retrospective Pilot Diagnostic Accuracy Study.

BRCA mutation and homologous recombination deficiency (HRD) are the criteria for the administration of PARP inhibitor (PARPi) maintenance therapy. It is known that PARPi efficacy is related to platinum sensitivity and that the latter can be demonstrated from the CA-125 elimination rate constant (KELIM). This study aims to investigate if KELIM can be another tool in the identification of patients that could be benefit from PARPi therapy. Retrospective analysis of patients with high-grade serous advanced ovarian cancer that underwent cytoreduction and was further tested for HRD status. The HRD status was tested either by myChoice HRD CDx assay or by RediScore assay. KELIM score was measured in both neoadjuvant and adjuvant settings with the online tool biomarker-kinetics.org. A total of 39 patients had available data for estimating both HRD status and KELIM score. When assuming KELIM as a binary index test with the value 1 as the cut-off point, the sensitivity was 0.86, 95% CI (0.64-0.97) and the specificity was 0.83, 95% CI (0.59-0.96). On the other hand, when assuming KELIM as a continuous index test, the area under the curve (AUC) was 81% and the optimal threshold, using the Youden index, was identified as 1.03 with a sensitivity of 85.7% and a specificity of 83.3%. KELIM score seems to be a new, cheaper, and faster tool to identify patients that can benefit from PARPi maintenance therapy.

Homologous Recombination Repair Gene Mutations in Prostate Cancer: Prevalence and Clinical Value.

Despite advances in our understanding of the molecular landscape of prostate cancer and the development of novel biomarker-driven therapies, the prognosis of patients with metastatic prostate cancer that is resistant to conventional hormonal therapy remains poor. Data suggest that a significant proportion of patients with metastatic castration-resistant prostate cancer (mCRPC) have mutations in homologous recombination repair (HRR) genes and may benefit from poly(ADP-ribose) polymerase (PARP) inhibitors. However, the adoption of HRR gene mutation testing in prostate cancer remains low, meaning there is a missed opportunity to identify patients who may benefit from targeted therapy with PARP inhibition, with or without novel hormonal agents. Here, we review the current knowledge regarding the clinical significance of HRR gene mutations in prostate cancer and discuss the efficacy of PARP inhibition in patients with mCRPC. This comprehensive overview aims to increase the clinical implementation of HRR gene mutation testing and inform future efforts in personalized treatment of prostate cancer.

Homologous Recombination Repair Deficiency in Metastatic Prostate Cancer: New Therapeutic Opportunities.

More than 20% of metastatic prostate cancer carries genomic defects involving DNA damage repair pathways, mainly in homologous recombination repair-related genes. The recent approval of olaparib has paved the way to precision medicine for the treatment of metastatic prostate cancer with PARP inhibitors in this subset of patients, especially in the case of BRCA1 or BRCA2 pathogenic/likely pathogenic variants. In face of this new therapeutic opportunity, many issues remain unsolved. This narrative review aims to describe the relationship between homologous recombination repair deficiency and prostate cancer, the techniques used to determine homologous recombination repair status in prostate cancer, the crosstalk between homologous recombination repair and the androgen receptor pathway, the current evidence on PARP inhibitors activity in metastatic prostate cancer also in homologous recombination repair-proficient tumors, as well as emerging mechanisms of resistance to PARP inhibitors. The possibility of combination therapies including a PARP inhibitor is an attractive option, and more robust data are awaited from ongoing phase II and phase III trials outlined in this manuscript.

Design, synthesis, and biological evaluation of novel chrysin derivatives as poly(ADP-ribose) polymerase 1 (PARP1) inhibitors for the treatment of breast cancer.

In this study, we reported the discovery and structure-activity relationship analysis of chrysin derivatives as a new class of inhibitors targeting poly (ADP-ribose) polymerase 1 (PARP1). Among these derivatives, compound 5d emerged as the most effective chrysin-based inhibitor of PARP1, with an IC50 value of 108 nmol·L-1. This compound significantly inhibited the proliferation and migration of breast cancer cell lines HCC-1937 and MDA-MB-436 by inducing DNA damage. Furthermore, 5d induced apoptosis and caused an extended G1/S-phase in these cell lines. Molecular docking studies revealed that 5d possesses a strong binding affinity toward PARP1. In vivo, in a xenograft model, 5d effectively reduced tumor growth by downregulating PARP1 expression. Overall, compound 5d shows promise as a potential therapeutic agent for the treatment of BRCA wild-type breast cancer.

Beyond monotherapy: An era ushering in combinations of PARP inhibitors with immune checkpoint inhibitors for solid tumors.

The introduction of PARP inhibitors (PARPis) and immune checkpoint inhibitors (ICIs) has marked a significant shift in the treatment landscape for solid tumors. Emerging preclinical evidence and initial clinical trials have indicated that the synergistic application of PARPis and ICIs may enhance treatment efficacy and potentially improve long-term patient outcomes. Nonetheless, how to identify specific tumor types and molecular subgroups most likely to benefit from this combination remains an area of ongoing research. This review thoroughly examines current studies on the co-administration of PARPis and ICIs across various solid tumors. It explores the underlying mechanisms of action, evaluates clinical efficacy, identifies potential responder populations, and delineates common adverse events alongside strategic management approaches. The aim is to offer a detailed understanding of this combination therapy, potentially guiding future therapeutic strategies for solid tumors.

PARP1-DOT1L transcription axis drives acquired resistance to PARP inhibitor in ovarian cancer.

BACKGROUND: Poly (ADP-ribose) polymerase inhibitor (PARPi) resistance poses a significant challenge in ovarian carcinoma (OC). While the role of DOT1L in cancer and chemoresistance is acknowledged, its specific role in PARPi resistance remains unclear. This study aims to elucidate the molecular mechanism of DOT1L in PARPi resistance in OC patients. METHODS: This study analyzed the expression of DOT1L in PARPi-resistant cell lines compared to sensitive ones and correlated it with clinical outcomes in OC patients. Comprehensive in vitro and in vivo functional experiments were conducted using cellular and mouse models. Molecular investigations, including RNA sequencing, chromatin immunoprecipitation (ChIP) and Cleavage Under Targets and Tagmentation (CUT&Tag) assays, were employed to unravel the molecular mechanisms of DOT1L-mediated PARPi resistance. RESULTS: Our investigation revealed a robust correlation between DOT1L expression and clinical PARPi resistance in non-BRCA mutated OC cells. Upregulated DOT1L expression in PARPi-resistant tissues was associated with diminished survival in OC patients. Mechanistically, we identified that PARP1 directly binds to the DOT1L gene promoter, promoting transcription independently of its enzyme activity. PARP1 trapping induced by PARPi treatment amplified this binding, enhancing DOT1L transcription and contributing to drug resistance. Sequencing analysis revealed that DOT1L plays a crucial role in the transcriptional regulation of PLCG2 and ABCB1 via H3K79me2. This established the PARP1-DOT1L-PLCG2/ABCB1 axis as a key contributor to PARPi resistance. Furthermore, we discovered that combining a DOT1L inhibitor with PARPi demonstrated a synergistic effect in both cell line-derived xenograft mouse models (CDXs) and patient-derived organoids (PDOs). CONCLUSIONS: Our results demonstrate that DOT1L is an independent prognostic marker for OC patients. The PARP1-DOT1L/H3K79me2-PLCG2/ABCB1 axis is identified as a pivotal contributor to PARPi resistance. Targeted inhibition of DOT1L emerges as a promising therapeutic strategy for enhancing PARPi treatment outcomes in OC patients.

NRDE2 deficiency impairs homologous recombination repair and sensitizes hepatocellular carcinoma to PARP inhibitors.

To identify novel susceptibility genes for hepatocellular carcinoma (HCC), we performed a rare-variant association study in Chinese populations consisting of 2,750 cases and 4,153 controls. We identified four HCC-associated genes, including NRDE2, RANBP17, RTEL1, and STEAP3. Using NRDE2 (index rs199890497 [p.N377I], p = 1.19 × 10-9) as an exemplary candidate, we demonstrated that it promotes homologous recombination (HR) repair and suppresses HCC. Mechanistically, NRDE2 binds to the subunits of casein kinase 2 (CK2) and facilitates the assembly and activity of the CK2 holoenzyme. This NRDE2-mediated enhancement of CK2 activity increases the phosphorylation of MDC1 and then facilitates the HR repair. These functions are eliminated almost completely by the NRDE2-p.N377I variant, which sensitizes the HCC cells to poly(ADP-ribose) polymerase (PARP) inhibitors, especially when combined with chemotherapy. Collectively, our findings highlight the relevance of the rare variants to genetic susceptibility to HCC, which would be helpful for the precise treatment of this malignancy.

C5aR1 inhibition reprograms tumor associated macrophages and reverses PARP inhibitor resistance in breast cancer.

Although Poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi) have been approved in multiple diseases, including BRCA1/2 mutant breast cancer, responses are usually transient requiring the deployment of combination therapies for optimal efficacy. Here we thus explore mechanisms underlying sensitivity and resistance to PARPi using two intrinsically PARPi sensitive (T22) and resistant (T127) syngeneic murine breast cancer models in female mice. We demonstrate that tumor associated macrophages (TAM) potentially contribute to the differential sensitivity to PARPi. By single-cell RNA-sequencing, we identify a TAM_C3 cluster, expressing genes implicated in anti-inflammatory activity, that is enriched in PARPi resistant T127 tumors and markedly decreased by PARPi in T22 tumors. Rps19/C5aR1 signaling is selectively elevated in TAM_C3. C5aR1 inhibition or transferring C5aR1hi cells increases and decreases PARPi sensitivity, respectively. High C5aR1 levels in human breast cancers are associated with poor responses to immune checkpoint blockade. Thus, targeting C5aR1 may selectively deplete pro-tumoral macrophages and engender sensitivity to PARPi and potentially other therapies.

Evaluating homologous recombination activity in tissues to predict the risk of hereditary breast and ovarian cancer and olaparib sensitivity.

Homologous recombination (HR) repairs DNA damage including DNA double-stranded breaks and alterations in HR-related genes results in HR deficiency. Germline alteration of HR-related genes, such as BRCA1 and BRCA2, causes hereditary breast and ovarian cancer (HBOC). Cancer cells with HR deficiency are sensitive to poly (ADP-ribose) polymerase (PARP) inhibitors and DNA-damaging agents. Thus, accurately evaluating HR activity is useful for diagnosing HBOC and predicting the therapeutic effects of anti-cancer agents. Previously, we developed an assay for site-specific HR activity (ASHRA) that can quantitatively evaluate HR activity and detect moderate HR deficiency. HR activity in cells measured by ASHRA correlates with sensitivity to the PARP inhibitor, olaparib. In this study, we applied ASHRA to lymphoblastoid cells and xenograft tumor tissues, which simulate peripheral blood lymphocytes and tumor tissues, respectively, as clinically available samples. We showed that ASHRA could be used to detect HR deficiency in lymphoblastoid cells derived from a BRCA1 pathogenic variant carrier. Furthermore, ASHRA could quantitatively measure the HR activity in xenograft tumor tissues with HR activity that was gradually suppressed by inducible BRCA1 knockdown. The HR activity of xenograft tumor tissues quantitatively correlated with the effect of olaparib. Our data suggest that ASHRA could be a useful assay for diagnosing HBOC and predicting the efficacy of PARP inhibitors.

Targeting PARG induces tumor cell growth inhibition and antitumor immune response by reducing phosphorylated STAT3 in ovarian cancer.

BACKGROUND: Ovarian cancer is the most lethal gynecological malignancy, with limited treatment options after failure of standard therapies. Despite the potential of poly(ADP-ribose) polymerase inhibitors in treating DNA damage response (DDR)-deficient ovarian cancer, the development of resistance and immunosuppression limit their efficacy, necessitating alternative therapeutic strategies. Inhibitors of poly(ADP-ribose) glycohydrolase (PARG) represent a novel class of inhibitors that are currently being assessed in preclinical and clinical studies for cancer treatment. METHODS: By using a PARG small-molecule inhibitor, COH34, and a cell-penetrating antibody targeting the PARG's catalytic domain, we investigated the effects of PARG inhibition on signal transducer and activator of transcription 3 (STAT3) in OVCAR8, PEO1, and Brca1-null ID8 ovarian cancer cell lines, as well as in immune cells. We examined PARG inhibition-induced effects on STAT3 phosphorylation, nuclear localization, target gene expression, and antitumor immune responses in vitro, in patient-derived tumor organoids, and in an immunocompetent Brca1-null ID8 ovarian mouse tumor model that mirrors DDR-deficient human high-grade serous ovarian cancer. We also tested the effects of overexpressing a constitutively activated STAT3 mutant on COH34-induced tumor cell growth inhibition. RESULTS: Our findings show that PARG inhibition downregulates STAT3 activity through dephosphorylation in ovarian cancer cells. Importantly, overexpression of a constitutively activated STAT3 mutant in tumor cells attenuates PARG inhibitor-induced growth inhibition. Additionally, PARG inhibition reduces STAT3 phosphorylation in immune cells, leading to the activation of antitumor immune responses, shown in immune cells cocultured with ovarian cancer patient tumor-derived organoids and in immune-competent mice-bearing mouse ovarian tumors. CONCLUSIONS: We have identified a novel antitumor mechanism underlying PARG inhibition beyond its primary antitumor effects through blocking DDR in ovarian cancer. Furthermore, targeting PARG activates antitumor immune responses, thereby potentially increasing response rates to immunotherapy in patients with ovarian cancer.