Combining PARP with ATR inhibition overcomes PARP inhibitor and platinum resistance in ovarian cancer models

Inhibiting PRMT5 induces DNA damage and increases anti-proliferative activity of Niraparib, a PARP inhibitor, in models of breast and ovarian cancer

BACKGROUND

Inhibitors of Poly (ADP-Ribose) Polymerases (PARP) provide clinical benefit to patients with breast and ovarian cancers, by compromising the DNA repair activity of cancer cells. Although these agents extend progression-free survival in many patients, responses can be short lived with many patients ultimately progressing. Identification of combination partners that increase dependence of cancer cells to the DNA repair activity of PARPs may represent a strategy to increase the utility of PARP inhibitors. Protein arginine methyltransferase 5 (PRMT5) regulates DNA damage response pathways through splicing and protein modification, and inhibitors of PRMT5 have recently entered clinical trials.

METHODS

The effect of PRMT5 inhibition on the levels of DNA damage and repair markers including γH2AX, RAD51, and 53BP1 was determined using high content immunofluorescent imaging. The anti-proliferative activity of the combination of PRMT5 and PARP inhibitors was evaluated using in vitro models of breast and ovarian cancers using both cell lines and ex vivo patient derived xenografts. Finally, the combinations of PRMT5 and PARP inhibitors were evaluated in cell line xenograft models in vivo.

RESULTS

Inhibition of PRMT5 by GSK3326595 led to increased levels of markers of DNA damage. The addition of GSK3326595 to the PARP inhibitor, niraparib, resulted in increased growth inhibition of breast and ovarian cancer cell lines and patient derived spheroids. In vivo, the combination improved the partial effects on tumor growth inhibition achieved by either single agent, producing complete tumor stasis and regression.

CONCLUSION

These data demonstrate that inhibition of PRMT5 induced signatures of DNA damage in models of breast and ovarian cancer. Furthermore, combination with the PARP inhibitor, Niraparib, resulted in increased anti-tumor activity in vitro and in vivo. Overall, these data suggest inhibition of PRMT5 as a mechanism to broaden and enhance the clinical application of PARP inhibitors.

An ATR-PrimPol pathway confers tolerance to oncogenic KRAS-induced and heterochromatin-associated replication stress

Activation of the KRAS oncogene is a source of replication stress, but how this stress is generated and how it is tolerated by cancer cells remain poorly understood. Here we show that induction of KRASG12V expression in untransformed cells triggers H3K27me3 and HP1-associated chromatin compaction in an RNA transcription dependent manner, resulting in replication fork slowing and cell death. Furthermore, elevated ATR expression is necessary and sufficient for tolerance of KRASG12V-induced replication stress to expand replication stress-tolerant cells (RSTCs). PrimPol is phosphorylated at Ser255, a potential Chk1 substrate site, under KRASG12V-induced replication stress and promotes repriming to maintain fork progression and cell survival in an ATR/Chk1-dependent manner. However, ssDNA gaps are generated at heterochromatin by PrimPol-dependent repriming, leading to genomic instability. These results reveal a role of ATR-PrimPol in enabling precancerous cells to survive KRAS-induced replication stress and expand clonally with accumulation of genomic instability.

Studying the DNA damage response pathway in hematopoietic canine cancer cell lines, a necessary step for finding targets to generate new therapies to treat cancer in dogs

Background

Dogs present a significant opportunity for studies in comparative oncology. However, the study of cancer biology phenomena in canine cells is currently limited by restricted availability of validated antibody reagents and techniques. Here, we provide an initial characterization of the expression and activity of key components of the DNA Damage Response (DDR) in a panel of hematopoietic canine cancer cell lines, with the use of commercially available antibody reagents.

Materials and methods

The techniques used for this validation analysis were western blot, qPCR, and DNA combing assay.

Results

Substantial variations in both the basal expression (ATR, Claspin, Chk1, and Rad51) and agonist-induced activation (p-Chk1) of DDR components were observed in canine cancer cell lines. The expression was stronger in the CLBL-1 (B-cell lymphoma) and CLB70 (B-cell chronic lymphocytic leukemia) cell lines than in the GL-1 (B-cell leukemia) cell line, but the biological significance of these differences requires further investigation. We also validated methodologies for quantifying DNA replication dynamics in hematopoietic canine cancer cell lines, and found that the GL-1 cell line presented a higher replication fork speed than the CLBL-1 cell line, but that both showed a tendency to replication fork asymmetry.

Conclusion

These findings will inform future studies on cancer biology, which will facilitate progress in developing novel anticancer therapies for canine patients. They can also provide new knowledge in human oncology.

Combination ATR (ceralasertib) and PARP (olaparib) Inhibitor (CAPRI) Trial in Acquired PARP Inhibitor-Resistant Homologous Recombination-Deficient Ovarian Cancer

PURPOSE

Addition of ataxia telangiectasia and Rad3-related kinase inhibitors (ATRi) to PARP inhibitors (PARPi) overcomes PARPi resistance in high-grade serous ovarian cancer (HGSOC) cell and mouse models. We present the results of an investigator-initiated study of combination PARPi (olaparib) and ATRi (ceralasertib) in patients with acquired PARPi-resistant HGSOC.

PATIENTS AND METHODS

Eligible patients had recurrent, platinum-sensitive BRCA1/2 mutated or homologous recombination (HR)-deficient (HRD) HGSOC and clinically benefited from PARPi (response by imaging/CA-125 or duration of maintenance therapy; > 12 months first-line or > 6 months ≥ second-line) before progression. No intervening chemotherapy was permitted. Patients received olaparib 300 mg twice daily and ceralasertib 160 mg daily on days 1 to 7 of a 28-day cycle. Primary objectives were safety and objective response rate (ORR).

RESULTS

Thirteen patients enrolled were evaluable for safety and 12 for efficacy; 62% (n = 8) had germline BRCA1/2 mutations, 23% (n = 3) somatic BRCA1/2 mutations, and 15% (n = 2) tumors with positive HRD assay. Prior PARPi indication was treatment for recurrence (54%, n = 7), second-line maintenance (38%, n = 5) and first-line treatment with carboplatin/paclitaxel (8%, n = 1). There were 6 partial responses yielding an ORR of 50% (95% confidence interval, 0.15-0.72). Median treatment duration was 8 cycles (range 4-23+). Grade (G) 3/4 toxicities were 38% (n = 5); 15% (n = 2) G3 anemia, 23% (n = 3) G3 thrombocytopenia, 8% (n = 1) G4 neutropenia. Four patients required dose reductions. No patient discontinued treatment due to toxicity.

CONCLUSIONS

Combination olaparib and ceralasertib is tolerable and shows activity in HR-deficient platinum-sensitive recurrent HGSOC that benefited and then progressed with PARPi as the penultimate regimen. These data suggest that ceralasertib resensitizes PARPi-resistant HGSOCs to olaparib, warranting further investigation.

The clinical challenges of homologous recombination proficiency in ovarian cancer: from intrinsic resistance to new treatment opportunities

Ovarian cancer is the most lethal gynecologic cancer. Optimal cytoreductive surgery followed by platinum-based chemotherapy with or without bevacizumab is the conventional therapeutic strategy. Since 2016, the pharmacological treatment of epithelial ovarian cancer has significantly changed following the introduction of the poly (ADP-ribose) polymerase inhibitors (PARPi). BRCA1/2 mutations and homologous recombination deficiency (HRD) have been established as predictive biomarkers of the benefit from platinum-based chemotherapy and PARPi. While in the absence of HRD (the so-called homologous recombination proficiency, HRp), patients derive minimal benefit from PARPi, the use of the antiangiogenic agent bevacizumab in first line did not result in different efficacy according to the presence of homologous recombination repair (HRR) genes mutations. No clinical trials have currently compared PARPi and bevacizumab as maintenance therapy in the HRp population. Different strategies are under investigation to overcome primary and acquired resistance to PARPi and to increase the sensitivity of HRp tumors to these agents. These tumors are characterized by frequent amplifications of Cyclin E and MYC, resulting in high replication stress. Different agents targeting DNA replication stress, such as ATR, WEE1 and CHK1 inhibitors, are currently being explored in preclinical models and clinical trials and have shown promising preliminary signs of activity. In this review, we will summarize the available evidence on the activity of PARPi in HRp tumors and the ongoing research to develop new treatment options in this hard-to-treat population.

Establishment and Molecular Characterization of an In Vitro Model for PARPi-Resistant Ovarian Cancer

Overcoming PARPi resistance is a high clinical priority. We established and characterized comparative in vitro models of acquired PARPi resistance, derived from either a BRCA1-proficient or BRCA1-deficient isogenic background by long-term exposure to olaparib. While parental cell lines already exhibited a certain level of intrinsic activity of multidrug resistance (MDR) proteins, resulting PARPi-resistant cells from both models further converted toward MDR. In both models, the PARPi-resistant phenotype was shaped by (i) cross-resistance to other PARPis (ii) impaired susceptibility toward the formation of DNA-platinum adducts upon exposure to cisplatin, which could be reverted by the drug efflux inhibitors verapamil or diphenhydramine, and (iii) reduced PARP-trapping activity. However, the signature and activity of ABC-transporter expression and the cross-resistance spectra to other chemotherapeutic drugs considerably diverged between the BRCA1-proficient vs. BRCA1-deficient models. Using dual-fluorescence co-culture experiments, we observed that PARPi-resistant cells had a competitive disadvantage over PARPi-sensitive cells in a drug-free medium. However, they rapidly gained clonal dominance under olaparib selection pressure, which could be mitigated by the MRP1 inhibitor MK-751. Conclusively, we present a well-characterized in vitro model, which could be instrumental in dissecting mechanisms of PARPi resistance from HR-proficient vs. HR-deficient background and in studying clonal dynamics of PARPi-resistant cells in response to experimental drugs, such as novel olaparib-sensitizers.

Investigation of PARP Inhibitor Resistance Based on Serially Collected Circulating Tumor DNA in Patients With BRCA-Mutated Ovarian Cancer

PURPOSE

Patient-specific molecular alterations leading to PARP inhibitor (PARPi) resistance are relatively unexplored. In this study, we analyzed serially collected circulating tumor DNA (ctDNA) from patients with BRCA1/2 mutations who received PARPis to investigate the resistance mechanisms and their significance in postprogression treatment response and survival.

EXPERIMENTAL DESIGN

Patients were prospectively enrolled between January 2018 and December 2021 (NCT05458973). Whole-blood samples were obtained before PARPi administration and serially every 3 months until progression. ctDNA was extracted from the samples and sequenced with a 531-gene panel; gene sets for each resistance mechanism were curated.

RESULTS

Fifty-four patients were included in this analysis. Mutation profiles of genes in pre-PARPi samples indicating a high tumor mutational burden and alterations in genes associated with replication fork stabilization and drug efflux were associated with poor progression-free survival on PARPis. BRCA hypomorphism and reversion were found in 1 and 3 patients, respectively. Among 29 patients with matched samples, mutational heterogeneity increased postprogression on PARPis, showing at least one postspecific mutation in 89.7% of the patients. These mutations indicate non-exclusive acquired resistance mechanisms-homologous recombination repair restoration (28%), replication fork stability (34%), upregulated survival pathway (41%), target loss (10%), and drug efflux (3%). We observed poor progression-free survival with subsequent chemotherapy in patients with homologous recombination repair restoration (P = 0.003) and those with the simultaneous involvement of two or more resistance mechanisms (P = 0.040).

CONCLUSIONS

Analysis of serial ctDNAs highlighted multiple acquired resistance mechanisms, providing valuable insights for improving postprogression treatment and survival.

PBRM1 mutations might render a subtype of biliary tract cancers sensitive to drugs targeting the DNA damage repair system

Polybromo-1 (PBRM1) loss of function mutations are present in a fraction of biliary tract cancers (BTCs). PBRM1, a subunit of the PBAF chromatin-remodeling complex, is involved in DNA damage repair. Herein, we aimed to decipher the molecular landscape of PBRM1 mutated (mut) BTCs and to define potential translational aspects. Totally, 1848 BTC samples were analyzed using next-generation DNA-sequencing and immunohistochemistry (Caris Life Sciences, Phoenix, AZ). siRNA-mediated knockdown of PBRM1 was performed in the BTC cell line EGI1 to assess the therapeutic vulnerabilities of ATR and PARP inhibitors in vitro. PBRM1 mutations were identified in 8.1% (n = 150) of BTCs and were more prevalent in intrahepatic BTCs (9.9%) compared to gallbladder cancers (6.0%) or extrahepatic BTCs (4.5%). Higher rates of co-mutations in chromatin-remodeling genes (e.g., ARID1A 31% vs. 16%) and DNA damage repair genes (e.g., ATRX 4.4% vs. 0.3%) were detected in PBRM1-mutated (mut) vs. PBRM1-wildtype (wt) BTCs. No difference in real-world overall survival was observed between PBRM1-mut and PBRM1-wt patients (HR 1.043, 95% CI 0.821-1.325, p = 0.731). In vitro, experiments suggested that PARP ± ATR inhibitors induce synthetic lethality in the PBRM1 knockdown BTC model. Our findings served as the scientific rationale for PARP inhibition in a heavily pretreated PBRM1-mut BTC patient, which induced disease control. This study represents the largest and most extensive molecular profiling study of PBRM1-mut BTCs, which in vitro sensitizes to DNA damage repair inhibiting compounds. Our findings might serve as a rationale for future testing of PARP/ATR inhibitors in PBRM1-mut BTCs.

Exploring the tumor immune microenvironment in ovarian cancer: a way-out to the therapeutic roadmap

INTRODUCTION

Despite cancer treatment strides, mortality due to ovarian cancer remains high globally. While immunotherapy has proven effective in treating cancers with low cure rates, it has limitations. Growing evidence suggests that both tumoral and non-tumoral components of the tumor immune microenvironment (TIME) play a significant role in cancer growth. Therefore, developing novel and focused therapy for ovarian cancer is critical. Studies indicate that TIME is involved in developing ovarian cancer, particularly genome-, transcriptome-, and proteome-wide studies. As a result, TIME may present a prospective therapeutic target for ovarian cancer patients.

AREAS COVERED

We examined several TIME-targeting medicines and the connection between TIME and ovarian cancer. The key protagonists and events in the TIME and therapeutic strategies that explicitly target these events in ovarian cancer are discussed.

EXPERT OPINION

We highlighted various targeted therapies against TIME in ovarian cancer, including anti-angiogenesis therapies and immune checkpoint inhibitors. While these therapies are in their infancy, they have shown promise in controlling ovarian cancer progression. The use of 'omics' technology is helping in better understanding of TIME in ovarian cancer and potentially identifying new therapeutic targets. TIME-targeted strategies could account for an additional treatment strategy when treating ovarian cancer.

PARP Inhibitors in Ovarian Cancer: A Review

Poly(ADP-ribose) polymerase (PARP) inhibitors (PARPis) have transformed the ovarian cancer (OC) treatment landscape. This narrative review provides a comprehensive overview of data for the PARPis olaparib, niraparib, and rucaparib in patients with OC and discusses their role in disease management, with a focus on the use of PARPis as maintenance therapy in the United States (US). Olaparib was the first PARPi to be approved as first-line maintenance monotherapy in the US, with maintenance niraparib subsequently approved in the first-line setting. Data also support the efficacy of rucaparib as first-line maintenance monotherapy. PARPi maintenance combination therapy (olaparib plus bevacizumab) also provides benefit in patients with newly diagnosed advanced OC whose tumors tested positive for homologous recombination deficiency (HRD). Biomarker testing is critical in the newly diagnosed setting to identify patients most likely to benefit from PARPi maintenance therapy and guide treatment decisions. Clinical trial data support the use of PARPis (olaparib, niraparib, rucaparib) as second-line or later maintenance therapy in patients with platinum-sensitive relapsed OC. Although distinct differences in tolerability profile were observed between PARPis, they were generally well tolerated, with the majority of adverse events managed by dose modification. PARPis had no detrimental effect on patients' health-related quality of life. Real-world data support the use of PARPis in OC, although some differences between PARPis are apparent. Data from trials investigating novel combination strategies, such as PARPis plus immune checkpoint inhibitors, are awaited with interest; the optimal sequencing of novel therapies in OC remains to be established.

Novel therapies for metastatic prostate cancer

INTRODUCTION

Patients with metastatic prostate cancer, especially in the castrate-resistant setting, have a poor prognosis. Many agents have been approved for metastatic prostate cancer, such as androgen receptor pathway inhibitors, taxane-based chemotherapy, radiopharmaceuticals, and immunotherapy. However, prostate cancer remains the leading cause of cancer deaths in nonsmoking men. Fortunately, many more novel agents are under investigation.

AREAS COVERED

We provide an overview of the broad group of novel therapies for metastatic prostate cancer, with an emphasis on active and recruiting clinical trials that have been recently published and/or presented at national or international meetings.

EXPERT OPINION

The future for patients with metastatic prostate cancer is promising, with further development of novel therapies such as radiopharmaceuticals. Based on a growing understanding of prostate cancer biology, novel agents are being designed to overcome resistance to approved therapies. There are many trials using novel agents either as monotherapy or in combination with already approved agents with potential to further improve outcomes for men with advanced prostate cancer.

Prudent tactics to sail the boat of PARP inhibitors as therapeutics for diverse malignancies

INTRODUCTION

PARP inhibitors block the DNA-repairing mechanism of PARP and represent a promising class of anti-cancer therapy. The last decade has witnessed FDA approvals of several PARP inhibitors, with some undergoing advanced-stage clinical investigation. Medicinal chemists have invested much effort to expand the structure pool of PARP inhibitors. Issues associated with the use of PARP inhibitors that make their standing disconcerting in the pharmaceutical sector have been addressed via the design of new structural assemblages.

AREA COVERED

In this review, the authors present a detailed account of the medicinal chemistry campaigns conducted in the recent past for the construction of PARP1/PARP2 inhibitors, PARP1 biased inhibitors, and PARP targeting bifunctional inhibitors as well as PARP targeting degraders (PROTACs). Limitations associated with FDA-approved PARP inhibitors and strategies to outwit the limitations are also discussed.

EXPERT OPINION

The PARP inhibitory field has been rejuvenated with numerous tractable entries in the last decade. With numerous magic bullets in hand coupled with unfolded tactics to outwit the notoriety of cancer cells developing resistance toward PARP inhibitors, the dominance of PARP inhibitors as a sagacious option of targeted therapy is highly likely to be witnessed soon.

Role of epithelial-mesenchymal transition factor SNAI1 and its targets in ovarian cancer aggressiveness

Ovarian cancer remains the most lethal gynecologic malignancy in the USA. For over twenty years, epithelial-mesenchymal transition (EMT) has been characterized extensively in development and disease. The dysregulation of this process in cancer has been identified as a mechanism by which epithelial tumors become more aggressive, allowing them to survive and invade distant tissues. This occurs in part due to the increased expression of the EMT transcription factor, SNAI1 (Snail). In the case of epithelial ovarian cancer, Snail has been shown to contribute to cancer invasion, stemness, chemoresistance, and metabolic changes. Thus, in this review, we focus on summarizing current findings on the role of EMT (specifically, factors downstream of Snail) in determining ovarian cancer aggressiveness.

Homologous Recombination Deficiency (HRD) in Cutaneous Oncology

Skin cancers, including basal cell carcinoma (BCC), cutaneous squamous cell carcinoma (SCC), and melanoma, are the most common malignancies in the United States. Loss of DNA repair pathways in the skin plays a significant role in tumorigenesis. In recent years, targeting DNA repair pathways, particularly homologous recombination deficiency (HRD), has emerged as a potential therapeutic approach in cutaneous malignancies. This review provides an overview of DNA damage and repair pathways, with a focus on HRD, and discusses major advances in targeting these pathways in skin cancers. Poly(ADP-ribose) polymerase (PARP) inhibitors have been developed to exploit HRD in cancer cells. PARP inhibitors disrupt DNA repair mechanisms by inhibiting PARP enzymatic activity, leading to the accumulation of DNA damage and cell death. The concept of synthetic lethality has been demonstrated in HR-deficient cells, such as those with BRCA1/2 mutations, which exhibit increased sensitivity to PARP inhibitors. HRD assessment methods, including genomic scars, RAD51 foci formation, functional assays, and BRCA1/2 mutation analysis, are discussed as tools for identifying patients who may benefit from PARP inhibitor therapy. Furthermore, HRD has been implicated in the response to immunotherapy, and the combination of PARP inhibitors with immunotherapy has shown promising results. The frequency of HRD in melanoma ranges from 18% to 57%, and studies investigating the use of PARP inhibitors as monotherapy in melanoma are limited. Further research is warranted to explore the potential of PARP inhibition in melanoma treatment.

"DEPHENCE" system-a novel regimen of therapy that is urgently needed in the high-grade serous ovarian cancer-a focus on anti-cancer stem cell and anti-tumor microenvironment targeted therapies

Ovarian cancer, especially high-grade serous type, is the most lethal gynecological malignancy. The lack of screening programs and the scarcity of symptomatology result in the late diagnosis in about 75% of affected women. Despite very demanding and aggressive surgical treatment, multiple-line chemotherapy regimens and both approved and clinically tested targeted therapies, the overall survival of patients is still unsatisfactory and disappointing. Research studies have recently brought some more understanding of the molecular diversity of the ovarian cancer, its unique intraperitoneal biology, the role of cancer stem cells, and the complexity of tumor microenvironment. There is a growing body of evidence that individualization of the treatment adjusted to the molecular and biochemical signature of the tumor as well as to the medical status of the patient should replace or supplement the foregoing therapy. In this review, we have proposed the principles of the novel regimen of the therapy that we called the "DEPHENCE" system, and we have extensively discussed the results of the studies focused on the ovarian cancer stem cells, other components of cancer metastatic niche, and, finally, clinical trials targeting these two environments. Through this, we have tried to present the evolving landscape of treatment options and put flesh on the experimental approach to attack the high-grade serous ovarian cancer multidirectionally, corresponding to the "DEPHENCE" system postulates.

BLM overexpression as a predictive biomarker for CHK1 inhibitor response in PARP inhibitor-resistant BRCA-mutant ovarian cancer

Poly(ADP-ribose) polymerase inhibitors (PARPis) have changed the treatment paradigm in breast cancer gene (BRCA)-mutant high-grade serous ovarian carcinoma (HGSC). However, most patients eventually develop resistance to PARPis, highlighting an unmet need for improved therapeutic strategies. Using high-throughput drug screens, we identified ataxia telangiectasia and rad3-related protein/checkpoint kinase 1 (CHK1) pathway inhibitors as cytotoxic and further validated the activity of the CHK1 inhibitor (CHK1i) prexasertib in PARPi-sensitive and -resistant BRCA-mutant HGSC cells and xenograft mouse models. CHK1i monotherapy induced DNA damage, apoptosis, and tumor size reduction. We then conducted a phase 2 study (NCT02203513) of prexasertib in patients with BRCA-mutant HGSC. The treatment was well tolerated but yielded an objective response rate of 6% (1 of 17; one partial response) in patients with previous PARPi treatment. Exploratory biomarker analyses revealed that replication stress and fork stabilization were associated with clinical benefit to CHK1i. In particular, overexpression of Bloom syndrome RecQ helicase (BLM) and cyclin E1 (CCNE1) overexpression or copy number gain/amplification were seen in patients who derived durable benefit from CHK1i. BRCA reversion mutation in previously PARPi-treated BRCA-mutant patients was not associated with resistance to CHK1i. Our findings suggest that replication fork-related genes should be further evaluated as biomarkers for CHK1i sensitivity in patients with BRCA-mutant HGSC.

The evolving role of DNA damage response in overcoming therapeutic resistance in ovarian cancer

Epithelial ovarian cancer (EOC) is treated in the first-line setting with combined platinum and taxane chemotherapy, often followed by a maintenance poly (ADP-ribose) polymerase inhibitor (PARPi). Responses to first-line treatment are frequent. For many patients, however, responses are suboptimal or short-lived. Over the last several years, multiple new classes of agents targeting DNA damage response (DDR) mechanisms have advanced through clinical development. In this review, we explore the preclinical rationale for the use of ATR inhibitors, CHK1 inhibitors, and WEE1 inhibitors, emphasizing their application to chemotherapy-resistant and PARPi-resistant ovarian cancer. We also present an overview of the clinical development of the leading drugs in each of these classes, emphasizing the rationale for monotherapy and combination therapy approaches.

Adjuvant Therapy with Oncolytic Adenovirus Delta-24-RGDOX After Intratumoral Adoptive T-cell Therapy Promotes Antigen Spread to Sustain Systemic Antitumor Immunity

Cancer cell heterogeneity and immunosuppressive tumor microenvironment (TME) pose a challenge in treating solid tumors with adoptive cell therapies targeting limited tumor-associated antigens (TAA), such as chimeric antigen receptor T-cell therapy. We hypothesize that oncolytic adenovirus Delta-24-RGDOX activates the TME and promote antigen spread to potentiate the abscopal effect of adoptive TAA-targeting T cells in localized intratumoral treatment. Herein, we used C57BL/6 mouse models with disseminated tumors derived from B16 melanoma cell lines to assess therapeutic effects and antitumor immunity. gp100-specific pmel-1 or ovalbumin (OVA)-specific OT-I T cells were injected into the first subcutaneous tumor, followed by three injections of Delta-24-RGDOX. We found TAA-targeting T cells injected into one subcutaneous tumor showed tumor tropism. Delta-24-RGDOX sustained the systemic tumor regression mediated by the T cells, leading to improved survival rate. Further analysis revealed that, in mice with disseminated B16-OVA tumors, Delta-24-RGDOX increased CD8⁺ leukocyte density within treated and untreated tumors. Importantly, Delta-24-RGDOX significantly reduced the immunosuppression of endogenous OVA-specific CTLs while increasing that of CD8⁺ leukocytes and, to a lesser extent, adoptive pmel-1 T cells. Consequently, Delta-24-RGDOX drastically increased the density of the OVA-specific CTLs in both tumors, and the combination synergistically enhanced the effect. Consistently, the splenocytes from the combination group showed a significantly stronger response against other TAAs (OVA and TRP2) than gp100, resulted in higher activity against tumor cells. Therefore, our data demonstrate that, as an adjuvant therapy followed TAA-targeting T cells in localized treatment, Delta-24-RGDOX activates TME and promotes antigen spread, leading to efficacious systemic antitumor immunity to overcome tumor relapse.

Significance

Adjuvant therapy with oncolytic viruses promotes antigen spread to potentiate localized intratumoral adoptive T-cell therapy with limited TAA targets, leading to sustainable systemic antitumor immunity to overcome tumor relapse.

Discovery of Ruthenium(II) Metallocompound and Olaparib Synergy for Cancer Combination Therapy

Synergistic drug combinations can extend the use of poly(ADP-ribose) polymerase inhibitors (PARPi) such as Olaparib to BRCA-proficient tumors and overcome acquired or de novo drug resistance. To identify new synergistic combinations for PARPi, we screened a "micro-library" comprising a mix of commercially available drugs and DNA-binding ruthenium(II) polypyridyl complexes (RPCs) for Olaparib synergy in BRCA-proficient triple-negative breast cancer cells. This identified three hits: the natural product Curcumin and two ruthenium(II)-rhenium(I) polypyridyl metallomacrocycles. All combinations identified were effective in BRCA-proficient breast cancer cells, including an Olaparib-resistant cell line, and spheroid models. Mechanistic studies indicated that synergy was achieved via DNA-damage enhancement and resultant apoptosis. Combinations showed low cytotoxicity toward non-malignant breast epithelial cells and low acute and developmental toxicity in zebrafish embryos. This work identifies RPC metallomacrocycles as a novel class of agents for cancer combination therapy and provides a proof of concept for the inclusion of metallocompounds within drug synergy screens.

Emerging strategies for cancer therapy by ATR inhibitors

DNA replication stress (RS) causes genomic instability and vulnerability in cancer cells. To counteract RS, cells have evolved various mechanisms involving the ATR kinase signaling pathway, which regulates origin firing, cell cycle checkpoints, and fork stabilization to secure the fidelity of replication. However, ATR signaling also alleviates RS to support cell survival by driving RS tolerance, thereby contributing to therapeutic resistance. Cancer cells harboring genetic mutations and other changes that disrupt normal DNA replication increase the risk of DNA damage and the levels of RS, conferring addiction to ATR activity for sustainable replication and susceptibility to therapeutic approaches using ATR inhibitors (ATRis). Therefore, clinical trials are currently being conducted to evaluate the efficacy of ATRis as monotherapies or in combination with other drugs and biomarkers. In this review, we discuss recent advances in the elucidation of the mechanisms by which ATR functions in the RS response and its therapeutic relevance when utilizing ATRis.

Dramatic, durable response to therapy in gBRCA2-mutated pancreas neuroendocrine carcinoma: opportunity and challenge

Poorly differentiated pancreatic neuroendocrine tumors (PDNEC), are a subtype of pancreatic cancer encompassing both small cell and large cell neuroendocrine carcinoma subtypes, and are characterized as distinct in terms of biology and prognosis compared to the more common pancreatic adenocarcinoma. Until recently, there has been a paucity of data on the genomic features of this cancer type. We describe a male patient diagnosed with PDNEC and extensive metastatic disease in the liver at diagnosis. Genomic analysis demonstrated a germline pathogenic variant in BRCA2 with somatic loss-of-heterozygosity of the BRCA2 wild-type allele. Following a favorable response to platinum-based chemotherapy (and the addition of immunotherapy), the patient received maintenance therapy with olaparib, which resulted in a further reduction on follow-up imaging (Fig. 1). After seventeen months of systemic control with olaparib, the patient developed symptomatic central nervous system metastases, which harboured a BRCA2 reversion mutation. No other sites of disease progression were observed. Herein, we report an exceptional outcome through the incorporation of a personalized management approach for a patient with a pancreatic PDNEC, guided by comprehensive genomic sequencing.

PARP Inhibitors in Breast and Ovarian Cancer

Poly (ADP-ribose) polymerase (PARP) inhibitors are one of the most successful examples of clinical translation of targeted therapies in medical oncology, and this has been demonstrated by their effective management of BRCA1/BRCA2 mutant cancers, most notably in breast and ovarian cancers. PARP inhibitors target DNA repair pathways that BRCA1/2-mutant tumours are dependent upon. Inhibition of the key components of these pathways leads to DNA damage triggering subsequent critical levels of genomic instability, mitotic catastrophe and cell death. This ultimately results in a synthetic lethal relationship between BRCA1/2 and PARP, which underpins the effectiveness of PARP inhibitors. Despite the early and dramatic response seen with PARP inhibitors, patients receiving them often develop treatment resistance. To date, data from both clinical and preclinical studies have highlighted multiple resistance mechanisms to PARP inhibitors, and only by understanding these mechanisms are we able to overcome the challenges. The focus of this review is to summarise the underlying mechanisms underpinning treatment resistance to PARP inhibitors and to aid both clinicians and scientists to develop better clinically applicable assays to better select patients who would derive the greatest benefit as well as develop new novel/combination treatment strategies to overcome these mechanisms of resistance. With a better understanding of PARP inhibitor resistance mechanisms, we would not only be able to identify a subset of patients who are unlikely to benefit from therapy but also to sequence our treatment paradigm to avoid and overcome these resistance mechanisms.

Olaparib-Resistant BRCA2MUT Ovarian Cancer Cells with Restored BRCA2 Abrogate Olaparib-Induced DNA Damage and G2/M Arrest Controlled by the ATR/CHK1 Pathway for Survival

The PARP inhibitor (PARPi) olaparib is currently the drug of choice for serous ovarian cancer (OC), especially in patients with homologous recombination (HR) repair deficiency associated with deleterious BRCA1/2 mutations. Unfortunately, OC patients who fail to respond to PARPi or relapse after treatment have limited therapeutic options. To elucidate olaparib resistance and enhance the efficacy of olaparib, intracellular factors exploited by OC cells to achieve decreased sensitivity to PARPi were examined. An olaparib-resistant OC cell line, PEO1-OR, was established from BRCA2MUT PEO1 cells. The anticancer activity and action of olaparib combined with inhibitors of the ATR/CHK1 pathway (ceralasertib as ATRi, MK-8776 as CHK1i) in olaparib-sensitive and -resistant OC cell lines were evaluated. Whole-exome sequencing revealed that PEO1-OR cells acquire resistance through subclonal enrichment of BRCA2 secondary mutations that restore functional full-length protein. Moreover, PEO1-OR cells upregulate HR repair-promoting factors (BRCA1, BRCA2, RAD51) and PARP1. Olaparib-inducible activation of the ATR/CHK1 pathway and G2/M arrest is abrogated in olaparib-resistant cells. Drug sensitivity assays revealed that PEO1-OR cells are less sensitive to ATRi and CHK1i agents. Combined treatment is less effective in olaparib-resistant cells considering inhibition of metabolic activity, colony formation, survival, accumulation of DNA double-strand breaks, and chromosomal aberrations. However, synergistic antitumor activity between compounds is achievable in PEO1-OR cells. Collectively, olaparib-resistant cells display co-existing HR repair-related mechanisms that confer resistance to olaparib, which may be effectively utilized to resensitize them to PARPi via combination therapy. Importantly, the addition of ATR/CHK1 pathway inhibitors to olaparib has the potential to overcome acquired resistance to PARPi.

Computational Methods Summarizing Mutational Patterns in Cancer: Promise and Limitations for Clinical Applications

Since the rise of next-generation sequencing technologies, the catalogue of mutations in cancer has been continuously expanding. To address the complexity of the cancer-genomic landscape and extract meaningful insights, numerous computational approaches have been developed over the last two decades. In this review, we survey the current leading computational methods to derive intricate mutational patterns in the context of clinical relevance. We begin with mutation signatures, explaining first how mutation signatures were developed and then examining the utility of studies using mutation signatures to correlate environmental effects on the cancer genome. Next, we examine current clinical research that employs mutation signatures and discuss the potential use cases and challenges of mutation signatures in clinical decision-making. We then examine computational studies developing tools to investigate complex patterns of mutations beyond the context of mutational signatures. We survey methods to identify cancer-driver genes, from single-driver studies to pathway and network analyses. In addition, we review methods inferring complex combinations of mutations for clinical tasks and using mutations integrated with multi-omics data to better predict cancer phenotypes. We examine the use of these tools for either discovery or prediction, including prediction of tumor origin, treatment outcomes, prognosis, and cancer typing. We further discuss the main limitations preventing widespread clinical integration of computational tools for the diagnosis and treatment of cancer. We end by proposing solutions to address these challenges using recent advances in machine learning.

DNA Repair and Therapeutic Strategies in Cancer Stem Cells

First-line cancer treatments successfully eradicate the differentiated tumour mass but are comparatively ineffective against cancer stem cells (CSCs), a self-renewing subpopulation thought to be responsible for tumour initiation, metastasis, heterogeneity, and recurrence. CSCs are thus presented as the principal target for elimination during cancer treatment. However, CSCs are challenging to drug target because of numerous intrinsic and extrinsic mechanisms of drug resistance. One such mechanism that remains relatively understudied is the DNA damage response (DDR). CSCs are presumed to possess properties that enable enhanced DNA repair efficiency relative to their highly proliferative bulk progeny, facilitating improved repair of double-strand breaks induced by radiotherapy and most chemotherapeutics. This can occur through multiple mechanisms, including increased expression and splicing fidelity of DNA repair genes, robust activation of cell cycle checkpoints, and elevated homologous recombination-mediated DNA repair. Herein, we summarise the current knowledge concerning improved genome integrity in non-transformed stem cells and CSCs, discuss therapeutic opportunities within the DDR for re-sensitising CSCs to genotoxic stressors, and consider the challenges posed regarding unbiased identification of novel DDR-directed strategies in CSCs. A better understanding of the DDR mediating chemo/radioresistance mechanisms in CSCs could lead to novel therapeutic approaches, thereby enhancing treatment efficacy in cancer patients.

BRCA Mutations-The Achilles Heel of Breast, Ovarian and Other Epithelial Cancers

Two related tumor suppressor genes, BRCA1 and BRCA2, attract a lot of attention from both fundamental and clinical points of view. Oncogenic hereditary mutations in these genes are firmly linked to the early onset of breast and ovarian cancers. However, the molecular mechanisms that drive extensive mutagenesis in these genes are not known. In this review, we hypothesize that one of the potential mechanisms behind this phenomenon can be mediated by Alu mobile genomic elements. Linking mutations in the BRCA1 and BRCA2 genes to the general mechanisms of genome stability and DNA repair is critical to ensure the rationalized choice of anti-cancer therapy. Accordingly, we review the literature available on the mechanisms of DNA damage repair where these proteins are involved, and how the inactivating mutations in these genes (BRCAness) can be exploited in anti-cancer therapy. We also discuss a hypothesis explaining why breast and ovarian epithelial tissues are preferentially susceptible to mutations in BRCA genes. Finally, we discuss prospective novel therapeutic approaches for treating BRCAness cancers.

Tumor suppressor role of RBM22 in prostate cancer acting as a dual-factor regulating alternative splicing and transcription of key oncogenic genes

Prostate cancer (PCa) is one of the leading causes of cancer-related deaths among men. Consequently, the identification of novel molecular targets for treatment is urgently needed to improve patients' outcomes. Our group recently reported that some elements of the cellular machinery controlling alternative-splicing might be useful as potential novel therapeutic tools against advanced PCa. However, the presence and functional role of RBM22, a key spliceosome component, in PCa remains unknown. Therefore, RBM22 levels were firstly interrogated in 3 human cohorts and 2 preclinical mouse models (TRAMP/Pbsn-Myc). Results were validated in in silico using 2 additional cohorts. Then, functional effects in response to RBM22 overexpression (proliferation, migration, tumorspheres/colonies formation) were tested in PCa models in vitro (LNCaP, 22Rv1, and PC-3 cell-lines) and in vivo (xenograft). High throughput methods (ie, RNA-seq, nCounter PanCancer Pathways Panel) were performed in RBM22 overexpressing cells and xenograft tumors. We found that RBM22 levels were down-regulated (mRNA and protein) in PCa samples, and were inversely associated with key clinical aggressiveness features. Consistently, a gradual reduction of RBM22 from non-tumor to poorly differentiated PCa samples was observed in transgenic models (TRAMP/Pbsn-Myc). Notably, RBM22 overexpression decreased aggressiveness features in vitro, and in vivo. These actions were associated with the splicing dysregulation of numerous genes and to the downregulation of critical upstream regulators of cell-cycle (i.e., CDK1/CCND1/EPAS1). Altogether, our data demonstrate that RBM22 plays a critical pathophysiological role in PCa and invites to suggest that targeting negative regulators of RBM22 expression/activity could represent a novel therapeutic strategy to tackle this disease.

CRISPR screens reveal genetic determinants of PARP inhibitor sensitivity and resistance in prostate cancer

Prostate cancer harboring BRCA1/2 mutations are often exceptionally sensitive to PARP inhibitors. However, genomic alterations in other DNA damage response genes have not been consistently predictive of clinical response to PARP inhibition. Here, we perform genome-wide CRISPR-Cas9 knockout screens in BRCA1/2-proficient prostate cancer cells and identify previously unknown genes whose loss has a profound impact on PARP inhibitor response. Specifically, MMS22L deletion, frequently observed (up to 14%) in prostate cancer, renders cells hypersensitive to PARP inhibitors by disrupting RAD51 loading required for homologous recombination repair, although this response is TP53-dependent. Unexpectedly, loss of CHEK2 confers resistance rather than sensitivity to PARP inhibition through increased expression of BRCA2, a target of CHEK2-TP53-E2F7-mediated transcriptional repression. Combined PARP and ATR inhibition overcomes PARP inhibitor resistance caused by CHEK2 loss. Our findings may inform the use of PARP inhibitors beyond BRCA1/2-deficient tumors and support reevaluation of current biomarkers for PARP inhibition in prostate cancer.

Combined inhibition of PARP and ATR synergistically potentiates the antitumor activity of HER2-targeting antibody-drug conjugate in HER2-positive cancers

The therapeutic management of various HER2-positive malignancies involves the use of HER2-targeted antibody-drug conjugates (ADCs). The primary mechanism of action of ADCs is the release of cytotoxic chemicals, which leads to single- or double-strand DNA breaks and cell death. Since both endogenous and exogenous sources of DNA damage are unavoidable, cells have evolved DNA damage-repair mechanisms. Therefore, combining inhibitors of DNA damage repair and HER2-targeted ADCs may be a practical strategy for treating HER2-positive cancers. Effects of the HER2-targeted ADC, DS-8201, in combination with PARPi (AZD2281), a DNA damage repair inhibitor that targets poly(ADP-ribose) polymerase, and ATRi (BAY1895344), which inhibits the serine/threonine kinase ATR, were determined by assessing cell-growth inhibition, apoptosis and cell-cycle arrest, as well as using in vivo pharmacodynamic studies. Combined use of AZD2281 and BAY1895344 synergistically potentiated the inhibitory effects of DS-8201 on the growth of HER2-positive cancer cells, inducing DNA damage and apoptosis, but had no effect on HER2-negative MDA-MB-231 breast cancer cells. Our data demonstrate that DS-8201 and DNA damage repair inhibitors together have synergistic anticancer effects in NCI-N87 xenograft models, effects that may reflect upregulation of γ-H2AX protein in tumor tissues. Collectively, our results indicate that the combination of DS-8201, BAY1895344, and AZD2281 exerts significant synergistic antitumor activity, suggesting that DNA damage-repair inhibitors in combination with HER2-targeted ADCs is a potential approach for treating HER2-positive malignancies, offering a promising strategy for future clinical applications.

DNA Damage Response Alterations in Ovarian Cancer: From Molecular Mechanisms to Therapeutic Opportunities

The DNA damage response (DDR), a set of signaling pathways for DNA damage detection and repair, maintains genomic stability when cells are exposed to endogenous or exogenous DNA-damaging agents. Alterations in these pathways are strongly associated with cancer development, including ovarian cancer (OC), the most lethal gynecologic malignancy. In OC, failures in the DDR have been related not only to the onset but also to progression and chemoresistance. It is known that approximately half of the most frequent subtype, high-grade serous carcinoma (HGSC), exhibit defects in DNA double-strand break (DSB) repair by homologous recombination (HR), and current evidence indicates that probably all HGSCs harbor a defect in at least one DDR pathway. These defects are not restricted to HGSCs; mutations in ARID1A, which are present in 30% of endometrioid OCs and 50% of clear cell (CC) carcinomas, have also been found to confer deficiencies in DNA repair. Moreover, DDR alterations have been described in a variable percentage of the different OC subtypes. Here, we overview the main DNA repair pathways involved in the maintenance of genome stability and their deregulation in OC. We also recapitulate the preclinical and clinical data supporting the potential of targeting the DDR to fight the disease.

Clinical application of PARP inhibitors in ovarian cancer: from molecular mechanisms to the current status

As a kind of gynecological tumor, ovarian cancer is not as common as cervical cancer and breast cancer, but its malignant degree is higher. Despite the increasingly mature treatment of ovarian cancer, the five-year survival rate of patients is still less than 50%. Based on the concept of synthetic lethality, poly (ADP- ribose) polymerase (PARP) inhibitors target tumor cells with defects in homologous recombination repair(HRR), the most significant being the target gene Breast cancer susceptibility genes(BRCA). PARP inhibitors capture PARP-1 protein at the site of DNA damage to destroy the original reaction, causing the accumulation of PARP-DNA nucleoprotein complexes, resulting in DNA double-strand breaks(DSBs) and cell death. PARP inhibitors have been approved for the treatment of ovarian cancer for several years and achieved good results. However, with the widespread use of PARP inhibitors, more and more attention has been paid to drug resistance and side effects. Therefore, further research is needed to understand the mechanism of PARP inhibitors, to be familiar with the adverse reactions of the drug, to explore the markers of its efficacy and prognosis, and to deal with its drug resistance. This review elaborates the use of PARP inhibitors in ovarian cancer.

ATF6-Mediated Signaling Contributes to PARP Inhibitor Resistance in Ovarian Cancer

High-grade serous ovarian cancer (HGSOC) is the deadliest ovarian cancer histotype due in-part to the lack of therapeutic options for chemotherapy-resistant disease. PARP inhibitors (PARPi) represent a targeted treatment. However, PARPi resistance is becoming a significant clinical challenge. There is an urgent need to overcome resistance mechanisms to extend disease-free intervals. We established isogeneic PARPi-sensitive and -resistant HGSOC cell lines. In three PARPi-resistant models, there is a significant increase in AP-1 transcriptional activity and DNA repair capacity. Using RNA-sequencing and an shRNA screen, we identified activating transcription factor 6 (ATF6) as a mediator of AP-1 activity, DNA damage response, and PARPi resistance. In publicly available datasets, ATF6 expression is elevated in HGSOC and portends a poorer recurrence-free survival. In a cohort of primary HGSOC tumors, higher ATF6 expression significantly correlated to PARPi resistance. In PARPi-resistant cell lines and a PDX model, inhibition of a known ATF6 regulator, p38, attenuated AP-1 activity and RAD51 foci formation, enhanced DNA damage, significantly inhibited tumor burden, and reduced accumulation of nuclear ATF6.

IMPLICATIONS

This study highlights that a novel p38-ATF6-mediated AP-1 signaling axis contributes to PARPi resistance and provides a clinical rationale for combining PARPi and AP-1 signaling inhibitors.

Targeting replication stress in cancer therapy

Replication stress is a major cause of genomic instability and a crucial vulnerability of cancer cells. This vulnerability can be therapeutically targeted by inhibiting kinases that coordinate the DNA damage response with cell cycle control, including ATR, CHK1, WEE1 and MYT1 checkpoint kinases. In addition, inhibiting the DNA damage response releases DNA fragments into the cytoplasm, eliciting an innate immune response. Therefore, several ATR, CHK1, WEE1 and MYT1 inhibitors are undergoing clinical evaluation as monotherapies or in combination with chemotherapy, poly[ADP-ribose]polymerase (PARP) inhibitors, or immune checkpoint inhibitors to capitalize on high replication stress, overcome therapeutic resistance and promote effective antitumour immunity. Here, we review current and emerging approaches for targeting replication stress in cancer, from preclinical and biomarker development to clinical trial evaluation.

Strategies for the Management of Patients with Pancreatic Cancer with PARP Inhibitors

A subset of patients with pancreatic adenocarcinomas (PDAC) harbor mutations that are exploitable in the context of DNA-damage response and repair (DDR) inhibitory strategies. Between 8-18% of PDACs harbor specific mutations in the DDR pathway such as BRCA1/2 mutations, and a higher prevalence exists in high-risk populations (e.g., Ashkenazi Jews). Herein, we will review the current trials and data on the treatment of PDAC patients who harbor such mutations and who appear sensitive to platinum and/or poly ADP ribose polymerase inhibitor (PARPi) based therapies due to a concept known as synthetic lethality. Although this current best-in-class precision treatment shows clinical promise, the specter of resistance limits the extent of therapeutic responses. We therefore also evaluate promising pre-clinical and clinical approaches in the pipeline that may either work with existing therapies to break resistance or work separately with combination therapies against this subset of PDACs.

Mechanisms of PARP Inhibitor Resistance

Poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi) represent the first medicines based on the targeting of the DNA damage response (DDR). PARPi have become standard of care for first-line maintenance treatment in ovarian cancer and have also been approved in other cancer indications including breast, pancreatic and prostate. Despite their efficacy, resistance to PARPi has been reported clinically and represents a growing patient population with unmet clinical need. Here, we describe the various mechanisms of PARPi resistance that have been identified in pre-clinical models and in the clinic.

Chk1 inhibition-induced BRCAness synergizes with olaparib in p53-deficient cancer cells

Although targeting DNA-damage repair by inhibition of PARP exhibits weak or modest single-agent activity due to the existence of functional BRCA1/2 alleles, PARP inhibitors have been gradually applicable in BRCA-proficient cancers. Checkpoint kinase 1 (Chk1) inhibition selectively disrupts homologous recombination (HR)-mediated DNA repair and confers synthetic lethality in p53-deficient tumors, we therefore aim at expounding the chemopotentiating effects of Chk1 inhibition on PARPi in BRCA-proficient and p53-deficient cancer cells. Initially, BRCA wild-type, p53-null cells including AsPC-1 and H1299 demonstrated innate resistance to PARP inhibitor olaparib compared to BRCA1-mutant, p53-null MDA-MB-436 cells. We quantified the interaction between olaparib and a selective Chk1 inhibitor MK-8776, which produced synergistic effects under sub-IC50 concentrations in p53-depleted AsPC-1 and H1299 cells. Olaparib in combination with MK-8776 showed enhanced antitumor effects through prohibiting proliferation and secondarily inducing apoptosis in two cell lines. Of note, we observed that MK-8776 significantly sensitized cells to olaparib by broad DNA and chromosomal breaks. Mechanistically, MK-8776 abrogated olaparib-induced BRCA1 intranuclear foci formation, MCM7-mediated replication machineries, and ultimately triggered an accumulation of γH2AX, a well-recognized marker of DNA double-strand breaks. Additionally, we established ectopic expression of hotspot mutant p53 in H1299 cells. Introduction of p53R175 H promoted olaparib resistance as single-agent treatment, but the synergy between olaparib and MK-8776 was still achievable and the region of synergy was produced by lower combination concentrations. These data provide insight into how Chk1 inhibition could be effectively targeted and confer sensitivity to olaparib toward p53-deficient and HR-proficient cancers.

Human PARP1 substrates and regulators of its catalytic activity: An updated overview

Poly (ADP-ribose) polymerase 1 (PARP1) is a key DNA damage sensor that is recruited to damaged sites after DNA strand breaks to initiate DNA repair. This is achieved by catalyzing attachment of ADP-ribose moieties, which are donated from NAD⁺, on the amino acid residues of itself or other acceptor proteins. PARP inhibitors (PARPi) that inhibit PARP catalytic activity and induce PARP trapping are commonly used for treating BRCA1/2-deficient breast and ovarian cancers through synergistic lethality. Unfortunately, resistance to PARPi frequently occurs. In this review, we present the novel substrates and regulators of the PARP1-catalyzed poly (ADP-ribosyl)ation (PARylatison) that have been identified in the last 3 years. The overall aim is the presentation of protein interactions of potential therapeutic intervention for overcoming the resistance to PARPi.

High expression of CETN2 is associated with platinum resistance and poor prognosis in epithelial ovarian cancer

PURPOSE

The poor prognosis of ovarian cancer is largely due to platinum resistance. It has been demonstrated that nucleotide excision repair (NER) involving centrin-2(CETN2) is connected to platinum resistance in ovarian cancer. The molecular mechanism of CETN2 in ovarian cancer and the mechanism affecting the outcome of chemotherapy are unknown.

METHODS

The protein-protein interaction (PPI) network was mapped after obtaining the interacting proteins of CETN2, and the interacting genes were subjected to enrichment analysis. To examine the relationship between CETN2 and platinum resistance, gene microarray data and clinical data related to platinum resistance in ovarian cancer were downloaded. The possible signaling pathway of CETN2 was investigated by Gene set enrichment analysis (GSEA). Immune infiltration analysis was performed. Immunohistochemistry (IHC) and quantitative real-time PCR (QRT-PCR) were used to examine the expression of CETN2 in clinical samples in relation to the effectiveness of chemotherapy. The capacity of CETN2 to predict chemotherapy results was proven by receiver operating characteristic (ROC) curves after the construction of two prediction models, the logistic regression model and the decision tree model. The impact of CETN2 on prognosis was examined using the Kaplan-Meier technique.

RESULTS

CETN2 was associated with NER, oxidative phosphorylation (OXPHOS) and cell cycle pathways in ovarian cancer drug-resistant samples. In clinical samples, CETN2 showed its possible correlation with immune infiltration. The protein expression level of CETN2 was significantly higher in platinum-resistant patients than that in platinum-sensitive patients, and the expression level had some predictive value for chemotherapy outcome, and high CETN2 protein expression was associated with poorer progression-free survival.

CONCLUSIONS

CETN2 protein had a significant effect on ovarian cancer platinum sensitivity and prognosis, which may be related to the activation of NER, OXPHOS and cell cycle pathways upon CETN2 upregulation. Further research is necessary to determine the therapeutic application value of CETN2, which may be a new biomarker of chemoresponsiveness.

Identification of an Individualized Prognostic Biomarker for Serous Ovarian Cancer: A Qualitative Model

Serous ovarian cancer is the most common type of ovarian epithelial cancer and usually has a poor prognosis. The objective of this study was to construct an individualized prognostic model for predicting overall survival in serous ovarian cancer. Based on the relative expression orderings (Ea > Eb/Ea ≤ Eb) of gene pairs closely associated with serous ovarian prognosis, we tried constructing a potential individualized qualitative biomarker by the greedy algorithm and evaluated the performance in independent validation datasets. We constructed a prognostic biomarker consisting of 20 gene pairs (SOV-P20). The overall survival between high- and low-risk groups stratified by SOV-P20 was statistically significantly different in the training and independent validation datasets from other platforms (p < 0.05, Wilcoxon test). The average area under the curve (AUC) values of the training and three validation datasets were 0.756, 0.590, 0.630, and 0.680, respectively. The distribution of most immune cells between high- and low-risk groups was quite different (p < 0.001, Wilcoxon test). The low-risk patients tended to show significantly better tumor response to chemotherapy than the high-risk patients (p < 0.05, Fisher’s exact test). SOV-P20 achieved the highest mean index of concordance (C-index) (0.624) compared with the other seven existing prognostic signatures (ranging from 0.511 to 0.619). SOV-P20 is a promising prognostic biomarker for serous ovarian cancer, which will be applicable for clinical predictive risk assessment.

Genomic and expressional dynamics of ovarian cancer cell lines in PARPi treatment revealed mechanisms of acquired resistance

BACKGROUND

Patients with epithelial ovarian cancer (EOC) can benefit from poly- (ADP ribose) polymerase inhibitors (PARPi) therapy. However, PARPi resistance has become a challenge in clinical practice, and its mechanism requires further exploration.

METHODS

We established three PARPi-resistant cell strains following olaparib exposure. CCK-8, clonogenic survival, transwell, wound healing, cell cycle, RT-qPCR and western blot assays were performed to explore the functional phenotype of the resistant cells. Whole-exome sequencing and RNA-sequencing were performed to identify the altered genes. Stable knockdown and overexpression were used to investigate the role of EP300, an upstream regulator of E-cadherin and epithelial-mesenchymal transition (EMT), in cell lines. We further validated the finding in clinical ovarian cancer samples by immunohistochemistry.

RESULTS

We combined public datasets to obtain an integrated PARPi sensitivity profile in EOC cells, which indicated that primary PARPi resistance could not be fully explained by mutations in BRCA1/2 or homologous recombination deficiency related genes. Genomic and transcriptome analyses revealed distinct mechanisms between primary and acquired resistance. Long-term PARPi treatment induced accumulation of de novo single nucleotide variants (SNV), and the complete frame-shift deletion of PARP1 was detected in the A2780 resistant strain. Additionally, the depressed histone acetyltransferase of EP300 could cause resistant phenotype through activated EMT process in vitro, and associated with PARPi-resistance in EOC patients.

CONCLUSION

Long-term PARPi treatment led to evolutionary genomic and transcriptional alterations that were associated with acquired resistance, among which depressed EP300 partly contributed to the resistant phenotype.

Resistance to Poly (ADP-Ribose) Polymerase Inhibitors (PARPi): Mechanisms and Potential to Reverse

PURPOSE OF REVIEW

This review will focus on the most common mechanisms for poly (ADP-ribose) polymerase inhibitors' (PARPi) resistance and the main strategies for overcoming acquired or de novo PARPi resistance.

RECENT FINDINGS

Initial approvals for PARPi as part of treatment for advanced epithelial ovarian cancer (EOC) started in 2014 with patient with recurrent cancer characterized by BRCA mutations in the 3rd and 4th line and now have approvals for front-line maintenance in both the BRCA mutated and BRCAwt populations. As with all therapies, patients will eventually develop resistance to treatment. The most common mechanisms for PARPi resistance include reversion mutations, methylation events, and restoration of homologous recombination deficiency (HRD) through combinations and targeting replication stress. As more and more patients receive initial treatment (and potential retreatment with PARPi), we need to better understand the mechanisms in which tumors acquire PARPi resistance.

Mechanisms of PARP1 inhibitor resistance and their implications for cancer treatment

The discovery of synthetic lethality as a result of the combined loss of PARP1 and BRCA has revolutionized the treatment of DNA repair-deficient cancers. With the development of PARP inhibitors, patients displaying germline or somatic mutations in BRCA1 or BRCA2 were presented with a novel therapeutic strategy. However, a large subset of patients do not respond to PARP inhibitors. Furthermore, many of those who do respond eventually acquire resistance. As such, combating de novo and acquired resistance to PARP inhibitors remains an obstacle in achieving durable responses in patients. In this review, we touch on some of the key mechanisms of PARP inhibitor resistance, including restoration of homologous recombination, replication fork stabilization and suppression of single-stranded DNA gap accumulation, as well as address novel approaches for overcoming PARP inhibitor resistance.

Clinical Application of Poly(ADP-ribose) Polymerase (PARP) Inhibitors in Prostate Cancer

Approximately a quarter of men with metastatic castrate resistant prostate cancer (mCRPC) have alterations in homologous recombination repair (HRR). These patients exhibit enhanced sensitivity to poly(ADP-ribose) polymerase (PARP) inhibitors. Leveraging the synthetic lethality between PARP inhibition and HRR deficiency, studies have established marked clinical benefit and a survival advantage from PARP inhibitors (PARPi) in mCRPC, most notably in cancers with BRCA1/2 alterations. The role of PARPi is evolving beyond patients with HRR alterations, with studies increasingly focused on exploiting synergistic effects from combination therapeutics. Strategies combining PARP inhibitors with androgen receptor pathway inhibitors, radiation, radioligand therapy, chemotherapy and immunotherapy demonstrate potential additional benefits in mCRPC and these approaches are rapidly moving into the metastatic hormone sensitive treatment paradigm. In this review we summarise the development and expanding role of PARPi in prostate cancer including biomarkers of response, the relationship between the androgen receptor and PARP, evidence for combination therapeutics and the future directions of PARPi in precision medicine for prostate cancer.

ATR Inhibitors in Platinum-Resistant Ovarian Cancer

Platinum-resistant ovarian cancer (PROC) is one of the deadliest types of epithelial ovarian cancer, and it is associated with a poor prognosis as the median overall survival (OS) is less than 12 months. Targeted therapy is a popular emerging treatment method. Several targeted therapies, including those using bevacizumab and poly (ADP-ribose) polymerase inhibitor (PARPi), have been used to treat PROC. Ataxia telangiectasia and RAD3-Related Protein Kinase inhibitors (ATRi) have attracted attention as a promising class of targeted drugs that can regulate the cell cycle and influence homologous recombination (HR) repair. In recent years, many preclinical and clinical studies have demonstrated the efficacy of ATRis in PROC. This review focuses on the anticancer mechanism of ATRis and the progress of research on ATRis for PROC.

DNA Damage Response in Cancer Therapy and Resistance: Challenges and Opportunities

Resistance to chemo- and radiotherapy is a common event among cancer patients and a reason why new cancer therapies and therapeutic strategies need to be in continuous investigation and development. DNA damage response (DDR) comprises several pathways that eliminate DNA damage to maintain genomic stability and integrity, but different types of cancers are associated with DDR machinery defects. Many improvements have been made in recent years, providing several drugs and therapeutic strategies for cancer patients, including those targeting the DDR pathways. Currently, poly (ADP-ribose) polymerase inhibitors (PARP inhibitors) are the DDR inhibitors (DDRi) approved for several cancers, including breast, ovarian, pancreatic, and prostate cancer. However, PARPi resistance is a growing issue in clinical settings that increases disease relapse and aggravate patients' prognosis. Additionally, resistance to other DDRi is also being found and investigated. The resistance mechanisms to DDRi include reversion mutations, epigenetic modification, stabilization of the replication fork, and increased drug efflux. This review highlights the DDR pathways in cancer therapy, its role in the resistance to conventional treatments, and its exploitation for anticancer treatment. Biomarkers of treatment response, combination strategies with other anticancer agents, resistance mechanisms, and liabilities of treatment with DDR inhibitors are also discussed.

Role of Rucaparib in the Treatment of Prostate Cancer: Clinical Perspectives and Considerations

Prostate cancer is one of the most common types of cancer worldwide and has strong genetic associations. This is important for the development of therapeutics for the condition, as metastatic castrate-resistant prostate cancer (mCRPC) is resistant to standard androgen deprivation therapy (ADT) and has a relatively poor prognosis. We conducted a literature review on rucaparib, a poly(adenosine diphosphate-ribose) polymerase (PARP) inhibitor that is currently indicated for the treatment of patients with mCRPC who harbor mutations in BRCA1/2 (homologous recombination repair [HRR] genes) and who have already tried androgen receptor-axis-targeted therapies (ARAT) and a taxane chemotherapy. We describe rucaparib's FDA approval, which was based on the results of the single-arm, open-label, Phase II TRITON2 clinical trial, which found an objective response rate (ORR) of 43.5%, a duration of response (DOR) of over six months in length and an acceptable safety profile. Rucaparib's dosage and clinical considerations for use were also discussed. We also compared rucaparib's use and safety profile with Olaparib, niraparib and talazoparib, three other PARP inhibitors tested for the treatment of mCRPC. Overall, initial results show that the safety profile of all four drugs in mCRPC was relatively similar, and further testing is currently indicated for all four. Differences in their metabolism, however, also warrant further research. The clinical validity of rucaparib will be tested by the follow-up TRITON3 clinical trial, which is comparing the effect of rucaparib compared to standard therapies for mCRPC harboring BRCA1/2 or ATM mutations. Other than TRITON3, other clinical trials are testing rucaparib's ability against other cancers (prostate or otherwise) with HRR mutations, and also the efficacy of combination therapies involving rucaparib. Finally, more research is needed to elucidate rucaparib's effect on HRR mutations other than BRCA1/2. Advancements in understanding the genetic landscape of mCRPC will also assist in understanding rucaparib's full therapeutic potential.

Differences in Durability of PARP Inhibition by Clinically Approved PARP Inhibitors: Implications for Combinations and Scheduling

Six PARP inhibitors (PARPi) are approved for cancer therapy as monotherapy agents in daily or twice-daily continuous dosing schedules to maintain the necessary continuous suppression of PARP activity. Continuous PARP inhibition is required for single-agent anticancer activity. To investigate if such intense schedules are necessary, we determined the durability of PARP inhibition up to 72 h after a 1 h pulse of 1 µM of five of the approved PARPi, rucaparib, olaparib, niraparib, talazoparib and pamiparib, in IGROV-1 and ES-2 (human ovarian cancer) cells. Rucaparib caused the most persistent inhibition of PARP activity when maintained at ≥75% at 72 h after drug withdrawal in both IGROV-1 and ES-2 cells, but inhibition was more rapidly lost with the other PARPi. PARPi are also under clinical investigation with ATR inhibitors, and thus, we evaluated the implications for scheduling with an ATR inhibitor (VE-821). Rucaparib enhanced VE-821 cytotoxicity in co-exposure, sequential and delayed (24 h drug-free) schedules in IGROV-1 and ES-2 cells. Olaparib and niraparib enhanced VE-821 cytotoxicity only in co-exposed cells and not in sequential exposures. These data have clinical implications for the scheduling of PARPi as a monotherapy and in combination with ATR inhibitors and other cytotoxic drugs.

Splicing factor BUD31 promotes ovarian cancer progression through sustaining the expression of anti-apoptotic BCL2L12

Dysregulated expression of splicing factors has important roles in cancer development and progression. However, it remains a challenge to identify the cancer-specific splicing variants. Here we demonstrate that spliceosome component BUD31 is increased in ovarian cancer, and its higher expression predicts worse prognosis. We characterize the BUD31-binding motif and find that BUD31 preferentially binds exon-intron regions near splicing sites. Further analysis reveals that BUD31 inhibition results in extensive exon skipping and a reduced production of long isoforms containing full coding sequence. In particular, we identify BCL2L12, an anti-apoptotic BCL2 family member, as one of the functional splicing targets of BUD31. BUD31 stimulates the inclusion of exon 3 to generate full-length BCL2L12 and promotes ovarian cancer progression. Knockdown of BUD31 or splice-switching antisense oligonucleotide treatment promotes exon 3 skipping and results in a truncated isoform of BCL2L12 that undergoes nonsense-mediated mRNA decay, and the cells subsequently undergo apoptosis. Our findings reveal BUD31-regulated exon inclusion as a critical factor for ovarian cancer cell survival and cancer progression.

ATM- and ATR-induced primary ciliogenesis promotes cisplatin resistance in pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal cancers because of its late diagnosis and chemoresistance. Primary cilia, the cellular antennae, are observed in most human cells to maintain development and differentiation. Primary cilia are gradually lost during the progression of pancreatic cancer and are eventually absent in PDAC. Here, we showed that cisplatin-resistant PDAC regrew primary cilia. Additionally, genetic or pharmacological disruption of primary cilia sensitized PDAC to cisplatin treatment. Mechanistically, ataxia telangiectasia mutated (ATM) and ATM and RAD3-related (ATR), tumor suppressors that initiate DNA damage responses, promoted the excessive formation of centriolar satellites (EFoCS) and autophagy activation. Disruption of EFoCS and autophagy inhibited primary ciliogenesis, sensitizing PDAC cells to cisplatin treatment. Collectively, our findings revealed an unexpected interplay among the DNA damage response, primary cilia, and chemoresistance in PDAC and deciphered the molecular mechanism by which ATM/ATR-mediated EFoCS and autophagy cooperatively regulate primary ciliogenesis.

The synthetic lethality of targeting cell cycle checkpoints and PARPs in cancer treatment

Continuous cell division is a hallmark of cancer, and the underlying mechanism is tumor genomics instability. Cell cycle checkpoints are critical for enabling an orderly cell cycle and maintaining genome stability during cell division. Based on their distinct functions in cell cycle control, cell cycle checkpoints are classified into two groups: DNA damage checkpoints and DNA replication stress checkpoints. The DNA damage checkpoints (ATM-CHK2-p53) primarily monitor genetic errors and arrest cell cycle progression to facilitate DNA repair. Unfortunately, genes involved in DNA damage checkpoints are frequently mutated in human malignancies. In contrast, genes associated with DNA replication stress checkpoints (ATR-CHK1-WEE1) are rarely mutated in tumors, and cancer cells are highly dependent on these genes to prevent replication catastrophe and secure genome integrity. At present, poly (ADP-ribose) polymerase inhibitors (PARPi) operate through “synthetic lethality” mechanism with mutant DNA repair pathways genes in cancer cells. However, an increasing number of patients are acquiring PARP inhibitor resistance after prolonged treatment. Recent work suggests that a combination therapy of targeting cell cycle checkpoints and PARPs act synergistically to increase the number of DNA errors, compromise the DNA repair machinery, and disrupt the cell cycle, thereby increasing the death rate of cancer cells with DNA repair deficiency or PARP inhibitor resistance. We highlight a combinational strategy involving PARP inhibitors and inhibition of two major cell cycle checkpoint pathways, ATM-CHK2-TP53 and ATR-CHK1-WEE1. The biological functions, resistance mechanisms against PARP inhibitors, advances in preclinical research, and clinical trials are also reviewed.