Understanding Resistance Mechanisms and Expanding the Therapeutic Utility of PARP Inhibitors

Induction of the DNA-Repair Gene POLQ only in BRCA1-mutant Breast-Cancer Cells by Methionine Restriction

BACKGROUND/AIM

BRCA1/2 mutations in breast cancer cells impair homologous recombination and promote alternative end joining (Alt-EJ) for DNA-damage repair. DNA polymerase theta, encoded by POLQ, plays a crucial role in Alt-EJ, making it a potential therapeutic target, particularly in BRCA1/2-mutant cancers. Methionine restriction is a promising approach to target cancer cells due to their addiction to this amino acid. The present study investigated the expression of POLQ in BRCA1/2 wild-type and BRCA1-mutant breast cancer cells under methionine restriction.

MATERIALS AND METHODS

POLQ mRNA expression was measured using qRT-PCR in BRCA1/2 wild-type (MDA-MB-231) and BRCA1- mutant (HCC1937 and MDA-MB-436) breast-cancer cells under normal, or serum-restricted, or serum- and methionine-restricted conditions.

RESULTS

Compared to BRCA1/2 wild-type cells, BRCA1-mutant cells displayed significantly higher basal POLQ expression in normal medium. Methionine restriction further increased POLQ expression in the BRCA1-mutant cells but decreased it in the BRCA1/2 wild-type cells.

CONCLUSION

The present findings suggest that methionine restriction showed differential effects on POLQ expression, potentially impacting Alt-EJ activity, in BRCA1/2 wild-type and BRCA1-mutant breast-cancer cells. Further investigation is needed to explore the potential of combining methionine restriction with DNA-repair inhibitors, such as PARP inhibitors, to overcome drug resistance in BRCA1/2 mutant cancers.

PARP Inhibitors: Strategic Use and Optimal Management in Ovarian Cancer

Poly (ADP-ribose) polymerase (PARP) inhibitors have become an established part of the anticancer armamentarium. Discovered in the 1980s, PARP inhibitors (PARPis) were initially developed to exploit the presence of BRCA mutations, which disrupt the homologous recombination repair of deoxyribonucleic acid (DNA) via synthetic lethality, an intrinsic vulnerability caused by the cell's dependence on other DNA repair mechanisms for which PARP is an essential contributor. PARPi use expanded with the demonstration of clinical benefit when other mechanisms of high-fidelity DNA damage response were present in cancer cells called homologous repair deficiency (HRD). Recently, new data have resulted in the voluntary withdrawal of later-line treatment indications for all the available PARPis used in ovarian cancer because of a negative impact on overall survival (OS). PARPi switch maintenance to consolidate a response to platinum-based therapy is recommended for earlier treatment lines to have the greatest impact on the chance of cure and length of survival. This article reviews the clinical utility of PARPis and how to integrate them into best practices.

PSPC1 Inhibition Synergizes with Poly(ADP-ribose) Polymerase Inhibitors in a Preclinical Model of BRCA-Mutated Breast/Ovarian Cancer

Poly (ADP-ribose) polymerase (PARP) inhibitors are effective against BRCA1/2-mutated cancers through synthetic lethality. Unfortunately, most cases ultimately develop acquired resistance. Therefore, enhancing PARP inhibitor sensitivity and preventing resistance in those cells are an unmet clinical need. Here, we investigated the ability of paraspeckle component 1 (PSPC1), as an additional synthetic lethal partner with BRCA1/2, to enhance olaparib sensitivity in preclinical models of BRCA1/2-mutated breast and ovarian cancers. In vitro, the combined olaparib and PSPC1 small interfering RNA (siRNA) exhibited synergistic anti-proliferative activity in BRCA1/2-mutated breast and ovarian cancer cells. The combination therapy also demonstrated synergistic tumor inhibition in a xenograft mouse model. Mechanistically, olaparib monotherapy increased the expressions of p-ATM and DNA-PKcs, suggesting the activation of a DNA repair pathway, whereas combining PSPC1 siRNA with olaparib decreased the expressions of p-ATM and DNA-PKcs again. As such, the combination increased the formation of γH2AX foci, indicating stronger DNA double-strand breaks. Subsequently, these DNA-damaged cells escaped G2/M checkpoint activation, as indicated by the suppression of p-cdc25C (Ser216) and p-cdc2 (Tyr15) after combination treatment. Finally, these cells entered mitosis, which induced increased apoptosis. Thus, this proves that PSPC1 inhibition enhances olaparib sensitivity by targeting DNA damage response in our preclinical model. The combination of olaparib and PSPC1 inhibition merits further clinical investigation to enhance PARP inhibitor efficacy.

Targetable lesions and proteomes predict therapy sensitivity through disease evolution in pediatric acute lymphoblastic leukemia

Childhood acute lymphoblastic leukemia (ALL) genomes show that relapses often arise from subclonal outgrowths. However, the impact of clonal evolution on the actionable proteome and response to targeted therapy is not known. Here, we present a comprehensive retrospective analysis of paired ALL diagnosis and relapsed specimen. Targeted next generation sequencing and proteome analysis indicate persistence of actionable genome variants and stable proteomes through disease progression. Paired viably-frozen biopsies show high correlation of drug response to variant-targeted therapies but in vitro selectivity is low. Proteome analysis prioritizes PARP1 as a pan-ALL target candidate needed for survival following cellular stress; diagnostic and relapsed ALL samples demonstrate robust sensitivity to treatment with two PARP1/2 inhibitors. Together, these findings support initiating prospective precision oncology approaches at ALL diagnosis and emphasize the need to incorporate proteome analysis to prospectively determine tumor sensitivities, which are likely to be retained at disease relapse.

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.

Recent Insights into PARP and Immuno-Checkpoint Inhibitors in Epithelial Ovarian Cancer

Ovarian cancer is one of the most common gynecologic cancers and has the highest mortality rate of any other cancer of the female reproductive system. Epithelial ovarian cancer (EOC) accounts for approximately 90% of all ovarian malignancies. The standard therapeutic strategy includes cytoreductive surgery accompanied by pre- or postoperative platinum-based chemotherapy. Nevertheless, up to 80% of the patients relapse within the following 12-18 months from the completion of the treatment and then receive first-line chemotherapy depending on platinum sensitivity. Mutations in BRCA1/2 genes are the most significant molecular aberrations in EOC and serve as prognostic and predictive biomarkers. Poly ADP-ribose polymerase (PARP) inhibitors exploit defects in the DNA repair pathway through synthetic lethality. They have also been shown to trap PARP1 and PARP2 on DNA, leading to PARP-DNA complexes. Olaparib, rucaparib, and niraparib have all obtained Food and Drug Administration (FDA) and/or the European Medicine Agency (EMA) approval for the treatment of EOC in different settings. Immune checkpoint inhibitors (ICI) have improved the survival of several cancers and are under evaluation in EOC. However, despite the success of immunotherapy in other malignancies, the use of antibodies inhibiting the immune checkpoint programmed cell death (PD-1) or its ligand (PD-L1) obtained modest results in EOC so far, with median response rates of up to 10%. As such, ICI have not yet been approved for the treatment of EOC. We herein provided a comprehensive insight into the most recent progress in synthetic lethality PARP inhibitors, along with the mechanisms of resistance. We also summarised data regarding the role of immune checkpoint inhibitors, the use of vaccination therapy, and adoptive immunotherapy in treating epithelial ovarian cancer.

Homologous Recombination Deficiency in Ovarian, Breast, Colorectal, Pancreatic, Non-Small Cell Lung and Prostate Cancers, and the Mechanisms of Resistance to PARP Inhibitors

Homologous recombination (HR) is a highly conserved DNA repair mechanism that protects cells from exogenous and endogenous DNA damage. Breast cancer 1 (BRCA1) and breast cancer 2 (BRCA2) play an important role in the HR repair pathway by interacting with other DNA repair proteins such as Fanconi anemia (FA) proteins, ATM, RAD51, PALB2, MRE11A, RAD50, and NBN. These pathways are frequently aberrant in cancer, leading to the accumulation of DNA damage and genomic instability known as homologous recombination deficiency (HRD). HRD can be caused by chromosomal and subchromosomal aberrations, as well as by epigenetic inactivation of tumor suppressor gene promoters. Deficiency in one or more HR genes increases the risk of many malignancies. Another conserved mechanism involved in the repair of DNA single-strand breaks (SSBs) is base excision repair, in which poly (ADP-ribose) polymerase (PARP) enzymes play an important role. PARP inhibitors (PARPIs) convert SSBs to more cytotoxic double-strand breaks, which are repaired in HR-proficient cells, but remain unrepaired in HRD. The blockade of both HR and base excision repair pathways is the basis of PARPI therapy. The use of PARPIs can be expanded to sporadic cancers displaying the “BRCAness” phenotype. Although PARPIs are effective in many cancers, their efficacy is limited by the development of resistance. In this review, we summarize the prevalence of HRD due to mutation, loss of heterozygosity, and promoter hypermethylation of 35 DNA repair genes in ovarian, breast, colorectal, pancreatic, non-small cell lung cancer, and prostate cancer. The underlying mechanisms and strategies to overcome PARPI resistance are also discussed.

PARP inhibitors chemopotentiate and synergize with cisplatin to inhibit bladder cancer cell survival and tumor growth

Background

Management of bladder cancer (BLCA) has not changed significantly in the past few decades, with platinum agent chemotherapy being used in most cases. Chemotherapy reduces tumor recurrence after resection, but debilitating toxicities render a large percentage of patients ineligible. Recently approved immunotherapy can improve outcomes in only a third of metastatic BLCA patients. Therefore, more options for therapy are needed. In this study, we explored the efficacy of PARP inhibitors (PARPi) as single agents or as combinations with platinum therapy.

Methods

We treated BLCA cells with PARPi (olaparib, niraparib, rucaparib, veliparib, or talazoparib) alone or as the combination of cisplatin with PARPi. We then measured their survival, proliferation, apoptosis, as well as their ability to form colonies. BLCA xenografts in male SCID mice were treated similarly, followed by the assessment of their growth, proliferation, and apoptosis.

Results

PARPi niraparib and talazoparib were effective in reducing BLCA cell survival as single agents. Combinations of Cisplatin with talazoparib and niraparib effectively reduced the survival of BLCA cells, while veliparib was not effective even at high concentrations. In vivo, the combinations of cisplatin with niraparib, rucaparib, or talazoparib reduced BLCA xenograft growth significantly.

Conclusions

We provide evidence that PARPi can be effective against BLCA as single agents or as combinatorial therapy with cisplatin.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-09376-9.

Dual-target inhibitors based on PARP1: new trend in the development of anticancer research

PARP1 is a hot target, and its inhibitors have been approved for cancer therapy. However, some undesirable properties restrict the application of PARP1 inhibitors, including drug resistance, side effects and low efficiency. For multifactorial diseases, dual-target drugs have exhibited excellent synergistic effects, such as reduced drug resistance, low side effects and high therapeutic efficacy, by simultaneously regulating the main pathogenic and compensatory signal pathways of diseases. In recent years, several dual-target inhibitors based on PARP1 have been reported and have demonstrated unique advantages. In this review we summarize the research progress in dual-target inhibitors based on PARP1 and discuss the related drug design strategies and structure-activity relationships. This work is expected to provide references for the development of PARP1 inhibitors.

Poly(ADP-ribose) polymerase inhibitors in prostate cancer: a cornerstone in precision oncology

Poly-(ADP-ribose) polymerase (PARP) inhibitors act in cells with defects in homologous recombination DNA repair (HRR) caused by genomic aberrations such as BRCA mutations. This phenomenon called synthetic lethality is known now to be more common in prostate cancer than previously thought. Olaparib and rucaparib, two PARP inhibitors, were successfully tested in clinical trials for HRR-deficient metastatic castration-resistant prostate cancer. They received a breakthrough US FDA approval in HRR altered metastatic castration-resistant prostate cancer in May 2020. Consequently, the combination of PARP inhibitors with other agents such as androgen receptor pathway inhibitors, immune checkpoint inhibitors or DNA damage inducing chemotherapy are being currently largely studied. In our review, we aim to summarize the key PARP inhibitors published and ongoing trials in prostate cancer.

PARP inhibitors in gastric cancer: beacon of hope

Defects in the DNA damage response (DDR) can lead to genome instability, producing mutations or aberrations that promote the development and progression of cancer. But it also confers such cells vulnerable to cell death when they inhibit DNA damage repair. Poly (ADP-ribose) polymerase (PARP) plays a central role in many cellular processes, including DNA repair, replication, and transcription. PARP induces the occurrence of poly (ADP-ribosylation) (PARylation) when DNA single strand breaks (SSB) occur. PARP and various proteins can interact directly or indirectly through PARylation to regulate DNA repair. Inhibitors that directly target PARP have been found to block the SSB repair pathway, triggering homologous recombination deficiency (HRD) cancers to form synthetic lethal concepts that represent an anticancer strategy. It has therefore been investigated in many cancer types for more effective anti-cancer strategies, including gastric cancer (GC). This review describes the antitumor mechanisms of PARP inhibitors (PARPis), and the preclinical and clinical progress of PARPis as monotherapy and combination therapy in GC.

Research progress of PARP inhibitor monotherapy and combination therapy for endometrial cancer

Endometrial cancer is one of the three most common malignant tumors in the female reproductive system. Advanced and recurrent endometrial cancers have poor prognoses and lack effective treatments. Poly(ADP-ribose) polymerase (PARP) inhibitors have been applied to many different types of tumors, and they can selectively kill tumor cells that are defective in homologous recombination repair. Endometrial cancer is characterized by mutations in homologous recombination repair genes; accordingly, PARP inhibitors have achieved positive results in off-label treatments of endometrial cancer cases. Clinical trials of PARP inhibitors as monotherapies and within combination therapies for endometrial cancer are ongoing. For this review, we searched PubMed with "endometrial cancer" and "PARP inhibitor" as keywords, and we used "olaparib", "rucaparib", "niraparib" and "talazoparib" as search terms in clinicaltrials.gov for ongoing trials. The literature search ended in October 2020, and only English-language publications were selected. Multiple studies confirm that PARP inhibitors play an important role in killing tumor cells with defects in homologous recombination repair. Its combination with immune checkpoint inhibitors, PI3K/AKT/mTOR pathway inhibitors, cell cycle checkpoint inhibitors, and other drugs can improve the treatment of endometrial cancer.

Advances in the Treatment of Ovarian Cancer Using PARP Inhibitors and the Underlying Mechanism of Resistance

The standard treatment for advanced ovarian cancer is cytoreductive surgery followed by cytotoxic chemotherapy. However, it has high risk of recurrence and poor prognosis. Poly(ADPribose) polymerase (PARP) inhibitors selectively target DNA double-strand breaks (DSBs) in tumor cells that cannot be repaired and induce the synthetic lethality of BRCA1/2 mutation cancers. PARP inhibitors are clinically used to treat recurrent ovarian cancer and show significant efficacy in ovarian cancer patients with homologous recombination repair (HRR) pathway defects. PARP inhibitors also have significant clinical benefits in patients without HR defects. With the increasingly extensive clinical application of PARP inhibitors, the possibility of acquiring drug resistance is high. Therefore, clinical strategies should be adopted to manage drug resistance of PARP inhibitors. This study aims to summarize the indications and toxicity of PARP inhibitors, the mechanism of action, targeted treatment of drug resistance, and potential methods to manage drug-resistant diseases. We used the term "ovarian cancer" and the names of each PARP inhibitor as keywords to search articles published in the Medical Subject Headings (MeSH) on Pubmed, along with the keywords "clinicaltrials.gov" and "google.com/patents" as well as "uspto.gov." The FDA has approved olaparib, niraparib, and rucaparib for the treatment of recurrent epithelial ovarian cancer (EOC). Talazoparib and veliparib are currently in early trials and show promising clinical results. The mechanism underlying resistance to PARP inhibitors and the clinical strategies to overcome them remain unclear. Understanding the mechanism of resistance to PARP inhibitors and their relationship with platinum resistance may help with the development of antiresistance therapies and optimization of the sequence of drug application in the future clinical treatment of ovarian cancer.

Adjuvant therapy for endometrial cancer in the era of molecular classification: radiotherapy, chemoradiation and novel targets for therapy

Endometrial cancer is primarily treated with surgery. Adjuvant treatment strategies for endometrial cancer, such as external beam pelvic radiotherapy, vaginal brachytherapy, chemotherapy, and combined chemotherapy and radiotherapy, have been studied in several randomized trials. Adjuvant treatment is currently based on the presence of clinico-pathological risk factors. Low-risk disease is adequately managed with surgery alone. In high-intermediate risk endometrial cancer, adjuvant vaginal brachytherapy is recommended to maximize local control, with only mild side effects and without impact on quality of life. For high-risk endometrial cancer, recent large randomized trials support the use of pelvic radiotherapy, especially in stage I-II endometrial cancer with risk factors. For women with serous cancers and those with stage III disease, chemoradiation increased both recurrence-free and overall survival, while GOG-258 showed similar recurrence-free survival compared with six cycles of chemotherapy alone, but with better pelvic and para-aortic nodal control with combined chemotherapy and radiotherapy. Recent molecular studies, most notably the work from The Cancer Genome Atlas (TCGA) project, have shown that four endometrial cancer molecular classes can be distinguished; POLE ultra-mutated, microsatellite instable hypermutated, copy-number-low, and copy-number-high. Subsequent studies, using surrogate markers to identify groups analogous to TCGA sub-classes, showed that all four endometrial cancer sub-types are found across all stages, histological types, and grades. Moreover, the molecular sub-groups have proved to have a stronger prognostic impact than histo-pathological tumor characteristics. This introduces an new era of molecular classification based diagnostics and treatment approaches. Integration of the molecular factors and new therapeutic targets will lead to molecular-integrated adjuvant treatment including targeted treatments, which are the rationale of new and ongoing trials. This review presents an overview of current adjuvant treatment strategies in endometrial cancer, highlights the development and evaluation of a molecular-integrated risk profile, and briefly discusses ongoing developments in targeted treatment.

Drug repurposing studies of PARP inhibitors as a new therapy for inherited retinal degeneration

The enzyme poly-ADP-ribose-polymerase (PARP) has important roles for many forms of DNA repair and it also participates in transcription, chromatin remodeling and cell death signaling. Currently, some PARP inhibitors are approved for cancer therapy, by means of canceling DNA repair processes and cell division. Drug repurposing is a new and attractive aspect of therapy development that could offer low-cost and accelerated establishment of new treatment options. Excessive PARP activity is also involved in neurodegenerative diseases including the currently untreatable and blinding retinitis pigmentosa group of inherited retinal photoreceptor degenerations. Hence, repurposing of known PARP inhibitors for patients with non-oncological diseases might provide a facilitated route for a novel retinitis pigmentosa therapy. Here, we demonstrate and compare the efficacy of two different PARP inhibitors, BMN-673 and 3-aminobenzamide, by using a well-established retinitis pigmentosa model, the rd1 mouse. Moreover, the mechanistic aspects of the PARP inhibitor-induced protection were also investigated in the present study. Our results showed that rd1 rod photoreceptor cell death was decreased by about 25-40% together with the application of these two PARP inhibitors. The wealth of human clinical data available for BMN-673 highlights a strong potential for a rapid clinical translation into novel retinitis pigmentosa treatments. Remarkably, we have found that the efficacy of 3 aminobenzamide was able to decrease PARylation at the nanomolar level. Our data also provide a link between PARP activity with the Wnt/β-catenin pathway and the major intracellular antioxidant concentrations behind the PARP-dependent retinal degeneration. In addition, molecular modeling studies were integrated with experimental studies for better understanding of the role of PARP1 inhibitors in retinal degeneration.

PARP and PD-1/PD-L1 checkpoint inhibition in recurrent or metastatic endometrial cancer

The prognosis of recurrent or metastatic endometrial cancer is poor, with five-year survival of only 10-20 %. First-line therapy consists of either platinum-based chemotherapy or hormonal therapy. No standard subsequent-line therapy has been identified. In recent years, significant progress has been made in the knowledge on underlying molecular biology of endometrial cancer and potential targets for therapy have been identified. Targeted therapies as poly (ADP-ribose) polymerase (PARP) inhibitors and immunotherapy as PD-1/PD-L1 checkpoint inhibitors have the potential to be effective against specific subtypes of endometrial cancer. Preclinical studies have shown that combining these agents may result in a synergistic effect. In this review, we focus on the molecular basis of checkpoint inhibition and targeted therapy as PARP inhibition in endometrial cancer and summarize available clinical data, and ongoing and planned clinical trials that investigate these agents as mono- or combination therapies in endometrial cancer and where relevant, other gynecological cancers.

Novel therapeutics: response and resistance in ovarian cancer

Here we review the latest pre-clinical and clinical developments for treatment of ovarian cancer, presented at the American Association of Cancer Research/Rivkin Center Ovarian Cancer Research Symposium held at the University of Washington in September 2018. Abstracts and presentations pertaining to the 'Novel Therapeutics' session were reviewed and are summarized here. The session featured a keynote presentation from Dr Ursula Matulonis, who summarized the current state of the art of treatment of ovarian cancer, including recent clinical trials incorporating the use of novel agents, including poly-ADP-ribose polymerase (PARP) inhibitors, other DNA-damaging agents, vascular endothelial growth factor receptor inhibitors, mirvetuximab soravtansine, and immune checkpoint blockade. Dr Jung-Min Lee then summarized the rationale and the results of early studies for targeting cell cycle checkpoint kinases for anti-cancer therapy. Eight submissions were selected for oral presentations, and 36 abstracts were presented as posters. The topics covered a range of clinical and pre-clinical strategies and biomarkers, including immunotherapy, mechanisms of chemotherapy, and PARP inhibitor resistance, DNA-damaging agents, and other novel therapeutic strategies. Key studies have highlighted that resistance to chemotherapy and PARP inhibitors remain a major challenge in therapy of ovarian cancer. Cancer stem cells represent an important mechanism of chemoresistance and strategies to target these cells may be a pathway to prevention of ovarian cancer relapse. Advancement of novel therapeutics targeting DNA damage, cell metabolism, and endoplasmic reticulum present some of the novel strategies in the pipeline. Emerging compelling pre-clinical data with novel antibody-drug conjugates targeting various surface receptors in ovarian cancer alone and in combination with immune checkpoint blockade generate a strong enthusiasm for rapid translation of these strategies to clinic.

PARP1 expression in soft tissue sarcomas is a poor-prognosis factor and a new potential therapeutic target

Soft tissue sarcomas (STSs) are aggressive tumors with few efficient systemic therapies. Poly(ADP‐ribose) polymerase‐1 (PARP1) inhibitors represent an emerging therapeutic option in tumors with genomic instability. The genomics of STSs is complex in more than half of cases, suggesting a high level of inherent DNA damage and genomic instability. Thus, STSs could be efficiently targeted with PARP inhibitors. Promising preclinical results have been reported, but few data are available regarding PARP1 expression in clinical samples. We examined PARP1 mRNA expression in 1464 clinical samples of STS, including 1432 primary tumors and 32 relapses, and searched for correlations with clinicopathological features, including metastasis‐free survival (MFS). Expression was heterogeneous across the samples, not different between primary and secondary tumors, and was correlated to PARP1 DNA copy number. In the 1432 primary tumors, the ‘PARP1‐high’ samples were associated with younger patients, more frequent locations at the extremities, superficial trunk and head and neck, more leiomyosarcomas and other STSs and less liposarcomas and myxofibrosarcomas, more grade 3, more high‐risk CINSARC tumors, and more ‘chromosomically instable’ tumors. They were associated with shorter MFS, independently of other significant prognostic features, including the CINSARC signature. We found a strong involvement of genes overexpressed in the ‘PARP1‐high’ samples in cell cycle, DNA replication, and DNA repair. PARP1 expression refines the prediction of MFS in STSs, and similar expression exists in secondary and primary tumors, supporting the development of PARP1 inhibitors.

Overexpression of MEF2D contributes to oncogenic malignancy and chemotherapeutic resistance in ovarian carcinoma

The transcription factor MEF2 promotes survival in various cell types and a number of studies indicate that abnormal regulation of MEF2 is linked to oncogenicity in several carcinomas. We have found that MEF2D, a member of the MEF2 family, is upregulated in Ovarian Cancer (OC). Immunohistochemistry analysis of tumor sections of 402 OC patients revealed that MEF2D is significantly elevated at the protein level. We have also found that the expression level of MEF2D is associated with cisplatin-resistance and poor prognosis by a retrospective analysis. Furthermore, Downregulation of MEF2D by siRNA reduces proliferation and invasiveness of OC cells SKOV3 and OVCAR3, induces apoptosis in vitro, and abolishes OVCAR3 tumorigenicity in xenograft model. Mechanistic study via ChIP analysis identified two of MEF2D-targeted genes, HPSE and IKBKE, which are associated with tumor invasion and chemotherapy-resistance, in accord with MEF2D expression in OC. Remarkably, knock-down of MEF2D invariably lead to the downregulation of IKBKE and reversed cisplatin (DDP)-resistance in cisplatin-resistant cells SKOV3-DDP. Our results suggest that MEF2D promotes malignant biological behaviors and cisplatin-resistance in OC and establish MEF2D as a new therapeutic target in OC treatment.

DNA Repair: Translation to the Clinic

It has been well established that an accumulation of mutations in DNA, whether caused by external sources (e.g. ultraviolet light, radioactivity) or internal sources (e.g. metabolic by-products, such as reactive oxygen species), has the potential to cause a cell to undergo carcinogenesis and increase the risk for the development of cancer. Therefore, it is critically important for a cell to have the capacity to properly respond to and repair DNA damage as it occurs. The DNA damage response (DDR) describes a collection of DNA repair pathways that aid in the protection of genomic integrity by detecting myriad types of DNA damage and initiating the correct DNA repair pathway. In many instances, a deficiency in the DDR, whether inherited or spontaneously assumed, can increase the risk of carcinogenesis and ultimately tumorigenesis through the accumulation of mutations that fail to be properly repaired. Interestingly, although disruption of the DDR can lead to the initial genomic instability that can ultimately cause carcinogenesis, the DDR has also proven to be an invaluable target for anticancer drugs and therapies. Making matters more complicated, the DDR is also involved in the resistance to first-line cancer therapy. In this review, we will consider therapies already in use in the clinic and ongoing research into other avenues of treatment that target DNA repair pathways in cancer.

Medicinal chemistry approaches of poly ADP-Ribose polymerase 1 (PARP1) inhibitors as anticancer agents - A recent update

Poly (ADP-ribose) Polymerase1 (PARP1) is a member of 17 membered PARP family having diversified biological functions such as synthetic lethality, DNA repair, apoptosis, necrosis, histone binding etc. It is primarily a chromatin-bound nuclear enzyme that gets activated by DNA damage. It binds to DNA signal- and double-strand breaks, does parylation of target proteins (using NAD⁺ as a substrate) like histones and other DNA repair proteins and modifies them as a part of DNA repair mechanism. Inhibition of PARP1 prevents the DNA repair and leads to cell death. Clinically, PARP1 Inhibitors have shown their potential in treating BRCAm breast and ovarian cancers and trials are going on for the treatment of other solid tumors like pancreatic, prostate, colorectal etc. as a single agent or in combination. There are currently three FDA approved PARP1 inhibitors namely Olaparib, Rucaparib and Niraparib in the market while Veliparib and Talazoparib are in the late stage of clinical development. All these molecules are nonselective PARP1 inhibitors with concurrent inhibition of PARP2 with similar potency. In addition, resistance to marketed PARP1 inhibitors has been reported. Overall, looking at the success rate of PARP1 inhibitors into various solid tumors, there is an urge of a novel and selective PARP1 inhibitors. This review provides an update on various newer heterocyclic PARP1 inhibitors reported in last three years along with their structural design strategies. We classified them into two main chemical classes; NAD analogues and non-NAD analogues and discussed the medicinal chemistry approaches of each class. To understand the structural features required for in-silico designing of next-generation PARP1 inhibitors, we also reported the crucial amino acid interactions of these inhibitors at the target site. Thus, present review provides the insight on recent development on new lead structures as PARP1 inhibitors, their SAR, an overview of in-vitro and in-vivo screening methods, current challenges and opinion on future designing of more selective and safe PARP1 inhibitors.

Combinatorial therapy of immune checkpoint and cancer pathways provides a novel perspective on ovarian cancer treatment

An increasing number of studies have reported that immunotherapy serves a significant role in ovarian cancer treatment. In recent years, blockade of checkpoint pathways, including programmed death-ligand 1 (PD-L1)/programmed death-1 and cytotoxic T-lymphocyte-associated protein 4, has demonstrated significant clinical and preclinical benefits in the treatment of ovarian cancer. Additionally, tumor-associated angiogenesis and homologous recombination deficiency frequently occurs in patients with high-grade ovarian cancer, which makes cancer cells more susceptible to targeted therapies, including therapies targeting poly (ADP-ribose) polymerase inhibitor, and anti-angiogenic approaches. Additionally, targeted therapy has been associated with elevated PD-L1 expression in tumor cells, increased T-cell infiltration in tumors and dendritic cell stimulation. This synergistic effect provides the rationale for the joint application of targeted therapy and immunotherapy. Checkpoint blockades are able to elicit durable antitumor immune reactions and complement the transient antitumor effect of targeted therapies. The current review discusses the underlying mechanism of these therapies and novel developments in combined therapy for the treatment of ovarian cancer.

Restored replication fork stabilization, a mechanism of PARP inhibitor resistance, can be overcome by cell cycle checkpoint inhibition

Poly(ADP-ribose) polymerase (PARP) inhibition serves as a potent therapeutic option eliciting synthetic lethality in cancers harboring homologous recombination (HR) repair defects, such as BRCA mutations. However, the development of resistance to PARP inhibitors (PARPis) poses a clinical challenge. Restoration of HR competency is one of the many molecular factors contributing to PARPi resistance. Combination therapy with cell cycle checkpoint (ATR, CHK1, and WEE1) inhibitors is being investigated clinically in many cancers, particularly in ovarian cancer, to enhance the efficacy and circumvent resistance to PARPis. Ideally, inhibition of ATR, CHK1 and WEE1 proteins will abrogate G2 arrest and subsequent DNA repair via restored HR in PARPi-treated cells. Replication fork stabilization has recently been identified as a potential compensatory PARPi resistance mechanism, found in the absence of restored HR. ATR, CHK1, and WEE1 each possess different roles in replication fork stabilization, providing different mechanisms to consider when developing combination therapies to avoid continued development of drug resistance. This review examines the impact of ATR, CHK1, and WEE1 on replication fork stabilization. We also address the therapeutic potential for combining PARPis with cell cycle inhibitors and the possible consequence of combination therapies which do not adequately address both restored HR and replication fork stabilization as PARPi resistance mechanisms.

Targeted therapy and immunotherapy for platinum-refractory advanced ovarian adenosquamous carcinoma: a case report

Background

Ovarian adenosquamous carcinoma is an extremely rare type of ovarian histology. Platinum-refractory disease is also uncommon, but can be fatal because of the lack of available treatment options. To date, there is no study or case report on platinum-refractory ovarian adenosquamous carcinoma or its relevant treatment.

Case presentation

Herein, we report the case of a 38-year-old Chinese woman with platinum-refractory advanced ovarian adenosquamous carcinoma who received clinical benefit from poly adenosine diphosphate ([ADP] ribose) polymerase and programmed death-1 inhibitors after failure of prior multiline chemotherapies and antiangiogenic agents. The targeted therapy and immunotherapy-controlled disease deterioration and improved performance status. Thus far, the patient has survived longer than 15 months, and she is taking nivolumab as maintenance treatment.

Conclusion

Targeted therapy and immunotherapy may be options for rare categories of ovarian cancer, but this warrants more clinical evidence of efficacy and toxicity.

Soft tissue sarcomas: new opportunity of treatment with PARP inhibitors?

BACKGROUND

Poly(ADP-ribose) polymerases (PARP) are a large family of enzymes involved in several cellular processes, including DNA single-strand break repair via the base-excision repair pathway. PARP inhibitors exert antitumor activity by both catalytic PARP inhibition and PARP-DNA trapping, moreover PARP inhibition represents a potential synthetic lethal approach against cancers with specific DNA-repair defects. Soft tissue sarcoma (STSs) are a heterogeneous group of mesenchymal tumors with locally destructive growth, high risk of recurrence and distant metastasis.

OBJECTIVES

The purpuse of this review is to provide an overview of the main preclinical and clinical data on use of PARPi in STSs and of effect and safety of combination of PARPi with irradiation.

RESULTS

Due to numerous genomic alterations in STSs, the DNA damage response pathway can offer an interesting target for biologic therapy. Preclinical and clinical studies showed promising results, with the most robust evidences of PARPi efficacy obtained on Ewing sarcoma bearing EWS-FLI1 or EWS-ERG genomic fusions. The activity of PARP inhibitors resulted potentiated by chemotherapy and radiation. Although mechanisms of synergisms are not completely known, combination of radiation therapy and PARP inhibitors exerts antitumor effect by accumulation of unrepaired DNA damage, arrest in G2/M, activity both on oxic and hypoxic cells, reoxygenation by effect on vessels and promotion of senescence. Early trials have shown a good tolerance profile.

CONCLUSIONS

The use of PARP inhibitors in advanced stage STSs, alone or combined in multimodal treatments, is of great interest and warrants further investigations.

Ovarian Cancers: Genetic Abnormalities, Tumor Heterogeneity and Progression, Clonal Evolution and Cancer Stem Cells

Four main histological subtypes of ovarian cancer exist: serous (the most frequent), endometrioid, mucinous and clear cell; in each subtype, low and high grade. The large majority of ovarian cancers are diagnosed as high-grade serous ovarian cancers (HGS-OvCas). TP53 is the most frequently mutated gene in HGS-OvCas; about 50% of these tumors displayed defective homologous recombination due to germline and somatic BRCA mutations, epigenetic inactivation of BRCA and abnormalities of DNA repair genes; somatic copy number alterations are frequent in these tumors and some of them are associated with prognosis; defective NOTCH, RAS/MEK, PI3K and FOXM1 pathway signaling is frequent. Other histological subtypes were characterized by a different mutational spectrum: LGS-OvCas have increased frequency of BRAF and RAS mutations; mucinous cancers have mutation in ARID1A, PIK3CA, PTEN, CTNNB1 and RAS. Intensive research was focused to characterize ovarian cancer stem cells, based on positivity for some markers, including CD133, CD44, CD117, CD24, EpCAM, LY6A, ALDH1. Ovarian cancer cells have an intrinsic plasticity, thus explaining that in a single tumor more than one cell subpopulation, may exhibit tumor-initiating capacity. The improvements in our understanding of the molecular and cellular basis of ovarian cancers should lead to more efficacious treatments.