Laying a trap to kill cancer cells: PARP inhibitors and their mechanisms of action

PARP inhibition impedes the maturation of nascent DNA strands during DNA replication

Poly(ADP-ribose) polymerase 1 (PARP1) is implicated in the detection and processing of unligated Okazaki fragments and other DNA replication intermediates, highlighting such structures as potential sources of genome breakage induced by PARP inhibition. Here, we show that PARP1 activity is greatly elevated in chicken and human S phase cells in which FEN1 nuclease is genetically deleted and is highest behind DNA replication forks. PARP inhibitor reduces the integrity of nascent DNA strands in both wild-type chicken and human cells during DNA replication, and does so in FEN1-/- cells to an even greater extent that can be detected as postreplicative single-strand nicks or gaps. Collectively, these data show that PARP inhibitors impede the maturation of nascent DNA strands during DNA replication, and implicate unligated Okazaki fragments and other nascent strand discontinuities in the cytotoxicity of these compounds.

Combinations of ATR, Chk1 and Wee1 Inhibitors with Olaparib Are Active in Olaparib Resistant Brca1 Proficient and Deficient Murine Ovarian Cells

Simple Summary

Poly(ADP-ribose) polymerases inhibitors (PARPis), including olaparib, have been recently approved for ovarian carcinoma treatment and PARPi resistance has already been observed in the clinics. With the aim of dissecting the molecular mechanisms of PARPi resistance, we generated olaparib resistant cells lines, both in a homologous recombination (HR)-deficient and -proficient background by continuous in vitro drug treatment. In the HR proficient background, olaparib resistance was caused by overexpression of multidrug resistance 1 gene (MDR1), while multiple heterogeneous co-existing mechanisms were found in olaparib resistant HR-deficient cells, including overexpression of MDR1, a decrease in PARP1 protein level and partial reactivation of HR repair. We found that combinations of ATR, Chk1 and Wee1 inhibitors with olaparib were synergistic in sensitive and resistant sublines, regardless of the HR status. These new olaparib resistant models will be instrumental to screen new therapeutic options for PARPi-resistant ovarian tumors.

Abstract

Background. Poly(ADP-ribose) polymerases inhibitor (PARPi) have shown clinical efficacy in ovarian carcinoma, especially in those harboring defects in homologous recombination (HR) repair, including BRCA1 and BRCA2 mutated tumors. There is increasing evidence however that PARPi resistance is common and develops through multiple mechanisms. Methods. ID8 F3 (HR proficient) and ID8 Brca1-/- (HR deficient) murine ovarian cells resistant to olaparib, a PARPi, were generated through stepwise drug concentrations in vitro. Both sensitive and resistant cells lines were pharmacologically characterized and the molecular mechanisms underlying olaparib resistance. Results. In ID8, cells with a HR proficient background, olaparib resistance was mainly caused by overexpression of multidrug resistance 1 gene (MDR1), while multiple heterogeneous co-existing mechanisms were found in ID8 Brca1-/- HR-deficient cells resistant to olaparib, including overexpression of MDR1, a decrease in PARP1 protein level and partial reactivation of HR repair. Importantly, combinations of ATR, Chk1 and Wee1 inhibitors with olaparib were synergistic in sensitive and resistant sublines, regardless of the HR cell status. Conclusion. Olaparib-resistant cell lines were generated and displayed multiple mechanisms of resistance, which will be instrumental in selecting new possible therapeutic options for PARPi-resistant ovarian tumors.

Combining Carbon-Ion Irradiation and PARP Inhibitor, Olaparib Efficiently Kills BRCA1-Mutated Triple-Negative Breast Cancer Cells

Background:

Triple-negative breast cancer (TNBC) exhibits poor prognosis due to the lack of targets for hormonal or antibody-based therapies, thereby leading to limited success in the treatment of this cancer subtype. Poly (ADP-ribose) polymerase 1 (PARP1) is a critical factor for DNA repair, and using PARP inhibitor (PARPi) is one of the promising treatments for BRCA-mutated (BRCA mut) tumors where homologous recombination repair is impaired due to BRCA1 mutation. Carbon ion (C-ion) radiotherapy effectively induces DNA damages in cancer cells. Thus, the combination of C-ion radiation with PARPi would be an attractive treatment for BRCA mut TNBC, wherein DNA repair systems can be severely impaired on account of the BRCA mutation. Till date, the effectiveness of C-ion radiation with PARPi in BRCA mut TNBC cell killing remains unknown.

Purpose:

Triple-negative breast cancer cell lines carrying either wild type BRCA1, BRCA wt, (MDA-MB-231), or the BRCA1 mutation (HCC1937) were used, and the effectiveness of PARPi, olaparib, combined with C-ion beam or the conventional radiation, or X-ray, on TNBC cell killing were investigated.

Methods:

First, effective concentrations of olaparib for BRCA mut (HCC1937) cell killing were identified. Using these concentrations of olaparib, we then investigated their radio-sensitizing effects by examining the surviving fraction of MDA-MB-231 and HCC1937 upon X-ray or C-ion irradiation. In addition, the number of γH2AX (DSB marker) positive cells as well as their expression levels were determined by immunohistochemistry, and results were compared between X-ray irradiated or C-ion irradiated cells. Furthermore, PARP activities in these cells were also observed by performing immunohistochemistry staining for poly (ADP-ribose) polymer (marker for PARP activity), and their expression differences were determined.

Results:

Treatment of cells with 25 nM olaparib enhanced radio-sensitivity of X-ray irradiated HCC1937, whereas lower dose (5 nM) olaparib showed drastic effects on increasing radio-sensitivity of C-ion irradiated HCC1937. Similar effect was not observed in MDA-MB-231, not possessing the BRCA1 mutation. Results of immunohistochemistry showed that X-ray or C-ion irradiation induced similar number of γH2AX-positive HCC1937 cells, but these induction levels were higher in C-ion irradiated HCC1937 with increased PARP activity compared to that of X-ray irradiated HCC1937. Elevated induction of DSB in C-ion irradiated HCC937 may fully activate DSB repair pathways leading to downstream activation of PARP, subsequently enhancing the effectiveness of PARPi, olaparib, with lower doses of olaparib exerting noticeable effects in cell killing of C-ion irradiated HCC1937.

Conclusions:

From this study, we demonstrate that C-ion irradiation can exert significant DSB in BRCA mut TNBC, HCC1937, with high PARP activation. Thus, PARPi, olaparib, would be a promising candidate as a radio-sensitizer for BRCA mut TNBC treatment, especially for C-ion radiotherapy.

Pre-activation of autophagy impacts response to olaparib in prostate cancer cells

Poly (ADP-ribose) polymerase 1 (PARP1) plays an essential role in DNA repair and is targeted by anticancer therapies using PARP inhibitors (PARPi) such as olaparib. PARPi treatment in prostate cancer (PC) is currently used as a monotherapy or in combination with standard therapies (hormonotherapy) in clinical trials for patients with DNA damage response mutation. Unfortunately, 20% of these patients did not respond to this new treatment. This resistance mechanism in PC is still not well understood. Here, we report that autophagy affects differently the response of PC cell lines to olaparib depending on its activation status. Pre-activation of autophagy before olaparib resulted in an increase of DNA repair activity by homologous recombination (HR) to repair double-strand breaks induced by olaparib and enhanced cell proliferation. When autophagy was activated after olaparib treatment, or completely inhibited, PC cells demonstrated an increased sensitivity to this PARPi. This autophagy-mediated resistance is, in part, regulated by the nuclear localization of sequestrosome 1 (SQSTM1/p62). Decrease of SQSTM1/p62 nuclear localization due to autophagy pre-activation leads to an increase of filamin A (FLNA) protein expression and BRCA1/Rad51 recruitment involved in the HR pathway. Our results reveal that autophagy basal levels may in part determine amenability to PARPi treatment.

Pre-activation of autophagy mediates resistance to olaparib by decreasing nuclear SQSTM1/p62, which increases homologous recombination-mediated repair through filamin A expression and BRCA1/Rad51 recruitment.

Recent Progress of the research on the benzimidazole PARP-1 inhibitors

Poly (ADP-ribose) polymerase-1 (PARP-1) is a multifunctional protein that plays an important role in DNA repair and genome integrity. PARP-1 inhibitors can be used as effective drugs not only to treat BRCA-1/2 deficient cancers because of the effect of synthetically lethal, but also to treat non-BRCA1/2 deficient tumours because of the effect of PARP capture. Therefore, the PARP inhibitors have become a focus of compelling research. Among these inhibitors, substituted benzimidazole derivatives were mainly concerned lead compounds. However, the commercial available benzimidazole PARP-1 inhibitors have some shortcomings such as serious toxicity in combination with chemotherapy drugs, in vivo cardiovascular side effects such as anemia. Therefore it's crucial for scientists to explore more structure-activity relationships of the benzimidazole PARP-1 inhibitors and access safer and more effective PARP inhibitors. As the binding region of PARP-1 and the substrates is usually characterized as NI site and AD site, the modification of benzimidazoles mainly occurs on the benzimidazole skeleton (NI site), and the side chain of benzimidazole on 2-C position (AD site). Herein, the recent progresses of the researches of benzamides PARP inhibitors were introduced. We noticed that even though many efforts were taken to the modification of NI sites, there were still lacks of optimistic and impressive results. However, the structure-activity relationships of the modification of AD sites have not thoroughly discovered yet. We hope that enlightened by the previous researches, more researches of AD site should be occurred and more effective benzimidazole PARP-1 inhibitors could be designed, synthesized, and applied to clinics.

Targeting DNA Damage Response and Immune Checkpoint for Anticancer Therapy

Deficiency in DNA damage response (DDR) genes leads to impaired DNA repair functions that will induce genomic instability and facilitate cancer development. However, alterations of DDR genes can serve as biomarkers for the selection of suitable patients to receive specific therapeutics, such as immune checkpoint blockade (ICB) therapy. In addition, certain altered DDR genes can be ideal therapeutic targets through adapting the mechanism of synthetic lethality. Recent studies indicate that targeting DDR can improve cancer immunotherapy by modulating the immune response mediated by cGAS-STING-interferon signaling. Investigations of the interplay of DDR-targeting and ICB therapies provide more effective treatment options for cancer patients. This review introduces the mechanisms of DDR and discusses their crucial roles in cancer therapy based on the concepts of synthetic lethality and ICB. The contemporary clinical trials of DDR-targeting and ICB therapies in breast, colorectal, and pancreatic cancers are included.

Inhibitors of PARP: Number crunching and structure gazing

PARP is an important target in the treatment of cancers, particularly in patients with breast, ovarian, or prostate cancer that have compromised homologous recombination repair (i.e., BRCA^−/−). This review about inhibitors of PARP (PARPi) is for readers interested in the development of next-generation drugs for the treatment of cancer, providing insights into structure–activity relationships, in vitro vs. in vivo potency, PARP trapping, and synthetic lethality.

Selective inhibitors of PARP1 and PARP2 (PARP1/2) are used to treat cancer patients with deficiencies in the repair of DNA via homologous recombination. Here we provide a perspective on the reported potencies of the most studied of these inhibitors (olaparib, talazoparib, niraparib, rucaparib, and veliparib) in vitro and in vivo and how these numbers relate to the known structures of these inhibitors bound to the active sites of PARP1 and PARP2. We suggest that the phenomenon of PARP trapping is primarily due to the inhibition of the catalytic activity of PARP1 and that the basis for the higher potency of talazoparib compared to the other inhibitors lies in its more extensive network of interactions with conserved residues in the active site. We also consider the potential role of the recently characterized protein “Histone PARylation Factor 1” (HPF1), which interacts with PARP1/2 to form a shared active site, for the design of the next generation of inhibitors of PARP1/2.

PARP Inhibitors Resistance: Mechanisms and Perspectives

Simple Summary

This review aims to analyze the emerging issue regarding PARP inhibitor’s resistance in tumors and their consequence on disease prognosis and treatment. Besides, we evaluate possible strategies and new therapeutic approaches to overcome PARPis resistance.

Abstract

PolyADP-ribose polymerase (PARP) inhibitors (PARPis) represent the first clinically approved drugs able to provoke “synthetic lethality” in patients with homologous recombination-deficient (HRD) tumors. Four PARPis have just received approval for the treatment of several types of cancer. Besides, another three additional PARPis underlying the same mechanism of action are currently under investigation. Despite the success of these targeted agents, the increasing use of PARPis in clinical practice for the treatment of different tumors raised the issue of PARPis resistance, and the consequent disease relapse and dismal prognosis for patients. Several mechanisms of resistance have been investigated, and ongoing studies are currently focusing on strategies to address this challenge and overcome PARPis resistance. This review aims to analyze the mechanisms underlying PARPis resistance known today and discuss potential therapeutic strategies to overcome these processes of resistance in the future.

Nucleases and Co-Factors in DNA Replication Stress Responses

DNA replication stress is a constant threat that cells must manage to proliferate and maintain genome integrity. DNA replication stress responses, a subset of the broader DNA damage response (DDR), operate when the DNA replication machinery (replisome) is blocked or replication forks collapse during S phase. There are many sources of replication stress, such as DNA lesions caused by endogenous and exogenous agents including commonly used cancer therapeutics, and difficult-to-replicate DNA sequences comprising fragile sites, G-quadraplex DNA, hairpins at trinucleotide repeats, and telomeres. Replication stress is also a consequence of conflicts between opposing transcription and replication, and oncogenic stress which dysregulates replication origin firing and fork progression. Cells initially respond to replication stress by protecting blocked replisomes, but if the offending problem (e.g., DNA damage) is not bypassed or resolved in a timely manner, forks may be cleaved by nucleases, inducing a DNA double-strand break (DSB) and providing a means to accurately restart stalled forks via homologous recombination. However, DSBs pose their own risks to genome stability if left unrepaired or misrepaired. Here we focus on replication stress response systems, comprising DDR signaling, fork protection, and fork processing by nucleases that promote fork repair and restart. Replication stress nucleases include MUS81, EEPD1, Metnase, CtIP, MRE11, EXO1, DNA2-BLM, SLX1-SLX4, XPF-ERCC1-SLX4, Artemis, XPG, and FEN1. Replication stress factors are important in cancer etiology as suppressors of genome instability associated with oncogenic mutations, and as potential cancer therapy targets to enhance the efficacy of chemo- and radiotherapeutics.

Phenethyl Isothiocyanate Enhances the Cytotoxic Effects of PARP Inhibitors in High-Grade Serous Ovarian Cancer Cells

While PARP inhibitor (PARPi) therapies have shown promising results in the treatment of high-grade serous ovarian cancer (HGSOC) harboring homologous recombination deficiencies, primary resistance to PARPi frequently occurs and even initial responders may eventually become resistant. Therefore, the development of novel effective combinatorial strategies to treat HGSOC is urgently needed. Here, we report that H₂O₂-induced oxidative stress sensitized HGSOC cells to PARPi BMN 673. Furthermore, Phenethyl isothiocyanate (PEITC) as a ROS-inducing agent significantly enhanced the cytotoxic effects of BMN 673. Mechanistically, combined use of PEITC and BMN 673 resulted in ROS overproduction and accumulation, enhanced DNA damage, G2/M arrest and apoptosis, all of which were significantly reversed by the ROS scavenger N-Acetyl-L-cysteine. We also showed that while PEITC did not further enhance the ability of BMN 673 on PARP1 trapping in HGSOC cells, the therapeutic effects of the PEITC/BMN 673 combination were at least in part dependent on the presence of PARP1. Importantly, the PEITC/BMN 673 combination potently abrogated the growth of HGSOC tumor spheroids and patient-derived organoid models of HGSOC and cervical cancer. Our findings provide a basis for further investigation of the utility of PARPi combination regimen in HGSOC and cervical cancer through ROS-mediated mechanisms.

Everything Comes with a Price: The Toxicity Profile of DNA-Damage Response Targeting Agents

Simple Summary

DNA damage induces genome instability, which may elicit cancer development. Defects in the DNA repair machinery further enhance cancer predisposition, but can also be exploited as a therapeutic target. Indeed, targeted agents against specific components of DNA repair, such as PARP inhibitors, are employed in various tumor types, while others, such as ATR, CHK1 or WEE1 inhibitors, are in clinical development. Even though these molecules have proven to be effective in different settings, they display several on- and off-target toxicities, shared by the whole pharmacological class or are drug specific. Among these effects, hematological and gastrointestinal toxicities are the most common, while others are less frequent but potentially life-threatening (e.g., myelodysplastic syndromes). Particular caution is needed in the case of combinatorial therapeutic approaches, which are currently being developed in clinical trials. In any case, it is necessary to recognize and properly manage adverse events of these drugs. This review provides a comprehensive overview on the safety profile of DDR-targeting agents, including indications for their management in clinical practice.

Abstract

Targeting the inherent vulnerability of cancer cells with an impaired DNA Damage Repair (DDR) machinery, Poly-ADP-Ribose-Polymerase (PARP) inhibitors have yielded significant results in several tumor types, eventually entering clinical practice for the treatment of ovarian, breast, pancreatic and prostate cancer. More recently, inhibitors of other key components of DNA repair, such as ATR, CHK1 and WEE1, have been developed and are currently under investigation in clinical trials. The inhibition of DDR inevitably induces on-target and off-target adverse events. Hematological and gastrointestinal toxicities as well as fatigue are common with all DDR-targeting agents, while other adverse events are drug specific, such as hypertension with niraparib and transaminase elevation with rucaparib. Cases of pneumonitis and secondary hematological malignancies have been reported with PARP inhibitors and, despite being overly rare, they deserve particular attention due to their severity. Safety also represents a crucial issue for the development of combination regimens incorporating DDR-targeting agents with other treatments, such as chemotherapy, anti-angiogenics or immunotherapy. As such, overlapping and cumulative toxicities should be considered, especially when more than two classes of drugs are combined. Here, we review the safety profile of DDR-targeting agents when used as single agents or in combination and we provide principles of toxicity management.

The DNA-PK inhibitor AZD7648 sensitizes patient derived ovarian cancer xenografts to pegylated liposomal doxorubicin and olaparib preventing abdominal metastases

Ovarian cancer is the deadliest gynaecological cancer, with a 5 year survival rate of 30%, when the disease has spread throughout the peritoneal cavity. We investigated the efficacy to delay disease progression by the DNA-dependent protein kinase (DNA-PKcs)inhibitor AZD7648, administered in combination with two of the therapeutic options for patient management: either pegylated liposomal doxorubicin (PLD) or the poly(adenosine diphosphate-ribose)polymerase (PARP) inhibitor olaparib. Patient-derived ovarian cancer xenografts (OC-PDXs) were transplanted subcutaneously to evaluate the effect of treatment on tumour growth, or orthotopically in the peritoneal cavity to evaluate the effect on metastatic spread. AZD7648 was administered orally (po)in combination with PLD (dosed intravenously) or with olaparib (po). To prove the inhibition of DNA-PK in the tumours, we measured pDNA-PKcs, pRPA32 and γH2AX, biomarkers of DNA-PK activity. AZD7648 enhanced the therapeutic efficacy of PLD in all the OC-PDXs tested, regardless of their BRCA status, sensitivity to cisplatin or PLD. The treatment caused disease stabilization, that persisted despite therapy discontinuation for tumours growing subcutaneously, and significantly impaired the abdominal metastatic dissemination, prolonging the lifespan of mice implanted orthotopically. AZD7648 potentiated the efficacy of olaparib in BRCA-deficient OC-PDXs, but did not sensitize BRCA-proficient OC-PDXs to olaparib, despite an equivalent inhibition of DNA-PK, suggesting the need of a pre-existing olaparib activity to benefit from the addition of AZD7648. This work suggests that AZD7648, an inhibitor of DNA-PK, dosed in combination with PLD or olaparib is an exciting therapeutic option that could benefit ovarian cancer patients and should be explored in clinical trials.

Therapeutic Targeting of DNA Damage Response in Cancer

DNA damage response (DDR) is critical to ensure genome stability, and defects in this signaling pathway are highly associated with carcinogenesis and tumor progression. Nevertheless, this also provides therapeutic opportunities, as cells with defective DDR signaling are directed to rely on compensatory survival pathways, and these vulnerabilities have been exploited for anticancer treatments. Following the impressive success of PARP inhibitors in the treatment of BRCA-mutated breast and ovarian cancers, extensive research has been conducted toward the development of pharmacologic inhibitors of the key components of the DDR signaling pathway. In this review, we discuss the key elements of the DDR pathway and how these molecular components may serve as anticancer treatment targets. We also summarize the recent promising developments in the field of DDR pathway inhibitors, focusing on novel agents beyond PARP inhibitors. Furthermore, we discuss biomarker studies to identify target patients expected to derive maximal clinical benefits as well as combination strategies with other classes of anticancer agents to synergize and optimize the clinical benefits.

Targeting PARP proteins in acute leukemia: DNA damage response inhibition and therapeutic strategies

The members of the Poly(ADP‐ribose) polymerase (PARP) superfamily are involved in several biological processes and, in particular, in the DNA damage response (DDR). The most studied members, PARP1, PARP2 and PARP3, act as sensors of DNA damages, in order to activate different intracellular repair pathways, including single-strand repair, homologous recombination, conventional and alternative non-homologous end joining. This review recapitulates the functional role of PARPs in the DDR pathways, also in relationship with the cell cycle phases, which drives our knowledge of the mechanisms of action of PARP inhibitors (PARPi), encompassing inhibition of single-strand breaks and base excision repair, PARP trapping and sensitization to antileukemia immune responses. Several studies have demonstrated a preclinical activity of the current available PARPi, olaparib, rucaparib, niraparib, veliparib and talazoparib, as single agent and/or in combination with cytotoxic, hypomethylating or targeted drugs in acute leukemia, thus encouraging the development of clinical trials. We here summarize the most recent preclinical and clinical findings and discuss the synthetic lethal interactions of PARPi in acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL). Despite the low frequency of genomic alterations of PARP and other DDR-related genes in acute leukemia, selective vulnerabilities have been reported in several disease subgroups, along with a “BRCAness phenotype.” AML carrying the RUNX1-RUNX1T1 or PML-RARA fusion genes or mutations in signaling genes (FLT3-ITD in combination with TET2 or TET2 and DNMT3A deficiency), cohesin complex members (STAG2), TP53 and BCOR as co-occurring lesions, IDH1/2 and ALL cases expressing the TCF3-HLF chimera or TET1 was highly sensitive to PARPi in preclinical studies. These data, along with the warning coming from the observation of cases of therapy-related myeloid malignancies among patients receiving PARPi for solid tumors treatment, indicate that PARPi represents a promising strategy in a personalized medicine setting. The characterization of the clonal and subclonal genetic background and of the DDR functionality is crucial to select acute leukemia patients that will likely benefit of PARPi-based therapeutic regimens.

Radiotherapy-Poly(ADP-ribose) Polymerase Inhibitor Combinations: Progress to Date

Radiation resistance remains a huge clinical problem for cancer patients and oncologists in the 21st century. In recent years, the mammalian DNA damage response (DDR) has been extensively characterized and shown to play a key role in determining cellular survival following ionizing radiation exposure. Genomic instability due to altered DDR is a hallmark of cancer, with many tumors exhibiting abnormal DNA repair or lack of redundancy in DDR. Targeting the abnormal DDR phenotype of tumor cells could lead to substantial gains in radiotherapy efficacy, improving local control and survival for patients with cancers that are refractory to current therapies. Poly(ADP-ribose) polymerase inhibitors (PARPi) are the most clinically advanced DDR inhibitors under investigation as radiosensitisers. Preclinical evidence suggests that PARPi may provide tumor specific radiosensitisation in certain contexts. In addition to inhibition of DNA single strand break repair, PARPi may offer other benefits in combination treatment including radiosensitisation of hypoxic cells and targeting of alternative repair pathways such as microhomology mediated end joining which are increasingly recognized to be upregulated in cancer. Several early phase clinical trials of PARPi with radiation have completed or are in progress. Early reports have highlighted tumor specific challenges, with tolerability dependent upon anatomical location and use of concomitant systemic therapies; these challenges were largely predicted by preclinical data. This review discusses the role of PARP in the cellular response to ionizing radiation, summarizes preclinical studies of PARPi in combination with radiotherapy and explores current early phase clinical trials that are evaluating these combinations.

Utilization of Poly(ADP-Ribose) Polymerase Inhibitors in Ovarian Cancer: A Retrospective Cohort Study of US Healthcare Claims Data

Introduction

We aimed to characterize real-world utilization of poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi) in women with ovarian cancer (OC).

Methods

This retrospective observational study of claims data from US MarketScan^® Commercial/Medicare Supplemental databases included women with OC initiating olaparib, niraparib, or rucaparib from January 1, 2017, to May 31, 2019. Patients were observed from first outpatient prescription until at least 30 days’ follow-up. Clinical events of interest (CEIs), based on adverse reactions in PARPi prescribing information, were identified from claims using ICD-9/10 codes. Other outcomes included dose modification, persistence, adherence, healthcare resource utilization (HCRU), and cost.

Results

Overall, 303, 348, and 162 women with OC received olaparib, niraparib, and rucaparib, respectively. During follow-up, risk of any CEI was higher with niraparib versus olaparib (odds ratio 3.36 [95% confidence interval 2.00–5.65]) and niraparib versus rucaparib (2.09 [1.10–3.95]), with no significant difference between rucaparib and olaparib (1.61 [0.93–2.79]). PARPi dose decreases were observed in 21.1%, 35.1%, and 30.2% of olaparib-, niraparib-, and rucaparib-treated patients, respectively. Persistence (no treatment gaps of more than 90 days) was significantly higher (P < 0.05) with olaparib (62.2%) versus niraparib (35.9%) and rucaparib (48.7%); adherence (medication possession ratio, MPR ≥ 80%) was 80.2% versus 38.6% and 63.2%, respectively (P < 0.001). Inpatient admissions and outpatient service use were higher with niraparib and rucaparib versus olaparib, reflected in mean (± standard deviation) total medical costs (excluding pharmacy) of $5393 ± 8828 for olaparib, $7732 ± 14,054 for niraparib, and $6868 ± 7929 for rucaparib.

Conclusion

Differences between the licensed PARPi were observed in the risk of experiencing a CEI, likelihood of dose modifications, ability to receive continuous PARPi therapy, HCRU, and costs.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12325-021-01959-5.

Pharmacokinetics and Pharmacodynamics of PARP Inhibitors in Oncology

Olaparib, niraparib, rucaparib, and talazoparib are poly (ADP-ribose) polymerase (PARP) inhibitors approved for the treatment of ovarian, breast, pancreatic, and/or prostate cancer. Poly (ADP-ribose) polymerase inhibitors are potent inhibitors of the PARP enzymes with comparable half-maximal inhibitory concentrations in the nanomolar range. Olaparib and rucaparib are orally dosed twice a day, extensively metabolized by cytochrome P450 enzymes, and inhibitors of several enzymes and drug transporters with a high risk for drug-drug interactions. Niraparib and talazoparib are orally dosed once a day with a lower risk for niraparib and a minimal risk for talazoparib to cause drug-drug interactions. All four PARP inhibitors show moderate-to-high interindividual variability in plasma exposure. Higher exposure is associated with an increase in toxicity, mostly hematological toxicity. For talazoparib, exposure-efficacy relationships have been described, but for olaparib, niraparib, and rucaparib this relationship remains inconclusive. Further studies are required to investigate exposure-response relationships to improve dosing of PARP inhibitors, in which therapeutic drug monitoring could play an important role. In this review, we give an overview of the pharmacokinetic properties of the four PARP inhibitors, including considerations for patients with renal dysfunction or hepatic impairment, the effect of food, and drug-drug interactions. Furthermore, we focus on the pharmacodynamics and summarize the available exposure-efficacy and exposure-toxicity relationships.

PARP mediated DNA damage response, genomic stability and immune responses

Poly (ADP-ribose) polymerase (PARP) enzymes, especially PARP1, play important roles in the DNA damage response and in the maintenance of genome stability, which makes PARPis a classic synthetic lethal therapy for BRCA-deficient tumors. Conventional mechanisms suggest that PARPis exert their effects via catalytic inhibition and PARP-DNA trapping. Recently, PARP1 has been found to play a role in the immune modulation of tumors. The blockade of PARP1 is able to induce innate immunity through a series of molecular mechanisms, thus allowing the prediction of the feasibility of PARPis combined with immune agents in the treatment of tumors. PARPis combined with immunomodulators may have a stronger tumor suppressive effect on inhibiting tumor growth and blocking immune escape. This article is protected by copyright. All rights reserved.

RAD51 as a functional biomarker for homologous recombination deficiency in cancer: a promising addition to the HRD toolbox?

INTRODUCTION

Carcinomas with defects in the homologous recombination (HR) pathway are sensitive to PARP inhibitors (PARPi). A robust method to identify HR-deficient (HRD) carcinomas is therefore of utmost clinical importance. Currently available DNA-based HRD tests either scan HR-related genes such as BRCA1 and BRCA2 for the presence of pathogenic variants or identify HRD-related genomic scars or mutational signatures by using whole-exome or whole-genome sequencing data. As an alternative to DNA-based tests, functional HRD tests have been developed that assess the actual ability of tumors to accumulate RAD51 protein at DNA double strand breaks as a proxy for HR proficiency.

AREAS COVERED

This review presents an overview of currently available HRD tests and discuss the pros and cons of the different methodologies including their sensitivity for the identification of HRD tumors, their concordance with other HRD tests, and their capacity to predict therapy response.

EXPERT OPINION

With the increasing use of PARP inhibitors in the treatment of several cancers there is an urgent need to implement HRD testing in routine clinical practice. To this end, calibration of HRD thresholds and clinical validation of both DNA-based and RAD51-based HRD tests should have top-priority in the coming years.

Opportunities for Utilization of DNA Repair Inhibitors in Homologous Recombination Repair-Deficient and Proficient Pancreatic Adenocarcinoma

Pancreatic cancer is rapidly progressive and notoriously difficult to treat with cytotoxic chemotherapy and targeted agents. Recent demonstration of the efficacy of maintenance PARP inhibition in germline BRCA mutated pancreatic cancer has raised hopes that increased understanding of the DNA damage response pathway will lead to new therapies in both homologous recombination (HR) repair-deficient and proficient pancreatic cancer. Here, we review the potential mechanisms of exploiting HR deficiency, replicative stress, and DNA damage-mediated immune activation through targeted inhibition of DNA repair regulatory proteins.

Expanding the Use of PARP Inhibitors as Monotherapy and in Combination in Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) is the most aggressive subtype of breast cancer, and is known to be associated with a poor prognosis and limited therapeutic options. Poly (ADP-ribose) polymerase inhibitors (PARPi) are targeted therapeutics that have demonstrated efficacy as monotherapy in metastatic BRCA-mutant (BRCAMUT) TNBC patients. Improved efficacy of PARPi has been demonstrated in BRCAMUT breast cancer patients who have either received fewer lines of chemotherapy or in chemotherapy-naïve patients in the metastatic, adjuvant, and neoadjuvant settings. Moreover, recent trials in smaller cohorts have identified anti-tumor activity of PARPi in TNBC patients, regardless of BRCA-mutation status. While there have been concerns regarding the efficacy and toxicity of the use of PARPi in combination with chemotherapy, these challenges can be mitigated with careful attention to PARPi dosing strategies. To better identify a patient subpopulation that will best respond to PARPi, several genomic biomarkers of homologous recombination deficiency have been tested. However, gene expression signatures associated with PARPi response can integrate different pathways in addition to homologous recombination deficiency and can be implemented in the clinic more readily. Taken together, PARPi have great potential for use in TNBC patients beyond BRCAMUT status, both as a single-agent and in combination.

Genomic Features and Clinical Implications of Intraductal Carcinoma of the Prostate

Intraductal carcinoma of the prostate (IDC-P) is a rare and unique form of aggressive prostate carcinoma, which is characterized by an expansile proliferation of malignant prostatic epithelial cells within prostatic ducts or acini and the preservation of basal cell layers around the involved glands. The vast majority of IDC-P tumors result from adjacent high-grade invasive cancer via the retrograde spreading of tumor cells into normal prostatic ducts or acini. A subset of IDC-P tumors is rarely derived from the de novo intraductal proliferation of premalignant cells. The presence of IDC-P in biopsy or surgical specimens is significantly associated with aggressive pathologic features, such as high Gleason grade, large tumor volume, and advanced tumor stage, and with poor clinical courses, including earlier biochemical recurrence, distant metastasis, and worse survival outcomes. These architectural and behavioral features of IDC-P may be driven by specific molecular properties. Notably, IDC-P possesses distinct genomic profiles, including higher rates of TMPRSS2–ERG gene fusions and PTEN loss, increased percentage of genomic instability, and higher prevalence of germline BRCA2 mutations. Considering that IDC-P tumors are usually resistant to conventional therapies for prostate cancer, further studies should be performed to develop optimal therapeutic strategies based on distinct genomic features, such as treatment with immune checkpoint blockades or poly (adenosine diphosphate–ribose) polymerase inhibitors for patients harboring increased genomic instability or BRCA2 mutations, as well as genetic counseling with genetic testing. Patient-derived xenografts and tumor organoid models can be the promising in vitro platforms for investigating the molecular features of IDC-P tumor.

Therapeutic vulnerability to PARP1,2 inhibition in RB1-mutant osteosarcoma

Loss-of-function mutations in the RB1 tumour suppressor are key drivers in cancer, including osteosarcoma. RB1 loss-of-function compromises genome-maintenance and hence could yield vulnerability to therapeutics targeting such processes. Here we demonstrate selective hypersensitivity to clinically-approved inhibitors of Poly-ADP-Polymerase1,2 inhibitors (PARPi) in RB1-defective cancer cells, including an extended panel of osteosarcoma-derived lines. PARPi treatment results in extensive cell death in RB1-defective backgrounds and prolongs survival of mice carrying human RB1-defective osteosarcoma grafts. PARPi sensitivity is not associated with canonical homologous recombination defect (HRd) signatures that predict PARPi sensitivity in cancers with BRCA1,2 loss, but is accompanied by rapid activation of DNA replication checkpoint signalling, and active DNA replication is a prerequisite for sensitivity. Importantly, sensitivity in backgrounds with natural or engineered RB1 loss surpasses that seen in BRCA-mutated backgrounds where PARPi have established clinical benefit. Our work provides evidence that PARPi sensitivity extends beyond cancers identifiable by HRd and advocates PARP1,2 inhibition as a personalised strategy for RB1-mutated osteosarcoma and other cancers.

Olaparib with or without bevacizumab or bevacizumab and 5-fluorouracil in advanced colorectal cancer: Phase III LYNK-003

Oxaliplatin-based chemotherapy with a regimen such as FOLFOX with or without targeted therapy is a standard of care option for advanced colorectal cancer; however, long-term exposure to oxaliplatin is associated with cumulative toxicity. Growing evidence suggests maintenance therapy with a less intensive regimen after platinum-based induction therapy can provide continuing benefit with reduced toxicity. We describe the rationale and design of the Phase III LYNK-003 trial, which will evaluate the efficacy and safety of olaparib with or without bevacizumab compared with 5-fluoruracil plus bevacizumab in patients with unresectable or metastatic colorectal cancer that has not progressed on an induction course of FOLFOX plus bevacizumab. The primary end point is progression-free survival by independent central review; secondary end points include overall survival, objective response, duration of response and safety. Clinical trial registration: NCT04456699.

Tackling PARP inhibitor resistance

Homologous recombination-deficient (HRD) tumours, including those harbouring mutations in the BRCA genes, are hypersensitive to treatment with inhibitors of poly(ADP-ribose) polymerase (PARPis). Despite high response rates, most HRD cancers ultimately develop resistance to PARPi treatment through reversion mutations or genetic/epigenetic alterations to DNA repair pathways. Counteracting these resistance pathways, thereby increasing the potency of PARPi therapy, represents a potential strategy to improve the treatment of HRD cancers. In this review, we discuss recent insights derived from genetic screens that have identified a number of novel genes that can be targeted to improve PARPi treatment of HRD cancers and may provide a means to overcome PARPi resistance.

Targeting DNA repair pathway in cancer: Mechanisms and clinical application

Over the last decades, the growing understanding on DNA damage response (DDR) pathways has broadened the therapeutic landscape in oncology. It is becoming increasingly clear that the genomic instability of cells resulted from deficient DNA damage response contributes to the occurrence of cancer. One the other hand, these defects could also be exploited as a therapeutic opportunity, which is preferentially more deleterious in tumor cells than in normal cells. An expanding repertoire of DDR-targeting agents has rapidly expanded to inhibitors of multiple members involved in DDR pathways, including PARP, ATM, ATR, CHK1, WEE1, and DNA-PK. In this review, we sought to summarize the complex network of DNA repair machinery in cancer cells and discuss the underlying mechanism for the application of DDR inhibitors in cancer. With the past preclinical evidence and ongoing clinical trials, we also provide an overview of the history and current landscape of DDR inhibitors in cancer treatment, with special focus on the combination of DDR-targeted therapies with other cancer treatment strategies.

Understanding and overcoming resistance to PARP inhibitors in cancer therapy

Developing novel targeted anticancer therapies is a major goal of current research. The use of poly(ADP-ribose) polymerase (PARP) inhibitors in patients with homologous recombination-deficient tumours provides one of the best examples of a targeted therapy that has been successfully translated into the clinic. The success of this approach has so far led to the approval of four different PARP inhibitors for the treatment of several types of cancers and a total of seven different compounds are currently under clinical investigation for various indications. Clinical trials have demonstrated promising response rates among patients receiving PARP inhibitors, although the majority will inevitably develop resistance. Preclinical and clinical data have revealed multiple mechanisms of resistance and current efforts are focused on developing strategies to address this challenge. In this Review, we summarize the diverse processes underlying resistance to PARP inhibitors and discuss the potential strategies that might overcome these mechanisms such as combinations with chemotherapies, targeting the acquired vulnerabilities associated with resistance to PARP inhibitors or suppressing genomic instability.

Veliparib monotherapy following carboplatin/paclitaxel plus veliparib combination therapy in patients with germline BRCA-associated advanced breast cancer: results of exploratory analyses from the phase III BROCADE3 trial

BACKGROUND

In the BROCADE3 trial (NCT02163694), addition of the poly(ADP-ribose) polymerase (PARP) inhibitor, veliparib, to carboplatin/paclitaxel improved progression-free survival (PFS) (hazard ratio [HR] 0.71, 95% confidence interval 0.57-0.88; P=0.002) in patients with advanced HER2-negative, germline BRCA1/2-mutated breast cancer. A subset of patients discontinued both carboplatin and paclitaxel prior to progression and continued on veliparib/placebo maintenance monotherapy until progression. Analyses in this patient subgroup are reported.

PATIENTS AND METHODS

Patients were randomized 2:1 to veliparib plus carboplatin/paclitaxel or placebo plus carboplatin/paclitaxel. Veliparib (120 mg twice daily [BID]) or placebo was given on days -2 to 5, carboplatin (AUC 6 mg/mL) on day 1, and paclitaxel (80 mg/m²) on days 1, 8, and 15 of 21-day cycles. Patients who discontinued both carboplatin and paclitaxel prior to progression received blinded study drug monotherapy at an increased dose of 300-400 mg BID continuously. PFS was the primary endpoint. Exploratory analyses were performed in the subgroup of patients who received blinded study drug as monotherapy. A time-varying Cox model including data from all patients was also used to evaluate treatment effect in the combination and monotherapy phases.

RESULTS

136 of 337 patients randomized to veliparib plus carboplatin/paclitaxel and 58/172 patients randomized to placebo plus carboplatin/paclitaxel discontinued both carboplatin and paclitaxel prior to progression and continued on blinded veliparib or placebo monotherapy. In this blinded monotherapy subgroup, investigator-assessed median PFS from randomization was 25.7 months with veliparib versus 14.6 months with placebo. HRs from a time-varying Cox model favored veliparib during both combination therapy and monotherapy. Any-grade adverse events (AEs) occurring in the monotherapy phase were primarily gastrointestinal. The most common grade ≥3 AEs were neutropenia and anemia (4% each with veliparib; 5% and 2%, respectively, with placebo).

CONCLUSIONS

Veliparib maintenance monotherapy had a tolerable safety profile and may extend PFS following combination chemotherapy.

Update on Poly ADP-Ribose Polymerase Inhibitors in Ovarian Cancer With Non-BRCA Mutations

Poly ADP-ribose polymerase inhibitor (PARPi) has become an important maintenance therapy for ovarian cancer after surgery and cytotoxic chemotherapy, which has changed the disease management model of ovarian cancer, greatly decreased the risk of recurrence, and made the prognosis of ovarian cancer better to certain extent. The three PARPis currently approved by the United States Food and Drug Administration (FDA) and the European Medicines Agency (EMA) for the treatment of ovarian cancer are Olaparib, Niraparib and Rucaparib. With the incremental results from new clinical trials, the applicable population of PARPi for ovarian cancer have expanded to population with non-BRCA mutations. Although BRCA mutated population are still the main beneficiaries of PARPi, recent clinical trials indicated PARPis' therapeutic potential in non-BRCA mutated population, especially in homologous recombination repair deficiency (HRD) positive population. However, lack of unified HRD status detection method poses a challenge for the accurate selection of PARPi beneficiaries. The reversal of homologous recombination (HR) function during the treatment will not only cause resistance to PARPis, but also reduce the accuracy of the current method to determine HRD status. Therefore, the development of reliable HRD status detection methods to determine the beneficiary population, as well as rational combination treatment are warranted. This review mainly summarizes the latest clinical trial results and combination treatment of PARPis in ovarian cancer with non-BRCA mutations, and discusses the application prospects, including optimizing combination therapy against drug resistance, developing unified and accurate HRD status detection methods for patient selection and stratification. This review further poses an interesting topic: the efficacy and safety in patients retreated with PARPis after previous PARPi treatment---"PARPi after PARPi".

Homologous Recombination Repair Gene Mutation Characterization by Liquid Biopsy: A Phase II Trial of Olaparib and Abiraterone in Metastatic Castrate-Resistant Prostate Cancer

Simple Summary

Mutations in homologous recombination repair (HRR) genes are frequent in advanced prostate cancer and tumours harbouring these mutations have known sensitivity to PARP inhibitors, such as olaparib. In the randomized double-blind Phase II study (NCT01972218), olaparib and abiraterone prolonged radiographic progression-free survival (rPFS) versus placebo and abiraterone in patients with metastatic castration-resistant prostate cancer (mCRPC) unselected by HRR status. The study was designed to prioritize tumour samples for a pre-specified analysis of HRR status but was challenged by a low tissue submission rate. Circulating tumour DNA (ctDNA) and germline testing were initiated to supplement the assessment. Here, we present data from further germline and ctDNA analyses which increase the number of patients with confirmed HRR status. Our results support prior findings that patients with mCRPC benefit from olaparib and abiraterone treatment regardless of HRR status and highlight the value of ctDNA testing as a complement to tumour tissue sequencing.

Abstract

Background: Phase III randomized trial data have confirmed the activity for olaparib in homologous recombination repair (HRR) mutated metastatic castration-resistant prostate cancer (mCRPC) post next-generation hormonal agent (NHA) progression. Preclinical data have suggested the potential for a combined effect between olaparib and NHAs irrespective of whether an HRR gene alteration was present. NCT01972217 was a randomised double-blind Phase II study which evaluated olaparib and abiraterone versus placebo and abiraterone in mCRPC patients who had received prior chemotherapy containing docetaxel. The study showed that radiologic progression was significantly delayed by the combination of olaparib and abiraterone regardless of homologous recombination repair mutation (HRRm) status. The study utilized tumour, blood (germline), and circulating tumour DNA (ctDNA) analysis to profile patient HRRm status, but tumour tissue provision was not mandated, leading to relatively low tissue acquisition and DNA sequencing success rates not representative of real-world testing. Patients and methods: Further analysis of germline and ctDNA samples has been performed for the trial to characterize HRRm status more fully and robustly analyse patient response to treatment. Results: Germline and plasma testing increased the HRRm characterized population from 27% to 68% of 142 randomized patients. Tumour-derived variants were detectable with high confidence in 78% of patients with a baseline plasma sample (71% of randomized patients). There was high concordance across methodologies (plasma vs. tumour; plasma vs. germline). The HR for the exploratory analysis of radiographic progression-free survival was 0.54 (95% CI: 0.32–0.93) in favour of olaparib and abiraterone in the updated HRR wild type (HRRwt) group (n = 73) and 0.62 (95% CI: 0.23–1.65) in the HRRm group (n = 23). Conclusion: Our results confirm the value of plasma testing for HRRm status when there is insufficient high-quality tissue for multi-gene molecular testing. We show that patients with mCRPC benefit from the combination of olaparib and abiraterone treatment regardless of HRRm status. The combination is currently being further investigated in the Phase III PROpel trial.

Phase 1A/1B dose-escalation and -expansion study to evaluate the safety, pharmacokinetics, food effects and antitumor activity of pamiparib in advanced solid tumours

BACKGROUND

Pamiparib, a PARP1/2 inhibitor, demonstrated antitumor activity in preclinical models.

METHODS

This Phase 1A/1B dose-escalation/dose-expansion study enrolled adults (≥18 years) with advanced/metastatic cancer. The dose-escalation phase evaluated the recommended Phase 2 dose (RP2D), maximum tolerated dose (MTD), and pharmacokinetics; the dose-expansion phase evaluated the antitumor activity and food effects.

RESULTS

Patients (N = 101) were enrolled in dose-escalation (n = 64) and dose-expansion (n = 37). During BID dose-escalation, dose-limiting toxicities were Grade 2 nausea (n = 1, 40 mg; n = 1, 80 mg); Grade 2 nausea and Grade 2 anorexia (n = 1, 120 mg), Grade 2 nausea, Grade 3 fatigue and Grade 3 paraesthesia (n = 1, 120 mg); MTD was 80 mg BID and RP2D was 60 mg BID. Common adverse events (AEs) were nausea (69.3%), fatigue (48.5%) and anaemia (35.6%); the most common Grade ≥3 AE was anaemia (24.8%). There was a dose-proportional increase in pamiparib exposure; no food effects on pharmacokinetics were observed. In the efficacy-evaluable population (n = 77), objective response rate (ORR) was 27.3% (95% CI, 17.7-38.6%). Median duration of response was 14.9 months (95% CI, 8.7-26.3). In the epithelial ovarian cancer (EOC)-evaluable population (n = 51), ORR was 41.2% (95% CI, 27.6-55.8%).

CONCLUSIONS

Pamiparib was tolerated with manageable AEs, and antitumor activity was observed in patients with EOC. CLINICALTRIALS.

GOV IDENTIFIER

NCT02361723.

Targeted protein degraders from an oncologist point of view: The Holy Grail of cancer therapy?

In the era of precision medicine, monoclonal antibodies and small molecule inhibitors are the mainstays of the biological therapy in patients with solid tumors. However, resistance to treatment and the "undruggability" of certain key oncogenic proteins emerged as major limitations and jeopardize the clinical benefit of modern therapeutic approaches. Targeted protein degraders are novel molecules entering the early phase of clinical development that exploit the intracellular ubiquitine-proteasome system to promote a specific degradation of target proteins. Since the peculiar mechanism of action, targeted protein degraders have the potential to limit and overcome resistance to treatment and to allow a full actionability of certain cancer drivers that are actually elusive targets. Here, we discuss the state-of-the-art and the open issues in the development of these emerging biological agents from a clinical perspective and with a focus on solid tumors.

Efficacy and Safety of PARP Inhibitors in Advanced or Metastatic Triple-Negative Breast Cancer: A Systematic Review and Meta-Analysis

Purpose

Poly (ADP-ribose) polymerase (PARP) inhibitors have shown promising results in metastatic triple-negative breast cancers (TNBCs). We therefore performed a systematic review and meta-analysis to evaluate the efficacy and safety of this drug in patients with advanced or metastatic TNBC.

Methods

A systematic literature search of PubMed, Embase, Scopus, Web of Science, and Cochrane Central Register of Controlled Trials for synonyms of "PARP inhibitors" and "breast cancer" was carried out. All published phase II/III clinical studies of PARP inhibitors in patients with advanced/metastatic TNBC were screened. Data were extracted independently by two authors and analyzed using Review Manager software version 5.3. End points include overall response rate (ORR), progression-free survival (PFS), and adverse events.

Results

Ten clinical trials were identified, with a total of 1,495 patients included. Pooled analyses showed that PARP inhibitors could provide a significant improvement of ORR [risk ratio (RR) = 2.00; 95% confidence interval (CI), 1.14-3.50; p = 0.02) and PFS [hazard ratio (HR) = 0.68; 95%Cl, 0.59-0.77; p < 0.0001) compared to chemotherapy in the whole population. In subgroup analysis, patients with BRCA mutation had a higher objective response to PARP inhibitor, with an RR of 2.85 (95%CI, 1.34-6.06; p = 0.007) compared to BRCA wild-type patients. However, no significant difference in ORR was observed between the homologous recombination deficiency (HRD) positive and non-HRD subgroups (RR = 1.82; 95%CI, 0.81-4.08; p = 0.14). Hematological toxicity is a common adverse event of PARP inhibitors.

Conclusions

PARP inhibitors are effective options for the treatment of patients with advanced or metastatic TNBC. Compared with patients without germline BRCA mutation, patients with germline BRCA mutation could benefit more from PARP inhibitors. In clinical setting, hematological toxicity associated with PARP inhibitors should be monitored regularly.

WRN helicase safeguards deprotected replication forks in BRCA2-mutated cancer cells

The tumor suppressor BRCA2 protects stalled forks from degradation to maintain genome stability. However, the molecular mechanism(s) whereby unprotected forks are stabilized remains to be fully characterized. Here, we demonstrate that WRN helicase ensures efficient restart and limits excessive degradation of stalled forks in BRCA2-deficient cancer cells. In vitro, WRN ATPase/helicase catalyzes fork restoration and curtails MRE11 nuclease activity on regressed forks. We show that WRN helicase inhibitor traps WRN on chromatin leading to rapid fork stalling and nucleolytic degradation of unprotected forks by MRE11, resulting in MUS81-dependent double-strand breaks, elevated non-homologous end-joining and chromosomal instability. WRN helicase inhibition reduces viability of BRCA2-deficient cells and potentiates cytotoxicity of a poly (ADP)ribose polymerase (PARP) inhibitor. Furthermore, BRCA2-deficient xenograft tumors in mice exhibited increased DNA damage and growth inhibition when treated with WRN helicase inhibitor. This work provides mechanistic insight into stalled fork stabilization by WRN helicase when BRCA2 is deficient.

DoE Optimization Empowers the Automated Preparation of Enantiomerically Pure [18F]Talazoparib and its In Vivo Evaluation as a PARP Radiotracer

Given the clinical potential of poly(ADP-ribose) polymerases (PARP) imaging for the detection and stratification of various cancers, the development of novel PARP imaging probes with improved pharmacological profiles over established PARP imaging agents is warranted. Here, we present a novel 18F-labeled PARP radiotracer based on the clinically superior PARP inhibitor talazoparib. An automated radiosynthesis of [18F]talazoparib (RCY: 13 ± 3.4%; n = 4) was achieved using a "design of experiments" (DoE) optimized copper-mediated radiofluorination reaction. The chiral product was isolated from the reaction mixture using 2D reversed-phase/chiral radio-HPLC (>99% ee). (8S,9R)-[18F]Talazoparib demonstrated PARP binding in HCC1937 cells in vitro and showed an excellent tumor-to-blood ratio in xenograft-bearing mice (10.2 ± 1.5). Additionally, a favorable pharmacological profile in terms of excretion, metabolism, and target engagement was observed. This synthesis of [18F]talazoparib exemplifies how DoE can enable the radiosyntheses of synthetically challenging radiolabeled compounds of high interest to the imaging community.

A wake-up call for cancer DNA damage: the role of Schlafen 11 (SLFN11) across multiple cancers

DNA-damaging agents exploit increased genomic instability, a hallmark of cancer. Recently, inhibitors targeting the DNA damage response (DDR) pathways, such as PARP inhibitors, have also shown promising therapeutic potential. However, not all tumors respond well to these treatments, suggesting additional determinants of response are required. Schlafen 11 (SLFN11), a putative DNA/RNA helicase that induces irreversible replication block, is emerging as an important regulator of cellular response to DNA damage. Preclinical and emerging clinical trial data suggest that SLFN11 is a predictive biomarker of response to a wide range of therapeutics that cause DNA damage including platinum salts and topoisomerase I/II inhibitors, as well as PARP inhibitors, which has raised exciting possibilities for its clinical application. In this article, we review the function, prevalence, and clinical testing of SLFN11 in tumor biopsy samples and circulating tumor cells. We discuss mounting evidence of SLFN11 as a key predictive biomarker for a wide range of cancer therapeutics and as a prognostic marker across several cancer types. Furthermore, we discuss emerging areas of investigation such as epigenetic reactivation of SLFN11 and its role in activating immune response. We then provide perspectives on open questions and future directions in studying this important biomarker.

Homologous Recombination Deficiency Assays in Epithelial Ovarian Cancer: Current Status and Future Direction

Epithelial ovarian cancer (EOC) patients are generally diagnosed at an advanced stage, usually relapse after initial treatments, which include debulking surgery and adjuvant platinum-based chemotherapy, and eventually have poor 5-year survival of less than 50%. In recent years, promising survival benefits from maintenance therapy with poly(ADP-ribose) polymerase (PARP) inhibitor (PARPi) has changed the management of EOC in newly diagnosed and recurrent disease. Identification of BRCA mutations and/or homologous recombination deficiency (HRD) is critical for selecting patients for PARPi treatment. However, the currently available HRD assays are not perfect predictors of the clinical response to PARPis in EOC patients. In this review, we introduce the concept of synthetic lethality, the rationale of using PARPi when HRD is present in tumor cells, the clinical trials of PARPi incorporating the HRD assays for EOC, the current HRD assays, and other HRD assays in development.

BRCA1/ATF1-Mediated Transactivation is Involved in Resistance to PARP Inhibitors and Cisplatin

Homologous recombination (HR)-deficient cells are sensitive to PARP inhibitors through a synthetic lethal effect. We previously developed an HR activity assay named Assay of Site-Specific HR Activity (ASHRA). Here, we evaluated the HR activity of 30 missense variants of BRCA1 by ASHRA and found that several BRCA1 variants showed intermediate HR activity, which was not clearly discerned by our previous analyses using a conventional method. HR activity measured by ASHRA was significantly correlated with sensitivity to olaparib. However, cells expressing the severely HR-deficient BRCA1-C61G variant were resistant to olaparib, and resistance was dependent on high expression of activating transcription factor 1 (ATF1), which binds to BRCA1 and activates the transcription of target genes to regulate cell proliferation. The BRCA1-C61G variant bound to ATF1 and stimulated ATF1-mediated transactivation similar to wild-type BRCA1. High expression of ATF1 conferred resistance to olaparib and cisplatin activating BRCA1/ATF1-mediated transcription without affecting HR activity in BRCA2-knockdown or RAD51-knockdown cells, but not in BRCA1-knockdown cells. These results suggest that ASHRA is a useful method to evaluate HR activity in cells and to predict the sensitivity to PARP inhibitors. The expression level of ATF1 might be an important biomarker of the effect of PARP inhibitors and platinum agents on HR-deficient tumors with the BRCA1-C61G variant or alteration of non-BRCA1 HR factors such as BRCA2 and RAD51.

Significance

ASHRA could evaluate HR activity in cells and predict the sensitivity to PARP inhibitors. High expression level of ATF1 may predict the resistance of BRCAness tumors with alterations of non-BRCA1 HR factors to PARP inhibitors and platinum agents.

Revisiting the BRCA-pathway through the lens of replication gap suppression: "Gaps determine therapy response in BRCA mutant cancer"

The toxic lesion emanating from chemotherapy that targets the DNA was initially debated, but eventually the DNA double strand break (DSB) ultimately prevailed. The reasoning was in part based on the perception that repairing a fractured chromosome necessitated intricate processing or condemned the cell to death. Genetic evidence for the DSB model was also provided by the extreme sensitivity of cells that were deficient in DSB repair. In particular, sensitivity characterized cells harboring mutations in the hereditary breast/ovarian cancer genes, BRCA1 or BRCA2, that function in the repair of DSBs by homologous recombination (HR). Along with functions in HR, BRCA proteins were found to prevent DSBs by protecting stalled replication forks from nuclease degradation. Coming full-circle, BRCA mutant cancer cells that gained resistance to genotoxic chemotherapy often displayed restored DNA repair by HR and/or restored fork protection (FP) implicating that the therapy was tolerated when DSB repair was intact or DSBs were prevented. Despite this well-supported paradigm that has been the impetus for targeted cancer therapy, here we argue that the toxic DNA lesion conferring response is instead single stranded DNA (ssDNA) gaps. We discuss the evidence that persistent ssDNA gaps formed in the wake of DNA replication rather than DSBs are responsible for cell killing following treatment with genotoxic chemotherapeutic agents. We also highlight that proteins, such as BRCA1, BRCA2, and RAD51 known for canonical DSB repair also have critical roles in normal replication as well as replication gap suppression (RGS) and repair. We review the literature that supports the idea that widespread gap induction proximal to treatment triggers apoptosis in a process that does not need or stem from DSB induction. Lastly, we discuss the clinical evidence for gaps and how to exploit them to enhance genotoxic chemotherapy response.

PARP inhibitors: clinical relevance and the role of multidisciplinary cancer teams on drug safety

INTRODUCTION

Cancer is considered one of the most devastating causes of death for humanity. Innovative and targeted therapies have become urgent in the treatment of this large subset of diseases. Over the last decade, the development of PARP (poly (ADP-Ribose) polymerase) inhibitors has emerged as a new target in cancer therapy.

AREAS COVERED

The authors conducted a review focusing on the clinic relevance and adverse effects of the four drugs already approved by drug regulatory agencies, namely: olaparib, rucaparib, niraparib and talazoparib. Despite the targeted action of this drug class, the adverse effects should be carefully monitored for the adequate safety of cancer patients taking them. The role of multidisciplinary cancer teams is crucial to help more and more patients to benefit from these revolutionary agents.

EXPERT OPINION

PARP (poly (ADP-Ribose) polymerase) inhibitors are drugs with great potential in the treatment of several types of cancer. However, their toxicity profiles often lead to treatment interruption or early discontinuation. The daily monitoring of these cancer patients by multidisciplinary cancer teams is essential for the success of therapy and for the promotion of a better quality of life.

PARP Inhibitors and Haematological Malignancies-Friend or Foe?

Simple Summary

PARP inhibitors are a class of orally active drugs that kill a range of cancer types by inducing synthetic lethality. The usefulness of PARP inhibitors for the treatment of haematological malignancies has begun to be explored in a variety of both pre-clinical models and human clinical trials. Despite being largely considered safe and well tolerated, secondary haematological malignancies have arisen in patients following treatment with PARP inhibitors, raising concerns about their use. In this review, we discuss the potential benefits and risks for using PARP inhibitors as treatments for haematological malignancies.

Abstract

Since their introduction several years ago, poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi) have become the standard of care for breast and gynaecological cancers with BRCA gene mutations. Given that PARPi act by exploiting defective DNA repair mechanisms within tumour cells, they should be ideally suited to combatting haematological malignancies where these pathways are notoriously defective, even though BRCA mutations are rare. To date, despite promising results in vitro, few clinical trials in humans for haematological malignancies have been performed, and additional investigation is required. Paradoxically, secondary haematological malignancies have arisen in patients after treatment with PARPi, raising concerns about their potential use as therapies for any blood or bone marrow-related disorders. Here, we provide a comprehensive review of the biological, pre-clinical, and clinical evidence for and against treating individual haematological malignancies with approved and experimental PARPi. We conclude that the promise of effective treatment still exists, but remains limited by the lack of investigation into useful biomarkers unique to these malignancies.

Discovery of 5-{4-[(7-Ethyl-6-oxo-5,6-dihydro-1,5-naphthyridin-3-yl)methyl]piperazin-1-yl}-N-methylpyridine-2-carboxamide (AZD5305): A PARP1-DNA Trapper with High Selectivity for PARP1 over PARP2 and Other PARPs

Poly-ADP-ribose-polymerase (PARP) inhibitors have achieved regulatory approval in oncology for homologous recombination repair deficient tumors including BRCA mutation. However, some have failed in combination with first-line chemotherapies, usually due to overlapping hematological toxicities. Currently approved PARP inhibitors lack selectivity for PARP1 over PARP2 and some other 16 PARP family members, and we hypothesized that this could contribute to toxicity. Recent literature has demonstrated that PARP1 inhibition and PARP1-DNA trapping are key for driving efficacy in a BRCA mutant background. Herein, we describe the structure- and property-based design of 25 (AZD5305), a potent and selective PARP1 inhibitor and PARP1-DNA trapper with excellent in vivo efficacy in a BRCA mutant HBCx-17 PDX model. Compound 25 is highly selective for PARP1 over other PARP family members, with good secondary pharmacology and physicochemical properties and excellent pharmacokinetics in preclinical species, with reduced effects on human bone marrow progenitor cells in vitro.

Replication-dependent cytotoxicity and Spartan-mediated repair of trapped PARP1-DNA complexes

The antitumor activity of poly(ADP-ribose) polymerase inhibitors (PARPis) has been ascribed to PARP trapping, which consists in tight DNA-protein complexes. Here we demonstrate that the cytotoxicity of talazoparib and olaparib results from DNA replication. To elucidate the repair of PARP1-DNA complexes associated with replication in human TK6 and chicken DT40 lymphoblastoid cells, we explored the role of Spartan (SPRTN), a metalloprotease associated with DNA replication, which removes proteins forming DPCs. We find that SPRTN-deficient cells are hypersensitive to talazoparib and olaparib, but not to veliparib, a weak PARP trapper. SPRTN-deficient cells exhibit delayed clearance of trapped PARP1 and increased replication fork stalling upon talazoparib and olaparib treatment. We also show that SPRTN interacts with PARP1 and forms nuclear foci that colocalize with the replicative cell division cycle 45 protein (CDC45) in response to talazoparib. Additionally, SPRTN is deubiquitinated and epistatic with translesion synthesis (TLS) in response to talazoparib. Our results demonstrate that SPRTN is recruited to trapped PARP1 in S-phase to assist in the excision and replication bypass of PARP1-DNA complexes.

Development of poly(ADP-ribose) polymerase inhibitor and immunotherapy combinations: progress, pitfalls, and promises

The efficacy of poly(ADP-ribose) polymerase inhibitors (PARPi) is restricted by inevitable drug resistance, while their use in combination with chemotherapy and targeted agents is commonly associated with dose-limiting toxicities. Immune checkpoint blockade (ICB) has demonstrated durable responses in different solid tumors and is well-established across multiple cancers. Despite this, single agent activity is limited to a minority of patients and drug resistance remains an issue. Building on the monotherapy success of both drug classes, combining PARPi with ICB may be a safe and well-tolerated strategy with the potential to improve survival outcomes. In this review, we present the preclinical, translational, and clinical data supporting the combination of DNA damage response (DDR) and ICB as well as consider important questions to be addressed with future research.

Determinants of Homologous Recombination Deficiency in Pancreatic Cancer

Pancreatic cancer is a treatment-resistant malignancy associated with high mortality. However, defective homologous recombination (HR), a DNA repair mechanism required for high-fidelity repair of double-strand DNA breaks, is a therapeutic vulnerability. Consistent with this, a subset of patients with pancreatic cancer show unique tumor responsiveness to HR-dependent DNA damage triggered by certain treatments (platinum chemotherapy and PARP inhibitors). While pathogenic mutations in HR genes are a major driver of this sensitivity, another layer of diverse tumor intrinsic and extrinsic factors regulate the HR deficiency (HRD) phenotype. Defining the mechanisms that drive HRD may guide the development of novel strategies and therapeutics to induce treatment sensitivity in non-HRD tumors. Here, we discuss the complexity underlying HRD in pancreatic cancer and highlight implications for identifying and treating this distinct subset of patients.

Targeting the p53 Pathway in CLL: State of the Art and Future Perspectives

The p53 pathway is a desirable therapeutic target, owing to its critical role in the maintenance of genome integrity. This is exemplified in chronic lymphocytic leukemia (CLL), one of the most common adult hematologic malignancies, in which functional loss of p53 arising from genomic aberrations are frequently associated with clonal evolution, disease progression, and therapeutic resistance, even in the contemporary era of CLL targeted therapy and immunotherapy. Targeting the 'undruggable' p53 pathway therefore arguably represents the holy grail of cancer research. In recent years, several strategies have been proposed to exploit p53 pathway defects for cancer treatment. Such strategies include upregulating wild-type p53, restoring tumor suppressive function in mutant p53, inducing synthetic lethality by targeting collateral genome maintenance pathways, and harnessing the immunogenicity of p53 pathway aberrations. In this review, we will examine the biological and clinical implications of p53 pathway defects, as well as our progress towards development of therapeutic approaches targeting the p53 pathway, specifically within the context of CLL. We will appraise the opportunities and pitfalls associated with these therapeutic strategies, and evaluate their place amongst the array of new biological therapies for CLL.