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

Exploiting interconnected synthetic lethal interactions between PARP inhibition and cancer cell reversible senescence

Senescence is a tumor suppression mechanism defined by stable proliferation arrest. Here we demonstrate that the known synthetic lethal interaction between poly(ADP-ribose) polymerase 1 inhibitors (PARPi) and DNA repair triggers p53-independent ovarian cancer cell senescence defined by senescence-associated phenotypic hallmarks including DNA-SCARS, inflammatory secretome, Bcl-XL-mediated apoptosis resistance, and proliferation restriction via Chk2 and p21 (CDKN1A). The concept of senescence as irreversible remains controversial and here we show that PARPi-senescent cells re-initiate proliferation upon drug withdrawal, potentially explaining the requirement for sustained PARPi therapy in the clinic. Importantly, PARPi-induced senescence renders ovarian and breast cancer cells transiently susceptible to second-phase synthetic lethal approaches targeting the senescence state using senolytic drugs. The combination of PARPi and a senolytic is effective in preclinical models of ovarian and breast cancer suggesting that coupling these synthetic lethalities provides a rational approach to their clinical use and may together be more effective in limiting resistance.

Sequential Therapy with PARP and WEE1 Inhibitors Minimizes Toxicity while Maintaining Efficacy

We demonstrate that concurrent administration of poly(ADP-ribose) polymerase (PARP) and WEE1 inhibitors is effective in inhibiting tumor growth but poorly tolerated. Concurrent treatment with PARP and WEE1 inhibitors induces replication stress, DNA damage, and abrogates the G₂ DNA damage checkpoint in both normal and malignant cells. Following cessation of monotherapy with PARP or WEE1 inhibitors, effects of these inhibitors persist suggesting that sequential administration of PARP and WEE1 inhibitors could maintain efficacy while ameliorating toxicity. Strikingly, while sequential administration mirrored concurrent therapy in cancer cells that have high basal replication stress, low basal replication stress in normal cells protected them from DNA damage and toxicity, thus improving tolerability while preserving efficacy in ovarian cancer xenograft and patient-derived xenograft models.

New medical approaches in advanced ovarian cancer

Ovarian cancer is the fifth leading cause of cancer death among women and the most lethal gynecologic malignancy. Most women with advanced epithelial ovarian cancer will experience many episodes of recurrent disease with progressively shorter disease-free intervals. For women whose disease continues to respond to platinum-based drugs, the disease can often be controlled for 5 years or more. Enormous progress has been made in the management of this disease, and new targeted treatments such as antiangiogenic drugs, poly(adenosine diphosphate-ribose) polymerase inhibitors, and immune checkpoint inhibitors offer potential for improved survival. A variety of combination strategies are being evaluated to leverage these agents. The objective of this review is to summarize results from clinical trials that tested cytotoxic drugs and target strategies for the treatment of ovarian cancer with particular attention to Phase III and ongoing trials.

Olaparib as maintenance treatment for patients with platinum-sensitive relapsed ovarian cancer

Poly (ADP-ribose) polymerase (PARP) inhibitors were developed with the intention of treating patients with homologous recombination repair deficiency (HRD), specifically for patients with tumours that harbour a BRCA mutation (BRCAm). Evidence from clinical trials to date has demonstrated that patients with a BRCAm derive the greatest benefit from PARP inhibitors. However, clinical studies have also shown that PARP inhibitors provide benefit to women with ovarian cancer who do not have a BRCAm. The recent updated approvals of olaparib, niraparib and rucaparib by the US Food and Drug Administration and the European Medicines Agency for the treatment of all platinum-sensitive relapsed (PSR) ovarian-cancer populations, regardless of their BRCAm status, support this. Long-term tolerability and efficacy of olaparib have been demonstrated in patients both with and without a BRCAm, with 13% of patients receiving maintenance olaparib for at least 5 years in one study, which is unprecedented in the relapsed ovarian-cancer setting (versus 1% on placebo). Further studies should be performed to elucidate which non-BRCAm patients are deriving benefit and what molecular processes are enabling this, so that patients continue to receive optimal treatment for their disease. Here, we review clinical and molecular markers of HRD, the long-term clinical safety and efficacy of PARP inhibitors in ovarian cancer, with a focus on olaparib and the current approved indications for PARP inhibitors, as well as guidance on treatment decisions for patients with PSR ovarian cancer.

DNA double-strand break repair pathway choice - from basic biology to clinical exploitation

Mutations in genes encoding components of the DNA damage response (DDR) are among the most frequent aberrations in human tumors. Moreover, a large array of human syndromes is caused by mutations in genes involved in DDR pathways. Among others, homologous recombination repair (HR) of DNA double-strand breaks (DSB) is frequently affected by disabling mutations. While impaired HR is clearly promoting tumorigenesis, it is also associated with an actionable sensitivity against PARP inhibitors. PARP inhibitors have recently received FDA approval for the treatment of breast- and ovarian cancer. However, as with all molecularly targeted agents, acquired resistance limits its use. Both pharmaco-genomic approaches and the study of human genome instability syndromes have led to a profound understanding of PARP inhibitor resistance. These experiments have revealed new insights into the molecular mechanisms that drive mammalian DSB repair. Here, we review recent discoveries in the field and provide a clinical perspective.

Discovery of 2-(1-(3-(4-Chloroxyphenyl)-3-oxo- propyl)pyrrolidine-3-yl)-1H-benzo[d]imidazole-4-carboxamide: A Potent Poly(ADP-ribose) Polymerase (PARP) Inhibitor for Treatment of Cancer

A series of benzimidazole carboxamide derivatives have been synthesized and characterized by ¹H-NMR, ¹³C-NMR and HRMS. PARP inhibition assays and cellular proliferation assays have also been carried out. Compounds 5cj and 5cp exhibited potential anticancer activities with IC₅₀ values of about 4 nM against both PARP-1 and PARP-2, similar to the reference drug veliparib. The two compounds also displayed slightly better in vitro cytotoxicities against MDA-MB-436 and CAPAN-1 cell lines than veliparib and olaparib, with values of 17.4 µM and 11.4 µM, 19.8 µM and 15.5 µM, respectively. The structure-activity relationship based on molecular docking was discussed as well.

Targeting PARP-1 with Alpha-Particles Is Potently Cytotoxic to Human Neuroblastoma in Preclinical Models

Alpha-emitters can be pharmacologically delivered for irradiation of single cancer cells, but cellular lethality could be further enhanced by targeting alpha-emitters directly to the nucleus. PARP-1 is a druggable protein in the nucleus that is overexpressed in neuroblastoma compared with normal tissues and is associated with decreased survival in high-risk patients. To exploit this, we have functionalized a PARP inhibitor (PARPi) with an alpha-emitter astatine-211. This approach offers enhanced cytotoxicity from conventional PARPis by not requiring enzymatic inhibition of PARP-1 to elicit DNA damage; instead, the alpha-particle directly induces multiple double-strand DNA breaks across the particle track. Here, we explored the efficacy of [²¹¹At]MM4 in multiple cancers and found neuroblastoma to be highly sensitive in vitro and in vivo Furthermore, alpha-particles delivered to neuroblastoma show antitumor effects and durable responses in a neuroblastoma xenograft model, especially when administered in a fractionated regimen. This work provides the preclinical proof of concept for an alpha-emitting drug conjugate that directly targets cancer chromatin as a therapeutic approach for neuroblastoma and perhaps other cancers.

NAMPT as a Dedifferentiation-Inducer Gene: NAD+ as Core Axis for Glioma Cancer Stem-Like Cells Maintenance

Glioma Cancer Stem-Like Cells (GSCs) are a small subset of CD133⁺ cells with self-renewal properties and capable of initiating new tumors contributing to Glioma progression, maintenance, hierarchy, and complexity. GSCs are highly resistant to chemo and radiotherapy. These cells are believed to be responsible for tumor relapses and patients' fatal outcome after developing a recurrent Glioblastoma (GBM) or High Grade Glioma (HGG). GSCs are cells under replicative stress with high demands on NAD⁺ supply to repair DNA, maintain self-renewal capacity and to induce tumor plasticity. NAD⁺ feeds Poly-ADP polymerases (PARP) and NAD⁺-dependent deacetylases (SIRTUINS) contributing to GSC phenotype. This energetic core axis is mainly controlled by the rate-limiting enzyme nicotinamide phosphoribosyltransferase (NAMPT), an important oncogene contributing to tumor dedifferentiation. Targeting GSCs depicts a new frontier in Glioma therapy; hence NAMPT could represent a key regulator for GSCs maintenance. Its inhibition may attenuate GSCs properties by decreasing NAD⁺ supply, consequently contributing to a better outcome together with current therapies for Glioma control.

Discovery of Stereospecific PARP-1 Inhibitor Isoindolinone NMS-P515

Poly(ADP-ribose) polymerase-1 (PARP-1) is an enzyme involved in signaling and repair of DNA single strand breaks. PARP-1 employs NAD⁺ to modify substrate proteins via the attachment of poly(ADP-ribose) chains. PARP-1 is a well established target in oncology, as testified by the number of marketed drugs (e.g., Lynparza, Rubraca, Zejula, and Talzenna) used for the treatment of ovarian, breast, and prostate tumors. Efforts in investigating an uncharted region of the previously identified isoindolinone carboxamide series delivered (S)-13 (NMS-P515), a potent inhibitor of PARP-1 both in biochemical (K _d: 0.016 μM) and cellular (IC₅₀: 0.027 μM) assays. Cocrystal structure allowed explaining NMS-P515 stereospecific inhibition of the target. After having ruled out potential loss of enantiopurity in vitro and in vivo, NMS-P515 was synthesized in an asymmetric fashion. NMS-P515 ADME profile and its antitumor activity in a mouse xenograft cancer model render the compound eligible for further optimization.

Targeting dePARylation selectively suppresses DNA repair-defective and PARP inhibitor-resistant malignancies

While poly(ADP-ribosyl)ation (PARylation) plays an important role in DNA repair, the role of dePARylation in DNA repair remains elusive. Here, we report that a novel small molecule identified from the NCI database, COH34, specifically inhibits poly(ADP-ribose) glycohydrolase (PARG), the major dePARylation enzyme, with nanomolar potency in vitro and in vivo. COH34 binds to the catalytic domain of PARG, thereby prolonging PARylation at DNA lesions and trapping DNA repair factors. This compound induces lethality in cancer cells with DNA repair defects and exhibits antitumor activity in xenograft mouse cancer models. Moreover, COH34 can sensitize tumor cells with DNA repair defects to other DNA-damaging agents, such as topoisomerase I inhibitors and DNA-alkylating agents, which are widely used in cancer chemotherapy. Notably, COH34 also efficiently kills PARP inhibitor-resistant cancer cells. Together, our study reveals the molecular mechanism of PARG in DNA repair and provides an effective strategy for future cancer therapies.

PARP Inhibitors as a Therapeutic Agent for Homologous Recombination Deficiency in Breast Cancers

Poly (ADP-ribose) polymerases (PARPs) play an important role in various cellular processes, such as replication, recombination, chromatin remodeling, and DNA repair. Emphasizing PARP's role in facilitating DNA repair, the PARP pathway has been a target for cancer researchers in developing compounds which selectively target cancer cells and increase sensitivity of cancer cells to other anticancer agents, but which also leave normal cells unaffected. Since certain tumors (BRCA1/2 mutants) have deficient homologous recombination repair pathways, they depend on PARP-mediated base excision repair for survival. Thus, inhibition of PARP is a promising strategy to selectively kill cancer cells by inactivating complementary DNA repair pathways. Although PARP inhibitor therapy has predominantly targeted BRCA-mutated cancers, this review also highlights the growing conversation around PARP inhibitor treatment for non-BRCA-mutant tumors, those which exhibit BRCAness and homologous recombination deficiency. We provide an update on the field's progress by considering PARP inhibitor mechanisms, predictive biomarkers, and clinical trials of PARP inhibitors in development. Bringing light to these findings would provide a basis for expanding the use of PARP inhibitors beyond BRCA-mutant breast tumors.

Aberrations in DNA repair pathways in cancer and therapeutic significances

Cancer cells show various types of mutations and aberrant expression in genes involved in DNA repair responses. These alterations induce genome instability and promote carcinogenesis steps and cancer progression processes. These defects in DNA repair have also been considered as suitable targets for cancer therapies. A most effective target so far clinically demonstrated is "homologous recombination repair defect", such as BRCA1/2 mutations, shown to cause synthetic lethality with inhibitors of poly(ADP-ribose) polymerase (PARP), which in turn is involved in DNA repair as well as multiple physiological processes. Different approaches targeting genomic instability, including immune therapy targeting mismatch-repair deficiency, have also recently been demonstrated to be promising strategies. In these DNA repair targeting-strategies, common issues could be how to optimize treatment and suppress/conquer the development of drug resistance. In this article, we review the extending framework of DNA repair response pathways and the potential impact of exploiting those defects on cancer treatments, including chemotherapy, radiation therapy and immune therapy.

PARP Trapping Beyond Homologous Recombination and Platinum Sensitivity in Cancers

Poly(ADP-ribose) polymerase inhibitors (PARPis) have recently been approved for the treatment of ovarian and breast cancers with BRCA mutations, as well as for maintenance therapies regardless of BRCA mutation for ovarian and primary peritoneal cancers that previously responded to platinum-based chemotherapy. The rationale of these indications is derived from the facts that cancer cells with BRCA mutations are defective in homologous recombination (HR), which confers synthetic lethality with PARPis, and that some of the sensitivity-determining factors for PARPis are shared with platinums. Although BRCA1 and BRCA2 are central for HR, more players within and beyond HR are emerging as response determinants to PARPis. Furthermore, there are similarities as well as differences in the DNA lesions and repair pathways induced by PARPis, platinums, and camptothecin topoisomerase 1 (TOP1) inhibitors. Here we review the sensitivity-determining factors for PARPis and the rationale for using PARPis as single agents and in combination therapy for cancers.

Resistance to PARP Inhibitors: Lessons from Preclinical Models of BRCA-Associated Cancer

Inhibitors of poly(ADP-ribose) polymerase (PARP) have recently entered the clinic for the treatment of homologous recombination–deficient cancers. Despite the success of this approach, resistance to PARP inhibitors (PARPis) is a clinical hurdle, and it is poorly understood how cancer cells escape the deadly effects of PARPis without restoring BRCA1/2 function. By synergizing the advantages of next-generation sequencing with functional genetic screens in tractable model systems, novel mechanisms providing useful insights into DNA damage response (DDR) have been identified. BRCA1/2 models not only are tools to explore therapy escape mechanisms but also yield basic knowledge about DDR pathways and PARPis’ mechanism of action. Moreover, alterations that render cells resistant to targeted therapies may cause new synthetic dependencies that can be exploited to combat resistant disease.

Niraparib activates interferon signaling and potentiates anti-PD-1 antibody efficacy in tumor models

PARP inhibitors have been proven clinically efficacious in platinum-responsive ovarian cancer regardless of BRCA1/2 status and in breast cancers with germline BRCA1/2 mutation. However, resistance to PARP inhibitors may preexist or evolve during treatment in many cancer types and may be overcome by combining PARP inhibitors with other therapies, such as immune checkpoint inhibitors, which confer durable responses and are rapidly becoming the standard of care for multiple tumor types. This study investigated the therapeutic potential of combining niraparib, a highly selective PARP1/2 inhibitor, with anti-PD-1 immune checkpoint inhibitors in preclinical tumor models. Our results indicate that niraparib treatment increases the activity of the type I (alpha) and type II (gamma) interferon pathways and enhances the infiltration of CD8⁺ cells and CD4⁺ cells in tumors. When coadministered in immunocompetent models, the combination of niraparib and anti-PD-1 demonstrated synergistic antitumor activities in both BRCA-proficient and BRCA-deficient tumors. Interestingly, mice with tumors cured by niraparib monotherapy completely rejected tumor growth upon rechallenge with the same tumor cell line, suggesting the potential establishment of immune memory in animals treated with niraparib monotherapy. Taken together, our findings uncovered immunomodulatory effects of niraparib that may sensitize tumors to immune checkpoint blockade therapies.

Increased PARP1-DNA binding due to autoPARylation inhibition of PARP1 on DNA rather than PARP1-DNA trapping is correlated with PARP1 inhibitor's cytotoxicity

PARP1 inhibitors (PARPis) are used clinically during cancer therapy and are thought to exert their cytotoxicity through PARP1 polymerase inhibition and PARP1-DNA trapping. Here, we showed no significant correlation between PARP1-DNA trapping and cytotoxicity induced by PARPis. We complemented PARP1-knockout sublines with wild-type PARP1 and 11 mutants with different point mutations that affect the polymerase activity. When examining the PARPi talazoparib, the induced cytotoxicity was highly significantly correlated with cellular PARP1 polymerase activity, but not with its PARP1-DNA trapping or polymerase inhibition. Similarly, talazoparib's PARP1-DNA trapping revealed significant correlation with the polymerase activity rather than its inhibition. Differently, however, when evaluating purified wild-type and mutated PARP1, we identified an almost linear relationship between PARPis' inhibiting PARP1 dissociation from DNA and their cytotoxicity in 17 cancer cell lines. In contrast, no significant correlation existed between PARP1 polymerase inhibition in the histone-based systems and the cytotoxicity. After careful comparisons on different methods and detection targets, we conclude that the PARPi-mediated increase in PARP1-DNA binding by inhibiting autoPARylation of PARP1 on DNA rather than in PARP1-DNA trapping is correlated with PARPi's cytotoxicity. Accordingly, we established a new PARPi screening model that more closely predicts cytotoxicity.

53BP1 as a potential predictor of response in PARP inhibitor-treated homologous recombination-deficient ovarian cancer

OBJECTIVE

Poly(ADP-ribose) polymerase (PARP) inhibitors have shown substantial activity in homologous recombination- (HR-) deficient ovarian cancer and are undergoing testing in other HR-deficient tumors. For reasons that are incompletely understood, not all patients with HR-deficient cancers respond to these agents. Preclinical studies have demonstrated that changes in alternative DNA repair pathways affect PARP inhibitor (PARPi) sensitivity in ovarian cancer models. This has not previously been assessed in the clinical setting.

METHODS

Clonogenic and plasmid-based HR repair assays were performed to compare BRCA1-mutant COV362 ovarian cancer cells with or without 53BP1 gene deletion. Archival biopsies from ovarian cancer patients in the phase I, open-label clinical trial of PARPi ABT-767 were stained for PARP1, RAD51, 53BP1 and multiple components of the nonhomologous end-joining (NHEJ) DNA repair pathway. Modified histochemistry- (H-) scores were determined for each repair protein in each sample. HRD score was determined from tumor DNA.

RESULTS

53BP1 deletion increased HR in BRCA1-mutant COV362 cells and decreased PARPi sensitivity in vitro. In 36 women with relapsed ovarian cancer, responses to the PARPi ABT-767 were observed exclusively in cancers with HR deficiency. In this subset, 7 of 18 patients (39%) had objective responses. The actual HRD score did not further correlate with change from baseline tumor volume (r = 0.050; p = 0.87). However, in the HR-deficient subset, decreased 53BP1 H-score was associated with decreased antitumor efficacy of ABT-767 (r = -0.69, p = 0.004).

CONCLUSION

Differences in complementary repair pathways, particularly 53BP1, correlate with PARPi response of HR-deficient ovarian cancers.

53BP1 as a potential predictor of response in PARP inhibitor-treated homologous recombination-deficient ovarian cancer

OBJECTIVE

Poly(ADP-ribose) polymerase (PARP) inhibitors have shown substantial activity in homologous recombination- (HR-) deficient ovarian cancer and are undergoing testing in other HR-deficient tumors. For reasons that are incompletely understood, not all patients with HR-deficient cancers respond to these agents. Preclinical studies have demonstrated that changes in alternative DNA repair pathways affect PARP inhibitor (PARPi) sensitivity in ovarian cancer models. This has not previously been assessed in the clinical setting.

METHODS

Clonogenic and plasmid-based HR repair assays were performed to compare BRCA1-mutant COV362 ovarian cancer cells with or without 53BP1 gene deletion. Archival biopsies from ovarian cancer patients in the phase I, open-label clinical trial of PARPi ABT-767 were stained for PARP1, RAD51, 53BP1 and multiple components of the nonhomologous end-joining (NHEJ) DNA repair pathway. Modified histochemistry- (H-) scores were determined for each repair protein in each sample. HRD score was determined from tumor DNA.

RESULTS

53BP1 deletion increased HR in BRCA1-mutant COV362 cells and decreased PARPi sensitivity in vitro. In 36 women with relapsed ovarian cancer, responses to the PARPi ABT-767 were observed exclusively in cancers with HR deficiency. In this subset, 7 of 18 patients (39%) had objective responses. The actual HRD score did not further correlate with change from baseline tumor volume (r = 0.050; p = 0.87). However, in the HR-deficient subset, decreased 53BP1 H-score was associated with decreased antitumor efficacy of ABT-767 (r = -0.69, p = 0.004).

CONCLUSION

Differences in complementary repair pathways, particularly 53BP1, correlate with PARPi response of HR-deficient ovarian cancers.

53BP1 as a potential predictor of response in PARP inhibitor-treated homologous recombination-deficient ovarian cancer

OBJECTIVE

Poly(ADP-ribose) polymerase (PARP) inhibitors have shown substantial activity in homologous recombination- (HR-) deficient ovarian cancer and are undergoing testing in other HR-deficient tumors. For reasons that are incompletely understood, not all patients with HR-deficient cancers respond to these agents. Preclinical studies have demonstrated that changes in alternative DNA repair pathways affect PARP inhibitor (PARPi) sensitivity in ovarian cancer models. This has not previously been assessed in the clinical setting.

METHODS

Clonogenic and plasmid-based HR repair assays were performed to compare BRCA1-mutant COV362 ovarian cancer cells with or without 53BP1 gene deletion. Archival biopsies from ovarian cancer patients in the phase I, open-label clinical trial of PARPi ABT-767 were stained for PARP1, RAD51, 53BP1 and multiple components of the nonhomologous end-joining (NHEJ) DNA repair pathway. Modified histochemistry- (H-) scores were determined for each repair protein in each sample. HRD score was determined from tumor DNA.

RESULTS

53BP1 deletion increased HR in BRCA1-mutant COV362 cells and decreased PARPi sensitivity in vitro. In 36 women with relapsed ovarian cancer, responses to the PARPi ABT-767 were observed exclusively in cancers with HR deficiency. In this subset, 7 of 18 patients (39%) had objective responses. The actual HRD score did not further correlate with change from baseline tumor volume (r = 0.050; p = 0.87). However, in the HR-deficient subset, decreased 53BP1 H-score was associated with decreased antitumor efficacy of ABT-767 (r = -0.69, p = 0.004).

CONCLUSION

Differences in complementary repair pathways, particularly 53BP1, correlate with PARPi response of HR-deficient ovarian cancers.

Regulating Immunity via ADP-Ribosylation: Therapeutic Implications and Beyond

Innate immune cells express pattern recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs) and endogenous danger-associated molecular patterns (DAMPs). Upon binding, PAMPs/DAMPs can initiate an immune response by activating lymphocytes, amplifying and modulating signaling cascades, and inducing appropriate effector responses. Protein ADP-ribosylation can regulate cell death, the release of DAMPs, as well as inflammatory cytokine expression. Inhibitors of ADP-ribosylation (i.e. PARP inhibitors) have been developed as therapeutic agents (in cancer), and are also able to dampen inflammation. We summarize here our most recent understanding of how ADP-ribosylation can regulate the different phases of an immune response. Moreover, we examine the potential clinical translation of pharmacological ADP-ribosylation inhibitors as putative treatment strategies for various inflammation-associated diseases (e.g. sepsis, chronic inflammatory diseases, and reperfusion injury).

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.

Loss of the p12 subunit of DNA polymerase delta leads to a defect in HR and sensitization to PARP inhibitors

Human DNA polymerase δ is normally present in unstressed, non-dividing cells as a heterotetramer (Pol δ4). Its smallest subunit, p12, is transiently degraded in response to UV damage, as well as during the entry into S-phase, resulting in the conversion of Pol δ4 to a trimer (Pol δ3). In order to further understand the specific cellular roles of these two forms of Pol δ, the gene (POLD4) encoding p12 was disrupted by CRISPR/Cas9 to produce p12 knockout (p12KO) cells. Thus, Pol δ4 is absent in p12KO cells, leaving Pol δ3 as the sole source of Pol δ activity. GFP reporter assays revealed that the p12KO cells exhibited a defect in homologous recombination (HR) repair, indicating that Pol δ4,  but not Pol δ3, is required for HR. Expression of Flag-tagged p12 in p12KO cells to restore Pol δ4 alleviated the HR defect. These results establish a specific requirement for Pol δ4 in HR repair. This leads to the prediction that p12KO cells should be more sensitive to chemotherapeutic agents, and should exhibit synthetic lethal killing by PARP inhibitors. These predictions were confirmed by clonogenic cell survival assays of p12KO cells treated with cisplatin and mitomycin C, and with the PARP inhibitors Olaparib, Talazoparib, Rucaparib, and Niraparib. The sensitivity to PARP inhibitors in H1299-p12KO cells was alleviated by expression of Flag-p12. These findings have clinical significance, as the expression levels of p12 could be a predictive biomarker of tumor response to PARP inhibitors. In addition, small cell lung cancers (SCLC) are known to exhibit a defect in p12 expression. Analysis of several SCLC cell lines showed that they exhibit hypersensitivity to PARP inhibitors, providing evidence that loss of p12 expression could represent a novel molecular basis for HR deficiency.

PARP inhibition prevents escape from a telomere-driven crisis and inhibits cell immortalisation

Telomeric crisis is the final replicative barrier to cell immortalisation; it is characterised by genome instability and cell death and is triggered when telomeres become critically short and are subjected to fusion. Pre-cancerous lesions, or early stage cancers, often show signs of a telomere crisis, suggesting that escape from telomere crisis is a prerequisite for disease progression. Telomeric crisis therefore represents an attractive, and as yet unexplored, opportunity for therapeutic intervention. Here, we show that two clinically approved PARP inhibitors, selectively eliminate human cells undergoing a telomere-driven crisis. Clonal populations of a colorectal cancer cell line (HCT116), or the plasma cell leukaemia cell line (JJN-3), expressing a dominant-negative telomerase, entered a telomere-driven crisis at defined population doubling points and telomere lengths. The addition of the PARP inhibitors, olaparib or rucaparib prevented these cells from escaping crisis. PARP inhibition did not alter cellular proliferation prior to crisis, rates of telomere erosion or the telomere length at which crisis was initiated, but affected repair of eroded telomeres, resulting in an increased in intra-chromosomal telomere fusion. This was accompanied by enhanced DNA damage checkpoint activation and elevated levels of apoptosis. We propose that PARP inhibitors impair the repair of dysfunctional telomeres and/or induce replicative stress at telomeres to inhibit escape from a telomere crisis. This is the first demonstration that a drug can selectively kill cells experiencing telomeric crisis. We propose that this type of drug, which we term 'crisolytic', has the potential to eliminate pre-cancerous lesions and tumours exhibiting short dysfunctional telomeres.

Synthesis, preliminarily biological evaluation and molecular docking study of new Olaparib analogues as multifunctional PARP-1 and cholinesterase inhibitors

A series of new Olaparib derivatives was designed and synthesized, and their inhibitory activities against poly (ADP-ribose) polymerases-1 (PARP-1) enzyme and cancer cell line MDA-MB-436 in vitro were evaluated. The results showed that compound 5l exhibited the most potent inhibitory effects on PARP-1 enzyme (16.10 ± 1.25 nM) and MDA-MB-436 cancer cell (11.62 ± 2.15 μM), which was close to that of Olaparib. As a PARP-1 inhibitor had been reported to be viable to neuroprotection, in order to search for new multitarget-directed ligands (MTDLs) for the treatment of Alzheimer’s disease (AD), the inhibitory activities of the synthesized compounds against the enzymes AChE (from electric eel) and BChE (from equine serum) were also tested. Compound 5l displayed moderate BChE inhibitory activity (9.16 ± 0.91 μM) which was stronger than neostigmine (12.01 ± 0.45 μM) and exhibited selectivity for BChE over AChE to some degree. Molecular docking studies indicated that 5l could bind simultaneously to the catalytic active of PARP-1, but it could not interact well with huBChE. For pursuit of PARP-1 and BChE dual-targeted inhibitors against AD, small and flexible non-polar groups introduced to the compound seemed to be conducive to improving its inhibitory potency on huBChE, while keeping phthalazine-1-one moiety unchanged which was mainly responsible for PARP-1 inhibitory activity. Our research gave a clue to search for new agents based on AChE and PARP-1 dual-inhibited activities to treat Alzheimer’s disease.

PARPs in genome stability and signal transduction: implications for cancer therapy

The poly(ADP-ribose) polymerase (PARP) superfamily of enzymes catalyses the ADP-ribosylation (ADPr) of target proteins by using nicotinamide adenine dinucleotide (NAD+) as a donor. ADPr reactions occur either in the form of attachment of a single ADP-ribose nucleotide unit on target proteins or in the form of ADP-ribose chains, with the latter called poly(ADP-ribosyl)ation. PARPs regulate many cellular processes, including the maintenance of genome stability and signal transduction. In this review, we focus on the PARP family members that possess the ability to modify proteins by poly(ADP-ribosyl)ation, namely PARP1, PARP2, Tankyrase-1, and Tankyrase-2. Here, we detail the cellular functions of PARP1 and PARP2 in the regulation of DNA damage response and describe the function of Tankyrases in Wnt-mediated signal transduction. Furthermore, we discuss how the understanding of these pathways has provided some major breakthroughs in the treatment of human cancer.

A comparative pharmacokinetic study of PARP inhibitors demonstrates favorable properties for niraparib efficacy in preclinical tumor models

Niraparib is an orally bioavailable and selective poly (ADP-ribose) polymerase (PARP)-1/-2 inhibitor approved for maintenance treatment of both BRCA mutant (mut) and BRCA wildtype (wt) adult patients with recurrent epithelial ovarian, fallopian tube, or primary peritoneal cancers who have demonstrated a complete or partial response to platinum-based chemotherapy. In patients without germline BRCA mutations (non-gBRCAmut), niraparib improved progression-free survival (PFS) by 5.4 months, whereas another PARP inhibitor (PARPi) olaparib supplied only 1.9 months of improvement in a similar patient population. Previous studies revealed higher cell membrane permeability and volume of distribution (VD) as unique features of niraparib in comparison to other PARPi including olaparib. Here, we explore the potential correlation of these pharmacokinetic properties to preclinical antitumor effects in BRCAwt tumors. Our results show that at steady state, tumor exposure to niraparib is 3.3 times greater than plasma exposure in tumor xenograft mouse models. In comparison, the tumor exposure to olaparib is less than observed in plasma. In addition, niraparib crosses the blood-brain barrier and shows good sustainability in the brain, whereas sustained brain exposure to olaparib is not observed in the same models. Consistent with its favorable tumor and brain distribution, niraparib achieves more potent tumor growth inhibition than olaparib in BRCAwt models and an intracranial tumor model at maximum tolerated doses (MTD). These findings demonstrate favorable pharmacokinetic profiles and potent antitumor effects of niraparib in BRCAwt tumors, consistent with its broader clinical effect in patients with both BRCAmut and BRCAwt tumors.

Positioning High-Throughput CETSA in Early Drug Discovery through Screening against B-Raf and PARP1

Methods to measure cellular target engagement are increasingly being used in early drug discovery. The Cellular Thermal Shift Assay (CETSA) is one such method. CETSA can investigate target engagement by measuring changes in protein thermal stability upon compound binding within the intracellular environment. It can be performed in high-throughput, microplate-based formats to enable broader application to early drug discovery campaigns, though high-throughput forms of CETSA have only been reported for a limited number of targets. CETSA offers the advantage of investigating the target of interest in its physiological environment and native state, but it is not clear yet how well this technology correlates to more established and conventional cellular and biochemical approaches widely used in drug discovery. We report two novel high-throughput CETSA (CETSA HT) assays for B-Raf and PARP1, demonstrating the application of this technology to additional targets. By performing comparative analyses with other assays, we show that CETSA HT correlates well with other screening technologies and can be applied throughout various stages of hit identification and lead optimization. Our results support the use of CETSA HT as a broadly applicable and valuable methodology to help drive drug discovery campaigns to molecules that engage the intended target in cells.

A phase 2 randomised study of veliparib plus FOLFIRI±bevacizumab versus placebo plus FOLFIRI±bevacizumab in metastatic colorectal cancer

Background

Metastatic colorectal cancer (mCRC) has low survival rates. We assessed if addition of veliparib, concurrent to FOLFIRI, improves survival in patients with previously untreated mCRC.

Methods

This study compared veliparib (200 mg BID for 7 days of each 14-day cycle) to placebo, each with FOLFIRI. Bevacizumab was allowed in both arms. The primary endpoint was progression-free survival (PFS).

Results

Patients were randomised to receive veliparib (n = 65) or placebo (n = 65) in combination with FOLFIRI. Median PFS was 12 vs 11 months (veliparib vs placebo) [HR = 0.94 (95% CI: 0.60, 1.48)]. Median OS was 25 vs 27 months [HR = 1.26 (95% CI: 0.74, 2.16)]. Response rate was 57% vs 62%. Median DOR was 11 vs 9 months [HR = 0.73 (95% CI: 0.38, 1.40)]. AEs with significantly higher frequency (p < 0.05) in the veliparib group were anaemia (39% vs 19%, p = 0.019) and neutropenia (66% vs 37%, p = 0.001) for common AEs (≥20%); neutropenia (59% vs 22%, p < 0.001) for common Grade 3/4 AEs (≥5%); none in serious AEs. Haematopoietic cytopenias were more common with veliparib (79% vs 52%, p = 0.003). Fourteen percent of patients on veliparib and 15% on placebo discontinued treatment due to AEs.

Conclusion

Veliparib added to FOLFIRI ± bevacizumab demonstrated similar efficacy as FOLFIRI ± bevacizumab in frontline mCRC patients. No unexpected safety concerns occurred.

Current Practice and Emerging Molecular Imaging Technologies in Oral Cancer Screening

Oral cancer is one of the most common cancers globally. Survival rates for patients are directly correlated with stage of diagnosis; despite this knowledge, 60% of individuals are presenting with late-stage disease. Currently, the initial evaluation of a questionable lesion is performed by a conventional visual examination with white light. If a lesion is deemed suspicious, a biopsy is taken for diagnosis. However, not all lesions present suspicious under visual white light examination, and there is limited specificity in differentiating between benign and malignant transformations. Several vital dyes, light-based detection systems, and cytology evaluation methods have been formulated to aid in the visualization process, but their lack of specific biomarkers resulted in high false-positive rates and thus limits their reliability as screening and guidance tools. In this review, we will analyze the current methodologies and demonstrate the need for specific intraoral imaging agents to aid in screening and diagnosis to identify patients earlier. Several novel molecular imaging agents will be presented as, by result of their molecular targeting, they aim to have high specificity for tumor pathways and can support in identifying dysplastic/cancerous lesions and guiding visualization of biopsy sites. Imaging agents that are easy to use, inexpensive, noninvasive, and specific can be utilized to increase the number of patients who are screened and monitored in a variety of different environments, with the ultimate goal of increasing early detection.

Replication Stress: An Achilles' Heel of Glioma Cancer Stem-like Cells

Glioblastoma (GBM) is a highly aggressive form of cancer that is resistant to standard therapy with concurrent radiation and temozolomide, two agents that work by inducing DNA damage. An underlying cause of this resistance may be a subpopulation of cancer stem-like cells that display a heightened DNA damage response (DDR). Although this DDR represents an attractive therapeutic target for overcoming the resistance of GBMs to radiotherapy, until now, the cause of this DDR upregulation has not been understood. In a previous issue of Cancer Research, Carruthers and colleagues investigated DNA replication stress as an underlying mechanism responsible for upregulation of the DDR and hence the radiation resistance of glioma stem-like cells. Furthermore, the authors explore the efficacy of combined ataxia telangiectasia and Rad3-related kinase and PARP inhibitors as a strategy to leverage these mechanisms and overcome radiation resistance.See related article by Carruthers and colleagues, Cancer Res; 78(17); 5060-71.

SUMOylation of PCNA by PIAS1 and PIAS4 promotes template switch in the chicken and human B cell lines

DNA damage tolerance (DDT) releases replication blockage caused by damaged nucleotides on template strands employing two alternative pathways, error-prone translesion DNA synthesis (TLS) and error-free template switch (TS). Lys164 of proliferating cell nuclear antigen (PCNA) is SUMOylated during the physiological cell cycle. To explore the role for SUMOylation of PCNA in DDT, we characterized chicken DT40 and human TK6 B cells deficient in the PIAS1 and PIAS4 small ubiquitin-like modifier (SUMO) E3 ligases. DT40 cells have a unique advantage in the phenotypic analysis of DDT as they continuously diversify their immunoglobulin (Ig) variable genes by TLS and TS [Ig gene conversion (GC)], both relieving replication blocks at abasic sites without accompanying by DNA breakage. Remarkably, PIAS1 ^-/- /PIAS4 ^-/- cells displayed a multifold decrease in SUMOylation of PCNA at Lys164 and over a 90% decrease in the rate of TS. Likewise, PIAS1 ^-/- /PIAS4 ^-/- TK6 cells showed a shift of DDT from TS to TLS at a chemosynthetic UV lesion inserted into the genomic DNA. The PCNA ^K164R/K164R mutation caused a ∼90% decrease in the rate of Ig GC and no additional impact on PIAS1 ^-/- /PIAS4 ^-/- cells. This epistatic relationship between the PCNA ^K164R/K164R and the PIAS1 ^-/- /PIAS4 ^-/- mutations suggests that PIAS1 and PIAS4 promote TS mainly through SUMOylation of PCNA at Lys164. This idea is further supported by the data that overexpression of a PCNA-SUMO1 chimeric protein restores defects in TS in PIAS1 ^-/- /PIAS4 ^-/- cells. In conclusion, SUMOylation of PCNA at Lys164 promoted by PIAS1 and PIAS4 ensures the error-free release of replication blockage during physiological DNA replication in metazoan cells.

PARP family enzymes: regulation and catalysis of the poly(ADP-ribose) posttranslational modification

Poly(ADP-ribose) is a posttranslational modification and signaling molecule that regulates many aspects of human cell biology, and it is synthesized by enzymes known as poly(ADP-ribose) polymerases, or PARPs. A diverse collection of domain structures dictates the different cellular roles of PARP enzymes and regulates the production of poly(ADP-ribose). Here we primarily review recent structural insights into the regulation and catalysis of two family members: PARP-1 and Tankyrase. PARP-1 has multiple roles in the cellular response to DNA damage and the regulation of gene transcription, and Tankyrase regulates a diverse set of target proteins involved in cellular processes such as mitosis, genome integrity, and cell signaling. Both enzymes offer interesting modes of regulating the production and the target site selectivity of the poly(ADP-ribose) modification.

Using PARP Inhibitors in the Treatment of Patients With Ovarian Cancer

OPINION STATEMENT

Use of poly(ADP-ribose) polymerase (PARP) inhibitors has greatly increased over the past 5 years. With several new Food and Drug Administration (FDA) approvals, three PARP inhibitors have entered into standard of care treatment for epithelial ovarian cancer (including ovarian, fallopian tube, and primary peritoneal cancer). Olaparib and rucaparib currently have indications for treatment of recurrent BRCA mutant ovarian cancer. Olaparib, rucaparib, and niraparib all have indications for maintenance therapy in recurrent platinum-sensitive ovarian cancer after response to platinum-based therapy. In our practice, we use both olaparib and rucaparib in the recurrent setting, and all three PARP inhibitors in the maintenance setting. Choice of which PARP inhibitor to use in either setting is largely based upon baseline laboratory values, number of prior therapies, and presence of a BRCA mutation and/or homologous recombination deficiency (HRD). As (HRD) and other biomarker assessments continue to improve, we anticipate being able to better identify which patients might most benefit from PARP inhibitor therapy in the future. The clinically available PARP inhibitors are currently undergoing extensive investigations in clinical trials. Other newer agents such as talazoparib, veliparib, 2X-121, and CEP-9722 are in earlier stages of development. As more FDA-approved indications for PARP inhibitor therapy in ovarian cancer become available, we anticipate the decision of which PARP inhibitor to use will become increasingly complex.

PARP1 Trapping by PARP Inhibitors Drives Cytotoxicity in Both Cancer Cells and Healthy Bone Marrow

PARP inhibitors have recently been approved as monotherapies for the treatment of recurrent ovarian cancer and metastatic BRCA-associated breast cancer, and ongoing studies are exploring additional indications and combinations with other agents. PARP inhibitors trap PARP onto damaged chromatin when combined with temozolomide and methyl methanesulfonate, but the clinical relevance of these findings remains unknown. PARP trapping has thus far been undetectable in cancer cells treated with PARP inhibitors alone. Here, we evaluate the contribution of PARP trapping to the tolerability and efficacy of PARP inhibitors in the monotherapy setting. We developed a novel implementation of the proximity ligation assay to detect chromatin-trapped PARP1 at single-cell resolution with higher sensitivity and throughput than previously reported methods. We further demonstrate that the PARP inhibitor-induced trapping appears to drive single-agent cytotoxicity in healthy human bone marrow, indicating that the toxicity of trapped PARP complexes is not restricted to cancer cells with homologous recombination deficiency. Finally, we show that PARP inhibitors with dramatically different trapping potencies exhibit comparable tumor growth inhibition at MTDs in a xenograft model of BRCA1-mutant triple-negative breast cancer. These results are consistent with emerging clinical data and suggest that the inverse relationship between trapping potency and tolerability may limit the potential therapeutic advantage of potent trapping activity. IMPLICATIONS: PARP trapping contributes to single-agent cytotoxicity of PARP inhibitors in both cancer cells and healthy bone marrow, and the therapeutic advantage of potent trapping activity appears to be limited.

Nanoemulsion-Based Delivery of Fluorescent PARP Inhibitors in Mouse Models of Small Cell Lung Cancer

The preclinical potential of many diagnostic and therapeutic small molecules is limited by their rapid washout kinetics and consequently modest pharmacological performances. In several cases, these could be improved by loading the small molecules into nanoparticulates, improving blood half-life, in vivo uptake and overall pharmacodynamics. In this study, we report a nanoemulsion (NE) encapsulated form of PARPi-FL. As a proof of concept, we used PARPi-FL, which is a fluorescently labeled sensor for olaparib, a FDA-approved small molecule inhibitor of the nuclear enzyme poly(ADP-ribose)polymerase 1 (PARP1). Encapsulated PARPi-FL showed increased blood half-life, and delineated subcutaneous xenografts of small cell lung cancer (SCLC), a fast-progressing disease where efficient treatment options remain an unmet clinical need. Our study demonstrates an effective method for expanding the circulation time of a fluorescent PARP inhibitor, highlighting the pharmacokinetic benefits of nanoemulsions as nanocarriers and confirming the value of PARPi-FL as an imaging agent targeting PARP1 in small cell lung cancer.

Cyclin D1 silencing impairs DNA double strand break repair, sensitizes BRCA1 wildtype ovarian cancer cells to olaparib

OBJECTIVE

Poly(ADP-ribose) polymerase inhibitors (PARPi) are active in cancer cells that have impaired repair of DNA by the homologous recombination (HR) pathway. Strategies that disrupt HR may sensitize HR-proficient tumors to PARP inhibition. As a component of the core cell cycle machinery, cyclin D1 has unexpected function in DNA repair, suggesting that targeting cyclin D1 may represent a plausible strategy for expanding the utility of PARPi in ovarian cancer.

METHODS

BRCA1 wildtype ovarian cancer cells (A2780 and SKOV3) were treated with a combination of CCND1 siRNA and olaparib in vitro. Cell viability was assessed by MTT. The effects of the combined treatment on DNA damage repair and cell cycle progression were examined to dissect molecular mechanisms. In vivo studies were performed in an orthotopic ovarian cancer mouse model. Animals were treated with a combination of lentivirus-mediated CCND1 shRNA and olaparib or olaparib plus scrambled shRNA. Molecular downstream effects were examined by immunohistochemistry.

RESULTS

Silencing of cyclin D1 sensitized ovarian cancer cells to olaparib through interfering with RAD51 accumulation and inducing cell cycle G0/G1 arrest. Treatment of lentivirus-mediated CCND1-shRNA in nude mice statistically significantly augmented the olaparib response (mean tumor weight ± SD, CCND1-shRNA plus olaparib vs scrambled shRNA plus olaparib: 0.172 ± 0.070 g vs 0.324 ± 0.044 g, P< 0.05).

CONCLUSIONS

Silencing of cyclin D1 combined with olaparib may lead to substantial benefit for ovarian cancer management by mimicking a BRCAness phenotype, and induction of G0/G1 cell cycle arrest.

Response of germline BRCA2-mutated advanced pancreatic acinar cell carcinoma to olaparib: A case report

RATIONALE

Pancreatic acinar cell carcinoma (PACC) is a relatively rare malignancy of the exocrine pancreas. BRCA2, a cancer susceptibility gene, has been widely studied in breast and ovarian carcinomas as mutation carriers for this gene are at a high risk for cancer development. Olaparib, an oral poly(adenosine diphosphate-ribose) polymerase (PARP) inhibitor, has been approved for the treatment of ovarian cancer with any BRCA 1/2 mutations. Herein, we report the first case of a germline BRCA2-mutated unresectable advanced PACC patient who responded well to olaparib treatment.

PATIENT CONCERNS

A 59-year-old male with a family history of cancer presented with a persistent epigastric dull pain for 3 months.

DIAGNOSIS

The patient was diagnosed with advanced PACC based on computed tomography (CT) scan, laparotomy, and pathology.

INTERVENTIONS

Exploratory laparotomy, intratumoral brachytherapy by radioiodine-125 seeds, modified FOLFIRINOX chemotherapy, and targeted therapy with olaparib were administered.

OUTCOMES

The patient responded well to olaparib until the occurrence of severe adverse drug reactions, he died as a result of multiple organ failure with an overall survival period of 12 months.

LESSONS

As a PARP inhibitor, olaparib has remarkable curative effect not only on breast and ovarian cancers, but also on other malignancies with BRCA mutations. Patients with advanced cancer could benefit from active targeted therapy with improvement in overall survival and quality of life.

Candidate biomarkers of PARP inhibitor sensitivity in ovarian cancer beyond the BRCA genes

BACKGROUND

Olaparib (Lynparza™) is a PARP inhibitor approved for advanced BRCA-mutated (BRCAm) ovarian cancer. PARP inhibitors may benefit patients whose tumours are dysfunctional in DNA repair mechanisms unrelated to BRCA1/2. We report exploratory analyses, including the long-term outcome of candidate biomarkers of sensitivity to olaparib in BRCA wild-type (BRCAwt) tumours.

METHODS

Tumour samples from an olaparib maintenance monotherapy trial (Study 19, D0810C00019; NCT00753545) were analysed. Analyses included classification of mutations in genes involved in homologous recombination repair (HRR), BRCA1 promoter methylation status, measurement of BRCA1 protein and Myriad HRD score.

RESULTS

Patients with BRCAm tumours gained most benefit from olaparib; a similar treatment benefit was also observed in 21/95 patients whose tumours were BRCAwt but had loss-of-function HRR mutations compared to patients with no detectable HRR mutations (58/95). A higher median Myriad MyChoice^® HRD score was observed in BRCAm and BRCAwt tumours with BRCA1 methylation. Patients without BRCAm tumours derived benefit from olaparib treatment vs placebo although to a lesser extent than BRCAm patients.

CONCLUSIONS

Ovarian cancer patients with tumours harbouring loss-of-function mutations in HRR genes other than BRCA1/2 may constitute a small, molecularly identifiable and clinically relevant population who derive treatment benefit from olaparib similar to patients with BRCAm.

Synthesis and Evaluation of a Mitochondria-Targeting Poly(ADP-ribose) Polymerase-1 Inhibitor

The poly(ADP-ribose) polymerase (PARP) family of enzymes plays a crucial role in cellular and molecular processes including DNA damage detection and repair and transcription; indeed, PARP inhibitors are under clinical evaluation as chemotherapeutic adjuncts given their capacity to impede genomic DNA repair in tumor cells. Conversely, overactivation of PARP can lead to NAD⁺ depletion, mitochondrial energy failure, and cell death. Since PARP activation facilitates genomic but impedes mitochondrial DNA repair, nonselective PARP inhibitors are likely to have opposing effects in these cellular compartments. Herein, we describe the synthesis and evaluation of the mitochondria-targeting PARP inhibitor, XJB-veliparib. Attachment of the hemigramicidin S pentapeptide isostere for mitochondrial targeting using a flexible linker at the primary amide site of veliparib did not disrupt PARP affinity or inhibition. XJB-veliparib was effective at low nanomolar concentrations (10-100 nM) and more potent than veliparib in protection from oxygen-glucose deprivation (OGD) in primary cortical neurons. Both XJB-veliparib and veliparib (10 nM) preserved mitochondrial NAD⁺ after OGD; however, only XJB-veliparib prevented release of NAD⁺ into cytosol. XJB-veliparib (10 nM) appeared to inhibit poly(ADP-ribose) polymer formation in mitochondria and preserve mitochondrial cytoarchitecture after OGD in primary cortical neurons. After 10 nM exposure, XJB-veliparib was detected by LC-MS in mitochondria but not nuclear-enriched fractions in neurons and was observed in mitoplasts stripped of the outer mitochondrial membrane obtained from HT22 cells. XJB-veliparib was also effective at preventing glutamate-induced HT22 cell death at micromolar concentrations. Importantly, in HT22 cells exposed to H₂O₂ to produce DNA damage, XJB-veliparib (10 μM) had no effect on nuclear DNA repair, in contrast to veliparib (10 μM) where DNA repair was retarded. XJB-veliparib and analogous mitochondria-targeting PARP inhibitors warrant further evaluation in vitro and in vivo, particularly in conditions where PARP overactivation leads to mitochondrial energy failure and maintenance of genomic DNA integrity is desirable, e.g., ischemia, oxidative stress, and radiation exposure.

The Combination of the PARP Inhibitor Olaparib and the WEE1 Inhibitor AZD1775 as a New Therapeutic Option for Small Cell Lung Cancer

Purpose: Introduced in 1987, platinum-based chemotherapy remains standard of care for small cell lung cancer (SCLC), a most aggressive, recalcitrant tumor. Prominent barriers to progress are paucity of tumor tissue to identify drug targets and patient-relevant models to interrogate novel therapies. Following our development of circulating tumor cell patient-derived explants (CDX) as models that faithfully mirror patient disease, here we exploit CDX to examine new therapeutic options for SCLC.Experimental Design: We investigated the efficacy of the PARP inhibitor olaparib alone or in combination with the WEE1 kinase inhibitor AZD1775 in 10 phenotypically distinct SCLC CDX in vivo and/or ex vivo These CDX represent chemosensitive and chemorefractory disease including the first reported paired CDX generated longitudinally before treatment and upon disease progression.Results: There was a heterogeneous depth and duration of response to olaparib/AZD1775 that diminished when tested at disease progression. However, efficacy of this combination consistently exceeded that of cisplatin/etoposide, with cures in one CDX model. Genomic and protein analyses revealed defects in homologous recombination repair genes and oncogenes that induce replication stress (such as MYC family members), predisposed CDX to combined olaparib/AZD1775 sensitivity, although universal predictors of response were not noted.Conclusions: These preclinical data provide a strong rationale to trial this combination in the clinic informed by prevalent, readily accessed circulating tumor cell-based biomarkers. New therapies will be evaluated in SCLC patients after first-line chemotherapy, and our data suggest that the combination of olaparib/AZD1775 should be used as early as possible and before disease relapse. Clin Cancer Res; 24(20); 5153-64. ©2018 AACR.

Advancing therapies in metastatic castration-resistant prostate cancer

INTRODUCTION

Prostate cancer is the second most common cause of cancer worldwide and is the most frequently detected cancer in the European Union in men over 50 years of age. Androgen deprivation therapy remains the cornerstone of treatment for recurrent or metastatic disease. Unfortunately, nearly all patients will develop resistance to androgen blockade leading to castration-resistant prostate cancer (CRPC). Over the last 10 years, new treatments have dramatically improved overall survival of men with mCRPC. Current therapies are based on AR-axis inhibitors and taxane-based chemotherapies, as well as radiopharmaceuticals and Sipuleucel T.

AREAS COVERED

The authors provide a review of the current field of systemic therapy in metastatic CRPC. This is followed by an in-depth analysis of recent developments in treatment, and the biological rationale behind these therapies.

EXPERT OPINION

Since several trials with docetaxel or novel hormonal agents showed improvement in overall survival in metastatic castration-sensitive prostate cancer, as well as in non-metastatic castration-resistant patients, it is expected that a growing subgroup of patients will be exposed earlier to chemotherapy and to AR targeted agents. It becomes then fundamental to find novel strategies to overcome drug resistance and further improve survival.

Synthetically Lethal Interactions of ATM, ATR, and DNA-PKcs

Synthetic lethality occurs when simultaneous perturbations of two genes or molecular processes result in a loss of cell viability. The number of known synthetically lethal interactions is growing steadily. We review here synthetically lethal interactions of ataxia-telangiectasia mutated (ATM), ATM- and Rad3-related (ATR), and DNA-dependent protein kinase catalytic subunit (DNA-PKcs). These kinases are appropriate for synthetic lethal therapies because their genes are frequently mutated in cancer, and specific inhibitors are currently in clinical trials. Understanding synthetically lethal interactions of a particular gene or gene family can facilitate predicting new synthetically lethal interactions, therapy toxicity, and mechanisms of resistance, as well as defining the spectrum of tumors amenable to these therapeutic approaches.

Olaparib tablets for the treatment of germ line BRCA-mutated metastatic breast cancer

INTRODUCTION

Germ line BRCA mutations (gBRCAm) are diagnosed in approximately 5% of unselected breast cancer patients. Olaparib is a new treatment option for patients with a gBRCAm who have metastatic HER2-negative breast cancer. Areas covered: Olaparib is an oral poly (ADP-ribose) polymerase inhibitor that has been shown in phase I-III clinical trials to have single-agent efficacy in breast cancer patients with gBRCAm. The recent phase III OlympiAD study demonstrated a statistically significant progression-free survival benefit compared with the chemotherapy control arm, although an overall survival benefit has not been demonstrated. The most common adverse events seen with olaparib include nausea, anemia, and vomiting. The most common grade 3 adverse events are anemia and neutropenia. Expert commentary: The US FDA-approved olaparib tablets in January 2018 for the treatment of patients with a gBRCAm and metastatic HER2-negative breast cancer. This is a well-tolerated and effective treatment option for this patient population, particularly in patients with triple-negative breast cancer in which chemotherapy is the only alternative. More data are needed to understand the role of olaparib in combination with endocrine therapy, other targeted agents, and chemotherapy, as well as sequentially with platinum chemotherapy in the metastatic setting.

The comings and goings of PARP-1 in response to DNA damage

Poly(ADP-ribose) polymerase (PARP) enzymes are broadly involved in the cellular response to DNA damage. PARP-1 is the chief human PARP enzyme involved in the DNA damage response, acting as a first responder that detects DNA strand breaks, and contributes to repair pathway choice and the efficiency of repair through modulation of chromatin structure and through interaction with and modification of a multitude of DNA repair factors. This perspective summarizes our knowledge of PARP-1 involvement in DNA repair pathways, and highlights recent structural and functional data regarding the activation of PARP-1 upon detecting DNA damage, and the release and trapping of PARP-1 at sites of DNA damage.

Poly(ADP-ribose) polymerase 1 searches DNA via a 'monkey bar' mechanism

Poly(ADP-ribose) polymerase 1 (PARP1) is both a first responder to DNA damage and a chromatin architectural protein. How PARP1 rapidly finds DNA damage sites in the context of a nucleus filled with undamaged DNA, to which it also binds, is an unresolved question. Here, we show that PARP1 association with DNA is diffusion-limited, and release of PARP1 from DNA is promoted by binding of an additional DNA molecule that facilitates a ‘monkey bar’ mechanism, also known as intersegment transfer. The WGR-domain of PARP1 is essential to this mechanism, and a point mutation (W589A) recapitulates the altered kinetics of the domain deletion. Demonstrating the physiological importance of the monkey bar mechanism for PARP1 function, the W589A mutant accumulates at sites of DNA damage more slowly following laser micro-irradiation than wild-type PARP1. Clinically relevant inhibitors of PARP1 did not alter the rate or mechanism of the release of PARP1 from DNA.

Our cells constantly withstand damage that can lead to breaks in the strands of our DNA. These cuts need to be fixed for the cell to stay healthy. When a break happens, one of the first responders to the scene is a protein known as PARP1. It binds to the ruptured strand (or strands) and then it recruits other repair agents to that location. But first, PARP1 needs to scan for cuts and notches amongst an overwhelming amount of DNA that is still intact. This is a complicated task, especially since the protein tends to bind both broken and unbroken DNA. How does it not stay ‘stuck’ on an undamaged portion of the genome?

Here, Rudolph et al. use a combination of biochemical techniques and cell biology to show that PARP1 travels through our genome by swinging from one DNA location to another, the way a child grabs onto monkey bars. One of the DNA-binding domains of PARP1, known as the WGR-domain, acts like an arm and initiates the movement by gripping onto a new segment of DNA. In fact, chopping off the WGR-domain or disabling it through mutations makes PARP1 worse at finding DNA breakages in the cell.

Unfixed DNA damage can lead to a cell becoming cancerous; ultimately, if the breakages keep accumulating, the cell does not survive. This makes PARP1 an important target for cancer treatment. Indeed, certain drugs already rely on trapping the protein so that tumor cells die. Understanding how cells cope with DNA damage and exactly how PARP1 works could help in the fight against cancer.