Development of PARP inhibitors in oncology

Counting the cost of public and philanthropic R&D funding: the case of olaparib

Background

Lack of transparency around manufacturing costs, who bears the bulk of research and development costs and how total costs relate to the pricing of products, continue to fuel debates. This paper considers the case of olaparib (Lynparza®), recently indicated for use among BRCA-mutant breast cancer patients, and estimates the extent of public and philanthropic R&D funding.

Methods

We know from previous work that attempting to ascertain the amount of public and philanthropic funding using purely bibliographic sources (i.e., authors’ declarations of funding sources and amounts traced through funders) is limited. Since we knew that a publically funded research unit was pivotal in developing olaparib, we decided to supplement bibliographic data with a Freedom of Information request for administrative records on research funding data from this research centre.

Research

In terms of stages of product development, work conducted in the pre-clinical research stage was the most likely to report non-industry funding (> 90% of pre-clinical projects received public or philanthropic funding). Clinical trials were least likely to be funded through non-industry sources—although even here, contrary to the popular assertion that this is wholly industry-financed, we found public or philanthropic funding declared by 23% of clinical trials. Using information reported in the publications, we identified approximately £128 million of public and philanthropic funding that may have contributed to the development of olaparib. However, this amount was less than one-third of the total amount received by one research institute playing a pivotal role in product discovery. The Institute of Cancer Research reported receiving 38 funding awards to support olaparib work for BRCA-mutant breast cancer totalling over £400 million.

Conclusions

Government or charitable funding of pharmaceutical product development is difficult to trace using publicly available sources, due to incomplete information provided by authors and/or a lack of consistency in funding information made available by funders. This study has shown that a Freedom of Information request, in countries where such requests are supported, can provide information to help build the picture of financial support. In the example of olaparib, the funding amounts directly reported considerably exceeded amounts that could be ascertained using publically available bibliographic sources.

Discovery of isoquinolinone and naphthyridinone-based inhibitors of poly(ADP-ribose) polymerase-1 (PARP1) as anticancer agents: Structure activity relationship and preclinical characterization

The exploitation of GLU988 and LYS903 residues in PARP1 as targets to design isoquinolinone (I & II) and naphthyridinone (III) analogues is described. Compounds of structure I have good biochemical and cellular potency but suffered from inferior PK. Constraining the linear propylene linker of structure I into a cyclopentene ring (II) offered improved PK parameters, while maintaining potency for PARP1. Finally, to avoid potential issues that may arise from the presence of an anilinic moiety, the nitrogen substituent on the isoquinolinone ring was incorporated as part of the bicyclic ring. This afforded a naphthyridinone scaffold, as shown in structure III. Further optimization of naphthyridinone series led to identification of a novel and highly potent PARP1 inhibitor 34, which was further characterized as preclinical candidate molecule. Compound 34 is orally bioavailable and displayed favorable pharmacokinetic (PK) properties. Compound 34 demonstrated remarkable antitumor efficacy both as a single-agent as well as in combination with chemotherapeutic agents in the BRCA1 mutant MDA-MB-436 breast cancer xenograft model. Additionally, compound 34 also potentiated the effect of agents such as temozolomide in breast cancer, pancreatic cancer and Ewing's sarcoma models.

Discovery of SK-575 as a Highly Potent and Efficacious Proteolysis-Targeting Chimera Degrader of PARP1 for Treating Cancers

The nuclear protein poly(ADP-ribose) polymerase-1 (PARP1) has a well-established role in the signaling and repair of DNA and is a validated therapeutic target for cancers and other human diseases. Here, we have designed, synthesized, and evaluated a series of small-molecule PARP1 degraders based on the proteolysis-targeting chimera (PROTAC) concept. Our efforts have led to the discovery of highly potent PARP1 degraders, as exemplified by compound 18 (SK-575). SK-575 potently inhibits the growth of cancer cells bearing BRCA1/2 mutations and induces potent and specific degradation of PARP1 in various human cancer cells even at low picomolar concentrations. SK-575 achieves durable tumor growth inhibition in mice when used as a single agent or in combination with cytotoxic agents, such as temozolomide and cisplatin. These data demonstrate that SK-575 is a highly potent and efficacious PARP1 degrader.

PARP inhibitor rucaparib induces changes in NAD levels in cells and liver tissues as assessed by MRS

Poly(adenosine diphosphate ribose) polymerases (PARPs) are multifunctional proteins which play a role in many cellular processes. Namely, PARP1 and PARP2 have been shown to be involved in DNA repair, and therefore are valid targets in cancer treatment with PARP inhibitors, such as rucaparib, currently in clinical trials. Proton magnetic resonance spectroscopy (¹ H-MRS) was used to study the impact of rucaparib in vitro and ex vivo in liver tissue from mice, via quantitative analysis of nicotinamide adenosine diphosphate (NAD⁺ ) spectra, to assess the potential of MRS as a biomarker of the PARP inhibitor response. SW620 (colorectal) and A2780 (ovarian) cancer cell lines, and PARP1 wild-type (WT) and PARP1 knock-out (KO) mice, were treated with rucaparib, temozolomide (methylating agent) or a combination of both drugs. ¹ H-MRS spectra were obtained from perchloric acid extracts of tumour cells and mouse liver. Both cell lines showed an increase in NAD⁺ levels following PARP inhibitor treatment in comparison with temozolomide treatment. Liver extracts from PARP1 WT mice showed a significant increase in NAD⁺ levels after rucaparib treatment compared with untreated mouse liver, and a significant decrease in NAD⁺ levels in the temozolomide-treated group. The combination of rucaparib and temozolomide did not prevent the NAD⁺ depletion caused by temozolomide treatment. The ¹ H-MRS results show that NAD⁺ levels can be used as a biomarker of PARP inhibitor and methylating agent treatments, and suggest that in vivo measurement of NAD⁺ would be valuable.

Therapeutic Targeting of Poly(ADP-Ribose) Polymerase-1 (PARP1) in Cancer: Current Developments, Therapeutic Strategies, and Future Opportunities

Poly(ADP-ribose) polymerase-1 (PARP-1) plays a central role in numerous cellular processes including DNA repair, replication, and transcription. PARP interacts directly, indirectly or via PARylation with various oncogenic proteins and regulates several transcription factors thereby modulating carcinogenesis. Therapeutic inhibition of PARP is therefore perceived as a promising anticancer strategy and a number of PARP inhibitors (PARPi) are currently under development and clinical evaluation. PARPi inhibit the DNA repair pathway and thus form the concept of synthetic lethality in cancer therapeutics. Preclinical and clinical studies have shown the potential of PARPi as chemopotentiator, radiosensitizer, or as adjuvant therapeutic agents. Recent studies have shown that PARP-1 could be either oncogenic or tumor suppressive in different cancers. PARP inhibitor resistance is also a growing concern in the clinical setting. Recently, changes in the levels of PARP-1 activity or expression in cancer patients have provided the basis for consideration of PARP-1 regulatory proteins as potential biomarkers. This review focuses on the current developments related to the role of PARP in cancer progression, therapeutic strategies targeting PARP-associated oncogenic signaling, and future opportunities in use of PARPi in anticancer therapeutics.

Development of PARP inhibitors in gynecological malignancies

PARP inhibitors demonstrate synthetic lethality in tumors with BRCA1/2 mutations and other homologous recombination repair deficiencies by interfering with DNA repair and causing direct toxicity to DNA through PARP trapping. PARP inhibitors have been shown to be beneficial in the treatment of BRCA1/2-mutated ovarian cancers, which has led to a shift in the treatment paradigm of this disease. Further studies to establish the role of PARP inhibitors during earlier stages of treatment are ongoing. The use of PARP inhibitors in other cancers with homologous recombination repair deficiencies, such as breast cancer and prostate cancer, is gradually evolving as well, including their use in the neoadjuvant and adjuvant settings. PARP inhibitor combination strategies with chemotherapy, targeted agents, radiotherapy, and immunotherapy are also being explored. The role of predictive biomarkers, including molecular signatures and homologous recombination deficiency scores based on loss of heterozygosity and other structural genomic aberrations, will be crucial to improved patient stratification to enhance the clinical utility of PARP inhibitors. This may also allow the use of PARP inhibitors to be extended beyond tumors with specific homologous recombination DNA repair gene mutations in the future. An improved understanding of the mechanisms underlying PARP inhibitor resistance will also be important to enable the development of new approaches to increase efficacy. This is a field rich in opportunity, and the coming years should see a better understanding of which patients we should be treating with PARP inhibitors and where these agents should come in over the course of treatment.

Novel PARP inhibitors sensitize human leukemic cells in an endogenous PARP activity dependent manner

Poly(ADP-ribose) polymerase (PARP) is a critical nuclear enzyme which helps in DNA repair. In this study we report, synthesis and biological studies of novel pyridazine derivatives as PARP inhibitors.

Overview on poly(ADP-ribose) immuno-biomedicine and future prospects

Poly(ADP-ribose), identified in 1966 independently by three groups Strassbourg, Kyoto and Tokyo, is synthesized by poly(ADP-ribose) polymerases (PARP) from NAD(+) as a substrate in the presence of Mg(2+). The structure was unique in that it has ribose-ribose linkage. In the early-1970s, however, its function in vivo/in vitro was still controversial and the antibody against it was desired to help clear its significance. Thereupon, the author tried to produce antibody against poly(ADP-ribose) in rabbits and succeeded in it for the first time in the world. Eventually, this success has led to the following two groundbreaking papers in Nature: "Naturally-occurring antibody against poly(ADP-ribose) in patients with autoimmune disease SLE", and "Induction of anti-poly(ADP-ribose) antibody by immunization with synthetic double-stranded RNA, poly(A)·poly(U)".On the way to the publication of the first paper, a reviewer gave me a friendly comment that there is "heteroclitic" fashion as a mechanism of the production of natural antibody. This comment was really a God-send for me, and became a train of power for publication of another paper, as described above. Accordingly, I thought this, I would say, episode is worth describing herein. Because of its importance in biomedical phenomena, a certain number of articles related to "heteroclitic" have become to be introduced in this review, although they were not always directly related to immuno-biological works on poly(ADP-ribose). Also, I tried to speculate on the future prospects of poly(ADP-ribose), product of PARP, as an immuno-regulatory molecule, including either induced or naturally-occurring antibodies, in view of "heteroclitic".

Addressing health disparities in Hispanic breast cancer: accurate and inexpensive sequencing of BRCA1 and BRCA2

BACKGROUND

Germline mutations in the BRCA1 and BRCA2 genes account for 20-25 % of inherited breast cancers and about 10 % of all breast cancer cases. Detection of BRCA mutation carriers can lead to therapeutic interventions such as mastectomy, oophorectomy, hormonal prevention therapy, improved screening, and targeted therapies such as PARP-inhibition. We estimate that African Americans and Hispanics are 4-5 times less likely to receive BRCA screening, despite having similar mutation frequencies as non-Jewish Caucasians, who have higher breast cancer mortality. To begin addressing this health disparity, we initiated a nationwide trial of BRCA testing of Latin American women with breast cancer. Patients were recruited through community organizations, clinics, public events, and by mail and Internet. Subjects completed the consent process and questionnaire, and provided a saliva sample by mail or in person. DNA from 120 subjects was used to sequence the entirety of BRCA1 and BRCA2 coding regions and splice sites, and validate pathogenic mutations, with a total material cost of $85/subject. Subjects ranged in age from 23 to 81 years (mean age, 51 years), 6 % had bilateral disease, 57 % were ER/PR+, 23 % HER2+, and 17 % had triple-negative disease.

RESULTS

A total of seven different predicted deleterious mutations were identified, one newly described and the rest rare. In addition, four variants of unknown effect were found.

CONCLUSIONS

Application of this strategy on a larger scale could lead to improved cancer care of minority and underserved populations.

hMSH2 expression is associated with paclitaxel resistance in ovarian carcinoma, and inhibition of hMSH2 expression in vitro restores paclitaxel sensitivity

The objective of the present study was to investigate the association between paclitaxel resistance, gene copy number, and gene expression in ovarian carcinoma, and to restore paclitaxel sensitivity in a paclitaxel-resistant ovarian carcinoma cell line by using hMSH2-targeting siRNA. Paclitaxel-resistant ovarian carcinoma cell lines OC3/TAX300 and OC3/TAX50 and their parental cell lines were analyzed by comparative genomic hybridization, and the expression levels of hMSH2 in ovarian carcinoma cell lines and tissues were determined. An siRNA targeted to hMSH2 mRNA was used to transfect a paclitaxel-resistant cell line. We assessed the morphological features, proliferation, and susceptibility to apoptosis of the transfected cells after paclitaxel treatment. Chromosome 2p21 (gene locus of hMSH2) was amplified in OC3/TAX300 cells. hMSH2 was overexpressed in 93.9 and 47.6% of paclitaxel-treated and untreated ovarian carcinoma tissue samples (P=0.0001), respectively. hMSH2 was overexpressed in 93.3 and 54.2% of low-differentiated and moderate-to-highly differentiated ovarian carcinoma tissue samples (P=0.0008), respectively. hMSH2 expression was inhibited in the OC3/TAX300 cells transfected with hMSH2 siRNA. hMSH2 siRNA increased paclitaxel sensitivity, inhibited OC3/TAX300 cell proliferation (G2/M arrest), and increased susceptibility to apoptosis. hMSH2 expression was upregulated in ovarian carcinoma cell lines and tissues after paclitaxel treatment. hMSH2 overexpression is related to paclitaxel resistance and poor prognosis. Inhibition of hMSH2 expression in vitro restores paclitaxel sensitivity in paclitaxel‑resistant ovarian carcinoma cell lines and indicates a new direction in adjuvant therapy for ovarian carcinoma.

Phase I/Ib study of olaparib and carboplatin in BRCA1 or BRCA2 mutation-associated breast or ovarian cancer with biomarker analyses

BACKGROUND

Olaparib has single-agent activity against breast/ovarian cancer (BrCa/OvCa) in germline BRCA1 or BRCA2 mutation carriers (gBRCAm). We hypothesized addition of olaparib to carboplatin can be administered safely and yield preliminary clinical activity.

METHODS

Eligible patients had measurable or evaluable disease, gBRCAm, and good end-organ function. A 3 + 3 dose escalation tested daily oral capsule olaparib (100 or 200mg every 12 hours; dose level1 or 2) with carboplatin area under the curve (AUC) on day 8 (AUC3 day 8), then every 21 days. For dose levels 3 to 6, patients were given olaparib days 1 to 7 at 200 and 400 mg every 12 hours, with carboplatin AUC3 to 5 on day 1 or 2 every 21 days; a maximum of eight combination cycles were permitted, after which daily maintenance of olaparib 400mg every12 hours continued until progression. Dose-limiting toxicity was defined in the first two cycles. Peripheral blood mononuclear cells were collected for polymorphism analysis and polyADP-ribose incorporation. Paired tumor biopsies (before/after cycle 1) were obtained for biomarker proteomics and apoptosis endpoints.

RESULTS

Forty-five women (37 OvCa/8 BrCa) were treated. Dose-limiting toxicity was not reached on the intermittent schedule. Expansion proceeded with olaparib 400mg every 12 hours on days 1 to 7/carboplatin AUC5. Grade 3/4 adverse events included neutropenia (42.2%), thrombocytopenia (20.0%), and anemia (15.6%). Responses included 1 complete response (1 BrCa; 23 months) and 21 partial responses (50.0%; 15 OvCa; 6 BrCa; median = 16 [4 to >45] in OvCa and 10 [6 to >40] months in BrCa). Proteomic analysis suggests high pretreatment pS209-eIF4E and FOXO3a correlated with duration of response (two-sided P < .001; Pearson's R (2) = 0.94).

CONCLUSIONS

Olaparib capsules 400mg every 12 hours on days 1 to 7/carboplatin AUC5 is safe and has activity in gBRCAm BrCa/OvCa patients. Exploratory translational studies indicate pretreatment tissue FOXO3a expression may be predictive for response to therapy, requiring prospective validation.

Expression of PARP-1 and its active polymer PAR in prostate cancer and benign prostatic hyperplasia in Chinese patients

BACKGROUND AND AIMS

Aberrant expression of PARP-1 has been reported in various human malignancies and was involved in the progression and metastasis of cancers. However, little is known about PARP-1 expression in prostate cancer (PCa). This study aimed to investigate the expression of PARP-1 and its active polymer poly(ADP-ribose) (PAR) in PCa and benign prostatic hyperplasia (BPH) tissues from Chinese patients.

METHODS

The expression of PARP-1 and PAR in PCa and benign prostate hyperplasia tissues was assessed by immunohistochemistry in 78 PCa patients and 49 BPH patients. The relationship between the expression of PARP-1 or PAR and clinicopathological parameters in PCa patients was also analyzed.

RESULTS

Both the positive and strong positive expression rates of PARP-1 in PCa tissues were significantly higher than those in BPH tissues. Although spearman correlations analysis showed the over-expression of PARP-1 and PAR in PCa tissues was not correlated with age, serum PSA level and Gleason scores (GS), an increasing trend was observed between over-expression of PARP-1 or PAR and the PSA levels (TPSA >20 vs TPSA ≤20) or GS grade (GS ≥8 vs GS ≤6).

CONCLUSION

PARP-1 and PAR expression is markedly elevated in PCa than that in BPH tissues, which may implicate that PARP-1 and PAR are involved in the development of PCa, and the possible expansion in the use of poly(ADP-ribose) polymerase inhibitors for targeting therapy of PCa in select patients alone or combined with chemotherapy or radiation.

PARP and CHK inhibitors interact to cause DNA damage and cell death in mammary carcinoma cells

The present studies examined viability and DNA damage levels in mammary carcinoma cells following PARP1 and CHK1 inhibitor drug combination exposure. PARP1 inhibitors [AZD2281 ; ABT888 ; NU1025 ; AG014699] interacted with CHK1 inhibitors [UCN-01 ; AZD7762 ; LY2603618] to kill mammary carcinoma cells. PARP1 and CHK1 inhibitors interacted to increase both single strand and double strand DNA breaks that correlated with increased γH2AX phosphorylation. Treatment of cells with CHK1 inhibitors increased the phosphorylation of CHK1 and ERK1/2. Knock down of ATM suppressed the drug-induced increases in CHK1 and ERK1/2 phosphorylation and enhanced tumor cell killing by PARP1 and CHK1 inhibitors. Expression of dominant negative MEK1 enhanced drug-induced DNA damage whereas expression of activated MEK1 suppressed both the DNA damage response and tumor cell killing. Collectively our data demonstrate that PARP1 and CHK1 inhibitors interact to kill mammary carcinoma cells and that increased DNA damage is a surrogate marker for the response of cells to this drug combination.

Epigenetic targeting therapies to overcome chemotherapy resistance

It is now well established that epigenetic aberrations occur early in malignant transformation, raising the possibility of identifying chemopreventive compounds or reliable diagnostic screening using epigenetic biomarkers. Combinatorial therapies effective for the reexpression of tumor suppressors, facilitating resensitization to conventional chemotherapies, hold great promise for the future therapy of cancer. This approach may also perturb cancer stem cells and thus represent an effective means for managing a number of solid tumors. We believe that in the near future, anticancer drug regimens will routinely include epigenetic therapies, possibly in conjunction with inhibitors of "stemness" signal pathways, to effectively reduce the devastating occurrence of cancer chemotherapy resistance.

3-Nitro-naphthalimide and nitrogen mustard conjugate NNM-25 induces hepatocellular carcinoma apoptosis via PARP-1/p53 pathway

Hepatocellular carcinoma (HCC) is one of the main causes of death in cancer. Some naphthalimide derivatives exert high anti-proliferative effects on HCC. In this study, it is confirmed that 3-nitro-naphthalimide and nitrogen mustard conjugate (NNM-25), a novel compound conjugated by NNM-25, displayed more potent therapeutic action on HCC, both in vivo and in vitro, than amonafide, a naphthalimide drug in clinical trials. More importantly, preliminary toxicological evaluation also supported that NNM-25 exhibited less systemic toxicity than amonafide at the therapeutic dose. The antitumor mechanism of conjugates of naphthalimides with nitrogen mustard remains poorly understood up to now. Here, we first reported that apoptosis might be the terminal fate of cancer cells treated with NNM-25. Inhibition of p53 by siRNA resulted in a significant decrease of NNM-25-induced apoptosis, which corroborated that p53 played a vital role in the cell apoptosis triggered by NNM-25. NNM-25 inhibited the PARP-1 activity, AKT phosphorylation, up-regulated the protein expression of p53, Bad, and mTOR as well as down-regulating the protein expression of Bcl-2 and decreasing mitochondrial membrane potential. It also facilitated cytochrome c release from mitochondria to cytoplasm, activated caspase 8, caspase 9, and caspase 3 in HepG2 cells in vitro, as also authenticated in H22 tumor-bearing mice in vivo. Collectively, the conjugation of naphthalimides with nitrogen mustard provides favorable biological activity and thus is a valuable strategy for future drug design in HCC therapy.

Poly(ADP-ribose) polymerase 1 modulates the lethality of CHK1 inhibitors in mammary tumors

The present studies sought to define whether checkpoint kinase 1 (CHK1) inhibitors and poly(ADP-ribose) polymerase 1 (PARP1) inhibitors interact in vitro and in vivo to kill breast cancer cells. PARP1 and CHK1 inhibitors interacted to kill estrogen receptor (ER)+, ER+ fulvestrant-resistant, HER2+, or triple-negative mammary carcinoma cells in a manner that was not apparently affected by phosphatase and tensin homolog deleted on chromosome 10 functional status. Expression of dominant-negative CHK1 enhanced and overexpression of wild-type CHK1 suppressed the toxicity of PARP1 inhibitors in a dose-dependent fashion. Knockdown of PARP1 enhanced the lethality of CHK1 inhibitors in a dose-dependent fashion. PARP1 and CHK1 inhibitors interacted in vivo both to suppress the growth of large established tumors and to suppress the growth of smaller developing tumors; the combination enhanced animal survival. PARP1 and CHK1 inhibitors profoundly radiosensitized cells in vitro and in vivo. In conclusion, our data demonstrate that the combination of PARP1 and CHK1 inhibitors has antitumor activity in vivo against multiple mammary tumor types and that translation of this approach could prove to be a useful anticancer therapeutic approach.

The role of poly adenosine diphosphate ribose polymerase inhibitors in breast and ovarian cancer: current status and future directions

Poly adenosine diphosphate ribose polymerase (PARP) inhibitors have demonstrated single agent activity in the treatment of patients with recurrent BRCA1-mutated and BRCA2-mutated breast and ovarian cancers. They also appear to have a potential role as maintenance therapy following chemotherapy in patients with platinum sensitive recurrent sporadic and BRCA1/2 related high-grade serous ovarian cancers. The concept of BRCAness raises the possibility that PARP inhibitors may be active in selected patients with homologous recombination (HR) DNA repair-deficient tumors, even if they do not harbor a BRCA1/2 germline mutation. Further research will be required to identify the subset of patients with sporadic cancers who may benefit from PARP inhibitor therapy. Precise details on the mechanisms of action, relative potency and anti-cancer effects of different PARP inhibitors remain to be clarified and are being investigated. PARP inhibitors are known to inhibit the base excision repair (BER) pathway but in addition, recent reports indicate that aberrant activation of the error-prone non-homologous end-joining (NHEJ) pathway occurs in HR-deficient cells and that cell death provoked by PARP inhibition is dependent on NHEJ-induced genomic instability. Characterization of the precise molecular mechanisms responsible for PARP inhibitor activity should lead to the identification of predictive biomarkers of response and help identify which patients should be treated with PARP inhibitors. This is a very active field of research and the current status and future directions are reviewed.

Inhibition of poly(ADP-ribose) polymerase (PARP) induces apoptosis in lung cancer cell lines

We have tested PJ34, a potent inhibitor of poly(ADP-ribose) polymerase (PARP), against various lung cancer cell lines (Calu-6, A549, and H460) and normal human bronchial epithelial cells (HBECs). While using WST1 dye assay, lung cancer cells exhibited LD(50) values of approximately 30 μM PJ34 (72-hr assay). Molecular data showed that the effect of PJ34-induced apoptosis on lung cancer cells occurs via a caspase-dependent pathway. The present study has clearly shown that (a) PARP inhibitor can independently kill tumor cells, (b) caspase-3 has modest influence on PARP-inhibitor-mediated cancer-specific toxicity, and (c) a pan-caspase inhibitor decreases the apoptotic effect of PJ34.

Response of human prostate cancer cells and tumors to combining PARP inhibition with ionizing radiation

Radiation therapy remains a promising modality for curative treatment of localized prostate cancer, but dose-limiting toxicities significantly limit its effectiveness. Agents that enhance efficacy at lower radiation doses might have considerable value in increasing tumor control without compromising organ function. Here, we tested the hypothesis that the PARP inhibitor ABT-888 (veliparib) can enhance the response of prostate cancer cells and tumors to ionizing radiation (IR). Following exposure of DU-145 and PC-3 prostate cancer cell lines to the combination of 10 μmol/L ABT-888 and 6 Gy, we observed similar persistence between both cell lines of DNA damage foci and in vitro radiosensitization. We have previously observed that persistent DNA damage foci formed after ABT-888 plus IR efficiently promote accelerated cell senescence, but only PC-3 cells displayed the expected senescent response of G(2)-M arrest, induction of p21 and β-galactosidase expression, and accumulation as large flat cells. In turn, combining ABT-888 with 6 Gy resulted in delayed tumor regrowth compared with either agent alone only in PC-3 xenograft tumors, whereas DU-145 tumors continued to grow. By 7 days after treatment with ABT-888 plus IR, PC-3 tumors contained abundant senescent cells displaying persistent DNA damage foci, but no evidence of senescence was noted in the DU-145 tumors. That equivalent radiosensitization by ABT-888 plus IR in vitro failed to predict comparable results with tumors in vivo suggests that the efficacy of PARP inhibitors may partially depend on a competent senescence response to accumulated DNA damage.

Poly(ADP-ribose) polymerase (PARP) inhibitors: Exploiting a synthetic lethal strategy in the clinic

Poly(ADP-ribose) polymerase (PARP) is an attractive antitumor target because of its vital role in DNA repair. The homologous recombination (HR) DNA repair pathway is critical for the repair of DNA double-strand breaks and HR deficiency leads to a dependency on error-prone DNA repair mechanisms, with consequent genomic instability and oncogenesis. Tumor-specific HR defects may be exploited through a synthetic lethal approach for the application of anticancer therapeutics, including PARP inhibitors. This theory proposes that targeting genetically defective tumor cells with a specific molecular therapy that inhibits its synthetic lethal gene partner should result in selective tumor cell killing. The demonstration of single-agent antitumor activity and the wide therapeutic index of PARP inhibitors in BRCA1 and BRCA2 mutation carriers with advanced cancers provide strong evidence for the clinical application of this approach. Emerging data also indicate that PARP inhibitors may be effective in sporadic cancers bearing HR defects, supporting a substantially wider role for PARP inhibitors. Drugs targeting this enzyme are now in pivotal clinical trials in patients with sporadic cancers. In this article, the evidence supporting this antitumor synthetic lethal strategy with PARP inhibitors is reviewed, evolving resistance mechanisms and potential molecular predictive biomarker assays are discussed, and the future development of these agents is envisioned.

Helicobacter pylori activation of PARP-1: usurping a versatile regulator of host cellular health

Chronic infection of the human stomach by Helicobacter pylori is an important risk factor for gastric cancer. H. pylori produces a cache of virulence factors that promote colonization and persistence, which, in turn, contributes to a robust inflammatory response at the host-pathogen interface. Recently, we reported that H. pylori activates the abundant nuclear regulator poly(ADP-ribose) polymerase (PARP)-1, resulting in the production of the catabolite poly(ADP-ribose) (PAR). PARP-1 is emerging as a key player in establishing homeostasis at the host-pathogen interface. In this article, we summarize the discovery of H. pylori-dependent PARP-1 activation, and discuss potential roles for PARP-1 in H. pylori-mediated gastric disease. In light of the remarkable successes that have reported for treating inflammatory disorders and cancers with PARP-1 inhibitors, we discuss the prospects of targeting PARP-1 for treatment of H. pylori-associated gastric disease.

Poly(ADP-ribose) polymerase 1 modulates the lethality of CHK1 inhibitors in carcinoma cells

Prior studies have demonstrated that inhibition of CHK1 can promote the activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) and phosphorylation of histone H2AX and that inhibition of poly(ADP-ribose) polymerase 1 (PARP1) can affect growth factor-induced ERK1/2 activation. The present studies were initiated to determine whether CHK1 inhibitors interacted with PARP1 inhibition to facilitate apoptosis. Transient expression of dominant-negative CHK1 raised basal ERK1/2 activity and prevented CHK1 inhibitors from activating ERK1/2. CHK1 inhibitors modestly increased the levels of PARP1 ADP ribosylation and molecular or small-molecule inhibition of PARP1 blocked CHK1 inhibitor-stimulated histone H2AX phosphorylation and activation of ERK1/2. Stimulated histone H2AX phosphorylation was ataxia telangiectasia-mutated protein-dependent. Multiple CHK1 inhibitors interacted in a greater than additive fashion with multiple PARP1 inhibitors to cause transformed cell-killing in short-term viability assays and synergistically killed tumor cells in colony-formation assays. Overexpression of BCL-xL or loss of BAX/BAK function, but not the function of BID, suppressed CHK1 inhibitor + PARP1 inhibitor lethality. Inhibition of BCL-2 family protein function enhanced CHK1 inhibitor + PARP1 inhibitor lethality and restored drug-induced cell-killing in cells overexpressing BCL-xL. Thus, PARP1 plays an important role in regulating the ability of CHK1 inhibitors to activate ERK1/2 and the DNA damage response. An inability of PARP1 to modulate this response results in transformed cell death mediated through the intrinsic apoptosis pathway.

Poly(ADP-ribose) polymerase inhibitors as promising cancer therapeutics

The year of 2005 was a watershed in the history of poly(ADP-ribose) polymerase (PARP) inhibitors due to the important findings of selective killing in BRCA-deficient cancers by PARP inhibition. The findings made PARP inhibition one of the most promising new therapeutic approaches to cancers, especially to those with specific defects. With AZD2281 and BSI-201 entering phase III clinical trials, the final application of PARP inhibitors in clinic would come true soon. This current paper will review the major advances in targeting PARP for cancer therapy and discuss the existing questions, the answers to which may influence the future of PARP inhibitors as cancer therapeutics.

Poly(ADP-ribose) polymerase inhibitor induces accelerated senescence in irradiated breast cancer cells and tumors

Persistent DNA double-strand breaks (DSB) may determine the antitumor effects of ionizing radiation (IR) by inducing apoptosis, necrosis, mitotic catastrophe, or permanent growth arrest. IR induces rapid modification of megabase chromatin domains surrounding DSBs via poly-ADP-ribosylation, phosphorylation, acetylation, and protein assembly. The dynamics of these IR-induced foci (IRIF) have been implicated in DNA damage signaling and DNA repair. As an IRIF reporter, we tracked the relocalization of green fluorescent protein fused to a chromatin binding domain of the checkpoint adapter protein 53BP1 after IR of breast cancer cells and tumors. To block DSB repair in breast cancer cells and tumors, we targeted poly(ADP-ribose) polymerase (PARP) with ABT-888 (veliparib), one of several PARP inhibitors currently in clinical trials. PARP inhibition markedly enhanced IRIF persistence and increased breast cancer cell senescence both in vitro and in vivo, arguing for targeting IRIF resolution as a novel therapeutic strategy.

Structural genomics-impact on biomedicine and drug discovery

The field of structural genomics emerged as one of many 'omics disciplines more than a decade ago, and a multitude of large scale initiatives have been launched across the world. Development and implementation of methods for high-throughput structural biology represents a common denominator among different structural genomics programs. From another perspective a distinction between "biology-driven" versus "structure-driven" approaches can be made. This review outlines the general themes of structural genomics, its achievements and its impact on biomedicine and drug discovery. The growing number of high resolution structures of known and potential drug target proteins is expected to have tremendous value for future drug discovery programs. Moreover, the availability of large numbers of purified proteins enables generation of tool reagents, such as chemical probes and antibodies, to further explore protein function in the cell.

Inhibition of poly(ADP-ribose) polymerase enhances the effect of chemotherapy in an animal model of regional therapy for the treatment of advanced extremity malignant melanoma

BACKGROUND

Poly(ADP-ribose) polymerase (PARP) is an important regulator of programmed cell death in response to alkylating agents such as temozolomide (TMZ). The goal of this study was to determine if a systemically administered PARP-inhibitor (INO-1001) could augment the efficacy of TMZ in a rat model of extremity malignant melanoma.

MATERIALS AND METHODS

PARP activity was measured in vitro across a panel of 5 human malignant melanoma-derived cell lines. To evaluate tumor response to PARP inhibition in combination with regional isolated limb infusion (ILI) therapy with TMZ, two TMZ-resistant malignant melanoma cell lines were grown as xenografts in the hind limb of rats. INO-1001 (400 mg/kg) was injected intraperitoneally 7 times every 8 hours prior to ILI. Tumor volume was measured for up to 40 days.

RESULTS

In vitro inhibition of PARP activity by INO-1001 ranged from 25.5% to 65.6%. In a mismatch repair (MMR)-deficient xenograft, treatment with INO-1001 prior to ILI significantly (P < .04) increased the efficacy of TMZ. The increase in tumor volume at day 40 following TMZ-ILI with INO-1001 was only 22.6% compared with 322.8% with TMZ-ILI alone. In a xenograft that was MMR-proficient and had high levels of O(6)-methylguanine-DNA methyltransferase (MGMT) activity, there was little improvement in TMZ efficacy with INO-1001 treatment.

CONCLUSION

The PARP-inhibitor, INO-1001, can enhance the response of TMZ-resistant, MMR-deficient, malignant melanoma xenografts to intra-arterially administered TMZ in a regional treatment model of advanced extremity malignant melanoma.

Telomere shortening is associated to TRF1 and PARP1 overexpression in Duchenne muscular dystrophy

Telomere shortening is thought to contribute to premature senescence of satellite cells in Duchenne muscular dystrophy (DMD) muscle. Telomeric repeat binding factor-1 (TRF1) and poly (ADP-ribose) polymerase-1 (PARP1) are proteins known to modulate telomerase reverse transcriptase (TERT) activity, which controls telomere elongation. Here we show that an age-dependent telomere shortening occurs in DMD muscles and is associated to overexpression of mRNA and protein levels of TRF1 and PARP1. TERT expression and activity are detectable in normal control muscles and they slightly increase in DMD. This is the first demonstration of TRF1 and PARP1 overexpression in DMD muscles. They can be directly involved in replicative senescence of satellite cells and/or in the pathogenetic cascade through a cross-talk with oxidative stress and inflammatory response. Modulation of these events by TRF1 or PARP1 inhibition might represent a novel strategy for treatment of DMD and other muscular dystrophies.

Discovery of 2-{4-[(3S)-piperidin-3-yl]phenyl}-2H-indazole-7-carboxamide (MK-4827): a novel oral poly(ADP-ribose)polymerase (PARP) inhibitor efficacious in BRCA-1 and -2 mutant tumors

We disclose the development of a novel series of 2-phenyl-2H-indazole-7-carboxamides as poly(ADP-ribose)polymerase (PARP) 1 and 2 inhibitors. This series was optimized to improve enzyme and cellular activity, and the resulting PARP inhibitors display antiproliferation activities against BRCA-1 and BRCA-2 deficient cancer cells, with high selectivity over BRCA proficient cells. Extrahepatic oxidation by CYP450 1A1 and 1A2 was identified as a metabolic concern, and strategies to improve pharmacokinetic properties are reported. These efforts culminated in the identification of 2-{4-[(3S)-piperidin-3-yl]phenyl}-2H-indazole-7-carboxamide 56 (MK-4827), which displays good pharmacokinetic properties and is currently in phase I clinical trials. This compound displays excellent PARP 1 and 2 inhibition with IC(50) = 3.8 and 2.1 nM, respectively, and in a whole cell assay, it inhibited PARP activity with EC(50) = 4 nM and inhibited proliferation of cancer cells with mutant BRCA-1 and BRCA-2 with CC(50) in the 10-100 nM range. Compound 56 was well tolerated in vivo and demonstrated efficacy as a single agent in a xenograft model of BRCA-1 deficient cancer.

Discovery and SAR of novel, potent and selective hexahydrobenzonaphthyridinone inhibitors of poly(ADP-ribose)polymerase-1 (PARP-1)

A novel hexahydrobenzonaphthyridinone PARP-1 pharmacophore is reported, subsequent SAR exploration around this scaffold led to selective PARP-1 inhibitors with low nanomolar enzyme potency, displaying good cellular activity and promising rat PK properties.

BMS-536924 sensitizes human epithelial ovarian cancer cells to the PARP inhibitor, 3-aminobenzamide

OBJECTIVE

To evaluate the anti-neoplastic activity of BMS-536924, an IGF-1R inhibitor, in epithelial ovarian cancer and its capacity to potentiate the effect of a PARP inhibitor, 3-aminobenzamide.

METHODS

OVCAR-3, OVCAR-4, SKOV-3 and TOV-81D cell lines were investigated in low-serum tissue culture conditions (1%FBS). Cytotoxicity assays were performed in quadruplicates using the Alamar colorimetric assay in the presence of BMS-536924 and/or 3-aminobenzamide. The levels of phospho-AKT, phospho-S6, PARP-1 and phospho-H2AX were evaluated by western blotting in the presence of BMS-536924.

RESULTS

BMS-536924 induced a time and dose inhibitory effect on cell survival. This effect seemed to be mediated by a reduction of pAKT and pS6 in a dose-dependent manner. The drug also provoked cell death by apoptosis as suggested by the increase in PARP-1 cleavage. It also induces DNA damage as demonstrated by the increased phosphorylation of histone H2AX and the augmentation of the comet tail moment. Finally, BMS-536924 sensitized cells to the effect of the PARP inhibitor, 3-aminobenzamide.

CONCLUSION

Our study reinforces the concept that IGF-1R is a good therapeutic target in ovarian cancer. Moreover, it suggests that combination therapy using BMS-536924 with a PARP inhibitor might be an effective strategy to circumvent resistance to treatment in clinical settings.