The role of inhibitors of poly(ADP-ribose) polymerase as resistance-modifying agents in cancer therapy

Design, synthesis, and biological evaluation of a series of benzofuran[3,2-d]pyrimidine-4(3H)-one derivatives containing thiosemicarbazone analogs as novel PARP-1 inhibitors

Poly ADP ribose polymerase-1 (PARP-1), one of the most important members of the PARP protein family, plays a crucial role in DNA damage repair, gene transcription, and apoptosis of cancer cells. In this work, benzofuran[3,2-d]pyrimidine-4(3H)-one was used as a framework to design and synthesize a series of novel PARP-1 inhibitors by introducing thiosemicarbazone or its derivatives into the scafford. Among all the target compounds, 19b and 19c were found to exhibit more potent inhibitory activity and higher selectivity against PARP-1 than Olaparib, especially the latter had an IC50 value of 0.026 μM against PARP-1 enzyme and a PARP-2/PARP-1 selectivity of 85.19-fold over Olapanib. Apart from strong cytotoxicity against the tested cancer cell lines, 19c was most sensitive to SK-OV-3 cells, with an IC50 value of 4.98 μM superior to Olaparib. Anti-cancer mechanism studies revealed that 19c could inhibit DNA single-strand breakage repair and aggravate DNA double-strand breakage by inhibiting PARP-1 activity, and promote the apoptosis of cancer cells through the mitochondrial apoptosis pathway.

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.

SARC025 arms 1 and 2: A phase 1 study of the poly(ADP-ribose) polymerase inhibitor niraparib with temozolomide or irinotecan in patients with advanced Ewing sarcoma

Background

In preclinical Ewing sarcoma (ES) models, poly(adenosine diphosphate ribose) polymerase (PARP) inhibitors were identified as a potential therapeutic strategy with synergy in combination with cytotoxic agents. This study evaluated the safety and dosing of the PARP1/2 inhibitor niraparib (NIR) with temozolomide (TMZ; arm 1) or irinotecan (IRN; arm 2) in patients with pretreated ES.

Methods

Eligible patients in arm 1 received continuous NIR daily and escalating TMZ (days 2‐6 [D2‐6]) in cohort A. Subsequent patients received intermittent NIR dosing (cohort B), with TMZ re‐escalation in cohort C. In arm 2, patients were assigned to NIR (days 1‐7 [D1‐7]) and escalating doses of IRN (D2‐6).

Results

From July 2014 to May 2018, 29 eligible patients (23 males and 6 females) were enrolled in arms 1 and 2, which had 7 dose levels combined. Five patients experienced at least 1 dose‐limiting toxicity (DLT) in arm 1 (grade 4 [G4] neutropenia for >7 days or G4 thrombocytopenia), and 3 patients experienced at least 1 DLT in arm 2 (grade 3 [G3] colitis, G3 anorexia, or G3 alanine aminotransferase elevation). The maximum tolerated dose was NIR at 200 mg every day on D1‐7 plus TMZ at 30 mg/m² every day on D2‐6 (arm 1) or NIR at 100 mg every day on D1‐7 plus IRN at 20 mg/m² every day on D2‐6 (arm 2). One confirmed partial response was observed in arm 2; the median progression‐free survival was 9.0 weeks (95% CI, 7.0‐10.1 weeks) and 16.3 weeks (95% CI, 5.1‐69.7 weeks) in arms 1 and 2, respectively. The median decrease in tumor poly(ADP‐ribose) activity was 89% (range, 83%‐98%).

Conclusions

The combination of NIR and TMZ or IRN was tolerable, but at lower doses in comparison with conventional cytotoxic combinations. A triple‐combination study of NIR, IRN, and TMZ has commenced.

Preclinical evaluations have identified the EWS‐FLI1 translocation, pathognomonic of Ewing sarcoma, as a predictive factor of response to poly(adenosine diphosphate ribose) polymerase (PARP) inhibitors with synergistic cell death in vivo with DNA damaging agents. This phase 1 study examines the dosing and safety of a combination of the PARP inhibitor niraparib with temozolomide or irinotecan.

Poly(ADP-Ribose) Glycohydrolase (PARG) vs. Poly(ADP-Ribose) Polymerase (PARP) - Function in Genome Maintenance and Relevance of Inhibitors for Anti-cancer Therapy

Poly(ADP-ribose) polymerases (PARPs) are a family of enzymes that catalyze the addition of poly(ADP-ribose) (PAR) subunits onto themselves and other acceptor proteins. PARPs are known to function in a large range of cellular processes including DNA repair, DNA replication, transcription and modulation of chromatin structure. Inhibition of PARP holds great potential for therapy, especially in cancer. Several PARP1/2/3 inhibitors (PARPi) have had success in treating ovarian, breast and prostate tumors harboring defects in the homologous recombination (HR) DNA repair pathway, especially BRCA1/2 mutated tumors. However, treatment is limited to specific sub-groups of patients and resistance can occur, limiting the use of PARPi. Poly(ADP-ribose) glycohydrolase (PARG) reverses the action of PARP enzymes, hydrolysing the ribose-ribose bonds present in poly(ADP-ribose). Like PARPs, PARG is involved in DNA replication and repair and PARG depleted/inhibited cells show increased sensitivity to DNA damaging agents. They also display an accumulation of perturbed replication intermediates which can lead to synthetic lethality in certain contexts. In addition, PARG is thought to play an important role in preventing the accumulation of cytoplasmic PAR and therefore parthanatos, a caspase-independent PAR-mediated type of cell death. In contrast to PARP, the therapeutic potential of PARG has been largely ignored. However, several recent papers have demonstrated the exciting possibilities that inhibitors of this enzyme may have for cancer treatment, both as single agents and in combination with cytotoxic drugs and radiotherapy. This article discusses what is known about the functions of PARP and PARG and the potential future implications of pharmacological inhibition in anti-cancer therapy.

Role of Akt Activation in PARP Inhibitor Resistance in Cancer

Poly(ADP-ribose) polymerase (PARP) inhibitors have recently been introduced in the therapy of several types of cancers not responding to conventional treatments. However, de novo and acquired PARP inhibitor resistance is a significant limiting factor in the clinical therapy, and the underlying mechanisms are not fully understood. Activity of the cytoprotective phosphatidylinositol-3 kinase (PI3K)-Akt pathway is often increased in human cancer that could result from mutation, expressional change, or amplification of upstream growth-related factor signaling elements or elements of the Akt pathway itself. However, PARP-inhibitor-induced activation of the cytoprotective PI3K-Akt pathway is overlooked, although it likely contributes to the development of PARP inhibitor resistance. Here, we briefly summarize the biological role of the PI3K-Akt pathway. Next, we overview the significance of the PARP-Akt interplay in shock, inflammation, cardiac and cerebral reperfusion, and cancer. We also discuss a recently discovered molecular mechanism that explains how PARP inhibition induces Akt activation and may account for apoptosis resistance and mitochondrial protection in oxidative stress and in cancer.

Synthesis of disaccharide nucleoside analogues as potential poly(ADP-ribose) polymerase-1 inhibitors

Poly(ADP-ribose) polymerase-1 (PARP-1) is an important target in cancer therapy. We present the synthesis of novel disaccharide nucleoside analogues that resemble the central motif of poly(ADP-ribose) and test their inhibitory effects on human PARP-1. Some compounds show inhibition of enzymatic activity in vitro and thus might be interesting for further investigations.

Opportunities for the repurposing of PARP inhibitors for the therapy of non-oncological diseases

The recent clinical availability of the PARP inhibitor olaparib (Lynparza) opens the door for potential therapeutic repurposing for non-oncological indications. Considering (a) the preclinical efficacy data with PARP inhibitors in non-oncological diseases and (b) the risk-benefit ratio of treating patients with a compound that inhibits an enzyme that has physiological roles in the regulation of DNA repair, we have selected indications, where (a) the severity of the disease is high, (b) the available therapeutic options are limited, and (c) the duration of PARP inhibitor administration could be short, to provide first-line options for therapeutic repurposing. These indications are as follows: acute ischaemic stroke; traumatic brain injury; septic shock; acute pancreatitis; and severe asthma and severe acute lung injury. In addition, chronic, devastating diseases, where alternative therapeutic options cannot halt disease development (e.g. Parkinson's disease, progressive multiple sclerosis or severe fibrotic diseases), should also be considered. We present a preclinical and clinical action plan for the repurposing of PARP inhibitors.

LINKED ARTICLES

This article is part of a themed section on Inventing New Therapies Without Reinventing the Wheel: The Power of Drug Repurposing. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.2/issuetoc.

The clinically used PARP inhibitor olaparib improves organ function, suppresses inflammatory responses and accelerates wound healing in a murine model of third-degree burn injury

BACKGROUND AND PURPOSE

The PARP inhibitor olaparib has recently been approved for human use for the therapy of cancer. Considering the role of PARP in critical illness, we tested the effect of olaparib in a murine model of burn injury, in order to begin exploring the feasibility of repurposing olaparib for the therapy of burn patients.

EXPERIMENTAL APPROACH

Mice were subjected to scald burn injury and randomized into vehicle or olaparib (10 mg·kg^-1 ·day^-1 i.p.) groups. Outcome variables included indices of organ injury, clinical chemistry parameters, plasma levels of inflammatory mediators (at 24 h, 7 and 21 days) and burn wound size (at 21 days).

KEY RESULTS

Olaparib reduced myeloperoxidase levels in heart and lung homogenates and reduced malondialdehyde levels in all tissues 24 h post-burn. Olaparib also reduced circulating alkaline aminotransferase, amylase and blood urea nitrogen and creatinine levels, indicative of protection against hepatic, pancreatic and renal dysfunction. Pro-inflammatory mediator (TNF-α, IL-1β, IFN-γ, GCSF, GM-CSF, eotaxin, KC, MIP-1-α and IL-3, 6 and 12) levels as well as the levels of several mediators that are generally considered anti-inflammatory (IL-4, 10 and 13) were reduced by olaparib. Plasma troponin-I levels (an indicator of skeletal muscle damage) was also attenuated by olaparib. Finally, olaparib stimulated wound healing.

CONCLUSIONS AND IMPLICATIONS

The clinically approved PARP inhibitor olaparib improves organ function, suppresses inflammatory responses and accelerates wound healing in murine burn injury. The data raise the potential utility of olaparib for severe burn injury.

LINKED ARTICLES

This article is part of a themed section on Inventing New Therapies Without Reinventing the Wheel: The Power of Drug Repurposing. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.2/issuetoc.

Chemotherapeutic potential of diazeniumdiolate-based aspirin prodrugs in breast cancer

Diazeniumdiolate-based aspirin prodrugs have previously been shown to retain the anti-inflammatory properties of aspirin while protecting against the common side effect of stomach ulceration. Initial analysis of two new prodrugs of aspirin that also release either nitroxyl (HNO) or nitric oxide (NO) demonstrated increased cytotoxicity toward human lung carcinoma cells compared to either aspirin or the parent nitrogen oxide donor. In addition, cytotoxicity was significantly lower in endothelial cells, suggesting cancer-specific sensitivity. To assess the chemotherapeutic potential of these new prodrugs in treatment of breast cancer, we studied their effect both in cultured cells and in a nude mouse model. Both prodrugs reduced growth of breast adenocarcinoma cells more effectively than the parent compounds while not being appreciably cytotoxic in a related nontumorigenic cell line (MCF-10A). The HNO donor also was more cytotoxic than the related NO donor. The basis for the observed specificity was investigated in terms of impact on metabolism, DNA damage and repair, apoptosis, angiogenesis and metastasis. The results suggest a significant pharmacological potential for treatment of breast cancer.

Effect of p53 activity on the sensitivity of human glioblastoma cells to PARP-1 inhibitor in combination with topoisomerase I inhibitor or radiation

Poly (ADP-Ribose) polymerase-1 (PARP-1) is involved in the DNA repairing system by sensing and signaling the presence of DNA damage. Inhibition of PARP-1 is tested in combination with DNA damaging agents such as topoisomerase I inhibitors or ionizing radiations (RT) for the treatment of glioblastoma (GBM). Disruption of p53, widely prevalent in GBMs, plays a major role in DNA repairing system. The current study investigates whether p53 activity has an effect on the sensitivity of human GBM cells to PARP-1 inhibitors in combination with topoisomerase I inhibitor topotecan (TPT) and/or RT. Human GBM cell lines carrying a different functional status of p53 were treated with PARP-1 inhibitor NU1025, in combination with TPT and/or RT. Cytotoxic effects were examined by analyzing the antiproliferative activity, the cell cycle perturbations, and the DNA damage induced by combined treatments. PARP inhibition enhanced the antiproliferative activity, the cell cycle perturbations and the DNA damage induced by both TPT or RT in GBM cells. These effects were influenced by the p53 activity: cells carrying an active p53 were more sensitive to the combination of PARP inhibitor and RT, while cells carrying an inactive p53 displayed a higher sensitivity to the combination of PARP inhibitor and TPT. Our study suggests that p53 activity influences the differential sensitivity of GBM cells to combined treatments of TPT, RT, and PARP inhibitors. © 2014 International Society for Advancement of Cytometry.

Drug resistance and DNA repair in leukaemia

Most cytotoxic agents exert their action via damage of DNA. Therefore, the repair of such lesions is of major importance for the sensitivity of malignant cells to chemotherapeutic agents. The underlying mechanisms of various DNA repair pathways have extensively been studied in yeast, bacteria and mammalian cells. Sensitive and drug resistant cancer cell lines have provided models for analysis of the contribution of DNA repair to chemosensitivity. However, the validity of results obtained by laboratory experiments with regard to the clinical situation is limited. In both acute and chronic leukaemias, the emergence of drug resistant cells is a major cause for treatment failure. Recently, assays have become available to measure cellular DNA repair capacity in clinical specimens at the single-cell level. Application of these assays to isolated lymphocytes from patients with chronic lymphatic leukaemia (CLL) revealed large interindividual differences in DNA repair rates. Accelerated O(6)-ethylguanine elimination from DNA and faster processing of repair-induced single-strand breaks were found in CLL lymphocytes from patients nonresponsive to chemotherapy with alkylating agents compared to untreated or treated sensitive patients. Moreover, modulators of DNA repair with different target mechanisms were identified which also influence the sensitivity of cancer cells to alkylating agents. In this article, we review the current knowledge about the contribution of DNA repair to drug resistance in human leukaemia.

Therapeutic intervention by the simultaneous inhibition of DNA repair and type I or type II DNA topoisomerases: one strategy, many outcomes

Many anticancer drugs reduce the integrity of DNA, forming strand breaks. This can cause mutations and cancer or cell death if the lesions are not repaired. Interestingly, DNA repair-deficient cancer cells (e.g., those with BRCA1/2 mutations) have been shown to exhibit increased sensitivity to chemotherapy. Based on this observation, a new therapeutic approach termed 'synthetic lethality' has been developed, in which radiation therapy or cytotoxic anticancer agents are employed in conjunction with selective inhibitors of poly(ADP-ribose)polymerase-1 (PARP-1). Such combinations can cause severe genomic instability in transformed cells resulting in cell death. The synergistic effects of combining PARP-1 inhibition with anticancer drugs have been demonstrated. However, the outcome of this therapeutic strategy varies significantly between cancer types, suggesting that synthetic lethality may be influenced by additional cellular factors. This review focuses on the outcomes of the combined action of PARP-1 inhibitors and agents that affect the activity of DNA topoisomerases.

Novel targeted therapies in head and neck cancer

INTRODUCTION

Molecularly targeted therapy, with the potential for increased selectivity and fewer adverse effects, hold promise in the treatment of HNSCC.

AREAS COVERED

Targeted agents for HNSCC expected to improve the effectiveness of current therapy including HER family, Src-family kinase, cell cycle, MET, AKT, HDAC, PARP, COX inhibitors and antiangiogenesis.

EXPERT OPINION

Epidermal growth factor receptor inhibitors are established in HNSCC and the need now is to find biomarkers for sensitivity to better select patients. Moreover, other pathway inhibitors hold significant promise and are being tested in clinical trials. Angiogenesis inhibition is likely to yield only modest efficacy alone but may augment existing standards. Lastly, one clinical arena where targeted therapies may find secure purchase is in the adjuvant or prevention setting where minimal or preneoplastic disease can be affected by inhibition of a single or few targets.

Poly(ADP-ribosyl)ation polymerases: mechanism and new target of anticancer therapy

Poly(ADP-ribose)polymerase (PARP) is a ubiquitously present nuclear enzyme that is not only involved in many important cellular pathways but also contributes to chromosomal structure and genomic stability. The development of highly selective and potent PARP inhibitors has become of increasing clinical interest because of their promising efficacy in patients with breast or ovarian cancer. Furthermore, recent Phase I and Phase II trials have demonstrated that PARP inhibitors have low toxicity rates. In particular patients with either deficiency or dysfunction of BRCA, which is involved in DNA double strand break repair, appear to benefit from PARP inhibition. This article summarizes the present knowledge regarding the physiological function of PARP and ([poly]ADP-ribose) PAR, the functional product of PARP, the development of PARP inhibitors, the recent clinical data of PARP inhibitors in cancer treatment and the selection of patients who may benefit from PARP inhibition.

Characterization of nuclear factors modulating the apolipoprotein D promoter during growth arrest: implication of PARP-1, APEX-1 and ERK1/2 catalytic activities

Human Apolipoprotein D (apoD) is upregulated under several stress conditions and pathological situations such as neurodegenerative diseases and cancers. We previously showed that apoD mRNA expression is induced in growth-arrested cells and demonstrated the specific binding of nuclear proteins to the region −514 to −475 of the promoter. Such region contains a pair of Serum Responsive Elements (SRE), an Ets-Binding Site (EBS) and a Glucocorticoid Responsive Element (GRE). In this study, we show that Parp-1, HnRNP-U, CBF-A, BUB-3, Kif4, APEX-1 and Ifi204 bind these regulatory elements of the apoD promoter. Specific binding of HnRNP-U and Parp-1 was confirmed by Electrophoretic Mobility Shift Assay (EMSA). In a biotin pull-down assay, Kif4 and BUB-3 bind preferentially the SRE1 and the EBS-GRE sites, respectively, while APEX-1 seems recruited indirectly to these elements. We found that the mRNA expression of some of these binding factors is upregulated in growth-arrested cells and that these proteins also transactivate the apoD promoter. In agreement with these results, mutants of APEX-1 and of Parp-1 defective for their DNA-binding and catalytic activities could not transactivate the promoter. The knockdown of Parp-1 and HnRNP-U and the use of specific inhibitors of MEK1/2 and of Parp-1 also inhibited the induction of apoD gene expression. Moreover, ERK1/2 was found activated in a biphasic manner post serum-starvation and the inhibition of Parp-1 causes a sustained activation of ERK2 but not ERK1 for up to 2 h. Altogether, these findings demonstrate the importance of Parp-1, APEX-1 and ERK1/2 catalytic activities in the growth arrest-induced apoD gene expression.

Optimization of phenyl-substituted benzimidazole carboxamide poly(ADP-ribose) polymerase inhibitors: identification of (S)-2-(2-fluoro-4-(pyrrolidin-2-yl)phenyl)-1H-benzimidazole-4-carboxamide (A-966492), a highly potent and efficacious inhibitor

We have developed a series of phenylpyrrolidine- and phenylpiperidine-substituted benzimidazole carboxamide poly(ADP-ribose) polymerase (PARP) inhibitors with excellent PARP enzyme potency as well as single-digit nanomolar cellular potency. These efforts led to the identification of (S)-2-(2-fluoro-4-(pyrrolidin-2-yl)phenyl)-1H-benzimidazole-4-carboxamide (22b, A-966492). Compound 22b displayed excellent potency against the PARP-1 enzyme with a K(i) of 1 nM and an EC(50) of 1 nM in a whole cell assay. In addition, 22b is orally bioavailable across multiple species, crosses the blood-brain barrier, and appears to distribute into tumor tissue. It also demonstrated good in vivo efficacy in a B16F10 subcutaneous murine melanoma model in combination with temozolomide and in an MX-1 breast cancer xenograft model both as a single agent and in combination with carboplatin.

Increased cytotoxicity of an unusual DNA topoisomerase II inhibitor compound C-1305 toward HeLa cells with downregulated PARP-1 activity results from re-activation of the p53 pathway and modulation of mitotic checkpoints

Our previous studies have shown that murine fibroblast cells, in which PARP-1 gene was inactivated by gene disruption, are extremely sensitive to triazoloacridone compound C-1305, an inhibitor of DNA topoisomerase II with unusual properties. Here, we show that pharmacological inhibition of PARP-1 activity by its inhibitor compound NU1025, sensitizes human cervical carcinoma HeLa cells to compound C-1305 compared to treatment with drug alone. Cytotoxic effect of drug/NU1025 of other topoisomerase II inhibitors varied depending on the dose of PARP-1 inhibitor. Increased cytotoxicity of topoisomerase II inhibitor/NU1025 combinations was attributable to the re-activation of the p53 pathway in drug-treated HeLa cells. This lead to a more stringent cell cycle checkpoint control during G2 and M and enhanced cell death by mitotic catastrophe induced by drug/NU1025 combinations. Interestingly, treatment of HeLa cells with NU1025 alone also increased p53 expression. This effect is, at least in part, related to the inhibition of proteasome activity by drug treatments. Together, our results show that concomitant inhibition of topoisomerase II and PARP-1 leads to the synergistic cytotoxic effect toward tumor cells that may be important for combination therapies with NU1025 and topoisomerase II inhibitors. We also confirmed our earlier work and show the important role of PARP-1 activity in the maintenance of the G2 arrest induced by DNA damaging drugs. Finally, based on our studies we propose that NU1025 and possibly other inhibitors of PARP-1 may be used as non-genotoxic agents to activate p53 in tumor cells with non-functional p53 pathways.

Synthesis and SAR of novel tricyclic quinoxalinone inhibitors of poly(ADP-ribose)polymerase-1 (PARP-1)

Based on screening hit 1, a series of tricyclic quinoxalinones have been designed and evaluated for inhibition of PARP-1. Substitutions at the 7- and 8-positions of the quinoxalinone ring led to a number of compounds with good enzymatic and cellular potency. The tricyclic quinoxalinone class is sensitive to modifications of both the amine substituent and the tricyclic core. The synthesis and structure-activity relationship studies are presented.

Discovery of the Poly(ADP-ribose) polymerase (PARP) inhibitor 2-[(R)-2-methylpyrrolidin-2-yl]-1H-benzimidazole-4-carboxamide (ABT-888) for the treatment of cancer

We have developed a series of cyclic amine-containing benzimidazole carboxamide PARP inhibitors with a methyl-substituted quaternary center at the point of attachment to the benzimidazole ring system. These compounds exhibit excellent PARP enzyme potency as well as single-digit nanomolar cellular potency. These efforts led to the identification of 3a (2-[(R)-2-methylpyrrolidin-2-yl]-1H-benzimidazole-4-carboxamide, ABT-888), currently in human phase I clinical trials. Compound 3a displayed excellent potency against both the PARP-1 and PARP-2 enzymes with a K(i) of 5 nM and in a C41 whole cell assay with an EC(50) of 2 nM. In addition, 3a is aqueous soluble, orally bioavailable across multiple species, and demonstrated good in vivo efficacy in a B16F10 subcutaneous murine melanoma model in combination with temozolomide (TMZ) and in an MX-1 breast cancer xenograft model in combination with either carboplatin or cyclophosphamide.

QSAR study of 2-(1-Propylpiperidin-4-yl)-1H-benzimidazole-4-carboxamide as PARP inhibitors for treatment of cancer

Quantitative structure-activity relationship of the 2-(1-propylpiperidin-4-yl)-1H-benzimidazole-4-carboxamide as a potent inhibitor of poly(ADP-ribose) polymerase for cancer treatment was studied. A suitable set of molecular descriptors was calculated and the genetic algorithm was employed to select those descriptors that resulted in the best fitted models. Excellent results were obtained employing multiple linear regressions and critically discussed using a variety of statistical parameters. Furthermore, the model was validated using leave-one-out and leave-group-out cross-validation, external test set and chance correlation. A genetic algorithm-multiple linear regression model with seven selected descriptors was obtained. This model, with high statistical significance (R(2) = 0.935, Q(2)_(LOO)= 0.894, Q(2)_(LGO)= 0.875, F = 53.481), could be used to predict poly(ADP-ribose) polymerase inhibitor activity of the molecules.

The selective poly(ADP-ribose) polymerase-1(2) inhibitor, CEP-8983, increases the sensitivity of chemoresistant tumor cells to temozolomide and irinotecan but does not potentiate myelotoxicity

The effect of the potent and selective poly(ADP-ribose) (PAR) polymerase-1 [and PAR polymerase-2] inhibitor CEP-8983 on the ability to sensitize chemoresistant glioblastoma (RG2), rhabdomyosarcoma (RH18), neuroblastoma (NB1691), and colon carcinoma (HT29) tumor cells to temozolomide- and camptothecin-induced cytotoxicity, DNA damage, and G(2)-M arrest and on the potentiation of chemotherapy-induced myelotoxicity was evaluated using in vitro assays. In addition, the effect of the prodrug CEP-9722 in combination with temozolomide and/or irinotecan on PAR accumulation and tumor growth was also determined using glioblastoma and/or colon carcinoma xenografts relative to chemotherapy alone. CEP-8983 sensitized carcinoma cells to the growth-inhibitory effects of temozolomide and/or SN38 increased the fraction of and/or lengthened duration of time tumor cells accumulated in chemotherapy-induced G(2)-M arrest and sensitized tumor cells to chemotherapy-induced DNA damage and apoptosis. A granulocyte-macrophage colony-forming unit colony formation assay showed that coincubation of CEP-8983 with temozolomide or topotecan did not potentiate chemotherapy-associated myelotoxicity. CEP-9722 (136 mg/kg) administered with temozolomide (68 mg/kg for 5 days) or irinotecan (10 mg/kg for 5 days) inhibited significantly the growth of RG2 tumors (60%) or HT29 tumors (80%) compared with temozolomide or irinotecan monotherapy, respectively. In addition, CEP-9722 showed "stand alone" antitumor efficacy in these preclinical xenografts. In vivo biochemical efficacy studies showed that CEP-9722 attenuated PAR accumulation in glioma xenografts in a dose- and time-related manner. These data indicate that CEP-8983 and its prodrug are effective chemosensitizing agents when administered in combination with select chemotherapeutic agents against chemoresistant tumors.

The emerging role of DNA repair proteins as predictive, prognostic and therapeutic targets in cancer

Advanced cancer is the second leading cause of death in the western world. Chemotherapy and radiation are the two main treatment modalities currently available to improve patient outcomes, but treatment related toxicity and the emergence of resistance limit their effectiveness. Hence there is an urgent need to develop novel treatment strategies. Rapid advances in cancer biology have identified key pathways involved in the repair of DNA damage induced by chemotherapeutic agents and irradiation. Efficient DNA repair in the cancer cell is an important mechanism for therapeutic resistance. Up to 130 genes have been identified that are associated with human DNA repair. Several of these proteins are emerging as important predictive and prognostic factors in solid tumours. Inhibition of DNA repair has the potential to enhance the efficacy of currently available DNA damaging agents. In recent years, several promising drug targets have been identified and novel drugs synthesised that target specific DNA repair proteins. These agents have shown impressive anti-cancer effects in preclinical studies in combination with chemotherapy or irradiation. Their role in human cancer is now being investigated in early phase clinical trials in combination with chemotherapy. MGMT inhibitors, PARP inhibitors and methoxyamine are currently in early stages of clinical development. Innovative clinical trial designs are essential to evaluate the potential of DNA repair inhibitor in cancer therapy.

Identification of three critical acidic residues of poly(ADP-ribose) glycohydrolase involved in catalysis: determining the PARG catalytic domain

PARG [poly(ADP-ribose) glycohydrolase] catalyses the hydrolysis of alpha(1''-->2') or alpha(1'''-->2'') O-glycosidic linkages of ADP-ribose polymers to produce free ADP-ribose. We investigated possible mechanistic similarities between PARG and glycosidases, which also cleave O-glycosidic linkages. Glycosidases typically utilize two acidic residues for catalysis, thus we targeted acidic residues within a conserved region of bovine PARG that has been shown to contain an inhibitor-binding site. The targeted glutamate and aspartate residues were changed to asparagine in order to minimize structural alterations. Mutants were purified and assayed for catalytic activity, as well as binding, to an immobilized PARG inhibitor to determine ability to recognize substrate. Our investigation revealed residues essential for PARG catalytic activity. Two adjacent glutamic acid residues are found in the conserved sequence Gln755-Glu-Glu757, and a third residue found in the conserved sequence Val737-Asp-Phe-Ala-Asn741. Our functional characterization of PARG residues, along with recent identification of an inhibitor-binding residue Tyr796 and a glycine-rich region Gly745-Gly-Gly747 important for PARG function, allowed us to define a PARG 'signature sequence' [vDFA-X3-GGg-X6-8-vQEEIRF-X3-PE-X14-E-X12-YTGYa], which we used to identify putative PARG sequences across a range of organisms. Sequence alignments, along with our mapping of PARG functional residues, suggest the presence of a conserved catalytic domain of approx. 185 residues which spans residues 610-795 in bovine PARG.

Chemopotentiation by PARP inhibitors in cancer therapy

Poly(ADP-ribose) polymerases (PARP) constitute a family of enzymes involved in the regulation of many cellular processes such as DNA repair, gene transcription, cell cycle progression, cell death, chromatin functions and genomic stability. Among the 18 members identified so far, PARP-1 and PARP-2 are the only proteins stimulated by DNA strand breaks and implicated in the repair of DNA injury. Therefore, these molecules have been exploited as potential targets for the development of pharmacological strategies to increase the antitumor efficacy of chemotherapeutic agents, which induce DNA damage. PARP inhibitors have been shown to restore sensitivity of resistant tumors to methylating agents or topoisomerase I inhibitors, drugs presently used for the treatment of primary and secondary brain tumors or malignancies refractory to standard chemotherapy. Interestingly, PARP inhibitors may also provide protection from the untoward effects exerted by certain anticancer drugs, which cause oxidative stress and consequent PARP overactivation. The aim of this article is to provide a brief overview of the recent literature on preclinical studies with the specific and potent inhibitors newly synthesized.

Gene 33 inhibits apoptosis of breast cancer cells and increases poly(ADP-ribose) polymerase expression

The structure of the Gene 33 protein suggests that it plays a role in intracellular signaling and Gene 33 is induced by many mitogenic and stressful stimuli. Previously, we found that Gene 33 expression is significantly induced by retinoic acid (RA), insulin and synergistically by both in a liver-derived cell line. In the present study, we investigated the basal expression and regulation of Gene 33 in multiple human breast cancer cell lines. These cell lines expressed different levels of Gene 33 protein, but Gene 33 protein was not regulated by RA or insulin, either alone, or in combination. However, epidermal growth factor (EGF) induced Gene 33 expression in SK-BR-3 cells and this induction was inhibited by co-treatment with RA. There was a strong correlation between endogenous basal Gene 33 expression and doubling time. Exogenous expression of Gene 33 in MCF-7 cells did not affect cell cycle distribution, but inhibited apoptosis and specifically increased the level of Poly(ADP-ribose) Polymerase (PARP-1) protein. This suggests that Gene 33 promotes breast cancer cell growth by an anti-apoptotic rather than a mitogenic effect, possibly involving up-regulation of PARP-1.

DNA repair pathways in drug resistance in melanoma

Metastatic melanoma has a poor prognosis due to resistance to multiple chemotherapy regimens. The mainstay of treatment remains dacarbazine, with cisplatin being a commonly used alternative. Melanoma displays marked resistance to the DNA-damaging effects of these drugs. Intrinsic and acquired resistance involves multiple cellular pathways of damage recognition, repair and apoptosis. Increased understanding of these pathways is identifying novel targets that it is hoped will make inroads into the treatment of this lethal disease.

An enzymatic assay for poly(ADP-ribose) polymerase-1 (PARP-1) via the chemical quantitation of NAD(+): application to the high-throughput screening of small molecules as potential inhibitors

The enzyme poly(adenosine 5'-diphosphate (ADP)-ribose) polymerase (PARP-1) catalyzes the formation of (ADP)-ribose polymers on a variety of protein acceptors in a NAD+ -dependent manner. While PARP-1 is activated by DNA damage and plays a critical role in cellular survival mechanisms, its overactivation leads to a depletion of NAD+/ATP energy stores and ultimately to necrotic cell death. Due to this dual role of PARP in the cell, small-molecule inhibitors of the PARP family of enzymes have been widely investigated for use as potentiators of anticancer therapies and as inhibitors of neurodegeneration and ischemic injuries. Unfortunately, standard assays for PARP inhibition are not optimal for the high-throughput screening of compound collections or combinatorial libraries. Described herein is a highly sensitive, inexpensive, and operationally simple assay for the rapid assessment of PARP activity that relies on the conversion of NAD+ into a highly fluorescent compound. We demonstrate that this assay can readily detect PARP inhibitors in a high-throughput screen using 384-well plates. In addition, the assay can be used to determine IC50 values for PARP inhibitors that have a range of inhibitory properties. As existing PARP assays utilize specialized reagents such as radiolabeled/biotinylated NAD+ or antibodies to poly(ADP-ribose), the chemical quantitation method described herein offers a highly sensitive and convenient alternative for rapidly screening compound collections for PARP inhibition.

Increased Susceptibility of Poly(ADP-Ribose) Polymerase-1 Knockout Cells to Antitumor Triazoloacridone C-1305 Is Associated with Permanent G2 Cell Cycle Arrest

Triazoloacridone C-1305 is a novel inhibitor of DNA topoisomerase II, which exhibits potent antitumor activity toward solid tumors. In this study, antiproliferative action of C-1305 and its close analog C-1533 was investigated in nontransformed mouse fibroblasts and two mutant cell lines in which the PARP-1 gene was specifically disrupted. Unexpectedly, C-1305 very strongly affected proliferation of cells lacking poly(ADP-ribose) polymerase-1 (PARP-1), whereas the action of less active compound C-1533 toward normal and PARP-1-negative cells was comparable. The IC(50) concentration of C-1305 determined for PARP-1 knockout cells was approximately 150-fold lower than that determined for cells with functional PARP-1. Both studied triazoloacridones exhibited very low direct cytotoxicity as evidenced by accumulation of 7-amino-actinomycin D, and only low levels of apoptosis were observed after a 24-h exposure to studied drugs. Analysis of DNA damage induced by C-1305 by the Comet assay showed that this drug induced very low levels of DNA strand breaks. C-1305 strongly affected cell cycle progression in normal and PARP-1 mutant cells and arrested both cell types in G(2)-M phase. However, the G(2)-M arrest induced by C-1305 was greatly prolonged in PARP-1-deficient cells as compared with normal fibroblasts. Together, these results show that mouse cells lacking PARP-1 are extremely sensitive to C-1305, a new topoisomerase II inhibitor. This is in striking contrast with previous reports in which PARP-1-deficient cells were shown to be resistant to classical topoisomerase II inhibitors. Our data also suggest that the PARP-1 status might be essential for the maintenance of the G(2) arrest induced by C-1305.

Potentiation of cytotoxic drug activity in human tumour cell lines, by amine-substituted 2-arylbenzimidazole-4-carboxamide PARP-1 inhibitors

The synthesis and biological evaluation of a new series of amine-substituted 2-arylbenzimidazole-4-carboxamide inhibitors of the DNA-repair enzyme poly(ADP-ribose) polymerase-1 (PARP-1) is reported. The introduction of an amine substituent at the 2-aryl position is not detrimental to activity, with most inhibitors exhibiting K(i) values for PARP-1 inhibition in the low nanomolar range. Two compounds in this series were found to potentiate the cytotoxicity of the DNA-methylating agent temozolomide by 4-5-fold in a human colorectal cancer cell line.

Anticancer chemosensitization and radiosensitization by the novel poly(ADP-ribose) polymerase-1 inhibitor AG14361

BACKGROUND

Poly(ADP-ribose) polymerase-1 (PARP-1) facilitates the repair of DNA strand breaks. Inhibiting PARP-1 increases the cytotoxicity of DNA-damaging chemotherapy and radiation therapy in vitro. Because classical PARP-1 inhibitors have limited clinical utility, we investigated whether AG14361, a novel potent PARP-1 inhibitor (inhibition constant <5 nM), enhances the effects of chemotherapy and radiation therapy in human cancer cell cultures and xenografts.

METHODS

The effect of AG14361 on the antitumor activity of the DNA alkylating agent temozolomide, topoisomerase I poisons topotecan or irinotecan, or x-irradiation or gamma-radiation was investigated in human cancer cell lines A549, LoVo, and SW620 by proliferation and survival assays and in xenografts in mice by tumor volume determination. The specificity of AG14361 for PARP-1 was investigated by microarray analysis and by antiproliferation and acute toxicity assays in PARP-1-/- and PARP-1+/+ cells and mice. After intraperitoneal administration, the concentration of AG14361 was determined in mouse plasma and tissues, and its effect on PARP-1 activity was determined in tumor homogenates. All statistical tests were two-sided.

RESULTS

AG14361 at 0.4 micro M did not affect cancer cell gene expression or growth, but it did increase the antiproliferative activity of temozolomide (e.g., in LoVo cells by 5.5-fold, 95% confidence interval [CI] = 4.9-fold to 5.9-fold; P =.004) and topotecan (e.g., in LoVo cells by 1.6-fold, 95% CI = 1.3-fold to 1.9-fold; P =.002) and inhibited recovery from potentially lethal gamma-radiation damage in LoVo cells by 73% (95% CI = 48% to 98%). In vivo, nontoxic doses of AG14361 increased the delay of LoVo xenograft growth induced by irinotecan, x-irradiation, or temozolomide by two- to threefold. The combination of AG14361 and temozolomide caused complete regression of SW620 xenograft tumors. AG14361 was retained in xenografts in which PARP-1 activity was inhibited by more than 75% for at least 4 hours.

CONCLUSION

AG14361 is, to our knowledge, the first high-potency PARP-1 inhibitor with the specificity and in vivo activity to enhance chemotherapy and radiation therapy of human cancer.

Crystal structure of the catalytic fragment of murine poly(ADP-ribose) polymerase-2

Poly(ADP-ribose) polymerase-1 (PARP-1) has become an important pharmacological target in the treatment of cancer due to its cellular role as a 'DNA-strand break sensor', which leads in part to resistance to some existing chemo- and radiological treatments. Inhibitors have now been developed which prevent PARP-1 from synthesizing poly(ADP-ribose) in response to DNA-breaks and potentiate the cytotoxicity of DNA damaging agents. However, with the recent discoveries of PARP-2, which has a similar DNA-damage dependent catalytic activity, and additional members containing the 'PARP catalytic' signature, the isoform selectivity and resultant pharmacological effects of existing inhibitors are brought into question. We present here the crystal structure of the catalytic fragment of murine PARP-2, at 2.8 A resolution, and compare this to the catalytic fragment of PARP-1, with an emphasis on providing a possible framework for rational drug design in order to develop future isoform-specific inhibitors.

Poly(ADP-ribose) polymerase (PARP-1) is not involved in DNA double-strand break recovery

BACKGROUND

The cytotoxicity and the rejoining of DNA double-strand breaks induced by gamma-rays, H2O2 and neocarzinostatin, were investigated in normal and PARP-1 knockout mouse 3T3 fibroblasts to determine the role of poly(ADP-ribose) polymerase (PARP-1) in DNA double-strand break repair.

RESULTS

PARP-1-/- were considerably more sensitive than PARP-1+/+ 3T3s to induced cell kill by gamma-rays and H2O2. However, the two cell lines did not show any significant difference in the susceptibility to neocarzinostatin below 1.5 nM drug. Restoration of PARP-1 expression in PARP-1-/- 3T3s by retroviral transfection of the full PARP-1 cDNA did not induce any change in neocarzinostatin response. Moreover the incidence and the rejoining kinetics of neocarzinostatin-induced DNA double-strand breaks were identical in PARP-1+/+ and PARP-1-/- 3T3s. Poly(ADP-ribose) synthesis following gamma-rays and H2O2 was observed in PARP-1-proficient cells only. In contrast neocarzinostatin, even at supra-lethal concentration, was unable to initiate PARP-1 activation yet it induced H2AX histone phosphorylation in both PARP1+/+ and PARP-1-/- 3T3s as efficiently as gamma-rays and H2O2.

CONCLUSIONS

The results show that PARP-1 is not a major determinant of DNA double-strand break recovery with either strand break rejoining or cell survival as an endpoint. Even though both PARP-1 and ATM activation are major determinants of the cell response to gamma-rays and H2O2, data suggest that PARP-1-dependent poly(ADP-ribose) synthesis and ATM-dependent H2AX phosphorylation, are not inter-related in the repair pathway of neocarzinostatin-induced DNA double-strand breaks.

Information decay in molecular docking screens against holo, apo, and modeled conformations of enzymes

Molecular docking uses the three-dimensional structure of a receptor to screen a small molecule database for potential ligands. The dependence of docking screens on the conformation of the binding site remains an open question. To evaluate the information loss that occurs as the active site conformation becomes less defined, a small molecule database was docked against the holo (ligand bound), apo, and homology modeled structures of 10 different enzyme binding sites. The holo and apo representations were crystallographic structures taken from the Protein Data Bank (PDB), and the homology-modeled structures were taken from the publicly available resource ModBase. The database docked was the MDL Drug Data Report (MDDR), a functionally annotated database of 95000 small molecules that contained at least 35 ligands for each of the 10 systems. In all sites, at least 99% of the molecules in the MDDR were treated as nonbinding decoys. For each system, the holo, apo, and modeled structures were used to screen the MDDR, and the ability of each structure to enrich the known ligands for that system over random selection was evaluated. The best overall enrichment was produced by the holo structure in seven systems, the apo structure in two systems, and the modeled structure in one system. These results suggest that the performance of the docking calculation is affected by the particular representation of the receptor used in the screen, and that the holo structure is the one most likely to yield the best discrimination between known ligands and decoy molecules, but important exceptions to this rule also emerge from this study. Although each of the holo, apo, and modeled conformations led to enrichment of known ligands in all systems, the enrichment did not always rise to a level judged to be sufficient to justify the effort of a docking screen. Using a 20-fold enrichment of known ligands over random selection as a rough guideline for what might be enough to justify a docking screen, the holo conformation of the enzyme met this criterion in eight of 10 sites, whereas the apo conformation met this criterion in only two sites and the modeled conformation in three.

Tricyclic benzimidazoles as potent poly(ADP-ribose) polymerase-1 inhibitors

Novel tricyclic benzimidazole carboxamide poly(ADP-ribose) polymerase-1 (PARP-1) inhibitors have been synthesized. Several compounds were found to be powerful chemopotentiators of temozolomide and topotecan in both A549 and LoVo cell lines. In vitro inhibition of PARP-1 was confirmed by direct measurement of NAD+ depletion and ADP-ribose polymer formation caused by chemically induced DNA damage.

Combined treatment with temozolomide and poly(ADP-ribose) polymerase inhibitor enhances survival of mice bearing hematologic malignancy at the central nervous system site

Temozolomide (TZM) is a DNA-methylating agent that has recently been introduced into various clinical trials for treatment of solid or hematologic neoplasias, including brain lymphomas. In the current study, we have investigated whether the antitumor activity of TZM could be selectively enhanced at the central nervous system (CNS) site by intracerebral injection of a poly(ADP-ribose) polymerase (PARP) inhibitor. Mice were injected intracranially with lymphoma cells. The PARP inhibitor NU1025 (1 mg/animal) was delivered intracerebrally, whereas TZM was given as a single or a fractionated dose of 200 mg/kg by intraperitoneal administration. Results indicated that this drug combination significantly enhanced the survival of tumor-bearing mice and that this fractionated modality of treatment was the most effective schedule. Increased survival time was related to a marked reduction of tumor growth, as evidenced by histologic studies. Treatment with TZM alone was ineffective. This is the first report exploring in vivo the combination of TZM with PARP inhibitor for intracerebral neoplasias.

Potential clinical applications of poly(ADP-ribose) polymerase (PARP) inhibitors

Poly(ADP-ribose) polymerases (PARPs) are defined as cell signaling enzymes that catalyze the transfer of ADP-ribose units from NAD(+)to a number of acceptor proteins. PARP-1, the best characterized member of the PARP family, that presently includes six members, is an abundant nuclear enzyme implicated in cellular responses to DNA injury provoked by genotoxic stress (oxygen radicals, ionizing radiations and monofunctional alkylating agents). Due to its involvement either in DNA repair or in cell death, PARP-1 is regarded as a double-edged regulator of cellular functions. In fact, when the DNA damage is moderate, PARP-1 participates in the DNA repair process. Conversely, in the case of massive DNA injury, elevated PARP-1 activation leads to rapid NAD(+)/ATP consumption and cell death by necrosis. Excessive PARP-1 activity has been implicated in the pathogenesis of numerous clinical conditions such as stroke, myocardial infarction, shock, diabetes and neurodegenerative disorders. PARP-1 could therefore be considered as a potential target for the development of pharmacological strategies to enhance the antitumor efficacy of radio- and chemotherapy or to treat a number of clinical conditions characterized by oxidative or NO-induced stress and consequent PARP-1 activation. Moreover, the discovery of novel functions for the multiple members of the PARP family might lead in the future to additional clinical indications for PARP inhibitors.

Modeling of poly(ADP-ribose)polymerase (PARP) inhibitors. Docking of ligands and quantitative structure-activity relationship analysis

Poly(ADP-ribose)polymerase-1 (PARP-1) is a nuclear enzyme that has recently emerged as an important player in the mechanisms leading to postischemic neuronal death, and PARP inhibitors have been proposed as potential neuroprotective agents. With the aim of clarifying the structural basis responsible for PARP inhibition, we carried out a computational study on 46 inhibitors available through the literature. Our computational approach is composed of three parts. In the first one, representative PARP inhibitors have been docked into the crystallographic structure of the catalytic domain of PARP by using the Autodock 2.4 program. The docking studies thus carried out have provided an alignment scheme that has been instrumental for superimposing all the remaining inhibitors. Upon the basis of this alignment scheme, a quantitative structure-activity relationship (QSAR) analysis has been carried out after electrostatic and steric interaction energies have been computed with the RECEPTOR program. The QSAR analysis yielded a predictive model able to explain much of the variance of the 46-compound data set. The inspection of the QSAR coefficients revealed that the major driving force for potent inhibition is given by the extension of the contact surface between enzyme and inhibitors while electrostatic energy and hydrogen bonding capability play a minor role. Finally, the projection of the QSAR coefficients back onto the X-ray structure of the catalytic domain of PARP provides insights into the role played by specific amino acid residues. This information will be useful to address the design of new selective and potent PARP inhibitors.

Expression in yeast and purification of functional recombinant human poly(ADP-ribose)polymerase (PARP). Comparative pharmacological profile with that of the rat enzyme

Human poly(ADP-ribose)polymerase (PARP) was expressed in the yeast line JEL1 under the control of a GAL promoter. Proteins were extracted and human recombinant PARP purified to apparent homogeneity. The pharmacological profile of this human enzyme was characterised in terms of the effects of known inhibitors of PARP belonging to various chemical families and this was compared with that of the rat enzyme purified from rat testes, using the same purification protocol. The rat and the human enzymes appeared very similar in terms of their sensitivities to those selected inhibitors.

Resistance-modifying agents. 9. Synthesis and biological properties of benzimidazole inhibitors of the DNA repair enzyme poly(ADP-ribose) polymerase

The nuclear enzyme poly(ADP-ribose) polymerase (PARP) facilitates the repair of DNA strand breaks and is implicated in the resistance of cancer cells to certain DNA-damaging agents. Inhibitors of PARP have clinical potential as resistance-modifying agents capable of potentiating radiotherapy and the cytotoxicity of some forms of cancer chemotherapy. The preclinical development of 2-aryl-1H-benzimidazole-4-carboxamides as resistance-modifying agents in cancer chemotherapy is described. 1H-Benzimidazole-4-carboxamides, particularly 2-aryl derivatives, are identified as a class of potent PARP inhibitors. Derivatives of 2-phenyl-1H-benzimidazole-4-carboxamide (23, K(i) = 15 nM), in which the phenyl ring contains substituents, have been synthesized. Many of these derivatives exhibit K(i) values for PARP inhibition < 10 nM, with 2-(4-hydroxymethylphenyl)-1H-benzimidazole-4-carboxamide (78, K(i) = 1.6 nM) being one of the most potent. Insight into structure-activity relationships (SAR) for 2-aryl-1H-benzimidazole-4-carboxamides has been enhanced by studying the complex formed between 2-(3-methoxyphenyl)-1H-benzimidazole-4-carboxamide (44, K(i) = 6 nM) and the catalytic domain of chicken PARP. Important hydrogen-bonding and hydrophobic interactions with the protein have been identified for this inhibitor. 2-(4-Hydroxyphenyl)-1H-benzimidazole-4-carboxamide (45, K(i) = 6 nM) potentiates the cytotoxicity of both temozolomide and topotecan against A2780 cells in vitro (by 2.8- and 2.9-fold, respectively).

Purification and cloning of pierisin-2, an apoptosis-inducing protein from the cabbage butterfly, Pieris brassicae

The cabbage butterfly Pieris rapae contains a strong apoptosis-inducing substance, pierisin, against human cancer cell lines, which is thought to act via ADP-ribosylation. Here we report the purification and cloning of an apoptosis-inducing substance, designated as pierisin-2, from another cabbage butterfly, Pieris brassicae. Pierisin-2 was purified from pupae by sequential chromatography and its cytotoxic and apoptosis-inducing activities to various cancer cells were similar to those of pierisin, designated as pierisin-1, from P. rapae. cDNA cloning of pierisin-2 was performed on the basis of the partial amino-acid sequence. The nucleotide sequence indicated that the cDNA encodes an 850-amino-acid protein with a calculated molecular mass of 97 986. The deduced amino-acid sequence of pierisin-2 was 91% identical with that of pierisin-1. In vitro expressed protein in the reticulocyte lysate exhibited apoptosis-inducing activities against human gastric carcinoma TMK-1 and cervical carcinoma HeLa cells, similar to the purified native pierisin-2 from the pupae. Pierisin-2 shows regional sequence similarities with certain ADP-ribosylating toxins such as the A-subunit of cholera toxin. The results from site-directed mutagenesis at Glu165, a conserved residue among ADP-ribosylating enzymes necessary for NAD binding, and from experiments with ADP-ribosylating enzyme inhibitors suggested that pierisin-2 could be considered as an ADP-ribosylating toxin like pierisin-1.