Tricyclic Benzimidazoles as Potent Poly(ADP-ribose) Polymerase-1 Inhibitors

Self Cite

112

Poly(ADP-ribose) polymerase (PARP) inhibitors enhance DNA topoisomerase I (topo I) poisoninduced cytotoxicity and antitumor activity in vitro and in vivo, but the mechanism has not been defined. We investigated the role of PARP-1 in the response to topo I poisons using PARP-1 À/À and PARP-1 +/+ mouse embryonic fibroblasts and the potent PARP-1 inhibitor, AG14361 (K i < 5 nmol/L). PARP-1 À/À mouse embryonic fibroblasts were 3-fold more sensitive to topotecan than PARP-1 +/+ mouse embryonic fibroblasts (GI 50 , 21 and 65 nmol/L, respectively). AG14361caused a >3-fold sensitization of PARP-1 +/+ cells to topotecan compared with a <1.4-fold sensitization in PARP-1 À/À cells. In human leukemia K562 cells, AG14361 caused a 2-fold sensitization to camptothecin-induced cytotoxicity. AG14361 did not affect the cellular activity of topo I as determined by measurement of cleavable complexes and topo I relaxation activity, showing that sensitization was not due to topo I activation. In contrast, repair of DNA following camptothecin removal, normally very rapid, was significantly retarded by AG14361, resulting in a 62% inhibition of repair 10 minutes after camptothecin removal. This led to a 20% increase in the net accumulation of camptothecin-induced DNA strand break levels in cells coexposed to AG14361 for 16 hours. We investigated the DNA repair mechanism involved using a panel of DNA repair^deficient Chinese hamster ovary cells. AG14361 significantly potentiated camptothecin-mediated cytotoxicity in all cells, except the base excision repair^deficient EM9 cells. Therefore, the most likely mechanism for the potentiation of topo I poison-mediated cytotoxicity by AG14361 is via PARP-1-dependent base excision repair.

Section: Methodsmentioning

confidence: 99%

The Novel Poly(ADP-Ribose) Polymerase Inhibitor, AG14361, Sensitizes Cells to Topoisomerase I Poisons by Increasing the Persistence of DNA Strand Breaks

Smith¹,

Willmore²,

Austin

et al. 2005

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112

“…These observations have stimulated considerable interest in these agents as anticancer therapy. AG-14361 is a potent inhibitor of PARP-1, with an inhibition constant in the nanomolar range, and AG-014699, a potent tricyclic indole with an inhibition constant of <5 nmol/L, is a close analogue and the first PARP inhibitor to enter clinical trials in patients with cancer (67). AG-014699 was developed through a collaborative program between the University of Newcastle upon Tyne, United Kingdom, and Pfizer.…”

Section: Parp Inhibitorsmentioning

confidence: 99%

Mechanisms of Chemoresistance to Alkylating Agents in Malignant Glioma

Sarkaria

Kitange

James

et al. 2008

320

276

Intrinsic or acquired chemoresistance to alkylating agents is a major cause of treatment failure in patients with malignant brain tumors. Alkylating agents, the mainstay of treatment for brain tumors, damage the DNA and induce apoptosis, but the cytotoxic activity of these agents is dependent on DNA repair pathways. For example, O 6 -methylguanine DNA adducts can cause double-strand breaks, but this is dependent on a functional mismatch repair pathway. Thus, tumor cell lines deficient in mismatch repair are resistant to alkylating agents. Perhaps the most important mechanism of resistance to alkylating agents is the DNA repair enzyme O 6 -methylguanine methyltransferase, which can eliminate the cytotoxic O 6 -methylguanine DNA adduct before it causes harm. Another mechanism of resistance to alkylating agents is the base excision repair (BER) pathway. Consequently, efforts are ongoing to develop effective inhibitors of BER. Poly(ADP-ribose)polymerase plays a pivotal role in BER and is an important therapeutic target. Developing effective strategies to overcome chemoresistance requires the identification of reliable preclinical models that recapitulate human disease and which can be used to facilitate drug development. This article describes the diverse mechanisms of chemoresistance operating in malignant glioma and efforts to develop reliable preclinical models and novel pharmacologic approaches to overcome resistance to alkylating agents.Intrinsic or acquired resistance to cytotoxic agents remains the greatest obstacle to the successful treatment of human cancer, and a variety of mechanisms of drug resistance have been identified in human tumors. High-grade malignant gliomas, including glioblastoma and anaplastic astrocytoma, are among the most rapidly growing and devastating cancers. Standard treatment consists of surgery followed by radiotherapy and chemotherapy with alkylating agents. Alkylating agents, including carmustine (bischloroethyl nitrosourea, or BCNU), lomustine (chloroethylnitrosourea), and temozolomide, readily cross the blood-brain barrier and have shown the most activity against malignant glioma. However, despite the ability of these agents to achieve therapeutic concentrations in the brain, malignant gliomas are often resistant to alkylating agents. Identifying and understanding the diverse mechanisms of chemoresistance operating in human tumors and developing effective strategies to overcome resistance is the focus of ongoing preclinical and clinical research.One important mechanism of resistance to alkylating agents is mediated by the DNA repair enzyme O 6 -methylguanine methyltransferase (MGMT; refs. 1, 2), which repairs O 6 -alkylguanine adducts. BCNU and chloroethylnitrosourea are bifunctional alkylating agents that form lethal double-strand cross-links (3). In contrast, the activity of methylating agents such as temozolomide and dacarbazine results in persistent O 6 -methylguanine adducts that initiate futile cycling of the DNA mismatch repair (MMR) pathway, which ultimately caus...

“…1), which is Ͼ1000-fold more potent than 3-aminobenzamide (Ki Ͻ 5 nM; Ref. 19) and enhances the cytotoxicity and antitumor activity of temozolomide in vitro and in vivo (20); and secondly, genetically matched cell lines with defined MMR phenotypes have been developed using microcell transfer techniques. The MMR defect of cells lacking hMLH1 can be corrected by transfer of chromosome 3, on which the gene is located (21).…”

Section: Introductionmentioning

confidence: 99%

Novel Poly(ADP-ribose) Polymerase-1 Inhibitor, AG14361, Restores Sensitivity to Temozolomide in Mismatch Repair-Deficient Cells

Curtin

Wang

Yiakouvaki

et al. 2004

Self Cite

152

Purpose: Mismatch repair (MMR) deficiency confers resistance to temozolomide, a clinically active DNA-methylating agent. The purpose of the current study was to investigate the reversal mechanism of temozolomide resistance by the potent novel poly(ADP-ribose) polymerase (PARP)-1 inhibitor, AG14361, in MMR-proficient and -deficient cells.Experimental Design: The effects of AG14361, in comparison with the methylguanine DNA methyltransferase inhibitor, benzylguanine, on temozolomide-induced growth inhibition were investigated in matched pairs of MMR-proficient (HCT-Ch3, A2780, and CP70-ch3) and -deficient (HCT116, CP70, and CP70-ch2) cells.Results: AG14361 enhanced temozolomide activity in all MMR-proficient cells (1.5-3.3-fold) but was more effective in MMR-deficient cells (3.7-5.2-fold potentiation), overcoming temozolomide resistance. In contrast, benzylguanine only increased the efficacy of temozolomide in MMR-proficient cells but was ineffective in MMR-deficient cells. The differential effect of AG14361 in MMR-deficient cells was not attributable to differences in PARP-1 activity or differences in its inhibition by AG14361, nor was it attributable to differences in DNA strand breaks induced by temozolomide plus AG14361. MMR-deficient cells are resistant to cisplatin, but AG14361 did not sensitize any cells to cisplatin. PARP-1 inhibitors potentiate topotecan-induced growth inhibition, but AG14361 did not potentiate topotecan in MMR-deficient cells more than in MMR-proficient cells.Conclusions: MMR defects are relatively common in sporadic tumors and cancer syndromes. PARP-1 inhibition represents a novel way of selectively targeting such tumors. The underlying mechanism is probably a shift of the cytotoxic locus of temozolomide to N 7 -methylguanine and N 3 -methyladenine, which are repaired by the base excision repair pathway in which PARP-1 actively participates.