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 repai r 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 develop ment of 2-aryl-1H-benzimidazole-4-carboxamides as resistance-modifying agen ts in cancer chemotherapy is described. 1H-Benzimidazole-4-carboxamides, pa rticularly 2-aryl derivatives, are identified as a class of potent PARP inh ibitors. Derivatives of 2-phenyl-1H-benzimidazole-4-carboxamide (23, K-i = 15 nM), in which the phenyl ring contains substituents, have been synthesiz ed. Many of these derivatives exhibit K-i values for PARP inhibition < 10 n M, with 2-(4-hydroxymethylphenyl)-1H-benzimidazole-4-carboxamide (78, K-i = 1.6 nM) being one of the most potent. Insight into structure-activity rela tionships (SAR) for 2-aryl-1H-benzimidazole-4-carboxamides has been enhance d by studying the complex formed between 2-(3-methoxyphenyl)-1H-benzimidazo le-4-carboxamide (44, K-i = 6 nM) and the catalytic domain of chicken PARP. Important hydrogen-bonding and hydrophobic interactions with the protein h ave been identified for this inhibitor. 2-(4-Hydroxyphenyl)-1H-benzimidazol e-4-carboxamide (45, K-i = 6 nM) potentiates the cytotoxicity of both temoz olomide and topotecan against A2780 cells in vitro (by 2.8- and 2.9-fold, r espectively).