Cancer Therapy: Preclinical Iniparib Nonselectively Modifies Cysteine-Containing Proteins in TumorCells and IsNot aBona FidePARP Inhibitor

Purpose:PARP inhibitors are beingdeveloped as therapeutic agents for cancer.More than six compounds have entered clinical trials. The majority of these compounds are b-nicotinamide adenine dinucleotide (NADþ)-competitive inhibitors. One exception is iniparib, which has been proposed to be a noncompetitive PARP inhibitor. In this study, we compare the biologic activities of two different structural classes of NADþ-competitive compounds with iniparib and its C-nitroso metabolite. Experimental Design: Two chemical series of NADþ-competitive PARP inhibitors, iniparib and its C-nitroso metabolite, were analyzed in enzymatic and cellular assays. Viability assays were carried out in MDA-MB-436 (BRCA1-deficient) and DLD1 / (BRCA2-deficient) cells together with BRCA-proficient MDA-MB-231 and DLD1þ/þ cells. Capan-1 and B16F10 xenograft models were used to compare iniparib and veliparib in vivo. Mass spectrometry and the H-labeling method were used to monitor the covalent modification of proteins. Results: All NADþ-competitive inhibitors show robust activity in a PARP cellular assay, strongly potentiate the activity of temozolomide, and elicit robust cell killing in BRCA-deficient tumor cells in vitro and in vivo. Cell killingwas associatedwith an induction ofDNAdamage. In contrast, neither iniparib nor its C-nitroso metabolite inhibited PARP enzymatic or cellular activity, potentiated temozolomide, or showed activity in a BRCA-deficient setting.We find that the nitrosometabolite of iniparib forms adducts withmany cysteine-containing proteins. Furthermore, both iniparib and its nitroso metabolite form protein adducts nonspecifically in tumor cells. Conclusions: Iniparib nonselectively modifies cysteine-containing proteins in tumor cells, and the primary mechanism of action for iniparib is likely not via inhibition of PARP activity. Clin Cancer Res; 18(2); 1–14. 2011 AACR.