Iniparib non-selectively modifies cysteine-containing proteins in tumor cells and is not a bona fide PARP inhibitor
Menée in vitro et in vivo, cette étude met en évidence un mécanisme d'action de l'iniparib et suggère, en contradiction avec des travaux précédents, que cette molécule n'est pas un inhibiteur de PARP
Purpose: PARP inhibitors are being developed as therapeutic agents for cancer. More than six compounds have entered clinical trials. The majority of these compounds are NAD+-competitive inhibitors. One exception is iniparib which has been proposed to be a non-competitive PARP inhibitor. Here we compare the biological activities of two different structural classes of NAD+-competitive compounds with iniparib and its C-nitroso metabolite. Experimental Design: Two chemical series of NAD+-competitive PARP inhibitors, iniparib and its C-nitroso metabolite were analyzed in enzymatic and cellular assays. Viability assays was carried out in MDA-MB-436 (BRCA1 deficient) and DLD1-/- cells (BRCA2 deficient) along with BRCA proficient MDA-MB-231 and DLD1+/+ cells. Capan-1 and B16F10 xenograft models were used to compare iniparib and veliparib in vivo. Mass spectrometry and 3H-labeling method were used to monitor the covalent modification of proteins. Results:All NAD+-competitive inhibitors demonstrate 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 killing was associated with an induction of DNA damage. 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 nitroso metabolite of iniparib forms adducts with many cysteine-containing proteins. Furthermore, both iniparib and its nitroso metabolite form protein adducts non-specifically in tumor cells. Conclusions:Iniparib non-selectively modifies cysteine-containing proteins in tumor cells and the primary mechanism of action for iniparib is likely not via inhibition of PARP activity.