Niraparib tosylate (MK-4827 (tosylate))(with IC50 of 3.8 nM/2.1 nM) is a selective PARP1/PARP2 inhibitor.

Micromolar concentrations of niraparib radiosensitizes tumor cell lines derived from lung, breast, and prostate cancers independently of their p53 status but not cell lines derived from normal tissues. Niraparib also sensitizes tumor cells to Water2 and converts Water2-induced single strand breaks (SSBs) into DSBs during DNA replication[5].

MK-4827 strongly enhances the effect of radiation on a variety of human tumor xenografts, both p53 wild type and p53 mutant. MK-4827 reduces PAR levels in tumors by 1 h after administration which persisted for up to 24 h[1]. In vivo treatment with MK-4827 and radiation prolonged survival (p<0.01) compared to single modalities. In vivo superiority of MK-4827 plus radiation is further documented by significant elevations of cleaved caspase-3 and γ-H2AX in tumors from the combination group compared to single modality cohorts[4].

Enzyme assay is conducted in buffer containing 25 mM Tris, pH 8.0, 1 mM DTT, 1 mM spermine, 50 mM KCl, 0.01% Nonidet P-40, and 1 mM MgCl2. PARP reaction contains 0.1 μCi [3H]NAD+ (200 000 DPM), 1.5 μM NAD+, 150 nM biotinylated NAD+, 1 μg/mL activated calf thymus, and 1?5 nM PARP-1. Autoreactions utilizing SPA bead-based detection are carried out in 50 μL volumes in white 96-well plates. Compounds (e.g., MK-4827) are prepared in 11-point serial dilution in 96-well plate, 5 μL/well in 5% DMSO/Water (10× concentrated). Reactions are initiated by adding first 35 μL of PARP-1 enzyme in buffer and incubating for 5 min at room temperature and then 10 μL of NAD+ and DNA substrate mixture. After 3 h at room temperature, these reactions are terminated by the addition of 50 μL of streptavidin-SPA beads (2.5 mg/mL in 200 mM EDTA, pH 8). After 5 min, they are counted using a TopCount microplate scintillation counter. IC50 data is determined from inhibition curves at various substrate concentrations[1].

V-C8 (BRCA2-negative) Chinese hamster cells are treated with the PARP inhibitor MK-4827 for 24 h, washed and incubated in drug-free medium for 5-7 days until colonies formed. (Only for Reference)