Question: Which enzyme is inhibited by niraparib?

Context:
While olaparib is the first PARP inhibitor to receive approval for ovarian cancer treatment, others including rucaparib and niraparib are clearly effective in this disease and, within the next year or two, the results of ongoing randomised trials will clarify their respective roles. 
Similar trials with other PARP inhibitors (rucaparib, niraparib and veliparib) are in progress and include non-BRCA-mutated ovarian cancer.
We report here that while the activities of the four human sirtuin isoforms SIRT1, SIRT2, SIRT3 and SIRT6 are blocked by sirtuin inhibitor Ex527 in vitro, they are unaffected by the seven clinical and commonly used PARP inhibitors niraparib, olaparib, rucaparib, talazoparib, veliparib, PJ34, and XAV939. 
Results from a phase III trial indicate that maintenance therapy with the PARP inhibitor niraparib is more effective than placebo in slowing the progression of recurrent platinum-sensitive ovarian cancer.
In vivo anti-tumor activity of the PARP inhibitor niraparib in homologous recombination deficient and proficient ovarian carcinoma.
OBJECTIVE: Poly(ADP-ribose) polymerase (PARP) inhibitors have yielded encouraging responses in high-grade serous ovarian carcinomas (HGSOCs), but the optimal treatment setting remains unknown. We assessed the effect of niraparib on HGSOC patient-derived xenograft (PDX) models as well as the relationship between certain markers of homologous recombination (HR) status, including BRCA1/2 mutations and formation of RAD51 foci after DNA damage, and response of these PDXs to niraparib in vivo.
There are a number of other PARP inhibitors in late phase clinical development in ovarian cancer including rucaparib, niraparib, veliparib, and talazoparib. 
Background Niraparib is an oral poly(adenosine diphosphate [ADP]-ribose) polymerase (PARP) 1/2 inhibitor that has shown clinical activity in patients with ovarian cancer.
Treatment with the PARP inhibitor, niraparib, sensitizes colorectal cancer cell lines to irinotecan regardless of MSI/MSS status.
METHODS: Here we examine the response to niraparib, a potent PARP-1/PARP-2 inhibitor currently under clinical evaluation, in MSI versus microsatellite stable (MSS) CRC cell lines in vitro and in vivo. 
Several clinical-stage PARP inhibitors, including veliparib, rucaparib, olaparib, niraparib, and talazoparib, have been evaluated for their PARP-trapping activity. 
Niraparib: A Poly(ADP-ribose) Polymerase (PARP) Inhibitor for the Treatment of Tumors with Defective Homologous Recombination.
We describe the rationale for this approach and the design and discovery of niraparib, a potent PARP-1/2 inhibitor with good cell based activity, selectivity for cancer over normal cells, and oral bioavailability. 
Niraparib (50 mg/kg) inhibited PARP1 activity in vivo and extended survival of mice with orthotopic pHGA xenografts, when administered before IR (20 Gy, fractionated), relative to control mice (40 vs. 25 days)
Niraparib (MK-4827), a novel poly(ADP-Ribose) polymerase inhibitor, radiosensitizes human lung and breast cancer cells.
The poly(ADP-ribose) polymerase inhibitor niraparib (MK4827) in BRCA mutation carriers and patients with sporadic cancer: a phase 1 dose-escalation trial.
Niraparib was characterized in a number of preclinical models before moving to phase I clinical trials, where it showed excellent human pharmacokinetics suitable for once a day oral dosing, achieved its pharmacodynamic target for PARP inhibition, and had promising activity in cancer patients. 
The aim of this study was to assess niraparib (MK-4827), a novel poly(ADP-Ribose) polymerase (PARP) inhibitor, for its ability to radiosensitize human tumor cells. 
Niraparib (MK4827) is an oral potent, selective PARP-1 and PARP-2 inhibitor that induces synthetic lethality in preclinical tumour models with loss of BRCA and PTEN function. 
Moreover, the potency in trapping PARP differed markedly among inhibitors with niraparib (MK-4827) > olaparib (AZD-2281) >> veliparib (ABT-888), a pattern not correlated with the catalytic inhibitory properties for each drug. 
Several clinical-stage PARP inhibitors, including veliparib, rucaparib, olaparib, niraparib, and talazoparib, have been evaluated for their PARP-trapping activity.
These results indicate that human tumor cells are significantly radiosensitized by the potent and selective PARP-1 inhibitor, niraparib, in the in vitro setting.
Niraparib (MK4827) is an oral potent, selective PARP-1 and PARP-2 inhibitor that induces synthetic lethality in preclinical tumour models with loss of BRCA and PTEN function.
Moreover, the potency in trapping PARP differed markedly among inhibitors with niraparib (MK-4827)>olaparib (AZD-2281)>>veliparib (ABT-888), a pattern not correlated with the catalytic inhibitory properties for each drug.
Niraparib (50 mg/kg) inhibited PARP1 activity in vivo and extended survival of mice with orthotopic pHGA xenografts, when administered before IR (20 Gy, fractionated), relative to control mice (40 vs.
Niraparib: A Poly(ADP-ribose) Polymerase (PARP) Inhibitor for the Treatment of Tumors with Defective Homologous Recombination.
Treatment with the PARP inhibitor, niraparib, sensitizes colorectal cancer cell lines to irinotecan regardless of MSI/MSS status.
Other PARP inhibitors under clinical trials include rucaparib, niraparib, veliparib, and the "PARP-trapping" BMN-673.
The high-throughput screens identified multiple clinical poly (ADP-ribose) polymerase 1 and 2 (PARP1/2) inhibitors, such as olaparib (AZD-2281), niraparib (MK-4827) and BMN 673, as being selective for ERCC1 deficiency.
Inhibition of Poly(ADP-ribose) Polymerase1 (PARP1) impairs DNA damage repair, and early generation PARP1/2 inhibitors (olaparib, niraparib, etc.) have demonstrated clinical proof of concept for cancer treatment.
Moreover, the potency in trapping PARP differed markedly among inhibitors with niraparib (MK-4827) > olaparib (AZD-2281) >> veliparib (ABT-888), a pattern not correlated with the catalytic inhibitory properties for each drug.

Answer: Niraparib is a Poly(ADP-ribose) Polymerase (PARP) Inhibitor. It is used for ovarian cancer treatment.
Exact answer: Poly(ADP-ribose) Polymerase