Background: Poly (ADP-ribose) polymerase (PARP) mediates DNA damage response; niraparib is an investigational PARP1/2-selective inhibitor. This Phase 0 study evaluates newly-diagnosed glioblastoma (GBM) tumor pharmacokinetics (PK) and pharmacodynamics (PD), graduating patients with O6-methylguanine methyltransferase (MGMT) unmethylated tumors into a therapeutic regimen of niraparib plus fractionated radiotherapy when high unbound drug concentrations are present in gadolinium-nonenhancing tumor. Methods: Patients with presumed newly-diagnosed GBM were enrolled in a phase 0 study receiving 4 days of niraparib (300/200 mg QD) prior to planned resection 3-5 or 8-12 hours following the last dose. Tumor tissue (enhancing and non-enhancing regions), cerebrospinal fluid (CSF), and plasma were collected. Total and unbound niraparib concentrations were measured using validated LC-MS/MS methods. PARP inhibition was assessed by quantification of PAR induction after 10 Gy ex vivo irradiation in surgical tissue compared to non-irradiated control tissue. A PK ‘trigger’ determined eligibility for the therapeutic expansion phase and was defined as unbound [niraparib] > 5-fold biochemical IC50 (i.e., 19 nM) in non-enhancing tumor. Patients with MGMT unmethylated tumors exceeding this PK threshold were eligible for expansion phase dosing of niraparib plus radiotherapy followed by a maintenance phase of niraparib. Results: All patients (n=35) enrolled in the phase 0 portion of the study met the PK threshold. In non-enhancing tumor regions, the mean unbound concentration of niraparib was 253.2 nM in 32 evaluable GBM patients. The suppression of PAR levels after ex vivo radiation was observed in 75% of the patients (18/24). Eleven out of 18 patients with unmethylated tumors enrolled in phase 2. Five of the 6 initial patients enrolled in phase 2 experienced thrombocytopenia related to niraparib, and 3/5 cases were deemed serious and life-threatening. Consequently, starting dose in both phases was lowered to 200 mg, and no serious AEs were observed thereafter. At a median follow-up of 8.1 months [range: 6.0-12.9 months], PFS6 was 64% (n=11) with 4 patients remaining on treatment and 5 patients ongoing survival follow-up. Conclusions: Niraparib achieves pharmacologically-relevant concentrations in non-enhancing, newly-diagnosed GBM tissue in excess of any other studied PARP inhibitor. Clinical trial information: NCT05076513.