A phase 0/2 trigger trial of niraparib in patients with newly diagnosed glioblastoma

Abstract 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 PK threshold is met. METHODS Presumed newly-diagnosed GBM patients received 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 ex vivo irradiation of surgical vs non-irradiated 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 and positive PK were eligible for phase 2 dosing of niraparib plus radiotherapy followed by a maintenance phase of niraparib. RESULTS All phase 0 patients (n = 44) met the PK threshold. In non-enhancing tumor regions, the mean unbound concentration of niraparib was 226.7 nM (n = 38). PAR suppression after ex vivo radiation was observed in 71% of the patients (20/28). Eighteen out of 26 patients with unmethylated tumors enrolled in phase 2. Five of the 18 initial patients in phase 2 experienced grade 4 thrombocytopenia related to niraparib. All adverse events were resolved without sequelae. For patients who completed at least 6 months of follow-up, PFS6 was 64% (n = 11) with 9.2-month median follow-up. Median follow-up for all patients (n = 18) was 4.2 months with 8 patients remaining on treatment and 2 patients ongoing PFS6 follow-up. CONCLUSIONS Niraparib achieves pharmacologically-relevant concentrations in non-enhancing, newly-diagnosed GBM tissue in excess of any other studied PARP inhibitor.