CTNI-50. CLINICAL EFFICACY AND PK/PD RESPONSE IN A PHASE 0/2 STUDY OF NIRAPARIB PLUS RADIOTHERAPY FOR NEWLY-DIAGNOSED, MGMT-UNMETHYLATED GLIOBLASTOMA

Abstract BACKGROUND Poly (ADP-ribose) polymerase (PARP) mediates DNA damage response; niraparib is a PARP1/2-selective inhibitor. This Phase 0/2 study evaluates tumor pharmacokinetics (PK), pharmacodynamics (PD), and clinical efficacy of niraparib plus fractionated radiotherapy in newly diagnosed, O6-methylguanine methyltransferase (MGMT)-unmethylated glioblastoma (GBM). METHODS Newly-diagnosed GBM patients received 4 days of niraparib (300/200 mg QD) prior to planned resection 3-5 (cohort 1) or 8-10 hours (cohort 2) following the last dose. Tumor tissue (gadolinium-enhancing and non-enhancing regions), cerebrospinal fluid (CSF), and plasma were collected. Total and unbound drug concentrations were measured using validated LC-MS/MS methods. PARP inhibition was assessed by quantification of PAR induction after ex vivo irradiation vs non-irradiated control. A PK response was defined as unbound [niraparib] > 5-fold biochemical IC50 (19 nM) in non-enhancing tumor. Patients with MGMT-unmethylated tumors and a PK response were eligible for Phase 2 dosing of niraparib plus radiotherapy followed by niraparib monotherapy. RESULTS All Phase 0 patients (n=46) met the PK threshold. The mean unbound concentrations of niraparib in non-enhancing tumor regions were 335.1 nM (cohort 1; n=43) and 331.9 nM (cohort 2; n=3). Ex vivo PAR suppression was observed in 73% of the patients (24/33). 71.4% patients with unmethylated tumors were enrolled into Phase 2. Five patients in Phase 2 experienced Grade 4 thrombocytopenia related to niraparib and all adverse events resolved without sequelae. At a mean clinical follow-up of 13.4 months, median overall survival was 20.3 months. CONCLUSIONS Niraparib achieves pharmacologically relevant concentrations in non-enhancing, newly-diagnosed GBM tissue. Accompanying PD effects were observed in tumor tissue, no new safety signals were observed, and clinical outcomes were promising. A global Phase 3, open-label, randomized 2-arm study comparing niraparib versus temozolomide in adult patients with newly diagnosed, MGMT-unmethylated glioblastoma is underway.