CTNI-48. A PHASE 0 ‘TRIGGER’ TRIAL OF CDK4/6 PLUS ERK1/2 INHIBITORS IN RECURRENT GLIOBLASTOMA

Abstract

Abstract BACKGROUND This dual-drug Phase 0 study (NCT04391595) evaluates the tumor pharmacokinetics (PK) and tumor pharmacodynamics (PD) of abemaciclib, a selective CDK4/6-inhibitor, plus LY3214996, a selective ERK1/2 inhibitor, in recurrent GBM patients. METHODS Adult recurrent GBM patients (n=10) with intact RB expression, > 30% pERK expression, and CDKN2A/B deletion or CDK4/6 amplification received six days of abemaciclib (150mg BID) plus LY3214996 (200mg QD) prior to a planned resection at 3-5 or 7-9 hour time interval after the final drug dose in a Time-Escalation Arm. Tumor tissue (gadolinium [Gd]-enhancing and nonenhancing regions), cerebrospinal fluid (CSF), and plasma were collected. Total and unbound drug concentrations were measured using validated LC-MS/MS methods. Tumor PD effects, including RB and RSK phosphorylation, were compared to matched archival or pre-treatment biopsied tissue. A PK ‘trigger’ (i.e., unbound concentration > 5x biochemical IC50) was set for each drug. Gd-nonenhancing tumor tissue exhibiting abemaciclib and LY3214996 concentrations in excess of their trigger threshold qualified patients for postoperative dual-drug therapy. RESULTS No dose-limiting toxicities were observed. In Gd-nonenhancing tumor regions, median unbound concentrations of abemaciclib were 31.2 nM (3-5 hour cohort) and 25.1 nM (7-9 hour cohort) while median unbound concentrations of LY3214996 were 52.0 nM (3-5 hour cohort) and 10.2 nM (7-9 hour cohort). Tumor RB and RSK phosphorylation decreased in 6/10 and 2/10 patients, respectively. Tumor proliferation (MIB-1) was decreased in 8/10 patients. 5/10 patients exceeded PK thresholds for both abemaciclib (12 nM) and LY3214996 (25 nM), thereby entering the study’s therapeutic expansion phase. CONCLUSION Abemaciclib and LY3214996 achieve pharmacologically-relevant concentrations in Gd-non-enhancing GBM tissue and are associated with suppression of RB pathway and tumor proliferation. The Optimal Time Interval (OTI) for tissue sampling was 3-5 hours after the final drug dose. Based on this interim analysis, the trial will accrue an additional 25 patients at this OTI.