CTNI-58. A PHASE 0/2 TRIAL OF ABEMACICLIB PLUS LY3214996 IN RECURRENT GLIOBLASTOMA

Abstract BACKGROUND This Phase 0/2 study (NCT04391595) evaluates tumor pharmacokinetics (PK), tumor pharmacodynamics (PD), and clinical efficacy of abemaciclib, a CDK4 and 6 inhibitor, plus LY3214996, an ERK 1/2 inhibitor, in recurrent glioblastoma (rGBM). METHODS Adult rGBM patients (n=42) with intact RB, >30% pERK expression, and CDKN2A/B deletion or CDK4/6 amplification received six days of abemaciclib plus LY3214996 prior to a planned resection. Time-escalation cohorts (3-5 and 7-9 hour intervals from last dose to tumor resection) identified the optimal time interval (OTI). Subsequent Phase 0 patients had tumor tissue (gadolinium [Gd]-enhancing and nonenhancing regions), cerebrospinal fluid (CSF), and plasma collected at the OTI. Total and unbound drug concentrations were measured using validated LC-MS/MS methods. Tumor PD was compared to matched archival or pre-treatment biopsied tissue. A PK ‘trigger’ (i.e., unbound concentration > 5 x IC50) was set for each drug. Gd-nonenhancing tumor demonstrating PK response to both drugs qualified patients for the Phase 2 component of the study. RESULTS Forty two patients were accrued to Phase 0 and no dose-limiting toxicities were observed. Median unbound concentrations in Gd-nonenhancing tumor regions were 31 nM and 10 nM for abemaciclib and LY3214996, respectively. Tumor RB, RSK phosphorylation, and tumor proliferation (MIB-1) decreased in (47.5%), (25%) and (72.5%) patients, respectively. Nine of the 40 (22.5%) patients exceeded PK thresholds for both abemaciclib (12 nM) and LY3214996 (25 nM), qualifying them for Phase 2. Here, median PFS and median OS were 2.4 and 7.2 months, respectively. CONCLUSION Abemaciclib and LY3214996 achieved pharmacologically-relevant concentrations in Gd-non-enhancing GBM tissue in 80% and 30% of the patients, respectively. Suppression of the RB pathway and tumor proliferation, but not ERK pathway was observed. Inadequate CNS penetration and tumor PD modulation by LY3214996 contributed to the unremarkable clinical efficacy profile of this drug combination. Targeted ERK inhibition remains an unmet need in experimental GBM therapy.