Phase I first-in-man trial of a novel bromodomain and extra-terminal domain (BET) inhibitor (BI 894999) in patients (Pts) with advanced solid tumors.

2504 Background: The BET family (BRD2, BRD3, BRD4) regulates transcription, epigenetic memory and cell growth, emerging as a novel therapeutic strategy. BI 894999 is a highly potent and selective orally available BET inhibitor. Methods: BI 894999 was given once a day, continuously (1 cycle = 3 weeks; Arm A). An intermittent schedule was explored: once a day, D1-14 Q21 days (1 cycle = 3 weeks; Arm B). Bayesian Logistic Regression Model was used to guide dose escalation. HEXIM1, HIST2H2BF and CCR2 gene expressions were used as pharmacodynamic (PD) markers. Results: 28 pts were treated: 21 in Arm A, 7 in Arm B. Median number of cycles was 2 (range: 1-12). Pts were treated at 6 dose levels in Arm A (0.2-5 mg) and 2 dose levels in Arm B (1.5 and 2 mg). The maximum tolerated dose (MTD) was exceeded at 2 mg in Arm A. In Arm B, dose escalation was halted due to the observation of ECG changes in 3 pts and raised serum troponin in 8 pts, pending cardiology review. MTD in Arm A was defined as 1.5 mg. The most frequent (≥10%) treatment-related adverse events were: fatigue (50%), thrombocytopenia (29%), decreased appetite (21%), diarrhea (18%), increased troponin T (18%), dysgeusia (14%), nausea (14%), stomatitis (14%), increased CK (11%), neutropenia (11%) and vomiting (11%). DLTs included: thrombocytopenia grade (G) 4 (n=3), increased troponin G3 (n=1), hypophosphatemia G3 (n=1), multiple G2 events in 1 pt preventing adequate dose intensity in cycle 1. Of 27 evaluable pts, 3 had partial response (PR; one was confirmed) and 1 had stable disease (SD) lasting >4 cycles. C max and AUC increased with dose in a greater than linear fashion particularly at higher dose levels. Mean terminal T 1/2 was ~1 day with high interpatient variability. PD analyses showed target engagement in all 3 genes at doses ≥1 mg in both schedules. Conclusions: BI 894999 showed target engagement at doses ≥ 1 mg and demonstrated clinical activity (3 PRs and 1 SD lasting >4 cycles). Thrombocytopenia prevented continuous dosing and 1.5 mg was defined the MTD for Arm A, whilst dose escalation was halted in Arm B due to cardiac findings. Mitigating hematological toxicity of BI 894999 via synergistic drug combinations should be explored. Clinical trial information: NCT02516553.