Dose Escalation Study of BET Inhibitor PLX2853 in Patients with Relapsed or Refractory Acute Myeloid Leukemia or High Risk Myelodysplastic Syndrome

Background: PLX2853 is an orally available, non-benzodiazepine BET (bromodomain and extraterminal domain) inhibitor that exhibits low nanomolar potency and a modest preference for binding to the second bromodomain (BD2) of the BET proteins. By regulating genes (e.g. MYC and BCL2) critical to leukemic cell growth and survival, PLX2853 demonstrated broad anti-leukemic activity both as a single agent and in combination with other therapeutic agents in preclinical models. The pharmacokinetic (PK) profile in solid tumor patients revealed a short half-life (< 3 hour) enabling high peak plasma concentrations and nearly complete elimination from the plasma 9 hour post dose. Since strong and prolonged suppression of BET proteins have often untoward effects in normal tissues, the PLX2853 PK profile is hypothesized to be associated with improved tolerability by allowing transient target engagement followed by time for recovery after daily dosing.

Methods: We are conducting an open-label, Phase 1b (Ph1b) study of PLX2853 as a single oral agent administered daily in adult patients with relapsed or refractory acute myeloid leukemia (AML) or high risk myelodysplastic syndrome (MDS) using a modified continuous reassessment model (mCRM) with escalation with overdose control (EWOC) to determine the recommended phase 2 dose (RP2D). Up to 36 patients are expected to enroll. The dosing cycle and dose limiting toxicity window (DLT) is 21 days. Primary objectives include safety and PK. Secondary objectives include measures of preliminary efficacy, and exploratory objectives include pharmacodynamics (PD) biomarker assessments in various tissues. Enrollment through Cohort 2 (40 mg QD) is ongoing as of July 2019.

Results: Five subjects with relapsed or refractory AML (median age 65 years) have received PLX2853 in escalating doses from 20 to 40 mg QD. Among these first 5 patients treated, the most common treatment emergent adverse events (AEs) regardless of causality in > 1 patient: decreased appetite (n=3), nausea (n=2), diarrhea (n=2), peripheral edema (n=2), cough (n=2), oropharyngeal pain (n=2), blood bilirubin increase (n=2), anemia (n=2), febrile neutropenia (n=2), fatigue (n=2), bacteremia (n=2), headache (n=2), dyspnea (n=2), and hypertension (n=2). Most were grade (G) 1-2. Treatment emergent AEs > G2 in > 1 patient included: anemia (n=2), febrile neutropenia (n=2) and hypertension (n=2). No treatment-related serious AEs or DLTs have been observed. Following a 20 mg daily dose of PLX2853, median time to reach maximal plasma concentrations (T_max) is 1 hour and the absorption half-life (T_1/2) is < 3 hours.

Conclusions: In an ongoing Ph1b study, PLX2853 has now completed its first dosing cohort for patients with relapsed or refractory AML or high risk MDS, and no DLT has been observed yet. As dose escalation continues, PK, PD, preliminary safety and efficacy data will be assessed further to determine the clinical significance of target engagement. This clinical trial is registered at clinicaltrials.gov: NCT03787498.