Phase 1b Study of BET Inhibitor PLX2853 in Patients with Relapsed or Refractory Acute Myeloid Leukemia or High Risk Myelodysplastic Syndrome

Background: Relapsed and refractory myeloid malignancies including myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML) remain a clinical challenge due to high morbidity and mortality as well as a paucity of effective therapeutic agents. PLX2853 is an orally available, non-benzodiazepine bromodomain and extraterminal domain (BET) 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 has demonstrated broad anti-leukemic activity both as a single agent and in combination with other therapeutic agents in preclinical models. The pharmacokinetic (PK) profiles in patients with AML or MDS revealed high peak plasma concentrations, a short terminal half-life (T_1/2 < 3 hour), and nearly complete elimination from the plasma by 9 hour post dose. This PK profile is hypothesized to enable improved tolerability by allowing transient target engagement followed by time for recovery after daily dosing.

Methods: This is an open-label, Phase 1b (Ph1b) study of PLX2853 as a single oral agent administered daily for 21 day cycles in adult patients with relapsed or refractory AML or high risk MDS. A modified continuous reassessment model with escalation with overdose control is employed to determine the recommended phase 2 dose (RP2D). Primary objectives include safety and PK. Secondary objectives include measures of preliminary efficacy, and exploratory objectives include pharmacodynamics (PD) biomarker assessments. Enrollment through Cohort 5 (140 mg QD) is ongoing as of July 2020.

Results: 9 subjects with relapsed or refractory AML and 7 subjects with high risk MDS (median age 65.7 years, range 47-77 years old; median number of prior therapies 2.3, range 1-5 prior therapies) have received PLX2853 in escalating doses from 20 to 140 mg QD. Through the data cut-off of 19 Jul 2020 (n=16), the most common treatment emergent adverse events (AEs) regardless of causality occurring in ≥20% of patients (n≥4) are: decreased appetite (n=9), (n=8), fatigue (N=8), hyperbilirubinemia (N=8), nausea (n=7), constipation (n=6), hypokalemia (n=6), leukopenia (n=6), vomiting (n=5), international normalised ratio increased (n=5), proteinuria (n=5), and dyspnea (n=5). Most were grade (G) 1-2. Treatment emergent AEs > G2 in > 1 patients are: anemia (n=7), leukopenia (n=5) hyperbilirubinemia (n=4), febrile neutropenia (n=4), sepsis (n=3), thrombocytopenia (n=3), hypertension (n=2), lymphocyte count decreased (n=3), and neutrophil count decreased (n=3). No dose limiting toxicity (DLT) has been observed. Following a daily dose of PLX2853 the median time to reach maximal plasma concentration (T_max) is 1 hour, and no accumulation observed at steady state, which is consistent with the short T_1/2 (< 3 hours). Dose-dependent increases in exposures were observed across the dose range tested (20-80 mg daily). Fifteen of 16 subjects have completed at least 1 cycle of treatment. The following best overall responses have been observed: 1 subject with a confirmed marrow complete remission, 1 subject with a partial remission (myeloid sarcoma), 10 subjects with stable disease, 1 subject with progressive disease, and 3 subjects not evaluable.

Conclusions: In an ongoing Ph1b study of PLX2853, four cohorts have been completed with continuous, once daily dosing in patients with relapsed or refractory AML or high risk MDS, and no DLTs have been observed yet. As dose escalation continues, PK, PD, preliminary safety, and efficacy data will be reviewed further to determine the clinical significance of this BET inhibitor and identify the RP2D. This clinical trial is registered at clinicaltrials.gov: NCT03787498.