Phase 1 Study of CB-839, a First-in-Class, Orally Administered Small Molecule Inhibitor of Glutaminase in Patients with Relapsed/Refractory Leukemia

Background: Malignant cells alter metabolism in order to enable their highly anabolic state. In addition to a massive increase in glycolysis, neoplastic cells frequently depend on glutamine to feed the TCA cycle and provide key building blocks for cell growth and proliferation. CB-839 is a first-in-class potent and selective inhibitor of the glutaminase (GLS), the first step in glutamine metabolism, and has broad in vitro and in vivo anti-tumor activity in solid and heme malignancies. Acute myeloid leukemia (AML) cells are particularly dependent on glutamine, and treatment of AML cell lines and primary AML cells with CB-839 results in growth arrest and/or cell death (Padre et al., ASH, 2014 and Jacque et al., Blood, 2015). CB-839 causes a decrease in glutamate and glutamate-derived metabolites, such as TCA cycle intermediates and glutathione, crippling the proliferative capacity of these cells and in some cases inducing apoptosis.

Methods: CX-839-003 is an ongoing Phase 1 (Ph1) trial of CB-839 in patients with relapsed/refractory hematologic malignancies. This study includes a dose escalation cohort to identify a recommended Phase 2 dose (RP2D), followed by expansion cohorts of patients (pts) with relapsed/refractory AML (IDH wt and mutant), acute lymphocytic leukemia, and IDH-mutant myelodysplastic syndrome (MDS). Pharmacokinetics (PK) is being monitored on Days 1 and 15.

Results: Safety data are available for a total of 20 patients (18 AML, 1 ALL, and 1 mixed-lineage leukemia) that have enrolled across the dose escalation and expansion cohorts. These patients have received a median of 3 prior lines of systemic induction therapy, with 5 pts (25%) having received prior allogeneic bone marrow transplantation. Patients' median age was 75 (range 35 to 84). Dose levels from 100-1000 mg TID were explored during the dose escalation. CB-839 has been well tolerated to date, with no DLTs occurring on study. One patient discontinued due to an adverse event (AE) of elevated transaminases, and 5 pts (25%) had G3/4 AEs considered possibly related to study drug. Across three Ph1 studies for CB-839 in solid tumors and heme malignancies, reversible asymptomatic elevations in transaminases have been the primary G3 AE. Although initially CB-839 was given three times daily (TID) without food, the drug is now being given twice daily (BID) with meals based on PK and safety data generated across these 3 studies. Liver abnormalities occurred primarily on the TID schedule in 6/59 (10.2%) pts, but in only one of 60 pts (1.7%) receiving the BID regimen. Pts are currently being enrolled in expansion cohorts at the RP2D, 600 mg BID. Inhibition of GLS activity was evaluated across all 3 Ph1 studies and established clear exposure-dependent inhibition of the target in peripheral blood 4 hr after the first dose of CB-839, with >90% inhibition being maintained for most patients at the RP2D. Preliminary evidence of clinical activity has been noted, including reductions in blast counts in the bone marrow and/or periphery in 2 pts, one of whom achieved a CRi and remains on study for more than one year. 5 (31%) of 16 evaluable pts have remained on study for at least 4 cycles (range 4-17). Additional data from >8 additional patients have recently started on the "BID with meals" regimen will be presented. Correlative studies on clinical samples will also be presented.

Conclusions: CB-839 has been well tolerated at and above doses that produced robust inhibition of GLS in blood platelets and in tumors. Dosing BID with food has mitigated the frequency and severity of LFT elevations, which has been the primary safety signal to date. Reductions in marrow and peripheral blast counts in a heavily pretreated acute leukemia population, with a significant number of patients tolerating therapy and remaining on study for a prolonged period, provide the rationale for continued evaluation of CB-839 as a single agent, and in combination with azacitidine, in AML patients.