A Phase I Trial of the Small Molecule Pan-Bcl-2 Family Inhibitor Obatoclax Mesylate (GX15-070) Administered by Continuous Infusion for up to Four Days to Patients with Hematological Malignancies.

Obatoclax antagonizes the BH3-binding groove of the bcl-2 family of anti apoptotic proteins. It is active in chronic lymphocytic leukemia(CLL; O’Brien et al, ASH 2005) with a recommended phase II dose of 28 mg/m² given over 3 h every 3 weeks with DLT of grade 3 infusional CNS toxicities. We evaluated prolonged infusions to minimize these toxicities while maintaining clinical activity. An initial trial established a Phase II dose of 28 mg/m² over 24 h every 2 weeks with dose limiting toxicities (DLTs) at 40 mg/m² and responses in 4/8 MDS patients (Borthakur et al., ASH 2006). We report on 21 patients who received obatoclax 20–28 mg/m²/24 h weekly (n=9) or 20 mg/m²/24 h for 2, 3 or 4 consecutive days every 2–3 weeks (n=12). The final dose level of 28 mg/m²/24 h for 4 days is now open. Median age was 61 (range 26–75) and 11 patients were male.

Diagnoses included: acute myelogenous leukemia (AML; 13), myelodysplastic syndromes (MDS; 6), acute lymphocytic leukemia (ALL; 1) and CLL (1). A total of 180 infusions of 24 h were administered. The most common adverse events (AE) were euphoric mood (57%), fatigue (57%), febrile neutropenia (43%), gait disturbance (43%), chills (33%), diarrhea (33%), nausea (33%), somnolence (33%), dizziness (29%). All were of Grades 1–2 with the exception of 7 grade 3 AEs of febrile neutropenia (unrelated to obatoclax administration) and 1 each of diarrhea, fatigue and gait disturbance. There were no DLTs. Multiple day infusions were well tolerated; for the 20 mg/m²/24 h x 4days dose level, 4/6 patients received ≥ 4 cycles, with two ongoing at this time. Plasma concentrations of obatoclax reached a steady state before end of infusion. Mean C_max values at 20 mg/m²/24 h during1x24, 2x24, 3x24,4x24 h, and 28 mg/m²/24 h during 1x24 h infusions was 15.3, 8.4, 10.1, 15.7 and 10.8 ng/mL, respectively. The mean AUC_(0–tlast) values at 20 mg/m²/24 h associated with 1x24, 2x24, 3x24, 4x24 h, and 28 mg/m²/24 h associated with 1x24 h infusions were 264.3, 396.9, 624.3, 1109.3 and 211.1 ng · hr/mL, respectively, increasing as expected with the duration of the infusion. One patient with treatment-related AML with a t(9;11)(p22;q23) translocation achieved a cytogenetic complete response (CR) with complete hematological recovery and transfusion independence on Day 9 following the start of weekly 24 h infusions of obatoclax. This CR was sustained > 8 months while the patient received a total of 35 weekly infusions of 20 mg/m² without cumulative toxicities.

Conclusions: obatoclax can be administered by prolonged infusions without eliciting novel or cumulative toxicities. The dramatic response observed in a patient with AML with immediate recovery of peripheral blood counts supports that the cytotoxic effects of obatoclax are specific to malignant cells while sparing normal bone marrow, as did our earlier reports of improvement in cytopenias in patients with MDS and CLL. The infusional schedules described here will be attractive to combine with standard chemotherapy regimens for AML and other indications to improve treatment outcomes where they may be limited by overexpression of Bcl-2 family members.