A Phase II Trial of Bexarotene, a Retinoid X Receptor Agonist, in Non-M3 Acute Myeloid Leukemia

In vitro, bexarotene inhibits the proliferation of non-M3 AML cell lines and induces differentiation of leukemic blasts. Our previous phase I study in non-M3 AML showed evidence of leukemic response as manifested by reduction in bone marrow blast counts (15% response rate), improved platelet counts (41%) and improved neutrophil counts (26%). Based on these results, a phase II trial in non-M3 AML was initiated at the phase I MTD. In the current phase II trial, bexarotene (300mg/m²) was administered daily as monotherapy until disease progression or unacceptable side effects occurred. Fourteen patients have been enrolled: 8M/6F, median age 74 (range 20–83), 9 secondary AML (MDS or prior chemotherapy), 9 primary refractory or relapsed < 1 year after induction, 5 no prior induction chemotherapy, 5 requiring hydroxyurea at the time of enrollment for leukemic blast control, 4 prior allogeneic stem cell transplant, 12 blood transfusion dependent, 11 platelet transfusion dependent, and 8 neutropenic. Overall, no significant adverse events were noted. All patients received prophylactic antihyperlipidemic agents and achieved good lipid control. Two patients developed mild hypothyroidism related to bexarotene. Five patients were evaluable with bone marrow biopsy at 2 months: 1 50% reduction in absolute blasts, 1 SD and 3 PD. Similar to data from our prior phase I study, evidence of clinical activity was manifested as platelet count response in 1 patient and neutrophil increases attributable to bexarotene in 2 patients. When combining the results of our phase I experience (27 patients) with our phase II data (14 patients), there is a suggestion of increased activity in patients with 5q minus abnormalities with 4/7 (57%) benefiting (2 BM response, 4 neutrophil improvements and 1 platelet response). Conversely rates of clinical benefit were lower in patients with multiple (>3) cytogenetic abnormalities (2/13), relapse after stem cell transplant (1/9) or requiring hydroxyurea for peripheral blast control at the time of study enrollment (0/10). Bexarotene is very well tolerated at the dose level studied. Early evidence for clinical activity has been seen as exemplified by improvement in platelet count, increased neutrophil counts and decreased bone marrow blasts. In summary, we conclude that bexarotene is an active agent in a subgroup of patients with AML. Study enrollment continues with amended inclusion criteria to focus on patients more likely to benefit from treatment.