Hypoxia-induced upregulation of BMX kinase mediates therapeutic resistance in acute myeloid leukemia.

Oncogenic addiction to the Fms-like tyrosine kinase 3 (FLT3) is a hallmark of acute myeloid leukemia (AML) that harbors the FLT3-internal tandem duplication (FLT3-ITD) mutation. While FLT3 inhibitors like sorafenib show initial therapeutic efficacy, resistance rapidly develops through mechanisms that are incompletely understood. Here, we used RNA-Seq-based analysis of patient leukemic cells and found that upregulation of the Tec family kinase BMX occurs during sorafenib resistance. This upregulation was recapitulated in an in vivo murine FLT3-ITD-positive (FLT3-ITD+) model of sorafenib resistance. Mechanistically, the antiangiogenic effects of sorafenib led to increased bone marrow hypoxia, which contributed to HIF-dependent BMX upregulation. In in vitro experiments, hypoxia-dependent BMX upregulation was observed in both AML and non-AML cell lines. Functional studies in human FLT3-ITD+ cell lines showed that BMX is part of a compensatory signaling mechanism that promotes AML cell survival during FLT3 inhibition. Taken together, our results demonstrate that hypoxia-dependent upregulation of BMX contributes to therapeutic resistance through a compensatory prosurvival signaling mechanism. These results also reveal the role of off-target drug effects on tumor microenvironment and development of acquired drug resistance. We propose that the bone marrow niche can be altered by anticancer therapeutics, resulting in drug resistance through cell-nonautonomous microenvironment-dependent effects.

Phase I pharmacokinetic and pharmacodynamic study of the multikinase inhibitor sorafenib in combination with clofarabine and cytarabine in pediatric relapsed/refractory leukemia.

PURPOSE: To assess the toxicity, pharmacokinetics, and pharmacodynamics of multikinase inhibitor sorafenib in combination with clofarabine and cytarabine in children with relapsed/refractory leukemia. PATIENTS AND METHODS: Twelve patients with acute leukemia (11 with acute myeloid leukemia [AML]) received sorafenib on days 1 to 7 and then concurrently with cytarabine (1 g/m(2)) and clofarabine (stratum one: 40 mg/m(2), n = 10; stratum two [recent transplantation or fungal infection]: 20 mg/m(2), n = 2) on days 8 to 12. Sorafenib was continued until day 28 if tolerated. Two sorafenib dose levels (200 mg/m(2) and 150 mg/m(2) twice daily) were planned. Sorafenib pharmacokinetic and pharmacodynamic studies were performed on days 7 and 8. RESULTS: At sorafenib 200 mg/m(2), two of four patients in stratum one and one of two patients in stratum two had grade 3 hand-foot skin reaction and/or rash as dose-limiting toxicities (DLTs). No DLTs were observed in six patients in stratum one at sorafenib 150 mg/m(2). Sorafenib inhibited the phosphorylation of AKT, S6 ribosomal protein, and 4E-BP1 in leukemia cells. The rate of sorafenib conversion to its metabolite sorafenib N-oxide was high (mean, 33%; range, 17% to 69%). In vitro, the N-oxide potently inhibited FLT3-internal tandem duplication (ITD; binding constant, 70 nmol/L) and the viability of five AML cell lines. On day 8, sorafenib decreased blast percentages in 10 of 12 patients (median, 66%; range, 9% to 95%). After combination chemotherapy, six patients (three FLT3-ITD and three FLT3 wild-type AML) achieved complete remission, two (both FLT3-ITD AML) had complete remission with incomplete blood count recovery, and one (FLT3 wild-type AML) had partial remission. CONCLUSION: Sorafenib in combination with clofarabine and cytarabine is tolerable and shows activity in relapsed/refractory pediatric AML.