Cancer Therapy: Clinical Emergence of Polyclonal FLT3 Tyrosine Kinase Domain Mutations during Sequential Therapy with Sorafenib and Sunitinib in FLT3-ITD–Positive Acute Myeloid Leukemia

Purpose: To evaluate the clinical activity of sequential therapy with sorafenib and sunitinib in FMS-like tyrosine kinase 3 (FLT3)-internal tandem duplication (ITD)-positive acute myelogenous leukemia (AML) andmonitor the emergence of secondary FLT3 tyrosine kinase domain (TKD)mutations during treatment. Experimental Design: Six children with relapsed/refractory AML were treated with sorafenib in combination with clofarabine and cytarabine, followed by single-agent sorafenib if not a candidate for transplantation. Sunitinib was initiated after sorafenib relapse. Bone marrow samples were obtained for assessment of FLT3 TKDmutations by deep amplicon sequencing. The phase of secondary mutations with ITD alleles was assessed by cloning and sequencing of FLT3 exons 14 through 20. Identifiedmutations were modeled in Ba/F3 cells, and the effect of kinase inhibitors on FLT3 signaling and cell viability was assessed. Results: Four patients achieved complete remission, but 3 receivingmaintenance therapy with sorafenib relapsed after 14 to 37 weeks. Sunitinib reduced circulating blasts in two patients andmarrow blasts in one. Two patients did not respond to sorafenib combination therapy or sunitinib. FLT3 mutations at residues D835 and F691 were observed in sorafenib resistance samples on both ITD-positive and -negative alleles. Deep sequencing revealed low-level mutations and their evolution during sorafenib treatment. Sunitinib suppressed leukemic clones with D835H and F691Lmutations, but not D835Y. Cells expressing sorafenibresistant FLT3 mutations were sensitive to sunitinib in vitro. Conclusions: Sunitinibhas activity inpatients that are resistant to sorafenib andharbor secondary FLT3TKD mutations. The use of sensitive methods to monitor FLT3 mutations during therapy may allow individualized treatment with the currently available kinase inhibitors. Clin Cancer Res; 19(20); 5758–68. 2013 AACR.