Sharyn D. Baker

Purpose: To evaluate the safety, activity, and emergence of FLT3-kinase domain (KD) mutations with combination therapy of crenolanib and sorafenib in acute myeloid leukemia (AML) with FLT3-internal tandem duplication (ITD). Patients and Methods: After in vitro and xenograft efficacy studies using AML cell lines that have FLT3-ITD with or without FLT3-KD mutation, a pilot study was performed with crenolanib (67 mg/m2/dose, three times per day on days 1–28) and two dose levels of sorafenib (150 and 200 mg/m2/day on days 8–28) in 9 pediatric patients with refractory/relapsed FLT3-ITD–positive AML. Pharmacokinetic, pharmacodynamic, and FLT3-KD mutation analysis were done in both preclinical and clinical studies. Results: The combination of crenolanib and sorafenib in preclinical models showed synergy without affecting pharmacokinetics of each agent, inhibited p-STAT5 and p-ERK for up to 8 hours, and led to significantly better leukemia response (P < 0.005) and survival (P < 0.05) compared with single agents. Fewer FLT3-KD mutations emerged with dose-intensive crenolanib (twice daily) and low-intensity sorafenib (three times/week) compared with daily crenolanib or sorafenib (P < 0.05). The crenolanib and sorafenib combination was tolerable without dose-limiting toxicities, and three complete remissions (one with incomplete count recovery) and one partial remission were observed in 8 evaluable patients. Median crenolanib apparent clearance showed a nonsignificant decrease during treatment (45.0, 40.5, and 20.3 L/hour/m2 on days 1, 7, and 14, respectively) without drug–drug interaction. Only 1 patient developed a FLT3-KD mutation (FLT3 F691L). Conclusions: The combination of crenolanib and sorafenib was tolerable with antileukemic activities and rare emergence of FLT3-TKD mutations, which warrants further investigation.