Recurrent Mutations in Cyclin D3 Confer Clinical Resistance to FLT3 Inhibitors in Acute Myeloid Leukemia.

Purpose: Biomarkers of response and resistance to FLT3 tyrosine kinase inhibitors (TKI) are still emerging, and optimal clinical combinations remain unclear. The purpose of this study is to identify co-occurring mutations that influence clinical response to the novel FLT3 inhibitor pexidartinib (PLX3397).

Experimental Design: We performed targeted sequencing of pretreatment blasts from 29 patients with internal tandem duplication (ITD) mutations treated on the phase I/II trial of pexidartinib in relapsed/refractory -ITD+ acute myeloid leukemia (AML). We sequenced 37 samples from 29 patients with available material, including 8 responders and 21 non-responders treated at or above the recommended phase II dose of 3,000 mg.

Results: Consistent with other studies, we identified mutations in , and a variety of epigenetic and transcriptional regulators only in non-responders. Among the most frequently mutated genes in non-responders was Cyclin D3 (. A total of 3 individual mutations in (Q276*, S264R, and T283A) were identified in 2 of 21 non-responders (one patient had both Q276* and S264R). No mutations were found in pexidartinib responders. Expression of the Q276* and T283A mutations in -ITD MV4;11 cells conferred resistance to apoptosis, decreased cell-cycle arrest, and increased proliferation in the presence of pexidartinib and other FLT3 inhibitors. Inhibition of CDK4/6 activity in mutant MV4;11 cells restored pexidartinib-induced cell-cycle arrest but not apoptosis.

Conclusions: Mutations in , a gene not commonly mutated in AML, are a novel cause of clinical primary resistance to FLT3 inhibitors in AML and may have sensitivity to CDK4/6 inhibition.