Acquired BRAF V600E Mutation as Resistant Mechanism after Treatment with Osimertinib.

INTRODUCTION: AZD9291 (osimertinib) is designed for acquired T790M mutation after first- and second-generation EGFR) tyrosine kinase inhibitors have been used. Some of the resistance mechanisms that present after osimertinib treatment, including a newly acquired EGFR C797S mutation, have been identified. It is unclear, however, whether the bypass pathway is also a mechanism of resistance in patients after osimertinib treatment. METHODS: Cells from malignant pleural effusion were collected and cultured at the time of progression in a patient being treated with osimertinib. Tumor genotyping was done by matrix-assisted laser desorption ionization-time of flight mass spectrometry. EGFR, AKT, MEK, and ERK phosphorylation were determined. An anchorage-dependent colony formation assay was used for drug sensitivity. RESULTS: An acquired mutation, BRAF V600E, was found in the patient at the time of progression while being treated with osimertinib. Cells grown from malignant pleural effusion were sensitive to BRAF V600E inhibitor and were more vulnerable to a combination treatment with osimertinib. CONCLUSIONS: A potential mechanism of acquired resistance to osimertinib in patients with T790M is through the BRAF pathway. Simultaneous blockade of the BRAF and EGFR had a significant inhibitory effect.