Establishment and characterization of an EGFR-mutant NSCLC XPDX model representing first-line osimertinib resistance through an acquired TRIM24-BRAF fusion

Background: Osimertinib is a third generation EGFR inhibitor approved as first-line treatment for patients with exon 19 or specific exon 21 variants. While this therapy has proven efficacy, resistance to treatment inevitably occurs in patients. One reported resistance mechanism following first-line osimertinib treatment is an acquired TRIM24-BRAF fusion. While molecular studies to identify this and other genomic alterations have been performed, no actionable in vivo model of this resistance mechanism has been widely available for testing. To this end we have established a non-small cell lung XenoSTART Patient-Derived Xenograft (XPDX) model designated ST5570 from a patient post first-line osimertinib which harbors an EGFR exon 19 deletion and TRIM24-BRAF fusion. This model was characterized for receptor expression, genomic alterations, and in vivo drug sensitivities toward multiple chemotherapies and targeted agents. Methods: ST5570 was established from a biopsy of a recurrent lung lesion collected from a 70-year-old African-American non-smoker female following two years of osimertinib treatment. The resulting model was passaged and characterized using immunohistochemistry and genomic analysis including WES and RNAseq. In vivo studies were performed testing various chemotherapy and targeted agents; study endpoints included tumor volume and time from treatment initiation with %T/C values and tumor regression reported at study completion; a %T/C of ≤20 versus control was considered sensitive. Tumor regression (%T/C < 0) versus Day 0 tumor volume was also reported. Results: Immunohistochemistry analysis of ST5570 confirmed disease morphologically similar to the patient’s archival clinical sample and a distinct profile for various receptor and signaling targets. WES analysis confirmed the exon 19 deletion at aa746–750, while RNA sequencing revealed a TRIM24-BRAF gene fusion, independently confirmed using molecular studies. In vivo, ST5570 was resistant to weekly cisplatin, paclitaxel or combination. Oral EGFR inhibitors osimertinib, afatinib or erlotinib were ineffective as was twice weekly cetuximab injections. Dabrafenib or encorafenib treatment was ineffective; however single agent trametinib resulted in tumor regressions. Conclusion: We have established and characterized an EGFR-mutant NSCLC XPDX model with an acquired TRIM24-BRAF fusion sensitive in vivo to the MEK inhibitor trametinib but not BRAF or EGFR inhibitors. This model is a valuable tool to study osimertinib-resistant lung cancer. No conflict of interest.