An acquired CCDC6:: gene fusion as resistance mechanism for Osimertinib in exon 21 -mutated non-small cell lung cancer and its successful management with Osimertinib and Selpercatinib: a case report and review of literature.

Epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKI) are the recommended front-line therapy for treatment-naïve patients with advanced stage EGFR mutated Non-Small Cell Lung Cancer (NSCLC), with better tolerance and outcomes compared to chemotherapy. However, patients inevitably develop resistance to EGFR-TKI. The extent of progression free survival depends on intrinsic or acquired on-target/off-target mechanisms of EGFR-TKI resistance. Overcoming these acquired rearrangements remains challenging in modern precision medicine. In case of disease progression during treatment with an EGFR-TKI, rebiopsy is recommended to search for a potential resistance mechanism. However, the therapeutic potential of these resistance mechanisms represents an unmet need in thoracic oncology. We present a case of a 78-year-old woman with stage IVB EGFR-mutated NSCLC in whom an acquired RET Gene Fusion was identified as the -independent resistance mechanism. Additionally, a combined therapy of Osimertinib and Selpercatinib showed a durable oncological response with 14 months of progression free survival in the absence of adverse events. Addition of Selpercatinib to Osimertinib in an EGFR-mutated NSCLC patient with an acquired RET fusion was well tolerated and created a clinical benefit. Further prospective investigation into these novel combination strategies is needed as resistance mechanisms could serve as possible targets for new therapy approaches.