The unavoidable progression of EGFR-mutated NSCLC on EGFR-TKIs forces us to discover solutions for further therapies. Herein, we discuss the necessity of accurate genomic mapping of progressive disease illustrated by a patient case. Tumor rebiopsies at progression are strongly needed to characterize acquired resistance to EGFR-TKI. The necessary data, however, may be reliably obtained only by deep targeted next generation sequencing (NGS) of both DNA and RNA. The reported case is a patient with EGFR-mutant NSCLC, who progressed during second line Osimertinib with subsequent targeted treatment determined by the detection of an acquired intergenic ANK3-RET-fusion with concomitant PTEN-mutation and MDM2-amplification. These three acquired gene alterations represent potential mechanisms of TKI-resistance, not previously reported on second line Osimertinib. Yet, while PTEN-mutations and MDM2-amplification are currently undruggable, the ANK3-RET fusion allowed further personalized treatment by combining continuation of Osimertinib with the RET-TKI Pralsetinib, which resulted in objective partial response, so far for 7 months, and significant clinical improvement. Hence, complementary DNA- and RNA-based targeted NGS may be of importance in clinical routine to better reveal the current molecular state of the disease and contribute to the identification of further targeted therapy strategies. Indeed, further cases with acquired RET-fusions, including ANK3-RET, should be investigated to fully determine the effectiveness of RET-TKI-Osimertinib combinations.

Activating mutations in the Epidermal Growth Factor Receptor gene occur as early cancer-driving clonal events in a subset of patients with non-small cell lung cancer (NSCLC) and result in increased sensitivity to EGFR-tyrosine-kinase-inhibitors (EGFR-TKIs). Despite very frequent and often prolonged clinical response to EGFR-TKIs, virtually all advanced EGFR-mutated (EGFRM+) NSCLCs inevitably acquire resistance mechanisms and progress at some point during treatment. Additionally, 20-30% of patients do not respond or respond for a very short time (< 3 months) because of intrinsic resistance. While several mechanisms of acquired EGFR-TKI-resistance have been determined analyzing tumor specimens obtained at disease progression, the factors causing intrinsic TKI-resistance are less understood. However, recent comprehensive molecular-pathological profiling of advanced EGFRM+ NSCLC at baseline has illustrated the co-existence of multiple genetic, phenotypic, and functional mechanisms that may contribute to tumor progression and cause intrinsic TKI-resistance. Several of these mechanisms have been further corroborated by preclinical experiments. Intrinsic resistance can be caused by mechanisms inherent EGFR or by EGFR-independent processes, including genetic, phenotypic or functional tumor changes. This comprehensive review describes the identified mechanisms connected with intrinsic EGFR-TKI-resistance and differences and similarities with acquired resistance and among clinically implemented EGFR-TKIs of different generations. Additionally, the review highlights the need for extensive pre-treatment molecular profiling of advanced NSCLC for identifying inherently TKI-resistant cases and designing potential combinatorial targeted strategies to treat them.