Tumor heterogeneity involving SCLC with a single exon deletion in RB1 and adenocarcinoma with EGFR-L718V mutation in EGFR-TKI-resistant lung cancer.

[INTRODUCTION] EGFR tyrosine kinase inhibitors (EGFR-TKIs) have significantly improved the outcomes of EGFR-mutant non-small cell lung cancer (NSCLC); however, resistance commonly develops. While secondary EGFR mutations (e.g., C797S) and histological transformations are known mechanisms, their coexistence at distinct sites is rarely documented.

[METHODS] We established patient-derived cell lines from pleural effusion and mediastinal lymph node metastasis before and after osimertinib resistance in a patient with EGFR L858R-mutated NSCLC. Resistance mechanisms were characterized using cell viability assays, Western blotting, Sanger sequencing, RNA sequencing, and whole-exome sequencing.

[RESULTS] Pleural effusion-derived cells (JFCR-330-4 and -7) retained adenocarcinoma morphology and harbored an acquired EGFR L718V mutation, causing resistance to first-, third- and next-generation TKIs but sensitivity to second-generation TKI afatinib. Conversely, mediastinal lymph node-derived cells (JFCR-330-5) exhibited small cell lung cancer (SCLC)-like morphology, low EGFR expression, high neuroendocrine marker expression, and maintained EGFR L858R. RNA sequencing revealed SCLC-associated transcriptional signatures, while whole-exome and Sanger sequencing identified a small deletion in RB1 gene around exon 19 and a concurrent TP53 mutation, collectively suggesting transformation into SCLC.

[CONCLUSION] Spatially distinct and simultaneous resistance mechanisms, namely, EGFR L718V mutation and SCLC transformation, contributed to osimertinib resistance. The detection of RB1 2.4 kb small deletion in RB1 gene highlights the importance of deep genomic profiling. Multi-site biopsy and molecular diagnostics are necessary to guide treatment decisions in EGFR-TKI-resistant NSCLC.