Lineage Plasticity and Histologic Transformation in EGFR-TKI Resistant Lung Cancer

Lineage plasticity, the ability of cancer cells to alter their differentiated state through transcriptional and epigenetic reprogramming, has emerged as a key mechanism of therapeutic resistance across cancers. This adaptive process can manifest in multiple ways, including epithelial-mesenchymal transition, acquisition of stem-like features, and histological transformation, the most striking and clinically apparent example. In <i>EGFR</i>-mutant lung adenocarcinoma (LUAD), lineage plasticity is increasingly recognized as a prevalent mechanism of acquired resistance to tyrosine kinase inhibitors (TKIs). Among its visible manifestations, histologic transformation into small-cell lung cancer (SCLC) is the most frequent, while squamous transformation and other phenotypic shifts also occur. Transformed tumors typically retain the initiating <i>EGFR</i> mutation but lose EGFR dependence, acquire neuroendocrine features, and display aggressive clinical behavior with poor clinical outcomes compared with both de novo SCLC and non-transformed LUAD. Recent studies show that plasticity arises through combined genomic, transcriptomic, and epigenetic reprogramming, often foreshadowed by molecular alterations before overt histological change. Spatial and single-cell profiling reveal heterogeneous trajectories and intermediate states, while functional models and multi-omics approaches have begun to identify therapeutic vulnerabilities distinct from both de novo <i>EGFR</i>-mutated SCLC and classical <i>EGFR</i>-mutated LUAD. Thus, lineage plasticity, whether manifested as histologic transformation or through more subtle epigenetic reprogramming, represents a formidable resistance mechanism in NSCLC. Defining its molecular basis and temporal dynamics will be essential for early detection, prognostication, and the development of tailored therapies.