Resistance Mechanisms and Treatment Sequencing

Dr. Wistuba explains that acquired resistance to ROS1-targeted therapies frequently involves on-target mutations, with G2032R representing the most frequently observed resistance mutation among 10 to 12 described mutations that affect the ATP binding site and reduce drug binding efficiency. Off-target resistance mechanisms include targetable options like CMET amplification and histologic transformations including small cell lung cancer transformation and adenocarcinoma to squamous cell transformation, with small cell transformation being more clinically relevant. Additional mechanisms include targetable alterations like KRAS mutations and complex phenotypic changes such as epithelial-mesenchymal transition or signaling pathway activation including EGFR that may not be based on genomic abnormalities.

Dr. Rodriguez reviews how different generations of ROS1 TKIs demonstrate variable ability to target acquired resistance mutations. First-generation crizotinib led to identification of on-target resistance including G2032R, whereas second-generation entrectinib continued showing substantial on-target resistance. Newer agents including repotrectinib and taletrectinib demonstrate excellent activity against resistance mutations with over 60% responses in second-line settings for patients previously treated with first-generation agents. This understanding emphasizes starting with drugs that target resistance mechanisms from initial treatment rather than sequential approaches, as many patients never reach better drugs due to clinical deterioration.

Dr. Leal discusses sequencing considerations, noting that although G2032R mutation data provides the strongest evidence for treatment selection, newer agents like repotrectinib and taletrectinib cover this resistance mutation, potentially making specific resistance identification less relevant for initial treatment selection. However, understanding resistance patterns remains important given multiple on-target and off-target mechanisms affecting care, supporting re-biopsy at progression for optimal sequencing decisions. Clinical trials demonstrate activity for sequencing with other ROS1 TKIs regardless of specific resistance mutations, though some agents may lack sufficient potency following highly potent agents like taletrectinib.

Emerging data with agents like zidesamtinib show activity in patients previously treated with taletrectinib, suggesting viable sequencing options. Dr. Rotow notes similarity to ALK-positive disease where acquired resistance mutations enrich for responses but activity occurs even without identified mutations, reflecting disease heterogeneity, incomplete resistance mechanism identification, and potency differences beyond specific acquired resistance detection.