What is the role of resistance testing in guiding treatment for non-small cell lung cancer?

Resistance Testing in Non-Small Cell Lung Cancer

At disease progression on targeted therapy, tissue biopsy with comprehensive DNA/RNA-based next-generation sequencing (NGS) is mandatory to identify actionable resistance mechanisms and guide subsequent treatment decisions. 1

Primary Role: Identifying Actionable Resistance Mechanisms at Progression

Tissue Biopsy is the Gold Standard

• Tissue re-biopsy is required at progression to assess both molecular resistance mechanisms and histologic transformation, which occurs in approximately 15% of patients. 1
• Histologic transformation to small-cell lung cancer, squamous cell carcinoma, or sarcomatoid features can only be detected through tissue analysis and fundamentally changes treatment approach (e.g., platinum/etoposide for SCLC transformation). 1

Comprehensive NGS Panel Requirements at Resistance

• DNA/RNA-based NGS using comprehensive gene panels is strongly preferred over single-gene testing due to the heterogeneity of resistance patterns. 1
• The panel must detect on-target resistance mutations (EGFR C797X, G724S, L718Q, L792H, G769R after osimertinib; T790M after first/second-generation EGFR TKIs). 1
• Off-target resistance mechanisms require detection of MET amplification (most common at 24% of osimertinib progressors), ERBB2 amplification, BRAF mutations, MET exon 14 skipping, and oncogenic fusions (RET, ALK, ROS1, BRAF, FGFR3, NTRK1). 1

Specific Testing Algorithms by Clinical Scenario

For Patients Progressing on First/Second-Generation EGFR TKIs

• Mandatory T790M testing to identify candidates for osimertinib (third-generation EGFR TKI). 1
• Minimum testing if NGS unavailable: EGFR T790M mutation analysis and FISH for MET amplification. 1

For Patients Progressing on Osimertinib

• FISH may detect higher rates of MET amplification than NGS-based methods and should be considered, particularly given MET amplification is the most common resistance mechanism (up to 24%). 1
• Screen for diverse acquired EGFR mutations (C797X, G724S, L718Q, L792H, G769R) that may guide clinical trial enrollment for novel EGFR inhibitors. 1

For ALK-Positive Patients at Progression

• ALK resistance mutations require detection as newer-generation ALK inhibitors show differential efficacy against specific ALK mutations. 1
• MET amplification is an important resistance mechanism to ALK inhibitors and must be assessed. 1

Role of Liquid Biopsy (ctDNA Testing)

When to Use ctDNA

• ctDNA testing is recommended only when tissue biopsy is not feasible, recognizing significant limitations. 1
• ctDNA can identify EGFR alterations and some resistance mechanisms in both primary diagnostic and resistance settings. 1

Critical Limitations of ctDNA

• Cannot detect histologic transformation (SCLC, squamous, sarcomatoid), which affects 15% of progressors and fundamentally changes treatment. 1
• Limited sensitivity for amplifications (particularly MET) and fusions. 1
• Approximately 25% of NSCLC patients are "non-shedders" with insufficient circulating tumor DNA. 1
• Negative ctDNA results must be interpreted with extreme caution—patients with negative or low variant allele frequency (VAF) of sensitizing mutations should still undergo tissue biopsy. 1

Complementary Role

• ctDNA and tissue analyses provide complementary information—ctDNA may better represent tumor heterogeneity, while tissue is essential for histologic assessment. 1

Common Pitfalls to Avoid

Technical Considerations

• Use molecular assays with high analytical sensitivity since resistant cells are often subclonal. 1
• When both sensitizing and resistance mutations are not detected on ctDNA, consider the sample non-informative rather than negative—these patients likely have insufficient ctDNA levels. 1

Clinical Decision-Making Errors

• Do not rely solely on ctDNA when tissue is obtainable—you will miss histologic transformation and have reduced sensitivity for critical resistance mechanisms like MET amplification. 1
• Avoid single-gene resistance testing—the heterogeneity of resistance mechanisms demands comprehensive profiling. 1
• Do not assume resistance testing at initial diagnosis is sufficient—resistance mechanisms emerge under selective pressure and require re-testing at progression. 1