By Tony S. K. Mok, BMSc, MD, FRCPC
Patients must first be identified as having an epidermal growth factor receptor (EGFR) mutation before they can benefit from EGFR tyrosine kinase inhibitors (TKI). Furthermore, to benefit from third-generation EGFR TKI such as osimertinib, it is essential to identify the presence of resistance exon 20 T790M mutations (T790M). The challenge is how to optimize detection of these mutations in all patients with advanced stage pulmonary adenocarcinoma. Limited by availability of adequate tumor samples, a significant portion of lung cancer patients are never tested for EGFR mutations. A recent Asian study showed that the testing rate of EGFR mutation analysis in 22,193 patients with advanced nonsmall cell lung cancer (NSCLC) was only 31.8%.1 There is much room for improvement.
A promising solution to the problem is plasma cell-free DNA (cfDNA) testing. Multiple studies on various platforms have consistently demonstrated good sensitivity and high specificity with EGFR mutation analysis on plasma cfDNA. We performed a prospective study on the matched tumor and plasma samples from FASTACT 2, a randomized study that compared intercalated combination of chemotherapy and erlotinib with chemotherapy alone in unselected treatment-naive patients, using the cobas 4800 blood test and reported concordance rate, sensitivity, and specificity at 88%, 75%, and 96%.2 With digital PCR, we analyzed matched pair samples from the ASPIRATION study, a single-arm first-line study of erlotinib in 204 patients with EGFR mutation. Results were similar, with concordance rate, sensitivity, and specificity at 78%, 77%, and 92%, respectively. 3 Other studies have shown similar specificity, but the sensitivity is variable. Luo et al4 conducted a meta-analysis of 20 studies and reported aggregate sensitivity of 67% and specificity of 92%. These data confirm the clinical applicability of EGFR mutation analysis on plasma cfDNA. When the result is positive for EGFR mutation, there is more than a 90% chance that it is a true positive and that the patient will benefit from EGFR TKI. When the plasma test is negative, it is essential to pursue tumor biopsy, as the false negative rate is about 30%. As plasma testing becomes more efficient and available, it should be integrated as a standard test for patients with advanced stage adenocarcinoma.
To detect T790M promptly at time of radiological progression to EGFR TKI is a clinical challenge. In addition to the limitation by accessibility and risk, many patients are reluctant to have a re-biopsy. Detection of T790M mutation from plasma cfDNA is much more feasible than tissue testing. Oxnard et al5 utilized the matched tumor and plasma samples from AURA study, and used BEAM digital PCR for detection of T790M mutation from plasma. The major advantage of this study is that the investigators included samples from both T790M positive and negative patients, and all enrolled patients had received osimertinib. Based on this database, they were able to report the diagnostic utility and correlate clinical response to osimertinib with the biomarkers. Sensitivity and specificity for detection of T790M was 70.2% and 69%, respectively. Tumor response rate in patients with tumor tissue positive for T790M mutation was 62%, while the response rate in plasma positive cases was 63%. In other words, the plasma test shares similar predictive power as the tissue sample test. Clinically, a false positive result poses significant consequences. Oxnard et al were able to analyze the clinical outcomes of patients with positive T790M in plasma but negative in tumor sample. Out of 18 false positive cases, four patients attained partial response and five attained stable disease. This may potentially reflect on tumor heterogeneity. The tumor sample that tests negative for T790M may not be representative of the molecular status in all progressing sites.
The role of plasma cfDNA for detection of EGFR mutation at initial diagnosis and of T790M mutation at time of disease progression on EGFR TKI is well established. The remaining controversy is the application of plasma testing for monitoring of disease status. It remains unknown if persistent detection of EGFR mutation or early detection of T790M from plasma prior to radiological progression will have any impact on the course of the illness. We reported that the persistent presence of EGFR mutation after 3 months of therapy is associated with worse progression- free survival and overall survival.2 However, it is unknown if a change in therapy based on the plasma molecular status would influence survival. Further investigation on the role of molecular monitoring on plasma cfDNA is warranted.
Advances in technology have now allowed us to perform routine EGFR mutation analysis on plasma cfDNA. Both the US FDA and the European Medicines Evaluation Agency have granted approval for the use of plasma samples as companion diagnostics for NSCLC for the detection of EGFR mutation. Liquid biopsy is now viable, and should be integrated into clinical practice.
Professor Mok’s provided conflicts of interest are available upon request to [email protected].
References
1. Yatabe Y, Kerr K, Utomo A, et al. EGFR mutation testing practices within the Asia Pacific region: Results of a multicenter diagnostic survey. J Thorac Oncol. 2015; 10:438-445.
2. Mok T, Wu YL, Lee JS, et al. Detection and dynamic changes of EGFR mutations from circulating tumor DNA as a predictor of survival outcomes in NSCLC patients treated with first-line intercalated erlotinib and chemotherapy. Clin Cancer Res. 2015; 21:3196-3203.
3. Park K, Yu CJ, Kim SW, et al. First-line erlotinib therapy until and beyond response evaluation criteria in solid tumors progression in Asian patients with epidermal growth factor receptor mutation–positive non–small-cell lung cancer: The ASPIRATION Study. JAMA Oncol. 2016; 2:305-312.
4. Luo J, Shen L, Zheng D. Diagnostic value of circulating free DNA for the detection of EGFR mutation status in NSCLC: A systematic review and meta-analysis. Sci Rep. 2014; 4:6269.
5. Oxnard G, Thress K, Alden R, et al. Plasma genotyping for predicting benefit from osimertinib in patients with advanced NSCLC. European Lung Cancer Conference (ELCC); 2016; Geneva, Switzerland.
