Circulating tumor DNA (ctDNA) to monitor treatment response and progression in patients treated with tyrosine kinase inhibitors (TKIs) and immunotherapy for EGFR-mutant non-small cell lung cancer (NSCLC)

e20652 Background: Detection of EGFR mutations in ctDNA can help determine appropriateness of TKI therapy for patients with NSCLC. We investigated whether longitudinal monitoring of ctDNA levels can be used to assess response to therapy and disease progression, with a focus on EGFR mutation-positive patients treated with immunotherapy. Methods: Serially collected blood from patients with EGFR mutation-positive NSCLC treated with TKIs and/or immunotherapy was analyzed using an ultrasensitive 24-gene next-generation sequencing assay. Clinical characteristics and outcomes were analyzed retrospectively by chart review. Results: We studied quantitative changes in ctDNA levels during treatment by analyzing somatic mutations in 91 plasma samples from 8 patients with EGFR-mutant NSCLC, including samples collected around the time of disease progression for a subset of patients. Two patients treated with PD-1 inhibitor monotherapy experienced a rise in ctDNA harboring EGFR-sensitizing mutations prior to radiographic progression. A third patient was started on anti-PD-1 monotherapy following disease progression on erlotinib. Plasma levels of L858R, T790M, and TP53 mutations were detectable on treatment initiation and decreased with radiographic response. The levels of these mutations rose at progression,fell with response to EGFR-directed therapy, and increased again before disease progression. Another patient was found to have mutations in EGFR, T790M, and TP53 that fell upon treatment with combination TKI therapy. The remaining four patients studied were treated with concurrent TKI and immunotherapy. In all of these cases, sensitizing EGFR mutations were present in plasma at low levels during response to treatment. Two of the four patients had a rise in ctDNA level at the time of radiographic progression; the other two patients had durable responses with persistently low ctDNA levels. Analysis of additional cases is ongoing. Conclusions: Monitoring quantitative changes in ctDNA may enable assessment of response or disease progression in immunotherapy- and TKI-treated EGFR-mutant NSCLC patients.