Purpose: Small cell lung cancer (SCLC) is characterized by rapid progression after platinum resistance. Circulating tumor (ctDNA) dynamics early in treatment may help determine platinum sensitivity.
Materials and methods: Serial plasma samples were collected from patients receiving platinum-based chemotherapy for SCLC on the first 3 days of cycle one and on the first days of subsequent cycles with paired samples collected both before and again after infusions. Tumor-informed plasma analysis was carried out using CAncer Personalized Profiling by deep Sequencing (CAPP-Seq). The mean variant allele frequency (VAF) of all pretreatment mutations was tracked in subsequent blood draws and correlated with radiologic response.
Results: ctDNA kinetics were assessed in 122 samples from 21 patients. Pretreatment VAF did not differ significantly between patients who did and did not respond to chemotherapy (mean 22.5% v 4.6%, P = .17). A slight increase in ctDNA on cycle 1, day 1 immediately post-treatment was seen in six of the seven patients with available draws (fold change from baseline: 1.01-1.44), half of whom achieved a response. All patients who responded had a >2-fold decrease in mean VAF on cycle 2 day 1 (C2D1). Progression-free survival (PFS) and overall survival (OS) were significantly longer in patients with a >2-fold decrease in mean VAF after one treatment cycle (6.8 v 2.6 months, log-rank P = .0004 and 21.7 v 6.4 months, log rank P = .04, respectively).
Conclusion: A >2-fold decrease in ctDNA concentration was observed by C2D1 in all patients who were sensitive to platinum-based therapy and was associated with longer PFS and OS.