Circulating tumor cells as a predictive biomarker in patients with small cell lung cancer undergoing chemotherapy

Background There are no biomarkers for assessment of disease burden or activity of therapy in SCLC. Patients and methods We conducted a prospective study enumerating serial CTCs in patients with newly diagnosed limited disease (LD) and extensive stage (ED) SCLC. CTCs demonstrating DNA damage and apoptosis based on γH2AX and M30 staining were also assessed. We correlated CTC number with disease stage, survival outcomes and tumor burden by RECIST. Results Between 03/2011-10/2013, 50 evaluable patients were enrolled (20 LD). Baseline CTC number was higher for ED (median CTC 71 vs. 1.5 for LD; p 0.0004). Patients with <5 CTC had longer PFS but not OS (11 vs. 6.7 months, p 0.0259 and 15.5 vs. 12.9 months, p 0.4357). A higher cutoff (CTC<50 or CTC≥50) was significantly correlated with both OS (20.2 vs. 11.8 months, p 0.0116) and PFS (10 vs. 4.8 months, p 0.0002). Patients with <5 CTC on day 1 of cycle 2 had longer PFS (10 vs. 3.17 months, p<0.001) and OS (18 vs. 9 months, p 0.0001). Patients with an increase in γ2HAX-positive CTCs after chemotherapy had longer OS compared to patients without an increase (25.3 vs. 9 months, p 0.15). Conclusions This study demonstrates that CTCs at baseline and Cycle 2 of chemotherapy correlate with disease stage and survival in patients with SCLC, suggesting that CTCs may be used as a surrogate biomarker for clinical response. Confirmatory prospective clinical trials are needed before we can incorporate routine evaluation of CTCs into clinical practice.