Predictive value of circulating tumor-derived DNA (ctDNA) in patients with locally advanced rectal cancer (LARC) treated with neoadjuvant chemoradiotherapy (CT-RT): Preliminary results

Abstract

Introduction: In patients with LARC, neoadjuvant CT-RT followed by curative surgery is the standard of care. Risk-adopted treatment is based on MRI-predicting local T and N stage, radial margins and vascular involvement while no molecular predictive markers are available. In the present prospective study, we investigated the predictive role of serum ctDNA in patients with LARC. Methods: Patients with diagnosis of LARC (T3-T4, N0 or any T, N+, M0) adenocarcinoma receiving neoadjuvant standard CT-RT with capecitabine concomitant to 50,4 Gy followed after 8-10 weeks by surgery, are included. 18 ml of peripheral blood samples were collected for ctDNA analysis extracted by using QIAamp Circulating Nucleic Acid QIAGEN. Baseline tumor biopsies and serum ctDNA collected at different time points (before CT-RT, after CT-RT/before surgery, after surgery and at the end of adjuvant CT, if indicated) were assessed for mutations in KRAS/NRAS exons 2-3-4, BRAF exons 11-15, and PIK3CA exons 9-20. CEA serum level was evaluated at the same time points. This preliminary analysis reports results on first consecutive 28 patients out of the 88 patients planned in the study protocol. The outcome of interest was the pathological complete response (pCR). Area under the receiver operating characteristic curves (AUC) was used to evaluate the ability of pre- and post-treatment value of ctDNA and CEA serum levels in predicting pCR. AUCs were compared using the DeLong, DeLong and Clarke-Pearson non-parametric approach. Results: The first consecutive 28 patients with detectable mutations, out of 71 screened, completed study calendar of blood draw collection and were included in this preliminary analysis. Main patient characteristics were: 20/8 male/female ratio; median age (range): 62 (37-79); rectum adenocarcinoma site: 2 proximal, 11 middle and 15 distal; clinical T stage: 3 T2, 23 T3, 1 T4, 1 TX; clinical N stage: 1 N0, 16 N1, 11 N2; mutations detected on tumor biopsy/serum ctDNA: 18 KRAS, 1 BRAF(not V006E), 3 NRAS and 7 PIK3CA. Pre-CT-RT, median values of ctDNA and CEA were respectively 0.06 ng/2ml (range 0 - 17.4) and 3.5 ng/ml (range 0.8 - 7); post-CT-RT median values of ctDNA and CEA were respectively 0 ng/2ml (range 0 - 0.60) and 1.9 ng/2ml (range 0.6 - 11). Seven out of the 28 evaluated patients (25%) achieved pCR. All patients achieving pCR were concentrated at the lowest values of pre-treatment ctDNA, while no clear pattern emerged for CEA. The median value of pre-treatment ctDNA was 0.0 in patients achieving pCR, 0.11 in patients with residual tumor (p=0.18). The AUCs were 0.67 (95% CI: 0.47-0.87) for ctDNA and 0.51 (95% CI: 0.24-0.78) for CEA (p-value for difference between the two ROC curves: 0.45). Similar results were observed for posttreatment ctDNA and CEA values. Conclusion: Preliminary analysis conducted in 28 patients with LARC showed a promising role of low baseline value of ctDNA in predicting pCR. Statistically not significant results could be due to the small sample size considered in this interim analysis; however the study is ongoing and it is adequately powered to detect the suggested effect at final analysis.