Circulating Tumor DNA Longitudinal Analysis During Total Neoadjuvant Therapy and Non-operative Management for Locally Advanced Rectal Cancer: A Biomarker Study from the NOMINATE Trial

Abstract Purpose: The role of circulating tumor DNA (ctDNA) in total neoadjuvant therapy (TNT) and non-operative management (NOM) for locally advanced rectal cancer (LARC) remains unclear. We evaluated the association of ctDNA with clinical outcomes, including treatment response, local regrowth, and distant recurrence in patients undergoing TNT and NOM. Experimental Design: This biomarker companion analysis of the NOMINATE trial, a prospective, multicenter, randomized phase II study, enrolled 64 patients with T3-T4NanyM0 LARC between March 2021 and July 2023. Plasma samples (n=412) were collected at multiple time points: pre-treatment (T0), interim evaluations (T1, T2), final re-staging (T3), post-surgery or post-NOM (T4 and beyond). ctDNA was monitored using a tumor-informed mPCR-NGS assay (Signatera™). The association between ctDNA status and clinical outcomes was analyzed. Results: Baseline ctDNA detection was 98.4%, decreasing to 32% at T1, 15% at T2, 30% at T3, and 5% at T4. Among 25 patients achieving clinical complete response (cCR) or near cCR with NOM, ctDNA clearance was 100% at T2-T4, whereas 39 non-cCR patients showed lower clearance rates (75% at T2, 51% at T3). ctDNA at T3 had 100% specificity and positive predictive value for pathological residual disease and was associated with shorter disease-free survival (HR 6.7, P = .005). Local regrowth occurred in five NOM patients, with ctDNA detected in two during surveillance. Conclusions: This study highlights the potential of ctDNA as a predictive and prognostic biomarker in LARC patients undergoing TNT and subsequently managed by NOM. However, the modest sensitivity of ctDNA highlights the need for technological improvements.