Abstract 2305: Ultra-sensitive detection of circulating tumor DNA by whole-genome sequencing of blood samples from locally advanced rectal cancer patients receiving neoadjuvant therapy and enrolled in watch-and-wait strategies for organ preservation
Authors: Sanchez-Vega Francisco , Chen Chin-Tung , Afterman Danielle , Omer Dana , Darmofal Madison , de Bruijn Ino , Chatila Walid K. , Drescher Matthew , Tam Grittney , Lauterman Tomer , Kuzman Maja , Gonzalez Santiago , Glavas Dunja , Samdbeck James , Maloney Dillon , Levatic Jurica , Deochand Sunil , Yahalom Michael , Ptashkin Ryan , Tavassoly Iman , Donenhirsh Zohar , White Eric , Kandasamy Ravi , Alon Ury , Berger Michael F. , Loomis Brian , Polak Paz , Oklander Boris , Zviran Asaf , Garcia-Aguilar Julio 
Published: June 2023

Abstract
Background. We hypothesized that circulating tumor DNA (ctDNA) can be used as a prognostic biomarker, improve the assessment of response and enhance detection of minimal residual disease in patients with locally advanced rectal cancer (LARC) treated with neoadjuvant therapy (NAT).
Methods. We analyzed data from 31 LARC patients treated at Memorial Sloan Kettering as part of the Organ Preservation in Rectal Adenocarcinoma (OPRA) phase II clinical trial. Patients had stage II or III rectal adenocarcinoma and received NAT including chemoradiation and chemotherapy. Patients without a clinical complete response (cCR) underwent surgical resection, while patients with a cCR were enrolled in a watch-and-wait protocol for organ preservation. Complete response (CR) after NAT was defined as either pathological complete response or a cCR sustained for ≥2 years. Disease-free survival (DFS) was measured from the start of NAT. Median follow-up was 5.41 years [range 2.96-8.38]. ctDNA analyses were performed using the C2i Genomics platform. A patient-specific molecular profile was created by performing whole-genome sequencing (WGS) of their tumor and matched normal DNA (40x coverage). WGS (20x coverage) was performed on plasma samples collected at baseline (before NAT), interval evaluation (halfway through NAT), re-staging evaluation (8 weeks after NAT) and follow-up (3-6 months after NAT).
Results. Tumor was detected in plasma samples from 24/25 patients at baseline (96% sensitivity). The tumor fraction (TF) levels detected at baseline separated responders from non-responders (median TF 6.2e-4 vs 1.4e-3; p=0.055). Tumor detection at interval was associated with a lower rate of CR (25% vs. 75%, p=0.0095) and shorter time to recurrence (58.3% vs. 94.1% 3-year DFS, p=0.02). Tumor detection at follow-up was associated with a higher rate of recurrence (p=0.037) and tumor was detected at follow-up for all 5/5 patients who developed recurrence. Overall TF dynamics showed clearance of ctDNA down to the non-detection level throughout treatment in patients with a CR, while non-responders exhibited non-decreasing and often increasing estimates of ctDNA burden. Analysis of tissue WGS data identified multiple patients with colibactin associated mutational signatures, which provides additional insights into their cancer etiology.
Conclusions. The WGS-based approach for ctDNA analysis exhibited very high sensitivity for detection at baseline. TF across multiple time points separated responders from non-responders, suggesting potential value as a prognostic marker. Detection of ctDNA at follow-up for all patients who recurred is indicative of potential clinical utility for treatment de-escalation in the context of organ preservation strategies.
Citation Format: Francisco Sanchez-Vega, Chin-Tung Chen, Danielle Afterman, Dana Omer, Madison Darmofal, Ino de Bruijn, Walid K. Chatila, Matthew Drescher, Grittney Tam, Tomer Lauterman, Maja Kuzman, Santiago Gonzalez, Dunja Glavas, James Samdbeck, Dillon Maloney, Jurica Levatic, Sunil Deochand, Michael Yahalom, Ryan Ptashkin, Iman Tavassoly, Zohar Donenhirsh, Eric White, Ravi Kandasamy, Ury Alon, Michael F. Berger, Brian Loomis, Paz Polak, Boris Oklander, Asaf Zviran, Julio Garcia-Aguilar. Ultra-sensitive detection of circulating tumor DNA by whole-genome sequencing of blood samples from locally advanced rectal cancer patients receiving neoadjuvant therapy and enrolled in watch-and-wait strategies for organ preservation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2305.