There is limited information on the feasibility and clinical potential of ctDNA analysis in non-metastatic rectal cancer. We assessed whether detection and analysis of plasma ctDNA could help monitor tumor evolution during neoadjuvant chemoradiotherapy (NACRT) treatment for LARC.
27 LARC patients who were enrolled on the CTRIAL-IE (ICORG) 12-38 TRI-LARC clinical trial (NCT02151019) and received NACRT prior to surgery, had samples for ctDNA analysis taken at baseline pre-NACRT, during week 3 of radiotherapy (RT), during the final week of RT, prior to surgery, and 3-12 months post-surgery. DNA from baseline biopsy samples was genotyped for 86 hotspot mutations in BRAF, EGFR, KRAS, NRAS and PIK3CA using the iPLEX™ HS Colon Panel on the MassARRAY® System (Agena Bioscience). The UltraSEEK™ Colon Panel (Agena Bioscience) was used to track 107 hotspot mutations in serial ctDNA samples.
At least one mutation was identified in 67% (18/27) of baseline biopsy samples. 15 patients (56%) had a KRAS mutation. Identical mutations were found in baseline ctDNA of 11/15 patients (73%). The median KRAS mutant allele frequency (MAF) in baseline ctDNA samples was 0.9% (range 0.1-2%). This significantly decreased over treatment (0.15% week 3 RT, 0.35% final week RT, 0.1% prior to surgery) (p < 0.05). 10 patients had a KRAS mutation in their baseline ctDNA that was not detected in their biopsy, with a median MAF of 0.3% (range 0.1-1.3%). Mutations in NRAS and PIK3CA were identified in 3/27 (11%) and 2/27 (7%) of patients, respectively. NRAS and PIK3CA mutations were identified in the ctDNA of 2/3 (67%) and 1/2 (50%) patients. Post-operative ctDNA samples were available for 13 patients and residual mutations were detected in 10 patients (77%), 3 of whom (30%) had recurrence at median follow up of 20.1 months. There was no recurrence in any patient with negative ctDNA post-surgery.
ctDNA can identify clinically relevant biomarkers and could be used as a minimally invasive alternative to repeated tumor biopsies to monitor tumor evolution. ctDNA detection after resection may provide evidence of residual disease and could identify patients at high risk of recurrence.
Clinical trial identification
Legal entity responsible for the study
Cancer Trials Ireland.
St. Luke's Institute of Cancer Research.
A. Sartori, D. Irwin: Employee, stock owner: Agena Biosciences. S. Hummel: Employee: Agena Biosciences. All other authors have declared no conflicts of interest.