Predictive role of circulating tumor DNA in pMMR locally advanced rectal cancer patients receiving neoadjuvant chemoradiotherapy combined with sintilimab.

3528 Background: Circulating tumor DNA (ctDNA) has emerged as a potential biomarker for various solid tumors, including colorectal cancer (CRC). It offers the advantage of longitudinal and dynamic surveillance of the tumor-specific genetic characteristics, eliminating the need for repeated invasive biopsies. However, the predictive role of ctDNA in patients with proficient mismatch repair (pMMR) locally advanced rectal cancer (LARC) receiving neoadjuvant chemoradiotherapy (CRT) combined with immunotherapy remains to be explored. Methods: In this prospective single-arm, phase II trial, pMMR LARC patients (cT 3-4 N 0 M 0 and cT 1-4 N 1-2 M 0 ) with an intermediate or high immunoscore (IS B ) were enrolled (NCT05450029). Treatment-naïve patients received radiotherapy (50 Gy/25 f) and 6 cycles of mFOLFOX6 plus 5 cycles of sintilimab, followed by total mesorectal excision (TME) 6-8 weeks post-radiotherapy. Baseline tumor tissue DNA and serial ctDNA dynamic changes were evaluated using next-generation sequencing. Baseline (T0) maximal somatic variant allelic frequency (maxVAF) as well as its changes at the first (T1, two cycles after therapy) and the second clinical evaluation (T2, four cycles after therapy) were assessed. Results: Tumor somatic mutations and aligned ctDNA analyses were conducted in 43 patients. The most frequently mutated genes in tumor tissue samples were APC (67%, n = 29), TP53 (65%, n = 28), KRAS (47%, n = 20), and FBXW7 (28%, n = 12), which were also observed in plasma. Pathway analysis indicated that mutations in SWI_SNF were more likely to be detected in patients achieving pathological complete response (pCR) (P = 0.02 for tumor tissue, P = 0.08 for plasma), suggesting a potential sensitization to sintilimab combined with CRT. For the dynamic ctDNA analysis, 37 patients were assessed using a 950-gene panel relevant to cancer. A significant decline in maxVAF from T0 to T1 was observed in the pCR group. An optimal cut-off of 0.11 for the maxVAF ratio (T1/T0) was identified to discriminate complete responders from other patients (AUC = 0.768; sensitivity 83.3%; specificity 72.0%; P < 0.001; 95% confidence interval [CI] 0.597-939). Patients with a low maxVAF ratio ( < 0.11) were more likely to achieve pCR following CRT plus sintilimab therapy (OR = 12.86; 95% CI: 2.23-74.08; P = 0.004). Conclusions: ctDNA may serve as a potential biomarker of the response to CRT combined with immunotherapy in pMMR LARC. Further validation is warranted to confirm the predictive value of maxVAF and to identify additional biomarkers with potential predictive significance in pMMR LARC. Clinical trial information: NCT05450029 .