Dynamic circulating tumor DNA profiling for prediction of long-term clinical benefit in pMMR locally advanced rectal cancer receiving neoadjuvant chemoradiotherapy plus sintilimab.

3590 Background: Immune checkpoint inhibitors (ICIs) targeting PD-1 have significantly improved outcomes in locally advanced rectal cancer (LARC). Circulating tumor DNA (ctDNA) has emerged as a promising biomarker for treatment response monitoring and surveillance, capable of predicting long-term clinical outcomes. However, whether ctDNA serves as a prognostic predictor in patients with proficient mismatch repair (pMMR) LARC receiving neoadjuvant chemoradiotherapy (CRT) combined with immunotherapy remains to be explored. Methods: This prospective single-arm phase II trial enrolled 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 ) (NCT05450029). Treatment-naïve patients received radiotherapy (50 Gy/ 25 fractions), 6 cycles of mFOLFOX6, and 5 cycles of sintilimab, followed by total mesorectal excision (TME) 6-8 weeks post-radiotherapy. Next-generation sequencing was used to evaluate baseline tumor tissue DNA and serial ctDNA dynamic changes. Baseline (T0) maximal somatic variant allelic frequency (maxVAF) was assessed, along with its changes at the first (T1, two cycles after therapy) and second clinical evaluation (T2, four cycles after therapy). Results: Tumor somatic mutations and aligned ctDNA analyses were conducted in 43 patients. The pathologic complete response (pCR) rate was 65.2%, with an objective response rate (ORR) of 93.5%. The R0 resection rate was 97.8%. The 3-year event-free survival rate, and 3-year overall survival rate remain immature. For 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. Among patients who reached follow-up endpoints, circulating tumor DNA analysis reveals that patients with a > 50% decline in maxVAF from T0 to T1 demonstrated longer survival outcomes and higher response rates compared to those without such decline. Conclusions: Serial ctDNA analysis demonstrates applicability in predicting treatment response to CRT combined with immunotherapy in pMMR LARC. Dynamic changes in circulating ctDNA may serve as a potential prognostic indicator in this population. Future studies should integrate ctDNA profiling of comprehensive cancer-related gene panels with large-scale clinical data to refine patient stratification and optimize therapeutic management. Clinical trial information: NCT05450029 .