Prognostic value of circulating tumor DNA for progression-free survival in patients with locally advanced dMMR/MSI-H colorectal cancer managed without surgery.

256 Background: Immunotherapy has shown the ability to induce a high rate of pathologic and clinical complete response in patients with deficient mismatch repair (dMMR)/microsatellite instability-high (MSI-H) colorectal cancer (CRC). The role of circulating tumor DNA (ctDNA) as a prognostic biomarker for survival in patients who receive non-operative management (NOM) is unknown. Among patients enrolled in a phase II trial (NCT04082572) that evaluated the efficacy of pembrolizumab in patients with locally advanced dMMR/MSI-H solid tumors, we evaluated the impact of ctDNA on progression-free survival (PFS) among 12 patients with locally advanced dMMR/MSI-H CRC who received NOM. Methods: ctDNA testing was performed in the CLIA-certified laboratory at MD Anderson using a 70-gene liquid biopsy panel (LBP-70), which used digital sequencing of cell-free DNA. Formalin-fixed paraffin-embedded tumor samples and germline peripheral blood mononuclear cells were profiled using next-generation sequencing. Mutations in ctDNA were considered tumor-specific if they were confirmed with matched tumor tissue profiling. The lower limit of detection of the LBP-70 assay was a mutational variant allele frequency of <0.3%. "ctDNA clearance" and “ctDNA(-)” status were defined as the absence of any detectable tumor-specific mutations in ctDNA. Two-year progression-free survival (PFS) was estimated using Kaplan-Meier techniques and compared with log-rank tests. Results: The median follow-up between the first cycle of pembrolizumab and last radiographic assessment was approximately three years (36.7 mo; range (range 1.2 – 51.0 mo). Forty-nine LBP-70 assays were performed among the 12 patients over the course of treatment with pembrolizumab. The median number of cycles was 16 (range 1 – 16 cycles). Six patients were ctDNA(-) prior to pembrolizumab, all of whom remained ctDNA(-) after. None of these six patients experienced disease progression (PD) at their last follow-up. The other six patients were ctDNA(+) prior to pembrolizumab (median baseline VAF 0.4%, range 0.3%–4.0%). Of these six patients, three experienced ctDNA clearance while receiving pembrolizumab. None of the three patients who cleared ctDNA experienced PD as of their last follow-up. Of the three patients who did not clear ctDNA, two experienced PD in their primary tumor (at two and six months after their first cycle, respectively), while the third patient remains without PD at their last follow-up. The two-year PFS rate was 100% among the nine patients who were ctDNA(-) after pembrolizumab compared to 33% among the three patients who were ctDNA(+) after pembrolizumab (p = 0.03). Conclusions: ctDNA may be a valuable tool for assessing treatment response and improving the ability to tailor organ-sparing, curative strategies to patients with locally advanced dMMR/MSI-H CRC.