Correlative analysis of ctDNA in CheckMate-9X8 study of frontline treatment with mFOLFOX6, bevacizumab with or without nivolumab in metastatic colorectal cancer.

3584 Background: The CheckMate 9X8 study evaluated ctDNA as biomarker to predict treatment benefit in metastatic colorectal cancer (mCRC) patients receiving standard-of-care therapy with or without nivolumab. Although the phase 2/3 trial did not meet its primary PFS endpoint, the addition of nivolumab appeared to confer clinical benefit in a subset of patients. As predictive biomarkers for first-line nivolumab in mCRC remain unvalidated, this exploratory analysis identified mutation- and methylation-based ctDNA signatures that may help inform future patient stratification and disease management. Methods: Study design and methods were published previously (Lenz, et al. JITC 2024). As part of an exploratory biomarker analysis, paired plasma samples (baseline and on-treatment at C3D1) were evaluated for circulating tumor DNA (ctDNA) using PredicineATLAS (600-gene) comprehensive genomic profiling, PredicineSCORE (Low-pass whole-genome sequencing), and PredicineEPIC (methylation) assays and correlated with baseline patient characteristics and treatment efficacy. Results: Over 98% (138/140) of baseline patients had quantifiable ctDNA with Predicine ATLAS and approximately 83% (116/140) had detectable copy number variants (CNV) with PredicineSCORE. High concordance was observed between ctDNA-derived mutations and tissue-derived KRAS, TP53, and APC mutations and MSI/MSS status at baseline. 97% of patients with KRAS G12D/V/C mutations positively identified by local tissue testing were also positively identified by ctDNA. 6% of subjects considered KRAS wild-type by local tissue testing were identified as having KRAS G12D/V/C mutations by ctDNA. KRAS mutations, wild-type TP53, chromosomal loss of 18q21.33, high methylation scores and hypomethylated Tetraspanin 15 correlated with treatment efficacy. Neo-KRAS WT, or loss of KRAS mutations on treatment, emerged during treatment in 36% of patients with KRAS mutations at baseline and was associated with improved survival. Significant ctDNA level reduction was seen during treatment in both arms of the study irrespective of tumor response. These insights underscore the promise of ctDNA profiling in enhancing precision medicine by enabling real-time, dynamic assessment of tumor biology and therapeutic response with a potential to guide personalized treatment strategies for mCRC patients in the future. Conclusions: This study highlights the potential utility of liquid biopsy in the mCRC setting and provides novel insights from the ctDNA analysis, highlighting genetic and epigenetic ctDNA features that might be associated with clinical efficacy of SOC ± nivolumab in 1L mCRC. The emergence of Neo-KRAS WT in a substantial number of patients indicates a dynamic nature of KRAS alterations and highlights the additional value that longitudinal plasma ctDNA analysis could provide. Clinical trial information: NCT03414983 .