Impact in Prognosis of Circulating Tumor Dna Mutant Allele Fraction (maf) in Ras Mutant Metastatic Colorectal Cancer (Mcrc)

Background: Determination of RAS status based on circulating tumor DNA (ctDNA) is expected to offer advantages over standard tissue testing. A 90% concordance rate between tissue and plasma determination based on BEAMing (Beads, Emulsions, Amplification, Magnetics) has been reported by our group and others, with false-negatives in tissue samples explaining most of the discordance. RAS mutations detected in primary tumor tissue confer worse patient survival in mCRC. The prognostic impact of RAS-mutant ctDNA MAFs, a surrogate of the clonality of RAS mutations in metastatic lesions, has not been explored in detail. We aim to correlate RAS-mutant MAF data across different therapy lines with disease burden and patient outcome. Methods: BEAMing was performed in plasma samples from a total of 110 patients with mCRC from different institutions. The technique provides MAF quantification with a sensitivity of 0.01%. Plasma was obtained at different time points: diagnosis of metastasic disease, progression to first-line chemotherapy and later stages. The impact of MAFs on progression-free and overall survival will be assessed in univariate Cox models as a continuous variable or categories (<1%, 1-10% and >10% MAF). A multivariate model adjusting for treatment lines and disease burden (number and site of metastases) will be constructed. Results:RAS mutation was detected on plasma from 63 patients (57.3%). Plasma from patients with surgery of CRC metastasis were excluded. RAS-mutant plasma samples were obtained before front line chemotherapy (30 samples), before second line treatment (12 samples) and in refractory setting (6 samples).Preliminary results show a trend towards a better progression free survival (PFS) and overall survival in first line in patients with MAF< 10% and treatment without bevacizumab. Similarly, a statistically significant better PFS and OS (HR: (HR: 6.602 p< 0.001) in second line for patients with MAF < 10% in plasma was observed. An expanded Next Generation Sequencing (Amplicon Seq) analysis of 61 plasma samples of mCRC patients (35 front line, 16 second line, 10 refractory setting) will be presented. Conclusion:RAS mutation in mCRC can be easily detected by BEAMing in plasma. RAS-mutant ctDNA MAFs may dynamically change during metastasic disease course. A real-time quantification of RAS-mutant ctDNA MAFs, reflecting the clonality of the mutation irrespective of disease burden, may better define patient prognosis than traditional clinicopathological factors.