Abstract 3617: Prediction of irinotecan and flurouracil efficacy in metastatic colorectal cancer by early measurement of circulating tumor DNA

Background: Chemotherapy resistance in metastatic colorectal cancer (mCRC) is a major challenge, and development of biomarkers for determining therapy efficacy is vital to ensure optimal palliative treatment. Recently, quantification of changes in circulating tumor DNA (ctDNA) levels has attracted substantial attention as a predictive marker. Here, we provide evidence that ctDNA has the potential to be used as an early marker of therapeutic efficacy of irinotecan and fluorouracil (FOLFIRI). Experimental Procedures: Patients diagnosed with mCRC, and with indication for first-line combination chemotherapy, were prospectively enrolled in this phase II study. Blood was drawn at baseline, day 7, 14, 21, 40 and then monthly until progression. Response evaluation was done at eight-week intervals by CT scanning using RECIST criteria. Chemotherapy was administered at day one and two every two weeks. The levels of cell-free DNA (cfDNA) and ctDNA were determined using sensitive digital droplet PCR assays. ctDNA assays were designed either to mutations identified by targeted sequencing or on basis of known tumor mutations detected in primary or metastatic lesions using a panel of mutation-specific qPCR based assays (KRAS, NRAS, BRAF). Results: Twenty-four patients were enrolled. A KRAS, NRAS or BRAF mutation was observed in tumor tissue of 16 patients. In 12 of these the mutations were confirmed in baseline samples and used for assessing ctDNA. For the remaining 12 patients, a mutation for ctDNA quantification was identified by targeted tumor sequencing. Patients with high baseline ctDNA levels (75th centile) had significantly shorter progression-free survival (PFS) than patients with low baseline ctDNA levels. After first treatment cycle all patients experienced a decline in ctDNA levels demonstrating initial response to treatment; however, the reduction in ctDNA was not associated with prolonged PFS or overall survival (OS). Conversely, change in ctDNA levels from first to second cycle of treatment (three weeks after treatment start) predicted FOLFIRI efficacy. Patients with increasing ctDNA levels at this time point had a significantly shorter PFS and a shorter OS than patients with stable or decreasing ctDNA levels. Conclusion: In agreement with the literature, we show that baseline tumor DNA levels predicted PFS. The initial observed decline in ctDNA levels upon administration of FOLFIRI indicates that all tumors had cancer cells sensitive to treatment; however,