Longitudinal ctDNA measurements for treatment response monitoring in patients with metastatic colorectal cancer undergoing systemic therapy: The ORCA trial.

220 Background: Precise monitoring of treatment response may support and improve treatment decision-making in patients with metastatic colorectal cancer (mCRC). The PLCRC-ORCA study aimed to explore the clinical applicability of personalized ctDNA testing for treatment response monitoring in patients with mCRC. Methods: The prospective, observational ORCA study is a substudy of the Prospective Dutch ColoRectal Cancer cohort (PLCRC). After informed consent, blood samples were collected before initiating first line of treatment (baseline) and subsequently every 8-9 weeks during routine clinical care. ctDNA levels were analyzed using a clinically validated, personalized, tumor-informed 16-plex PCR NGS assay (Signatera™, Natera, Inc). Results: The majority of patients (45/49) had blood samples collected at both baseline and the first on-treatment time point. At baseline, 43/45 (95.6%) patients were ctDNA-positive with a median concentration of 63.5 mean tumor molecules per mL (MTM/mL; range 0.08 to 16,200). In a multivariate linear regression model including metastatic site at baseline and resection status of the primary tumor, ctDNA levels at baseline were lower in patients with metastatic disease limited to the lung (n = 4; p < 0.001) or peritoneum (n = 6; p = 0.002) compared to patients with liver-limited metastases. Moreover, primary tumor resection was independently associated with lower levels of ctDNA at baseline (p = 0.05). At the first measurement on-treatment, 13/45 patients (28.9%) were ctDNA-negative and 32/45 (71.1%) patients were ctDNA-positive with ctDNA levels ranging from 0.04 to 318 MTM/mL. Reduction in ctDNA levels relative to baseline was observed in 42/45 (85.7%) patients while two patients exhibited increased ctDNA levels. One patient had undetectable ctDNA levels at baseline and on treatment. Among the 30 patients with detectable but reduced ctDNA levels on treatment, the median ctDNA levels decreased 55-fold (95% CI: 26–600). Conclusions: The levels of ctDNA in patients with mCRC are influenced by the metastatic site and are lowest among patients with metastases limited to the lung and peritoneum. Our data suggest that clinical applications of ctDNA testing for treatment response monitoring should take the baseline ctDNA levels into account when interpreting ctDNA dynamics. Correlation of ctDNA status with radiological treatment response is underway and will be reported during the conference.