Background: Patients with advanced colorectal cancer after progressing through first line therapy, have several FDA-approved systemic therapies that are associated with clinical benefit for a substantive minority of patients. Current clinical practice is to trial these various treatments in a step-wise fashion using CT scans every 3 months to evaluate effectiveness. This process requires 3-4 months between therapeutic interventions from which the patient may ultimately derive no clinical benefit, may have a performance status decline; limiting the number of possible interventions and increases risk for physical and financial toxicity. An alternative to the traditional CT-scan guided approach for disease assessment is a circulating tumor DNA (ctDNA) intervention. The Signatera ctDNA assay which utilizes 16 truncal mutations derived from a patient’s tumor, can be assessed every 2 weeks for a rapid determination of the effectiveness of a systemic therapy. This may allow patients to be exposed to many treatments during a short time, limiting toxicity, allowing for a quicker determination of clinical benefit and personalization of treatment. The aim of this study will be to compare the traditional scan-driven approach vs an intervention guided by ctDNA assessments, both arms using a pre-specified order of chemotherapy treatments. Methods: This is a phase 2 randomized study of patients with refractory metastatic adenocarcinoma of the colon or rectum. Participants are eligible after progression or intolerance to first line chemotherapy or recurrence within 6 months of adjuvant oxaliplatin based chemotherapy. They must have RECIST measurable metastatic disease that is not eligible for definitive management. Tissue from the primary and/or metastatic deposit is required for Signatera NGS analysis and subjects must have measurable ctDNA at sampling. Participants must be ≥18 years old without major organ dysfunction and have an ECOG performance status of 0 to 2. Subjects with Microsatellite High, deficient in DNA mismatch repair genes, or BRAF V600E mutations are excluded. Subjects will be randomized 1:1 to Arm A (ctDNA guided intervention) or Arm B (scan-guided control group). Patients in both arms will undergo systemic treatments in a standardized pre-specified order. Arm A will have ctDNA assessments every 2 weeks until an intervention shows a significant decrease, then every 4 weeks until Progressive Disease (PD) by scan or significant ctDNA increase. CT imaging will be performed every 12 weeks. Those in Arm B, will have CT imaging every 12 weeks and blood collected for post-hoc analysis every 4 weeks until PD by scan. The primary endpoint is overall survival. Secondary endpoints include progression free survival and overall response. Exploratory analysis with be performed of the microbiome. Enrollment continues to a maximum of 78 patients. Clinical trial information: NCT04786600.