The central promise of liquid biopsy — that a tube of blood can tell you something about cancer that a tissue biopsy can't, or can tell you something equivalent with far less invasiveness — is now supported by a body of clinical evidence that would have seemed implausibly optimistic ten years ago. ctDNA (circulating tumor DNA) has moved from a research tool to an approved companion diagnostic to a clinical trial endpoint, sometimes all three simultaneously. The GALAXY trial in colon cancer showed that post-surgical ctDNA status is a stronger predictor of recurrence than any pathological staging variable in current use. Grail's PATHFINDER study demonstrated multi-cancer early detection from a single blood draw with acceptable specificity. What this means for how clinical trials are designed — and how patients are stratified within them — is changing rapidly.
This article is for informational purposes only and does not constitute medical advice. Liquid biopsy tests have variable FDA clearance status; some are approved companion diagnostics while others are LDTs or research use only. Consult your oncologist about which tests are appropriate for your clinical situation.
Summary
Guardant360 CDx is FDA-approved as a companion diagnostic for multiple solid tumor indications. GALAXY trial: post-surgical ctDNA clearance predicted disease-free survival in stage II–IV colon cancer better than pathological staging (ctDNA-positive: HR 0.79 for adjuvant chemotherapy benefit; ctDNA-negative: no adjuvant benefit detectable). PATHFINDER (Grail Galleri): multi-cancer early detection in 6,621 adults, 99.5% specificity, sensitivity 15.1% overall (71.4% in stage III–IV cancers). MCED tests are not yet FDA-approved but multiple registration trials are underway. ctDNA-guided de-escalation trials in breast (NEST) and intensification trials in colorectal cancer (CIRCULATE-US, DYNAMIC-II) will define the next clinical standard.
ClinicalMetric Analysis
- ctDNA as a surrogate endpoint in adjuvant trials has strong biological rationale but hasn't been prospectively validated as a trial endpoint — that distinction matters for regulatory submissions. The GALAXY data shows that ctDNA positivity after curative-intent surgery predicts clinical recurrence with high hazard ratios. But a surrogate biomarker becoming a primary endpoint requires prospective evidence that an intervention that improves the biomarker also improves clinical outcomes — the "surrogate validity" standard. DYNAMIC-II and CIRCULATE-US are designed exactly to generate this evidence: treat ctDNA-positive patients with intensified therapy and measure whether clinical recurrence decreases. If those trials are positive, ctDNA clearance can be submitted to the FDA as a validated surrogate endpoint, enabling future colon cancer trials to be smaller and faster. The oncology community is using ctDNA as if this validation has already happened; technically, it hasn't — and the regulatory pathway remains incomplete.
- The Galleri multi-cancer detection test's sensitivity numbers require careful contextualization: 15% overall sensitivity means it catches 1 in 7 early cancers — but those it catches disproportionately include high-lethality, hard-to-screen cancers. The PATHFINDER 99.5% specificity means roughly 1 in 200 people with a positive result have a false positive — which in a screening-level population generates a meaningful absolute number of unnecessary diagnostic workups. The 15.1% overall sensitivity is real but misses the critical stratification: sensitivity was 71.4% in stage III–IV disease (where early detection has less impact) and much lower in stage I–II (where early detection matters most). Sensitivity for stage I specifically is in the 5–15% range for most tumor types. The clinical utility case for MCED tests depends on whether the subset of early-stage cancers detected by ctDNA at an actionable stage is large enough to shift population-level mortality — a question that requires the large ongoing PATHFINDER 2 and NHS-Galleri RCT data, not the smaller observational studies that have been published so far.
- Tumor-informed versus tumor-naive ctDNA assays have meaningfully different analytical sensitivities, and the distinction matters when interpreting MRD studies. Tumor-informed assays (Signatera, PersonalizeDx) sequence the primary tumor, identify patient-specific somatic variants, and design a personalized PCR panel targeting those specific mutations. This allows detection of ctDNA at allele frequencies below 0.01% — substantially more sensitive than tumor-naive panels. Tumor-naive panels (Guardant360, FoundationOne Liquid CDx) use fixed gene panels covering common cancer mutations without patient-specific optimization. For MRD in the adjuvant setting — where ctDNA concentrations are very low — tumor-informed assays consistently outperform tumor-naive assays. For advanced metastatic disease biomarker profiling — where ctDNA levels are higher and variant diversity matters more — tumor-naive broader panels have advantages. Interpreting published ctDNA literature requires knowing which assay category was used and whether the sensitivity is appropriate for the clinical question.
GALAXY Trial: Colon Cancer MRD Standard
The GALAXY trial (NCT04008030, part of the CIRCULATE-Japan consortium) enrolled 1,039 patients with stage II–IV colon cancer resected with curative intent. ctDNA was measured by Signatera (tumor-informed, 16-variant personalized panel) at 4 weeks post-surgery and serially thereafter. Results published in Nature Medicine 2022 showed:
ctDNA-positive at 4 weeks: DFS HR 10.0 (95% CI 6.8–14.7). The ctDNA-positive hazard ratio is so large that it dwarfs conventional staging variables — stage III patients who were ctDNA-negative had better outcomes than stage II patients who were ctDNA-positive. For adjuvant chemotherapy analysis (observational, post-hoc): among ctDNA-positive patients, adjuvant chemotherapy was associated with improved DFS (HR 0.79). Among ctDNA-negative patients, no adjuvant chemotherapy benefit was detectable — raising the hypothesis that ctDNA-negative stage III patients could safely avoid chemotherapy. This is the basis for the DYNAMIC-II and CIRCULATE-US prospective randomized trials.
ctDNA in Early Breast Cancer: The De-escalation Question
The ARTEMIS-BEACON trial (NCT04915183) tests ctDNA-guided treatment in early breast cancer: patients who achieve ctDNA clearance after neoadjuvant chemotherapy are randomized to standard vs de-escalated adjuvant therapy. The hypothesis is that ctDNA negativity identifies patients who don't need intensive adjuvant therapy, potentially sparing chemotherapy toxicity without compromising cure rates. This is a clinical de-escalation trial using a biomarker endpoint — the design template that multiple other cancer types are now following.
The NEST-1 and NEST-2 trials (Natera-sponsored) similarly test whether ctDNA-guided intensification (treating ctDNA-positive patients with additional therapy) improves outcomes in early breast cancer. The Breast Cancer INDEX (BCI) and Signatera platform are being evaluated in parallel, creating a competitive landscape for MRD-guided breast cancer trials that should generate robust comparative data by 2027–2028.
Multi-Cancer Early Detection: PATHFINDER and the NHS Trial
Grail's Galleri test uses cfDNA methylation patterns (rather than somatic mutation detection) to identify cancer-derived DNA and predict tissue of origin. In PATHFINDER (NCT04241796), 6,621 participants ≥50 years had blood drawn for Galleri testing; participants with positive signals underwent diagnostic workup. 92 had positive results; 35 were confirmed cancers (38% PPV), with 21 of 35 being cancers with no standard screening recommendation — the clinical utility subset that justifies the test's existence. Overall sensitivity at 99.5% specificity was 15.1%.
The NHS-Galleri trial (UK, 140,000 participants) is the large RCT that will determine whether MCED screening with Galleri reduces stage IV cancer diagnosis and cancer mortality in a real-world population. Results are expected around 2026–2027 — this will be the most important MCED dataset to date. Grail has submitted a Breakthrough Device request to the FDA; approval before the NHS RCT results are available remains uncertain.
Liquid Biopsy in Clinical Trial Context
For patients currently enrolled in or considering clinical trials, liquid biopsy is increasingly appearing as either an enrollment requirement (Guardant360 CDx is a required companion diagnostic for several EGFR, RET, and PIK3CA-targeted therapy trials) or as an exploratory biomarker endpoint. Patients in colon cancer trials may be asked to provide serial blood draws for ctDNA monitoring as part of the MRD substudy. Many phase II/III trials now include a ctDNA translational component — not as a gating criterion but as an exploratory biomarker that may support label expansion or dose selection for follow-on trials. Participation in the liquid biopsy component of a trial typically adds only a blood draw at specific protocol-defined timepoints, not additional procedures.