PEGASUS Trial: ctDNA-Guided Adjuvant Therapy in Colon Cancer Misses Primary Endpoint

On September 14, 2026, researchers published findings in Nature Cancer detailing the phase 2 PEGASUS trial, which tested whether serial circulating tumor DNA testing can safely guide postoperative chemotherapy intensity in high-risk stage II and stage III colon cancer.

Screening Patient Cohorts Between July 2020 and July 2022

Investigators conducted the multicenter, single-arm, nonrandomized trial across 11 medical centers located in Italy and Spain. Between July 2020 and July 2022, clinical staff screened 156 patients who had completely resected, microsatellite-stable high-risk stage II or stage III colon cancer, ultimately enrolling 135 individuals into the final analysis.

At the initial postoperative liquid biopsy, 30 patients tested positive for circulating tumor DNA, while 105 tested negative. During a planned confirmatory assessment, five initially negative patients converted to positive results. This left a final landmark distribution of 35 ctDNA-positive patients and 100 ctDNA-negative patients.

Adapting Adjuvant Therapy According to Molecular Status

The trial utilized dynamic treatment protocols dictated by serial molecular findings rather than standard fixed regimens. Patients with confirmed negative test results received six months of capecitabine monotherapy. Conversely, individuals with initial molecular residual disease received three months of capecitabine plus oxaliplatin.

From neoadjuvant to ctDNA-guided adjuvant therapy, @AdrianaKahnMD @YaleCancer reviews elacestrant in early ER+/HER2- BC: ELIPSE reduced Ki67 by 52.9%, while ELEGANT and CATE are evaluating adjuvant use in high-risk and ctDNA-positive disease. #DAVABreast pic.twitter.com/t7AVkDlhHQ

— https://x.com/DAVAOnc/status/2090514317838012889

Further longitudinal monitoring guided subsequent adjustments. Patients who remained positive after initial combination therapy escalated to six months of folinic acid, fluorouracil, and irinotecan. Meanwhile, those who cleared their circulating tumor DNA received three additional months of capecitabine alongside continued surveillance.

Missing the Primary Endpoint at Two Years

Researchers designed the study to evaluate a primary end point of the 2-year relapse-free rate specifically among patients who maintained negative test results across two consecutive postsurgery assessments. Although the protocol initially targeted 134 patients in this subgroup, accrual finished with 100 evaluable individuals.

Within that group of 100 patients, 12 relapses occurred inside the two-year window. This corresponded to a relapse-free rate of 88%, with a 90% confidence interval ranging from 81% to 93%. Because this figure fell below the prespecified target of at least 92%, the trial failed to meet its primary statistical endpoint.

To contextualize efficacy, investigators compared trial outcomes against a matched historical cohort derived from the TOSCA trial. Two-year disease-free survival reached 79.7% in the PEGASUS cohort compared with 82.2% in the historical reference group. Because the trial was nonrandomized, researchers noted that the comparison cannot formally establish therapeutic equivalence.

Significant differences emerged regarding treatment-related side effects. The molecularly guided approach produced a 43.2% absolute reduction in neurotoxicity compared to the historical benchmark. Exploratory transcriptomic evaluations also linked consensus molecular subtype 4, which features stroma-rich tumor biology, to poorer patient prognoses.

Evaluating Treatment-Escalation Success Rates

Recurrences were observed in 15 of the 100 ctDNA-negative patients, representing a 15% recurrence rate.
Among the 35 ctDNA-positive patients, 13 experienced recurrences, translating to a 37% recurrence rate.
Estimated 2-year disease-free survival reached 87.7% for ctDNA-negative patients versus 61.8% for ctDNA-positive patients.
Median time to recurrence measured 14.8 months for the negative group and 10.9 months for the positive group.

Frequently Asked Questions About the PEGASUS Trial

How many clinical centers participated in the PEGASUS trial?

The investigator-initiated study was conducted across 11 clinical centers situated across Italy and Spain.

What specific chemotherapy regimen did patients with persistent molecular residual disease receive?

Patients who remained ctDNA positive following initial capecitabine and oxaliplatin treatment escalated their therapy to a six-month course of folinic acid, fluorouracil, and irinotecan.

What were the overall survival rates at three years for the trial participants?

Three-year overall survival reached 93.8% among ctDNA-negative patients and 83.9% among ctDNA-positive patients.

What biomarker subtype correlated with poorer clinical outcomes in exploratory analyses?

Exploratory transcriptomic analyses linked consensus molecular subtype 4, characterized by stroma-rich tumor biology, to worse prognoses.

Can prospective randomized studies confirm whether tailoring adjuvant chemotherapy entirely eliminates the need for oxaliplatin in biomarker-negative colon cancer patients, or will survival endpoints continue to require standard combination approaches?