News|Articles|March 17, 2026
GLSI-100 Vaccine Shows <1% Annual Recurrence Rate in Phase 3 Breast Cancer Trial
Author(s)Sabrina Serani
Fact checked by: Paige Britt
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Key Takeaways
- An open-label non–HLA-A*02 cohort showed a 0.7% annualized recurrence rate after primary immunization with GLSI-100, measured over an average 1.2 patient-years of exposure.
- Historical comparison to KATHERINE’s T-DM1 arm (~4% annual recurrence) yielded an estimated 83% recurrence-rate reduction, but lacked Kaplan-Meier analysis due to no placebo comparator.
Preliminary data from the fully enrolled, 250-patient, open-label non-HLA-A*02 arm of the ongoing phase 3 FLAMINGO-01 trial (NCT05232916) show a recurrence rate of less than 1% per year in patients with HER2-positive breast cancer treated with GLSI-100 following completion of the primary immunization series (PIS). The observed rate was statistically significantly lower than the approximately 4% annual recurrence rate reported for a comparable population in the KATHERINE trial (NCT01772472) treated with trastuzumab emtansine (T-DM1; Kadcyla).1
Because the non-HLA-A*02 arm lacks a direct placebo comparator, a Kaplan-Meier survival analysis was not possible. Instead, investigators compared the observed recurrence rate among treated patients to historical data from the KATHERINE trial. As of the current data cut, the annualized recurrence rate in non-HLA-A*02 patients who completed the PIS—measured over an average of 1.2 patient-years of exposure—was 0.7%, compared with approximately 4% per year observed over a similar follow-up period in the T-DM1–treated arm of the KATHERINE trial. This corresponds to an approximately 83% reduction in recurrence rate (Chi-square, P <.005).
The company noted that the majority of FLAMINGO-01 participants also received T-DM1 prior to enrollment in the trial, followed by GLSI-100 making the KATHERINE trial a relevant, if imperfect, reference population.
The company emphasized that this preliminary data will continue to be updated and cleaned, and that future and final results may vary. The data have not been fully reviewed and are not predictive of final study outcomes.
Consistency With Prior Results
The non-HLA-A*02 arm findings are described as trending similarly to results from the prior phase 2b study, in which HLA-A*02 patients treated with GLSI-100 demonstrated reductions in breast cancer recurrence of up to 80%, compared with a reported 20% to 50% reduction associated with other approved products. Immune response at baseline, the rising immune response trajectory during the PIS, and the safety profile of non-HLA-A*02 patients are also reported to be tracking comparably to the HLA-A*02 arms of FLAMINGO-01 and to phase 2b data. Virtually all non-HLA-A*02 patients enrolled in FLAMINGO-01 have completed the PIS.
Trial Design and Enrollment Status
FLAMINGO-01 is a phase 3 trial designed to evaluate the safety and efficacy of fast track-designated GLSI-100 in patients with HER2-positive breast cancer who had residual disease or high-risk pathologic complete response at surgery and who have completed both neoadjuvant and postoperative adjuvant trastuzumab-based treatment. The trial is led by Baylor College of Medicine and includes US and European sites from university-based hospitals and academic and cooperative networks, with plans for up to 150 sites globally.2
The trial includes two double-blinded arms in which approximately 500 HLA-A*02 patients are to be randomized to GLSI-100 or placebo, and a third open-label arm in which up to 250 patients of other HLA types receive GLSI-100. The study is powered to detect a hazard ratio of 0.3 in invasive breast cancer–free survival, with 28 required events. An interim analysis for superiority and futility is planned when at least 14 events have occurred.1
More than 1000 patients have been screened to date, with a current annualized screening rate of approximately 800 patients per year. The 250-patient non-HLA-A*02 arm is now fully enrolled. Every 6 months, approximately 110 patient-years are added to the non-HLA-A*02 database.
REFERENCES
1. Greenwich LifeSciences provides update showing continued reduction in recurrence rate in the open label arm of FLAMINGO-01. News release. Greenwich LifeSciences. March 17, 2026. Accessed March 17, 2026. https://tinyurl.com/25czk3px
2. Phase 3 study to evaluate the efficacy and safety of HER2/Neu peptide GLSI-100 (GP2 + GM-CSF) in HER2/Neu positive subjects (FLAMINGO-01). ClinicalTrials.gov. Updated October 3, 2025. Accessed March 17, 2026. https://clinicaltrials.gov/study/NCT05232916
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