Danish biotechnology company ExpreS2ion Biotech Holding AB (STO:EXPRS2) on Thursday announced initial immunogenicity data based on the first patient enrolled in its Phase I clinical trial of ES2B-C001, a therapeutic vaccine targeting HER2-expressing breast cancer.

The patient received two vaccine doses over six weeks, with blood samples taken before dosing and three weeks after each dose. Analysis showed a significant rise in HER2-specific antibodies following the second dose, indicating a vaccine-induced immune response.

Researchers highlighted that the antibody increase is independent of prior monoclonal antibody or antibody-drug conjugate therapies. This represents the first confirmation that ES2B-C001 can activate the human immune system and break tolerance toward HER2, consistent with its intended mechanism of action.

Investigators emphasised the preliminary nature of the findings, noting the results are limited to one patient. Additional data from the first cohort are expected after review by the Data Safety Monitoring Board later this year.

ExpreS2ion's leadership described the results as an important milestone, underscoring both the potential of ES2B-C001 and the value of the company's vaccine platform. The Phase I trial, which commenced in June 2025, continues to recruit patients.