Early Evidence of Immune Activation

The patient received two doses of the vaccine over six weeks. Blood samples taken before dosing (baseline) and three weeks after each dose and analysed for antibody levels against the HER2 protein.

Encouragingly, a significant increase in HER2-specific antibodies was observed three weeks after the second dose, compared to pre-dose baseline. The antibody measured is particularly relevant because it reflects a patient-generated, vaccine-induced immune response and is not affected by prior monoclonal antibody (mAb) or antibody-drug conjugate (ADC) therapies.

The results represent the first confirmation that the vaccine is able to activate the human immune system and break tolerance toward HER2, consistent with its intended mechanism of action.

"It is encouraging to observe early signs of immunogenicity in this first patient," said Dr. Rupert Bartsch, MD, PhD, Clinical Division of Oncology, Medical University of Vienna, Austria, Deputy Principal Investigator of the trial. "While we must remain cautious and await broader safety and efficacy data, the observed increase in HER2-specific antibodies is consistent with a vaccine-induced response. These initial findings support further evaluation of ES2B-C001 as a novel immunotherapeutic strategy for HER2-expressing breast cancer."

Lars Damstrup MD, PhD, Director of Clinical Development, commented: "It is important to underline that this observation about immunogenicity is based on one patient. That being said, the readout is significant, and I hope we will see the similar responses after the first cohort of three patients have been assessed in the Data Safety Monitoring Board towards the end of the year."

Bent U. Frandsen, CEO of ExpreS2ion, commented: "This first patient-level readout is an important milestone for ExpreS2ion and the ES2B-C001 program. It is our first observation of the immune system's reaction to our vaccine in humans. While these data are early, the results strengthen our belief in this vaccine and the platform's potential and validates the years of preclinical development that preceded it. We look forward to generating more data as the trial progresses."

About ES2B-C001 (HER2-VLP)

ES2B-C001 is an innovative immunotherapy developed to treat HER2-expressing cancers by stimulating a patient's own immune system, offering a novel alternative to existing antibody-based approaches. This novel approach combines ExpreS2ion's ExpreS2 production platform with AdaptVac's VLP technology, both of which have been proven in clinical Phase III. The HER2-VLP vaccine's design aims to stimulate a robust and durable polyclonal immune response against HER2-expressing tumours, offering a complementary strategy to current treatments. Preclinical studies, as highlighted by Ruzzi et al. (2022), have demonstrated the safety and efficacy across multiple animal models, significantly inhibiting tumour growth and improving survival rates. By leveraging the patient's immune system, ES2B-C001 addresses the complexities of HER2-expressing breast cancers, potentially offering enhanced therapeutic options and filling critical gaps in the current treatment landscape. The ES2B-C001 Phase I clinical trial started as announced in June 2025.