Abstract Background: Patients with HER2-overexpressing breast cancer often exhibit reduced immunity to the HER2 antigen. Therapeutic vaccines can enhance cytotoxic T cell responses against tumor-associated antigens, enabling selective tumor destruction. Both peptide- and DNA-based platforms have been explored for vaccine delivery, but direct comparisons of their safety and immunogenicity remain limited. We hypothesize that DNA-based vaccines may generate more robust and durable HER2-specific immunity than other vaccine platforms due to prolonged antigen expression from persistent plasmid presence in host tissues. Methods: We retrospectively analyzed 66 patients from a phase 1 DNA-vaccine trial and 38 from a phase 2 peptide-vaccine trial, all with stage III or IV HER2-positive breast cancer and either no evidence of disease or stable bone-only disease at enrollment. Both vaccines targeted HER2 intracellular domain (ICD) epitopes. Immunogenicity was assessed via IFN-γ ELISpot assays on cryopreserved PBMCs stimulated with HER2 ICD or extracellular domain (ECD) peptides as an indicator of epitope spreading. Changes in T-cell responses from baseline to peak post-vaccination were evaluated using the Wilcoxon test, and between-trial differences using the Mann-Whitney test. Patients were stratified by baseline HER2 immunity and immunologic responder status. Adverse events were recorded per NCI CTCAE v4.0. Results: The magnitude of T-cell immunity achieved to the HER2 ICD varied between the two modes of vaccination and was dependent on whether patients had preexisting immunity to HER2 when entering the study. Patients receiving the DNA vaccine showed significant increases in HER2 ICD immunity regardless of preexisting HER2 immunity (p=0.005 with, p0.001 without), while those receiving the peptide vaccine showed elevated responses only in patients without preexisting immunity (p0.001; p=0.76 with immunity). Both vaccines induced intramolecular epitope spreading, that is a broadening of the immune response to the HER2 ECD, representing the induction of endogenous immunity. DNA-vaccine recipients showed significant ECD immune responses both in those with (p=0.03) and without (p0.001) baseline HER2 immunity. In contrast, peptide-vaccine recipients showed increased ECD immunity only in those without baseline immunity (p=0.04; p=0.33 with immunity). The DNA group had higher ECD immunity at baseline (p=0.02) and post-vaccination (p=0.005). The elevation in T-cell immunity from baseline to peak post-vaccination was comparable between vaccine types for both HER2 ICD (p=0.37) and ECD (p=0.13), including among responders (p=0.35 for ICD, p=0.44 for ECD). However, among non-responders, DNA-vaccine recipients demonstrated greater increases in both HER2 ICD (p=0.005) and ECD (p=0.04) immunity. One grade 3 ALT elevation occurred in the DNA-vaccine trial; no grade 3–5 events were observed in the peptide-vaccine trial. Conclusion: Both DNA- and peptide-based HER2 vaccines were well tolerated and capable of inducing HER2-specific T-cell responses in patients with advanced HER2-positive breast cancer. However, the DNA vaccine elicited broader and more consistent immune activation, including stronger epitope spreading to HER2 ECD and greater responses among patients without preexisting immunity. Notably, DNA-vaccine recipients demonstrated enhanced immunity even among non-responders, suggesting a more robust platform for overcoming baseline immune tolerance. Citation Format: Y. Liu, S. Huang, Y. Dang, M. L. Disis. Dna-based HER2 vaccine induces broader and more durable t-cell responses than peptide vaccine in advanced HER2-positive breast cancer abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS5-01-17.
Liu et al. (Tue,) studied this question.