Abstract PS1-03-25: Incidence of hypersensitivity reactions in patients receiving trastuzumab biosimilars

Abstract Background: Currently multiple biosimilars of trastuzumab (Herceptin®) are approved for use in HER2-positive breast cancers. Accordingly, MD Anderson Cancer Center updated its formulary from trastuzumab (Herceptin®) to trastuzumab biosimilars: trastuzumab-qyyp (Trazimera®) and trastuzumab-anns (Kanjinti®), with trastuzumab-qyyp designated as the preferred agent. While these biosimilars have been approved based on extensive structural and functional characterization demonstrating similar clinical activity, differences in hypersensitivity reactions (HSRs) have been reported. We evaluated the incidence of HSRs in patients receiving trastuzumab biosimilars compared to the reference product. Methods: A retrospective chart review was conducted on patients who received trastuzumab products from January 1, 2019, to January 1, 2025. Patients with documented trastuzumab allergies in EMR were included in this analysis. Incidence of HSR between patients who received trastuzumab (Group A, n=612) and trastuzumab biosimilars (Group B, n=503) was compared. Results: HSRs were significantly more common in patients in patients who received trastuzumab biosimilars (Group B; n=86, 17.1%) compared to trastuzumab (Group A; n=22, 3.6%) (p0.001). Majority of the patients in Group B received Trastuzumab-qyyp (n=79, 92%). Chills and/or rigors were the most common HSR symptom across both groups (Group A: n=18, 75%); Group B: n=75, 87.2%). Severe symptoms, such as dyspnea (Group A: n=2 9.1% vs Group B: n=10 11.6%, p = 0.74) and hypotension (Group A: n=1, 4.5% vs Group B: n=5 5.8%, p=0.82), were not significantly different between the groups. Two patients in Group B required epinephrine, while no patients in Group A required epinephrine. Seven patients in Group B required a change in trastuzumab product. Conclusion: The incidence and severity of HSRs were higher in patients receiving biosimilar trastuzumab products compared to the reference product. Data collection for patients without documented trastuzumab allergies is ongoing. Based on this analysis, a prospective study utilizing prophylactic medications is being planned. Citation Format: E. M. Grannan, A. C. McGhie, A. Zheng, S. Cherian, A. B. Newman, R. M. Layman, S. Damodaran. Incidence of hypersensitivity reactions in patients receiving trastuzumab biosimilars 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 PS1-03-25.