Abstract Introduction: Trastuzumab has a major impact on the treatment of HER2 positive breast cancer (BC). Anti-HER2 biosimilars, such as OgivriTM, demonstrated safety and clinical equivalency to the reference product HerceptinTM in clinical trials. However, real-world reporting of side effects, quality of life (QoL) and outcome in patients treated with biosimilars has not yet been performed using electronic PROs. Here standardized and structured ePROs are dynamically recorded with the mediduxTM app during OgivriTM treatment and are applied to perform a comparative analysis with ePROs derived from 38 historical patients treated with HerceptinTM in two previous studies (NCT02004496, NCT03578731). Methodology: In this prospective observational study, patients with HER2-positive BC were treated with OgivriTM alone +/- Pertuzumab, +/- Chemotherapy and hormone therapy in neo-adjuvant, adjuvant and non-curative settings. Over an observational period of 6 weeks, patients used the mediduxTM app to dynamically record symptoms (according to CTCAE), well-being, EQ-5D-5L, cognitive capabilities, and vital parameters. Comparative analysis was performed with historical data from 38 patients that had reported 5217 ePROs and more than 1400 reports of well-being in similar therapeutic settings. Results: 52 females (age 37-86 yrs) were treated with OgivriTM. From 92 different symptoms available, 84 were entered (average > 2 symptoms/day) resulting in 10`532 individual patient entries. Among the most common symptoms reported in both groups were fatigue, taste disorder, nausea, diarrhea, dry mucosa, joint discomfort, tingling, sleep disorder, headache, appetite loss. As compared to HerceptinTM, overall, symptoms associated with OgivriTM were reported with a similar incidence and distribution among groups (p=0.68), although slightly but not significantly lower scores (p=0.24; CI: 0.08-0.31). Distribution of symptom grades in the OgivriTM cohort revealed that the vast majority of patients experienced mild and grade 1 toxicities, 13% grade 2, 2% grade 3 and 0.4% grade 4 toxicities, respectively. The latter finding could possibly also be attributed to a trend towards de-escalation in chemotherapeutic regimes during the past 5 years since data acquisition, affecting anthracyclines and taxanes, and that. Of note, when treatments were compared restricted to the trastuzumab antihormones and trastuzumab- paclitaxel therapies, this reported trend seamed weaker. Importantly, well-being of patients undergoing HER2 directed treatment did not differ (p=0.24; CI: 0.39-0.88) in both cohorts. Conclusion: No difference was reported for symptoms, adverse events, and well-being with respect to the Trastuzumab biosimilar OgivriTM in comparison with HerceptinTM. The integration of ePRO into research and clinical practice provides reliable information when investigating real world tolerability and outcomes of similar therapeutic compounds. Citation Format: Andreas Trojan, Sven Roth, Reinhard Zenhäusern, Yannick Kadvany, Christian Jackisch, Matti Aapro, Alexandru Eniu. HER2-directed biosimilar Ogivri in the treatment of breast cancer: real world reporting of symptoms and wellbeing using electronic patient reported outcome (ePRO) [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-17-04.