Background: Biosimilars for trastuzumab, a HER2 inhibitor (HER2I), have been available for use in the US since in 2019, yet information on their utilization and patient characteristics is limited. We assessed utilization and patient characteristics for the trastuzumab originator, biosimilars, and other HER2Is in the US. Methods: We analyzed healthcare claims for 10/1/2016-up to 2/29/2020 (end date varied across health plans) from the Biologics and Biosimilars Collective Intelligence Consortium’s Distributed Research Network ( > 95 million persons) using the FDA’s Sentinel distributed analysis tools. We conducted descriptive analyses on the number of incident users and patients’ characteristics for each HER2I. Adults continuously enrolled in their health plan with medical and drug coverage ≥365 days (baseline period) prior to their incident HER2I use were eligible for analysis. Results: Of the incident users (incident to any HER2Is), we identified 6,631 originator trastuzumab users, 122 trastuzumab-anns, 116 ado-trastuzumab emtansine, 54 neratinib, and 54 lapatinib users. Trastuzumab-dkst and trastuzumab/hyaluronidase-oysk had < 11 users each. Mean age was the highest for trastuzumab/hyaluronidase-oysk (73.7 years; SD, 18.6) and similar between the trastuzumab originator and biosimilars (52.5-59.0). The number of incident users/100,000 person-years decreased for the trastuzumab originator from 13.5 in 2016 to 9.4 in 2020 and increased for trastuzumab-anns from 0.4 in 2019 to 4.9 in 2020. Of the baseline clinical characteristics examined, Charlson/Elixhauser comorbidity score was the highest for lapatinib (2.0), lowest for trastuzumab-dkst and neratinib (0.5), and similar between the trastuzumab originator (1.1) and trastuzumab-anns (1.3). The proportion of patients who received any chemotherapy during the baseline period was 38.9% for lapatinib, 18.5% for the trastuzumab originator, and 14.8% for trastuzumab-anns. The proportion of endocrine therapy users was the highest for neratinib (63.0%) and similar between the trastuzumab originator (11.1%) and trastuzumab-anns (10.7%). Among incident users with metastatic breast cancer, endocrine therapy receivers during the baseline period accounted for 19.3% for the trastuzumab originator and 69.6% for lapatinib. Conclusions: Though full data were not available for 2019-2020 for all health plans, these preliminary findings suggest that utilization of biosimilar trastuzumab-anns increased whereas the trastuzumab originator use decreased over time and that there is a variation in patient characteristics between HER2Is and by metastatic status while the characteristics were generally similar between the trastuzumab originator and trastuzumab-anns. We plan to conduct ongoing assessment of HER2I utilization as more data become available to help inform clinical practices and health policies.