Abstract 939: Patient and oncologist factors associated with trastuzumab drug choice and administration route

Abstract Background: As biosimilars and subcutaneous (SQ) formulations of intravenous (IV) biologics enter the market, it is unclear how prescribers determine which formulation to give a patient. While patients may generally prefer SQ administration, IV biosimilars may be less expensive than newer SQ formulations. We focused on trastuzumab, which has name-brand SQ, name-brand IV, and biosimilar IV formulations, to assess potential association of patient and oncologist factors with drug choice and administration route. Methods: This cross-sectional study included patients with HER2+ breast or gastrointestinal (GI) cancer who received trastuzumab at the UChicago from 1/1/2021 to 12/31/2024. Patient factors, trastuzumab formulation and administration route, and prescriber were extracted from the electronic medical record. Prescriber demographics were obtained using public data from the Centers for Medicare & Medicaid Services. Multilevel multivariable logistic regression models assessed associations of patient and oncologist factors with IV vs SQ trastuzumab and brand-name IV vs biosimilar, nesting doses within patients and patients within oncologists. Models adjusted for patient insurance, age, race, sex, SVM quartile, ECOG score, BMI, cancer stage, and oncologist sex and years since graduation. The name-brand IV vs biosimilar model also adjusted for cancer type. The SQ vs IV model only included those with breast cancer as SQ trastuzumab is not approved for GI cancer, and it also accounted for concurrent IV chemotherapy administration. Results: A total of 35 oncologists and 368 patients were included, of which 319 (87%) had HER2+ breast cancer and 49 (13%) had HER2+ GI cancer. Most patients had private (47%) or Medicare (41%) primary insurance, and oncologists had a mean of 22 years of experience. IV biosimilars were given to 89% of patients. Patients with Medicaid had lower odds of receiving a biosimilar than patients with private insurance (aOR 0.11, 95% CI 0.04-0.34). No patients with GI cancer and 69 (22%) of patients with breast cancer received SQ. Patients with Medicare had lower odds of receiving SQ than those with private insurance (aOR 0.18, 95% CI 0.04-0.75). Except for concurrent IV chemotherapy administration in the SQ vs. IV model, no other patient factors beyond insurance and no oncologist factors were significantly associated with drug formulation or administration route. Conclusions: Drug choice and administration route were significantly associated with patient insurance. While administration route may be driven by the logistics of giving IV trastuzumab along with concurrent IV chemotherapy, no patient or oncologist factors were identified in prescribers’ decision-making about drug choice or administration route beyond insurance. It may be difficult to incorporate patient preferences or cost-saving measures into prescribing patterns if insurance does not allow for drug choice. Citation Format: Derek Hsu, Liam Schmitt, Austin Wesevich. Patient and oncologist factors associated with trastuzumab drug choice and administration route [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 939.