Table 1.

Attrition

Criterion	Description	N included	N excluded
Breast cancer patients with CTR-curated data availability	Syapse identifies potentially eligible patients via our structured data sources via International Classification of Disease diagnosis codes (ICD9 174.x or 175.x or ICD10 C50.x) Patient’s histology confirmed disease is CTR-verified prior to curation	15,689
Known dates of breast cancer diagnosis	Has breast cancer diagnosis date	14,383	1306
Age 18 years or older	Patient is 18 or older at breast cancer diagnosis	14,382	1
Evidence of mBC diagnosis	mBC - stage at diagnosis IV, IVa, IVb, IVc - or clinician assessed patient has progressed to metastatic disease	2490	11,892
Exposure to HER2-directed therapy in mBC setting by February 22, 2020a	HER2-directed therapies approved prior to December 2019: HER2-directed therapies include: Trastuzumab Pertuzumab Ado-Trastuzumab emtansine Neratinib Lapatinib Trastuzumab-anns Trastuzumab-dkst Trastuzumab-dttb Trastuzumab-pkrb Trastuzumab-qyyp	544	1946
No exposure to trastuzumab deruxtecan prior to study end date (May 22, 2020)		533	11

CTR certified tumor registrar, HER2 human epidermal growth factor receptor 2, ICD International classification of diseases, mBC metastatic breast cancer

^aIndex date defined by first exposure. This end date allows patients to have a minimum of 3 months of potential post-index follow-up prior to study end date