428P Palbociclib dose patterns in Swedish patients with metastatic breast cancer: 5-year update from the SIRI study

Although palbociclib combined with endocrine therapy is a well-established treatment option in patients with hormone receptor-positive (HR+), human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer (MBC), there is limited evidence on patterns of dose reductions in real-world setting. The Swedish Ibrance Registries Insights (SIRI) study investigated real-world dose patterns using a nationwide cohort of palbociclib-treated MBC patients. This was a retrospective study utilizing population-based Swedish Health Data Registers. The cohort included all patients ≥ 18 years with ≥ 1 dispensation of palbociclib from January 2017 – June 2022. Minimum follow-up was 3 months. Starting dose and dose changes for the full population and for different age groups, in total and over time, was investigated. 2058 patients were identified and the median (IQR) age at treatment initiation was 68.4 (15.7) years. Patients were stratified into three age groups: <50, 50-69, ≥70 years, with the following distribution: 9.9%, 46.1%, and 44.0%, respectively. Most patients were initiated on 125 mg (82.0%), with a lower share for older patients (≥70 years; 74.4%). In total, 45.5% of patients had ≥ 1 dose reduction, with a higher frequency for older patients (≥70 years; 48.5%). Median (IQR) time to first dose reduction was 2.4 (3.9) months, and 5.9 (7.5) months to second dose reduction, with little variation across age groups. The probability for dose reduction was equal across age groups during the initial 6 months of treatment, whereafter it slightly increased for older patients (≥70 years). For patients starting with lower dose (100 mg) compared to standard dose (125 mg) at treatment initiation, the probability for dose reduction was similar during the first 3 months of treatment, whereafter it increased. Most Swedish palbociclib-treated patients were initiated on the recommended dose, but a lower starting dose and a higher frequency of dose reductions were observed for older patients. The time to dose reduction was equal across age groups, whereas the probability for dose reduction increased over time for older patients, and for patients with a start dose of 100 mg.