201P The implementation of first-line pertuzumab and the impact on overall survival in patients with de novo HER2+ metastatic breast cancer in the Netherlands in 2008-2017: A population-based cohort study

This study aims to determine whether real-world overall survival (OS) of patients diagnosed with de Novo human epidermal growth factor receptor 2 positive (HER2+) advanced breast cancer (ABC) improved in the Netherlands since the introduction of pertuzumab in 2013. Patients systemically treated for de Novo HER2+ ABC in 2008-2017 were retrieved from the Netherlands Cancer Registry. Vital status was obtained on January 31, 2023. OS was estimated using the Kaplan-Meier method and compared between 2013-2017 and 2008-2012 through multivariable Cox proportional hazards modelling. The use of first-given HER2-targeted therapy was determined per period. Analyses were stratified for age, grade, hormone receptor status, number of organs with metastases, initial metastatic sites, and surgery of the primary tumour. Among 1, 458 systemically treated patients, OS was longer for patients diagnosed in 2013-2017 than in 2008-2012 (five-year OS rates were 46% versus 31%, adjusted hazard ratio=0.67, 95% confidence interval:0.59-0.76, p<0.001). OS improved for all subgroups except for patients with central nervous system (CNS) metastases. First-given HER2-targeted therapy was received by 88% of patients in 2013-2017 and 72% in 2008-2012. Pertuzumab use in 2013-2017 was 50%, which increased from 9% in 2013 to 76% in 2017. Pertuzumab use was highest in patients aged <50 years (63%) and lowest in patients aged ≥75 years (19%). OS has improved by 33% in Dutch patients with de Novo HER2+ ABC 2013-2017 versus 2008-2012. This advancement was observed in all subgroups except for patients with CNS metastases. The implementation of pertuzumab was slow and deserves attention.