202P Complete response rates, treatment patterns and survival outcomes in patients treated with first-line pertuzumab-based therapy in HER2-positive advanced breast cancer: A SONABRE study

Complete response (CR) rates on first-line pertuzumab-trastuzumab-chemotherapy (PTC) in HER2-positive (HER2+) advanced breast cancer (ABC) are unknown. We aimed to describe real world CR rates, predictors of CR, treatment patterns and outcome in patients treated with first-line PTC for HER2+ ABC. We included patients with HER2+ ABC who started first-line PTC in ten Dutch hospitals in 2013-2019 from the SONABRE registry (NCT-03577197). CR was defined as no disease evidence on imaging, or post-resection and/or radiotherapy. Multivariable logistic regression was used to identify predictors for CR. Treatment patterns in patients achieving CR were reported. Progression-free survival (PFS) and overall survival (OS) from start of first-line PTC were computed with the Kaplan-Meier method. We identified 159 patients treated with first-line PTC, of whom 84% were <65 years, 48% hormone receptor-positive disease, 51% de novo disease, 75% visceral metastases, and 11% CNS metastases. The chemotherapy backbone included taxanes (93%). The median follow-up duration of 62 months. Forty patients (25%) reached CR. Patients with de Novo ABC (Odds ratio (OR)=2.35, 95% confidence interval(CI):1.05-5.28, p=0.04) and single organ metastasis (OR=3.44, 95%CI:1.58-7.49, p=0.002) were more likely to reach CR. Among patients reaching CR, 21 (53%) stopped pertuzumab and trastuzumab, 11 (27%) stopped pertuzumab but continued trastuzumab and 8 (20%) continued both until progression or last follow-up. In patients who achieved CR, the 5-year PFS rate was 59% (95%CI:39%-74%), while in those who did not, it was 9% (95%CI:4%-17%). The 5-year OS rates were 83% (95%CI:63%-93%) and 30% (95%CI:21%-40%), respectively. In this real world cohort, one in four patients treated with first-line PTC reached CR. In 80% of the patients achieving CR, pertuzumab was stopped early. Patients reaching CR have a better prognosis than those not reaching CR. Prospective studies on the safety of HER2-targeted therapy discontinuation in patients with CR in HER2+ ABC are warranted.