Abstract PS1-09-08: Impact of locoregional treatment after 6 months of palbociclib + letrozole combination therapy in de novo er+, her2- metastatic breast cancer patients: first results of the prospective palatine trial

Abstract Background: De novo metastatic breast cancer (mBC) accounts for approximately 30% of ER+/HER2- mBC. Treatment relies on systemic therapy. Retrospective and prospective trials have attempted to demonstrate the benefit of locoregional treatment on overall survival (OS) with mixed results. However, all these trials were before the CDK4/6 inhibitors era. The primary objective of the present trial was to evaluate the overall survival (OS) rate at 24 months in ER+/HER2- de novo mBC patients receiving palbociclib plus letrozole combination with a planned locoregional therapy after 6 months of systemic treatment Material and methods: We conducted a prospective multicentric, single arm, phase II trial, including women from 23 centers with de novo ER+/HER2- mBC. Patients with advanced, symptomatic, visceral spread, at risk of short-term, life-threatening complications and visceral crisis, were excluded. All patients received palbociclib + letrozole for 24-26 weeks, followed by a locoregional treatment of the investigator's choice: surgery and/or radiation therapy (RT). Palbociclib and letrozole were continued until progression. Assuming that a 24-month overall survival (OS) rate of 88% or lower is not clinically meaningful, and aiming for 93% power to detect a 24-month OS rate of 94% or higher, a total of 191 patients are required (one-sided alpha of 7%). Results: From 2019-10-28 to 2022-08-01, 209 patients were screened, 199 were evaluable and 191 patients were analyzed in ITT. Median age was 56 years (range 26-86) and 115 patients (59%) were post-menopausal. For the 183 patients with unilateral BC, the median tumoral size was 37.5 mm (range