Abstract RF01-03: PARSIFAL-LONG: Extended follow-up of hormone receptor-positive/HER2-negative advanced breast cancer patients treated with fulvestrant and palbociclib vs. letrozole and palbociclib in the PARSIFAL study

Abstract Background: The Phase 2 PARSIFAL study assessed whether fulvestrant (FUL) or letrozole (LET) was the optimal endocrine partner for the cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) palbociclib (PAL) in patients (pts) with untreated, endocrine-sensitive, hormone receptor-positive (HR+)/HER2-negative (HER2-) advanced breast cancer (ABC) in the first-line setting. This trial failed to demonstrate an improvement in progression-free survival (PFS) of PAL+FUL over PAL+LET (Llombart-Cussac et al. Jama Oncol 2021). Following progression on CDK4/6i-based regimens in the metastatic setting, the EMERALD study (Bardia et al, SABCS 2022 GS3-01) identified early progressions (< 12 months [mo]) as a strong predictor of resistance to subsequent endocrine therapies. Here, we report updated PFS and overall survival (OS) from PARSIFAL, exploring a PFS <12 mo threshold as a prognostic factor for poor outcomes. Methods: This was an observational, international, multicenter study that included pts from the prospective PARSIFAL study, in which pts were randomly assigned (1:1) to receive PAL (oral 125 mg/day, 28-day cycles; 3 weeks on, 1 week off) plus FUL or LET at conventional doses. The primary objective was to extend the assessment of OS of PARSIFAL study with a longer median follow-up. Secondary objectives included extended PFS, other post-progression efficacy data, and the identification of new prognostic and predictive markers. The design had a planned recruitment of at least 388 pts with 195 deaths. The 2-sided stratified log-rank test (α = 0.05) had a 70% power to detect a hazard ratio ≤0.70 in favor of FUL + PAL arm. Results: A total of 389 pts (80.5%) from the PARSIFAL study were included in this analysis, involving 32 of the 47 original sites. Pts signed a new informed consent form according to local regulations. Demographic and baseline disease characteristics were similar between the PARSIFAL-LONG and the overall PARISFAL intention-to-treat populations. At the time of analysis, after a median follow up of 5.0 years (range, 0.1-7.3), 241 and 213 events were reported for PFS and OS, respectively. No differences in efficacy were observed between treatment arms whether for PFS (hazard ratio, 1.0, p=0.985) or OS (hazard ratio, 0.94, p=0.635). In accordance with the protocol for PARSIFAL-LONG, both arms were combined for subsequent analysis. The median PFS (mPFS) for the first-line PAL-based regimen population was 33.2 mo (95%CI, 27.7-39.5), with a median OS (mOS) of 65.4 mo (95%CI, 57.8-72.0). A total of 86 pts (22.1% of the population) had a mPFS time <12 mo (early progressors). mOS and mPFS for this early progressor subgroup were 24.0 mo (95%CI, 17.3-30.1) and 7.0 mo (95%CI, 5.6-8.3), respectively, and only 11 pts (12.8%) were still alive at the time of analysis. The remaining 303 pts (77.9%) were progression-free on PAL-based regimens at 12 mo (PFS≥12). The number of events for PFS and OS at this time were 165 (54.5%) and 138 (45.5%), respectively. mOS from randomization for the PFS≥12 subgroup was 81.5 mo (95%CI, 70.2-not achieved) and the mPFS was 49.8 mo (95%CI, 40.9-59.8). Following progression on PAL-based regimens, the PFS≥12 criteria was a strong predictor for mOS, with 27.0 vs 18.0 mo (hazard ratio, 0.67, 95%CI, 0.51-0.90, p=0.007) for PFS≥12 and early progressors, respectively. These differences may increase in the future, as 54.5% of pts with PFS≥12 are still alive compared to 12.8% of pts with an early progression. Conclusions: Extended follow-up analysis from PARSIFAL study confirms no major differences between LET or FUL when combined with PAL. mPFS and mOS results are consistent with those reported in other first-line trials involving different CDK4/6i. Progression within the first year of first-line CDK4/6i-based regimen for HR+/HER2- ABC pts may be prognostic of less favorable outcomes. Citation Format: Antonio Llombart-Cussac, José Manuel Pérez-García, Meritxell Bellet- Ezquerra, Florence Dalenc, Miguel Gil-Gil, Manuel Ruíz - Borrego, Joaquín Gavilá, Peter Schmid, Pilar Zamora, Duncan Wheatley, Eduardo Martínez-de Dueñas, Kepa Amillano, Antonio Antón, Paul Cottu, Gemma Viñas, Thierry Petit, Petra Tesarová, Juan Cueva, Marco Colleoni, Maria Purificación Martínez del Prado, Raquel Andrés, Marta Díaz Cabo, Susana Victorino, Miguel Sampayo-Cordero, Javier Cortés. PARSIFAL-LONG: Extended follow-up of hormone receptor-positive/HER2-negative advanced breast cancer patients treated with fulvestrant and palbociclib vs. letrozole and palbociclib in the PARSIFAL study [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr RF01-03.