Abstract PO1-16-08: Real-World Outcomes of Patients Receiving First-line Palbociclib Plus Endocrine Therapy in Spain: Subgroup Analysis Based on Tumor Grade, Progesterone Receptor, Ki-67 and Histological Subtype from PALBOSPAIN Study

Abstract Background PALBOSPAIN (NCT04874025) is an observational, multicenter study evaluating real-world patterns and outcomes of first-line treatment with palbociclib plus endocrine therapy (ET) in routine clinical practice in Spain. Results from real-world progression-free survival (rwPFS) and overall survival (OS) in the total population, as well as subgroup analysis according to endocrine sensitivity, age, menopausal status, location and number of metastatic sites have been previously reported. Here, we present the prognostic impact of pathological features in metastatic breast cancer patients treated with Palbociclib plus endocrine therapy. Methods Patients diagnosed with hormone-receptor positive and HER2 negative ABC who started first-line treatment with palbociclib plus ET between November 2017 and November 2019 were included. We retrospectively collected clinical-pathological and treatment response data from patient medical records. Estrogen receptor (ER), Progesterone receptor (PR), and Ki67 were assessed by immunohistochemistry (IHC). HER2 was assessed by IHC or fluorescence in situ hybridization (FISH). To evaluate the prognostic role of proliferation index, Ki-67 cut-off was set at 20%. If results of several biopsies were available, the most recent one was used for analysis. Survival analysis (rw-PFS, OS) were estimated using the Kaplan–Meier method with log-rank test. Times for event-free patients at the time of data cutoff in all survival analyses were calculated from the date of treatment start to the date of last follow-up. The association between prognostic factors and survival was examined using Cox proportional hazards regression model. Results A total of 762 patients from 35 hospitals met inclusion and exclusion criteria and were included in the analysis. The cut-off date for this analysis was July 2022 and the median duration of follow-up was 29 months. The distribution of the pathological features is shown in Table 1. Patients with histological grade 1 tumors had a median rwPFS of 33 months, while grade 2 tumors experienced a median rwPFS of 24 months and grade 3 tumors had the poorest outcome with a median rwPFS of 19 months. Median OS was 43, 42 and 40 months for grade 1, 2 and 3 respectively. Patients with Ki-67 < 20% had better rwPFS (35 vs 20 months; HR 0.60, p=0.004) and OS (not reached [NR] vs 35 months; HR 0.66, p=0.002) than those with Ki-67 ≥20% Patients with low ki-67 had better outcomes compared with those with high Ki-67. rwPFS (35 vs 20 months; HR 0.60, p=0.004) and OS (not reached [NR] vs 35 months; HR 0.66, p=0.002) for Ki 67 < 20% and ≥20% respectively. No differences were found according to histological subtype, with similar median rwPFS (23 vs 25 months; HR 0.96, p= 0.77) and OS (42 vs 43 months; HR 0.94, p=0.74) for ductal and lobular carcinomas respectively. Double hormone receptor positive tumors (ER+/PR+) had significant longer rwPFS (27 vs 18 months; HR 0.72, p=0.001) and OS (44 vs 40 months; HR 0.73, p= 0.01) compared with ER+/PR- tumors. Conclusions Grade 3 tumors, Ki 67 ≥20% and absence of PR are associated with reduced rw-PFS and OS in patients treated with palbociclib plus ET in clinical practice. Pathologic results have a prognostic role and may be useful to identify patients who derive more benefit with first-line CDK 4/6 inhibitors plus ET treatment. Table 1 Pathological Characteristics in PALBOSPAIN study Citation Format: Fernando Moreno, Meritxell Bellet- Ezquerra, Luis Manso, Fernando Henao, Antonio Antón, Serafin Morales Murillo, Pablo Tolosa, Verónica Obadía, Teresa Sampedro, Raquel Andrés, Lourdes Calvo, Elena Galve, Rafael López, Francisco Ayala de la Peña, Sara López-Tarruella, Blanca Hernando, Laia Boronat, J. Ignacio Chacón, Noelia Martínez-Jáñez. Real-World Outcomes of Patients Receiving First-line Palbociclib Plus Endocrine Therapy in Spain: Subgroup Analysis Based on Tumor Grade, Progesterone Receptor, Ki-67 and Histological Subtype from PALBOSPAIN 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 PO1-16-08.