Abstract PO5-05-01: ASSESING THE CLINICAL AND SURVIVAL RESULTS OF PATIENTS WITH HR-POSITIVE, HER2 NEGATIVE ADVANCED BREAST CANCER TREATED WITH CDK 4/6 INHIBITORS IN A SPANISH COHORT

Abstract BACKGROUND First line treatment of HR-positive, HER2 negative advanced breast cancer is based on endocrine therapy plus CDK 4/6 inhibitors (CDK 4/6i). Recently, there has been published in ASCO 2023 results from SONIA trial, which explores the use of CDK 4/6i in second-line treatment after monotherapy with endocrine therapy. This trial have shown using a CDK4/6i in the first-line setting did not significantly prolong the time from random assignment to progression on second-line therapy (PFS2) or overall survival (OS) when compared with deferred use until the second-line setting. METHODS We conducted a descriptive, observational and retrospective analyses of patients treated in our centre with CDK4/6i, comparing demographic, clinical and survival results. We also analysed different outcomes between first versus second line CDK4/6i. RESULTS A cohort of 153 patients were included, treated between January 2018 and June 2023, with a median age of 50 years. At diagnosis, 49% were stage I-II and 31% stage IV. 59% had visceral metastases at the start of CDK4/6i treatment. Palbociclib was the most commonly CDK4/6i used (55%), followed by Ribociclib (32%) and Abemaciclib (12.5%). The most widely endocrine therapy used was letrozole (60%), followed by fulvestrant (38%). 58% received CDK4/6i in first-line setting while 9% received them in second-line. The overall median progression-free survival (mPFS) was 18 months. In a subgroup analysis, the mPFS was 19 months with Palbociclib, 26 months with Ribociclib, and 31 months with Abemaciclib. Baseline characteristics were similar between subgroups of CDK4/6i. Better survival data were obtained with letrozole than with fulvestrant (not reached versus 29 months OS respectively). After CDK4/6i disease progression the most common subsequent treatment was chemotherapy (capecitabine, 23%). The median OS with palbociclib and abemaciclib was 37 months, while not reached with ribociclib. We analysed PFS2 using CDK4/6i in the first-line setting compared with deferred use until the second-line. The use of CDK4/6i in second-line showed a PFS2 of 45 months, while first-line PFS2 was 26 months. CONCLUSIONS Our study results are consistent with previous studies assesing the efficacy of cyclin inhibitors in first and second-line setting. Palbociclib seems to have a lower PFS rate. Patients treated with letrozole had a stadistically significant benefit in survival outcomes. Our data continue the trend observed in the SONIA trial in terms of PFS2. Using endocrine therapy as first-line monotherapy may be a valid treatment option. However, larger sample sizes are required to draw definitive conclusions. The identification of potential predictors of treatment response may help in the selection of the most effective treatment for patients with HR-positive HER2 negative advanced breast cancer. Citation Format: Alicia Cano-Jimenez, Rocio Urbano-Cubero, Ruben Garcia-Muñoz, Ana Maria Jaen-Morago, Maria Lomas-Garrido, Pedro Sánchez-Rovira. ASSESING THE CLINICAL AND SURVIVAL RESULTS OF PATIENTS WITH HR-POSITIVE, HER2 NEGATIVE ADVANCED BREAST CANCER TREATED WITH CDK 4/6 INHIBITORS IN A SPANISH COHORT [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 PO5-05-01.