Abstract PS10-54: Single-centre retrospective study of treatment choices and outcomes of metastatic breast cancer post-progression on the CDK4/6 inhibitor palbobiclib
Authors: Lagunes Maria Luisa Romero , Sickandar Faisal , Thawer Alia , Stjepanovic Neda , Jerzak Katarzyna , Trudeau Maureen , Eisen Andrea , Gandhi Sonal , Warner Ellen , Mata Danilo Giffoni M. M. , Al-Humiqani Abdullah MA , Lott Anthony , Pezo Rossanna C 
Published: February 2021

Abstract
Background: For Hormonal Receptor positive/HER2 negative (HR+/HER2-) metastatic Breast Cancer (mBC) the optimal treatment post-progression on a CDK4/6 inhibitor is still not defined. We aim to identify real-world patterns of systemic treatment choice following palbociclib use at a large tertiary cancer centre and to determine the outcomes for this patient population.
Methods: We identified HR+/HER2- mBC patients treated with palbociclib between January 2016 and June 2020 using the institutional computerized prescriber order entry (CPOE) system. Patients with a 2nd primary other than breast, male, or with HER 2 overexpression were excluded. Electronic medical records were retrospectively reviewed to determine clinical, pathological characteristics, and treatment patterns. Our primary outcome was Time to Treatment Failure (TTF) for the subsequent treatment, using the Kaplan Meier survival method.
Results: A total of 136 patients were included. At first diagnosis the median age was 52 years; 52% were premenopausal, the most prevalent primary tumor features were ductal histology (79%), and lymph node positive disease (54%). Regarding initial treatment 77% underwent surgery, 52% had adjuvant radiation therapy, 61% had chemotherapy (CT) and 67% had endocrine therapy (ET). The most frequent metastatic sites were bone 70%, liver 36%, and lung 33%. 63 tumors were rebiopsied, in 21 (33.3%) biomarkers had changed. Palbociclib was indicated as 1 Line in 45% of patients, 2 Line 26%, and ≥ 3 Line 27%. The most tolerated dose was 75 mg (44% v 33% tolerating 125 mg). The most prescribed endocrine backbones were AI (66%) and fulvestrant (30%). mTTF was 29.9 mo. (95%CI 12.68-47.11) for 1L, 33.2 mo. (95%CI 22.45-44.08) for 2L, 7.03 mo. (95%CI 2.29-11.76) for ≥ 3L. After a median follow-up of 18.7 mo, 74 patients (54%) had discontinued palbociclib due to progression (46%), toxicity (5%), or death (4%). Sites of progression were liver 37%, bone 29%, pleura 11%, lung 10% and peritoneum 6.5%. 63 patients had subsequent systemic therapy, with mTTF of 5.6 mo. 46 patients (34%) received chemotherapy, 29 patients had capecitabine, mTTF was 4.8 mo. 13 patients (9%) received endocrine therapy, 9 patients had fulvestrant +/- others, mTTF was 8.9 mo. For 5 patients (4%) the subsequent treatment was CDKi-based, 4 patients continued with palbociclib plus another endocrine backbone, mTTF was 16.5 mo. Only 1 patient (.7%) received everolimus-based subsequent treatment, with a TTF 5.6 mo.
Conclusion: In this real-world analysis, we found that palbociclib was most tolerated at 75 mg and most prescribed with an AI. After progression on CDKi, the tendency was to prescribe a chemotherapy-based subsequent line, mainly capecitabine.
Citation Format: Maria Luisa Romero Lagunes, Faisal Sickandar, Alia Thawer, Neda Stjepanovic, Katarzyna Jerzak, Maureen Trudeau, Andrea Eisen, Sonal Gandhi, Ellen Warner, Danilo Giffoni M. M. Mata, Abdullah MA Al-Humiqani, Anthony Lott, Rossanna C Pezo. Single-centre retrospective study of treatment choices and outcomes of metastatic breast cancer post-progression on the CDK4/6 inhibitor palbobiclib [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS10-54.