Abstract: Purpose: CDK4/6 inhibitors (CDK4/6i) combined with endocrine therapy (ET) are considered standard-of-care for first-line therapy of patients with hormone receptor positive, HER2 negative, advanced breast cancer (HR+/HER2- ABC). Superiority of combination therapy over endocrine monotherapy has been demonstrated in a multitude of randomized controlled phase III and IV clinical trials (RCTs). However, RCTs reflect clinical reality only to a limited extent, as narrow inclusion criteria lead to a selected patient collective. Here, we present real world data (RWD) on CDK4/6i treatment in patients with HR+/HER2- ABC at four certified German university breast cancer centers. Methods: Patients diagnosed with HR+/HER2- ABC who were treated in clinical routine with CDK4/6i between November 2016 and December 2020 at four certified German university breast cancer centers (Saarland University Medical Center, University Medical Center Charité Berlin, University Medical Center Bonn and University Medical Center Hospital Schleswig-Holstein, Campus Kiel) were identified and enrolled in this retrospective study. Clinicopathological characteristics and clinical outcomes were recorded with particular emphasis on CDK4/6i therapy course (progression-free survival (PFS) following treatment initiation, toxicity, dose reduction, therapy discontinuation, prior and subsequent therapy line). Results: Data from n=448 patients were evaluated. The mean patient age was 63 (± 12) years. Of these patients, n=165 (36.8 %) were primarily metastasized and n=283 (63.2 %) had secondary metastatic disease. n=319 patients (71.3 %) received Palbociclib, n=114 patients (25.4 %) received Ribociclib, and n=15 patients (3.3 %) received Abemaciclib, respectively. Dose reduction was performed in n=132 cases (29.5 %). n=57 patients (12.7 %) discontinued the treatment with CDK4/6i due to side effects. n=196 patients (43.8 %) experienced disease progression under CDK4/6i treatment. The median PFS was 17 months. Presence of hepatic metastases and prior therapy lines were associated with shorter PFS, whereas estrogen positivity and dose reduction due to toxicity were positively associated with PFS. Presence of bone and lung metastases, progesterone positivity, Ki67 index, grading, BRCA1/2 and PIK3CA mutation status, adjuvant endocrine resistance and age did not significantly impact on PFS. Conclusion: Our RWD analysis on CDK4/6i treatment in Germany supports data from RCTs regarding both treatment efficacy and safety of CDK4/6i for treatment of patients with HR+/HER2- ABC. In comparison to data from the pivotal RCTs, median PFS was lower but within the expected range for RWD which could result from inclusion of patients with more advanced disease (i.e., higher therapy lines) to our dataset.

The Author(s). Published by S. Karger AG, Basel