# Utilization and Safety of Concurrent Use of Abemaciclib and Radiation Therapy Among Patients With HR+, HER2− Metastatic Breast Cancer in the Real-World Setting

## TL;DR

This study examines how well patients with a specific type of breast cancer tolerate radiation therapy and abemaciclib together in real-world settings.

## Contribution

The study provides real-world evidence on the safety and tolerability of concurrent abemaciclib and radiation therapy in HR+, HER2− metastatic breast cancer patients.

## Key findings

- Most patients did not need dose modifications when using abemaciclib and radiation therapy together.
- Common side effects included diarrhea, fatigue, rash, and neutropenia, but most were manageable.

## Abstract

Real-world data on tolerability of concurrent radiation therapy (RT) and abemaciclib are limited. This study described real-world utilization and safety of concurrent RT and abemaciclib in patients with hormone receptor-positive (HR+) and human epidermal growth factor receptor 2-negative (HER2−) metastatic breast cancer (MBC).

This retrospective study accessed data from the Flatiron Health United States nationwide deidentified electronic health records-derived longitudinal database. Abemaciclib initiation date was the index date (September 2017-September 2021). Concurrent RT was defined as receipt of any RT with ≥1 day of overlap with abemaciclib therapy. The minimum follow-up time was 90 days. Patient characteristics, treatment patterns, and real-world adverse events (rwAEs) were presented descriptively; Kaplan–Meier methods were used to assess time to treatment discontinuation.

This study included 174 female patients with a median follow-up time of 17.5 (IQR, 10.1-26.5) months. The median age was 63.0 (IQR, 54.0-71.0) years, 8.0% had Eastern Cooperative Oncology Group Performance Status ≥ 2 at index and 31.0% had MBC. Prior to abemaciclib use, 20.1% and 28.2% of patients had chemotherapy or other CDK4/6 inhibitors, respectively. About half of the patients (47.7%) received abemaciclib in combination with fulvestrant, and 76.4% initiated abemaciclib at 150 mg twice daily. Overall, 151 patients (86.8%) initiated RT at or after initiation of abemaciclib. During concurrent RT and abemaciclib use, 76.4% of patients had no dose change in abemaciclib; 16.7% and 2.3% had a dose hold or dose reduction, respectively; 4.0% discontinued abemaciclib. The incidence of rwAEs during concurrent abemaciclib + RT were diarrhea (73.0%), fatigue (62.6%), rash (27.6%), and neutropenia (23.0%). The median (95% CI) time to treatment discontinuation was 368 (290-516) days.

Most patients did not require a dose modification or interruption with concurrent RT and abemaciclib. These findings suggest that the addition of RT to abemaciclib therapy is well tolerated in patients with HR+, HER2− MBC.