Drug-Induced Eosinophilic Pneumonia as an Adverse Event of Abemaciclib

In 2020, a 45-year-old woman was started on fulvestrant and abemaciclib therapy to treat breast cancer which had recurred in her left breast after surgery. We were able to control her cancer using this treatment; however, the ground-glass opacity in the lower lobe of her right lung expanded, along with an increase in her peripheral blood eosinophil count. She was referred to the respiratory medicine department for a detailed examination including bronchoscopy. We discovered a high proportion of eosinophils in her bronchoalveolar lavage fluid and diagnosed the condition as drug-induced eosinophilic pneumonia. The ground-glass change improved after steroid administration. In this case, the adverse effects of abemaciclib, a cyclin-dependent kinase 4/6 inhibitor playing an essential role in breast cancer treatment, were discovered by combining blood, imaging, and bronchoalveolar lavage fluid findings. This contributed to an early introduction of treatment and prevented the deterioration of her quality of life.