Abemaciclib-Associated Psychosis: A Case Report of a Rare Neuropsychiatric Adverse Effect during Breast Cancer Treatment.

Introduction: Abemaciclib is a cyclin-dependent kinase 4/6 inhibitor widely used in combination with endocrine therapy for the treatment of hormone receptor-positive, HER2-negative advanced and early breast cancer. While generally well-tolerated, neuropsychiatric adverse effects are uncommon but may be clinically significant. Case Presentation: We report the case of a 72-year-old Caucasian female with stage I endometrial cancer and invasive ductal carcinoma of the breast who developed severe psychotic symptoms approximately 2 weeks after initiating abemaciclib, as part of her adjuvant breast cancer treatment. Her prior treatment had included adjuvant carboplatin and paclitaxel chemotherapy with brachytherapy, and her endocrine therapy was switched from anastrozole to letrozole at the time abemaciclib was added. Despite having a history of anxiety and obsessive-compulsive tendencies, she had no prior psychotic episodes. Her symptoms ultimately required two emergency department visits and a 9-day psychiatric hospitalization. Treatment with mirtazapine, olanzapine, and clonazepam resulted in the complete resolution of psychotic symptoms, and abemaciclib was permanently discontinued. After approximately 9 months, she was able to discontinue psychiatric medications without relapse of her psychotic symptoms. Conclusion: This case highlights the importance of recognizing rare but serious neuropsychiatric adverse effects of abemaciclib and emphasizes the need for careful monitoring and multidisciplinary management.