<i>Pneumocystis jirovecii</i> pneumonia in a patient with HR⁺/HER2^‑ advanced breast cancer following abemaciclib combined with endocrine therapy: A case report

This case report presents the first occurrence of <i>Pneumocystis jirovecii</i> pneumonia in a patient with breast cancer during CDK4/6 inhibitor therapy. Abemaciclib serves as a first-line therapeutic option for hormone receptor-positive/human epidermal growth factor receptor 2-negative advanced breast cancer. However, there are no documented cases in the literature linking abemaciclib to <i>Pneumocystis jirovecii</i> pneumonia. Notably, although rare, abemaciclib-induced severe lymphopenia may predispose patients to potentially life-threatening opportunistic infections. Optimal patient management requires multidisciplinary collaboration and strict compliance with American Society of Clinical Oncology/European Society for Medical Oncology guidelines to optimize the balance between therapeutic efficacy and infection risk mitigation. Critical strategies include early intervention and proactive surveillance to mitigate morbidity and mortality in this high-risk cohort. Consequently, the present case advocates for serial monitoring of lymphocyte counts and CD4⁺ levels during abemaciclib treatment, with implementation of primary prophylaxis protocols for patients exhibiting elevated risk profiles.