biostudies-literatureUnknown30(5)Hao WThis 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 therapeOncology letters505https://www.ebi.ac.uk/biostudies/studies/S-EPMC12434459biostudies-literature&lt;i&gt;Pneumocystis jirovecii&lt;/i&gt; pneumonia in a patient with HR&lt;sup&gt;+&lt;/sup&gt;/HER2&lt;sup&gt;-&lt;/sup&gt; advanced breast cancer following abemaciclib combined with endocrine therapy: A case report.PMC12434459Bao JSun CWu CHao WQin YLiu Sfalse&lt;i&gt;Pneumocystis jirovecii&lt;/i&gt; pneumonia in a patient with HR&lt;sup&gt;+&lt;/sup&gt;/HER2&lt;sup&gt;-&lt;/sup&gt; 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 therape2025-01-01T00:00:00Z2025 Nov2026-04-08T19:52:33.955Z2026-04-08T14:35:41.843ZS-EPMC124344594095971210.3892/ol.2025.15251