CD4+ Lymphocytopenia and Pneumocystis Jirovecii Pneumonia in A Metastatic Breast Cancer Patient Treated with Palbociclib and Corticosteroids.

Background: Palbociclib, a cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor used to treat metastatic breast cancer, is known to cause myelotoxicity and primarily neutropenia, but its potential to induce severe lymphopenia and opportunistic infections is less understood. Case description: A 61-year-old woman with metastatic breast cancer treated with palbociclib and corticosteroids was admitted with fever and general weakness. Chest radiography and computed tomography showed bilateral consolidation. She was diagnosed with severe cellular immunodeficiency, very low CD4+ T cell count and several opportunistic infections including Pneumocystis jirovecii pneumonia (PCP). Conclusion: This case underscores the need for monitoring lymphocyte counts in patients on CDK4/6 inhibitors, particularly those on concomitant corticosteroid therapy. and considering the risk for opportunistic infections in relevant patients. LEARNING POINTS: Palbociclib, a CDK4/6 inhibitor, may be associated with severe immunosuppression and low CD4+ T cell count.There may a greater risk for patients with additional risk factors such as concomitant corticosteroid therapy.Significant opportunistic infections such as Pneumocystis jirovecii pneumonia should be considered in clinically relevant patients.Safety measures should be taken such as blood count monitoring and even consideration of an antibiotic prophylaxis in relevant patients.