Unusual Presentation of Secondary Chronic Myeloid Leukemia in a Metastatic Breast Cancer Patient Receiving Cyclin-dependent Kinase 4/6 Inhibitor Therapy: A Case Report

Abstract An association between breast cancer treatment and secondary chronic myeloid leukemia (CML) has rarely been reported. In this report, we present a 62-year-old postmenopausal woman initially diagnosed with hormone receptor (HR)-positive, HER2-negative metastatic breast cancer who subsequently developed CML with an atypical presentation. The patient was initially diagnosed with Stage IV invasive ductal carcinoma. Treatment included modified radical mastectomy, palliative radiotherapy, chemotherapy, followed by endocrine therapy and the cyclin-dependent kinase 4/6 (CDK4/6) inhibitor, palbociclib. Six years after the breast cancer diagnosis, regular blood tests revealed thrombocytosis and basophilia. Even though bone marrow studies only revealed megakaryocytic proliferation, chromosomal analysis and genetic testing confirmed the diagnosis of chronic phase CML. This case not only demonstrates the occurrence of CML with an atypical presentation under CDK4/6 inhibitor treatment but also further indicates that patients receiving long-term CDK4/6 inhibitor therapy may develop secondary malignancies. In addition, while radiotherapy and chemotherapy are established risk factors for therapy-related myeloid neoplasms, the association between CDK4/6 inhibitors and secondary CML development, as well as their influence on the clinical outcomes of CML, remains poorly understood. Regarding management, data on the concurrent administration of BCR-ABL1 tyrosine kinase inhibitors and CDK4/6 inhibitors are limited. With CDK4/6 inhibitors becoming standard treatment for metastatic HR-positive breast cancer, longer follow-up and further studies are needed to clarify their relationship with secondary CML and to optimize treatment for these patients.