Rheumatoid arthritis is a common autoimmune disease that requires new therapeutic agents. Cyclin-dependent kinase 4/6 inhibitors have recently been approved for metastatic breast cancer patients and have also been reported to improve the arthritis score in collagen-induced arthritis mouse models. We report a 56-year-old woman who had previously been diagnosed with rheumatoid arthritis and treated with methotrexate. At age 40, she underwent surgery with curative intent for breast cancer but subsequently developed lung metastases. Palbociclib, a cyclin-dependent kinase 4/6 inhibitor, was administered in combination with fulvestrant (anti-oestrogen drug) for metastatic breast cancer. One month later, serum matrix metalloproteinase-3 and C-reactive protein levels were markedly decreased, and her rheumatoid arthritis symptoms, which had worsened just prior to the detection of metastatic lung disease, showed amelioration. Methotrexate, which had been used to treat her rheumatoid arthritis, could subsequently be administered in a reduced dose. The cyclin-dependent kinase 4/6 inhibitor was also effective for the metastatic breast cancer, and, to date, the patient's disease has remained stable for more than one year. Based on the results of basic research, cyclin-dependent kinase 4/6 inhibitors are promising new therapeutic agents for rheumatoid arthritis patients, although these drugs have not, as yet, been used in a clinical setting. To the best of our knowledge, this is the first report to describe a patient whose rheumatoid arthritis responded to a cyclin-dependent kinase 4/6 inhibitor administered for metastatic breast cancer.
Keywords: CDK4/6 inhibitor; Rheumatoid arthritis; breast cancer; paraneoplastic syndrome; target therapy.