A case of Merkel cell carcinoma development under treatment with a Janus kinase inhibitor

IL: interleukin JAK: Janus kinase MCC: Merkel cell carcinoma MCPyV: Merkel cell polyomavirus RA: rheumatoid arthritis STAT: signal transducers and activator of transcription INTRODUCTION Merkel cell carcinoma (MCC) is an aggressive neuroendocrine skin cancer thought to be derived from Merkel cells present in the cutaneous basement layer; these cells have synaptic contacts with somatosensory afferents. MCC is a rare malignant tumor, the development of which may be associated with immunosuppressive conditions. Tofacitinib is a Janus kinase (JAK) inhibitor that exerts antiinflammatory activity by inhibiting JAK enzymes, thereby interfering with the JAK signal transducers and activator of transcription (STAT) signaling pathway. Tofacitinib is recommended for individuals with active moderate-to-severe rheumatoid arthritis (RA) that show an inadequate response to traditional disease-modifying antirheumatic drugs or biologic agents. Here we report a case of MCC in an RA patient thought to be caused by immunosuppression during treatment with tofacitinib.