A case in which baricitinib was effective for both rheumatoid arthritis and essential thrombocythemia.

We experienced a case of rheumatoid arthritis and JAK2V617F mutation-positive essential thrombocythemia treated with baricitinib. The patient was a 72-year-old male. He was diagnosed with rheumatoid arthritis at a local clinic in April 2018. Methotrexate (MTX) was started and the dose was increased to 16 mg/week. In October of the same year, anaemia was observed and MTX was reduced, but anaemia progressed. Blood tests showed pancytopenia, and he was referred to Rheumatology on suspicion of drug-induced pancytopenia. Pancytopenia improved with discontinuation of MTX and administration of folic acid. His platelet count was markedly increased to 1,400,000/µl at one point, decreased to 400,000/µl, and then gradually increased to 700,000-1,000,000/µl. Despite taking an antiplatelet drug, he developed cerebral infarction in June 2019. The JAK2V617F mutation was noted, and he was diagnosed with essential thrombocythemia. Hydroxycarbamide was started, but the effect was insufficient. Baricitinib, a JAK1/2 inhibitor indicated for rheumatoid arthritis, was started in August with the expectation that it would also be effective for essential thrombocythemia. The platelet count decreased to ∼400,000-600,000 cells/µl, and a decrease in the C-reactive protein level and the improvement of arthritis were noted. We report this case because it is considered to be a valuable case, suggesting that baricitinib may be effective for essential thrombocythemia.