Ruxolitinib is a small molecule Janus kinase inhibitor that is used in the treatment of intermediate or high risk myelofibrosis and resistant forms of polycythemia vera and graft-vs-host disease. Ruxolitinib is associated with transient and usually mild elevations in serum aminotransferase during therapy and to rare instances of self-limited, clinically apparent idiosyncratic acute liver injury as well as to cases of reactivation of hepatitis B in susceptible individuals.

Ruxolitinib is an orally bioavailable Janus-associated kinase (JAK) inhibitor with potential antineoplastic and immunomodulating activities. Ruxolitinib specifically binds to and inhibits protein tyrosine kinases JAK 1 and 2, which may lead to a reduction in inflammation and an inhibition of cellular proliferation. The JAK-STAT (signal transducer and activator of transcription) pathway plays a key role in the signaling of many cytokines and growth factors and is involved in cellular proliferation, growth, hematopoiesis, and the immune response; JAK kinases may be upregulated in inflammatory diseases, myeloproliferative disorders, and various malignancies.

Ruxolitinib is a pyrazole substituted at position 1 by a 2-cyano-1-cyclopentylethyl group and at position 3 by a pyrrolo[2,3-d]pyrimidin-4-yl group. Used as the phosphate salt for the treatment of patients with intermediate or high-risk myelofibrosis, including primary myelofibrosis, post-polycythemia vera myelofibrosis and post-essential thrombocythemia myelofibrosis. It has a role as an antineoplastic agent and an EC 2.7.10.2 (non-specific protein-tyrosine kinase) inhibitor. It is a nitrile, a pyrrolopyrimidine and a member of pyrazoles.