Filgotinib (GLPG0634), by the Belgian biotech company Galápagos NV, is a drug which is currently under investigation for the treatment of Rheumatoid Arthritis (RA) and Crohn's disease.
Filgotinib is a Janus kinase inhibitor with selectivity for subtype JAK1 of this enzyme. It is considered a promising agent as it inhibits JAK1 selectively. Less selective JAK inhibitors (e.g. tofacitinib) are already being marketed. They show long-term efficacy in the treatment of various inflammatory diseases. However, their lack of selectivity leads to dose-limiting side effects. It is thought that inhibition of all JAK isoenzymes is beneficial in rheumatoid arthritis. However, pan-JAK inhibition might also lead to unwanted side effects that might not outweigh its benefits. This is the rationale for the development of newer and more selective inhibitors like filgotinib.

The signal transmission of large numbers of proinflammatory cytokines is dependent on JAK1. Inhibition of JAK2 may also contribute to the efficacy against RA. Nonetheless it is thought that JAK2 inhibition might lead to anemia and thrombopenia by interference with erythropoietin and thrombopoietin and granulocyte-macrophage colony-stimulating factor. Therefore one might prefer to choose a more selective JAK1 inhibitor as a primary therapeutic option. Filgotinib exerts a 30-fold selectivity for JAK1 compared to JAK2. It is however still to be seen to what extent JAK2 inhibition should be avoided.