- Therapeutic drug monitoring of A77 1726, the active metabolite of leflunomide: serum concentrations predict response to treatment in patients with rheumatoid arthritis. van Roon, E.N., Jansen, T.L., van de Laar, M.A., Janssen, M., Yska, J.P., Keuper, R., Houtman, P.M., Brouwers, J.R. Ann. Rheum. Dis. (2005)