IN VIVO MECHANISMS OF ACTION OF LEFLUNOMIDE IN ALLO-TRANSPLANTATION AND XENO-TRANSPLANTATION.

Abstract

678 Background: Leflunomide (Lef) is effective in controlling autoimmune disease, cancer and transplantation rejection. Two activities have been identified for Lef; inhibition of dihydroorotate dehydrogenase at IC50≤ 5 μM; and inhibition of protein tyrosine kinases at IC50 ≥ 50μM. We have examined the in vivo mechanism of action of Lef in controlling allograft and xenograft rejection in Lewis rats. Methods: Lewis rats were transplanted with hearts from Brown Norway (BN) rats or Golden Syrian hamsters. Immunosuppression was Lef (5, 15 or 35 mg/kg/day; gavage) alone, or in combination with uridine (500 mg/kg/day; ip). The serum concentrations of the active metabolite of Lef, A77 1726, and uridine were quantified following Lef and uridine administration. All transplanted grafts were examined histologically at the end of the experiment, and scored following a modified grading of cellular rejection of cardiac biopsies. Results:TableConclusion: (1) ≥15 mg/kg Lef resulted in optimum immunosuppression. (2) Uridine partially antagonized the ability of ≤15 mg/kg Lef to control allograft rejection, but dramatically antagonized the ability of Lef to control xenograft rejection. (3) The effect of uridine on the immunosuppressive activity was significantly reduced at ≥15 mg/kg Lef;(4) Uridine was able to reduce the toxic effect of high-dose Lef. (5) The peak A77 1726 concentrations in the serum were 78, 266 and 478 μM for 5, 15 or 35 mg/kg Lef. (6) The control of T cell-mediated allograft rejection may be mediated by the inhibition of tyrosine phosphorylation as well as pyrimidine synthesis, while control of B cell-dependent xenograft rejection by Lef is primarily dependent on the inhibition of pyrimidine synthesis.