Leflunomide is a new disease modifying drug that is widely used in the therapy of rheumatoid arthritis (RA). The active metabolite of leflunomide A771726 leads to inhibition of dihydroorotate dehydrogenase, an enzyme necessary for pyrimidine de-novo synthesis. Since activated lymphocytes expand their pyrimidine pool, A771726 leads to a decrease of their proliferation. A771726 also suppresses TNF mediated nuclear factor kappaB activation. We were therefore interested if A771726 also would be capable to influence transendothelial migration (TEM) of peripheral mononuclear cells (PBMC).
We investigated the TEM of PBMC, which migrated through endothelial cell monolayers in an in-vitro model. Human umbilical vein endothelial cells (EC) were cultured to confluence on collagen gels and then incubated with human PBMC of healthy blood donors. PBMC were recollected in three groups: 1) cells that did not adhere to the endothelium, 2) cells that bound to the endothelium, 3) cells that had migrated through the endothelium, and then counted by microscope. Experiments in which PBMC as well as EC were treated with A771726 (in the absence or presence of uridine) were compared to simultaneously performed control experiments. No increased toxicity on the PBMC treated with the doses of A771726 used in our experiments, was observed.
24 hour incubation of PBMC and EC with 30 mg/ml A771726 led to a significant decrease in TEM (11 ± 3 % vs. 6 ± 3 % migrated cells, P = 0.016, paired T test). Uridine partly reversed the decrease in TEM when incubating PBMC and EC with both uridine and A771726.
Our results demonstrate that leflunomide may have direct anti-inflammatory effects by inhibiting the extravasation of PBMC. These data suggest, that leflunomide, besides its influences on lymphocyte proliferation, also reduces accumulation of cells at inflammatory sites.