Leflunomide is an immunomodulatory agent that was recently approved for the treatment of rheumatoid arthritis (RA). The mechanism of action is not fully understood. Nitric oxide (NO) plays an important role in the pathogenesis of RA. Leflunomide has been shown to cause cell specific inhibition of inducible nitric oxide synthase (iNOS) activation in animal models. We carried out this study to determine if there was alteration in NO production in patients with RA.
An 8-week open label study was carried out on patients with adult onset active RA. We measured levels of nitrite and citrulline spectrophotometrically as surrogate markers of NO production. Within-patient serum levels of nitrite and citrulline were compared with leflunomide therapy at three points of time (at 0, 4 and 8 weeks of therapy).
Thirty-three patients with active RA were enrolled for this study. These patients were a subset of 63 individuals who are studied for clinical efficacy of leflunomide. Three patients were lost to follow-up. Median nitrite levels were 817.2 (nmol/ml) at the start of therapy and this declined to 440.9 nmol/ml and 301.1 nmol/ml at 4 and 8 weeks of therapy. Median citrulline levels were 649.3 nmol/ml at the start of the study, which declined to 549.2 nmol/ml and 485.4 nmol/ml at 4 and 8 weeks, respectively. Statistically significant decrease in median values for serum nitrite and citrulline levels was documented after 4 weeks of leflunomide therapy (p<0.01), which was sustained at 8 weeks (p<0.01), although there was no further fall between 4 and 8 weeks (p>0.1).
Leflunomide inhibits nitric oxide production in patients with active RA. Inhibition of NO synthesis may be one of the mechanisms responsible for the immunomodulatory activity of leflunomide.