Effects of leflunomide on immune responses and models of inflammation

Conclusions

Leflunomide is an antiphlogistic and immunomodulating agent that has been shown to be effective in preventing and healing autoimmune disorders and reactions leading to organ graft rejection. From our preliminary clinical data [4], we now have hopes that these effects, observed in experimental animals, can truly be transferred to humans.

Although we are far from understanding the mode of action of leflunomide, we are slowly gathering some insight. A good many of the immunosuppressive effects of leflunomide can be attributed to the antagonistic effects it has on responses to many cytokines, most likely through receptor expression and signal transduction (tyrosine kinase inhibition). The inhibition of transplant rejection could be explained by interference with the activity of IL-2 and IL-4, i.e. the interference of cytotoxic T cell formation. Considering, further, that increased IL-3-like activity has been reported in autoimmune MRL/lpr mice [23], and that it is felt that this amplified activity may contribute to the pathology of their illness [23], then the interference of leflunomide with IL-3 may, together with the antagonistic activity of TRF and specifically IL-4, explain some of the disease modifying properties of this drug in animals with SLE-like and other autoimmune diseases. Also, interference with responses to IL-6 (Germann, personal communication) could be responsible for the suppression of acute-phase proteins observed in adjuvant-diseased rats [24].

Our data concerning tyrosine kinase inhibition as a hypothetical mechanism for the non-cytotoxic and reversible antiproliferative activity of A77 1726 are in many ways, intriguing. First of all, many known receptors for growth factors are associated with tyrosine kinase, i.e. EGF [35], IL-2 (the high binding, 75 kDa chain) [21], IL-3 [26], G-CSF, GM-CSF and TNF-α [9]. Leflunomide antagonizes all of these mediators. On the other hand, IL-1, which is not antagonized by leflunomide, is not associated with tyrosine kinase, but with threonine and serine kinase [11]. Much more work must be conducted before we can be sure that tyrosine kinase inhibition is important for the mode of action of leflunomide.

Another important aspect of this drug is its inhibitory effect on the release of histamine from basophils and mast cells, because of its role in inflammatory reactions. Relating to our findings on the activity of leflunomide on murine SLE-like disorders, it has been reported recently that SLE patients often exhibit abnormal production of antibodies to IgE, and that these autoantibodies may, by activating mast cells and basophils, play a consequential part in the release of vasoactive amines, thus leading to generalized tissue injury [15].

We are confident that leflunomide will prove to be an effective drug in combating human autoimmune disorders. Indeed, we already have preliminary evidence for this, from studies of its effects on humans suffering from autoimmune diseases. Moreover, this drug provides a tool for gaining new insights into both the mechanisms leading to such ailments and their therapeutic control, and as such may facilitate the discovery of even more proficient drugs or other means to modulate these malfunctioning immune reactions.