Effect of leflunomide on constitutive and inducible pathways of cellular eicosanoid generation

Leflunomide exerts its effects primarily via the immunomodulating and antiphlogistic activities of its major metabolite A 77 1726. Our investigation in several eicosanoid forming systems revealed that in human white blood cells the metabolite did not cause any alteration on Ca-ionophore stimulated metabolism of membrane bound arachidonic acid to cis-, trans- and epi-LTB4. Thus, the involved enzyme systems phospholipase A2, 5-lipoxygenase and LTA4-hydrolase can be ruled out as a target of the drug. However, in several cellular systems the drug weakly inhibited the generation of 5-HETE and LTB4 from exogenous arachidonic acid, possibly by interfering with the exogenous substrate's access to the 5-lipoxygenase. In order to get information about the cyclooxygenase (COX-2) which is inducible in human PMNL by inflammatory mediators via de novo protein biosynthesis, we activated the cells with LPS for 18 h. A 77 1726 and indomethacin had no influence on the enzyme activity of the newly induced COX-2. However, both drugs in low concentrations were able to blunt the long term activation process resulting in PGE2 generation. In contrast, the prostaglandins generated by constitutive enzymes (COX-1) are probably involved in maintaining vital functions, and their inhibition by indomethacin and other nonsteroidal antiinflammatory drugs (NSAIDs) account for numerous adverse effects, for instance gastric erosion. Our study revealed that leflunomide and A 77 1726 are not to be regarded as COX-1-inhibitors, and thus cannot be associated with the typical adverse effects of the NSAIDs.