Lead leflunomide

Leflunomide undergoes rapid conversion, both in the intestine and in the plasma, to its active metabolite, A77-1726. This metabolite inhibits dihydroorotate dehydrogenase, leading to a decrease in ribonucleotide synthesis and the arrest of stimulated cells in the Gi phase of cell growth. Consequently, leflunomide inhibits T-cell proliferation and production of autoantibodies by cells. Secondary effects include increases of interleukin-10 receptor mRNA, decreased interleukin-8 receptor type A mRNA, and decreased TNF-a-dependent nuclear factor kappa (NF- ) activation. 

The diverse and different pharmacological effects on several central biosynthetic pathways and signaling cascades that are reported for Leflunomide recommended this small molecule drug for employment of the SMART approach for the identification of alternative targets and finally, lead stmcture identification. 

The results described in recent studies with differential proteomics approaches (Dax 1996 Dax et ah, 1998 Ortlepp et al, 1998 Kirschbaum et al., 1999 Mangold et al, 1999) finally lead to additional insight into the mode of action of Leflunomide (Arava), a new antiproliferative and immonoregulatory drug for the treatment of RA. In the present paper we show that affinity chromatography led to the identification of ten intracellular potential binding partners of the active Leflunomide metabolite A 77 1726. 

Leflunomide (HWA 486), a novel immunomodulating compound for the treatment of autoimmune disorders and reactions leading to transplantation rejection. Agents Actions 31(1/2), 10-21. 

Usually, overdosage and adverse events can be managed by dosage reduction, the addition of colestyramine, and symptomatic therapy (36). However, in one study in patients with rheumatoid arthritis, leflunomide 10 mg/ day compared with 20 mg/day was associated with less efficacy and more adverse events leading to treatment withdrawal (24). Colestyramine 3x8 g/day for 11 days is recommended to wash out leflunomide, if A77 1726 plasma concentrations do not fall to 0.02 mg/1 or less, additional colestyramine is advised. Without this washout procedure, it can take up to 2 years to reach A77 1726 plasma concentrations of 0.02 mg/1. Oral activated charcoal 50 g every 6 hours for 24 hours also reduced plasma A77 1726 concentrations (80). 

JNK activation may be a mechanism that is associated with the initiation of mitochondrial permeability transition (MPT) (Hanawa et al. 2008 Latchoumycan-dane et al. 2006, 2007). As discussed above, both JNK activation (Matsumaru et al. 2003) and MPT (Lemasters 1998) are known to occur as a result of increased oxidative stress. MPT leads to additional oxidative stress with loss of mitochondrial membrane potential and loss of the ability of the hepatocyte to synthesize ATP. Latchoumycandane et al. (2006, 2007) found that leflunomide protected mice from mitochondrial permeabilization. Direct evidence for a role of JNK activation in acetaminophen-induced MPT was recently reported by Hanawa et al. (2008). A time course of events indicated GSH depletion by 1-2 h, JNK activation in liver homogenate by 2-4- h, JNK translocation to mitochondria by 4 h, and increased toxicity (serum ALT by 6 h). The JNK inhibitor did not alter GSH depletion but blocked JNK activation in homogenate, JNK translocation to mitochondria, and toxicity. Mitochondria from liver of acetaminophen-treated mice showed decreased State III respiration and decreased respiratory control ratios, whereas mice treated with acetaminophen plus JNK inhibitor were partially protected from these losses. Addition of activated JNKl or JNK2 to mitochondria from acetaminophen-treated mice plus JNK inhibitor showed a decrease in State 111 respiration and decreased respiratory control ratio. Addition of the MPT inhibitor cyclosporine A prevented these decreases. It was hypothesized that activated JNK is an important mediator of acetaminophen-induced MPT (Hanawa et al. 2008). 

The manufacturers advise caution if leflunomide is given with phenytoin or tolbutamide. The reason is that the active metabolite of leflunomide (A771726) has been shown by in vitro studies to be an inhibitor of the cytochrome P450 isoenzyme CYP2C9, which is concerned with the metabolism of these two drugs. If this inhibition were to occur in vivo it could possibly lead to a decrease in their metabolism and an increase in their toxicity. Although so far there appear to be no clinical reports of an interaction, the manufacturers made a similar prediction with warfarin, another CYP2C9 substrate, which has, in isolated cases, been borne out in practice. See Coumarins + Leflunomide , p.423. 

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