Leflunomide Increases Hepatic Exposure to Methotrexate and Its Metabolite by Differentially Regulating Multidrug Resistance-Associated Protein Mrp2/3/4 Transporters via Peroxisome Proliferator-Activated Receptor α Activation.

Methotrexate (MTX) is the gold standard drug for the treatment of rheumatoid arthritis (RA), and it is frequently combined with leflunomide (LEF) to enhance its clinical efficacy. However, this combination can exacerbate liver toxicity, and the underlying mechanism has not yet been clarified. We investigated whether LEF affects the pharmacokinetics of MTX and its primary toxic metabolite, 7-hydroxyl methotrexate (7OH MTX), in mice. LEF significantly increased the plasma concentration (area under the plasma concentration-time curve) of MTX and 7OH MTX (2.4 and 4.5 times, respectively), decreased their bile excretion, and increased their accumulation in the liver and kidneys. When we investigated the effect of LEF on the MTX absorption, distribution, metabolism, and excretion process, we found that LEF had little effect on liver aldehyde oxidase and 7OH MTX formation. However, LEF significantly decreased the expression of the apical efflux transporter multidrug resistance-associated protein 2 (Mrp2) and increased that of the basolateral efflux transporters Mrp3/4, except there was no significant change in Mrp4 protein expression. Mrp2/3/4 alteration changed the distribution of MTX and 7OH MTX in plasma and tissues. Further studies suggested that LEF indirectly activated peroxisome proliferator-activated receptor α (PPARα), which was likely responsible for the Mrp2/3/4 alteration in the liver. The MTX plasma concentration change induced by LEF was reversed by the PPARα-specific antagonist GW6471. These results may partially explain the exacerbated liver toxicity caused by combination treatment with MTX and LEF and may raise concerns regarding the risk of potential drug-drug interactions between PPARα agonists and Mrp substrates in the clinic.