Research On Synthetic Process Optimization And Quality Control Of Leflunomide

Leflunomide (LEF) is a new immunomodulatory agent and a novel anti-rheumatoid arthritis drug, which is developed by Hoechst Marion Roussel, LEF is the first oral medication for the treatment of anti-rheumatoid arthritis and came into United States market in1998. Compared to the conventional therapeutic drugs, Leflunomide processes fewer side effects, not only ease the symptoms of rheumatoid arthritis, but also effectively inhibit the progression of the disease.Ethyl5-methylisoxazole-4-carboxylate was synthesized from ethyl acetoacetate and triethyl orthoformate by two steps including the reaction of condenzation and cyclization, then was hydrolyzed to acid without purification.5-methylisoxazole-4-carbonyl chloride, a low boiling piont compound acylating from the acid by vacuum distillation, was dissolved with toluene and added to toluene-water solution of p-trifluoromethyl aniline for synthsis of leflunomide with an overall yeild of40.3%, the products were purifed by recrystallizing from ethanol/water (2:1, v/v) with yield83.2%and purity99.6%(HPLC). The study concentrated on optimization of the cyclization reaction in terms of bases, pH of reaction solution, reaction temperature. Other four steps was optimized including the reaction of condenzation/hydrolyzation/chloration/acylation with per step's average yield84.0%,66.6%,89.0%and83.9%.2-cyano-N-(4-(trifluoromethyl)phenyl)acetamide, a related impurity of Leflunomide in tablets was found exist in the acid aqueous of (Z)-2-cyano-3-hydroxy-N-(4-(trifluoromethyl)-phenyl)but-2-enamide, and was isolated by preparative chromatography and characterized by MS and1H NMR. Monitoring by HPLC, Leflunomide isomer was prepared by repeated recrystallizing from enriched mother liquor. Other three commonly related impurities of LEF were also prepared and characterized by1H NMR.A reliable and efficient RP-HPLC method was developed for determinining5-methylisoxazole-4-carboxylic acid and it's isomer. The optimum separation was carried out on Agilent TC-C18at30℃, the HPLC-UV method uses a mobile phase consisting of methanol and a KH2PO4-buffer (25mM, pH adjusted to3.00by phosphoric acid)(20:80. v/v), at a flow rate of1mL/min. The UV detection was at210nm and injection volume was20μl.5-methylisoxazole-4-carboxylic acid was linear over the concentration range of50～150 μg/ml(r2=0.9997)with RSD of precision0.13%,LOD0.011μg/ml,average recovery100.82%. It's isomer demonstrated good linearity(r2=0.9992)in the range of2～6μg/ml with RSD of precision1.03％,LOD0.016μg/ml,average recovery99.47%.