Presidential Green Chemistry Challenge: 2012 Greener Synthetic Pathways Award
Professor Yi Tang of the University of California, Los Angeles
An Efficient Biocatalytic Process to Manufacture Simvastatin
Summary of Technology: Simvastatin, a leading cholesterol lowering drug, was originally developed by Merck under the brand name Zocor®. In 2005, Zocor® was Merck's best selling drug and the second-largest selling statin in the world with about $5 billion in sales. After Zocor® went off patent in 2006, simvastatin became the most-prescribed statin, with 94 million prescriptions filled in 2010, according to IMS Health.
Simvastatin is a semisynthetic derivative of lovastatin, a fungal natural product. Simvastatin contains an additional methyl group at the C2' position of the lovastatin side chain. Introduction of this methyl group in lovastatin using traditional methods requires a multistep chemical synthesis. In one route, lovastatin is hydrolyzed to the triol, monacolin J, which is protected by selective silylation, esterified with dimethylbutyryl chloride, and deprotected. Another route involves protecting the carboxylic acid and alcohol, methylating the C2' with methyl iodide, and deprotecting. Despite considerable optimization, these processes have overall yields of less than 70 percent, are mass-intensive due to protection/deprotection, and require copious amounts of toxic and hazardous reagents.
Professor Yi Tang and his group at UCLA conceived a new simvastatin manufacturing process and identified both a biocatalyst for regioselective acylation and a practical, low-cost acyl donor. The biocatalyst is LovD, an acyltransferase that selectively transfers the 2-methylbutyryl side chain to the C8 alcohol of monacolin J sodium or ammonium salt. The acyl donor, dimethylbutyryl-S-methylmercaptopropionate (DMB-SMMP), is very efficient for the LovD-catalyzed reaction, is safer than traditional alternatives, and is prepared in a single step from inexpensive precursors. Codexis licensed this process from UCLA and subsequently optimized the enzyme and the chemical process for commercial manufacture. Codexis carried out nine iterations of in vitro evolution, creating 216 libraries and screening 61,779 variants to develop a LovD variant with improved activity, in-process stability, and tolerance to product inhibition. The approximately 1,000-fold improved enzyme and the new process pushed the reaction to completion at high substrate loading and minimized the amounts of acyl donor and of solvents for extraction and product separation.
In the new route, lovastatin is hydrolyzed and converted to the water-soluble ammonium salt of monacolin J. Then a genetically evolved variant of LovD acyltransferase from E coli. uses DMB-SMMP as the acyl donor to make the water-insoluble ammonium salt of simvastatin. The only coproduct of simvastatin synthesis is methyl 3-mercaptopropionic acid, which is recycled. The final yield of simvastatin ammonium salt is over 97 percent at a loading of 75 grams per liter of monacolin J. The nominated technology is practical and cost-effective. It avoids the use of several hazardous chemicals including tert-butyl dimethyl silane chloride, methyl iodide, and n-butyl lithium. Customers have evaluated the simvastatin produced biocatalytically and confirmed that it meets their needs. Over 10 metric tons of simvastatin have been manufactured using this new process.
Podcast on the technology:(MP3, 1.03 MB, 1:07 minutes), Narrator: Dr. Richard Engler, US EPA
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