Advances in biocatalytic synthesis of pharmaceutical intermediates
Introduction
The molecular complexity of active pharmaceutical ingredients (APIs) is rising and is characterized by a growing number of functional groups and chiral centers. To synthesize an advanced intermediate (AI) to assemble such a complex API requires catalysts that are highly selective and that operate under conditions compatible with frequently sensitive functional groups. Enzymes, as highly selective catalysts operating under ambient conditions, are particularly suited for this purpose, and numerous industrial applications have been reviewed recently [1•, 2, 3, 4, 5].
A long-known but illustrative example of an API with multiple chiral centers created by biocatalytic means is Omapatrilat, an antihypertensive drug that acts both as an inhibitor of angiotensin converting enzyme and neutral endopeptidase [5]. Omapatrilat contains four chiral centers as well as several sensitive functional groups (Figure 1). The molecule can be built-up from alternatively (S)-6-hydroxynorleucine (1) or (S)-allysine ethylene acetal (2), and d-phenylalanine (3) as key intermediates. The latter is accessible via a dynamic kinetic resolution using d-hydantoinase and d-N-carbamoylase technology. An elegant route to (S)-6-hydroxynorleucine from the racemate has been developed that makes use of d-amino acid oxidase to oxidize the (R)-enantiomer to the corresponding 2-keto-6-hydroxyhexanoic acid, which undergoes reductive amination by glutamate dehydrogenase with cofactor regeneration by glucose dehydrogenase. Routes to (2) include aminopeptidase resolution of the amino acid amide, but asymmetric synthesis by reductive amination of the corresponding ketoacid acetal by phenylalanine dehydrogenase with formate dehydrogenase to regenerate NADH has been implemented [5]. Even the closure of the rings to the thiazepine (5) has been investigated. l-lysine-ɛ-aminotransferase is able to catalyze the conversion of the (S)-homocystein-(S)-lysine dipeptide (4) to the corresponding aldehyde, which closes to the ring under acidic conditions, and the third chiral center is thereby configured correctly.
The Omapatrilat case illustrates the use of enzymes and multi-enzyme systems to address both chirality and sensitive functional groups in the synthesis of a complex API. In the following text, we discuss other examples and novel technological means to enable the use of enzymes in a wider range of applications in the pharmaceutical area.
Section snippets
Biocatalytic routes to HMG-CoA reductase inhibitors
Another prominent example of the versatility of biocatalysis for manufacturing a complex pharmaceutical ingredient is the stereoselective synthesis of the 3,5-dihydroxyhexanoate side chain of 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) reductase inhibitors (statins). These cholesterol-lowering drugs represent a multibillion-dollar business, and the market demand has led to the development of numerous routes to synthesize the side chain of statins. The need for high stereochemical quality at both
Improvements in pyruvate decarboxylase for ephedrine production
In the (R)-phenylacetycarbinol (R-PAC) process, a thiamine diphosphate-dependent pyruvate decarboxylase (PDC) activates acetaldehyde, which is carboligated to externally provided benzaldehyde (Figure 3). This classical biotransformation process suffers from several drawbacks: it relies on living yeast cells to provide pyruvate, and the viability of the cells — and therefore the pyruvate supply — is decreased by the added benzaldehyde. Finally, the process leads to prominent by-products, mostly
Fermentative production of (non-natural) intermediates
Rather than using single enzyme reactions on a chemically produced intermediate, such pharmaceutically relevant intermediates could, in selected cases, be made by fermentation. A very prominent example here is the production of shikimic acid as an intermediate for Roche's neuraminidase inhibitor Tamiflu® (Roche) with engineered E. coli strains (Figure 4). Shikimic acid is a regular intermediate in aromatic amino acid synthesis, which makes fermentative production a very appealing option.
Conclusions
Biocatalysis has more to offer than providing a solution when the chemistry doesn’t work. The examples given in this review go beyond more established enzyme classes such as the hydrolases and ketone reductases, and illustrate the enormous potential of enzymatic catalysis in single-step reactions, combined enzyme reactions and metabolic engineering.
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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