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Review- Alternatives for the Separation of Drug Enantiomers: Ibuprofen As a Model Compound

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Review- Alternatives for the Separation of Drug Enantiomers: Ibuprofen As a Model Compound Brazilian Journal of Chemical ISSN 0104-6632 Printed in Brazil Engineering www.abeq.org.br/bjche Vol. 23, No. 03, pp. 291 - 300, July - September, 2006 REVIEW- ALTERNATIVES FOR THE SEPARATION OF DRUG ENANTIOMERS: IBUPROFEN AS A MODEL COMPOUND P. O. Carvalho1*, Q. B. Cass2, S. A. Calafatti3, F. J. Contesini1 and R. Bizaco1 1Faculdade de Farmácia, Universidade São Francisco, Phone: +(55) (11) 4034-8298, Av. São Francisco de Assis 218, CEP 12916-900, Bragança Paulista - SP, Brazil. 2Departamento de Química, Universidade Federal de São Carlos, Phone: +(55)(16) 260-8208, Cx. Postal 676, CEP 13565-905, São Carlos-SP, Brazil 3Unidade Integrada de Farmacologia e Gastroenterologia Clínica, Universidade São Francisco, 12916-900 Bragança Paulista-SP, Brazil. E-mail: [email protected] (Received: March 5, 2005 ; Accepted: June 12, 2006) Abstract - Given the inherent dangers associated with racemic pharmaceuticals, exhaustive studies of techniques designed to separate enantiomers have been conducted. This paper reports a brief review of the different physical, chemical, and enzymatic methods used for the enantiomeric separation of rac-ibuprofen. The possible contribution of each technique to the preparation of enantiomerically pure drugs is reviewed in the context of competitive approaches that depend on the scale of application, with special emphasis on the recent progress achieved in this particular area. Keywords: Separation of drug enantiomers; Enzimatic methods. INTRODUCTION important. Another factor that is often overlooked in small-scale experiments is the volume yield or There are two possible approaches to the productivity (kilograms of product per unit reactor preparation of optically active compounds by volume per unit time). For an economically viable chemical transformation of optically inactive starting process, it is generally essential to obtain high materials: kinetic resolution and asymmetric chemical and optical yields at high substrate synthesis. Kinetic resolution depends on the fact that concentrations and it is sometimes necessary to give the two enantiomers of a racemate react at different percentage points on the former to the benefit of the rates with a chiral reagent or catalyst, such as an latter. enzyme. Asymmetric synthesis, on the other hand, Although a number of stereoselective syntheses involves the creation of an asymmetric (stereogenic) have been described and applied to the production of center, which, in this case, is by chiral discrimination single-enantiomer substances, relatively few are of equivalent groups in an achiral starting material selected for large-scale preparation, particularly at (prochiral). the early stages of development of new drugs. At The cost of these processes is affected by the these stages, the use of an asymmetric synthesis price of substrate, and this can have a bearing on the would be both expensive and time-consuming; thus choice of method. Similarly, the price and ease of preparative techniques for the separation of recycling of the resolving agent or biocatalyst and enantiomers have an interesting potential (Maier et the reduction of waste streams are obviously al., 2001). *To whom correspondence should be addressed 292 P. O. Carvalho, Q. B. Cass, S. A. Calafatti, F. J. Contesini and R. Bizaco CHIRALITY pharmaceuticals entering the commercial market. The motivation for this single-isomer trend has been By definition, a chiral material is one which lacks provided in part by the FDA (De Castro, 1997) and reflectional symmetry, i.e. it has a nonsuperimposable in part by the production of a host of mirror image structure and is referred to as being pharmaceuticals previously protected by 17-20 year “handed”. The most common type of chiral patent production laws. Those particular chiral drugs, compounds are enantiomers. These materials are whose patents are attracting a multitude of overseas typically characterized by an asymmetric, tetrahedral producers, would provide price competition and carbon atom located at the center of the molecule. increase the “generic brand” availability from These molecules can be as stable, observable producers with large-scale capacities. Chirally active stereoisomers if their energy barrier of conversion drugs also represent a large share of the exceeds 80 KJ/mole. In addition, compounds that are pharmaceutical market. Single-isomer drug sales enantiomers have nearly identical physical and reached $ 115 billion dollars worldwide in 1999, up chemical properties in an achiral (symmetrical) 16% from $ 99 billion dollars in 1998. It was environment. The specific distinguishing physical estimated that this class of drugs would grow at an property of enantiomers is the rotation of plane- average annual rate of 8% up to $ 146 billion in 2003 polarized light. If the rotation is to the right (Stinson, 2000). (clockwise) the substance is dextrorotatory ((+) or (d)); if the rotation is to the left (counterclockwise) it PHARMACOLOGICAL IMPLICATIONS is levorotatory ((-) or (l)). When these enantiomers are present in equimolar amounts within a mixture, Drug action is the result of pharmacological and the resultant mixture is termed racemic. These pharmacokinetic processes, by which the drug enters, preparations are optically inactive, because the net interacts with and leaves the body. There is a broad rotation of plane-polarized light is counterbalanced range of examples where the stereoisomers of drugs by equal concentrations of each enantiomer. are different in terms of their bioavailability, and Diastereoisomers are nonmirror image stereoisomers distribution, metabolic and excretion behavior and that have more than one asymmetric center. Unlike where stereochemical parameters have a fundamental enantiomers, diastereoisomers may be individually significance in their action and disposition in isolated because they have different as physical and biological systems (Norbert et al., 2001). chemical properties, such solubility and melting The high degree of stereoselectivity of many point (Sheldon, 1993). Enantiomers may be biological processes implies that when a given transformed into diastereoisomers by either covalent racemic mixture is administered as a drug, the two or noncovalent coupling of the enantiomers of a enantiomers should not have to be equally potent. In racemic mixture to another chiral molecule having at fact, very often one of them is the more active least one asymmetric center. This methodology isomer for a given action (eutomer), while the other defines a separation route by which two previously one (distomer), might even be active in a different inseparable material may be isolated by conventional way, contributing to side effects, displaying toxicity, techniques. or acting as an antagonist (Wainer, 1993). For The interest in chirality and its consequences is example, it has been established that the S-(+) not a new phenomenon. However, during the past enantiomer of ibuprofen is almost entirely decade it has raised increasing expectations for responsible for the anti-inflammatory effects of rac- scientific and economic reasons, with the ibuprofen (Mayer and Testa, 1997). It has been pharmaceutical industry being the main contributor reported that the S-(+) enantiomer is 160 times more and driving force. active than its antipode in the in vitro synthesis of prostaglandins (Adams et al., 1976). To avoid the unwanted effects of the R-enantiomer, the use of PHARMACEUTICAL TRENDS pure S-enantiomer of the profens is desirable. Furthermore, some of the profens generally undergo Strong emphasis has been placed on the search a unidirectional in vivo chiral inversion from the for therapeutic benefits with the goal of developing inactive R-enantiomer to the active S-form (Caldwell safer and more effective drugs. The importance of et al., 1988). This peculiar stereoselectivity, which determining the pharmacological activity of each can occur in many of the metabolic routes of a given component in a drug has gained full acceptance, as drug, has to be carefully considered when its shown by the substantial number of single-isomer development is envisaged or its safety is evaluated. Brazilian Journal of Chemical Engineering Alternatives for the Separation of Drug Enantiomers 293 IBUPROFEN AS A MODEL COMPOUND administered as a racemate the distomer is converted in vivo into the eutomer, while the latter is In the anti-inflammatory class of pharmaceuticals, unaffected. However, the situation is more complex, rac-ibuprofen ((R,S)-2-(isobutylphenyl)- propionic as becomes evident upon examining the mechanism acid) is an interesting example of a drug that is still of metabolic inversion (Figure 2). sold as a racemic mixture (Figure 1). The single- The enantiospecificity of the inversion is isomer product has a higher commercial value than controlled by the enzyme acylcoenzime-A synthetase the racemic mixture, but production has been limited that converts (R)-ibuprofen to the corresponding by legal and process difficulties. In addition, coenzyme A thioester. Racemization of the latter and ibuprofen is the one member of the nonsteroidal anti- subsequent hydrolysis yields (S)-ibuprofen. In inflammatory drug (NSAID) family that has proven to competition with hydrolysis, the thioester be the most challenging to enantiomerically purify. A intermediate can also undergo acyl exchange with large investment has been made in order to research the endogenous triacylglycerols. This results in and develop specific techniques for ibuprofen. accumulation
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