Direct Enantioselective HPLC Monitoring of Lipase-Catalyzed Kinetic Resolution of Tiaprofenic Acid in Nonstandard HPLC Organic Solvents ASHRAF GHANEM, 1 * MOHAMMED NABIL ABOUL-ENEIN, 2 AIDA EL-AZZOUNY, 2 MOHAMMED F. EL-BEHAIRY, 1,2 EBTESSAM AL-HUMAIDI, 1 ALWALEED A. ALAIDAN, 1 KAMILIA AMIN, 3 AND MOHAMMED N. AL-AHDAL 1 1 Biomedicinal Chemistry Unit, Biological and Medical Research Department, King Faisal Specialist Hospital and Research Centre, Riyadh, Saudi Arabia 2 Medicinal and Pharmaceutical Chemistry Department, Pharmaceutical and Drug Industries Research Division, National Research Centre (NRC), Dokki-Cairo, Egypt 3 Pharmaceutical Chemistry Department, Faculty of Pharmacy, Cairo University, Cairo, Egypt ABSTRACT The first straightforward lipase-catalyzed enantioselective access to enantiomerically enriched tiaprofenic acid as a versatile method in chiral separation of racemates is demonstrated. The latter was directly monitored by enantioselective HPLC using a 3,5-dimethylphenylcarbamate derivative of cellulose-based chiral stationary phase namely Chiralpak IB (the immobilized version of Chiralcel OD). Non-standard HPLC organic solvents were used as diluent to dissolve the ‘‘difficult to dissolve’’ enzyme substrate (the acid) and as eluent for the simultaneous enantioselective HPLC baseline separation of both substrate and product in one run without any further deriva- tization. The existence of a non-standard HPLC organic solvent (e.g., methyl tert-butyl ether) in the mobile phase composition is mandatory to accomplish the simultaneous enantioselective HPLC baseline separation of both substrate and product. Chirality 20:871–877, 2008. V V C 2008 Wiley-Liss, Inc. KEY WORDS: Chiralpak IB; direct monitoring; enantioselective separation; HPLC; kinetic resolution; lipases; tiaprofenic acid INTRODUCTION Nonsteroidal anti-inflammatory drugs (NSAIDs) of the 2- arylpropionic acid (2-APA) class represent one of the most commercially successful and important classes of analgesic anti-inflammatory drugs. A common structural feature of 2- APA NSAIDs is a sp3-hybridized tetrahedral chiral carbon heteroatom within the propionic acid side chain moiety, with the S-enantiomer possessing most of the beneficial anti-inflammatory activity. In vivo, however, some of this class of compounds, namely, profens undergoes, to a limited degree, inversion from the (R) to the (S) form. Examples include the (R)-(2)-enantiomers of ibuprofen and naproxen which were converted in blood into the corresponding (S)- (1)-enantiomers. 1 However, only naproxen and flunaxapro- fen are administered as the pure (S)-(1)-enantiomer. Tiaprofenic acid (5-benzoyl-a-methyl-2-thiopheneacetic acid) belongs to this class of compounds having potent anti-inflammatory and analgesic properties. 2 Several stud- ies in healthy and arthritic subjects have involved the administration of the racemate. However, tiaprofenic acid has been suggested to exhibit limited pharmacokinetic stereoselectivity. 3 In view of the increasing legislative concern regarding the development and use of single enantiomeric drug in studying the pharmacokinetics and pharmacodynamics of each separate enantiomer, an enan- tioselective straightforward route to access to separate enantiomers is required. The enantioselective resolution of tiaprofenic acid was reported using gas chromatography GC, high performance liquid chromatography HPLC and capillary electrochromatography (CEC). 4–10 Most of these assays are indirect which involve the formation of diaster- eomers through reaction of the carboxylic acid moiety in tiaprofenic acid with a coupling reagent, 2,2,2-trichlor- oethyl chloroformate, to form a mixed anhydride. The lat- ter is followed by the formation of an amide using L-leuci- namide. However, in addition to racemization, partial or complete chiral conversion might occur. 4 In fact, none of the reported procedure used to access to enantiomercially pure/enriched tiaprofenic acid did include a straightfor- ward enzymatic resolution of the racemates. This is prob- *Correspondence to: Ashraf Ghanem, Biomedicinal Chemistry Unit, Bio- logical & Medical Research Department, King Faisal Specialist Hospital & Research Centre, MBC-03-95, P.O. Box 3354, Riyadh 11211, Kingdom of Saudi Arabia. E-mail: ghanem@kfshrc.edu.sa Dedicated to Professor Nina Berova on the occasion of her receiving the Chirality medal 2007. Contract grant sponsor: King Abdul-Aziz City for Science and Technology; Contract grant number: KACST 25/08. Received for publication 12 September 2007; Accepted 29 November 2007 DOI: 10.1002/chir.20533 Published online 1 February 2008 in Wiley InterScience (www.interscience.wiley.com). CHIRALITY 20:871–877 (2008) V V C 2008 Wiley-Liss, Inc.