Chirality and Nonsteroidal Anti-Inflammatory Drugs

Drugs 1996; 52 Suppl. 5: 47-58 0012-6667/96/=5-0047/$06.00/0

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Chirality and Nonsteroidal Anti-Inflammatory Drugs Peter J. Hayball Pharmacy Department, Repatriation General Hospital, Daw Park, South Australia, Australia

Summary The nonsteroidal anti-inflammatory drugs (NSAIDs) are of significant clinical importance and include congeners of many chemical classes, some of which incorporate an asymmetric or chiral carbon atom_ With very few exceptions, chiral NSAIDs have been marketed for clinical use as racemates_ However, it is apparent that differences, sometimes major, exist between enantiomers in terms of their pharmacological and toxicological properties_ With regard to the ability of chiral NSAIDs to inhibit cyclo-oxygenase, their chief mechanism of action, major or exclusive activity is confined to enantiomers of the S-stereoconfiguration. Accordingly, it is questionable whether the R-antipodes should be included in the final drug product for use in the clinic. In addition to differences between enantiomers in terms of their pharmacodynamic properties, pharmacokinetic differences are possible for chiral NSAID isomers, and these may modulate preexisting enantioselectivities at the site of action of such compounds. As a consequence, a considerably simpler pharmacological profile is likely to result from the use of single enantiomers versus racemic mixtures.

Nonsteroidal anti-inflammatory drugs (NSAIDs) are widely prescribed agents for alleviating pain and inflammation associated with tissue injury. NSAIDs are a structurally diverse group of therapeutic agents and include a subclass of compounds possessing molecular asymmetric or chiral elements. Such chiral NSAIDs are the focus of this review and currently include congeners from 3 distinct chemical groups: (a) the 2-arylpropionic acids (2-APAs) or 'profens'; (b) other arylalkanoic acids (ketorolac, indobufen, etodolac, etc.); and (c) miscellaneous compounds including agents such as oxyphenbutazone and azapropazone. With the exception of sulindac, which contains a chiral sulphur tricoordinate centre, the molecular asymmetry of these chiral NSAIDs results from the presence of a chiral carbon atom, i.e. a carbon covalently attached to 4 different chemical groups.

Figure 1 depicts this for the most commonly used subcategory of chiral NSAIDs, the 2-APAs, a group that also encompasses the greatest number of clinically used congeners.

It was recognised early in the development of chiral NSAIDs that pharmacological differences existed between the optical isomers or enantiomers of these drugs. In terms of the classically defined pharmacological activity of NSAIDs, mediated via inhibition of cyclo-oxygenase,[ll major or exclusive activity was afforded to 2-APA enantiomers of the S-stereoconfigurationPl Indeed, this work prompted the development by Syntex of the highly successful isomerically pure agent, S-naproxen, in the early 1970s. However, enantioselectivity (technically defined below) slid into the unconscious until the mid-1980s, a time that heralded the advent of more widely available enantioselective syn-

48

thetic and enantiospecific bioanalytical methodologies. This apparent neglect of the important role that molecular asymmetry played in defining the pharmacological and toxicological properties of chiral NSAIDs led to the majority of such compounds being used clinically as the more readily synthesised racemic mixture containing an equal proportion of the 2 enantiomers.

This review discusses the ramifications associated with the use of chiral NSAIDs, most notably when administered as the racemate. Accordingly, an evaluation of literature pertaining to the pharmacodynamics, pharmacokinetics and toxicology of the individual enantiomers of representative agents is discussed below. Particular emphasis has been placed on the clinical applicability of acquired knowledge on this therapeutically and economically important class of drugs. In terms of the latter, the worldwide market for combined chiral and achiral NSAIDs in 1995 has been estimated to exceed £1000 million.!3]

1. Definition of Enantioselectivity

Biologically active agents often show a high degree of selectivity as a result of the discriminatory ability of molecular sites of action. This discriminatory ability requires complementarity between

R-2-Arylpropionic acid S-2-Arylpropionic acid

Fig. 1. Stereochemical depictions of the R- and 8-enantiomers of congeners of 2-arylpropionic acid. The chiral carbon is indicated by an asterisk and all chemical bonds are oriented in the plane of the page unless indicated by a solid or dashed wedge, which denotes substituents lying above and below the plane of the page, respectively. The identity and attachment site of the aryl substituent (X) varies according to the particular 2-APA. For example, in the case of ketoprofen, X is a benzoyl group attached to the 3-position on the aromatic nucleus. The arrows illustrate the unidirectional nature of the metabolic chiral inversion reaction.

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Hayball

the bioactive agent and the molecular site of action or receptor, most commonly a protein, which in tum is constructed of chiral amino acid subunits.

Inherent enantioselectivity in action can be accounted for on the basis of as few as 3 groups in the active molecule participating in a structurespecific fashion with the effector site (the so-called '3-point' interaction). Accordingly, for chiral drugs, it is hardly surprising to find enantioselectivity the rule rather than the exception in nature, since the multitude of enantiomerically pure asymmetric interaction sites present in organisms make them a veritable 'chiral jungle' with which drugs must come into contact.

Differences between enantiomers in the pharmacological response to a given dose may arise from differences in drug delivery to the receptor site(s) [enantioselective pharmacokinetics], most notably in terms of unbound drug (drug not bound to protein). Alternatively, differences may exist between enantiomers in terms of their relative complementarity and subsequent activity at effector site( s) [enantioselecti ve pharmacodynamics]. Both enantioselective pharmacodynamics and pharmacokinetics are important in describing the influence of optical isomerism on clinical pharmacology; the relative contribution of each is a complex function of many factors. The term 'enantiospecific' has usually been used in the literature to confer exclusivity to one enantiomer over its optical antipode, i.e. total enantioselectivity. Typically, the synthesis of enantiomers of high optical purity and exclusive activity of one enantiomer (the other isomer being devoid of activity) are referred to as enantiospecific processes.

2. Enantioselective Pharmacodynamics of Chiral NSAIDs

It has been generally thought that NSAIDs exert their predominant pharmacological effects by inhibiting the synthesis of prostaglandins (mediators of pain and inflammation) at one or more steps in the endoperoxide biosynthetic pathway.!4] A variety of enzymes catalyse the conversion of cell membrane-derived arachidonic acid to prosta-

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Chirality and NSAIDs 49

Table I. Comparative in vitro data on the potencies olthe separate enantiomers and racemic forms of the 2-arylpropionic acid NSAIDs derived from human eicosanoid-dependent test systems

Chiral NSAID Test system SIR potency ratio RSIS potency ratio Reference

Carprofen Inhibition of platelet aggregation > 24 0.58 9

Fenoprofen Inhibition of platelet aggregation 35 0.50 10

Ibuprofen Inhibition of platelet thromboxane production 0.50 11

Indobufen Inhibition of platelet thromboxane production 16 0.50 12

Ketoprofen Inhibition of platelet thromboxane production 0.50 13

Ketoprofen Inhibition of polymorph thromboxane production 3000 0.50 14

a After accounting for optical contamination in the 'pure' R-isomer, no inhibitory activity was detected for R-ketoprofen, nor did it modulate the activity of the active 8-enantiomer when present together in the test system.

glandins, thromboxanes, leukotrienes and hydroxy acids.l5] The cyclo-oxygenase subunit of prostaglandin synthetase is the primary specific site at which NSAIDs bind and block. In terms of chiral NSAIDs, it is well characterised that for 2-APA congeners and other arylalkanoic acids,[6-8] major or exclusive in vitro inhibition of prostaglandin synthesis is elicited by enantiomers of the Sstereoconfiguration; corresponding pharmacodynamic data for azapropazone and oxyphenbutazone are currently lacking. The situation in vivo needs to be tempered in light of the unique, unidirectional metabolic biotransformation of the Renantiomers of 2-APAs to their corresponding pharmacologically active S-antipodes (see section 3). Accordingly, depending on the particular 2-APA congener and the animal species (which together determine the extent of R to S conversion), in vivo activity may be falsely attributed to the R-enantiomer.

Representative in vitro data pertaining to humanderived test systems are surveyed in table I. High eudismic ratios have been observed for each chiral NSAID studied. Furthermore, inhibitory activity of the racemate was very close to 50% of the corresponding pure S-isomer value, indicating a lack of interaction between enantiomers. Hayball et aU13] have suggested that, in most cases, activity attributed to isomers of the R-stereoconfiguration is likely to be due to optical contamination of this enantiomeric test compound. Clearly, caution is needed when interpreting the data in table I, since they are indices rather than direct measures of inhibitory activity on cyclo-oxygenase. Of particular note is the variability of the concentrations of iso-


mers required to elicit a 50% inhibition (IC50) of human platelet thromboxane production.D5] For example, even after taking into account fluctuations in protein binding of S-ketoprofen, the estimated in vitro IC50 value for unbound S-ketoprofen ranged from 0.044 to 0.508 J!g/L in a heterogeneous group of 14 elderly patients with rheumatoid arthritis (mean ± SD = 0.216 ± 0.143 J!glL) compared with an IC50 range of 0.262 to 0.402 gIL (mean ± SD = 0.320 ± 0.062 J!glL) in 4 young healthy drug-free volunteers.[l3,16]

Recently, cyclo-oxygenase has been shown to be encoded by 2 genes giving rise to 2 distinct forms of this enzyme (COX-l and COX-2).[17] COX-I is expressed in most tissues under normophysiological conditions and is involved in cellular homeostatic or 'housekeeping' functions, such as maintaining the microvascular integrity of the gastrointestinal tract. In contrast, COX-2 is chiefly produced in inflammatory cells in response to tissue trauma and inflammation. Accordingly, COX-2-derived prostanoids are more likely to be the autacoids responsible for tissue damage and pain.[18]

It has been suggested that selective COX-2 inhibition, at the expense of COX-I blockade, would be a logical therapeutic goal, thereby limiting the well recognised gastrointestinal adverse effects of NSAIDs.l17,19] Hayllar and Bjarnason[20] have surveyed data on the selectivity of in vitro COX-l and COX-2 inhibition versus relative risks of serious upper gastrointestinal adverse effects. Limited data have shown that ligands with less inhibitory activity for COX-I, such as S-naproxen and indo-


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methacin, are less likely to cause gastrointestinal complications than NSAIDs such as piroxicam, which are more selective COX-I antagonists. However, the multifactorial nature of the pathophysiology of NSAID-induced adverse effectsl21] potentially limits the clinical utility of such in vitro observations, particularly in light of the discrepancies in magnitude between COX-lICOX-2 blockade and relative gastrointestinal risks; agents with vastly different in vitro ratios have similar gastrointestinal toxicity.

The active metabolite of the achiral NSAID prodrug, nabumetone, has been shown in in vitro animal studies to be a 7-fold more selective inhibitor of COX-2 than of COX-I ,[22] yet significantly less selectivity was observed in vivo and ex vivo in healthy volunteers receiving nabumetone. l23] In vitro human studies with the enantiomers of indobufenl24] and ketoprofenl25] have shown greater COX-I inhibition compared with COX-2 for each drug enantiomer; similar degrees of enantioselectivity (S > R) were noted in these studies for the inhibition of each COX isoform. Such in vitro data, while not necessarily mimicking the situation in vivo, provide useful screening information for lead compound selection in the search for better tolerated NSAIDs.

Evidence of NSAID-induced cyclo-oxygenase inhibition as the means by which these drugs exert their pharmacological and toxicological actions is a reasonable rank-order correlation between inhibition of prostaglandin synthesis in vitro or ex vivo and anti-inflammatory or analgesic effects in vivoP6-28] However, notable exceptions to this generalisation have prompted speCUlation that NSAID pharmacological activity is elicited, at least in part, by mechanisms independent of prostaglandin synthesis.l29-31] Brune and co-workersl32] established that the analgesic activity of the enantiomers of flurbiprofen were equal in an in vivo rat model of nociception, yet R-flurbiprofen, in contrast to its optical antipode, had imperceptible effects on prostaglandin synthesis. These results could not be explained in terms of potential chiral inversion (see above and section 3), and since this

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Hayball

is a negligible metabolic pathway for flurbiprofen in the rat, the investigators suggested that there were additional molecular mechanisms of analgesia that remained to be elucidated.l32] The existence of a third, centrally located isoform of COX, selective for paracetamol, has been postulatedJI7] In the case of the oldest NSAID, salicylic acid, which does not inhibit cyclo-oxygenase at analgesic concentrationsl33] or dosages,l34] Vanel17] has suggested that its mechanism of action may include suppression of the induction of COX. Clearly, there remains much to be elucidated regarding COX isoforms and their pharmacological modulation by NSAIDs.

Postulated mechanisms of action of NSAIDs independent of their effects on prostanoid production include the following: (a) the ability of NSAIDs (including aspirin and salicylate) to lower sulfated glycosaminoglycan synthesis in articular cartilage in vitro,l35] although this mechanism is unlikely to contribute to the typical analgesic antiinflammatory actions of these drugs; (b) suppression of neutrophil aggregation, chemotaxis, degranulation and resultant superoxide (020-) generationl36] and inhibition of inflammatory oedema by an action on polymorphonuclear leucocytes;l37] and (c) inhibition of mitochondrial ~-oxidation offatty acidsl38-40] (a mechanism more likely to convey toxicity than efficacy). The clinical significance of most of these alternative postulated mechanisms of action remains to be fully elucidated. However, it has been suggestedl29] that the effects of NSAIDs on stimulus-response coupling in neutrophils may explain the antiinflammatory activity of salicylic acid. Furthermore, a recent study showed that NSAIDs could be divided into 3 distinct categories based on their effects on stimulus-induced neutrophil oxidative burst. It was suggested that some NSAIDs, while providing temporary relief of arthritic symptoms, could exacerbate the underlying inflammatory condition as a result of 020--mediated tissue damage.l41 ]

The prostaglandin-independent effects of NSAIDs are thought to result from their ability to


Chirality and NSAIDs

partition (or dissolve) into cellular phospholipid bilayers, whereupon they physically disrupt intercellular signalling and subcellular protein-protein interactions.[Z9] This mode of action would be expected to lack the degree of structural specificity inherent in the cyclo-oxygenase mechanism of NSAID action and, as suggested by Evans,[6] is likely to occur without the same degree of enantioselectivity. Physical properties such as lipid or aqueous solubility are identical for individual enantiomers of a chiral drug, and thus the process of membrane partitioning should be nonenantioselective, assuming that bilayer sequestered (carrier) proteins are not involved. This appears to be supported by the lack of enantioselectivity observed with the uncoupling of oxidative phosphorylation in rat liver mitochondria and Ozoproduction in macrophages induced by clindanac enantiomers,[4Z] and a recent report of equipotent inhibition by R-, S- and RS-ibuprofen of human polymorphonuclear cell function in vitro.l43 ]

In summary, it is likely that inhibition of prostaglandin synthesis is the primary mechanism by which NSAIDs exert their pharmacological and toxicological actions. More recent reports of prostaglandin-independent biological properties of these drugs are beginning to contribute to a more detailed understanding of their clinical pharmacology. However, it needs to be recognised that, to date, there is no proven connection between any of these alternative mechanisms and the clinical or toxic effects of NSAIDs.

3. Enantioselective Pharmacokinetics

Interactions of chiral drugs with chiral macromolecules may occur during drug absorption, distribution, metabolism and excretion, and hence each of these pharmacokinetic processes may be enantioselective.l44-46] Enantioselectivity at the dispositional level is often clouded by the complexity of the biological system. Discerning the identity of the enantioselective process in operation is not always possible because of interfering 'chiral noise'. Pharmacokinetic differences between enantiomers may alter pre-existing differ-

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ences at the effector site, either tempering or accentuating pharmacodynamic enantioselectivity. An exhaustive review of enantioselective pharmacokinetics of chiral NSAIDs (see Evans[6]) is beyond the scope of this paper. Accordingly, a brief discussion of representative data is provided for clinically important pharmacokinetic processes and the way in which they impact on the therapeutic efficacy of enantiomeric NSAIDs.

Pharmacokinetic studies of chiral NSAIDs must be performed using enantioselective analytical methods if meaningful dispositional and drug concentration-effect data are to be obtained.l47,48]

Much of the earlier work in this area was performed using nonstereoselective (achiral) assays, which quantified plasma and urinary drug concentrations of chiral NSAIDs as their unresolved enantiomers (i.e. total drug); therefore, pharmacokinetic data from these studies appear to reflect administration of a single compound rather than a mixture. Accordingly, the pharmacokinetic parameters of chiral NSAIDs administered as racemates, generated using nonstereoselective analytical methods, are unlikely to reflect the true pharmacokinetic parameters of the individual enantiomers.

Examples of dramatic differences between enantiomers in terms of their respective pharmacokinetic parameters include etodolac[49] and ketorolac.l50] For instance, following administration of a single intramuscular bolus dose of RSketorolac to healthy volunteers, the systemic exposure to S-ketorolac [as measured by area under the plasma concentration-time profile (AUC)] was approximately 50% of that of its optical antipode.l50] Similarly, both the distribution volume and elimination half-life of the separate enantiomers were different. Taken in conjunction with the probable involvement ofhigbly enantioselective prostaglandindependent processes in both the activity and toxicity of this drug,l51] plasma concentrations of individual isomers are required to correlate plasma levels with therapeutic effect.

Stereo selectivity in drug metabolism is an important determinant of possible differences in the

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disposition of pairs of enantiomers,[52-54) and the literature is replete with examples of enantioselectivity in metabolic transformations, including the chiral NSAIDs. In general, it appears that while drug absorption and excretion do not show significant enantioselectivity (except for actively transported drugs), and plasma protein binding exhibits differences for some compounds,[16,55,56) the largest enantiomeric differences in disposition are the result of hepatic-metabolising activity. In the case of the 2-APA class of chiral NSAIDs, the R-enantiomer may be subject to a unique, highly enantioselective biotransformation process, leading to chiral inversion to the corresponding S-enantiomer, the extent of which is both substrateand species-dependent (fig. 1).[7) For example, fenoprofen undergoes significant chiral inversion in man,[IO) which contributes to the substantially greater AVC observed for the pharmacologically active S-isomer. In contrast, ketoprofen is minimally inverted in man[5?) and its disposition is largely nonstereoselective; the AVC profiles for the individual isomers are virtually superimposable after racemic drug administration. [1 6)

The mechanism of the inversion reaction is thought to involve the stereospecific formation of a coenzyme A thioester [catalysed by acyl coenzyme A ligase(s)] from the R-enantiomer of the 2-APA analogue, which then undergoes a number of alternative fates (see CaldweW58) and Shirley et al.l59) for detailed surveys): (a) racemisation of the chiral centre in the 2-APA moiety, followed by hydrolysis to yield a mixture of enantiomers of the parent drug; (b) hydrolysis with retention of stereoconfiguration yielding the original R-2-APA; or (c) acyl transfer of the 2-APA moiety to glycerol, resulting in the formation of a hybrid triglyceride. Interestingly, recent studies in dogs given the archetypal 2-APA compound, 2-phenylpropionic acid, have demonstrated that both enantiomers are probable substrates for canine hepatic acyl coenzyme A ligase(s) and thus undergo bidirectional inversion.l60) Both R- and S-2-phenylpropionic acid were present in plasma after dosing with either antipode.


Hayball

In addition to chiral inversion, enantioselectivity may be observed for other routes of metabolism, for example, oxidation and conjugation with glucuronic acid. In the case of glucuronidation, chiral NSAIDs such as ketoprofen,[61) naproxen,[61) ketorolac[62) and ximoprofen[63) are subject to significant metabolism by hepatic uridine diphosphoglucuronosyltransferase yielding acyl glucuronide metabolites. Interestingly, acyl glucuronides are intrinsically reactive species, which are capable of undergoing a variety of chemical reactions under physiological pH and temperature conditions.l64) These reactions include hydrolysis of the conjugate back to the parent drug (aglycone), intramolecular rearrangement yielding isomeric (positional) glucuronide conjugates, and formation of covalent adducts with biological macromolecules.

The facile hydrolysis of acyl-linked glucuronides of NSAIDs has potential clinical implications, particularly in situations where such metabolites accumulate systemically. Although such NSAIDs have negligible percentages of their doses excreted into urine as unchanged drug, paradoxically a number of them have diminished clearance in patients with renal dysfunction, in elderly patients in whom renal function is expected to be diminished, and in subjects receiving concurrent probenecid.l65-6?) It has been hypothesised that such observations are consistent with a decrease in the renal excretion of acyl glucuronides, leading to the systemic accumulation of these labile conjugates followed by their facile hydrolysis back to the parent drug (fig. 2).l68) As discussed in a recent review, [69) the systemic regeneration of parent NSAID in the above scenario will have unique stereochemical implications for 2-APA congeners subject to metabolic chiral inversion. In such cases after racemic drug administration, a greatly exaggerated exposure to the pharmacologically active deconjugated S-isomer would be predicted, as has been shown in animal studies with the archetypal 2-APA, 2-phenylpropionic acid.l68) Consistent with this systemic cycling hypothesis, delayed clearance of R- and S-ketoprofen in


Chirality and NSAIDs 53

R-Aglycone 5-Aglycone

H fH3 0

-()'t-c~ ~OH -:?' " HO OH x~1 0 0 COO-

R-Glucuronide S-Glucuronide

! ! Urine Urine

Fig. 2. The systemic cycle originally proposed by Metlin et al.l68] for the formation, hydrolysis and renal clearance of epimeric acyl glucuronides of 2-arylpropionic acid NSAIDs that undergo metabolic chiral inversion. Not shown in this simplified model are: (a) additional biotransformation pathways for both aglycones and glucuronides; (b) regeneration of aglycone from rearranged isomers of biosynthetic glucuronides and, to a lesser extent (quantitatively), from irreversibly bound drug-protein adduct; and (c) biliary excretion of epimeric glucuronides[69] (reproduced with the permission of John Wiley & Sons, New York).

rheumatoid arthritis patients with diminished creatinine clearance has been observed.l161

The ability of acyl glucuronide metabolites of both chiral and achiral NSAIDs to form covalent attachments with protein is also likely to have clinical consequences, particularly when the clearance of these metabolites is delayed. The putative toxicological sequelae of adduct formation are discussed below.

4. Toxicology of Chiral NSAID Enantiomers

Inhibition of arachidonic acid metabolism via the cyclo-oxygenase pathway is the cause of a range of well documented adverse effects of congeners of this drug class. Gastropathy induced by NSAIDs is the principal adverse effect and is thought to result from blockade of COX-I, the enzyme subtype responsible for generating cyto-

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protective 'housekeeping' prostaglandins such as PGE2 and PGIz, which increase gastric mucosa blood flow and mucin production.[21) It would be expected that S-enantiomers are primarily responsible for eliciting the gastrointestinal adverse effects of these drugs. Using the enantiomers of flurbiprofen, animal studies have demonstrated that R-flurbiprofen caused minimal gastrointestinal toxicity, which in tum correlated with its marginal inhibitory effect on local prostaglandin production. [32.70) After long term administration of either racemic or enantiomerically pure flurbiprofen to rats, Wechter and co-workers[71) found that the Renantiomer alone did not elicit gastrointestinal lesions, while the racemate proved to be 2 to 4 times more ulcerogenic in enantiomerically equivalent doses than the S-enantiomer. This suggests a possible attenuation of NSAID-induced gastropathy from the prostaglandin-independent


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actions of R-flurbiprofen. It has been speculated that the R-enantiomer might exacerbate gastrointestinal toxicity by inducing leucocyte adherence in postcapillary venules, and thereby exacerbating the tissue destruction resulting from local ischaemia induced by cyclo-oxygenase antagonismPl,72]

Epidemiological studies have revealed a doseresponse relationship for gastrointestinal toxicity, in addition to a relationship between the duration of exposure and toxicity, so that the risks are greatest for patients who receive between 2 and 4 prescriptions or therapy lasting 2 to 4 monthsP3,74] The toxicity referred to in these studies is serious upper gastrointestinal bleeding or perforation. Additional risk factors include old age, where it is generally accepted that NSAID-induced gastropathy operates on an increased basal risk of peptic ulceration, thereby giving an overall increased risk of complications of peptic ulceration in the elderly. It remains to be elucidated whether systemic cycling of NSAIDs metabolised to renally eliminated acyl glucuronides (see section 3) enhances the risk of gastrointestinal toxicity in elderly, renally compromised patients.

It has become apparent that an alternative arachidonic acid catabolic pathway generates products that profoundly affect cellular and vascular reactions associated with inflammationP5] The lipoxygenase pathway generates hydroperoxyeicosatetraenoic acids (HPETEs), which are reduced by peroxidase to the corresponding hydroxy acids. One of the hydroperoxy derivatives,5-HPETE, is the precursor of a group of intensely acti ve autacoids referred to as leukotrienes. Accordingly, NSAID-induced blockade of cyclo-oxygenase activity, while inhibiting production of prostaglandins and thromboxanes, may putatively increase the catabolism of arachidonic acid via lipoxygenase pathways. This could lead to overproduction of leukotrienes, hydroxy-fatty acids and tissue-destructive free oxygen radicals.

Theoretical advantages might therefore be conferred upon the clinical use of agents that block both cyclo-oxygenase and lipoxygenase pathways of arachidonate catabolism,[75] such as

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Hayball

ketoprofenP6] However, this hypothesis has been challenged by Somasundaram et al.,[21] who argue that metabolic diversion of arachidonic acid would be unlikely, since cyclo-oxygenase and lipoxygenase have different subcellular locations (cytosol and endoplasmic reticulum, respectively[22]). In addition to drug toxicity predicted from inhibition of eicosanoid production, the participation of 2-APA analogues in the metabolic chiral inversion process, as well as the generation of covalent drug-protein adducts from those acidic NSAIDs that are metabolised to reactive acyl glucuronides, have each been hypothesised to lead to toxic sequelae.

The generation of R-2-arylpropionyl coenzyme A thioesters, as obligate intermediates in the chiral inversion reaction (section 3), has been suggested as having toxicological significance.£58]It has been shown by use of racemic mixtures of a number of 2-APA analogues that these drugs are capable of inhibiting cholesterogenesis and fatty acid synthesis in vitro, and that this activity is correlated with their ability to form hybrid triacylglycerolsP7] As suggested by Caldwell and Marsh,[78] the formation of such hybrid lipids and their incorporation into cell membranes have the potential to alter membrane structure and function. Accordingly, the involvement of R-profens in coenzyme A thioester intermediate formation might confer an advantage for the use of the S-enantiomers alone in clinical practice. However, there is no direct clinical evidence of corresponding toxicity to date. Indeed, as argued by Williams and Day,[79] there is little evidence to suggest that S-naproxen is significantly less toxic than those 2-APAs used clinically as the racemate. Moreover, Evans[6] makes the point that the efficacy and safety of racemic ibuprofen, which is subject to significant chiral inversion in man, is noteworthy.

In humans, many chiral NSAIDs are metabolised to renally eliminated acyl glucuronides to a significant extent (section 3). It has been argued that the ability of such drugs (without exception) to form covalent drug-protein adducts, via these reactive metabolites, may lead to immunotoxicological responses.£64] A significant proportion of



those drugs withdrawn from the US and UK markets over the last 2 decades because of induction of severe hypersensitivity reactions were carboxylic acids subject to significant glucuronide conjugation.l80] Ding et aU81] have recently identified Lys-199 as the most prominent covalent binding site for tolmetin glucuronide on the human serum albumin molecule; Lys-199 is a lysine E-amino group located in a hydrophobic region notable as a target for covalent binding of recognised immunogens such as penicillin-derived benzylpenicilloyl radicals. [82]

It has been further argued that the ubiquitous involvement of the glucuronic acid moiety in acyl glucuronide-mediated adduct formation may explain the apparent immunological cross-reactivity with several NSAIDs in susceptible patients.[81] Clearly, the ability of both enantiomers of chiral NSAIDs to participate in adduct formation[69] suggests that marketing of drugs as the sole enantiomer will not negate covalent drug-protein generation, although adduct density is likely to be diminished. It should also be noted that increased systemic exposure to acyl glucuronides, due to conjugate recycling, may result in higher adduct formation, as has been shown in humans after inhibition of the renal elimination of zomepirac glucuronide by probenecid.l83] However, the link between adduct formation in humans and immunotoxicity remains to be conclusively proven.

5. Conclusions

Chiral NSAIDs, in particular 2-APA analogues, exhibit well characterised enantioselective pharmacokinetic, pharmacodynamic and toxicological properties, the degree of enantioselectivity being dependent on the particular pharmacological process under investigation. Accordingly, a need for a 3-dimensional understanding of the pharmacological properties is implicit in understanding the interaction between chiral NSAIDs and biological environments. It is clear, given the existence of the chiral inversion reaction for some 2-APAs, that the activity and disposition of such drugs (when administered as racemates) are unlikely to be the

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simple sum of activities and disposition of the individual enantiomers.

There has been considerable debate as to whether all chiral NSAIDs should be marketed as the pure S-enantiomer for use in the treatment of inflammatory conditions, since it is generally accepted that the activity of such drugs in this context is confined to isomers. of the S-stereoconfiguration. Benefits of using the S-enantiomer alone might be the avoidance of so-called 'isomeric ballast'[47] and a reduction in the metabolic load to the patient. Furthermore, for those agents that are administered as race mates and undergo extensive chiral inversion (e.g. ibuprofen[67] and fenoprofen[lO]), the effective dose of the active agent is unknown, which is clearly an undesirable situation. In addition, to the extent that a knowledge of clinical pharmacology facilitates dosage selection, single isomers might be preferred because of less complex pharmacokinetic profiles, drug interactions and concentration-effect relationships; however, very few studies have correlated plasma concentrations of NSAIDs with assessments of therapeutic response.l84] Moreover, the interindividual variations in the pharmacokinetics of the S-isomer of the metabolically complex 2-APA congener, ibuprofen, were similar irrespective of whether the drug was administered as the racemate or as the separate S-enantiomer.[85] As commented upon by Williams,P5] the limited clinical studies of 2-APA analogues administered as the separate S-isomers, while demonstrating efficacy as anti-inflammatory agents, have not yet facilitated attempts to establish concentration-effect relationships for these drugs. Furthermore, the possible role ascribed to Renantiomers of chiral NSAIDs in eliciting prostaglandin-independent actions in blunting pain and inflammation complicate the 'enantiomer versus racemate' debate for this chiral drug class.

It would appear that there are at least theoretical advantages to marketing chiral NSAIDs as enantiomerically pure drug products, if only to minimise the potential additional toxicity mediated by the optical antipode. Birkett,[86] arguing from a drug regulatory standpoint, has concluded that the


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balance of the argument is overwhelmingly in favour of the use of separate enantiomers. Indeed, the pharmaceutical industry appears to be moving away from developing racemates in favour of enantiomerically pure or achiral drugs)87J While obviously benefiting the economies of pharmaceutical companies, the reworking of traditionally safe and efficacious racemic NSAIDs into patentable S-isomeric formulations is likely to contribute significantly to our understanding of the basic and clinical pharmacology of these therapeutically important drugs. Modern marketing applications for enantiomerically 'cleaned-up' drugs must, by necessity, be accompanied by extensive basic and clinical pharmacological data on both the safety and efficacy of these drugs, irrespective of what has been previously learnt about the racemate. This additional knowledge should enhance more rational prescribing of chiral NSAIDs and, in the process, enable us to learn more about pain and inflammatory processes and their response to pharmacological intervention.

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3. Lewis AJ, Furst DE. Non-steroidal anti-inflammatory drugs: mechanisms and clinical uses. 2nd ed. New York: Marcel Dekker, 1994: 1-2

4. Vane JR, Botting RM. The mode of action of anti-inflammatory drugs. Postgrad Med J 1990; 66 Suppl. 4: S2-17

5. Cashman J, McAnulty G. Nonsteroidal anti-inflammatory drugs in peri surgical pain management. Drugs 1995; 49: 51-70

6. Evans AM. Enantioselective pharmacodynamics and pharmacokinetics of chiral non-steroidal anti-inflammatory drugs. Eur J Clin Pharmacol1992; 42: 237-56

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Correspondence and reprints: Dr Peter Haybal/, School of Pharmacy and Medical Sciences, University of South Australia, North Terrace, South Australia 5000, Australia.


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Chirality and Nonsteroidal Anti-Inflammatory Drugs