Enantiomers in arthritic disorders

Pharmac. Ther. Vol. 46, pp. 273-295, 1990 Printed in Great Britain. All rights reserved

0163-7258/90 $0.00 + 0.50 © 1990 Pergamon Press plc

Associate Editor: M. ORME

ENANTIOMERS IN ARTHRITIC DISORDERS KENNETH M. WILLIAMS Department of Clinical Pharmacology and Toxicology, St Vincent's Hospital, Victoria Street, Darlinghurst, NSW, 2010, Australia A~traet--Drugs which have a center of asymmetry are often administered as an equal mixture of the two possible enantiomeric forms i.e. a racemate. However, there are frequently large pharmacodynamic and pharmacokinetic differences between enantiomers. Consequently, it is possible that while one enantiomer mediates the antiinflammatory or antirheumatic action, the other enantiomer, although adding little to the efficacy of the drug, may contribute to its adverse effects. Asymmetric drugs may also serve as sensitive pharmacological probes of the mechanisms underlying the action of drugs and the inflammatory processes which they modulate. These concepts are the focus for this review. CONTENTS I. Introduction 1.1. General 1.2. Terminology 1.3. Scope of review 2. Nonsteroidal Antiinflammatory Drugs 2.h 2-Arylpropionic acids 2.2. Heteroarylacetic acids--sulindac 2.3. Enolic acids 2.3.1. Oxyphenbutazone 2.3.2. Etodolac 2.3.3. Azapropazone 3. Immunomodulating Drugs 3. I. o-Penicillamine (S-peniciUamine) 3.1.1. Clinical experience with penicillamine enantiomers 3.1.2. Animal toxicity data 3.1.3. Pyridoxine and thiazolidine formation 3.1.4. Effects on collagen synthesis 3.1.5. Immunological activity of penicillamine enantiomers 3.1.6. Pharmacokinetics and disposition 3.2. Thalidomide 3.3. Cyclosporine 3.4. Levamisole 4. Antimalarial Drugs 4.1. Chloroquine, hydroxychloroquine 4.2. Primaquine, quinacrine 5. Gold Compounds 6. Cytotoxic Drug Therapy 6. h Cyclophosphamide 6.2. Methotrexate 7. Prostaglandin-E 2 Analogs 8. Conclusions and Recommendations Acknowledgements References

1. I N T R O D U C T I O N

273 273 274 275 275 275 280 280 280 281 281 281 281 282 282 282 283 283 283 283 285 285 285 285 286 286 287 287 287 288 288 289 289

of drinking "looking-glass milk" he (unwittingly?) posed a question which has great biochemical, toxicological and pharmacological implications. Just 10 years previously, Pasteur's study of the tartrates had led him to conclude that all the biochemically important molecular building blocks were asymmetric (i.e. molecules whose mirror images were nonsuperimposable). His conclusion that there was a unique relationship between life and asymmetry was further

I.I. GENERAL Perhaps looking-glass milk isn't good to drink ... (Through the Looking Glass and what Alice Found There? m Lewis Carroll, 1872.) When Lewis Carroll (Charles Lutwidge Dodgson: 1832-1898) speculated through Alice about the value 273

274

K.M. WILLIAMS

reinforced by later observations, for example the ability of the 'gustatory nerves' to discriminate between the mirror-image (enantiomeric) molecules of arginine. One enantiomer was sweet, the other insipid or tasteless. Since that time, all evidence, such as the ability of the proteolytic enzymes to recognize an unnatural amino acid within a sequence of natural amino acids, have confirmed that the body can indeed discriminate between 'looking-glass' and 'normal' milk. Subsequent to Pasteur, studies of enantiomeric substrates, notably the work of Cushney (1926) and Easson and Stedman (1933), established the discriminatory capacity of receptors for both the endogeneous sympathetic neurohormones and related drug agonists and antagonists. However, the translation of the basic understanding of differences between the activities of enantiomers by medical chemists and basic pharmacologists to the clinical relevance and potential importance of these differences has been slow. Only recently has it been generally realized that molecular asymmetry may be a more important consideration than other structural characteristics. Consequently, those drugs which are administered as an equal mixture (racemate) of each of the mirrorimage, isomeric molecules (enantiomers) constitute a particularly interesting group. A number of reviews have addressed the general area of enantioselective drug disposition (Jenner and Testa, 1973; Simonyi, 1984; Williams and Lee, 1985; Trager and Testa, 1985; Vermeulen, 1986; Williams, 1989) and some have specifically addressed the disposition of the 2-arylpropionate nonsteroidal antiinflammatory drugs (NSAIDs; Hutt and Caldwell, 1983, 1984; Williams, 1987; Caldwell et al., 1988). In addition to this important group of antiinflammatory agents, there are a significant number of other asymmetric antirheumatic drugs (Table 1). 1.2. TERMINOLOGY As intimated above, molecules which are mirror image reflections of each other, but which are not superimposable, in the way that the left and right hands are related, are asymmetric, enantiomeric or chiral. Pairs of enantiomers have identical physical characteristics such as polarity, solubility, lipophilicity, and melting and boiling points. Physically they are distinguishable by the effect which a solution of the enantiomer has on plain polarized light. In this

tl

I°

view

o%,,o < o R-configuration

view

T*

> o,o,;,o o S-configuration

FIG. I. Assignment of absolute configuration after Cahn et al. (1956). A pair of enantiomeric (mirror-image) molecules are depicted. C* denotes the asymmetric carbon atom to which 4 different functional groups or atoms (a~:l) are attached. These 4 substituents are assigned a priority ( a > b > c > d ) according to the Sequence Rules. The molecule is viewed such that the lowest priority function (d) is away from the observer. If the direction a b-c is to the right the molecule has the configuration R (rectus), if the direction is to the left then it has the configuration S (sinister). environment one enantiomer rotates the light to the left (levorotatory, depicted by the symbol, l or ( - ) ) while the other enantiomer rotates the light equally but in the opposite direction (dextrorotatory, d or (+)). The magnitude and even direction of rotation is affected by pH, ionic strength, concentration and wavelength of the light. It is important to note that the direction of rotation does not reflect the absolute shape of the molecule (absolute configuration). This can be determined by X-ray crystallography, for example. Two systems of nomenclature have been developed for describing the absolute configuration of asymmetric molecules. The first is one which uses two reference compounds, the natural levorotatory enantiomer of serine (designated L-serine) and the natural dextrorotatory enantiomer of glyceraldehyde (designated D-glyceraldehyde). Amino acids or structurally related compounds were assigned a configuration relative to that of serine. The sugars were related to glyceraldehyde. This system although still in use, was not sufficiently universal. Further, some compounds could be designated D or L depending on which of the two reference compounds was used. The second system of nomenclature for absolute configuration was that introduced by Cahn et al. (1956). It is based on a set of rules for assigning an order of decreasing priority (a~t) to each of the substituents attached to the asymmetric carbon, decreasing atomic weight being the simplest situation. The molecule is then viewed with the lowest priority group away from the viewer (Fig. 1). If the direction of rotation from highest to lowest priority is to the left the enantiomers can be described as S (sinister)

TABLE [. Asymmetric Drugs used in the Rheumatic' Diseases 2-Arylpropionates benoxaprofen carprofen fenoprofen flurbiprofen ibuprofen indoprofen ketoprofen naproxen* pirprofen tiaprofenic acid

Enolic acids azapropazone etodolac oxyphenbutazone Gold auranofin* aurothiglucose* aurothiomalate

Heteroarylacetates sulindac

Cytotoxic's cyclophosphamide methotrexate*

*Used as the single enantiomer; other drugs racemic.

lmmunomodulat ors cyclosporine levamisole* penicillamine* thalidomide Antimalarials" chloroquine hydroxychloroquine primaquine quinacrine

275

Enantiomers in arthritic disorders or if the direction of rotation is to the right then the molecule is R (rectus). The nomenclature of Cahn et al. is unambiguous and is applicable to any asymmetric molecule. Throughout this review, this nomenclature will be employed where the absolute configuration has been determined (with the exception of penicillamine because of the common usage of the D- and L-nomenclature). Elsewhere, the enantiomers will be designated ( + ) or ( - ) according to their optical rotation in solution. A racemic drug is described as (+), D,L or R,S. Two final points should be stressed; not all L-amino acids are of the S-series and, the nomencalture D and L, and R and S do not imply anything about the direction which solutions of the enantiomers will rotate polarized light. Conversely, nothing can be inferred about the absolute configuration of a molecule from the direction a solution of the substance rotates polarized light. 1.3. SCOPEOF REVIEW Differences in the therapeutic ratios between pairs of enantiomeric antiinflammatory drugs will depend on differences in the pharmacokinetics, metabolism and pharmacodynamics of the enantiomers. This in turn will be dictated by the enantioselectivity of the enzymes, receptors and to a lesser extent the tissues and plasma proteins of the organism. For the nonsteroidal antiinflammatory drugs, differences in activities are determined primarily by the difference in ability to inhibit prostaglandin synthetase, an activity which resides exclusively in the S-enantiomers. For other drugs such as penicillamine, preferential use of one enantiomer (D-penicillamine) is based on the apparent greater toxicity of the other. The purpose of this paper is to review the literature dealing with the enantiomers of drugs used in the treatment of arthritis and, where possible, to place into perspective the possible clinical implications of the use of these asymmetric antiinflammatory and antirheumatic drugs, especially those used as their racemates. In general, however, little data are available on the relative potencies of these enantiomers. There are even less data dealing with the relative activities of enantiomers in models of inflammation. From this point of view, it may be premature to consider writing a review dealing with enantiomers in arthritis. However, it is hoped that by reviewing the data, often obtained from other applications of these drugs, the opportunities to increase the therapeutic indices of these asymmetric antiinflammatory and antirheumatic drugs by selective use of enantiomers will be highlighted. Ultimately, a comparison of the relative activities of enantiomers in the rheumatic diseases must be assessed by proper clinical trials. When the data are finally available, we may well find that qooking-glass' drugs or a mixture of a drug and its mirror image isomer, are not good to eat. One difficulty encountered in reviewing the literature which contrasts the activities of enantiomers, is the frequent absence of data validating the optical purities of the enantiomers used. This is illustrated, for example, in the discussion of the relative efficacy and toxicity of chloroquine where in some studies the so-called 'enantiomers' were little more than

racemates. By necessity, all discussion in this review assumes optical purity of the drug enantiomers. Thus differences in efficacy and toxicity represent minimum differences. Differences in drug disposition are clear where enantiospecific assays have been used but otherwise similar arguments hold. Asymmetric drugs used in the management of the rheumatic diseases are grouped for discussion purposes according to their chemical classification or according to their general pharmacological actions.

2. NONSTEROIDAL A N T I I N F L A M M A T O R Y DRUGS 2.1. 2-ARYLPROPIONICACIDS Early structure activity studies established that the activity of 2-methyl substituted arylacetic acids (2-arylpropionates, 2-APAs; Fig. 2), as determined by inhibition of prostaglandin synthetase in vitro, resided in enantiomers of the S-configuration (Shen, 1972). However, subsequently studies of activity in vivo did not always find such significant enantiomeric differences. Thus while S-ibuprofen (Fig. 2) was 160 times more potent than R-ibuprofen in inhibiting microsomal prostaglandin synthetase from bovine seminal vesicles, their activities were not significantly different in experimental models (Adams et aL, 1967; Adams et al., 1976; Table 2). The authors concluded that there was almost complete inversion of the R- to the S-enantiomer. These data confirmed observations that the urinary metabolites of ibuprofen in man were dextrorotatory (Adams et aL, 1967; Vangiessen and Kaiser, 1975). A comparison of the in vitro and in vivo activities of 2-APA enantiomers (Table 2) suggests that inversion does not occur universally or at least may be substantially incomplete (e.g. carprofen). Additionally, there appears to be significant interspecies differences in ability to invert 2-APAs (Fournel and Caldwell, 1986). The data for clidanac (Tamura et al., 1981b) and other preliminary data for flurbiprofen (Williams, unpublished data) for example, suggest that the guinea pig inverts more drug than rats or mice. Pharmacokinetic studies establishing unequivocal inversion in man have been published for ibuprofen (Figs 3 and 4; Lee et al., 1985; Mills et al., 1973; Kaiser et al., 1976), fenoprofen (Rubin et al., 1985) and benoxaprofen (Bopp et aL, 1979). Equally, however, data demonstrating no significant inversion in man have been reported for indoprofen (Tamassia et al., 1984) and flurbiprofen (Jamali et al., 1988a). Other 2-APAs which are also probably not inverted in man are tiaprofenic acid (Singh et al., 1986),

CH 3

:.

HO R(-)-ibuprofen

S(+)-ibuprofen

FIG. 2. The structures of S( + )-ibuprofen and R (-)-ibuprofen. The S-enantiomers of the 2-arylpropionates inhibit prostaglandin synthesis. The R-enantiomers are inactive in this respect.

276

K. M. WILLIAMS TABLE 2. Relative Potencies o f the Enantiomers o f 2-Arylpropionic Acids In Vitro and In Vivo Relative ( S / R ) potency

Arylpropionate carprofen

in vitro:

clidanac

in vivo: in vitro: in vivo:

fenoprofen

in vitro: in civo: 01 vitro:

flurbiprofen ibuprofen

0l vitro: in vivo:

indoprofen

in vitro: in vivo: in vitro:

naproxen

in tivo:

tiaprofenic acid

in vitro:

> 16 > 23 14 1000 2.5 12 > 12 1 35 1 900 5 7 160 1.3 100 20 130 70 14 20 28 120

Test of activity

Reference

IPG synthesis (SSV) Platelet aggregation AA (rat) IPG synthesis (GPS) CPE, GP (rats); PW (mice) Antipyresis (rat) UVE (guinea pig) Inhibit PG synthesis (HP) AA, UVE, CPE (rat) Antagonism SRS-A (guinea pig) Guinea pig anaphylaxis IPG synthesis (BSV) UVE (guinea pig); AW (mouse) PT (rat) IPG synthesis (SSV) CPE and GP (rats); PW (mice) IPG synthesis (SSV) IPG synthesis (BSV) CPE, analgesia, antipyresis CPE (rat) IPG synthesis (MPM)

Gaut e t a l . , 1975 Tamura e t a l . , 1981a Kuzuna et al., 1974 Rubin et al., 1985 Nickander e t a l . , 1971 Greig and Griffin, 1975 Adams et al., 1976 Buttinoni el al., 1983 Ku and Wasvary, 1975 Tomlinson et al., 1972 Tomlinson e t a l . , 1972 Harrison et al., 1970 Siebler and Fenner, 1985

AA = adjuvant arthritis; AW = acetylcholine induced writhing (analgesia); BSV = bovine seminal vesicle microsomes; CPE =carrageenin induced paw edema; GP = granuloma pouch model; GPSM = guinea pig skin microsomes; HP = human platelets; IPG = inhibition prostaglandin; MPM = mouse peritoneal macrophages (zymosan stimulated); PT = pain threshold (yeast-inflamed foot); PW = phenyl-quininone induced writhing (analgesia); SSV =sheep seminal vesicle microsomes; UVE = ultraviolet erythema. (Data drawn from above references in conjunction with similar tables by Caldwell et al., 1988 and Lee, 1986.) ketoprofen (Foster et al., 1988a, b; Sallustio et al., 1988b) and carprofen (Stoltenborg et al., 1981). Synovial fluid concentrations of active enantiomer give some insight into the time course of action of these drugs. Data for ibuprofen, for example (Fig. 5; Day e t a l . , 1988a) clearly explain why this drug, despite its short plasma half-life of approximately 2 hr, may be dosed on a 3 times or even twice daily regimen. Concentrations of S-ibuprofen in the synovium may fluctuate to an even smaller degree. In addition, our studies of the enantiomeric disposition of ibuprofen have given some insight into the mechanisms by which drug equilibrates between plasma and synovial fluid. The data suggested that ibuprofen diffuses into the synovial fluid in the unbound form but that a proportion of the drug (10-20%) may

diffuse out of the synovial compartment bound to albumin (Day e t a l . , 1988b). A relationship between concentration and response for reversibly acting antiinflammatory drugs has only been established in a limited number of cases (Day et al., 1982a; Dunagan et al., 1986). Although the difficulty in accurately assessing disease activity, study design and the limited dosage ranges studied have thwarted other attempts to validate this relationship (Day e t a l . , 1987), it is perhaps more than coincidental that a plasma concentration-response relationship was demonstrated for naproxen (Day

lOO

-~ 5o

{5°

5

%o =="~ • ~,

1.0

.

0,5

5 c~

i

;%% • •

~,o

loo

01

1.C

o 0

0.5

TIME, h

FIG. 3. Plasma concentrations of R (D) and S (11) ibuprofen following oral administration of R-ibuprofen (400 mg) to a healthy volunteer (Lee et al, 1984). Reprinted with the permission of the copyright holder, the American Pharmaceutical Association, Washington, D.C.

2

4

§

8 TIME, h

10

12

14

16

FIG. 4. Plasma concentrations of R ([7) and S (m) ibuprofen following oral administration of S-ibuprofen (400 rag) to a healthy volunteer (Lee et al., 1984). The very low concentrations of the R-enantiomer probably represent the small amount of impurity of the R-enantiomer in the administered S-enantiomer, rather than inversion of S-ibuprofen to R-ibuprofen. Reprinted with the permission of the copyright holder, the American Pharmaceutical Association, Washington, D.C.

Enantiomers in arthritic disorders

277

IN

2 E v Z

G

I-z

1.~

~

0.5

O.l

i 8

2

4

6

8

IQ

12

14

TIME Chr)

FIG. 5. Plasma concentrations of R (l-q) and S fro) ihuprofen, and synovial fuid concentrations of R (O) and S (0) ibuprofen in a patient at steady state treated with R,S-ibuprofen (800 mg twice daily). The last dose was at time zero (Day et al., 1988a). et al., 1982a), the only 2-APA administered as the

S-enantiomer. Interpreting pharmacokinetic data based on nonenantioselective assays when chiral drugs, especially racemates, are administered, is a tenuous process. Studies of dose, concentration and effect relationships require measurement of the relevant active species. There has been some interest in the site of the in vivo inversion of 2-APAs (Table 3). It has been suggested that inversion occurs in the gut and that the longer the absorption time then the greater the inversion (Jamali et al., 1988c; Mehvar and Jamali, 1988). Early evidence had demonstrated that everted rat gut preparations inverted R-benoxaprofen (Simmonds et aL, 1980) while it was concluded that no inversion occurred in the liver. Although these latter data support the gut first-pass inversion hypothesis, other data are not consistent with this view. For example liver homogenate (Nakamura et al., 1981; Knihinicki et al., 1989) and perfused liver (Cox et al.,

1985) from rats have been shown to invert ibuprofen, while kidney slices were also active with 2-phenylpropionic acid as a substrate (Yamaguchi and Nakamura, 1987; Nakamura and Yamaguchi, 1987). The literature is not always in concordance, with some authors for example, finding inversion of ibuprofen in liver preparations and others not observing inversion with this tissue (Table 3). Clearly, tissue preparation is critical. However, data negating the hypothesis that ibuprofen is inverted by the gut in humans, have been recently presented in abstract form (Cox, 1988). In this study of 12 healthy adult males, there was no difference between the fractional inversion of R-ibuprofen when the drug was given orally and by intravenous administration. The data for phenylpropionic acid incubated with kidney slices was of particular interest because there was greater inversion by the kidney than for slices of liver (Nakamura and Yamaguchi, 1987). These data may be of relevance to the renal complications asso-

TABLE3. Site o f Inversion o f 2-Arylpropionic Acids Reference (drug)

Organ*

Conditions

Inversion

Simmonds et al., 1980 (benoxaprofen)

liver gut

Nakamura et al., 1981 (ibuprofen) Cox et al., 1985 (ibuprofen) Nakamura and Yamaguchi, 1987 (phenylpropionate)

liver

homogenate microflora everted perfused homogenate perfused

no no yes yes yes yes

homogenate slices slices slices slices homogenate subcellular§ homogenate homogenate homogenate

no yes yes ?? no no no yes no no

Mayer et al., 1988 (flurbiprofen, naproxen, suprofen, ibuprofen) Knihinicki et al., 1989

liver liver kidney intestine othert liver++ liver kidney gut

*Organs from rat. tHeart, lung, spleen, testes. ++Rat and guinea pig. §Mitochondria, microsomes, cytosol + mitochondria.

278

K.M. WILLIAMS Disposition

Metabolic

Pathways

Consequences

lipids Lipid incorporation 2

/ R-CoA.

, ve sJ

! Perturbation of lipid

~N~ ~

II Direct toxicity (?)

S-CoA

It, oxia,,on,,L R

Renal clearance 3 (futile cycle)

biochemistries (?)

S

It

ll

R.glue

S-glue

NN~

~ PG synthetase

inhibition

~

Drug-protein adducts Immunological effects

/ CLrenal

1.

2.

3.

The R-enantiomers of the 2-APA's (R) are stereospecifically converted to their CoA thioesters (R-CoA). The CoA thioester is racemised by a racemase and the CoA thioesters are hydrolysed to release the R or S-2-APA (Nakamura et al., 1981). CoA thioesters may have direct toxicity or they may be enzyme inhibitors. The activated 2-APA-CoA thioesters may enter lipid metabolic pathways to form hybrid lipids (Fears, 1985; Williams et al., 1986; Sallustio et al., 1988) with unknown biochemical sequelae. Futile cycle (Meffin, 1985). A reduction in the clearance of the acyl-glucuronides due to a change in renal function will lead to accumulation of the glucuronides (R-gluc; S-gluc) with subsequent hydrolysis to release the parent drug. More R-enantiomer is expected to be inverted (Spahn et at., 1987) or shunted into the lipid biochemical pathways. Glucuronides may also react with plasma and other proteins to form drug-protein adducts with the potential for stimulating an immune response (Van Breemen and Fenselau, 1985).

FIG. 6. (See footnotes.) ciated with use of the 2-APAs, particularly in those patients with compromised renal function. Locally higher concentrations of the active S-enantiomer may cause a greater depression of renal prostaglandin concentrations thus contributing to the associated adverse effects. The mechanism of inversion and its consequence are of some interest. It was first suggested that inversion proceeded via a methylene intermediate (Wechter et al., 1974). However, it is almost certain that inversion proceeds according to the schema proposed by Nakamura et al. (1981; incorporated into Fig. 6). Very recent data from an excellent pharmacokinetic study demonstrating that inversion of R-ibuprofen, which had been trideuterated in the 2-methyl position, proceeded with retention of the deuterium atoms, further supports this schema (Baillie et al., 1989). The first step in the inversion of 2-arylpropionates is enantiospecific formation of the CoA thioester of the R-enantiomer. Additional evidence for the specificity of this reaction has been recently published (Knights et al., 1988; Knihinicki et al., 1989). The CoA thioester is then racemized with subsequent hydrolysis to release the R- and S-enantiomers. If the first step is bypassed by synthesizing the thioester of the S-enantiomer, the S-enantiomer also is racemized (Nakamura et al., 1981). There are several consequences of this mechanism of inversion which are also illustrated (Fig. 6) and which are of potential clinical interest. Firstly, the extent of inversion between individuals is likely to be variable. Variability in inversion is effectively a variability in the dose of active drug with the consequence of interpatient variability in the extent of prostaglandin synthetase inhibition. This may be

one of the factors contributing to the variability in response to treatment with some members of this group of drugs (Williams and Day, 1985, 1988; Day et al., 1988a). Secondly, CoA thioesters are able to form hybrid triglycerides (Fig. 7; Fears et al., 1978; Fears, 1985), a process which we (Williams et al., 1986) and others (Sallustio et al., 1988a) have demonstrated to occur enantiospecifically for the R-enantiomers as predicted by the above schema (Fig. 6). Those 2-APAs which do not participate in the lipid metabolic pathways are probably not substrates for the CoA synthetase and consequently these 2-APAs also are not inverted (Fig. 6); i.e. we might predict that there will be a correlation between the uptake of 2-APA into hybrid triglycerides in vitro and the clearance of 2-APAs by inversion in vivo (Binskin et al., 1987). The order of inversion (expressed as clearance by inversion, CLR~) for 2-APAs in the rabbit (Table 4) is the same order of uptake as reported for incorporation into hybrid lipids by rat gut preparations (Fears et al., 1978). Given the species variability in inversion discussed previously,

CH2-O--ibuprofen CH2-O--palmltyl CH2-O-- palmltyl FIG. 7. A typical hybrid lipid. Following formation of the activated CoA thioester of the 2-APA, it competes for uptake into the triglyceride where it replaces one (or more) of the usual fatty acyl groups, illustrated for ibuprofen. The effect of formation of such hybrid lipids on cellular function is unknown. The enantioselectivity of this incorporation is probably controlled primarily by the enantioselectivity of the CoA synthetase.

Enantiomers in arthritic disorders

279

TABLE 4. Comparison of Clearances by Inversion of 2-Arylpropionic Acids in the Rabbit CLRi*

2-Arylpropionate fenoprofen ibuprofen ketoprofen flurbiprofen

(ml/min/kg)

Reference

5.5 4.5 0.4 0.3

Hayball and Metfin, 1987 Williams (unpublished) Abas and Meffin, 1987 Binskin et aL, 1987

*CLR~ = clearance of the R-enantiomer by inversion.

however, it is not too surprising that the correlation is not exact. Thus no lipid incorporation of flurbiprofen was observed in rat tissue preparations (Fears et al., 1978) while we found a low inversion of flurbiprofen in the rabbit (Binskin et al., 1987), but there was significant inversion in the guinea pig (Williams, unpublished data). Other hybrid conjugates, such as the cholesterol esters, have been identified for several xenobiotics but not as yet with the 2-APAs. In preliminary experiments we have been unable to demonstrate that ibuprofen is incorporated into phospholipids (unpublished data). Although the consequences of these data are uncertain, it has been speculated that some toxicity may be mediated via formation of these hybrid lipids (Caldwell and Marsh, 1983; Fears, 1985) and the CoA thioesters themselves may be toxic (Sherratt and Osmundsen, 1976; Sherratt, 1986). Formation of hybrid phospholipids would be of particular interest as these would be more likely to lead to changes in membrane characteristics and function. Central side effects could potentially be related to these abnormal lipids. Thus, while not inhibitors of prostaglandin synthesis, the R-2-APAs are not metabolically inert and although they may be viewed as prodrugs in some specific cases, it may be better to use the active S-enantiomer alone. (This argument assumes that the R-enantiomers do not have an antiinflammatory action mediated by nonprostaglandin-dependent pathways, a consideration which has not been fully investigated.) In fact, naproxen is one nonsteroidal antiinflammatory drug which is administered as the S-enantiomer. It has to be admitted that at this time there is no clear clinical evidence that naproxen has toxicities different to the racemic 2-APAs. The third interesting relationship of potential significance is the effect of renal dysfunction on the enantioselective disposition of the 2-APAs. Although the 2-APAs are not cleared by the kidneys, their glucuronide metabolites are renally excreted. The overall renal excretion may be enantioselective (Lee et al., 1985), a reflection of either enantioselective formation (Mouelhi et al., 1987; Fournel-Gigleux et al., 1988), hydrolysis or excretion, or a combination of these (Fournel-Gigleux et al., 1988; Baillie et al., 1989). The recent data of Baillie et al. (1989) suggest that conjugation of ibuprofen is nonstereoselective, although this does not take into account the contribution of enantioselective protein binding (Hansen et al., 1985; Evans et al., 1989). An additional interesting observation has been that one enantiomer may inhibit the glucuronidation of the other (Mouelhi et al., 1987). A decrease in renal function can result in accumulation of glucuronides which are then

hydrolyzed in the plasma causing a significant elevation of the parent drug, the so-called futile cycle (Fig. 6; Meffin, 1985). Merlin et al. (1986) demonstrated that there is a significant elevation of plasma concentrations of unbound S-phenylpropionic acid in rabbits following induction of renal dysfunction. More recently, it was shown that following a reduction of the renal excretion of benoxaprofen glucuronides by probenicid in man there was a concomitant increase in the ratio of S I R benoxaprofen (Spahn et aL, 1987). The data are suggestive that reduced clearance of benoxaprofen glucuronides in the elderly with resulting increased inversion of R- to S-benoxaprofen may have been one of the factors contributing to the adverse reactions which resulted in the withdrawal of this drug from the market. It is likely, therefore, that there will be significant interindividual variability in plasma concentrations and, consequently, response to treatment for 2-APAs for which acyl-glucuronidation is a significant pathway of elimination. However, there appears to be little inversion of either R-ketoprofen or R-indoprofen, two currently used 2-APAs which are primarily excreted as their glucuronides in man and this may not be a clinically important consideration at this time. An additional consequence of glucuronide accumulation is the potential for acylation of proteins (Fig. 6) via reaction with the glucuronide conjugates (Hyneck el al., 1988; Van Breeman and Fenselau, 1985). Again, although no firm associations have been established, it is believed that immunological sensitization may occur via formation of these adducts. The enantioselectivity of glucuronide formation is thus of some interest in this context. In conclusion, a number of 2-arylpropionates are inverted and it appears that inversion is specific for the R-enantiomers. There are large interspecies differences in the extent of inversion and in fact, rather than being a general metabolic pathway in man, it may be relatively specific for a limited number of 2-arylpropionates. A study of the mechanism of inversion has been particularly interesting and has resulted in some significant insights, particularly into the role of CoA thioester formation in the disposition of these drugs. Finally, it raises the potential for the design of 2-arylpropionates of the R-configuration which are inverted in vivo and which may thus be prodrugs. Alternatively, it may suggest that because of the potential for hybrid lipid formation, only the S-enantiomers should be employed. (For further discussion on the chiral inversion of 2-arylpropionates refer also to reviews by Caldwell et al. (1988), Hutt and Caldwell (1983, 1984) and Williams (1987).)

280

K. M.

/---COOH

/--COOH CHa

CHa ~ ,s HaC sulfide

"o s,

H3C sulfoxide (sulindac)

WILLIAMS

F

F -cOOH

/--COOH ]~CH3

CHa CH

~ °",s-- o

HaC suffone

F16.8. Sulindac, the sulfoxide, readily equilibrates with the active sulfide in a reversible manner. It is metabolized irreversibly to the inactive sulfone. 2 . 2 . HETEROARYLACET1C A C I D S - - S u L I N D A C

Sulindac (5-fluoro-2-methyl- l-[p-(methylsulfinyl)benzylidenyl]indene-3-acetic acid; Fig. 8) is a benzylidine analog of indomethacin (Brogden et al., 1978) and can exist as the c/s and trans geometric isomers. By comparison with data for an analog of sulindac (where F- is replaced by CH 3O- and -S-CH~ by Cl-), the cis-configuration is much more stable thermodynamically than the t r a n s - i s o m e r (Hoogsteen and Trenner, 1970; Gund and Shen, 1977). The c i s - i s o m e r is also the more potent antiinflammatory agent and is used in preparations of sulindac. Interestingly, t r a n s - s u l i n d a c may be the preferable isomer for the treatment of preeclampsia because of its selective actions on NAD-linked prostaglandin dehydrogenase (Jarabak, 1988) which might lead to a more favorable balance of the vasoconstrictive and vasodilatory metabolites of arachidonic acid. Additionally, the p-methylsulfinyl group (which very significantly increases the solubility of the drug), being tetrahedral in configuration, also makes the molecule asymmetric. Sulindac is thus more accurately R , S - c i s - 5 fluoro-2-methyl- 1-[p-(methylsulfinyl)-benzylidenyl]indene-3-acetic acid. Sulindac is a prodrug with its antiinflammatory activity residing in the sulfide metabolite (Fig. 8; Brogden et al., 1978). Equilibration between the sulfide and the sulfoxide accounts for the apparent renal sparing efl'ect of sulindac compared with other NSAIDs. The enantiomers of sulindac are said to be equally active within the limits of experimental error of animal assays and this equivalence has been attributed to the hypothesis that there is ready equilibration of each of the enantiomers with the active sulfide metabolite (Shen and Winter, 1977). Interconversion of the enantiomers is not a likely explanation as arylalkyl sulfoxides do not readily racemize, at least nonenzymatically (Rayner et al., 1968). Interestingly, the specific rotation obtained by Light et al. (1982) of [a]~,7 = +61.60 was much greater than that reported by Shen and Winter (1977; [~]~7= +22.60). Thus the apparent lack of enantiomeric purity in the latter study may discount the conclusion by Shen and Winter that the enantiomers were equally active. There is a paucity of published data dealing with the enantiomers of sulindac. It is known that the reoxygenation of the sulfide by mammalian FADcontaining monooxygenase to the parent prodrug is a stereospecific process (Light et al., 1982; Hajjar and Hodgson, 1982), the product being (+)-sulindac (probably of the R-configuration; Fig. 9). Light et al. (1982) believe that prostaglandin cyclooxygenase peroxidase may stereospecifically form S-sulindac

FAD-containing/ ~ ? H ~ prostaglandin monooxygenase/ S• )L..E/ ~ooxygenase HaC F /--COOH ~CH3 CH

.o,,,¢

CH3 R(+)-sulindac

F /--COOH ~CH3 CH

.o.#

CH3 S(-)-sulindac

FIG, 9. The sulfoxidation of the sulfide to reform sulidac occurs stereoselectively. FAD-containing monooxygenase forms R-sulindac while prostaglandin cyclooxygenase forms S-sulindac (Light et al., 1982; Shen, 1985). Reprinted with the permission of the author and the copyright holder, CRC Press, Inc., Boca Raton. sulfoxide. The net stereochemical outcome will thus be determined by the relative contribution of the flavoprotein and the heme protein (Fig, 9; Waxman et al., 1982). In conclusion, it is likely that the relative concentrations of the enantiomers of sulindac will vary with time but the clinical implications of this observation are unknown. The drug continues to be used as its racemate. 2.3. ENOLICAcIDs 2.3.1. O x y p h e n b u t a z o n e Oxyphenbutazone is the p-hydroxy metabolite of phenylbutazone (Fig. 10) and accounts for 2% of the dose after chronic dosing with phenylbutazone (Furst and Paulus, 1987). Because of the propensity for both drugs to cause bone marrow toxicity, they are being replaced by the newer NSAIDs. Synthetic

CH3CH(OH)CH2CH2 O

O,,.~N / ~ N ~

~

phenylbut ....e ~

y-hydroxyphenylbutazone

CHaCH2CH2CH: z

O

O~NIN~f.~

o×yphenbutazone FIG. 10. Phenylbutazone [4-butyl-l-4-(hydroxyphenyl)-2phenyl-3,5-pyrazolidinedione], a prochiral molecule, is metabolized to oxyphenbutazone (approx. 2%) and hydro×ypheny]butazone (approx. 12%; Furst and Paulus,

1987) which are both asymmetric. The enantioselectivity of this metabolism has not been studied.

Enantiomers in arthritic disorders oxyphenbutazone is racemic. This raises a potentially interesting difference between the oxyphenbutazone produced by metabolism of phenylbutazone in vivo and the synthetic drug. Phenylbutazone is prochiral, that is to say, although it is not chiral itself, when it is oxidized it forms the chiral molecule, oxyphenbutazone. An interesting example of stereoselective product formation, is the p-hydroxylation of phenytoin which occurs preferentially to give S-(5-hydroxyphenyl)5-phenylhydantoin (Poupaert et al., 1975). The p-hydroxy metabolites of phenytoin are subsequently excreted as their diastereomeric glucuronides. The ratio of S / R enantiomer excreted in this form lies between 10 (Butler et al., 1976) and 20 (Hermansson et al., 1982). Structural similarity between phenytoin and oxyphenbutazone are suggestive that the metabolism of phenylbutazone to oxyphenbutazone may also be stereoselective and occur via the 5-Sdihydrodiol (Maguire et aL, 1980). Consequently, oxyphenbutazone produced in vivo in the first instance is unlikely to be the same as that produced synthetically i.e. racemic. However, the structure of oxyphenbutazone as shown (ketone form; Fig. 10), is only one of several possible tautomeric forms (Wiley and Wiley, 1964). Tautomerism with the enol form means that even if the initial product of the oxidation of phenylbutazone is formed enantiospecifically, there will be subsequent racemization of the enantiomer. Thus although this example is of little clinical interest, it is a reminder that a synthetic drug (frequently racemic), should not be automatically assumed to be the same as the active metabolite or the active principal of a prodrug whose stereochemistry will be determined by the enantioselectivity of the metabolizing enzymes (and therefore, frequently not racemic). The ?-hydroxy metabolite of phenylbutazone (Fig. 10), which has uricosuric activity, is also asymmetric. 2.3.2. Etodolac R(+)-etodolac (Fig. 11; Humber et al., 1986) inhibits cyclooxygenase and in an adjuvant-induced polyarthritis rat model, the R-enantiomer was 100 times more active than the S-enantiomer (Demerson et al., 1983). The chemical structure suggests that, unlike the 2-arylpropionates, there is no possibility of inversion of configuration. Cyclooxygenase and peroxidase activities have been shown to copurify on a single protein (Ohki et al., 1979). In contrast to the enantioselectivity for inhibition of cyclooxygenase, there is no stereoselectivity for the activity of the enantiomers as substrates to support PGH synthase-catalyzed peroxide reduction (Markey et al., 1987). These data

~OH2cOOH CH3CH2 H (~H2CH 3 R(+)-etodolac FIG. 11. The structure of the active R(+)-enantiomer of etodolac, R-1,8-diethyl-l,3,4,9-tetrahydropyrano[3,4-b]indole- l-acetic acid. JPT,

46/2--I

281

12,.I

E

.o P g

10

8 6 4

O O

2

o

. 0

.

-2.0

.

.

.

2"9:::::=::B .

i

2,0

.

.

.

.

.

a

6.0

.

.

.

.

.

i

•

10.0

,

.

Time (h) FIG. 12. Steady-state plasma concentration-time profile of the active R-enantiomer of etodolac ( l l ) and its conjugate ([]), and S-etodolac (O) and its conjugate (O) following repeated oral administration (200 mg, twice daily) of R,Setodolac (Jamali et al., 1988c). Note that in this instance the inactive S-enantiomer contributes only a small proportion to the total concentration of etodolac in plasma. Reprinted with the permission of the authors and the copyright holder, the American Pharmaceutical Association, Washington, D.C.

illustrate the elegant use of enantiomers as probes as they allow the conclusion that the antiinflammatory activity of etodolac is mediated via the cyciooxygenase and independent from the peroxidasereducing activity. Limited pharmacokinetic studies have shown that the steady-state plasma concentrations of the inactive S-enantiomer are substantially lower than for the active R-enantiomer after treatment of healthy volunteers with racemic etodolac, the area under the plasma concentration-time curve for S-etodolac being only about 10% of that for R-etodolac (Fig. 12; Jamali et al., 1988c). In this situation, as is the case for fenoprofen (Rubin et al., 1985), where total drug and the active enantiomer concentrations are not too dissimilar, attempts to relate plasma concentrations to effect based on measurement of racemic drug may be more tenable. Another interesting feature of this study was the relative plasma concentrations of the etodolac conjugates (presumably glucuronides). For the S-enantiomer conjugated and unconjugated concentrations were similar, while for the R-enantiomer conjugate concentrations were only 20-25% of those of the unconjugated drug. S-etodolac conjugate was much more predominant in urine. The data are suggestive that there is enantioselective renal excretion of the S-conjugate. 2.3.3. Azapropazone The pharmacokinetic and pharmacodynamic properties of the enantiomers of azapropazone have yet to be investigated.

3. I M M U N O M O D U L A T I N G D R U G S 3.1. D-PENICILLAMINE(S-PENICILLAMINE) Penicillamine (Fig. 13) is used extensively in the treatment of Wilson's Disease and in rheumatoid arthritis. There is no clinical experience comparing

282

K. M.

H3C" H3C= " ~ , ,

COOH H

HS NH2 L-penicilamine

HOOC CH3 H , ~ CH3 H2N SH D-penicilamine

FIG. 13. The enantiomers of penicillamine. The clinically used D-enantiomer has the S-configuration. the relative efficacy/toxicity of the penicillamine enantiomers and the racemic drug in arthritis where the doses are generally much lower (approximately 750mg/day) than those used for Wilson's Disease (1-2 g/day). The choice of the D-enantiomer has been determined by the experience in Wilson's Disease and on the metabolic and toxicity data obtained in animal studies. These data are the basis for the following discussion. 3.1.1. Clinical Experience with Penicillamine Enantiomers

In contrast to the toxicity associated with some of the D-amino acids, it was the L-enantiomer of penicillamine which had greater toxicity. Sternlieb (1966) reported that all 8 patients suffering from Wilson's Disease who developed nephrotic syndrome were being treated with D,L-penicillamine. When 3 of these patients were rechallenged with the racemic drug they had a recurrence of the syndrome, while 2 other patients continued treatment with the Denantiomer without untoward effect. It was further noted in this report that the syndrome appeared to be unrelated to a penicillamine-induced deficiency of pyridoxine. The treatment of Wilson's disease with D,L-penicillamine has also been associated with optic neuritis. Toxicity was ameliorated in one instance by treatment with vitamin B 6 (Tu et al., 1963) but another patient was receiving daily pyridoxine supplements (2.5-5.0mg/day). In neither patient were there any overt signs of pyridoxine deficiency and pyridoxine deficiency was not generally associated with treatment of Wilson's patients with D,L-penicillamine (doses up to 1500rag/day) although these patients were receiving small pyridoxine supplements. It was suggested that optic neuritis was a metabolic abnormality produced by e-penicillamine and not involving pyridoxine (Goldstein et al., 1966). Finally, the occurrence of thrombocytopenia and/or leukopenia apparently became less prevalent when use of the racemic drug was discontinued (Scheinberg, 1968). Consequently, because the D-enantiomer had the same capacity for chelation of copper and apparently had the lesser toxicity, penicillamine was used as the D-enantiomer in the treatment of Wilson's Disease. Having been established as the apparent enantiomer of choice for this indication, D-penicillamine was adopted without further clinical study for the treatment of rheumatoid arthritis. 3.1.2. Animal Toxici O, Data Studies in animals have given further evidence of the greater toxicity of the L-enantiomer. The LDs0 of D-penicillamine in the rat exceeded 1200 mg/kg compared to 365 mg/kg for the racemic drug (Aposhian,

WILLIAMS

1958; Aposhian and Aposhian, 1959). L-Penicillamine inhibited the growth of rats on choline deficient diets with weight loss, intermittent fitting and death. In contrast, D-penicillamine did not display this spectrum of activity (Wilson and Vigneaud, 1948; Wilson and Vigneaud, 1950). The symptoms of toxicity were also those which had been associated with vitamin B6 deprivation. Indeed rats treated with L-penicillamine had a greater requirement for vitamin B6 with increased excretion of pyridoxine in urine (Kuchinskas and Vigneaud, 1957). This toxicity was reversed by administration of pyridoxine. However, both D,Land D-penicillamine induced a considerable increase in excretion of vitamin B 6 activity in urine of treated rats, although again the effect was somewhat greater for D,L-Penicillamine (Kajiwara and Matsuda, 1979). e-penicillamine also decreased rat liver transaminases, an effect which was also reversed by administration of pyridoxine (Kuchinskas et al., 1957). Although not having growth inhibitory activity in animals on normal diets, D-penicillamine (and D,L-penicillamine) had an antipyridoxine effect in man (Jaffe et al.. 1964) as determined by its effect on the metabolism of a loading dose of tryptophan. Equal doses of D-penicillamine and D,L-penicillamine elevated urinary kynurenine excretion approximately 2-5 times and 7 13 times, respectively. A similar elevation of xanthurenic acid excretion was observed following the racemate, with a lesser and more variable effect with D-penicillamine. It can be concluded that the effect is much greater for the L-enantiomer and is a reflection of a decrease in the pyridoxinedependent metabolism of 3-hydroxykynurenine to 3-hydroxyanthranilic acid. Metabolite excretion returned to control values when pyridoxine was administered with D,L-penicillamine. It was suggested on the basis of these data that patients treated with D-penicillamine should supplement their diets with daily pyridoxine. It was concluded in another study, however, that this was unnecessary for treatment with D-penicillamine but was recommended if D,L-penicillamine was used (Gibbs and Walshe, 1966). Pyridoxine supplementation has not been adopted in clinical practice. 3.1.3. Pyridoxine and Thiazolidine Formation

Penicillamine can condense with aldehydes such as pyridoxal to form thiazolidines (Heyl et al., 1948; Ueda et al., 1960; Howard-Lock et al., 1986b). This also results in the introduction of a second asymmetric center into the product (in the thiazolidine ring; Fig. 14). Thus, S-penicillamine will form the diastereomerically related thiazolidines of the S , R - and S,Sconfigurations i.e. they are not mirror-images and thus have different physical characteristics. Similarly, R-penicillamine yields the R,S- and R,R-thiazolidine diastereomers. Together, these represent two pairs of enantiomeric (RR and SS, and R S and S R ) molecules. Although the total amount of thiazolidine will be the same for the enantiomers, the distribution of the isomers will be different. The metabolic consequences of this are uncertain. However, the mechanism of the antipyridoxine activity cannot simply be depletion of pyridoxal-5-phosphate stores via

Enantiomers in arthritic disorders H3C HSH3C C "~. , COOH + 'NH= S-penLcillamine

HO

CH2OH ('H20)

H3C

(+H~O) pytiOoxal

H

S~c~NH

H=C S,R-thiazolidine S,S-thiazolidine

FIG. 14. Penicillamine can condense with aldehydes to form thiazolidines, here illustrated for the reaction of S-penicillamine with pyridoxal. The S-configuration contributed by the penicillamine is maintained. Additionally, a new asymmetric center is formed in the thiazolidine ring as indicated (*). The configuration at this carbon will be S or R. The ratio of the S,R-thiazolidine to the S,S-thiazolidine diastereomers need not be 1.0. formation of the thiazolidine, as both enantiomers would equally deplete pyridoxal by this mechanism. 3.1.4. Effects on Collagen Synthesis Howard-Lock et al. (1986a) have suggested that the toxicity of L-penicillamine may occur via "its incorporation into proteins in the place of L-cysteine or by blocking sites in proteins involved in the metabolism of L-cysteine". This conclusion was based on data which showed that L-penicillamine was bound more avidly to tissues and had a higher uptake into collagen- and elastin-rich organs (Planas-Bohne, 1981; Ruiz-Torres, 1974b). Furthermore, there was a drop in insoluble collagen synthesis and an increase in soluble collagen synthesis in rats treated with penicillamine, the effect being greater for the racemate than the D-enantiomer. This effect occurred despite diet supplementation with pyridoxine and was also associated with inhibition of wound healing and an increase in skin fragility (Nimni and Bavetta, 1965; Nimni et al., 1969). An excess of soluble collagen in tissues has also been reported to be associated with penicillamine-induced skin lesions in patients treated with either D,L or D-penicillamine. These data were the basis for the use of D-penicillamine in the treatment of scleroderma (Howard-Lock et al., 1986b). However, these data do not clearly separate the activities of D- and L-penicillamine in terms of their effects on collagen synthesis. 3.1.5. Immunological Activity of Penicillamine Enantiomers Toxicities associated with treatment with D-penicillamine (e.g. myasthenia gravis, glomerulonephritis) often have an immunological basis. Interestingly both enantiomers were reported to be haptens for specific T-cells using a mouse model. Although, therefore, there appears to be no specific advantage of the enantiomers over each other in terms of their relative immunogenicities, the data did reaffirm the sensitivity of biological systems to enantiomeric differences in structure. Thus proliferator responder cells which were primed with D-penicillamine were not restimulated by L-penicillamine and vice versa (Nagata et al,, 1986). Another measure of a difference in the potential toxicities of the enantiomers, is their relative mutagenicities. Glatt and Oesch (1985) have reported that while mutagenic activity could be

283

demonstrated for both enantiomers, the L-enantiomer was approximately 8 times more mutagenic than the D-enantiomer. 3.1.6. Pharmacokinetics and Disposition The difference in toxicity of the enantiomers may also have a pharmacokinetic component although no comparative pharmacokinetics of the enantiomers have been determined in humans. The elimination half-lives were found to be similar in rats, but higher concentrations were obtained for L-penicillamine in serum (based on total radioactivity) than for the o-enantiomer and there was a greater oral availability of the L-enantiomer (Ruiz-Torres, 1974a). L-PeniciUamine is actively transported across the gut mucosa and inhibits the rate of absorption of other L-amino acids, while the D-enantiomer is absorbed passively and has no significant effect on amino acid absorption (Ueda et al., 1960; Wass and Evered, 1970). This may in part, account for the higher availability of the L-enantiomer. However, there are insufficicent relative pharmacokinetic data on the enantiomers to make any firm conclusions. The limax for the S-methylation of penicillamine by human red blood cell membrane thiol methyltransferase is greater for L-penicillamine (Keith et al., 1985). Neither enantiomer is a substrate for L-cysteine desulfhydrase and both are poor substrates for the respective amino acid oxidases (Aposhian, 1971). Despite the apparent higher toxicity of the Lenantiomer, the relative therapeutic potency of the penicillamine enantiomers is not known, The chemical interactions of the enantiomers are expected to be the same. Thus (nonenzymically-mediated) formation of disulfides, reactions with ketones or aldehydes such as pyridoxal to form thiazolidines, chelation of metals, and ability to scavenge reactive species such as hypochlorite (Cuperus et al., 1985) will occur with equal propensity for both D- and L-penicillamine. Both enantiomers can act as haptens (as discussed above), although there may be clinically important differences here. In conclusion, penicillamine is used as the o-enantiomer because of the greater toxicity of the L-enantiomer. The difference may be a metabolic consequence of the greater activity of L-penicillamine as a pyridoxyl antagonist by mechanisms which are not clear and/or perhaps also because of its effects on collagen synthesis. However, the basis for the difference in toxicity is by no means understood. It would be interesting to compare the relative potencies of the enantiomers in a suitable animal model of arthritis. Similar activities in such a model would suggest a chemical rather than a receptor or immunologically mediated mechanism of action. Such studies could be of great interest in delineating the mechanism of the antirheumatic action of penicillamine. The L-enantiomer may have the higher potency as well as the higher toxicity. 3.2. THALIDOMIDE

The enantiomer with the therapeutic action may also be the enantiomer responsible for the toxicity of the drug, toxicity sometimes being simply an exten-

284

K.M. WILLIAMS O

HO O C , , ~ N // 0 2R-(2-phthaloyl)glutamic acid + HOOC

O

In vivo

~'

o O

O

thalidomide

H0 0 C'/'--~'H ~ HOOC

N ~ ] // 0

2S-(2-phthaloyl)gLutamicacid

FIG. 15. Metabolism of thalidomide with formation of two enantiomeric glutamates. It has been postulated that one or more of the S-glutamates is the primary teratogen. sion of the therapeutic action. However, the dissociation of some of the toxic manifestations of a drug may sometimes be attained by removing the less active but more toxic enantiomer. It has been implied that this is the case for the now much cited example of thalidomide (Fig. 15), a racemic hypnosedative found to be extremely nontoxic in acute animal studies and following overdose in man. However, the drug was withdrawn from the market for its now well-known adverse effect of phocomelia in infants born to mothers treated with this hypnosedative during the first trimester of pregnancy. Thalidomide is the treatment of choice for erythema nodosum leprosum (Sheskin, 1980; Naafs et al., 1982). More recently, data have suggested that thalidomide may be useful in the treatment of a number of dermatological and immunological diseases (Kaitin, 1988) including graft versus host disease (Heney et al., 1988) and rheumatioid arthritis. In a study of 7 patients with rheumatoid arthritis, Gutierrez-Rodriguez (1984) observed that treatment with thalidomide resulted in long lasting remission of disease in 4 subjects and impressive improvements in ESR and rheumatoid factor, and normalization of the articular index in all subjects. Clinical improvement was not without adverse effects which included leukopenia, constipation, erythema and edema of the lower limbs. Not unexpectedly, drowsiness was a problem, being severe in 3 patients. The mechanism of action is uncertain, but the drug is known to inhibit polymorphonuclear leucocyte chemotaxis (Faure et al., 1980). The adverse effects on fetal development prompted a more detailed study of the metabolism of the drug. Faigle et al. (1962) identified N-phthaloylglutamine and N-[O-carboxybenzoyl]-D,L-glutamic acid along with lesser quantities of other glutamine and glutamic acid derivatives in dogs treated with thalidomide. In particular these workers drew attention to the likely mix of derivatives of both the S- (unnatural) and R- (natural) enantiomers of glutamic acid and the potential for the S-series "interfering with the biochemical and physiological functions of natural glutamic acid or its derivatives". Attention was also drawn to the possibility that the thalidomide metabolites may be folic acid antagonists. Subsequent studies in rats and mice have presented data which suggest that, while both enantiomers are equally efficacious as hypnosedatives, it is the S-enantiomer which is

teratogenic (Blaschke et aL, 1979; Blaschke, 1980) via formation of the metabolites S ( - ) p h t h a l o y l g u t a m i c acid (Fig. 14) and S(-)phthaloylglutamine. This study has been criticized because these species were not sufficiently susceptible to the teratogenic action of thalidomide (Heger et al., 1988; Schmahl et al., 1989). However, the structurally related compound, S-phthaloylaspartic acid, was also embryotoxic and teratogenic in mice while these toxicities were not associated with administration of the R-enantiomer (Ockenfels et al., 1977). By contrast, a study in rabbits, a species sensitive to thalidomide teratogenicity, found both enantiomers to be equally teratogenic (Fabro et al., 1967). An interesting observation on this latter data by Simonyi (1984) was that both enantiomers were apparently less toxic than the racemic drug. Finally, both enantiomers were found to be equally active in depressing the blast transformation of human leucocytes cultured in t,itro (Roath et al., 1962). Racemic metabolites, carboxybenzylD,L-glutamine and carboxybenzyl-D,L-isoglutamine, and to a lesser extent N-phthaloyl-D,L-glutamic acid and N-carboxybenzyl-D,L-glutamic acid imide were also inhibitors of lymphocyte proliferation (Roath et al., 1963). The thalidomide analog [2-(2,6-dioxopiperidine3-yl)-phthalimidine (Fig. 16), racemizes significantly both in vivo and in vitro (Schmahl et al., 1988, 1989). Plasma concentrations of the antipode given to marmoset monkeys were 25% of the administered enantiomer at 5hr. Teratogenetic abnormalities were observed with both enantiomers. However, the S-enantiomer had the greater toxicity (Heger et al., 1988) and also the greater area under the plasma concentration-time curve. It was not clear from these data whether the greater toxicity of the S-enantiomer had a pharmacokinetic or pharmacodynamic explanation. Attempts to separate efficacy from toxicity for thalidomide are thus likely to be difficult since presumably the 2-hydrogen of thalidomide is also labile and would allow racemization to occur for this drug as well. (This racemization is similar to that observed for 5-phenylhydantoins. However, the 3-H should be much less acidic than the corresponding 5-H of 5-phenylhydantoin and consequently, less likely to racemize under physiological conditions.) In conclusion, the inference that use of R-thalidomide would have avoided the problems associated with the racemic drug is premature at this time. However, there is scope to examine in further detail the potential for the use of an enantiomerically pure preparation of thalidomide or analogs of this drug for the treatment of arthritis because of the potential for significant disease modifying activity. 0 N o

S(-)-EM 12

~"~o R(+)-EM 12

FIG. 16. The thalidomide analog [2-(2,6-dioxopiperidine3-yl)-phthalimidine] (EM-12). Separation of toxicity and efficacy by use of an enantiomer is limited by the nonenzymatic racemization of this drug in solution.

Enantiomers in arthritic disorders 3.3. CYCLOSPORINE

Cyclosporine, being a cyclic polypeptide (11 amino acid residues), has many asymmetric centers. Its amino acids, 7 of which are N-methylated, are of the natural L-configuration with the one exception, that of o-alanine in position 8. Some other D-amino acids, in particular D-glutamic acid, are immunosuppressants, but the pharmacologial importance of the D-alanine in the overall activity of the cyclosporine is unknown. On the subject of amino acids of the unnatural configuration, it is of rheumatological interest that the smallest subunit of bacterial cell walls which is able to induce adjuvant arthritis in rats is N-acetylmuramyl-L-alanine-D-isoglutamine. By contrast the diastereomer containing isoglutamine of the natural L-configuration has no adjuvant activity (Chang et al., 1981). 3.4. LEVAMISOLE Levamisole is the levorotatory enantiomer of tetramisole and has the S-configuration (Raeymakers et aL, 1967; Fig. 17). It is an antihelminthic drug and has greater potency in this respect than the dextrorotatory enantiomer, dextramisole (Bullock et al., 1968). Treatment with levamisole was first reported to result in significant objective and subjective improvement in patients with rheumatoid arthritis some 24 years ago (Schuermans, 1975). Disease assessment in this study was carried out after 6 months therapy. The time-course of action was similar to treatment with o-penicillamine and the gold agents. Its use is associated with a high incidence of adverse effects, notably agranulocytosis. However, adverse reactions to levamisole are generally reversible within I week of cessation of the drug (Veys et al., 1986). Levamisole has been shown to enhance monocyte chemotaxis (Anderson et al., 1976; Wright et al., 1977), to stimulate T-cell subpopulations and to augment the ability of the T-cells to respond to suboptimal doses of stimulus (Merluzzi et al., 1975). Levamisole together with dextramisole and tetramisole, activated the mononuclear phagocytic system as determined by carbon clearance studies in mice (Hoebeke et al., 1973). In this latter study, no selectivity was seen between the enantiomers. However, in another study, while levamisole increased the chemotactic response of human monocytes to a number of stimuli, dextramisole inhibited the action of its enantiomorph in a manner indicative of competitive binding to the monocyte (Pike and Snyderman, 1976).

..) S(-)-tetramisole (levamisole)

R(+)-tetramisole (dextramisole)

FIG. 17. Tetramisole is a racemic antihelminthic agent. The S( - )-enantiomer (levamisole) is also an antihelrninthic and is an antirheumatic drug with immunomodulatory properties,

285

The immune reactivity of the enantiomers has also been studied in mice immunized with sheep red blood cells as determined by plaque formation (Renoux and Renoux, 1974). Levamisole was immunostimulatory. An interesting finding was that while dextramisole did not elicit an immune response, it did result in synergistic effect when administered with the Renantiomer in the form of the racemate, tetramisole. This could be one example of where the racemic drug may have potential therapeutic advantage over the individual enantiomers. The importance of the clearly established enantiospecific activity of levamisole over dextramisole as an inhibitor of alkaline phosphatase in the context of the rheumatic diseases is uncertain. However, this distinction has been put to good use by Lerner and Granstr6m (1984). They demonstrated that inhibition of bone resorption (as assessed by effects on mineral mobilization and matrix degradation in a bone culture system) was effected by either enantiomer of tetramisole and, therefore, that inhibition of alkaline phosphatase was unrelated to this phenomenon. It is difficult to make any clear conclusions concerning the importance of the relative immunomodulatory properties of the enantiomers, particularly in view of the finding that even levamisole, which appears to be the more active enantiomer in most stituations, can be either immunostimulatory or immunosuppressive depending on the concentration employed (Renoux and Renoux, 1974). No studies have addressed the relative activities of levamisole and dextramisole for the management of the rheumatic disease. The combination of inhibitory activity on bone resorption together with a greater spectrum of immunostimulatory activity suggests that levamisole may be the preferred form. The potentiation of the immunostimulatory effect of levamisole by dextramisole deserves further attention.

4. A N T I M A L A R I A L D R U G S 4.1. CHLOROQUINE,HYDROXYCHLOROQUINE Chloroquine, hydroxychloroquine and quinacrine are racemic drugs used for the prophylaxis and treatment of malaria, amebic hepatitis, lupus erythamatosis and rheumatoid arthritis. These drugs are highly concentrated by various tissues and ocular toxicity has been the primary limitation of their use in the rheumatic diseases (Wickens and Paulus, 1987) although this risk has been overplayed and there is good evidence for their efficacy (Day et al., 1982b). The absolute configuration of the chloroquine enantiomers has been determined recently, the levorotatory enantiomer having the R-configuration (Fig. 18; Craig et al., 1988). The kinetics of the enantiomers have been investigated in rheumatoid patients after treatment with the racemic drug (Gustafsson et al., 1986) and the relative binding affinities of the enantiomers to plasma proteins have also been reported (Ofori-Adjei et al., 1986b). These data indicate differences in the disposition of the enantiomers with S-chloroquine being more avidly bound to plasma and albumin (66.6+ 1.9,

286

K.M. W[LLIAMS H3C H *" HN' ~ " ~

CH2CH3 N "-CH2CH3

R (@cNoroquine

H3C H

CH2CH3

HN' ~ , . * " ~

N~CH2CH20 H

hydroxychloroquine

F1o. 18. Structures of the antimalarial drugs, R ( - ) chloroquine and hydroxychloroquine. 45.9 __+0.8%) than R-chloroquine (48.5 _+ 2.4, 25.3 _ 0.6%) while the R-enantiomer binds more tightly to alpha-I acid glycoprotein. Both enantiomers are actively secreted (Gustafsson et al., 1986). Metabolites of chloroquine such as desethylchloroquine (McChesney et al., 1967; Fu et al., 1986), may play an important role in both the antimalarial and the antirheumatic activity of the drug. Metabolism of chloroquine to desethylchloroquine appears to be enantioselective and there may be enantioselective renal secretion in some individuals (OforiAdjei et al., 1986a; Gustafsson et al., 1986). It is only in the treatment of malaria that there is some knowledge of the activities of the individual enantiomers. Data from the earliest studies suggested that there was little difference between the toxicity or efficacy of the chloroquine enantiomers (Riegel and Sherwood, 1949). However, it was subsequently found that the enantiomers used in this study were little better than racemates (Blaschke et al., 1978; Haberkorn et al., 1979). Pure S-chloroquine was found to be more effective than the R-enantiomer at subcurative doses in a mouse model (P. b e r g h e i ) , although there was no difference at doses required for 100% survival. The acute toxicity was lower for the S-enantiomer. S-chloroquine was found to be approximately 3.5 times more active than the R-enantiomer against P. t, inckei (based on relative EDsos; Fink et al., 1979) but there were reported to be only minor differences in potency against two strains of P l a s m o d i u m f a l c i p a r u m (Fu et al., 1986). There appears to be less retinal toxicity associated with hydroxychloroquine than chloroquine (Fig. 18; Finbloom et al., 1985) but there are no data on the relative efficacy/toxicity of the enantiomers of hydroxychloroquine. 4.2. PRIMAQUINE, QUINACRINE

Primaquine (Fig. 19) inhibits the metabolism of other drugs but does so nonstereoselectively (Mihaly et al., 1985). The disposition of the enantiomers of primaquine (Ward et al., 1987) have been studied in a perfused rat liver model where there was no H H

N ~ CH3

C

H

N ~~N O

primaquine

H NH2

3

~ Cl"

v

HN~ * " * " , , . , , . , ~ N{CH2CH3}2 CH3 OCH3 "Nquinacrine

FIG. 19. Structures of the antimalarial drugs, primaquine and quinacrine.

difference between the enantiomers at low doses but enantioselective differences became apparent at higher doses. In another study there was a greater proportion of the (+)-enantiomer in urine after racemic drug administration to rats. This appeared to be due to enantioselective metabolism by mitochondria and it was suggested that the more rapid conversion of ( - ) - p r i m a q u i n e to less active metabolites might explain the lower toxicity of ( -)-primaquine in rodents (Baker and McChesney, 1988). Disparate activities of the enantiomers of the antimalarial drug, primaquine, have been reported (Schmidt et al., 1977). Again the enantiomers are equipotent with each other and with the racemate in their antimalarial activities. However, there were differences in toxicity between the enantiomers in acute and subacute studies in mice and monkeys, respectively. The acute toxicity of (+)-primaquine was found to be 4 times that of ( - ) - p r i m a q u i n e in mice but the subacute toxicity was 3-5 times greater for the ( - ) - e n a n t i o m e r in Rhesus monkeys (Schmidt et al., 1977). The authors believe that the Rhesus monkey data are the more relevant and have proposed that use of the (+)-enantiomer be investigated in man. A further study, however, found that there was no clear enantiomeric separation of schizontocidal activity and cytotoxicity (Brossi et al., 1987). Much larger doses of the racemic drug are ingested for the management of the rheumatic diseases than for malaria. Use of the drugs has been limited by their toxicity, primarily ocular toxicity. By contrast, toxicity observed in t, ivo in the above studies was primarily hepatotoxicity. An additional problem with primaquine is its propensity to induce hemolytic anemia in people with glucose-6-phosphate dehydrogenase deficiency. It has been shown recently, however, that there is differential toxicity of the enantiomers as determined by their effects on oxidation (levels of reduced glutathione) and membrane leakiness (hemaglobin release) of red blood cells (Agarwal et al., 1988). It was not clear from these data which enantiomer was most likely to have the better therapeutic index. The enantiomers of quinacrine (Fig. 19) are equally active against malaria. However, the (+)-enantiomer was half as toxic in animals and less toxic than the racemic drug in man (Gause, 1945). In conclusion, the increasing reliance on the antimalarials, particularly hydroxychloroquine, for the management of rheumatoid arthritis, suggests that it is time the enantiomers of these drugs were assessed in suitable animal models of inflammation. Not only might this give a basis for clinical trial of the enantiomers, but valuable mechanistic insights are also likely to be obtained.

5. G O L D COMPOUNDS Chrysotherapy has become an important part of the armamentarium for the treatment of patients with rheumatoid arthritis. Despite the generally accepted view that gold is the therapeutically important agent, it has been suggested that the activity may be mediated by the gold 'carriers' (thioglucose, thiomalate) by analogy with the activity of the thiol

Enantiomers in arthritic disorders el-~oH

287 C ~ * /_N(CHzCH2Cl)2

L-o

AuS~ C~ COOI~ I

H

Au

OH

cyclophosphamido

D-aurothioglucose

gold sodium thiomalate

oeoo~ auranofin FIG. 20. Asymmetric gold salts. Thioglucose and the pyranose o f auranofin (2,3,4,6-tetra-O-acety]-l-thio-]Y-glucopyranosato-S-triethylphosphine gold) are the natural D-configuration. Thiomalate is racemic.

disease-modifying drug, o-penicillamine (Jellum and Munthe, 1977; Munthe et al., 1978; Rudge et al., 1984a, b, c; Drury et al., 1984). These gold carriers are asymmetric, thiomalate is racemic while thioglucose and the glucopyranose (both of which have several asymmetric centers) of auranofin are (presumably) of the natural o-configuration (Fig. 20). Significant concentrations of free thiomalate are apparent in urine and plasma following treatment with Myocrisin (aurothiomalate), the data suggesting that the dissociation of aurothiomalate is complete (Jellum et al., 1980; Rudge et al., 1983; Rudge, et al., 1984a, b, c). Thiomalate is also bound extensively to tissue membranes and cells (Jellum and Munthe, 1982) and is excreted in the urine as unchanged drug and probably as the disulfide. Several studies have indicated that thiomalate has activities similar to penicillamine in models of adjuvant arthritis and in its effects on delayed hypersensitivity reactions (Arrigoni-Martelli et al., 1978). Thiomalate is apparently much less toxic (30 times) than D-penicillamine (Munthe et al., 1978). Even if the thiol carriers do not have antirheumatic activity, an enantiomeric form may be associated with less adverse effects than the racemate, or the L-sugars may have different activities from the present D-enantiomers. In conclusion, little attention has been given to the potential activity or toxicity of the gold 'carriers'. The possibility of differential activities of the enantiomers of these thiols has not yet been addressed, but may well be the basis for some interesting studies.

FIG. 21. The cytotoxic agent, cyclophosphamide. In contrast to other drugs discussed in this review, the asymmetric center is the phosphorus atom rather than a carbon. greater need to investigate the potential for an improvement in their therapeutic indices by comparing the relative efficacies of the enantiomers. 6.1. CYCLOPHOSPHAMIDE Cyclophosphamide (Fig. 21), an alkylating agent whose activity is mediated by its active metabolite phosphoramide mustard, has found some use in the management of severe rheumatological conditions such as polyarteritis, systemic lupus erythematosis and rheumatoid arthritis (Kovarsky, 1983). The drug has a narrow therapeutic index, however, and its use is limited by the potential for severe adverse effects such as hemorrhagic cystitis, infertility and carcinomas (Smyth et al., 1975). Toxicity is apparently dose related (Clements, 1987). The chiral centre of cyclophosphamide is the phosphorus atom rather than a carbon (Fig. 21). In view of the severe and dose-related toxicity of this drug, the potential to improve the therapeutic index by using one of the enantiomers is highly attractive. Initial studies in patients indicated that there was stereoselective metabolism (Cox et al., 1976) but later work discounted this observation (Jarman et al., 1979) and it was concluded that no significant therapeutic advantage was to be gained by using an individual enantiomer (Farmer, 1988). However, despite the apparent lack of metabolic enantioselectivity, studies using mice and rats suggest that there are differences in toxicity and antitumor activity (Kleinrok et al., 1986; Kusnierczyk et al., 1986; Paprocka et aL, 1986; Paprocka and Radzikowski, 1985). Again interpretation of this data is difficult because of species effects. In summary, it is an attractive proposition to examine the individual enantiomers for the treatment of arthritis because of the high toxicity of the racemic drug, but the data do not clearly indicate that there will be a positive outcome of such studies. 6.2. METHOTREXATE

6. CYTOTOXIC D R U G THERAPY Cytotoxic drug therapy has been used to manage rheumatic diseases since the first report that there was a positive response to treatment with nitrogen mustard in 1951 (Jiminez-Diaz et al., 1951). The results of such therapy are an acknowledgement of the proliferative and immunological basis of such diseases. Several asymmetric cytotoxic drugs, notably methotrexate and cyclophosphamide, are now in use for the management of the arthropathies. Because of the high toxicity of these agents, there is perhaps a

Methotrexate has been primarily used as a neoplastic agent and to a lesser extent for the treatment of psoriasis and for its immunosuppressive activity. More recently it has been used to treat rheumatoid arthritis, particularly in those patients unresponsive to other therapies. It is derived from L-glutamic acid and is enantiomerically pure (Fig. 22), although variable, but small proportions of the D-enantiomer may be present as an enantiomerie impurity (Cramer et al., 1984). D-Methotrexate is also a strong inhibitor of dihydrofolate reductase from murine and human leukemia cells (Lee et al., 1974), although Cramer et

288

K.M. WILLIAMS CH3 - - uCE~"1 A

JN,.v~

I[ "T

.oocc.,cH=--~'""

HN,~,,,,,~

I~N~*'Ny

NH2

.,,J~.,~N

""

T

NH2

~ O

OH

O L-methotrexate

FIG. 22. The cytotoxic folate antagonist, methotrexate, which is derived from L-glutamic acid.

o

al. (1984) reported a 30-fold greater 150 value for

OH

D-methotrexate on dihydrofolate reductase isolated from human leukemic spleen. However, D-methotrexate is a poor inhibitor of cell growth, partly perhaps because of its inability to form intracellular polyglutamates (McGuire and Bertino, 1981) which are active dihydrofolate reductase inhibitors and which also retain methotrexate within the cells when blood concentrations are virtually zero. L-Methotrexate is actively absorbed and transported into ceils. There is some uncertainty about the gastrointestinal absorption of D-methotrexate with reports that it is essentially not absorbed by humans (Hendel and Brodthagen, 1984) but is rapidly absorbed by dogs and mice (Crame r et al., 1984). Presumably there would be significant differences between the enantiomers for the active components of transport and absorption. This has been found to be the case for the leucovorin diastereomers (Straw et al., 1984) with the active isomer (derived from L-glutamate) being much more readily absorbed than its counterpart (derived from D-glutamate). Enantioselective transport may also contribute to the lesser inhibitory activity of D-methotrexate on cell growth. Thus the available, but limited data suggest that D-methotrexate is unlikely to be of benefit in the management of arthritis and the D-enantiomer is of less interest because the drug is used as the L-enantiomer. Nevertheless, D-methotrexate should prove a useful probe in further delineating the mechanisms of action of L-methotrexate.

7. PROSTAGLANDIN-E2 A N A L O G S Synthetic 15-methyl substituted prostaglandin-E 2 analogs such as arbaprostil [(15-R)-15-methylprostaglandin-E2] have been found to be efficacious in treating gastric and duodenal ulcers and drug-induced gastric mucosal injury by inhibiting gastric acid secretion (Robert et al., 1981; Karim et al., 1973; Gilbert et al., 1984). Substitution at the 15-carbon prevents inactivation by prostaglandin 15-dehydrogenase. Arbaprostil itself is a prodrug and is inactive but it is racemized in the acidic gastric environment to give the active 15-S-enantiomer (Fig. 23; as arbaprostil has more than one asymmetric carbon, inversion at the 15-carbon results in formation of a diastereomer or epimer, not an enantiomer. The process is thus more correctly described as epimerization ).

Epimerization is pH-dependent such that gastric acid secretion is significantly and increasingly inhib-

-

-

~

C

O

O

H

HO CHs 15(R)-15-methylprostaglandimE2

1l~ c o o H H3C OH 15(S)-15-methylprostaglandin-E 2

FIG. 23. Arbaprostil is the inactive lS-R-15-methylprostaglandin-Ez epimer. In the acidic environment of the stomach this prodrug epimerizes to form the active 15-Sepimer. ited as the pH falls below 5 (Reele, 1985). Thus the amount of active drug generated is controlled by the gastric pH and, consequently, this helps to minimize the amount of active drug available for systemic absorption and thus the potential for adverse sequelae (Cox et al., 1986).

8. CONCLUSIONS A N D RECOMMENDATIONS The chirality of a drug is a structural feature of great importance because the environment into which we place the drug is, itself, asymmetric. Synthetic drugs are frequently administered as racemates. However, although enantiomeric pairs may share the same activities, toxicities and metabolic fate, this is by far the exception. Frequently, the activity of interest will primarily reside in one of the enantiomers. As a consequence there is the possibility that the other enantiomer, while contributing little to the efficacy of the drug, may add to its adverse effects. The activities of the 2-APAs as inhibitors of prostaglandin synthesis are mediated via the Senantiomers. However, there are a number of examples for which there is a unique biotransformation of the inactive R-enantiomers into the Senantiomers in vivo. Inversion is substrate and species dependent. Only fenoprofen, ibuprofen and benoxaprofen have been unequivocally shown to be inverted in humans. Enantioselective coenzyme A thioester formation appears to determine the overall selectivity of inversion, only R-enantiomers being substrates for the CoA synthetase. Interesting insights and questions have been raised into the role of CoA thioesters in the disposition of the 2-APAs. These drugs, once activated to the CoA thioesters, may share common pathways of metabolism with lipids. The parent 2-APAs are not excreted renally. However, a decrease in renal function may lead to accumulation of acylglucuronides which can form protein-drug adducts. Subsequent hydrolysis of the glucuronides may result in an increase in inversion and thus an increase in the concentrations of the active S-enantiomers,

Enantiomers in arthritic disorders and increased formation of hybrid lipids. The toxicological consequences of these interrelationships are uncertain but may account, in part, for the toxicity associated with benoxaprofen in the elderly. However, there have been no studies as yet to indicate that the S-enantiomers alone are clinically superior to the racemates. Thus, naproxen, the only 2-arylpropionate administered as the S-enantiomer, does not appear to have toxicities clearly different to the racemic 2-arylpropionates. The active sulfide derived from sulindac is reoxygenated by the enantioselective actions of prostaglandin cyclooxygenase peroxidase and F A D containing monooxygenase such that the relative concentrations of the enantiomers of sulindac will vary with time. Oxyphenbutazone is probably enantioselectively formed from phenylbutazone in vivo, but is subsequently racemized. The consequences of these examples of enantioselective metabolism are unknown. The activity of etodolac is mediated via the inhibition of cyclooxygenase by its R-enantiomer. Retodolac also accounts for 90% of the average plasma concentrations at steady-state after administration of the racemic drug. Attempts to correlate plasma or tissue concentration with response for asymmetric drugs must take into account the concentrations of the active enantiomer, although for etodolac, these are not too dissimilar to total drug concentrations. There are limited data, but these suggest that there are enantioselective differences in the toxicities and/or pharmacodynamic effects of the immunomodulating, antimalarial and cytotoxic drugs. Given that the efficacy of these and the gold antirheumatic drugs, is frequently limited by their adverse effects, there is great potential to significantly improve their therapeutic ratios by use of pure enantiomers rather than racemates. But most importantly, there is a need to compare the activities of the enantiomers of the antirheumatic drugs in suitable animal models of inflammation. This includes the need to reexamine the rationale for the use of the enantiomers of penicillamine and levamisole, which although used as the S-enantiomers, have not been satisfactorily compared with the R-enantiomers for their relative antirheumatic activity. Hopefully the results of such studies will establish a basis for clinical trials of some enantiomers. In any event, comparative studies of the enantiomers will give valuable insights into the pathogenesis and mechanisms of the rheumatological diseases. Finally it must be emphasized that in many instances enantiomers are pharmacologically discrete entities and that 'looking-glass' drugs may not be good to eat. would like to thank Associate Professor Richard Day for reviewing the manuscript and Mr Costa Corm for help and discussion in preparing the chemical structure illustrations. Acknowledgements--I

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Enantiomers in arthritic disorders N O T E A D D E D IN P R O O F Since submission of this manuscript data describing the chiral inversion of flunoxaprofen in man has been published:

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