J Clin Pharmacol 1992;32:925-929 925

The Importance of Stereochemistryin Drug Action and Disposition

John Caldwell, PhD, DSc

Many biologically active synthetic drugs contain chiral centers, although they are usedas racemic mixtures, Enantiomers are hard to distinguish in the chemical laboratory butare readily discriminated in the body and differ in their biological activities and disposi-

tion. The pharmacokinetic profiles of enantiomers can be variable, especially for drugswith a first-pass effect and enantioselective pharmacokinetic monitoring should becarried out. Ultimately, whether to exploit a racemate or a single enantiomer in therapy

is a multi-faceted decision to which drug disposition data have important contributionsto make.

T he phenomenon of stereoisomerism, the exis-tence of two or more forms of a compound that

differ only in the three-dimensional arrangement ofthe same constituent atoms, is widespread amongpharmacologically active substances. Stereoisomer-ism arises from the occurrence within compounds of

chiral centers, also known as stereogenic or asym-metric centers. The number of possible stereoi-somers is 2”, where n is the number of chiral centers.

By far the most common type of chiral center ariseswhen a tetravalent atom, whose valencies are di-rected to the four corners of a regular tetrahedron,

carries four different substituents. Suth a compound

exists in two distinct and separate forms, or enan-tiomers (Figure 1), whose mirror images are not su-perimposable and cannot be interconverted withoutbreaking and reforming a bond. Of the various atomsthat can exhibit this phenomenon, those of greatestpharmacologic relevance are carbon, nitrogen, sul-fur, and phosphorus. Such chiral centers, notably

centered on carbon, are very commonly encoun-tered in natural products, where it is generally thecase that only one of the two possible forms occurs.Until the last 25 years, the importance of stereochem-

From the Department of Pharmacology and Toxicology, St. Mary’s Hos-

pital Medical School, Imperial College of Science, Technology and Medi-

cine, London, England. Presented at From Controversy to Resolution:

Bioequivalency of Chiral Drugs, Symposium at the American College of

Clinical Pharmacology 20th Annual Meeting, Atlanta, Georgia, October

16, 1991. Address for reprints: Professor J. CaIdwell, Department of

Pharmacology and Toxicology, St. Mary’s Hospital Medical School, Nor-

folk Place, London W2 1PG, England.

istry in pharmacology has thus been a largely theoret-ical consideration: although drugs based on naturalproducts almost always contain one or more chiral

centers, their stereochemistry is absolutely defined.This situation has been changed quite dramatically

with the impact of synthetic chemistry on drug de-velopment, and it now has been estimated that some75% of synthetic drugs currently in use contain a

single chiral center and thus exist as pairs of enan-tiomers.1 In the vast majority of cases, however, theracemate (an equimolar mixture of the two enan-

tiomers) is used therapeutically.Stereoisomers differ only in three dimensions and

thus may only be distinguished in a three-dimen-

sional environment with which they interact differ-entially.2 Analytically this may involve plane-polar-ized light (the original medium used by Pasteur),

chiral stationary phases in chromatography, chiralNMR shift reagents, or chiral derivatizing reagents.In biology, it must be appreciated that the body is

built of macromolecules in which chiral centersabound and that exist in only one of the two possibleforms: proteins are composed of L-amino acids exclu-

sively and carbohydrates are solely D-sugars. The in-teractions of chiral drugs with these various macro-molecules thus discriminate between stereoisomers,and this leads to differences between stereoisomersin their biologic effects and disposition.

Target sites for drug action such as receptors, en-zymes, and ion channels frequently interact differ-entially with stereoisomers so that they differ greatlyin terms of their biologic effects. The more active

enantiomer has been termed the eutomer and theless active the distomer.3 Although these terms have

a

b-\ - - d

a

b’\d’ C

b C

CALD WELL

926 5 J Clin Pharmacol 1992;32:925-929

Figure 1. lsonieric tetracoordinate carbon-centred compounds.

showing the two enantiomers and how their substituent groups (a-

d) are directed to the corners of a regular tetrahedron. The bold

lines show bonds in the plane of the paper, the dashed lines show

bonds directed away from the observer and the bold wedges repre-

sent bonds pointing towards the observer.

been valuable in focusing attention on the issuesraised by stereochemistry in drug development and

use, it can be suggested that they have now outlivedtheir usefulness, because this nomenclature carriesthe implication that one enantiomer is good and theother bad. There are numerous cases in which thedrug target does not discriminate between enan-tiomers, generally the result of the chiral center be-ing (relatively) uninvolved with the interaction withthe receptor. In other cases, the two enantiomers in-teract with different receptors, leading to differenteffects, this having been well known for many yearsamong the opiates, whose (-)-enantiomers are po-

tent narcotic analgesics whereas their (+)-antipodesare less potent (but still very useful) antitussives.4 Inaddition, the quantification of differences in activitybetween enantiomers, on which the eutomer/dis-tomer nomenclature is based, and the validity of ex-pressing these as eudismic ratios (activity of eu-tomer/activity of distomer) depends very much ontheir optical purity. Because the interaction betweena potent agonist or antagonist and its receptor is veryspecific and receptors have an exquisite ability todiscriminate between enantiomers, it might be sug-gested that there are no eudismic ratios less than in-finity! Contamination of the inactive enantiomerwith a small amount of its much more active anti-pode thus will lead to marked underestimates of thedrug’s stereoselectivity. This fact, although obvioustheoretically, until very recently remained withoutpractical verification. Work with the stereoisomers

of the fl-adrenergic agonist formoterol has shownthat the eudismic ratios of impure batches are verymarkedly less than those of which are >99.5% opti-

cally pure.5 This finding suggests that the same crite-ria should be set fOr optical purity as are commonlyencountered for chemical purity and clearly has im-

plications for the setting of specifications for the qual-ity of drug substances.

Enantiomeric discrimination in drug dispositiondepends on the mechanism of the process under con-sideration.6’7 Absorption, distribution, and excretionare generally passive processes that do not differen-tiate between enantiomers, but enzymic metabolism

and protein binding, to plasma or tissue proteins, canshow a high degree of stereoselectivity. The net re-sult of the interaction of the stereoselectivities ofthese various processes often obscures the impor-tance of the stereoselectivity of one.8 This is particu-larly the case for metabolism: although the ratios of

the total plasma clearance of the enantiomers of awide range of drugs never exceed 2, individual meta-

bolic pathways often show much greater stereoselec-tivity.

In terms of metabolism, chiral discrimination oc-curs at both substrate and product levels,6 giving riseto five distinct stereochemical courses for drug me-tabolism,7 namely (1) prochiral -‘ chiral, (2) chiral -‘

chiral, (3) chiral -‘ diastereoisomer, (4) chiral

achiral, and (5) chiral inversion. As a result, the meta-bolic and pharmacokinetic profiles of enantiomersafter administration of racemic drugs can be very

variable, so that the exposure to the two enantiomersmay be very different. There are by now an enor-mous number of examples of each of these possibili-ties in the literature, and the following represents a

small and personal selection to illustrate some of themore interesting possibilities.

The conversion of prochiral drugs to chiral metabo-lites, an example of what has been termed “product

enantioselectivity,” is of considerable interest. Insuch cases, the chirality of the products is solely afunction of the biologic system responsible for themetabolism and obviously cannot be influenced by

the drug substrate administered. There are well-doc-umented examples6 of prochiral -b chiral transfor-mations involving either metabolism at the prochiralcenter, e.g., the reduction of many ketones and thehydroxylation of prochiral methylene groups, both

yielding chiral secondary alcohols, or metabolismremote from the prochiral center inducing chirality,as is seen in the hydroxylation of the prochiral anti-convulsant diphenylhydantoin to its chiral metabo-

lite 4’-hydroxydiphenylhydantoin. If the chiral me-tabolite is biologically active and this activity showsenantioselectivity, a by no means unlikely situation,

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STEREOCHEMISTRY IN DRUG ACTION AND DISPOSITION

CHIRAL SYMPOSIUM 927

then animal interspecies and human interindividualvariation in metabolic enantioselectivity, which is

observed with increasing frequency, will have an

impact on the drug’s action.

It has been apparent for some years that the enan-tiomers of chiral drugs can be metabolized at differ-ent rates, sometimes along different routes. Numer-ous examples of enantiomers being metabolized at

different rates can be cited, one of the earliest beinghexobarbitone,9 but enantiomers being metabolizedalong different routes are somewhat less common.An early example is that of warfarin,1#{176} whose R-en-antiomer is preferentially hydroxylated at the 7-po-sition, whereas its S-antipode is reduced at the keto

group to products that are diastereoisomeric (see be-low), because the secondary alcohol resulting fromreduction is a new chiral center.

The enantiomers of the somewhat elderly anticon-vulsant mephenytoin are metabolized along differ-ent pathways. The R-enantiomer is N-demethylatedslowly (t#{189},70 hours), giving phenylethylhydantoin.whose elimination is even slower (tV2, 150-200hours), whereas the metabolism of the S-antipode,by aromatic hydroxylation at the 4’-position, ex-hibits a genetic polymorphism. Some 95% of Cau-casians are able to carry out this reaction rapidly,with a t1/2 of 1 hour, but about 5% are geneticallydeficient in the cytochrome P-450 isozyme responsi-ble for the reaction. In these, the S-isomer is slowlyN-demethylated so that the drug and its metabolitephenylethylhydantoin accumulate, giving rise tomarked adverse reactions. This is summarized in Fig-ure 2.

One means of the conversion of chiral drugs todiastereoisomeric metabolites has been illustratedabove with reference to warfarin. Another, morecommonly encountered, means is by conjugationwith an agent derived from the chiral pools of the

body. The most important of these are glucuronicacid, which is exclusively f3-D-glucopyranosiduronicacid, and glutathione. Such pairs of diastereoiso-meric conjugates differ in their physicochemicalproperties, and this is often reflected in their elimina-tion pharmacokinetics.12’13

Metabolism does not often result in the loss of achiral center, but does occur in cases like the oxida-tive deamination of the amphetamines, which canexhibit substrate stereoselectivity.14

The inversion of a chiral center cannot occur

without the breaking and reforming of a bond at thechiral center. There is a unique and biologically for-tuitous example of this occurring during the metabo-lism of the “profen” or 2-arylpropionic acid nonste-roidal anti-inflammatory agents. These simple car-boxylic acid derivatives contain a chiral center at the

Figure 2. Stereospecific metabolism of mephenvtoin and its genetic

polymorphism.

carbon atom bearing the carboxyl group and thusexist as readily separable pairs of enantiomers. Theiranti-inflammatory activity is due to inhibition of theconversion of arachidonic acid to prostaglandins and

other biologic mediators and this resides in their (+)-

S enantiomers. It is commonly the case, however,that there is little or no difference in the activities ofthe enantiomers when tested in vivo.15 This is due tothe metabolic chiral inversion of these compounds,in which the inactive R-enantiomers are inverted to

their active S-antipodes.12’16 The reaction involvesthe exchange of the a-methine proton, but there isotherwise no covalent change to the profen. Themechanism of this inversion is now known to in-volve the conversion of the profen to its Coenzyme Athioester, a high-energy intermediate in which the

a-methine proton is labile. The nature of the inver-sion and the possible participation of enzymes, how-ever, remains uncertain.12

Baillie and colleagues17 have investigated the in-version of ibuprofen, an archetypal molecule for the

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CALD WELL

928 5 J Clin Pharmacol 1992;32:925-929

study of this reaction, using the 2H-labeled formshown in Figure 3. When the R-enantiomer wasgiven to rats, both R- and S-ibuprofen were found inblood and urine. In confirmation of the findings of

others, the a-methine deuteron of the administeredR-ibuprofen was replaced by a proton in the S-ibu-profen formed, but they also noted that some of theR-ibuprofen had lost this cr-methine deuteron, de-spite the fact that no inversion had occurred. Themechanism of the inversion is thus not simply the

inversion of R to S, but seems to proceed by the mech-

anism shown in Figure 4, in which there is an obliga-tory enolate intermediate that can be the precursorof both R- and S-ibuprofen.

The true importance of stereoselective drug metab-

olism is seen with drugs whose action is stereoselec-tive, particularly those that show a substantial first-pass effect. In such cases, the enantiomeric composi-tion of the drug in plasma after administration of theracemate is a consequence of the route (and, per-haps, the rate) of administration. The earliest exam-ple of this is the archetypal j3-blocker propranolol,which is seemingly more potent when given orally

than intravenously.18 This paradox was resolvedwhen it became clear that the first-pass metabolism,most notably aromatic hydroxylation, of propranololwas markedly stereoselective for the essentially in-

active R-enantiomer.19 Since then, many further ex-amples have been uncovered,20 including verapamiland nilvadipine, in addition to numerous other [3-blockers.

Indacrinone is a racemate whose diuretic proper-ties reside in its (-)-isomer and its uricosuric actionis due to the (+)-antipode.21 When the racemate isgiven, the th/2 of the uricosuric isomer is too short toprevent the rise in serum uric acid that accompanies

diuresis. Manipulation of the relative proportions ofthe two enantiomers, however, showed that a 1:4combination of the (-) and (+) isomers gave accept-

Figure 3. (-)-fl-2H5-Ibuprofen.

Figure 4. Mechanism of the metabolic chirol inversion of ibupro.fen, as revealed by in viva studies with (-)-R-2H5.ibuprogen in rats

after Sanins et al.2’

able diuresis without changing serum uric acid

levels.21Major differences in the metabolic and pharmaco-

kinetic profiles of enantiomers may present a ratio-

nale for the use of only the more active enantiomer

of a racemic drug in therapy, and this is particularlythe case when there occurs a relative accumulationof the enantiomer with the less desirable activity.The use of single enantiomers may thus increase se-lectivity of drug action, reduce total exposure to thexenobiotic, and simplify dose-response relation-ships. The relative benefit of the use of single enan-

tiomers will be increased for drugs with steeperdose-response curves. There can be no hard-and-fastrules, however, about the desirability of pure enan-

tiomers over racemates, and decisions must be takenon a case-by-case basis. Among the varied criteria

that impinge on such a decision are the perceivedpharmacologic and therapeutic advantage, ease andeconomics of stereoselective synthesis, problems inextrapolation of animal data to humans, and likelymarketing advantage. When both enantiomers have

distinct and desirable effects or their effects are notstereoselective, the use of a racemate may be en-tirely justifiable. In any event, it is important to ap-preciate that there are three separate issues that areoften rolled into one. These are (1) the selection ofracemate or single enantiomer when nominating anewly discovered agent for further development: (2)when a racemate is in development and it becomes

apparent that its activity resides in one enantiomer,how can the switch to the active enantiomer bemade with the least wastage of time and data: and (3)

STEREOCHEMISTRY IN DRUG ACTION AND DISPOSITION

CHIRAL SYMPOSIUM 929

how to license and market the active enantiomer of a

therapeutically established racemate. In each ofthese cases, metabolic and pharmacokinetic infor-mation are essential, but their contribution changesfrom situation to situation. In particular, toxicokine-tic monitoring that shows the separate exposure toboth enantiomers of a racemic drug may well vali-

date a toxicity test of a racemate as an indicator ofrisk from the individual enantiomers by confirmingadequacy of exposure.

For the major research-orientated pharmaceuticalcompanies, the decision to develop only single enan-

tiomers de novo in most cases has already beentaken.22 For drugs further along the development

path and in the generic market, we may expect theracemate-versus-enantiomer debate to remain alively one for years to come, fuelled by emerging reg-ulations dealing with this issue in the principal juris-dictions.23 However these arguments are ultimatelyresolved, closer attention to the third dimensionmust improve the safety-in-use of chiral drugs. Nev-ertheless, the debate will only be closed when, as theZen masters teach, we learn to clap with one hand.

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