Table Of ContentInternatIonal journal of BIomedIcal scIence
REVIEW ARTICLE
Chiral Drugs: An Overview
Lien Ai Nguyen1, Hua He2, Chuong Pham-Huy3
1Department of Pharmacy, Lucile Salter Packard Children’s Hospital, Stanford University Medical Center, 725 Welch Road,
Palo Alto, USA; 2Department of Analytical Chemistry, China Pharmaceutical University, Nanjing, China; 3Laboratory of
Toxicology, Faculty of Pharmacy, University of Paris 5, 4 avenue de l’Observatoire, Paris, France
AbstrAct
About more than half of the drugs currently in use are chiral compounds and near 90% of the last ones
are marketed as racemates consisting of an equimolar mixture of two enantiomers. Although they have the
same chemical structure, most isomers of chiral drugs exhibit marked differences in biological activities
such as pharmacology, toxicology, pharmacokinetics, metabolism etc. some mechanisms of these properties
are also explained. therefore, it is important to promote the chiral separation and analysis of racemic drugs
in pharmaceutical industry as well as in clinic in order to eliminate the unwanted isomer from the prepara-
tion and to find an optimal treatment and a right therapeutic control for the patient. In this article, we review
the nomenclature, pharmacology, toxicology, pharmacokinetics, metabolism etc of some usual chiral drugs
as well as their mechanisms. Different techniques used for the chiral separation in pharmaceutical industry
as well as in clinical analyses are also examined.
Keywords: analysis; chiral drugs; chiral separation; chiral terms; enantioselective antibodies; metabolism; phar-
macokinetics; pharmacology; toxicology
INTRODUCTION and animals but also in pharmaceutical, agricultural
and other chemical industries. All proteins, enzymes,
Chiral chemistry was discovered by Louis Pasteur, amino acids, carbohydrates, nucleosides and a number
a French chemist and biologist, when he separated by of alkaloids and hormones are chiral compounds. In
hand for the first time, in 1848, the two isomers of pharmaceutical industries, 56% of the drugs currently
sodium ammonium tartrate (1, 2). However, it needed in use are chiral products and 88% of the last ones
about a century later to find that the phenomenon of are marketed as racemates consisting of an equimo-
chirality plays a key role not only in the life of plants lar mixture of two enantiomers (3-5). In contrast to
chiral artificial products, all natural compounds are
under single enantiomeric form, for example, all natu-
ral amino acids are l-isomer (levorotatory) as well as
all natural sugars (carbohydrates) are d-isomer (dex-
Corresponding author: Chuong Pham-Huy, Laboratory of Toxicology,
trorotatory). Although they have the same chemical
Faculty of Pharmacy, University of Paris V, 4 avenue de l’Observatoire,
75006 Paris, France. Fax: 33 1 4674 6644; E-mails: pham.huy.chuong@ structure, most enantiomers of racemic drugs exhibit
wanadoo.fr or [email protected]. marked differences in biological activities such as
Copyright: © 2006 Nguyen et al. This is an open-access article distribut-
pharmacology, toxicology, pharmacokinetics, metab-
ed under the terms of the Creative Commons Attribution License, which
permits unrestricted use, distribution, and reproduction in any medium, olism etc. The mechanisms of chiral drugs with bio-
provided the original author and source are credited. logical environment are now explained. Therefore, it is
85
chIral drugs: an overvIew
important to promote the chiral separation and analysis act as an asymmetric center. Sulfur, phosphorus and
of racemic drugs in pharmaceutical industry as well as nitrogen can sometimes form chiral molecules such as
in clinic in order to eliminate the unwanted isomer from omeprazole, cyclophosphamide and methaqualone, re-
the preparation and to find an optimal treatment and a spectively. Chiral molecules exhibit optical activity, so
right therapeutic control for the patient. enantiomers are also sometimes called optical isomers.
Chirality is now a top-class subject for academic The two enantiomers of such compounds may be classi-
research as well as for pharmaceutical development. fied as levorotary (l-isomer) or dextrorotary (d-isomer)
Accounting for the important role of chiral separation, depending on whether they rotate plane-polarized light
the 2001 Nobel Prize in Chemistry has been awarded to in a left (-) or right (+) -handed manner, respectively.
three scientists: Dr. William S. Knowles and Pr. K. Bar- An equimolar mixture (50/50) of the two enantiomers
ry Sharpless in USA and Pr. Ryori Nyori in Japan, for of a chiral compound is called a racemic mixture (race-
their development of asymmetric synthesis using chiral mate) with sign (±) or (d, l) that does not exhibit optical
catalysts in the production of single enantiomer drugs activity. Optical isomers or enantiomers are molecules
or chemicals (6, 7). Thanks to a wide range of new having the same chemical formula, the same physical
technologies for chiral separation, US Food and Drug and chemical properties, but differing in their optical
Administration (FDA) recently recommends the assess- activity and their spatial arrangement.
ments of each enantiomer activity for racemic drugs in Enantiomers are now determined by their spatial
body and promotes the development of new chiral drugs arrangement (3 dimensions) of substituents (groups)
as single enantiomers (7). around a chiral center (asymmetric carbon) in the mol-
In this article, we review the nomenclature of chi- ecule. This configuration follows the Cahn-Ingold-Pre-
ral compounds, the biological activities such as phar- log (CIP) convention (11) which is used to assign pri-
macology, toxicology, pharmacokinetics, metabolism orities to substituent groups. This system is based on
etc of some usual racemic drugs in therapeutics as well a set of rules for ordering the substituents attached to
as their mechanisms. Different techniques used for the the asymmetric atom by using sequence rules to assign
chiral separation in pharmaceutical industry as well as priorities. There are different rules, being the simplest:
in clinical analyses are also examined. “substituents of the higher atomic number precede those
with lower ones”. First, draw a picture so that the atom
NOMENCLATURE OF CHIRAL COMPOUNDS with lowest priority, for example H, seems below the
plane of the picture (Fig. 1). Then start counting from
The terminology used to describe different stereo- the highest priority (highest atomic number or highest
chemical properties is resumed as following (8-10). mass) to the lowest one, for example 35Br→17Cl →9F or
Chirality (also sometimes called stereoisomerism for another example Cl→CH CH → CH . If the count-
2 3 3
or enantiomerism or dissymmetry) is a property of an ing goes in a clockwise direction, the configuration is
object which is non-superimposable with its mirror im- designated as R (rectus or right); otherwise in a counter
age. The origin of the word chiral is Greek cheir, which clockwise direction, it is S (sinister or left). A racemate
means ‘handedness’. When a molecule cannot be super- is designated as R,S. Each R- and S-enantiomers can ro-
imposed on its mirror image, this molecule and its im- tate plane-polarized light, therefore they can be desig-
age are called chiral. It is like left and right hands. The nated as R(+) or R (-) and S (+) or S(-). The optical activ-
two non-superimposable mirror-image forms of chiral ity of an enantiomer is determined by a polarimeter or
molecules are called enantiomers. Enantiomers are by optical rotary dispersion and circular dichroism. The
most commonly formed when a carbon atom contains spatial arrangement of a chiral compound is determined
four different substituents (asymmetric carbon atom or by nuclear magnetic resonance or/and X-ray crystallog-
stereogenic carbon or also called chiral center). A chi- raphy diffraction. The R/S tridimensional configuration
ral molecule is a molecule having at least one asym- allows to explain the interaction of enantiomers with
metric carbon. Carbon is not the only atom that can their biologic receptors.
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chIral drugs: an overvIew
In a stereoselective synthesis, one of a set of isomers
is predominantly or exclusively formed whereas in a
stereospecifi c synthesis, one isomer leads to one prod-
uct while another isomer leads to the opposite product.
Homochirality is the biological chirality in which all
biologic compounds have the same chirality such as all
amino acids are levorotary isomers.
Chiral switch is a procedure used to transform an
old racemic drug into its single active enantiomer. This
new enantiomeric drug developed by a pharmaceutical
manufacturer will receive additional patent protection
and a new generic name.
Figure 1. H behind the plane of the paper.
PHARMACOLOGY
The body with its numerous homochiral compounds
being amazingly chiral selector, will interact with each
racemic drug differently and metabolize each enantio-
mer by a separate pathway to generate different phar-
Diastereomers are any molecules which have two or macological activity. Thus, one isomer may produce the
more chiral centers. A diastereomer with two chiral car- desired therapeutic activities, while the other may be
bon has four isomers. Unlike enantiomers, the physical inactive or, in worst cases, produce undesired or toxic
and chemical properties of diastereomers can differ and effects (9, 12, 13, 15, 25, 32).
consequently, their chemical characterization is easy In pharmacology area, only racemic drugs will be
and their biological activities are often different. This is examined and their activity can be divided into three
the basis for derivatization of enantiomers to form dia- main groups. The majority of racemic pharmaceuticals
stereomers in chiral separation and also for the explana- have one major bioactive enantiomer (called eutomer),
tion of enantiomer activities with their chiral receptors the other is inactive or less active (distomer) or toxic
in the body. Diastereomers, enantiomers and geomet- or can exert other desired or undesired pharmacologi-
ric isomers form a family called stereoisomers that are cal properties. The second category is intended to drugs
molecules having the same chemical formulas but dif- where the two enantiomers are equally active and have
fering only with respect to the spatial arrangement. the same pharmacodynamics. The last one is racemic
Eutomer refers to bioactive enantiomer or enantio- drugs having only one eutomer, but the distomer could
mer having higher pharmacological activity. Its oppo- be transformed in body into its bioactive antipode by
site is called distomer. chiral inversion (12-15).
Epimers are two diastereoisomers having a different
confi guration at only one chiral centre. Group 1. Racemic drugs with one major bioactive
Enantioselectivity is a property of a process where- enantiomer
by one enantiomer is expressed exclusively or predomi- In this group, there are a number of cardiovascular
nantly over the other. In pharmacological terms, that drugs, agents widely used for the treatment of hyper-
means a biological structure (enzyme, antibody or re- tension, heart failure, arrhythmias, and other diseases.
ceptor) which exhibits affi nity towards one enantiomer Among these are the β-adrenergic blocking agents, cal-
over the other. cium channel antagonists and angiotensin-converting
Enantioselective assay is an analytical method ca- enzyme (ACE) inhibitors.
pable of separating and quantifying enantiomers. Levorotary–isomer of all β-blockers is more potent
87
chIral drugs: an overvIew
in blocking β-adrenoceptors than their dextrorotary-iso- are sympathomimetic drug-selective β -adrenoceptor
2
mer, such as S(-)-propranolol is 100 times more active agonists mainly used as bronchodilators in the treatment
than its R(+)-antipode (16-18). A number of β-blockers of asthma. They are longtime marketed as racemate.
are still marketed as racemic form such as acebutolol, Pharmacologically, only their l-isomer or R (-)-isomer
atenolol, alprenolol, betaxolol, carvedilol, metopro- is effective and the other inactive d-or S (+)-isomer
lol, labetalol, pindolol, sotalol, etc, except timolol and may be responsible for the occasional unpleasant side-
penbutolol are used as single l-isomer. However, it has effects associated with the drug. The Food and Drug
been demonstrated that d,l- and d-propranolol can in- Administration recently approved a chiral switch drug,
hibit the conversion of thyroxin (T4) to triiodothyronin levalbuterol (the pure l-isomer of albuterol) as a pre-
(T3), contrary to its l-form (19-21). Therefore, single d- servative-free nebulizer solution. However, some clini-
propranolol might be used as a specific drug without cal studies recently reported that it is neither safer nor
β-blocking effects to reduce plasma concentrations of more effective than a same dose of racemic albuterol. In
T3 particularly in patients suffering from hyperthyroid- contrast, levalbuterol may cost as much as 5 times more
ism in which racemic propranolol cannot be adminis- than its racemate (26-27).
tered because of contraindications for β-blocking drugs In neurology and psychiatry, many pharmaceuticals
(16). It is to know that for a racemic drug, each enantio- used are chiral compounds and most of them are mar-
mer possesses its own pharmacological activities that keted as racemates. Hypnotics such as hexobarbital,
can be null, similar, different or opposite. secobarbital, mephobarbital, pentobarbital, thiopental,
Many calcium channel antagonists are used under thiohexital are racemic compounds and overall, only
racemic form such as verapamil, nicardipine, nimodip- l-isomer is hypnotic or sedative, the other is either in-
ine, nisoldipine, felodipine, mandipine etc, except active or excitative. For example, S(-)-secobarbital is
diltiazem is a diastereoisomer with two pairs of en- more potent as anesthetics than R(+)-secobarbital i.e. it
antiomers. For example, the pharmacological potency causes a smoother more rapid anesthetic effect.(13, 28)
of S(-)-verapamil is 10-20 times greater than its R(+)- Ketamine is an intravenous anesthetic. The (+)-isomer
antipode in terms of negative chromotropic effect on is more potent and less toxic than itS(-)-antipode, but
AV conduction and vasodilatator in man and animals unfortunately, ketamine is still used as racemic drug
(22, 23). On the other hand, verapamil has another pos- (5, 10). Isoflurane is an inhalational general anesthetic
sible application in cancer chemotherapy as a modifier widely used in surgical operations as a racemic mixture
of multidrug resistance. Unfortunately, for this purpose, of its two optical isomers. The (+) isomer of isoflurane
verapamil must be used at high concentrations leading is more effective than the (-) isomer at inhibiting cur-
to high cardiotoxicity. However, it was later found that rents induced by the bath application of acetylcholine
R(+)-verapamil has far less cardiotoxicity than S(-)- (24). In the treatment of depression, S (+)-citalopram is
verapamil. Therefore, the R-enantiomer would be pref- over 100-fold more potent as a selective serotonin reup-
erable as a modifier of multidrug resistance in cancer take inhibitor than R (-)-enantiomer (3).
chemotherapy, while the S-enantiomer or the racemate Methadone, a central-acting analgesic with high af-
would be preferable as a calcium channel blocker for finity for µ-opiod receptors, has been used to treat opi-
cardiovascular therapy (24). ate dependence and cancer pain. Methadone is a chi-
All ACE inhibitors such as captopril, benazepril, ral synthetic compound used in therapy under racemic
enalapril, idapril are chiral compounds under diastereo- mixture. In humans, R (-)-methadone is about [25-50]
isomeric form and most of them are marketed as single fold more potent as an analgesic than its S (+) antipode
isomer. Valsartan, an angiotensin II receptor antago- (3, 29, 30).
nist, is used as a single S-enantiomer and the activity of The list of racemic drugs with one eutomer is long.
the R-enantiomer is clearly lower than the S-enantiomer It includes anticonvulsants such as mephenytoine, etho-
(25). suximide; antiarrhythmics and local anesthetics such
Albuterol (salbutamol), salmeterol and terbutaline as propafenone, disopyramide, prilocaine, tocainide;
88
chIral drugs: an overvIew
antibiotics such as ofloxacin, moxalactam; anticoagu- cannot occur in vivo because each enantiomer is trans-
lants such as warfarine, acenocoumarol; antihistamin- ported by protein (albumin) with different affinity. The
ics such as terfenadine, loratadine; antihyperlipidemic binding affinities of the enantiomers to albumin may in-
such as atorvastatin; psychostimulants such as amphet- hibit the attack of hydroxyl ions (water) and thus retard
amine, metamphetamine; proton pump inhibitors such the epimerization and racemization in vivo. Therefore,
as omeprazole, pantoprazole, lansoprazole, etc (15, 31- R- and S-oxazepam concentrations can be found differ-
32). Some of these racemates recently undergo chiral ent in the serum of these treated rabbits. On the other
switch to single enantiomer such as levofloxacin (from hand, He et al (34) have also demonstrated that the in
ofloxacin), levalbuterol (from albuterol), escitazolam vitro chiral inversion of these benzodiazepine enantio-
(from citalopram), esomeprazole (from omeprazole), mers was temperature-dependent and was inhibited by
dexketoprophen (from ketoprophen), dexmethylpheni- lowering temperature of aqueous solution to about 10°C
date (from methylphenidate), etc. (33-34). The S (+)-oxazepam enantiomer is 100-200 fold
more potent as a tranquilizer and sedative than R (-)-ox-
Group 2. Racemic drugs with equally bioactive en- azepam (35).
antiomers Thalidomide is a former racemic sedative with-
There are only some racemic drugs that could be- drawn from the market in the 1960s due to severe tera-
long to this group such as cyclophosphamide (antineo- togenic effects (phocomelia, amelia). However, there
plastic), flecainide (antiarrhythmic), fluoxetine (antide- is renewed interest in restricted use of thalidomide be-
pressant) (15). cause of its immunomodulatory (36), anti-angiogenic,
and anti-inflammatory effects (15) Moreover, it strongly
Group 3. Racemic drugs with chiral inversion inhibits the tumor necrosis factor α (TNF-α). Thalido-
There are two kinds of drug chiral inversion: unidi- mide gave spectacular results in the treatment of ery-
rectional and bidirectional inversion (31). thema nodosum leprosum, aptosis, Behcet’s syndrome
Unidirectional enzyme mediated inversion was pre- and has been assayed for organ transplantation, some
viously described only with 2-arylpropionate nonsteroi- autoimmune diseases such as chronic lupus erythema-
dal anti-inflammatory drugs (NSAID), namely ibupro- tosus, rheumatoid arthritis, some forms of cancer, etc
fen, ketoprofen, fenprofen, benoxaprophen, etc. For this (15, 36). Single thalidomide enantiomers and its deriva-
group, only S-enantiomer is active i.e. has an analgesic tive, N-hydroxythalidomide, were also synthetized by
and anti-inflammatory effect. For example, S-ibuprofen asymmetric technique in order to study their individual
is over 100-fold more potent as an inhibitor of cyclo- biological and chemical activities (37, 38). It seems that
oxygenase I than (R)-ibuprofen. In the body, only in- a multitude of its pharmacological activities could be
active R-enantiomer can undergo chiral inversion by due not only to the mother molecule but also to its nu-
hepatic enzymes into the active S-enantiomer and not merous chiral and achiral metabolites. Because of this
vice-versa (9, 31). in vivo interconversion of thalidomide, it is difficult to
Bidirectional chiral inversion or racemization should determine exactly the pharmacological effect of each
be represented by 3-hydroxy-benzodiazepines (oxaze- enantiomer.
pam, lorazepam, temazepam) and thalidomide in which The main pharmacological potency observed from
R and S enantiomer can racemize in vitro by aqueous two isomers of some current racemic drugs is gathered
solution. However, in vivo this phenomenon could oc- in the Table 1.
cur with thalidomide, but not with hydroxyl-benzodiaz-
epines because of the differences in substituents around TOXICOLOGY
their chiral carbon. Some authors (33) have found for
the first time the difference in R- and S-oxazepam con- Since there are frequently large pharmacodynamic
centrations in treated rabbit serum. They explained that and pharmacokinetic differences between enantio-
the chiral inversion by tautomerization of oxazepam mers, it is not surprising that enantiomers may result
89
chIral drugs: an overvIew
Table 1. Comparison of isomer potency of some racemic drugs (l=levorotary, d=dextrorotary)
Main pharmacological effects of drugs Isomer potency
b -Adrenoreceptor blocking drugs (b-blockers): propranolol, acebutolol, atenolol, l > d (d = inactive)
alprenolol, betaxolol, carvedilol, metoprolol, labetalol, pindolol, sotalol, etc
Ex: S(-)-propranolol>R(+)-propranolol
Calcium channel antagonists: verapamil, nicardipine, nimodipine, nisoldipine, l > d
felodipine, mandipine etc
Ex: S(-)-verapamil>R(+)-verapamil
β-Adrenoceptor agonists: Bronchodilators: Albuterol (salbutamol), salmeterol l > d (d = inactive)
2
and terbutaline
Ex: R(-)-albuterol>S(+)- albuterol
Hypnotics, Sedatives: hexobarbital, secobarbital, mephobarbital, pentobarbital, l > d
thiopental, thiohexital
Ex: S(-)-secobarbital>R-(+)secobarbital
Anesthetics: Ketamine, isoflurane d > l (l = inactive)
Ex: S(+)-ketamine>R(-)-ketamine
S(+)-isoflurane > R(-)-isoflurane
Central-acting analgesic (μ-opiod receptors): Methadone Ex: R(-)-methadone>S(+)-methadone
d > l
Analgesics, Anti-inflammatory : (NSAID ): ibuprofen, ketoprofen, benoxapro-
phen, fenprofen, etc Ex: S(+)-ibuprofen>R(-)-ibuprofen
Tranquilizers: 3-hydroxy-benzodiazepines: oxazepam, lorazepam, temazepam d> l (l = inactive)
Ex: S(+)-oxazepam>R(-)- oxazepam
in stereoselective toxicity. In toxicology, the different drugs cited in the chapter of pharmacology are trans-
toxic effects of chiral drugs can reside either in one formed by chiral switch into their single active iso-
enantiomer only or in both ones. The toxicological mer because one of two isomer has side-effects and no
properties in a pair of enantiomers can be identical or pharmacological activity (9, 14, 15, 41, 43). However, a
entirely different. They can reside in the pharmaco- number of chiral drugs are still marketed under race-
logically active enantiomer or in the inactive one (9, mic form because either their chiral separation is dif-
12, 15, 25, 32, 40, 43). ficult, or their pharmacologic and toxic effects reside
Some following drugs are marketed as single enan- in the same enantiomer or their high cost production
tiomer solely because their toxicities reside almost in (9, 14, 15, 41). Toxicity of chiral drugs like ketamine
one of their two enantiomers. Dopa or dihydroxy-3,4 (anesthetic), penicillamine (chelating agent), etham-
phenylalanine is a precursor of dopamine that is effec- butol (antitubercular agent) reside exclusively in their
tive in the treatment of Parkinson disease. Dopa was distomer. For example, only R (-)-ketamine (distomer)
used under racemic form: d,l- dopa, but owing to the is responsible for agitation, hallucination, restlesness,
grave toxicity (agranulocytosis) of d-isomer, therefore, in contrast to S (+)-ketamine (eutomer) (40). However,
only levoratory form called L-Dopa is actually used in secobarbital enantiomers are equipotent as anticonvul-
therapeutics. Tetramisole is a nematocide, first used sants, but the S(-)-isomer is a more potent anesthetic
under racemic form. Because of numerous side-effects and is also more toxic than the R(+)-isomer.40 In the
(vertigo, headache, vomiting, abdominal pain) mainly case of cyclophosphamide, its two isomers exert the
due to d-isomer, therefore, only l-isomer called levam- same toxicity (42, 43). For thalidomide, theoretically,
isole is now used in medicine. Actually, some racemic only the inactive S(-)-isomer is teratogenic, but practi-
90
chIral drugs: an overvIew
cally, both isomers are genotoxic because of its in vivo is not surprising that a wide interindividual variabil-
interconversion and of its species-dependence (42, 43). ity exists in the metabolism of these drugs. Overall,
Tests with mice in 1961 suggested that only one enan- substantial stereoselectivity has been observed in both
tiomer was teratogenic while the other possessed the the pharmacokinetics and pharmacodynamics of chi-
therapeutic activity. Unfortunately, subsequent test ral antiarrhythmic agents. Because the effects of these
with rabbits showed that both enantiomers had both drugs are related to their plasma concentrations, this
teratogenicity. The S-isomer (in contrast to the R- information is of special clinical relevance (48).
isomer) has been linked to thalidomide’s teratogenic As reported by Stoschitzky et al (16), there are
effects. However, attempts to formulate the R-isomer marked pharmacokinetic differences between the d-
have not solved the problem of teratogenicity, as the and l-enantiomers of most β-blockers, particularly un-
two isomers are readily interconvertible in vivo (14, der exercise and when extensive and poor metabolisers
39). Moreover, toxicity of thalidomide could be due to are compared. Plasma concentrations of these d and
its numerous chiral and achiral metabolites of which l-enantiomers usually differ significantly and in wide
pharmacological and toxicological studies remain very ranges when the racemic mixture is administered oral-
scarce. ly or intravenously. Mehvar et al (49) also reported that
the β-blockers are quite diverse in pharmacokinetic
PHARMACOKINETICS AND METABOLISM profile, as they display a high range of values in plas-
ma protein binding, in percent of drug eliminated by
The processes of absorption, distribution, elimina- metabolism or unchanged in the urine, and in hepatic
tion and metabolism are crucial determinants of drug extraction ratio. With respect to plasma concentrations
action and can assume equal relevance to the actual bi- attained after oral or intravenous dosing, in most cases
ological effect of the drug at its receptor site. The po- the enantiomers of the β-blockers show only a mod-
tential for discrimination between enantiomers at each est degree of stereoselectivity. However, the relative
of these stages is therefore important and emphasizes magnitude of the concentrations of the enantiomers
the need for stereo-pharmacokinetic studies and ste- in plasma is not constant in all situations and varies
reospecific drug assays (44). Indeed, numerous studies from drug to drug. Further, various factors related to
have demonstrated that stereoisomers of a chiral drug the drug (e.g., dosing rate or enantiomer-enantiomer
often exhibited pronounced differences in their phar- interaction) or the patient (e.g., racial background, car-
macokinetic and metabolic profiles both quantitatively diovascular function, or the patient metabolic pheno-
and qualitatively (45-47). type) may affect the stereospecific pharmacokinetics
According to Mehvar et al. (48), many antiarrhyth- and pharmacodynamics of β-blockers (49). In another
mic drugs are marketed as racemates such as disopyra- study (50) the pharmacokinetic profile of propranolol
mide, encainide, flecainide, mexiletine, propafenone, enatiomers and their enantiomer sulfate and glucuro-
tocainide, etc. The absorption of chiral antiarrhyth- nide metabolites, in human serum and urine showed
mics appears to be nonstereoselective. However, their that the S/R ratios of mother molecule in serum and
distribution, metabolism and renal excretion usually urine were about 1.4, of sulfate conjugate about 2 and
favour one enantiomer versus the other. In terms of of major glucuronide metabolite about 3. But varia-
distribution, plasma protein binding is stereoselective tion of the ratios S(-)- active/R(+)-inactive isomers be-
for most of these drugs, resulting in up to two-fold tween individuals was also observed (50).
differences between the enantiomers in their unbound Methadone, a central-acting analgesic, used in the
fractions in plasma and volume of distribution. Hepatic treatment of opiate addicts is a racemate with predom-
metabolism plays a significant role in the elimination inantly active R(+)-isomer. In humans, the ratios of
of these antiarrhythmics. Additionally, in most cases, R/S methadone levels varied from 0.6 to 2.0 in serum
significant stereoselectivity is observed in different and 1.2- 2.0 in urine because of inter-individual differ-
pathways of metabolism of these drugs. Therefore, it ences in the pharmacokinetics of methadone enantio-
91
chIral drugs: an overvIew
mers (3, 30, 51, 52). Methadone is mainly metabolized an enantiomer with a protein is a delicate and difficult
by hepatic cytochrome P-450 3A4 and secondary by step of the immunogen preparation. The specificity
P-450 2D6 to a major methadone metabolite, EDDP, and sensitivity of an antibody against an enantiomer
(2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine). drug mainly depends on the hapten preparation and
In human urine, EDDP levels were always higher the conjugation technique. According to Sahui-Gnassi
(about 1.5-5 fold) than those of mother molecule be- et al. (56) and Chikki-Chorfi et al. (58) for the obten-
cause of its polarity, however its concentrations were tion of a specific antibody to a chiral compound, it
undetectable in human saliva and always lower than is necessary to keep intact the carbon asymmetry i.e.
those of methadone in serum (53). no direct linking of hapten with protein carrier at the
Although the in vitro chiral inversion of 3-hydroxy- chiral center. In the literature, some enantioselective
benzodiazepines in aqueous solution, some authors33 antibodies have been successfully used to separate en-
were the first to demonstrate the difference in se- antiomers, such as antibodies to propranolol enantio-
rum concentrations of two oxazepam enantiomers, mers (56) methadone enantiomers (58) amphetamine
with predominance of the active S (+) form in rabbits enantiomers (55) warfarin enantiomers (55) etc. How-
treated with pharmacological and toxic dosages, while ever, they are still scarce in comparison with numer-
the inactive R (-) antipode was higher in intoxicated ous antibodies to achiral drugs. The application of en-
rabbits under antidote treatment with flumazenil. Be- antioselective antibodies to chiral drugs plays a key
cause of the high binding of oxazepam enatiomers to role in biochemistry. They can be used as a specific
protein (97%), the hydroxyl ions of water in body can- reagent not only for immunoassays such as radioim-
not racemize each enantiomer. Differences in the bind- munoassay (55) enzymatic immunoassay (57) but also
ing affinities of each oxazepam enantiomer to protein for histologic immunoassay, immunoaffinity chroma-
may account for the selectivity of the drug and could tography, immunoextraction of chiral drugs, liquid
explain the variation of the ratios R/S enantiomer con- chromatography using antibodies as chiral selectors,
centrations in serum (33). etc. These immunoassays can be applied to pharma-
An understanding of the stereospecific pharmacoki- cokinetics, drug therapeutic monitoring, toxicological
netics of all chiral drugs may help clinicians to interpret diagnostic, drug pharmacological assessment, identi-
and predict differences among patients in pharmacologic fication of drug fixed on target organ, etc.
responses to the racemic drugs and to adjust their dosage
for each patient. MECHANISM OF BIOLOGICAL ACTIVITY
ENANTIOSELECTIVE ANTIBODIES TO The enantiomers of a chiral drug may vary in
CHIRAL DRUGS their interactions with chiral environments such as
enzymes, proteins, receptors, etc of the body. These
As many target organs can distinguish the two en- variations may lead to differences in biological activi-
antiomers of a chiral drug, it is not surprising that the ties such as pharmacology, pharmacokinetics, metabo-
immune system may recognize them in the same man- lism, toxicity, immune response etc. Indeed, biologi-
ner by producing selectively and specifically each cor- cal systems can recognize the two enantiomers as two
responding enantiomer antibody. Such enantioselec- different substances, and their interaction each other
tive antibody can distinguish even minor differences will therefore elicit different responses. But, why do
in composition or configuration of a chiral drug. As enantiomers have different biological activities? The
drugs are small molecules called haptens, they need reason for chiral recognition by drug receptors is a
to be conjugated to a support or to a carrier molecule three-point interaction of the drug with the receptor
(protein) by forming an immunogen so that the im- site proposed by Easson and Stedman (44, 47, 59). The
mune system (B cells) could produce corresponding difference between two enantiomers of a drug with its
antibody (54, 55). The conjugation of a hapten such as receptor is illustrated in Figure 2 (published by Mc-
92
chIral drugs: an overvIew
Figure 2. Easson-Stedman hypothetical interaction between the two enantiomers of a racemic drug with a receptor at the drug binding
sites. The three substituents A, B, C of the active enantiomer (left) can interact with three binding sites a, b, c of a receptor by forming
three contacts Aa, Bb and Cc, whereas the inactive enantiomer (right) cannot because the contact is insuffi cient. Note: This fi gure is in
the publication of McConalthy and Owens (47).
Conathy and Owens (47) using a hypothetical interac- different chiral discrimination by diastereomeric for-
tion between a chiral drug and its chiral binding site mations with a chiral environment (9, 12, 44, 47, 59).
(47). In this case, one enantiomer is biologically active
while the other enantiomer is not. The substituents of CHIRAL SEPARATION
the active enantiomer drug labeled A, B, and C must
interact with the corresponding regions of the bind- Chiral separation, also called chiral resolution, is
ing site labeled a, b, and c of the receptor in order to a procedure used to separe the two isomers of a ra-
have an alignment Aa, Bb, Cc. In this case, this fitting cemic compound in pharmaceutical industry as well
interaction can produce an active biological effect. In as in clinical analysis. Various methodologies used for
contrast, the inactive enantiomer cannot bind in the chiral separation on both analytical and preparative
same way with its receptor when it rotates in space, scales will be described below.
consequently, there is no active response (44, 47). To
resume this hypothesis, the attachment of an enantio- In pharmaceutical industry
mer to the chiral receptor is similar to a hand fitting During chemical synthesis, many drugs obtained
into a glove or to a key into a lock. Indeed, a right hand can be racemized in situ by a variety of chemical reac-
can only fit into a right hand glove, so a particular tions, even the procedure used has started with pure
enantiomer can only fit into a receptor site having the enantiomeric reagents. In industry, two main catego-
complimentary shape. The other enantiomer will not ries of techniques are often applied for chiral resolu-
fit, like a right hand in a left glove, but may fit into a tion: the classical methods and the modern technolo-
receptor site elsewhere in the body and cause an even- gies (60, 61).
tual unwanted or toxic effect. On the other hand, enan- For the classical approach, the most widely used
tiomers can show different chemical behaviour due to technique is the resolution by diastereomeric salt for-
93
chIral drugs: an overvIew
mation. In this strategy, an acid-base reaction is in- phase. Among a hundred HPLC CSPs commercially
volved between a racemic drug and a pure single en- available, only some types of chiral sorbents follow-
antiomer called resolving agent. This reaction leads ing are presently the most widely used for preparative
to the formation of two diastereomeric salts that now HPLC in industry: carbohydrate (cellulose, amylose),
have different physical and chemical properties. These polyacrilamide, diallyltartardiamide, Pirkle phases,
two diastereomers obtained can be easily separated ei- chirobiotic phases (vancomycin, teicoplanin) (64).
ther by crystallization or by filtration if one is soluble However, there is no single CSP that can be considered
and the other is insoluble. Finally, the salt is decom- universal, i.e., has the ability to separate all classes of
posed by treatment with either acid or base, then the racemic compounds. Choosing the right column for the
pure enantiomer is obtained (44, 60). The two dia- enantioseparation of a racemic compound is difficult.
stereomers formed can also be separated by classical The decision relies mostly on empirical data and expe-
achiral liquid chromatography. This method has been rience (60). However, the understanding of the recogn-
used in the resolution of -methyl-L-dopa, asparagine tion mechanisms of chiral selectors with enantiomers
and glutamic acid (44). Another classical approach is can help the chromatographists to resolve some prob-
the enzymatic or kinetic resolution. In this methodol- lems of resolution and to economize time-consuming.
ogy, resolution is achieved by means of biochemical According to Aboun-Enein and Ali (60) all chiral se-
process that destroys one enantiomeric form. Certain lectors provide a chiral surface to enantiomers, which
microorganisms such as yeasts, molds, bacteria can form with the selectors temporary complexes, having
only degrade one of two isomers of a racemate by en- different bonding energies. The enantiomers differ
zymatic assimilation, the other which is not digested in their binding energies because they fit differently
remains in solution, then it is isolated (60, 61). Enzy- into the chiral selector structures. Consequently, the
matic resolution has been used in the preparation of two enantiomers can be eluted at different times by
lotrafiban (benzodiazepine), levofoxacin (antibacterial the mobile phase and then separately collected. Brief-
drug), and S-naproxen (antiinflammatory drug) (44). tly, in general, the recognition mechanism on a chiral
For the modern technologies, preparative high-per- selector is based on a key-and-lock arrangement (60).
formance liquid chromatography (HPLC) is the method However, many other factors such as mobile phase
of choice for the enantiomer separation. Chiral HPLC composition (pH, electrolytes, solvent nature), size
has proven to be one of the best methods for the direct and length of column, temperature etc also play a key
separation and analysis of enantiomers. In chromato- role for chiral resolution.
graphic methods, two techniques are used: indirect Besides the direct chiral HPLC, a new technique
and direct. The indirect HPLC involves derivatization called simulated moving bed (SMB) chromatography is
of samples with a chiral derivatization reagent i.e. a recently developed for industry. As reported by Burke
pure single enantiomer, resulting in the formation of and Henderson (44) the basic concept of SMB tech-
two diastereomers which can be separated by a clas- nology is the continuous countercurrent movement of
sical reversed-phase column (62). This indirect HPLC stationary and mobile phases in which the movement
method is rarely used in industry, but frequently per- of a stationary phase is simulated. The small particles
formed in biological analysis because of its high sensi- in this component are packed into single columns and
tivity. On the other hand, the direct HPLC utilizes the connected to form a circle. Four external valves allow
chiral selector either in chiral stationary phases (CSPs) the addition and subtraction of feed and effluent. The
or in the mobile phase called chiral mobile phase addi- mobile phase is pumped through the circle and when
tives (CMPA) (60, 63, 64). The last technique is rarely it passes the stationary phase a slight separation oc-
used in industry because of its high cost and low effi- curs, the less absorbable compound running in front
ciency. Direct chiral separations using CSPs are more and the more absorbable compound staying behind.
widely used and are more predictable, in mechanistic When steady state is reached, the system can be oper-
terms, than those using chiral additives in the mobile ated continuously. If all flow rates and the shift time
94
