Role of racemization in optically active drugs development

Review Article
Role of Racemization in Optically Active Drugs Development
IMRAN ALI, 1 VINOD K. GUPTA, 2 HASSAN Y. ABOUL-ENEIN, 3 * PRASHANT SINGH, 4 AND BHAVTOSH SHARMA 4
1 Department of Chemistry, Faculty of Natural Sciences, Jamia Millia Islamia (University), Jamia Nagar, New Delhi, India
2 Department of Chemistry, Indian Institute of Technology, Roorkee, India
3 Department of Medicinal Chemistry, College of Pharmacy, University of Sharjah, Sharjah, United Arab Emirates
4 Department of Chemistry, D.A.V. (P.G.) College, Dehradun, India
ABSTRACT U.S. Food and Drug Administration issues certain guidelines for marketing
of optically active drugs as some enantiomers racemize into human body, leading
to the generation of other antipodes, which may be toxic or ballast to the human beings.
Moreover, racemization reduces the administrated dosage concentration as optically
active enantiomer converted into its inactive counter part. Therefore, the study of racemization
of such type of drugs is an important and urgent need of today. This article
describes in vitro and in vivo racemization of optically active drugs. The racemization
process of various optically active drugs has been discussed considering the effect of
different variables i.e. pH, temperature, concentration of the drug, ionic concentration,
etc. Attempts have also been made to discuss the mechanisms of racemization. Besides,
efforts have been made to suggest the safe dosages of such type of drugs too. Chirality
19:453–463, 2007. VC 2007 Wiley-Liss, Inc.
KEY WORDS: enantiomer; epimerization; harmful effects; in vitro; in vivo; optically
active drugs; racemization
INTRODUCTION
Nowadays, many drugs are used for the treatment of different
diseases in human beings. In case of optically active
drugs frequently only one of the two enantiomers is pharmaceutically
active while other may be inactive or toxic. 1,2
Administration of the racemic mixture of an optically
active drug into human body creates confusion of its dose
as about 50% is active while 50% inactive. Therefore, the
use of optically active enantiomer is recommended under
such conditions, especially in United State of America and
in European countries. Even then it has been observed
that, sometimes, optically active drug enantiomer racemizes
into its antipode, leading to various side effects and
diseases. For example, ( )-form of thalidomide is harmful
when administered into pregnant women and that is why
(þ)-form is given under such situation. But, unfortunately,
(þ)-form of thalidomide also racemizes into ( )-enantiomer,
which is responsible for morning sickness. 3 In
addition to this, many optically active enantiomers of a variety
of drugs racemize into body of living beings, resulting
into various side effects and diseases.
Therefore, it is very important to study the racemization
of optically active drugs. It is difficult to study racemization
of drugs in vivo and therefore, studying it in vitro is
another means to deal with this problem. Many factors
such as drug concentration, pH, temperature, ionic concentration,
etc. are controlling factors of racemization process.
Therefore, to provide exact, safe and correct dose of
an optically active pharmaceutical, it is important to have
VC 2007 Wiley-Liss, Inc.
CHIRALITY 19:453–463 (2007)
the knowledge of their racemization. Many workers studied
the racemization of such type of drugs. In view of the
importance of the racemization of optically active drugs,
attempts have been made to review in vitro and in vivo racemization
of such drugs. A thorough search of literature
was carried out through analytical and chemical abstracts
along with the consultation of various refereed journals.
The present article describes in vitro and in vivo racemization
of optically active drugs with respect to various
operational variables. The various side effects due to racemization
have been included in this article. Attempts
have been made to discuss the mechanisms of racemization.
Besides, attempts have been made to suggest the
safe dosages of such type of drugs too.
HARMFUL AND DIFFERENT THERAPEUTIC
EFFECTS OF OPTICALLY ACTIVE DRUGS
As discussed in the introduction part of this article, one
of the enantiomers may be toxic or inactive or ballast
while its antipode may be useful therapeutically. This is
Contract grant sponsor: Uttarakhand State Council for Science and Technology.
*Correspondence to: Professor Hassan Y. Aboul-Enein, College of
Pharmacy, University of Sharjah, P. O. Box 27272, Sharjah, United Arab
Emirates. E-mail: hyaboulenein@yahoo.com
Received for publication 5 December 2006; Accepted 6 February 2007
DOI: 10.1002/chir.20397
Published online 28 March 2007 in Wiley InterScience
(www.interscience.wiley.com).

454 ALI ET AL.
TABLE 1. Harmful effects of racemization of optically active drugs
Optically active drugs Therapeutic group Species Harmful effects
( )-Thalidomide Immunomodulatory
Man Malformation of embryos
& antiangiogenic
in pregnant woman 2
( )-Ibuprofen Antianalgesic Man Inactive 2
D-DOPA Parkinson’s disease Man Inactive 8
L-Sucrose Sweeting agent Man Non-metabolized 9
D-Ribose sugar Sugar Man Less therapeutics 10
L-Penicillamine Antiarthritic Man Toxic 2
(þ)-Warfarin Anticoagulant Man Inactive 2
L-Peptide (V13KD) Antimicrobial Candida ablicans Trypsin proteolysis 11
S-Albuterol Adrenergic Rat Partially active 12
(þ)-Clausenamide Synaptic transducer Rat Low synaptic Transmission 13
S-Propranolol b-Blockers Man Toxic 14
R-Ketoprofen Antiallodynic Man Inactive 15
R-( )-Vigabatrin Antiepileptic Man Highly toxic 16
S-Tiaprofenic acid NASID Man Inactive 17
N-Alkylated
Antitumor & antimetastic Man Inactive 18
dihydro-pyridine-( )-AC 394
(þ)-Tramadol Analgesic Man Nausea & vomiting 19
R-CC-4047 Immunomodulatory Man Inactive 5
(þ)-Thyroxine Hormone Man Inactive 2
S-Flurbiprofen NSAID Man Inactive 20
( )-Ketamine Anesthetic Rat Inactive 2
(þ)-Methadone Analgesic Man Inactive 2
(þ)-Morphine Analgesic Man Inactive 2
( )-Tetramisole Anthelmintic Man Inactive 2
S-2-[2,6-dioxopiperidine-3-yl]-phthalimidine Sedative Monkey Teratogenic 4
R-Fenoprofen NSAID Rat Inactive 6
( )-Fluoxetine Anti-depressant Man Inactive 2
(þ)-Verapamil Calcium channel blocker Man Inactive 2
NASID: Non steroidal anti-inflammatory drug.
very important information in medicinal and clinical sciences.
And many authorities are asking data on the harmful
effects of enantiomers. But unfortunately, much work
could not be done in this direction and is under progress.
A warn example is of ( )-thalidomide which is teratogenic
in nature. 2,3 It is interesting to mention that the stereoselectivity
of teratogenic properties of thalidomide (S-thalidomide
as teratogenic enantiomer) had been observed only
after intaperitoneal application, and this result still needs
to be reconfirmed. L-Penicillamine is more toxic than its Dform.
R-( )-Vigabatrin is highly toxic in comparison to
its enantiomers. S-2-[2,6-dioxopiperidine-3-yl]-phthalimidine
(EM 12) was found to be teratogenic in nature. 4
Besides, some enantiomers are inactive as for example
Teo et al. 5 reported R-enantiomer of CC-4047 as less effective
in human. Similarly, Berry and Jamali 6 reported R-( )fenoprofen
as inactive antipode. Yoshida et al. 7 reported
that S-enantiomer of KE-298 reflected twofold greater area
under curve (AUC) value (drug plasma concentration with
time) than R-enantiomers after oral dosage in rats. The different
therapeutic properties of some drug enantiomers
are summarized in Table 1.
RACEMIZATION OF OPTICALLY ACTIVE DRUGS
As indicated earlier the knowledge of racemization of
optically active drugs is essential to provide safe dosage to
Chirality DOI 10.1002/chir
human beings. Therefore, it is customary to study first
the racemization in vitro followed by in vivo and, hence,
the racemization of optically active drugs is discussed in the
two sections i.e. in vitro and in vivo. Reist et al. 21 described
the meaning and importance of racemization, enantiomerization,
diastereomerization, and epimerization. The authors
defined the racemization as a macroscopic and statistical
reaction of irreversible change of one enantiomer into the
racemic form. And enantiomerization is a microscopic process
of reversible change of one enantiomer into other.
Wsol et al. 22 reviewed the chiral inversion of drugs and discussed
coincidence and principle of this phenomenon. The
authors discussed chiral inversion of 2-arylpropionic acid
derivatives. Effect of various parameters on the chiral inversion
and its mechanisms was also highlighted.
In Vitro Racemization
The racemization in test tube is called as in vitro racemization,
which provides rough knowledge of the behavior of
optically pure drugs. However, some workers studied in
vitro racemization of optically active drugs, which is discussed
herein. The study of ketorolac racemization was
performed by Brandl et al. 23 in aqueous solution at 258C
and 808C temperatures and reported a U type pH rate outline
at 808C. The authors also reported a T 90 value of the
drug in a solution of 1.5% (R)-ketorolac tromethamine at
pH 7.4 and 258C temperature. Aso et al. 24 reported the

Fig. 1. The effect of cofactors on the inversion of R-ibuprofen with rat
liver homogenate. (*P ¼ 0.0002 decrease in R-ibuprofen concentration,
**P ¼ 0.0008 increase in S-ibuprofen concentration). 20
effects of a-, b-, g- and dimethyl-b-cyclodextrins (CDs) and
liposomes on epimerization and racemization of chiral
drugs such as etoposide, ethiazide, and carbenicillin. The
rate of epimerization of carbenicillin increases with a- and
b-CDs but no effect was observed on epimerization of etoposide
and ethiazide. Dimethyl-b-cyclodextrin diminishes
the epimerization of etoposide and ethiazide but does not
affect epimerization of carbenicillin. g-CD decreases and
increases the epimerization of etoposide and of carbenicllin
respectively. The authors indicated that inclusion complexes
are responsible for the retardation of racemization
as CDs prevented the attack by bases and buffers.
The polymyxins B and E have complex structures of
polypeptide antibiotics. Polymyxin B contains B 1,B 2,B 3,
and B 4 components as major 25 while isoleucine-polymyxin
B 1 26 and polymyxin B5 and B 6 27 are minor constituents.
Similarly, polymyxin E 1 (colistin A) and polymyxin E 2 (colistin
B) are the main parts of polymyxin E whereas
polymyxin E 3 and E 4, 25 norvaline-polymyxin E 1, valinepolymyxin
E2, 26 valine-polymyxin E1, isoleucine-polymyxin
E 1 and isoleucine polymyxin E 2 28 are minor identities.
Orwa et al. 27 described the decomposition of polymyxin
with racemization as the principal mechanism of decomposition
at pH 1.4 and 7.4 pH. The traits of decomposition
were studied in aqueous solution at different pHs and temperatures.
At constant pH and temperature it was found
that the decomposition kinetics of polymyxins follows
pseudo first order kinetics.
Knihinicki et al. 20 studied the racemization of R-ibuprofen
in rat liver homogenate in the presence of ATP, CoA
and (ATP and CoA both) and in the absence of ATP and
CoA both for 30 min at 378C separately and respectively. A
remarkable change occurred in the concentration of Rand
S-ibuprofen enantiomers in the presence of both ATP
and CoA together (see Fig. 1). The inversion of R-ibuprofen
was not reported into kidney and intestine homogenates of
rat under the above cited four conditions. R-Ibuprofen was
mixed with rat liver homogenates having cofactors and
was observed a decrease in the concentration of R-ibupro-
RACEMIZATION IN OPTICALLY ACTIVE DRUGS DEVELOPMENT
455
fen after 10 min. No decrease was observed in the concentration
of R-ibuprofen in absence of CoA. The racemization
of R- and S-ibuprofen-CoA in buffer {TRIS HCl [tris-(hydroxymethyl)
methylamine hydrochloride)] 50 mM, pH
7.4} and in fresh human plasma is shown in Figures 2a
and 2b, which indicates the different pattern of racemization
under varying conditions. Other authors also reported
the inversion of R-ibuprofen in guinea pigs 29 with the observation
of stereospecific inversion by CoA. Caldwell
et al. 30 also reported the spontaneous racemization of Ribuprofen-CoA.
In 1989, Knihinicki et al. 20 studied in vitro
racemization of (R)-ibuprofen only. Furthermore, Knihinicki
et al. 31 carried out the racemization of R- and S-enantiomers
of 2-arylpropionic acids. The racemization and hydrolysis
processes in vitro condition have been performed
using rat liver homogenate and subcellular fractions. The
authors reported that the rat liver homogenate showed
racemization of (R)-ibuprofen-CoA to (S)-ibuprofen-CoA
Fig. 2. The racemization of R- andS- ibuprofen-CoA in (a) buffer (tris
HCl, 50 mM, pH 7.4) and (b) fresh human plasma. [R-Ibuprofen (*), Sibuprofen
(l) after mild alkaline hydrolysis and R-ibuprofen concentrations
(D) before alkaline hydrolysis.] 16
Chirality DOI 10.1002/chir

456 ALI ET AL.
Time (min)
TABLE 2. Enantiomeric ratio of Ibuprofen as
CoA-thioesters and free after incubation in liver
mitochondrial fraction of rat 31
(R)-Ibuprofen-CoA (S)-Ibuprofen-CoA
S/R CoA S/R Free R/S CoA R/S Free
10 0.76 0.61 0.77 0.67
20 0.62 0.69 0.58 0.78
30 0.70 0.67 0.56 0.77
60 0.56 0.67 0.63 0.77
with the amount of (S)-ibuprofen formed equal to lost
amount of (R)-ibuprofen. In the same way, (S)-ibuprofen-
CoA racemized to (R)-ibuprofen-CoA, and their rate of hydrolysis
and racemization was reported same as for
(R)- and (S)-ibuprofen-CoA. The mitochondrial fraction
mediated the racemization and hydrolysis of (R)- and (S)ibuprofen-CoA
in the same pattern as rat liver homogenate.
The enantiomer ratio of ibuprofen-CoA with rat liver
mitochondrial fraction was not altered even after 1 h period
of experiment. Table 2 31 shows the ratios of S/R CoA
as free S/R, which indicates different pattern of racemization
in rat liver mitochondrial fraction. A rapid racemization
and hydrolysis of (R)-ibuprofen-CoA to (S)-ibuprofen-CoA
was reported in microsomal fraction. (S)-Ibuprofen-CoA
racemized and hydrolyzed to (R)-ibuprofen-CoA with
microsomal fraction. But it was observed that the rate
of hydrolysis of (S)-ibuprofen was slower than that of
(R)-ibuprofen-CoA. In rat liver microsomal fraction, a
difference in enantiomer ratios was reported and given
in Table 3. 31 Knadler and Hall 32 reported the racemization
and hydrolysis of CoA thioesters of (R)-ibuprofen
and (R)-fenoprofen with rat liver microsomal and mitochondrial
fraction, which supported the findings of
Nakamura et al. 33 in 1981. Skalova et al. 34 reported chiral
inversion of R-(þ)-flobufen and (2S;4S)-dihydroflobufen
(DHF) in human hepatocytes. The authors also
reported the unidirectional chiral inversion of the enantiomers
of dihydroflobufen (DHF) as (2S;4S)-DHF to
(2R;4S)-DHF and from (2R;4R)-DHF to (2S;4R)-DHF in
hepatocyte cultures. The chiral inversion has also been
discussed for 2-aryloxypropionates 35,36 and for D-leucine
37 in human hepatocytes. Wsol et al. 38 studied in
vitro stereoselective biotransformation of flobufen enantiomers
in hepatocytes of male rat. The authors also
reported that bidirectional chiral inversion occurred in
flobufen enantiomers with hepatocytes.
Yang et al. 39–47 carried out a remarkable work on the
racemization of various optically active drugs. The
authors 45 reported that the conjugated benzylic C-H at chiral
center was responsible for the racemization reaction in
oxazepam. Yang and Lu 44 studied the temperature dependent
racemization of 3-methoxy-N-desmethyldiazepam in
acetonitrile and methanol having 0.5 M H 2SO 4. Yang and
Bao 42 reported base catalyzed racemization of 3-O-acyloxazepam.
The authors studied the kinetics of racemization in
alkaline solutions with and without buffered conditions.
The authors also indicated that in aqueous solutions the
Chirality DOI 10.1002/chir
racemization process took place with changing rates at pH
7.5–14. The authors also suggested that this racemization
reaction occurred due to keto-enol tautomerism between
C 2 carbon of CO group and C 3 carbon catalyzed by a
base. Similarly, Yang 43 reported acid catalyzed racemization
of 3-O-methyloxazepam in ethanol and 3-O-ethyl oxazepam
in methanol. The authors suggested that the above
cited reaction occurred via C 3 carbocation intermediate.
Yang et al. 41 reported that the enantiomers of oxazepam
(OX) and temazepam (TMZ) showed the racemization
reactions 40 times faster than of hydrolysis of these racemates.
The authors added that only hydrolysis results
showed racemization reaction. Furthermore, Yang 40 studied
the kinetics of spontaneous racemization and stereoselective
conversion of temazepam (TMZ) enantiomers to
3-O-methyltemazepam (MeTMZ) and 3-O-ethyltemazepam
(EtTMZ). The authors also reported that N 4-protonated
and unprotonated enantiomers of (S)-TMZ showed spontaneous
racemization. A highly stereoselective nature was
shown by three S-OH group of (S)-TMZ giving EtTMZ as
a substituted product, which has greater proportion of (S)-
EtTMZ. Sulla 48 reported that various aliphatic acid chlorides
[RCH(R’)COCl], having a chiral center adjacent to
carboxylic group, showed a high degree of racemization
with strong acids.
Ferorelli et al. 49 studied racemization of optically active
acid chlorides of clofibric acid with 3-tropanol. The authors
reported that methanolic KOH hydrolysis of the methyl
esters of 2-(4-chlorophenylthio)propanoic acid was produced
as racemic mixture. The partial racemization of the
above cited drug was reported with aminoalcohols as free
base for the corresponding esters and no racemization
was reported with aminoalcohols as hydrochloride salts. 50
Mey et al. 51 reported 23–25 min as half life of racemization
for (þ)- and ( )-diethylpropion (DEP) in human plasma.
Tumambac et al. 52 reported half life of 2-benzoylcylohexanone
in hexanes and ethanol as 552 and 23.8 min respectively
at 668C. Fernandez et al. 53 reported increasing racemization
of zopiclone (ZOP) enantiomers with increasing
pH and temperature. Lamparter et al. 54 studied the racemization
of (þ)-chlorthalidone with variation of pH and
reported a minimum pH 3 for log K/pH curve. The
authors also discussed that the rate of racemization
decreased with increasing liposome concentration. Teo
et al. 5 reported the racemization of S-enantiomer of CC-
4047 in phosphate buffer. The authors claimed S-enan-
Time (min)
TABLE 3. Enantiomeric ratio of Ibuprofen as
CoA-thioesters and free after incubation in liver
microsomal fraction of rat 31
(R)-Ibuprofen-CoA (S)-Ibuprofen-CoA
S/R CoA S/R Free R/S CoA R/S Free
2 0.19 0.18 0.10 1.30
5 0.22 0.19 0.19 0.57
10 0.27 0.18 0.23 0.70
20 0.47 0.19 0.21 0.98
30 0.56 0.21 0.13 0.95
60 0.76 0.25 0.10 0.82

TABLE 4. Racemization of different drugs in phosphate
buffer (0.01 m) at pH 7.4 a62
Compound
tiomer as more powerful than racemic mixture. Murakami
et al. 55 reported the racemization of R-t-butyl-2-(3,4-O-carbonyldioxy-phenyl)-2-(phthalimidooxy)
acetate in diethylketone
in presence of small amount of 1,8-diazabicyclo[5.4.0]undec-
7-ene. Gaitani et al. 56 studied the epimerization of thioridazine
2-sulfoxide (THD 2-SO) in human plasma, buffer and
methanolic solutions under different experimental conditions
of incubation. The authors reported that both enantiomers
of THD-2-SO were stable at varying temperature,
pH and ionic strength but at pH 8.5 solubility problems
were reported. In the presence of the enantiomer of THD or
racemic mixture in the incubation mixture, the valuable differences
were reported in the formation of THD metabolites
in vitro stereoselective THD metabolism. 57 Knoche et al. 58
reported the racemization of thalidomide in phosphate buffer
of pH 7.4 at 378C temperature. 2-Phthalimidinoglutarimide
and 2-phthalimidoadipinimide (derivatives of thalidomide)
showed a rapid racemization. 59,60 Nunes et al. 61 described
the epimerization reaction of pilocarpine (2S:3R) at C-2
chiral atom at pH 7.4 and 358C with half life of 36 days.
Pepper et al. 62 reported in vitro racemization of aromatase
inhibitors i.e. 3-(4-aminophenyl)-pyrrolidine-2,5-dione, its Npentyl
analogue and antifungal econazoles (having benzylic
protons) at pH 7.4 in phosphate buffer. The authors
described that (þ)- and ( )-3-(4-aminophenyl)pyrrolidine-
2,5-dione (WSP-3; I), (þ)- and ( )-1-pentyl-3-(4-aminophenyl)pyrrolidine-2,5-dione
(WSP-3; II) and (þ)- and ( )-econazoles
(III) enantiomers showed racemization at physiological
pH (7.4) at room temperature (208C) and at 378C. But (þ)- and
( )-1-[(Benzofuran-2-yl)-4-chlorophenylmethyl] imidazole
(IV) enantiomers were stable at 378C. The results are given
in Table 4, 62 which indicate twofold racemization rates at
378C in comparison to room temperature. The following
equation was used for the calculation of rate constant and
half life in the conversion of an enantiomer to racemate:
k ¼ 2:303
t
0:5a
log
ð0:5a xÞ
t 1/2 (h)
Room temperature 378C
(þ)-WSP3 (I) 7 (60.12) 4 (60.11)
( )-WSP3 (I) 7 (60.14) 4 (60.12)
(þ)-Pentyl (II) 6 (60.17) 3 (60.10)
( )-Pentyl (II) 6 (60.19) 3 (60.09)
(þ)-Econazole (III) 5 (60.08) 2.64 (60.07)
( )-Econazole (III) 5 (60.10) 2.64 (60.09)
a Solutions (50 nM) of the pure enantiomers of (I), (II), and (III) in phosphate
buffer (0.01 M, pH 7.4) stored at 20 and 378C. Samples (20 ll) were
injected on the AGP column at pre-determined intervals. The mobile
phase was phosphate buffer (0.01 M) pH 7.4 having (I), 5% 2-propanol, (I),
12% 2-propanol, and (III), 10% acetonitrile, respectively.
where a, initial concentration of the enantiomer; x,
decrease in enantiomer concentration with time ’t’; k,
pseudo first order rate constant of the reaction.
RACEMIZATION IN OPTICALLY ACTIVE DRUGS DEVELOPMENT
457
In Vivo Racemization
After in vitro study, in vivo racemization of optically
active drugs is necessary to establish the exact mechanism
and effect of optically active drugs in animals
including human beings. It provides knowledge of drug
formulations of optically active pharmaceuticals. Therefore,
many workers tried to study the racemization of
enantiomers in various animals. Some research work carried
out in this area is discussed as follows.
The chiral inversion of R-ibuprofen in rat is studied
by Sanins et al. 63 and the authors reported that the CoA
thioester of R-ibuprofen was a principal metabolite in the
inversion reaction, which tautomerized during in vivo racemization.
Berry et al. 6 reported a chiral inversion of R-( )fenoprofen
in intestine and liver of rat. The chiral inversion
of ibuprofen through enzymes was reported in human
beings. 64,65 Further, Lee et al. 66 found that the inversion of
ibuprofen was 57%–71%. Many workers reported bidirectional
chiral inversion of ketoprofen in mice, 15 ibuprofen in
rats, guinea pigs and rabits 67 and tiaprofenic acid in rats. 17
Aboul-Enein et al. 68 established that many 2-arylpropionic
acid derivatives (also called profens) showed metabolic
chiral inversion. The chiral inversion of R-( )-enantiomers
was reported as an important in the metabolism of 2-arylpropionic
acids. 64,65,69
In various animal species and
humans, ibuprofen was most widely studied with unidirectional
chiral inversion. 31,63–65,69–77 The unidirectional chiral
inversion of R-fenoprofen to its active antipode, with
high interspecies changes in the inversion magnitude,
was reported in rat, 6,78–80 guinea pig, 80,81 cat 82 and humans.
83,84 In rats 85 and humans, 86 benoxaprofen showed
stereospecific inversion from R- toS-enantiomer with high
inversion rate in rats than humans. Trejtnar et al. 87 discussed
the chiral inversion of flobufen [4-(20 ,40-difluorobi phenyl-4-yl)-2-methyl-4-oxobutanoic acid] in male wistar
rat, after administration of racemic mixture and individual
enantiomers of the drug. The authors reported the ratio of
S/R enantiomer as 13.3 after racemic flobufen administration.
The authors suggested that the rapid changes in S/R
enantiomer ratio occurred due to the chiral inversion or
other stereoselective process in the pharmacokinetics of
flobufen enantiomers (see Fig. 3). In vivo chiral inversion
has been discussed for several 2-arylpropionic acids. 88,89
Pepper et al. 62 reported in vivo racemization of (þ)-econazoles
in rats and the authors described 1.24 h as half life.
Kaneko et al. 90 studied the racemization of non-toxic [D-
Ser (26)]-b-amyloid 1-40 to the toxic form and truncated
peptides in human, which increased the efficiency of neurons
towards those amino acids having exciting power. The
authors discussed the possibility of formation of the soluble
[D-Ser (26)] b-amyloid 1–40 in old men suffering from Alzheimer’s
disease. Further, Takekazu et al. 91 reported the
modification of racemization reaction of serine and asparagines
of b-amyloid. Schmahl et al. 92 studied the racemization
of 2-(2, 6-dioxopiperidine-3-yl)-phthalimidine (EM 12)
enantiomers in marmoset monkey. The highest plasma concentration
and plasma AUC values of both enantiomers;
produced through chiral inversion; were 13% and 21% and
24% and 30% respectively. The plasma pharmacokinetic data
Chirality DOI 10.1002/chir

458 ALI ET AL.
Fig. 3. Rapid changes in S/R enantiomer concentration ratio within
first minute after racemic flobufen administration in rat. 87
were reported in the same range. Further, in 1996 Schmahl
et al. 93 studied the racemization of EM 12 enantiomers in
monkey, rat and mouse. R- and S-Enantiomers showed their
appearance in embryo of marmoset monkey, wistar rat and
NMRI mouse in organogenesis duration. The gestation day
of monkey, rat, mouse were 61, 12, 10 respectively. The
presence of EM 12 enantiomers was reported in maternal
plasma, placenta and embryo in marmoset monkey and
wistar rat. Low concentrations of EM 12 metabolites were
identified in plasma and embryo of rat and monkey, evaluating
the teratogenic effect of parent drug. Cuyue et al. 94
reported that blood samples of rat have only S-stiripentol
[(S)-STP] after oral administration of [(S)-STP]. But with a
same dose of [(R)-STP], samples showed the presence of
both R- andS-enantiomers indicating the racemization of Rform
only. The authors also suggested the entry route of
chiral inversion of stiripentol R-enantiomer. After oral dose
of any enantiomer of the drug, the authors observed that
drug has become enriched with R-form; during the course
of intestinal movement. Furthermore, the authors described
that acid catalyzed racemization and enantioselectivity
were main factors contributing metabolic chiral inversion
of (R)-STP. The study of degradation and racemization
of thioridazine (THD) and thioridazine 2-sulfone [THD 2-
SO 2 ] in human plasma and aqueous solutions has been performed
by de Gaitani et al. 95
The rate of racemization of N-a-phthalimidoglutarimide
(thalidomide) was studied by Nishimura et al. 96 and a half
life period of 566 min was reported at pH 7.4 and 378C
temperature. Heger et al. 97 studied racemization of S-( )-
EM 12, which was responsible for limb defects as amelia,
phocomelia and radius aplasia. The authors also reported
that no exposed fetus was free from skeletal defects.
Papini et al. 98 studied the pharmacokinetic racemization of
lorazepam in pregnant women and data given in Table 5
about phamacokinetics of lorazepam and its metabolite
(lorazepam-glucuronide) in pregnant women indicate racemization
process. Similarly, Pham-Huy et al. 99 reported
racemization of lorazepam in polar media. Teo et al. 5
Chirality DOI 10.1002/chir
reported the racemization of S-enantiomer of CC-4047 in
human plasma. Hainzl et al. 100 studied the possibility of
racemization of carbamazepine derivatives in human.
To make in vivo study more clearly to the readers one
experimental methodology used by Papini et al. 98 is summarized
in this paragraph briefly. The authors selected
10 healthy parturients aged 18–37 yr with a gestational
age of 36–40.1 wk and treated with single oral dose of
2.0 mg racemic lorazepam 2 to 9 h before delivery. The
maternal venous blood samples were collected via a venous
catheter at time 0, 0.5, 1, 2, 3, 4, 6, 8, 12, 30, 36, and
48 h. The blood samples were also collected from the umbilical
vein after clamping. The urine samples were collected
at 12 h interval up to 48 h after lorazepam administration.
Heparin was used as an anticoagulant for blood
samples. The plasma and urine samples were separated by
centrifugation at 2000g for 10 min and stored at 208C.
The chiral analysis of lorazepam enantiomers was carried
out by using LC-MS technique.
EFFECT OF DIFFERENT VARIABLES ON
RACEMIZATION
In vitro racemization is controlled by pH of the solution,
temperature, ionic balance and concentration of the drug
itself. On the other hand, in vivo racemization is difficult to
control in animals; specially in human beings. However,
some workers tried to study the effect of various variables
on the racemization of optically active drugs. The effect of
some variables on racemization has been discussed in section
3.1 and 3.2 briefly but this section describes the effect
of these variables in detail. El-Nimr et al. 101 described the
racemization of L-noradrenaline bitartrate in the presence
of hydronium ion. Authors discussed the effect of ionic
concentration on the racemization of the above cited drug.
The kinetic study of racemization of chiral drugs such as
carbenicillin, ethiazide, etoposide and oxazepam acetate in
human serum albumin was studied by Aso et al. 102 Matsuo
et al. 103 studied the kinetics of racemization of meluadrine
tartrate at 1.2 to 12 pH and 40, 60, 808C in aqueous solu-
TABLE 5. Kinetic disposition of Lorazepam and its
metabolite glucuronide in parturients 98
Lorazepam
isomeric mixture
Lorazepam-glucuronide
isomeric mixture
Cmax (ng/ml) 12.96 (9.42–16.49) 35.55 (8.27–62.83)
tmax (h) 3.10 (2.57–3.63) 4.33 (2.90–5.77)
t1/2a (h) 3.16 (2.62–3.68) 1.37 (1.15–1.58)
Ka (h 1 ) 0.23 (0.19–0.28) 0.52 (0.44–0.59)
t1/2b (h) 10.35 (9.39–11.32) 18.17 (14.10–22.23)
b (h 1 ) 0.068 (0.061–0.075) 0.039 (0.032–0.047)
AUC 0-a
[(ng h)/mL]
175.25 (145.74–204.75) 481.19 (252.87–709.51)
ClT/ F [mL/
(min kg)]
2.61 (2.34–2.88) –
Vd/F (1) 178.78 (146.46–211.10) –

Fig. 4. The effect of pH on the K obs of the polymyxins B 1,E 1,andE 2 at
378C. 104
tion. The racemization rate constants were found to be
minimum between pH 4 and 6 and maximum at pH 9.
Orwa et al. 104 studied the effect of pH on the K obs of the
polymyxins B 1,E 1, and E 2 at 378C and is given in Figure 4,
which indicates high value of Kobs at high pH. Figure 5
shows the pseudo first order kinetics of the decomposition of
polymyxin B 1 at pH 1.4 and 608C. Guo et al. 105 described the
effects of solvent on 2,2-dimethyl-1,3-dioxalane-4-methanol
(DDM), which epimerizes according to relative stabilization
of reaction intermediate of DDM epimerization reaction.
Yang and Lu 44 studied the temperature dependent racemization
of 3-methoxy-N-desmethyldiazepam in acetonitrile and
methanol having 0.5 M H 2SO 4. The racemization reaction followed
first order kinetics. Thermodynamic parameters were
DE (18.8 Kcal/mol), DH (18.3 Kcal/mol), DS ( 14.8 Kcal/
mol) and DG (22.7 Kcal/mol) respectively. The authors
reported racemization through carbon intermediate.
Tumambac et al. 52 described the unexpected solvent
effect (mixtures of hexanes and ethanol) on the half-lives
(11.5 and 24.0) of 2-benzoylcylohexanone. The reason for
RACEMIZATION IN OPTICALLY ACTIVE DRUGS DEVELOPMENT
459
such behavior of solvent mixtures was described as complex
isomerization mechanisms with three possible interchanging
enol tautomers of 2-benzoylcyclohexanonone.
Welch et al. 106 reported inter-conversion of enantiomer of
5-aryl-thiazolidinedione with different conditions of solvents,
aqueous solutions irrespective of pHs. The authors
described a rapid racemization in many solvents and in
dog and human plasma. Mey et al. 51 studied the kinetics
of racemization of (þ)- and ( )-diethylpropion (DEP) and
reported that on increasing pH and phosphate buffer concentration,
the rate of racemization increased in aqueous
solution but in cyclodextrins (CDs), the racemization rates
of the enantiomers of DEP did not vary so much.
Blaschke et al. 107 studied the effect of cyclodextrins
(CDs) and its alkylated and hydroxyalkylated derivatives
on racemization of tropic acid alkaloids. The authors
reported that all different cyclodextrins, except a-cyclodextrin,
decreased racemization reaction. This may be due to
the inclusion of the drug in cyclodextrins, which prevents
the attack of OH and/or water molecule. The racemization
of these drugs was pH and temperature dependent.
Vakily et al. 108 studied the effect of pH and ionic strength
on the racemization of ketorolac in human and rat. The
authors reported that the racemization of the ketorolac
occurred at high pH and ionic strength. Gaitani et al. 95
reported the stability of the enantiomers of thioridazine
(THD) and thioridazine 2-sulfone [THD 2-SO (2)] in
human plasma at different temperature, pH and ionic
strength. The authors described that enantiomers of THD
showed a degradation with UV light but the enantiomers
of THD 2-SO 2 were found stable for UV as well as visible
light. The chiral inversion of certain anti-inflammatory
drugs was reported as unidirectional in rat, 109 rabbit and
humans. 110 The important factors affecting racemization of
these drugs were temperature, pH 43,46,111–115 and organic
solvent. 43,46,101,111–113 In drug racemization and epimerization,
the important action of human serum albumin (HSA)
was also reported. 102 It has been discussed that HSA retarded
the racemization of ( )-ethiazide to (þ)-ethiazide.
The effect of various variables on racemization of some
drugs is given in Table 6.
Fig. 5. Plot for the first order kinetics of the decomposition of polymyxin
B 1 at pH 1.4 and 608C. 104
Chirality DOI 10.1002/chir

460 ALI ET AL.
TABLE 6. Effect of experimental variables on racemization
Drugs Parameters
Effect of pH Low pH High pH
Zopiclone
enantiomers (ZOP)
Decrease Increase 53
Meluadrine tartrate Minimum Maximum 103
Diethylpropion(DEP) Decrease Increase 51
Polymyxin B1,E1, and E2 Decrease Increase 27
Ketorolac No Racemization Racemization 108
Effect of temperature Low
High
temperature temperature
Zopiclone enantiomers
(ZOP)
Low High 53
3-(4-Aminophenyl)
pyrrolidine-2,5-dione
Low High 62
1-Pentyl-3-(4-aminophenyl)
pyrrolidine-2,5-dione
Low High 62
Econazoles Low High 62
Diethylpropion(DEP) Low High 51
Effect of ionic
Low ionic conc. High ionic
concentration
conc.
Diethylpropion(DEP) High Low 51
Ketorolac No
Racemization
Racemization 108
Other effects
– Rate of racemization of chlorthalidone decreases with
increasing liposomes concentration. 54
– Except a-cyclodextrin, all CDs decreases the
racemization of tropic acid alkaloids. 107
– Human serum albumin retards the racemization of ethiazide. 102
– A decrease in racemization of R-ibuprofen was observed in the
presence of cofactors. 20
MECHANISMS OF RACEMIZATION
Mechanism is an integral part of any racemization,
which is required for the beneficial use of research for society.
Therefore, the mechanism of racemization was studied
by a number of workers and some of them are discussed
here. Sepetov et al. 116 reported a mechanism of
racemization of peptides in aqueous solution. The authors
observed broad racemization of amino acid in the proposed
mechanism. In this mechanism a diketopiperazinelike
[DKP-like] intermediate is formed having a secondary
amino group. Further, the intermediate forms a racemic
product by attack of OH group on the carbonyl group of
DKP-like ring. Lambert et al. 117 reported the kinetics and
mechanism of racemization process of ibutilide. After the
removal of hydroxyl group, a carbo-cation is formed as an
intermediate, which is attacked by nucleophilic amine.
The authors also reported that racemization reaction
occurred through a direct SN 2 reaction via carbo-cation intermediate.
The processes of racemization, stereoselective
nucleophilic substitution of the enantiomers and racemic
oxazepam (OX), 3-O-methyloxazepam (MeOX) and 3-Oethyloxazepam
(EtOX) in anhydrous acidic methanol and
ethanol, were reported by Yang et al. 39 The kinetics of racemization
and nucleophilic substitution processes were
also studied with deuterated and non-deuterated solvents.
Chirality DOI 10.1002/chir
The authors reported that many reactions occurred when
(S)-OX was dissolved in acidic methanol including spontaneous
racemization of (S)-OX. These contain (1) stereoselective
substitution of 3-methoxy group of (S)-MeOX by
methoxy group of methanol to give MeOX having (S)-
MeOX predominantly and (2) stereoselective substitution
of 3-methoxy group of (R)-MeOX by methoxy group of
methanol to give MeOX having (R)-MeOX predominantly.
A racemic MeOX was reported by recurrence of (1) and
(2) reactions. The authors also reported that same reactions
took place with acidic ethanol for the enantiomers
of OX. Orwa et al. 104 reported that in the decay process of
polymyxins, racemization was the main mechanism of
decomposition process in neutral and acidic conditions.
Knihinicki et al. 20 studied in vitro mechanism of the inversion
of 2-arylpropionic acid (ibuprofen) enantiomers by
using rat liver homogenates as medium and found the
inversion of R to S enantiomer but not vice versa. The
authors also reported that there is no inversion of R-ibuprofen
with kidney or small intestine homogenates. The
authors also studied the mechanism of inversion of arylpropionic
acids proposed by many workers (see Fig.
6). 22,31 Ferorelli et al. 49 described that racemization reactions
of amino alcohols occurred through intramolecular
hydrogen bonding followed by ketene intermediate. Severian
118 reported a dynamic equilibrium of the enantiomers
of chlorthalidone in aqueous medium via a carbonium ion
intermediate. The authors reported DG value as 21.6 Kcal/
mol for the inversion at carbon. Stepensky et al. 119 studied
the kinetics of sulfonation and racemization process of the
L-adrenaline decay with D-adrenaline, L- and D-adrenaline
sulfonate as decay products.
IMPORTANCE AND CONSEQUENCES OF
RACEMIZATION IN DRUG DEVELOPMENT
Chiral drug pharmacokinetics and pharmacodynamics
phenomenon have wide range implications in practical
therapeutics, health care and pharmacy and psychiatry
practices. Therefore, the need of optically active pure dosage
(homochiral drugs) market is increasing continuously.
The knowledge of racemization of optically active drugs is
very important in drug design and development. As discussed
above in detail that, sometimes, racemization leads
to new antipode, which may be inactive or toxic. Therefore,
the role of racemization in chiral drug design and development
is important to be studied. Sometimes, the implications
of racemization are serious, which result into lethal
side effects. In summary, racemization study of chiral drugs
is an essential issue to be considered in modern medical science
for the development of new homochiral drugs.
CONCLUSION
Because of strict regulations of FDA and some European
agencies regarding the marketing of optically active
pharmaceuticals, chiral drugs are sold as their pure forms
i.e. ( )- or (þ)-enantiomer. But before taking/injecting
such drugs into our bodies, it is important to know the

fate of the enantiomers, which may under go racemization,
resulting into another antipode (may be toxic). Therefore,
racemization study is becoming essential in modern pharmaceutical
science. As discussed in this article some workers
attempted in vitro and in vivo racemization of various
drugs but still need more studies. Briefly, racemization
study is the need of today, to formulate dosages of optically
active drugs, and should be carried out for all the
optically active drugs.
LITERATURE CITED
1. Günther K. Enantiomer separations. In: Sherma J, Fried B, editors.
Hand book of TLC. Marcel Dekker: New York; 1991. p 541.
2. Aboul-Enein HY, Ali I. Chiral separations by liquid chromatography
and related technologies. Marcel Dekker: New York; 2003.
3. Sergio F. Mechanism of the teratogenic action of thalidomide. Acta
Vitaminologica et Enzymologica 1967;21:123–132.
4. Heger W, Klug S, Schmahl HJ, Nau H, Merker HJ, Neubert D.
Embryotoxic effects of thalidomide derivatives on the non-human primate
Callithrix jacchus; 3. Teratogenic potency of the EM 12 enantiomers.
Arch Toxicol 1988;62:205–208.
5. Teo SK, Chen Y, Muller GW, Chen RS, Thomas SD, Stirling DI,
Chandula RS. Chiral inversion of the second generation IMiD CC-
4047(ACTIMID) in human plasma and phosphate buffered saline.
Chirality 2003;15:348–351.
6. Berry BW, Jamali F. Presystemic and systemic chiral inversion of R-
( )-fenoprofen in the rat. J Pharmacol Exp Ther 1991;258:695–701.
7. Yoshida H, Kohno Y, Endo H, Hasegawa M, Suwa T. Streoselective
pharmacokinetics of [ 14 C]-labelled KE-298, a new anti-rheumatic
drug, in rats. Chirality 1996;8:258–263.
8. Jakubke HD, Jeschkeit H. Amino acids, peptides and proteins. The
Macmillan Press: London; 1977.
9. Davies SG, Brown JM, Pratt AJ, Fleet G. Asymmetric synthesis—
Meeting the challenge. Chem Br 1989;25:259–263.
10. Tschamber T, Gessier F, Dubost E, Newsome J, Tarnus C, Kohler J,
Neuburger M, Streith J. Carbohydrate transition state mimics: synthesis
of imidazolo- pyrrolidinoses as potential nectrisine surrogates.
Biorg Med Chem 2003;11:3559–3568.
11. Hamamoto K, Kida Y, Zhang Y, Shimizu T, Kuwano K. Antimicrobial
activity and stability to proteolysis of small linear cationic peptides
with D-amino acid substitutions. Microbiol Immunol 2002;46:741–
749.
12. Gumbhir-Shah K, Kellerman DJ, DeGraw S, Koch P, Jusko WJ. Pharmacokinetic
and pharmacodynamic characteristic and safety of
inhaled albuterol enantiomers in healthy volunteers. J Clin Pharmacol
1998;38:1096–1106.
13. Liu S, Zhang J. Difference between the effects of ( ) clausenamide
and (þ) clausenamide on the synaptic transmission in the dentate
gyrus of anesthetized rats. Yao Xue Xue Bao 1998;33:254–258.
14. Davis WM, Hatoum NS. Lethal synergism between morphine or
other narcotic analgesics and propranolol. Toxicology 1979;14:141–
151.
15. Jamali F, Lovin R, Aberg G. Bi-directional chiral inversion of ketoprofen
in CD-1 mice. Chirality 1997;9:29–31.
RACEMIZATION IN OPTICALLY ACTIVE DRUGS DEVELOPMENT
Fig. 6. Mechanism of inversion of aryl propionic acids. 22,31
461
16. Sheean G, Schramm T, Anderson DS, Eadie MJ. Vigabatrin plasma
enantiomer concentrations and clinical effects. Clin Exp Neurol
1992;29:107–116.
17. Erb K, Burger R, Williams K, Geisslinger G. Stereoselective disposition
of tiaprofenic acid enantiomers in rats. Chirality 1999;11:103–108.
18. Ohishi K, Morinaga Y, Ohsumi K, Nakagawa R, Suga Y, Tsuji T,
Akiyama Y, Tsuruo T. Potentiation of antitumor and antimetastatic
activities of adriamycin by a novel N-alkylated dihydropyridine,
AC394, and its enantiomers in colon cancer-bearing mice. Cancer
Chemother Pharmacol 1996;38:446–452.
19. Raimundo JM, Sudo RT, Pontes LB, Antunes F, Trachez MM, Zapta-
Sudo G. In vitro and in vivo vasodilator activity of racemic tramadol
and its enantiomers in Wistar rats. Eur J Pharmacol 2006;530:117–
123.
20. Knihinicki RD, Williams KM, Day RO. Chiral inversion of 2-aylpropionic
acid non steroidal anti-inflammatry drugs-1: in vitro studies of
ibuprofen and flurbiprofen. Biochem Pharmacol 1989;38:4389–4395.
21. Reist M, Testa B, Carrupt PA, Jung M, Schuring V. Racemization,
enantiomerization, diastereomerization, and epimerization: their
meaning and pharmacological significance. Chirality 1995;7:396–400.
22. Wsol V, Skalova L, Szotakova B. Chiral inversion of drugs: coincidence
or principle? Curr Drug Metab 2004;5:517–533.
23. Brandl M, Conley D, Johnson D, Johnson D. Racemization of ketorolac
in aqueous solutions. J Pharm Sci 1995;84:1045–1048.
24. Aso Y, Yoshioka S, Takeda Y. Epimerization and racemization of
some chiral drugs in the presence of cyclodextrin and liposomes.
Chem Pharm Bull 1989;37:2786–2789.
25. Thomas AH, Thomas JM, Holloway J. Microbial and chemical analysis
of polymyxin B and polymyxin E (colistin) sulfates. J Analyst
1980;105:1068–1075.
26. Elverdam I, Larsen P, Lund E. Isolation and characterization of three
new polymyxins in polymyxins B and E by high performance liquid
chromatography. J Chromatogr 1981;218:653–661.
27. Orwa JA, Govaerts C, Bussan R, Roets E, Schepdael AV, Hoogmartens
J. Isolation and structural characterization of polymyxin B components.
J Chromatogr A 2001;912:369–373.
28. Ikai Y, Oka H, Hayakawa J, Kawamura N, Mayumi T, Suzuki M, Harada
K. Total structures of colistin minor components. J Antibiot
1998;51:492–498.
29. Greig ME, Griffin RL. Antagonism of slow reacting substance of anaphylaxis
(SRS-A) and other spasmogens on the guinea pig tracheal
chain by hydratropic acids and their effects on anaphylaxis. J Med
Chem 1975;18:112–116.
30. Caldwell J, Hutt AJ, Fournel-Gigleux S. The metabolic chiral inversion
and dispositional enantioselectivity of the 2-arylpropionic acids
and their biological consequences. Biochem Pharmacol 1988;37:105–
114.
31. Knihinicki RD, Day RO, Williams KM. Chiral inversion of 2-arylpropionic
acid non steroidal anti-inflammatry drugs-II: racemization and
hydrolysis of (R)- and (S)-ibuprofen-CoA thioesters. Biochem Pharmacol
1991;42:1905–1911.
32. Knadler MP, Hall SD. Stereoselective arylpropionyl-CoA thioester formation
in vitro. Chirality 1990;2:67–73.
33. Nakamura Y, Yamaguchi T, Takahashi S, Hashimoto S, Iwatani K,
Nakagawa Y. Optical isomerization mechanism of R( )-hydratropic
acid derivatives. Pharmacobiodyn 1981;4:S-1.
Chirality DOI 10.1002/chir

462 ALI ET AL.
34. Skalova L, Kral R, Szotakova B, Babu YN, Pichard-Garcia L, Wsol V.
Chiral aspects of metabolism of anti-inflammatory drug flobufen in
human hepatocytes. Chirality 2003;15:433–440.
35. Mason JP, Hutt AJ. Stereochemical aspect of drug metabolism. In:
Aboul-Enein HY, Wainer IW, editors. The impact of stereochemistry
on drug development and use. New York: Wiley; 1997. p 45–123.
36. Zheng H, Jiang C, Chiu MH, Covey JM, Chan KK. Chiral pharmacokinetics
and inversion of a new quinoxaline topoisomerase II b position
in the rat. Drug Metab Dispos 2002;30:344–348.
37. Hasegawa H, Matsukawa T, Shinohara Y, Hashimoto T. Assessment
of the metabolic chiral inversion of D-leucine in rat by gas chromatography-mass
spectrometry combined a stable isotop dilution analysis.
Drug Metab Dispos 2000;28:920–924.
38. Wsol V, Kral R, Skalova L, Szotakova B, Trejtnar F, Flieger M. Stereospecificity
and stereoselectivity of flobufen metabolic profile in male
rats in vitro and in vivo: phase I of biotransformation. Chirality
2001;13:754–759.
39. Yang SK, Bao Z, Shou M. Stereoselective nucleophilic substitution of
oxazepam and racemization in acidic methanol and ethanol. Chirality
1996;8:214S–223S.
40. Yang SK. Racemization and stereoselective alcoholysis of temazepam.
Chirality 1999;11:179–186.
41. Yang SK. Substitution and racemization of 3-hydroxy- and 3-alkoxy-
1,4-benzodiazepines in acidic aqueous solutions. Chirality 1995;7:
365–375.
42. Yang SK, Bao Z. Base-catalysed racemization of 3-O-acyloxazepam.
Chirality 1994;6:321–328.
43. Yang SK. Acid-catalysed stereoselective heteronucleophilic substitution
and racemization of 3-O-methyloxazepam and 3-O-ethyloxazepam.
Chirality 1994;6:175–184.
44. Yang SK, Lu XL. Acid-catalysed nucleophilic substitution and racemization
of 3-methoxy-N-desmethyldiazepam enantiomers in methanol.
Chirality 1993;5:91–96.
45. Yang SK, Lu XL. Racemization kinetics of enantiomeric oxazepams and
stereoselective hydrolysis of enantiomeric oxazepam 3-acetate in rat
liver microsomes and brain homogenate. J Pharm Sci 1989;78:789–795.
46. Yang SK, Lu XL. Resolution and stability of oxazepam enantiomers.
Chirality 1992;4:443–446.
47. Yang SK. Mechanism of racemization of 3-hydroxy-1,4-benzodiazepines
in alcoholic solvents. Enantiomer 1998;3:485–490.
48. Sulla TV. Racemizzazione in ambiente acido dei cloruri del tipo
RCH(R’)COCl. Ric Sci 1962;32:181–183.
49. Ferorelli S, Loiodice F, Longo A, Molfetta A, Tortorella V, Amoroso
R. Different behavior toward racemization in basic media from chiral
analogs of clofibric acid, the active metabolite of the antilipidemic
drug clofibrate. Chirality 2000;12:697–704.
50. Gualtieri F, Bottalico C, Calandrella A, Dei S, Giovannoni MP, Mealli
S, Romanelli MN, Scapecchi S, Teodori E, Galeotti N, Ghelardini C,
Giotti A, Bartolini A. Presynaptic cholinergic modulators as potent
cognition enhancers and analgesic drugs. 2. 2-Phenoxy-, 2-(Phenylthio)-,
and 2-(Phenylamino)alkanoic acid esters. J Med Chem
1994;37:1712–1719.
51. Mey B, Paulus H, Lamparter E, Blaschke G. Kinetics of racemization
of (þ)- and ( )-diethylpropion: studies in aqueous solution, with and
without the addition of cyclodextrins, in organic solvents and in
human plasma. Chirality 1998;10:307–315.
52. Tumambac GE, Francis CJ, Wolf C. Configurational stability of 2-benzoylcyclohexanone:
unexpected solvent effects on the rate of racemization.
Chirality 2005;17:171–176.
53. Fernandez C, Gimenez F, Mayrargue J, Thuillier A, Farinotti R. Degradation
and racemization of zopiclone enantiomers in plasma and
partially aqueous solutions. Chirality 1995;7:267–271.
54. Lamparter E, Blaschke G, Schluter J. Racemization of chlorthalidone
in the presence of liposomes. Chirality 1993;5:370–374.
55. Murakami K, Ohashi M, Matsunaga A, Yamamoto I, Nohira H. Asymmetric
transformation of a racemic-a(phthalimidooxy)arylacetic ester
by a combination of preferential crystallization and simultaneous racemization.
Chirality 1993;5:41–48.
Chirality DOI 10.1002/chir
56. de Gaitani CM, Martinez AS, Bonato PS. Degradation and configurational
changes of thioridazine 2-sulfoxide. J Pharm Biomed Anal
2004;36:601–607.
57. Eap CB, Koeb L, Baumann P. Artifacts in the analysis of Thioridazine
and other neuroleptics. J Pharm Biomed Anal 1993;11:451–457.
58. Knoche B, Blaschke G. Racemization of thalidomide in vitro. Presented
at the 4th International symposium on chiral discrimination,
September 19–22, 1993, Montreal, Canada. Abstract 144.
59. Schmahl HJ, Heger W, Nau H. The enantiomers of the teratogenic
thalidomide analogue EM 12. Toxicol Lett 1989;45:23–33.
60. Eger K, Jalalian M, Verspohl EJ, Lüpke NP. Synthesis, central nervous
system activity and teratogenicity of a homothalidomide. Arzneim-Forsch
(Drug Res) 1990;40:1073–1075.
61. Nunes MA, Brochmann-Hanssen E. Hydrolysis and epimerization
kinetics of pilocarpine in aqueous solution. J Pharm Sci 1974;63:716–721.
62. Pepper C, Smith HJ, Barrell KJ, Nicholls PJ, Hewlins MJE. Racemization
of drug enantiomers by benzylic proton abstraction at physiological
pH. Chirality 1994;6:400–404.
63. Sanins SM, Adams WJ, Kaiser DG, Halstead GW, Hosley J, Barnes
H, Baillie TA. Mechanistic studies on the metabolic chiral inversion
of R-ibuprofen in the rat. Drug Metab Dispos 1991;19:405–410.
64. Adams SS, Bresloff P, Mason CG. Pharmacological differences
between the optical isomers of ibuprofen: evidence for metabolic
inversion of the ( )-isomer. J Pharm Pharmacol 1976;28:256–257.
65. Kaiser DG, Van-Giessen GJ, Reischer RJ, Wechter WJ. Isomeric
inversion of ibuprofen (R)-enantiomer in humans. J Pharm Sci 1976;
65:269–273.
66. Lee E, Williams KM, Day R, Graham G, Champion D. Stereoselective
disposition of ibuprofen enantiomers in man. Br J Clin Pharmacol
1985;19:669–674.
67. Chen CS, Shieh WR, Lu PH, Harriman S, Chen CY. Metabolic stereoisomeric
inversion of ibuprofen in mammals. Biochim Biophys Acta
1991;1078:411–417.
68. Aboul-Enein HY, Wainer IW, editors. The impact of stereochemistry
on drug development and use. New York: Wiley; 1997.
69. Wechter WJ, Loughhead DG, Reischer RJ, VanGiessen GJ, Kaiser
DG. Enzymic inversion at saturated carbon. Nature and mechanism
of the inversion of R ( ) p-isobutyl hydratropic acid. Biochem
Biophys Res Commun 1974;61:833–837.
70. Reichel C, Brugger R, Bang H, Geisslinger G, Brune K. Molecular
cloning and expression of a 2-arylpropionyl-coenzyme A epimerase: a
key enzyme in the inversion metabolism of ibuprofen. Mol Pharmacol
1997;51:576–582.
71. Williams KM, Day RO. Stereoselective disposition-basis for variability
in response to NSAIDs. Agents Actions 1985;17:119–126.
72. Tracy TS, Hall SD. Metabolic inversion of (R)-ibuprofen. Epimerization
and hydrolysis of ibuprofenyl-coenzyme A. Drug Metab Dispos
1992;20:322–327.
73. Sanins SM, Adams WJ, Kaiser DG, Halstead GW, Baillie TA. Studies
on the metabolism and chiral inversion of ibuprofen in isolated rat
hepatocytes. Drug Metab Dispos 1990;18:527–533.
74. Knights KM, Addinall YF, Roberts BJ. Enhanced chiral inversion of
R-ibuprofen in liver from rats treated with clofibric acid. Biochem
Pharmacol 1991;41:1775–1777.
75. Davies NM. Clinical pharmacokinetics of ibuprofen. The first 30
years. Clin Pharmacokinet 1998;34:101–154.
76. Tan SC, Patel BK, Jackson SHD, Swift CG, Hutt AJ. Ibuprofen stereochemistry:
double-the-trouble? Enantiomer 1999;4:195–203.
77. Evans AM. Comparative pharmacology of S(þ)-ibuprofen and (RS)ibuprofen.
Clin Rheumatol 2001;1:9–14.
78. Kemal C, Casida JE. Coenzyme A esters of 2-aryloxyphenoxypropionate
herbicides and 2-arylpropionate anti-inflammatory drugs are
potent and stereoselective inhibitors of rat liver acetyl-CoA carboxylase.
Life Sci 1992;50:533–540.
79. Sallustio BC, Meffin PJ, Knights KM. The stereospecific in corporation
of fenoprofen in to rat hepatocytes and adipocyte triacylglycerols.
Biochem Pharmacol 1988;37:1919–1923.

80. Soraci A, Benoit E. In vitro fenoprofenyl-coenzyme A thioester formation:
interspecies variations. Chirality 1995;7:534–540.
81. San Martin MF, Soraci A, Fogel F, Tapia O, Islas S. Chiral inversion
of (R)-( )-fenoprofen in guinea-pigs pretreated with clofibrate. Vet
Res Commun 2002;26:323–332.
82. Castro EF, Soraci A, Fogel F, Tapia O. Chiral inversion of R-( )-fenoprofen
and ketoprofen enantiomers in cats. J Vet Pharmacol Ther
2000;23:265–271.
83. Rubin A, Knadler MP, Ho PP, Bechtol LD, Wolen RL. Stereoselective
inversion of (R)-fenoprofen to (S)-fenoprofen in humans. J Pharm Sci
1985;74:82–84.
84. Mehvar R, Jamali F. Stereospecific high-performance liquid chromatographic
(HPLC) assay of fenoprofen enantiomers in plasma and
urine. Pharm Res 1988;5:53–56.
85. Bopp RJ, Nash JF, Ridolfo AS, Shepard ER. Stereoselective inversion
of (R)-( ) benoxaprofen to the (S)-(þ) enantiomer in humans. Drug
Metab Dispos 1979;7:356–359.
86. Simmonds RG, Woodage TJ, Duff SM, Gren JN. Stereospecific inversion
of (R)-( )-benoxaprofen in rat and man. Eur J Drug Metab Pharmacokinet
1980;5:169–172.
87. Trejtnar F, Wsol V, Szotakova B, Skalova L, Pavek P, Kuchar M. Stereoselective
pharmacokinetics of flobufen in rats. Chirality
1999;11:781–786.
88. Hutt AJ, Caldwell J. The metabolic chiral inversion of 2-arylpropionic
acids. A novel route with pharmacological consequences. J Pharm
Pharmacol 1983;35:693–704.
89. Williams KM. Enantiomers in arthritic disorders. Pharmacol Ther
1990;46:273–295.
90. Kaneko I, Morimoto K, Kubo T. Drastic neuronal loss in vivo by bamyloid
racemized at Ser(26) residue: Conversion of non-toxic [D-
Ser(26)]b-amyloid 1-40 to toxic and proteinase-resistant fragments.
Neurosci 2001;104:1003–1011.
91. Takekazu K, Satoko N, Yoshihiro K, Isao K. In vivo conversion of
racemized b-amyloid ([D-Ser 26]A b 1-40) to truncated and toxic fragments
([D-Ser 26]A b 25-35/40) and fragment presence in the brains
of Alzheimer’s patients. J Neurosci Res 2002;70:474–483.
92. Schmahl HJ, Nau H, Neubert D. The enantiomers of the teratogenic
thalidomide analogue EM 12:1. Chiral inversion and plasma pharmacokinetics
in the marmoset monkey. Arch Toxicol 1988;62:200–204.
93. Schmahl HJ, Dencker L, Plum C, Chahoud I, Nau H. Stereoselective
distribution of the teratogenic thalidomide analogue EM 12 in the
early embryo of marmoset monkey, Wistar rat and NMRI mouse.
Arch Toxicol 1996;70:749–756.
94. Cuyue T, Kanyin Z, Francis L, Levy RH, Baillie TA. Metabolic chiral
inversion of stiripentol in the rat. II. Influence of route of administration.
Drug Metab Dispos 1994;22:554–560.
95. de Gaitani CM, Martinez AS, Bonato PS. Racemization and degradation
of thioridazine and thioridazine 2-sulfone in human plasma and
aqueous solutions. Chirality 2003;15:479–485.
96. Nishimura K, Hashimoto Y, Iwasaki S. (S)-form of a-methyl-N(a)phthalimidoglutarimide,
but not its (R)-form, enhanced phorbol esterinduced
tumor necrosis factor-a production by human leukemia cell
HL-60: Implication of optical resolution of thalidomidal effects. Chem
Pharm Bull 1994;42:1157–1159.
97. Heger W, Schmahl HJ, Klug S, Felies A, Nau H, Merker HJ, Neubert
D. Embryotoxic effects of thalidomide derivatives in the non-human primate
Callithrix jacchus. IV. Teratogenicity of micrograms/kg doses of
the EM 12: enantiomers. Teratog Carcinog Mutagen 1994;14:115–122.
98. Papini O, da Cunha SP, da Silva Mathes AC, Bertucci C, Moises
ECD, de Barros Duarte L, de Carvalho Cavalli R, Lanchote VL.
Kinetic disposition of lorazepam with focus on the glucuronidation
capacity, transplacental transfer in parturients and racemization in
biological samples. J Pharm Biomed Anal 2006;40:397–403.
99. Pham-Huy C, Villain-Pautet G, Hua H, Chikhi-Chorfi N, Galons H,
Thevenin M, Claude JR, Warnet JM. Separation of oxazepam, lorazepam
and temazepam enantiomers by HPLC on a derivated cyclodextrin-bonded
phase: application to the determination of oxazepam in
plasma. J Biochem Biophys Methods 2002;54:287–299.
RACEMIZATION IN OPTICALLY ACTIVE DRUGS DEVELOPMENT
463
100. Hainzl D, Parada A, Soares-da-Silva P. Metabolism of two new antiepileptic
drugs and their principal metabolites S(þ)-and R( )-10,11-dihydro
-10-hydroxy carbamazepine. Epilepsy Res 2001;44:197–206.
101. El-Nimr AE, Kassem AA, Kassem MA. On the racemization of L-noradrenaline
bitartrate. Pharmazie 1975;30:216–217.
102. Aso Y, Yoshioka S, Takeda Y. Epimerization and racemization of
some chiral drugs in the presence of human serum albumin. Chem
Pharm Bull 1990;38:180–184.
103. Matsuo K, Yamamoto Y, Kado N, Yamazaki M, Nagata O, Kato H,
Tsuji A. Racemization kinetics of meluadrine tartrate in aqueous solution.
Chem Pharm Bull 2001;49:794.
104. Orwa JA, Govaerts C, Gevers K, Roets E, Van Schepdael A, Hoogmartens
J. Study of the stability of polymyxins B1, E1 and E2 in aqueous
solution using liquid chromatography and mass spectrometry. J
Pharm Biomed Anal 2002;29:203–212.
105. Guo C, Shah RD, Mills J, Dukor RK, Cao X, Freedman TB, Nafie LA.
Fourier transform near-infrared vibrational circular dichroism used
for on-line monitoring the epimerization of 2,2-dimethyl-1,3-dioxolane-
4-methanol: a pseudo racemization reaction. Chirality 2006;18:775–
782.
106. Welch CJ, Kress MH, Beconi M, Mathre DJ. Studies on the racemization
of a stereolabile 5-aryl-thiazolidinedione. Chirality 2003;15:143–
147.
107. Blaschke G, Lamparter E, Schluter J. Racemization and hydrolysis of
tropic acid alkaloids in the presence of cyclodextrins. Chirality
1993;5:78–83.
108. Vakily M, Corrigan B, Jamali F. The problem of racemization in the
stereospecific assay and pharmacokinetic evaluation of ketorolac in
human and rats. Pharm Res 1995;12:1652–1657.
109. Iwakawa S, Spahn H, Benet LZ, Lin ET. Stereoselective disposition of
carprofen, flunoxaprofen and naproxen in rats. Drug Metab Dispos
1991;19:853–857.
110. el Mouelhi M, Ruelius HW, Fenselau C, Dulik DM. Species-dependent
enantioselective glucuronidation of three 2-arylpropionic acids.
Naproxen, ibuprofen and benoxaprofen. Drug Metab Dispos 1987;
15:767–772.
111. Aso Y, Yoshioka S, Shibazaki T, Uchiyama M. The kinetics of the racemization
of oxazepam in aqueous solution. Chem Pharm Bull 1988;
36:1834–1840.
112. Vree TB, Baars AM, Wuis EW. Direct high pressure liquid chromatographic
analysis and preliminary pharmacokinetics of enantiomers of
oxazepam and temazepam with their corresponding glucuronide conjugates.
Pharm Weekbl Sci 1991;13:142–147.
113. Ameyibor E, Stewart JT. Enantiomeric HPLC separation of selected
chiral drugs using native and derivatized b-cyclodextrins as chiral
mobile phase additives. J Liq Chromatogr Relat Technol 1997;20:855–
869.
114. Nishikawa T, Hayashi Y, Suzuki S, Kubo H, Ohtani H. On-column
enantiomerization of 3-hydroxybenzodiazepines during chiral liquid
chromatography with optical rotation detection. J Chromatogr A
1997;767:93–100.
115. Krause K, Girod M, Chankvetadze B, Blaschke G. Enantioseparations in
normal- and reversed-phase nano-high-performance liquid chromatography
and capillary electrochromatography using polyacrylamide and polysaccharide
derivatives as chiral stationary phases. J Chromatogr A
1999;837:51–63.
116. Sepetov NF, Krymsky MA, Ovchinnikov MV, Bespalova ZD, Isakova
OL, Soucek M, Lebl M. Rearrangement, racemization and decomposition
of peptides in aqueous solution. Pept Res 1991;4:308–313.
117. Lambert WJ, Timmer PG, Walters RR, Hsu Cy. Racemization and
intramolecular nucleophilic substitution reactions of ibutilide. J
Pharm Sci 1992;81:1028–1031.
118. Severin G. Spontaneous racemization of chlorthalidone: Kinetics and
activation parameters. Chirality 1992;4:222–226.
119. Stepensky D, Chorny M, Dabour Z, Schumacher I. Long-term stability
study of L-adrenaline injections: kinetics of sulfonation and
racemization pathways of drug degradation. J Pharm Sci 2004;93:
969–980.
Chirality DOI 10.1002/chir