Stereoselective binding of disopyramide to human plasma protein

The binding of racemic disopyramide and its two enantiomers to protein were compared in two samples of human plasma, two samples of freshly drawn serum and in a solution of alpha 1-acid glycoprotein. The binding of S(+)-disopyramide was higher at all concentrations as compared to to R(?)-disopyramide, and the binding of the racemate was intermediate. Differences in binding were due to differences in the association constant.     

Stereoselective binding and activity of oxotremorine analogs at muscarinic receptors in rat brain.

The activities of the enantiomers of BM-5 were examined to measure muscarinic cholinergic selectivity in the central nervous system. Autoradiographic studies assessed the ability of each enantiomer to inhibit the binding of [3H]-(R)-quinuclidinyl benzilate ([3H]-(R)-QNB) to muscarinic receptors in the rat brain. (+)-(R)-BM-5 inhibited [3H]-(R)-QNB binding to rat brain sections at concentrations below 1.0 microM, while 100-fold higher concentrations of (-)-(S)-BM-5 were required for comparable levels of inhibition. Analysis of the autoradiograms indicated that both stereoisomers had a similar distribution of high affinity binding sites. Each enantiomer displayed higher affinity for muscarinic receptors in the superior colliculi and lower affinity for receptors in the cerebral cortex and hippocampus. (+)-(R)-BM-5 and oxotremorine inhibited adenylyl cyclase activity in the cerebral cortex with efficacies comparable to that for acetylcholine. (+)-(R)-BM-5 was 26-fold more potent than (-)-(S)-BM-5 in inhibiting adenylyl cyclase. Oxotremorine-M and carbamylcholine stimulated phosphoinositide turnover in the cerebral cortex. Oxotremorine had lower activity and (+)-(R)-BM-5 was essentially inactive at comparable concentrations. The difference in activity of the two enantiomers indicates a remarkable stereochemical selectivity for muscarinic receptors. The stereoselectivity index is comparable for both the autoradiographic assays (48) and measures of adenylyl cyclase activity (26) in the cerebral cortex.     

Nonlinear stereoselective pharmacokinetics of ketoconazole in rat after administration of racemate

The stereoselective pharmacokinetics of ketoconazole (KTZ) enantiomers were studied in rat after i.v. and oral administration of (+/-)-KTZ. Sprague-Dawley rats were administered racemic KTZ as 10 mg/kg i.v. or orally over the range 10-80 mg/kg as single doses. Serial blood samples were collected over a 24-h period via surgically placed jugular vein cannulae. Plasma was assayed for KTZ enantiomer concentrations using stereospecific HPLC. Enantiomeric plasma protein binding was determined using an erythrocyte partitioning method at racemic concentrations of 10 and 40 mg/L. Stereoselective metabolism was tested by incubating the racemate (0.5-250 microM) with rat liver microsomes. In all rats, (+)-KTZ plasma concentrations were higher (up to 2.5-fold) than (-)-KTZ. The clearance and volume of distribution of the (-) enantiomer were approximately twofold higher than antipode. Half-life did not differ between the enantiomers. After oral doses the t(max) was not stereoselective. For both enantiomers with higher doses the respective half-life were found to increase. The mean unbound fraction of the (-) enantiomer was found to be up to threefold higher than that of the (+) enantiomer. At higher concentrations nonlinearity in plasma protein binding was observed for both enantiomers. There was no evidence of stereoselective metabolism by liver microsomes. Stereoselectivity in KTZ pharmacokinetics is attributable to plasma protein binding, although other processes such as transport or intestinal metabolism may also contribute.     

Stereoselective distribution of hydroxychloroquine in the rabbit following single and multiple oral doses of the racemate and the separate enantiomers

Hydroxychloroquine (HCQ) stereoselective distribution was investigated in rabbits after 20 mg/kg po of racemic-HCQ (rac-HCQ) and 20 mg/kg po of each enantiomer, 97% pure (?)-(R)-HCQ and 99% pure (+)-(S)-HCQ. Concentrations were 4 to 6 times higher in whole blood than in plasma. Melanin did not affect plasma and whole blood levels since concentrations did not differ between pigmented and nonpigmented animals. After single and multiple doses of the separate enantiomers, only 5–10% of the antipode could be measured, in blood or plasma. Therefore, there was no significant interconversion from one enantiomer into the other. Following rac-HCQ, plasma (+)-(S)-levels always surpassed (?)-(R)-ones while in whole blood, (?)-(R)-HCQ concentrations were always the highest. When the enantiomers were administered separately, blood concentrations achieved after (?)-(R)-HCQ were higher, especially after multiple doses. These observations suggest that (?)-(R)-HCQ is preferentially concentrated by cellular components of blood. This enantioselective distribution of HCQ could be secondary to a stereoselective protein binding to plasma proteins, although a more specific binding of (?)-(R)-HCQ to blood cells cannot be ruled out. Since in whole blood (?)-(R)-HCQ is retained in cellular components, metabolism would favour the more available (+)-(S)-enantiomer. © 1994 Wiley-Liss, Inc.     

19F nuclear magnetic resonance investigation of stereoselective binding of isoflurane to bovine serum albumin.

Whether proteins or lipids are the primary target sites for general anesthetic action has engendered considerable debate. Recent in vivo studies have shown that the S(+) and R(-) enantiomers of isoflurane are not equipotent, implying involvement of proteins. Bovine serum albumin (BSA), a soluble protein devoid of lipid, contains specific binding sites for isoflurane and other anesthetics. We therefore conducted 19F nuclear magnetic resonance measurements to determine whether binding of isoflurane to BSA was stereoselective. Isoflurane chemical shifts were measured as a function of BSA concentration to determine the chemical shift differences between the free and bound isoflurane. KD was determined by measuring the 19F transverse relaxation times (T2) as a function of isoflurane concentration. The binding duration was determined by assessing increases in 1/T2 as a result of isoflurane exchanging between the free and bound states. The S(+) and R(-) enantiomers exhibited no stereoselectivity in chemical shifts and KD values (KD = 1.3 +/- 0.2 mM, mean +/- SE, for S(+), R(-), and the racemic mixture). Nonetheless, stereoselectivity was observed in dynamic binding parameters; the S(+) enantiomer bound with slower association and dissociation rates than the R(-).     

Development and application of a chiral high performance liquid chromatography assay for pharmacokinetic studies of methadone

Methadone enantiomers and EDDP, the main metabolite of methadone, were separated (R(s) = 2.0 for methadone enantiomers) following liquid-liquid extraction from human serum and urine followed by reverse-phase high-performance liquid chromatography on a derivatized beta-cyclodextrin column and quantified at therapeutic concentrations with ultraviolet detection. Detector response was linear (r(2) > 0.98) to 1,000 and 2,500 ng x mL(-1) for methadone enantiomers and EDDP, respectively. The limit of quantification from a 1-mL biological sample was 2.5 and 5 ng x mL(-1) for methadone enantiomers and EDDP, respectively. Interday variation was <13% and intraday variation was <8% for the analytes of interest. The assay was applied to plasma protein and erythrocyte binding studies and a 96-h pharmacokinetic study in two healthy female volunteers following oral dosing with rac-methadone. The binding of methadone to plasma proteins was enantioselective with the active (-)-(R) enantiomer having the highest free fraction (mean +/- SD: 21.2+/-7.6% vs. 13.3+/-6.2% for (+)-(S)-methadone, n = 8). Binding of methadone to erythrocytes was not apparently enantioselective (38.6+/-1.3% and 38.1+/-1.4% bound for (-)-(R)- and (+)-(S)-methadone, respectively). The pharmacokinetic study revealed enantioselective disposition of methadone in one volunteer but not in the other. EDDP was observed in urine but was only in small or undetectable concentrations in serum. The method is applicable to in vitro and pharmacokinetic studies of rac-methadone disposition in humans.     

Discrimination of pheromone enantiomers by two pheromone binding proteins from the gypsy moth Lymantria dispar

The gypsy moth, Lymantria dispar, uses (7R, 8S)-cis-2-methyl-7, 8-epoxyoctadecane, (+)-disparlure, as a sex pheromone. The (-) enantiomer of the pheromone is a strong behavioral antagonist. Specialized sensory hairs, sensillae, on the antennae of male moths detect the pheromone. Once the pheromone enters a sensillum, the very abundant pheromone binding protein (PBP) transports the odorant to the sensory neuron. We have expressed the two PBPs found in gypsy moth antennae, PBP1 and PBP2, and we have studied the affinity of these recombinant PBPs for the enantiomers of disparlure. To study pheromone binding under equilibrium conditions, we developed and validated a binding assay. We have addressed the two major problems with hydrophobic ligands in aqueous solution: (1) concentration-dependent adsorption of the ligand on vial surfaces and (2) separation of the protein-bound ligand from the material remaining free in solution. We used this assay to demonstrate for the first time that pheromone binding to PBP is reversible and that the two PBPs from L. dispar differ in their enantiomer binding preference. PBP1 has a higher affinity for the (-) enantiomer, while PBP2 has a higher affinity for the (+) enantiomer. The PBP from the wild silk moth, Antheraea polyphemus (Apol-3) bound the disparlure enantiomers more weakly than either of the L. dispar PBPs, but Apol-3 was also able to discriminate the enantiomers. We have observed extensive aggregation of both L. dispar PBPs and an increase in pheromone binding at high (>2 microM) PBP concentrations. We present a model of disparlure binding to the two PBPs.     

Actions of terazosin and its enantiomers at subtypes of α- and α2-Adrenoceptors in vitro

Abstract

Terazosin and its enantiomers, antagonists of α1-adrenoceptors, were studied in radioligand binding and functional assays to determine relative potencies at subtypes of α1- and α2-adrenoceptors in vitro. The racemic compound and its enantiomers showed high and apparently equal affinity for subtypes of α1-adrenoceptors with K values in the low nanomolar range, and showed potent antagonism of α1-adrenoceptors in isolated tissues, with the enantiomers approximately equipotent to the racemate at each α1-adrenoceptor subtype. At α2b sites, R(+) terazosin bound less potently than either the S(-) enantiomer or racemate. R(+) terazosin was also less potent than the S(-) enantiomer or the racemate at rat atrial α2B receptors. These agents were not significantly different in their potencies at α2a or α2A sites. Since the high affinity for α2B sites of quinazoline-type α-adrenoceptor antagonists has been used to differentiate α2-adrenoceptor subtypes, the low affinity of R(+) terazosin for these sites was unexpected. Because terazosin or its enantiomers are approximately equipotent at α1 -adrenoceptor subtypes, the lower potency of R(+) terazosin at α2B receptors indicates a somewhat greater selectivity for α1- compared to α2B adrenoceptor subtypes. The possible pharmacological significance of this observation is discussed.     

Pharmacokinetics and pharmacodynamics of hydroxychloroquine enantiomers in patients with rheumatoid arthritis receiving multiple doses of racemate

Hydroxychloroquine, a slow acting antirheumatic drug, is administered as the racemic mixture. Blood concentrations of the two enantiomers of hydroxychloroquine were measured in two studies, one study of eight patients, in whom blood and urine concentrations were measured during the first 6 months of therapy with rac-hydroxychloroquine, and one of 43 patients who had received rac-hydroxychloroquine therapy for at least 6 months. In the latter study rheumatoid disease activity was also measured. The pharmacokinetics of hydroxychloroquine were found to be enantioselective. The concentrations of (?)-(R)-hydroxychloroquine were higher than those of the (+)-(S)-antipode in all patients at all time points, although the ratios of the two enantiomers did display a two to three fold variability between patients. Both total and renal clearance were greater for the (+)-(S)-enantiomer. From the observational, cross-sectional study design used, it was not possible to differentiate concentration–effect relationships of the two enantiomers. The 11-fold range of drug concentrations swamped any effect of variability between patients in enantiomer proportions. Blood concentrations of both enantiomers were significantly higher in groups of patients with less active disease. © 1994 Wiley-Liss, Inc.     

Protein binding of indobufen enantiomers: pharmacokinetics of free fraction-studies after single or multiple doses of rac-indobufen

The binding of the enantiomers of indobufen (INDB) to human serum proteins was investigated using the racemic mixture or the pure (+)-S-enantiomer in a concentration range of 2.5-100.0 mg/L. In addition, the pharmacokinetics of free (unbound) and total INDB enantiomers were studied 1) following administration of a single 200 mg rac-INDB tablet to healthy volunteers, and 2) in obliterative atherosclerosis patients at steady state. The free fraction of INDB was obtained by ultrafiltration. Using the racemic mixture, the binding parameters of the two enantiomers were different, showing enantioselectivity in protein binding. The (-)-R-enantiomer was bound more strongly to human serum albumin, with association constant K = 11.95 +/- 0.98 x 10(5) M(-1) and n = 0.72 +/- 0.02 binding sites. The comparable data for the (+)-S-enantiomer were K = 4.65 +/- 0.02 x 10(5) M(-1), n = 0.92 +/- 0.01. When the binding of (+)-S-enantiomer was studied alone, the association constant K (2.10 +/- 0.18 x 10(5) M(-1)) was lower and the number of binding sites was increased, to n = 1.87 +/- 0.17. Competition occurred between the enantiomers, with the (-)-R-enantiomer displacing its antipode. The fraction of both enantiomers bound to serum proteins was 99.0%, which increased with decreasing initial concentration of the enantiomers. In healthy volunteers the (+)-S-enantiomer was eliminated faster than its (-)-R antipode, resulting in a lower AUC for the (+)-S-enantiomer. Significant differences were observed in the total INDB enantiomer concentrations. The mean unbound fraction of (-)-R- and (+)-S-INDB was 0.45% and 0.43%, respectively. Levels of the free (+)-S-enantiomer were higher than its (-)-R-antipode at steady state in patients with obliterative atherosclerosis who also took other drugs. The free enantiomer fraction increased to around 1% upon repeated administration. We conclude that the more rapid elimination of the (+)-S enantiomer is associated with its weaker binding to serum proteins.     

Enantioselective plasma protein binding of bimoclomol

The binding of bimoclomol enantiomers to human plasma, its components, as well as to plasma from monkey, dog, rat, and mouse was investigated by ultrafiltration and equilibrium dialysis. The considerably stronger binding of the (-)-(S)-enantiomer found in human plasma is due to the alpha(1)-acid glycoprotein (AAG) component. The binding parameters for AAG (n(R)K(R) = 1.3 x 10(4) M(-1) and n(S)K(S) = 1.0 x 10(5) M(-1)) revealed high enantioselectivity, while the binding to human serum albumin was found to be weak (nK = 5 x 10(3) M(-1)) and not stereoselective. (-)-(S)-Bimoclomol was extensively displaced in the presence of specific marker ligands for the "FIS" subfraction of human AAG. Comparative binding studies indicated considerable differences between plasma of the five species investigated.     

Effects of (+) and (-) enantiomers of calcium channel agonist, Bay K 8644, on mechanical and electrical responses of frog skeletal muscle

The effects of (+) and (-) enantiomers of Bay K 8644, a Ca2+ channel agonist, on the mechanical and electrical properties of frog skeletal muscle fibers were investigated. In the concentration range of 10(-6) to 10(-5) M, both (+) and (-) enantiomers of Bay K 8644 significantly increased the maximum amplitudes of twitch responses. Both (+) and (-) enantiomers of Bay K 8644, at higher concentrations such as 10(-4) M, greatly depressed the amplitudes of twitches. Potentiating and depressing effects of (-) enantiomer of Bay K 8644 on twitch responses were significantly greater than those of the (+) enantiomer. At all concentrations used, both (+) and (-) enantiomers of Bay K 8644 significantly decreased the area under the tetanic force x time curve. In intracellular recordings, it was found that the depressing effects of both (+) and (-)-Bay K 8644 on tetanic contractions and twitch responses were due to the inhibition of action potentials. The inhibitory effect of (-) enantiomer of Bay K 8644 on action potentials also was significantly greater than that of the (+) enantiomer. In conclusion, present results suggest that, in contrast with cardiac muscle fibers, (+) and (-) enantiomers of Bay K 8644 have similar inhibitory effects on the electrical and mechanical properties of frog skeletal muscle fibers.     

cis-chlorobenzene dihydrodiol dehydrogenase (TcbB) from Pseudomonas sp. strain P51, expressed in Escherichia coli DH5alpha(pTCB149), catalyzes enantioselective dehydrogenase reactions

cis-Chlorobenzene dihydrodiol dehydrogenase (CDD) from Pseudomonas sp. strain P51, cloned into Escherichia coli DH5alpha(pTCB149) was able to oxidize cis-dihydrodihydroxy derivatives (cis-dihydrodiols) of dihydronaphthalene, indene, and four para-substituted toluenes to the corresponding catechols. During the incubation of a nonracemic mixture of cis-1,2-indandiol, only the (+)-cis-(1R,2S) enantiomer was oxidized; the (-)-cis-(S,2R) enantiomer remained unchanged. CDD oxidized both enantiomers of cis-1,2-dihydroxy-1,2,3, 4-tetrahydronaphthalene, but oxidation of the (+)-cis-(1S,2R) enantiomer was delayed until the (-)-cis-(1R,2S) enantiomer was completely depleted. When incubated with nonracemic mixtures of para-substituted cis-toluene dihydrodiols, CDD always oxidized the major enantiomer at a higher rate than the minor enantiomer. When incubated with racemic 1-indanol, CDD enantioselectively transformed the (+)-(1S) enantiomer to 1-indanone. This stereoselective transformation shows that CDD also acted as an alcohol dehydrogenase. Additionally, CDD was able to oxidize (+)-cis-(1R,2S)-dihydroxy-1, 2-dihydronaphthalene, (+)-cis-monochlorobiphenyl dihydrodiols, and (+)-cis-toluene dihydrodiol to the corresponding catechols.     

Albumin binding sites for etodolac enantiomers

Non-steroidal anti-inflammatory drugs (NSAIDs) are strongly bound to human serum albumin (HSA), mainly to sites I and II. The aim of this study was to characterize the binding site(s) of etodolac enantiomers under physiological conditions (580 μM HSA) using equilibrium dialysis. The protein binding of etodolac enantiomers, alone or in various ratios, was studied in order to evaluate the potential competition between them. Our results showed that (S)-etodolac was more strongly bound to HSA than (R)-etodolac. The displacement of one enantiomer by its antipode was observed only at high concentrations of the competitor, and was more pronounced for the (S)-form. Displacement studies of the enantiomers by specific probes of sites I and II of albumin, dansylamide, and dansylsarcosine, respectively, showed that (R)-etodolac was slightly displaced by both these probes whereas the free concentration of (S)-etodolac increased markedly in the presence of dansylsarcosine. Moreover, the binding of ligands to sites I and II is usually affected by alkaline pH, by chloride ions, and by fatty acids. For etodolac, the presence of 0.1 and 1 M chloride ions and increasing pH (5.5-9) decreased the binding of both enantiomers. The same result was obtained with addition of octanoic acid. Conversely, the addition of oleic, palmitic, or stearic acid to the protein solution increased the binding of (R)-etodolac, but decreased that of its antipode. All these findings suggest that (R)- and (S)-etodolac interact mainly with site II of HSA, and that the (R)-isomer is also bound to site I under physiological conditions. © 1996 Wiley-Liss, Inc.     

Development and validation of a chiral liquid chromatographic method, based on Chiralpak to quantify enantiomers of (+/-)-DRF 2725 in rat plasma: lack of inversion of ragaglitazar (S(-)-DRF 2725) to its antipode in plasma

A selective, accurate and reproducible high-performance liquid chromatographic (HPLC) method for the separation of individual enantiomers of DRF 2725 [R(+)-DRF 2725 and S(-)-DRF 2725 or ragaglitazar] was obtained on a chiral HPLC column (Chiralpak). During method optimization, the separation of enantiomers of DRF 2725 was investigated to determine whether mobile phase composition, flow-rate and column temperature could be varied to yield the base line separation of the enantiomers. Following liquid-liquid extraction, separation of enantiomers of DRF 2725 and internal standard (I.S., desmethyl diazepam) was achieved using an amylose based chiral column (Chiralpak AD) with the mobile phase, n-hexane-propanol-ethanol-trifluoro acetic acid (TFA) in the ratio of 89.5:4:6:0.5 (v/v). Baseline separation of DRF 2725 enantiomers and I.S., free from endogenous interferences, was achieved in less than 25 min. The eluate was monitored using an UV detector set at 240 nm. Ratio of peak area of each enantiomer to I.S. was used for quantification of plasma samples. Nominal retention times of R(+)-DRF 2725, S(-)-DRF 2725 and I.S. were 15.8, 17.7 and 22.4 min, respectively. The standard curves for DRF 2725 enantiomers were linear (R(2) > 0.999) in the concentration range 0.3-50 microg/ml for each enantiomer. Absolute recovery, when compared to neat standards, was 70-85% for DRF 2725 enantiomers and 96% for I.S. from rat plasma. The lower limit of quantification (LLOQ) for each enantiomers of DRF 2725 was 0.3 microg/ml. The inter-day precisions were in the range of 1.71-4.60% and 3.77-5.91% for R(+)-DRF 2725, S(-)-DRF 2725, respectively. The intra-day precisions were in the range of 1.06-11.5% and 0.58-12.7% for R(+)-DRF 2725, S(-)-DRF 2725, respectively. Accuracy in the measurement of quality control (QC) samples was in the range 83.4-113% and 83.3-113% for R(+)-DRF 2725, S(-)-DRF 2725, respectively. Both enantiomers and I.S. were stable in the battery of stability studies viz., bench-top (up to 6 h), auto-sampler (up to 12 h) and freeze/thaw cycles (n = 3). Stability of DRF 2725 enantiomers was established for 15 days at -20 degrees C. The application of the assay to a pharmacokinetic study of ragaglitazar [S(-)-DRF 2725] in rats is described. It was unequivocally demonstrated that ragaglitazar does not undergo chiral inversion to its antipode in vivo in rat plasma.     

Plasma protein binding of ketoprofen enantiomers in man: method development and its application.

The protein binding of ketoprofen enantiomers was investigated in human plasma at physiological pH and temperature by ultrafiltration. 14C-labelled (RS)-ketoprofen was synthesized and purified by high-performance liquid chromatography and utilized as a means of quantifying the unbound species. In vitro studies were conducted with plasma obtained from six healthy volunteers. The plasma was spiked with (R)-ketoprofen alone, (S)-ketoprofen alone, and (RS)-ketoprofen in the enantiomeric concentration range of 1.0 to 19.0 micrograms/ml. The plasma protein binding of ketoprofen was nonenantioselective. At a racemic drug concentration of 2.0 micrograms/ml the mean (+/- SD) percentage unbound of (R)-ketoprofen was 0.80 (+/- 0.15)%. The corresponding value for (S)-ketoprofen, 0.78 (+/- 0.18)%, was not statistically different (P greater than 0.05). At this racemic drug concentration (2.0 micrograms/ml) the percentage unbound of each enantiomer was unaffected (P greater than 0.05) by the presence of the glucuronoconjugates of ketoprofen (10 micrograms/ml) in plasma. At clinically relevant concentrations, the plasma binding of ketoprofen did not exhibit enantioselectivity or concentration dependence nor was the binding of either enantiomer influenced by its optical antipode (P greater than 0.05).     

Effects of pure enantiomers of flurbiprofen in comparison to racemic flurbiprofen on eicosanoid release from various rat organs ex vivo.

The effects of oral treatment of rats with pure enantiomers of flurbiprofen in comparison to racemic flurbiprofen on ex vivo release of eicosanoids from gastric mucosa, jejunum, lung, brain and clotting whole blood were investigated. With the S(+) enantiomer and the racemate dose-dependent inhibition of release of cyclooxygenase products of arachidonate metabolism in all tissues tested was observed, while release of leukotriene (LT) C4 was inhibited in gastric mucosa, but not in jejunum and lung. On the other hand, the R(-) enantiomer inhibited cyclooxygenase in the various tissues less potently and to a variable degree with no significant effect in the jejunum. The R(-) enantiomer had no effect on LTC4 release from any of the tissues investigated. Furthermore, the effect of a high dose of 25 mg/kg of the S(+) enantiomer on release of cyclooxygenase products from the various tissues was much longer lasting than that of an identical dose of the R(-) enantiomer. Stereoselective pharmacokinetics of the flurbiprofen enantiomers and/or organ specific cyclooxygenase activities could underly these results. The more potent cyclooxygenase inhibition by the S(+) enantiomer correlates with its higher anti-inflammatory activity and gastrointestinal toxicity. On the other hand, both enantiomers have been shown previously to be almost equally effective analgesics. Inhibition of brain cyclooxygenase might contribute to this effect.     

Enantiospecific disposition of pranoprofen in beagle dogs and rats

The pharmacokinetic characteristics of pranoprofen enantiomer were examined and compared with the disposition of the corresponding isomer after the administration of racemic pranoprofen to beagle dogs and rats. The plasma levels of (+)-(S)-isomer were significantly higher than those of (-)-(R)-isomer in dogs and rats by either intravenous or oral administration. Although the oral bioavailability and absorption rate constant between the (-)-(R)- and (+)-(S)-form was the same, the elimination rate constant of the (+)-(S)-form was significantly lower than that of the (-)-(R)-form in both dogs and rats. This discrepancy can be explained on the basis of differences in protein binding and the metabolism of the two enantiomers. The (-)-(R)-isomer was predominantly conjugated depending on its higher free plasma level and its faster metabolic rate than the (+)-(S)-form, and thus was excreted more rapidly in the urine and bile in the form of pranoprofen glucuronide. Furthermore, a (-)-(R)- to (+)-(S)-inversion occurred to the extent of 14% in beagle dogs, but not in rats. This chiral inversion might be an important factor in the slow elimination of the (+)-(S)-form in dogs. The most efficient organ for chiral inversion was the liver, followed by kidney and intestine.     

Disposition and absorption of hydroxychloroquine enantiomers following a single dose of the racemate

The disposition of hydroxychloroquine enantiomers has been investigated in nine patients with rheumatoid arthritis following administration of a single dose of the racemate. Blood concentrations of (?)-(R)-hydroxychloroquine exceed those of (+)-(S)-hydroxychloroquine following both an oral and intravenous dose of the racemate. Maximum blood concentrations of (?)-(R)-hydroxychloroquine were higher than (+)-(S) -hydroxychloroquine after oral dosing (121 ± 56 and 99 ± 42 ng/ml, respectively, P = 0.009). The time to maximum concentration and the absorption half-life, calculated using deconvolution techniques, were similar for both enantiomers. The fractions of the dose of each enantiomer absorbed were similar, 0.74 and 0.77 for (?)-(R)- and (+)-(S)-hydroxychloroquine, respectively (P = 0.77). The data suggest that absorption of hydroxychloroquine is not enantioselective. The stereoselective disposition of hydroxychloroquine appears to be due to enantioselective metabolism and renal clearance, rather than stereoselectivity in absorption and distribution. © 1994 Wiley-Liss, Inc.     

Stereoselective disposition and tissue distribution of carvedilol enantiomers in rats.

After intravenous bolus injection of rac-carvedilol at 2 mg/kg to the rat, the (+)-(R)- and (-)-(S)-enantiomer levels in the blood and tissues (liver, kidney, heart, muscle, spleen, and aorta) were measured by stereospecific HPLC assay. As compared with the (+)-(R), the (-)-(S) had a larger Vdss (3.32 vs. 2.21 liter/kg), MRT (33.4 vs. 25.6 min), and CLtot (96.1 vs. 83.8 ml/min/kg). AUC comparison after iv and po administration showed systemic bioavailability of the (-)-(S) to be about half that of its antipode, explained by the fact that the free fraction of the (-)-(S) in blood was 1.65-fold greater than that of the (+)-(R). Tissue-to-blood partition coefficient values for the (-)-(S) were 1.6- to 2.1-fold greater than those for the (+)-(R) in all tissues, showing that the (-)-(S) accumulates more extensively in the tissues. These results were consistent with the greater Vdss for the (-)-(S) estimated from systemic blood data. The stereoselective tissue distribution of carvedilol enantiomers results from an enantiomeric difference in plasma protein binding rather than in tissue binding.