ENANTIOSELECTIVE DISTRIBUTION OF VERAPAMIL AND NORVERAPAMIL INTO HUMAN AND RAT ERYTHROCYTES: THE ROLE OF PLASMA PROTEIN BINDING

In this in vitro study, the distribution of the enantiomers of verapamil (VER) and its active metabolite, norverapamil (NOR), into the red blood cells (RBCs) of humans and rats was investigated using a chiral liquid chromatographic assay. When plasma was replaced with buffer, the distribution of VER and NOR enantiomers into both human and rat RBCs was substantial (RBC:blood concentration ratios, 1·39–1·79), non-stereoselective, concentration (125–1000 ng mL⁻¹) linear, and species independent. However, in the presence of plasma, the RBC distribution of VER and NOR was stereoselective, with opposite stereoselectivity for human (S>R) and rat (R>S) blood. Additionally, the presence of plasma caused a reduction in the extent of RBC distribution for both VER and NOR enantiomers and in some cases resulted in nonlinearity in the RBC distribution of the enantiomers. Plasma protein binding studies revealed opposite stereoselectivity in the free fractions in human (S>R) and rat (R>S) plasma for both VER and NOR. These data suggest that the stereoselective protein binding is responsible for the apparent stereoselectivity in the RBC distribution of VER and NOR. The data are also in agreement with the opposite stereoselectivity in the plasma concentrations of VER observed in vivo in rats and humans.     

In vitro and in vivo studies on the stereoselective pharmacokinetics and biotransformation of an (S)-(–)- and (R)-(+)-pyrazolotriazine sulfoxide in the male rat

1. BOF-4272 (a pyrazolotriazine sulfoxide) is a new drug for the treatment of hyperuricemia. The pharmacokinetics and biotransformation of both BOF-4272 enantiomers were investigated in rat. 2. Plasma concentrations after intravenous or oral administration of racemic BOF-4272 to rat were significantly higher for (S) - than for (R) -BOF-4272. 3. The concentration of (S) -BOF-4272 in hepatocyte culture medium 24 h after the addition of racemic BOF-4272 was higher than that of (R) -BOF-4272. 4. Liver concentrations after oral adminstration of racemic BOF-4272 to rat were significantly higher for (R) - than for (S) -BOF-4272. Kidney concentrations were significantly higher for (S) - than for (R) -BOF-4272. 5. Hepatic biotransformation from BOF-4272 to unknown metabolites, possibly conjugates, is stereoselective. Biotransformation of both enantiomers to the sulfone metabolite by cytochrome P450 in rat liver may also be stereoselective. 6. Biotransformation of the sulfide metabolite of BOF-4272 to BOF-4272 may be stereoselective, possibly due to the stereospecificity of flavin-containing mono-oxidase and/or cyrochrome P450. 7. The stereoselectivity of plasma concentrations of racemic BOF-4272 after intravenous or oral administration appears to be due to differences in the hepatic uptake of the two enantiomers as well as the stereoselective biotransformation of the sulfide metabolite to BOF-4272 in rat liver. Biotransformation of BOF-4272 in rat liver may also be stereoselective.     

In vitro and in vivo studies on the stereoselective pharmacokinetics and biotransformation of an (S)-(-)- and (R)-(+)-pyrazolotriazine sulfoxide in the male rat

1. BOF-4272 (a pyrazolotriazine sulfoxide) is a new drug for the treatment of hyperuricemia. The pharmacokinetics and biotransformation of both BOF-4272 enantiomers were investigated in rat. 2. Plasma concentrations after intravenous or oral administration of racemic BOF-4272 to rat were significantly higher for (S)- than for (R)-BOF-4272. 3. The concentration of (S)-BOF-4272 in hepatocyte culture medium 24 h after the addition of racemic BOF-4272 was higher than that of (R)-BOF-4272. 4. Liver concentrations after oral adminstration of racemic BOF-4272 to rat were significantly higher for (R)- than for (S)-BOF-4272. Kidney concentrations were significantly higher for (S)- than for (R)-BOF-4272. 5. Hepatic biotransformation from BOF-4272 to unknown metabolites, possibly conjugates, is stereoselective. Biotransformation of both enantiomers to the sulfone metabolite by cytochrome P450 in rat liver may also be stereoselective. 6. Biotransformation of the sulfide metabolite of BOF-4272 to BOF-4272 may be stereoselective, possibly due to the stereospecificity of flavin-containing mono-oxidase and/or cyrochrome P450. 7. The stereoselectivity of plasma concentrations of racemic BOF-4272 after intravenous or oral administration appears to be due to differences in the hepatic uptake of the two enantiomers as well as the stereoselective biotransformation of the sulfide metabolite to BOF-4272 in rat liver. Biotransformation of BOF-4272 in rat liver may also be stereoselective.     

Stereoselective disposition of S-8666, a novel uricosuric antihypertensive diuretic, and its N-monodemethylated metabolite in a perfused rat liver preparation. Effect of protein binding on the kinetics of S-8666.

S-8666 (5-dimethyl-sulfamoyl-6,7-dichloro-2,3-dihydrobenzofuran-2-carboxylic acid), a novel uricosuric antihypertensive diuretic, and its N-monodemethylated metabolite (M-I) were studied in a single pass perfused rat liver preparation under constant perfusate flow (ca. 16 ml/min). During perfusion with 100 nmol/ml of racemic S-8666 not containing bovine serum albumin (BSA), the steady-state hepatic extraction ratio of R(+)-S-8666 was two times higher (0.65 +/- 0.08) than that of S(-)-S-8666 (0.34 +/- 0.08). R(+)- and S(-)-M-I in the effluent perfusate plasma accounted for 64 and 18% of the influx rate of each enantiomeric S-8666, respectively. The N-monodemethylation was found to be responsible for the hepatic extraction of S-8666 enantiomers. S(-)-S-8666 was excreted into bile at a more rapid rate than the R(+)-enantiomer. Biliary excretion of R(+)-M-I was faster than S(-)-M-I, although the excretion rates of M-I were slower than those of S-8666 for both enantiomers. The steady-state extractions of preformed R(+)- and S(-)-M-I were low and a significant difference [S(-) greater than R(+)] was observed during the perfusion of 100 nmol/ml preformed racemic M-I without BSA. Increasing the concentration of BSA in the perfusate led to decreases in the extraction ratios of S-8666 enantiomers and biliary excretion rates of all chemicals, which was due to the decreases in the free fractions of S-8666 and M-I enantiomers. The binding of S-8666 and M-I enantiomers to BSA also showed stereoselectivity [R(+) less than S(-)].(ABSTRACT TRUNCATED AT 250 WORDS)     

Enantioselective pharmacokinetics of etodolac in the rat: tissue distribution, tissue binding, and in vitro metabolism.

The nonsteroidal anti-inflammatory agent etodolac (ET) exhibits stereoselectivity in its pharmacokinetics following administration to humans and rats. To underline the factors responsible for this stereoselectivity, the tissue distribution, in vitro tissue binding, and microsomal metabolism of ET enantiomers were studied in the rat. Following iv administration of racemic ET, the S:R AUC ratios in tissues were stereoselective, and different from that in plasma. Binding of enantiomers to tissues was stereoselective, although it did not relate well with in vivo tissue distribution. Rather, the tissue distribution of enantiomers appeared to be better explained by the unbound fractions of enantiomers in plasma. With respect to in vitro glucuronidation by liver microsomes, the Vmax of S-ET was 3.4-fold greater than that of R-ET; the enantiomers possessed similar Km. There appeared to be stereoselectivity in the oxidative metabolism of ET enantiomers by liver and kidney microsomes, in favor of the R-enantiomer. The lower AUC in rat plasma of pharmacologically active S-ET as compared with its antipode is due to its relatively greater distribution to tissues, owing to a lesser degree of binding to plasma proteins, and to its higher rate of glucuronidation.     

Stereoselective plasma protein binding of ibuprofen enantiomers

Summary We have developed a novel and reproducible method for determining the plasma protein binding of the two ibuprofen enantiomers in the presence of each other. The method involves the use of radiolabelled racemic ibuprofen, equilibrium dialysis, derivatization of the enantiomers to diastereomeric amides, high-performance liquid chromatography, and radiochemical analysis.We have determined the plasma protein binding of R(–)- and S(+)-ibuprofen in 6 healthy male volunteers after the oral administration of 800 mg racemic ibuprofen.The mean time-averaged percentage unbound of the R(–)-enantiomer, 0.419 was significantly less than that of the S(+)-enantiomer, 0.643, consistent with stereoselective plasma protein binding.The percentage unbound of each ibuprofen enantiomer was concentration-dependent over the therapeutic concentration range and was influenced by the presence of its optical antipode.     

Stereoselective pharmacokinetics of tocainide in human uraemic patients and in healthy subjects

Summary The disposition of tocainide enantiomers were examined in healthy human subjects and uraemic patients following a single i. v. dose (200 mg) of racemic tocainide hydrochloride.In the healthy subjects, the total body clearance of R(–)-tocainide was significantly greater than that of S(+)-tocainide (2.62 vs 1.70 ml·min^–1·kg^–1). Renal clearance also favoured R(–)-tocainide and appeared to contribute significantly to the stereoselective total body clearance. The volume of distribution of the enantiomers did not differ significantly.Uraemia produced a marked decrease in the total body clearance with no apparent effect on the volume of distribution of both enantiomers. The S/R ratio for total body clearance decreased significantly from 0.66 in healthy subjects to 0.54 in the uraemics, while the ratio for terminal elimination half-life significantly increased from 1.43 to 1.59.These results indicate that uraemia alters the degree of stereoselectivity in the pharmacokinetic parameters of tocainide enantiomers.     

Disposition of the enantiomers of hydroxychloroquine in patients with rheumatoid arthritis following multiple doses of the racemate.

In eight patients with rheumatoid arthritis receiving racemic hydroxychloroquine, blood and urine concentrations of the enantiomers of hydroxychloroquine and its major metabolites were measured each month over the first 6 months of therapy. Plasma concentrations of hydroxychloroquine enantiomers were measured in five of these patients. In all patients, the blood concentration of (R)-hydroxychloroquine exceeded that of the (S)-enantiomer, the mean (R)/(S) ratio being 2.2 (range 1.6-2.9). A similar excess of (R)-hydroxychloroquine was found in the plasma, the mean (R)/(S) ratio being 1.6 (range 1.2-1.9). The mean enantiomer blood concentration ratio (R)/(S) for the metabolite desethylhydroxychloroquine was 0.45 (range 0.34-0.58) and for desethylchloroquine it was 0.56 (range 0.35-0.86) suggesting stereoselective metabolism of hydroxychloroquine. (S)-hydroxychloroquine had a mean (+/- s.d.) renal clearance from blood of 41 +/- 11 ml min-1, approximately twice that of (R)-hydroxychloroquine. The predicted unbound renal clearance was also higher for (S)-hydroxychloroquine. The clinical implications of enantioselective disposition of hydroxychloroquine are currently not known.     

Stereoselective renal tubular secretion of a new uricosuric diuretic, 6,7-dichloro-5-(N,N-dimethylsulfamoyl)-2,3-dihydro-2-benzofurancarboxyl ic acid (S-8666), in cynomolgus monkeys.

Plasma concentration-time curves and urinary excretion of individual enantiomers of unchanged S-8666 and its N-monodemethylated metabolite, M-1, in male cynomolgus monkeys were measured after oral administration of racemic S-8666 at doses of 5, 10, and 50 mg/kg and also after intravenous injection at doses of 1, 5, and 10 mg/kg. The Tmax values for individual enantiomers of S-8666 in fasted male monkeys were 30 min after oral administration. The AUC values for S(-)-S-8666 were greater than those for R(+)-S-8666 and oral dose-AUC relationships of both enantiomers showed a linearity over the dose range used. Most of the S-8666 and a trace of M-1 were excreted in the first 24-hr urine, with no evidence of stereoselectivity from the amounts excreted. Since large portions were recovered from the urine as unchanged S-8666 after intravenous injection, most excretion occurs via the kidney. The t1/2 beta values and the Vdss for S(-)-S-8666 after intravenous injection were smaller than those for R(+)-S-8666. The CLt and the CLr values decreased with increasing intravenous doses, indicating saturation at a renal excretion process at high plasma concentrations of S-8666. The CLt and CLr values for R(+)-S-8666 were greater than those for the S(-)-enantiomer. The unbound fraction of R(+)-S-8666 in plasma was significantly greater than that of S(-)-S-8666 [21.6% for R(+), 12.0% for S(-)]. Renal clearance for the unbound fraction of S(-)-S-8666 was greater than that of R(+)-S-8666, suggesting stereoselective renal tubular secretion.     

Enantioselective disposition and protein binding of [(5,6-dichloro-9a-propyl-3-oxo-2,3,9,9a-tetrahydro-1-H-fluoren-7 -yl)-oxy] acetic acid, a new cerebral antiedemic agent, in rats

[(5,6-Dichloro-9a-propyl-3-oxo-2,3,9,9a-tetrahydro-1-H-fluoren-7-y l)-oxy] acetic acid (DPOFA) is an agent capable of reducing the swelling of astroglial cells in brain tissues. In vitro studies have demonstrated that the (R)-(+)-form of DPOFA is more effective than its (S)-(-)-form in inhibiting tissue swelling. The purpose of this study is to compare the elimination kinetics of the enantiomers. A new stereoselective HPLC procedure was developed for the simultaneous quantitation of (R)-(+)- and (S)-(-)-enantiomers in plasma and bile samples. After iv administration of the racemic mixture (40 mg/kg), rats cleared the (R)-(+)-enantiomer more rapidly than the (S)-(-)-isomer; time-averaged total plasma clearances were 8.88 +/- 0.55 and 4.20 +/- 0.70 ml/min/kg (mean +/- SD), respectively. Similar results were observed when the individual isomers were administered (20 mg/kg iv). Both (R)-(+)- and (S)-(-)-enantiomers were highly bound to plasma protein. The (R)-(+)-isomer had a higher unbound fraction (2%) than did the (S)-(-)-enantiomer (0.8%). The intrinsic clearance of unbound drug for (R)-(+)- and (S)-(-)-enantiomers were 434 +/- 27 and 490 +/- 84 ml/min/kg, respectively, suggesting that the differences in the elimination of the enantiomers in rats were attributable to stereoselectivity in plasma protein binding rather than to enzyme activity. In vitro studies with isolated hepatocytes supported the hypothesis that there was no stereoselectivity in metabolism of the enantiomers.     

Stereoselective glucuronidation of formoterol by human liver microsomes

AIMS: Formoterol is a beta2-adrenoceptor agonist marketed as a racemic mixture of the active (R; R)- and inactive (S; S)-enantiomers (rac-formoterol). The drug produces prolonged bronchodilation by inhalation but there is significant interpatient variability in duration of effect. Previous work has shown that in humans formoterol is metabolized by conjugation with glucuronic acid but little is known about the stereoselectivity of this reaction. The aim of the present study was to investigate the glucuronidation of formoterol enantiomers in vitro by human liver microsomes. METHODS: The kinetics of formation of formoterol glucuronides during incubation of racemate and of single formoterol enantiomers with human liver microsomes (n=9) was characterized by chiral h.p.l.c. assay. RESULTS: The kinetics of glucuronidation of the two formoterol enantiomers obeyed the Michaelis-Menten equation. Glucuronidation of formoterol was stereoselective and occurred more than two times faster for (S; S)-formoterol than for (R; R)-formoterol. In incubations with single formoterol enantiomers, the median (n=9) Km values for (R; R)-glucuronide and (S; S)-glucuronide were 827.6 and 840.4 microm, respectively, and the median V max values were 2625 and 4304 pmol min-1 mg-1, respectively. Corresponding values determined in incubations with rac-formoterol were 357.2 and 312.1 microm and 1435 and 2086 pmol min-1 mg-1 for (R; R)- and (S; S)-glucuronide, respectively. Interindividual variation was large with the ratio of V max/Km (S; S/R; R) ranging from 0.57 to 6.90 for incubations with rac-formoterol. CONCLUSIONS: Our study demonstrates that glucuronidation of formoterol by human liver microsomes is stereoselective and subject to high interindividual variability. These findings suggest that clearance of formoterol in humans is subject to variable stereoselectivity which could explain the variation in duration of bronchodilation produced by inhaled formoterol in patients with asthma.     

Selective Effect of Adjuvant Arthritis on the Disposition of Propranolol Enantiomers in Rats Detected Using a Stereospecific HPLC Assay

Nonstereospecific studies have indicated that the pharmacokinetics of propranolol (PR) are altered in inflammatory conditions such as arthritis. However, as the kinetics and dynamics of PR are stereoselective, we examined the effect of adjuvant arthritis (AA) on the disposition of the individual enantiomers. A novel normal-phase stereospecific HPLC assay for PR was developed involving chiral derivatization with S-(naphthyl)ethyl isocyanate and fluorescence detection. Oral and iv doses of racemic PR were administered to control and AA rats (n = 6). AA had no significant effect on either clearance or S:R ratio after iv doses. On the other hand, after oral doses, clearance was significantly decreased in AA. Although significant for both enantiomers, this effect was more pronounced on the less active R-enantiomer. The AUC R:S ratio was, therefore, significantly altered (AA, 14 ± 3.0; control, 4.3 ± 1.2). Increased total (S + R) plasma concentrations of PR in AA, possibly due to a reduced intrinsic clearance, therefore, reflect mainly increased concentrations of the less active R-enantiomer.     

Steady-state pharmacokinetics of (R)- and (S)-methadone in methadone maintenance patients

AIMS: To investigate the steady-state pharmacokinetics of (R)- and (S)-methadone in a methadone maintenance population. METHODS: Eighteen patients recruited from a public methadone maintenance program underwent an interdosing interval pharmacokinetic study. Plasma and urine samples were collected and analysed for methadone and its major metabolite (EDDP) using stereoselective h.p.l.c. Methadone plasma protein binding was examined using ultrafiltration, and plasma alpha1-acid glycoprotein concentrations were quantified by radial immunoassay. RESULTS: (R)-methadone had a significantly (P < 0.05) greater unbound fraction (mean 173%) and total renal clearance (182%) compared with (S)-methadone, while maximum measured plasma concentrations (83%) and apparent partial clearance of methadone to EDDP (76%) were significantly (P < 0.001) lower. When protein binding was considered (R)-methadone plasma clearance of the unbound fraction (59%) and apparent partial intrinsic clearance to EDDP (44%) were significantly (P < 0.01) lower than for (S)-methadone, while AUCtau_?u?ss (167%) was significantly (P < 0. 001) greater. There were no significant (P > 0.2) differences between the methadone enantiomers for AUCtauss, steady-state plasma clearance, trough plasma concentrations and unbound renal clearance. Patients excreted significantly (P < 0.0001) more (R)-methadone and (S)-EDDP than the corresponding enantiomers. Considerable interindividual variability was observed for the pharmacokinetic parameters, with coefficients of variation of up to 70%. CONCLUSIONS: Steady-state pharmacokinetics of unbound methadone are stereoselective, and there is large interindividual variability consistent with CYP3A4 mediated metabolism to the major metabolite EDDP; the variability did not obscure a significant dose-plasma concentration relationship. Stereoselective differences in the pharmacokinetics of methadone may have important implications for pharmacokinetic-pharmacodynamic modelling but is unlikely to be important for therapeutic drug monitoring of methadone, in the setting of opioid dependence.     

Stereoselective pharmacokinetics and metabolism of the enantiomers of cyclophosphamide. Preliminary results in humans and rabbits

[R(+),S(-)]-Cyclophosphamide [(R,S)-CP] is an anticancer drug, containing a chiral phosphorous atom, which is prepared and used clinically as the racemic mixture. A new high-performance liquid chromatographic assay suitable for pharmacokinetic studies of CP enantiomers in plasma has been reported recently by this laboratory (Reid et al., Anal Chem 61: 441-446, 1989). Briefly, the assay involves ethyl acetate extraction of CP enantiomers from plasma followed by derivatization to diastereomers in a two-step process utilizing chloral and (+)-naproxen acid chloride. Chromatographic analysis was performed on a reversed phase (ODS) column with detection at 232 nm. In the present study, preliminary results on the applicability of this assay to pharmacokinetic studies are presented. Several rabbits were used to compare the influence of i.p., i.v., and oral routes of administration on the stereoselective disposition of (R,S)-CP. Following i.p. administration, S-CP was cleared faster than R-CP. Following oral administration, only R-CP was detectable in plasma, while i.v. administration resulted in minor or no stereoselective disposition. These results indicated that there was a marked stereoselective metabolism of the S-CP enantiomer, with the i.p. and oral routes producing the greatest differences due to first-pass metabolism. Incubation of rabbit-liver microsomes with (R,S)-CP demonstrated that the monooxygenase system can exhibit marked stereoselectivity in its metabolism of CP. The ratio of R-CP to S-CP in the incubation medium increased during the incubation period from 1:1 initially to 4.5:1 after 60 min. The results from the experiments with rabbits indicate that the first-pass metabolism of this drug is highly stereoselective; in contrast, cancer patients who had received (R,S)-CP as an i.v. infusion showed no stereoselectivity in the elimination of the enantiomers. Pharmacokinetic studies with cancer patients, receiving (R,S)-CP as an oral dose, are in progress in order to determine if stereoselective first-pass metabolism of this drug also occurs in humans.     

Species-related stereoselective disposition of ofloxacin in the rat, dog and monkey.

1. The stereoselective disposition of ofloxacin (OFLX) was studied in rats, dogs and monkeys after oral administration of racemic OFLX. 2. In rats serum concentrations of (R)-(+)-OFLX were much greater than those of (S)-(-)-OFLX, which is the active form of OFLX. In monkeys, by contrast, serum concentrations of (S)-(-)-OFLX predominated over (R)-(+)-OFLX levels. In dogs there were no differences in AUC or Cmax between the enantiomers. Thus, there exists a species-related difference in the stereoselective disposition of OFLX. 3. In rats the stereoselective differences were mainly due to stereoselective glucuronidation; OFLX is hardly metabolized in dogs, monkeys and humans. 4. In monkeys the AUC of (S)-(-)-OFLX was increased by co-administration of the (R)-(+)-form, indicating that the stereoselectivity of OFLX disposition in monkeys may be caused by competition between the enantiomers for renal excretion, especially for renal tubular secretion.     

Metoprolol-paroxetine interaction in human liver microsomes: stereoselective aspects and prediction of the in vivo interaction.

This study in human liver microsomes was undertaken to establish whether paroxetine stereoselectively inhibits the oxidative metabolism of metoprolol in vitro, and whether the in vivo observed magnitude of the paroxetine-metoprolol interaction was predictable from these in vitro data. Two distinct approaches were used: inhibitory effect of paroxetine on 1) the formation of alpha-hydroxymetoprolol and O-desmethylmetoprolol from the individual metoprolol enantiomers and 2) on the depletion of the enantiomers from the incubation mixture. Nonspecific binding of both metoprolol and paroxetine to human liver microsomes was also investigated. Whereas metoprolol displayed negligible binding, paroxetine was extensively bound to microsomal proteins. This was taken into account in order to obtain unbiased K(i) values and unbound concentrations of paroxetine. In the substrate depletion experiments, the intrinsic clearance (CL(int)) of (R)-metoprolol was larger than that of (S)-metoprolol. Paroxetine caused a concentration-dependent decrease in CL(int) of both enantiomers and abolished the stereoselectivity. In the metabolite formation experiments paroxetine did not stereoselectively affect alpha-hydroxylation, but preferentially inhibited the O-demethylation of the (R)-enantiomer versus the (S)-enantiomer. The use of unbound paroxetine concentrations in the two in vitro methods yielded comparable predicted increases in area under the curve (1.7-1.9 and 2.2-2.5 for (S)- and (R)-metoprolol, respectively) but underestimated the in vivo observed changes of about 7- and 10-fold, respectively. In conclusion, this study showed that paroxetine abolishes the stereoselective metabolism of metoprolol due to a stereoselective inhibition of the O-demethylation toward (R)-metoprolol. Furthermore, the extent of the in vivo metoprolol-paroxetine interaction was substantially underestimated by either one of the two in vitro approaches used when a competitive mechanism was assumed.     

Simultaneous analysis of the enantiomers of verapamil and norverapamil in rat plasma by liquid chromatography-tandem mass spectrometry

An enantioselective micromethod for the simultaneous analysis of verapamil (VER) and norverapamil (NOR) in plasma was developed, validated and applied to the study of the kinetic disposition of VER and NOR after the administration of a single oral dose of racemic-VER to rats. VER, NOR and the internal standard (paroxetine) were extracted from only 100-μL plasma samples using n-hexane and the enantiomers were resolved on a Chiralpak AD column using n-hexane:isopropanol:ethanol:diethylamine (88:6:6:0.1) as the mobile phase. The analyses were performed in the selected reaction monitoring mode. Transitions 456 > 166 for VER enantiomers, 441 > 166 for NOR enantiomers and 330 > 193 for the internal standard were monitored and the method had a total chromatographic run time of 12 min. The method allows the determination of VER and NOR enantiomers at plasma levels as low as 1.0 ng/mL. Racemic VER hydrochloride (10 mg/kg) was given to male Wistar rats by gavage and blood samples were collected from 0 to 6.0 h (n = 6 at each time point). The concentration of (−)-(S)-VER was three folds higher than (+)-(R)-VER, with an AUC ratio (−)/(+) of 2.66. Oral clearance values were 12.17 and 28.77 L/h/kg for (−)-(S)-VER and (+)-(R)-VER, respectively. The pharmacokinetic parameters of NOR were not shown to be enantioselective.     

Effects of the enantiomers of disopyramide and its major metabolite on the electrophysiological characteristics of the guinea-pig papillary muscle

Summary Disopyramide, a Class la antiarrhythmic drug, is clinically used as a racemic mixture; R(–)disopyramide and S(+)disopyramide. The major metabolite in man is desisopropyldisopyramide: R(–)desisopropyl-disopyramide and S(+)desisopropyldisopyramide. The effects of the four compounds were compared on the electrophysiological characteristics of the guinea-pig papillary muscle using the standard microelectrode technique. At an external K⁺ concentration of 5.4 mmol/l and a stimulation frequency of 1 Hz, S(+)disopyramide (20 mol/l) increased action potential duration (APD) by more than 18%, while it was diminished by 6% in the presence of R(–)disopyramide. Resting membrane potential amounted to –87.1 ± 0.5 mV (n = 14) and –85.6 ± 1.2 mV (n = 10), respectively. Also a small but significant difference in effect on the maximal rate of depolarization was observed, R(–)disopyramide being more potent, related with a slower recovery of the maximal rate of depolarization.The enantiomers of the metabolite appeared to be three times less potent than those of the parent drug in their effect on the maximal rate of depolarization. The characteristics of the enantiomers of the metabolite correlated with those of the parent drug: also the R(–)-enantiomer was more potent in decreasing the maximal rate of depolarization and caused more shortening of the action potential than the S(+)enantiomer.Time constants for onset and recovery of/from rate dependent block of the maximal rate of depolarization were dependent upon the external K⁺ concentration, both for the enantiomers of the parent drug and those of the metabolite. Onset slowed down while recovery accelerated when external K⁺ was increased. Time constants were lower for the metabolite.When stimulation interval was shortened, the effect on the maximal rate of depolarisation increased. Only for the metabolite statistical significant stereoselective differences were observed at all stimulation intervals. The effects on the action potential duration were dependent upon stimulation interval; for all enantiomers the action potential duration tended to be relatively (% of control) higher at short stimulation intervals than at large stimulation intervals. The effect on the maximal rate of depolarization was also voltage dependent, but no significant differences were observed between the enantiomers, for the parent drug as well as for the metabolite. Send offprint requests to N. Verbeke at the above address     

Stereoselective pharmacokinetics of BOF-4272 racemate after oral administration to rats and dogs

The stereoselective pharmacokinetics of BOF-4272 enantiomers in rats and dogs was investigated by simultaneously measuring concentrations in arterial, portal, and venous plasma, the liver, and the kidney at 2 h after the oral administration of the racemic drug. The concentrations of BOF-4272 enantiomers were measured using high-performance liquid chromatography. The concentrations of the S(-) enantiomer in arterial, portal, and venous plasma were higher than those of the R(+) enantiomer in rats, but the opposite was found in dogs. In rats, absorption from the intestinal tract into the portal system was almost the same for the two enantiomers, whereas the hepatic uptake of the R(+) enantiomer was greater than that of the S(-) enantiomer. In dogs, absorption from the intestinal tract into the portal system was greater for the R(+) enantiomer than for the S(-) enantiomer, whereas hepatic uptake was comparable for the two enantiomers. The stereoselectivity of the renal uptake of BOF-4272 enantiomers had little effect on the stereoselectivity of enantiomers in the systemic circulation in both rats and dogs. The stereoselectivity in the systemic circulation of BOF-4272 enantiomers is therefore related to hepatic uptake in rats, and to absorption from the intestinal tract into the portal system in dogs.     

外消旋普罗帕酮在经诱导的大鼠肝微粒体中N-去丙基化的立体选择性

采用对照及β-萘黄酮(β-NF)或地塞米松(Dex)诱导的大鼠肝微粒体,应用GITC柱前衍生化,反相高效液相色谱法研究了消旋普罗帕酮〔(R/S)-PPF〕体外代谢的立体选择性. 实验结果表明,在Dex,β-NF诱导的微粒体中有N-去丙基普罗帕酮生成。在β-NF,Dex预处理组,(R/S)-PPF低浓度时的经肝微粒体代谢具有立体选择性,R(-)对映体的清除大于S(+)对映体,高浓度时的代谢无立体选择性. R(-)对映体的K_m值显著低于S(+)对映体,而V_max值无显著性差异. 在Dex预处理组中的立体选择性大于β-NF组. 在对照组中代谢无立体选择性,且K_m,V_max值均小于β-NF,Dex预 处理组。结果提示,CYP1A,CYP3A4亚族对普罗帕酮(PPF)的N-去丙基化有贡献. (R/S)-PPF的N-去丙基化具有浓度依赖性的立体选择性.