Chiral HPLC determination and stereoselective pharmacokinetics of tetrahydroberberine enantiomers in rats

Tetrahydroberberine (THB), a racemic mixture of (+)‐ and (?)‐enantiomer, is a biologically active ingredient isolated from a traditional Chinese herb Rhizoma corydalis (yanhusuo). A chiral high performance liquid chromatography method has been developed for the determination of THB enantiomers in rat plasma. The enantioseparation was carried out on a Chiral®‐AD column using methanol:ethanol (80:20, v/v) as the mobile phase at the flow rate 0.4 ml/min. The ultraviolet detection was set at 230 nm. The calibration curves were linear over the range of 0.01–2.5 μg/ml for (+)‐THB and 0.01‐5.0 μg/ml for (?)‐THB, respectively. The lower limit of quantification was 0.01 μg/ml for both (+)‐THB and (?)‐THB. The stereoselective pharmacokinetics of THB enantiomers in rats was studied after oral and intravenous administration at a dose of 50 and 10 mg/kg racemic THB (rac‐THB). The mean plasma levels of (?)‐THB were higher at almost all time points than those of (+)‐THB. (?)‐THB also exhibited greater C_max, and AUC_0–∞, smaller CL and V_d, than its antipode. The (?)/(+)‐enantiomer ratio of AUC_0–∞ after oral and intravenous administration were 2.17 and 1.43, respectively. These results indicated substantial stereoselectivity in the pharmacokinetics of THB enantiomers in rats. Chirality, 2012. © 2012 Wiley Periodicals, Inc.     

Stereoselective chiral inversion of pantoprazole enantiomers after separate doses to rats

(±)-Pantoprazole ((±)-PAN), (±)-5-(difluoromethoxy)-2-[[(3.4-dimethoxy-2-pyridinyl)methyl]sulfinyl]-1H-benzimidazole is a chiral sulfoxide that is used clinically as a racemic mixture. The disposition kinetics of (+)-PAN and (−)-PAN given separately has been studied in rats. Serum levels of (+)- and (−)-PAN and its metabolites, pantoprazole sulfone (PAN-SO₂), pantoprazole sulfide (PAN-S), 4′-O-demethyl pantoprazole sulfone (DMPAN-SO₂), and 4′-O-demethyl pantoprazole sulfide (DMPAN-S) were measured by HPLC. Following single intravenous or oral administration, both enantiomers were rapidly absorbed and metabolized, resulting in similar serum concentrations, suggesting that the two enantiomers have approximately the same disposition kinetics. The major metabolite of both (+)- and (−)-PAN was PAN-SO₂, while DMPAN-SO₂ was also detected as a minor metabolite. Serum levels of PAN-S and DMPAN-S could not be quantified after intravenous or oral administration of either enantiomer. Significant chiral inversion occurred after intravenous and oral administration of (+)-PAN. The AUCs of (−)-PAN after intravenous and oral dosing of (+)-PAN were 36.3 and 28.1%, respectively of those of total [(+) + (−)] PAN. In contrast, the serum levels of (+)-PAN were below quantitation limits after intravenous or oral administration of (−)-PAN. Therefore, chiral inversion was observed only after administration of (+)-PAN, supporting the hypothesis that stereoselective inversion from (+)-PAN to (−)-PAN occurs in rats. Chirality 10:747–753, 1998. © 1998 Wiley-Liss, Inc.     

Stereoselective disposition of tiaprofenic acid enantiomers in rats

The pharmacokinetics and metabolic chiral inversion of the S(+)‐ and R(−)‐enantiomers of tiaprofenic acid (S‐TIA, R‐TIA) were assessed in vivo in rats, and in addition the biochemistry of inversion was investigated in vitro in rat liver homogenates. Drug enantiomer concentrations in plasma were investigated following administration of S‐TIA and R‐TIA (i.p. 3 and 9 mg/kg) over 24 hr. Plasma concentrations of TIA enantiomers were determined by stereospecific HPLC analysis. After administration of R‐TIA it was found that 1) there was a time delay of peak S‐TIA plasma concentrations, 2) S‐TIA concentrations exceeded R‐TIA concentrations from ∼2 hr after dosing, 3) C_max and AUC_(0‐∞) for S‐TIA were greater than for R‐TIA following administration of S‐TIA, and 4) inversion was bidirectional but favored inversion of R‐TIA to S‐TIA. Bidirectional inversion was also observed when TIA enantiomers were incubated with liver homogenates up to 24 hr. However, the rate of inversion favored transformation of the R‐enantiomer to the S‐enantiomer. In conclusion, stereoselective pharmacokinetics of R‐ and S‐TIA were observed in rats and bidirectional inversion in rat liver homogenates has been demonstrated for the first time. Chiral inversion of TIA may involve metabolic routes different from those associated with inversion of other 2‐arylpropionic acids such as ibuprofen. Chirality 11:103–108, 1999. © 1999 Wiley‐Liss, Inc.     

Analysis of Oxcarbazepine and the 10‐Hydroxycarbazepine Enantiomers in Plasma by LC‐MS/MS: Application in a Pharmacokinetic Study

Oxcarbazepine is a second‐generation antiepileptic drug indicated as monotherapy or adjunctive therapy in the treatment of partial seizures or generalized tonic–clonic seizures in adults and children. It undergoes rapid presystemic reduction with formation of the active metabolite 10‐hydroxycarbazepine (MHD), which has a chiral center at position 10, with the enantiomers (S)‐(+)‐ and R‐(?)‐MHD showing similar antiepileptic effects. This study presents the development and validation of a method of sequential analysis of oxcarbazepine and MHD enantiomers in plasma using liquid chromatography with tandem mass spectrometry (LC‐MS/MS). Aliquots of 100 μL of plasma were extracted with a mixture of methyl tert‐butyl ether: dichloromethane (2:1). The separation of oxcarbazepine and the MHD enantiomers was obtained on a chiral phase Chiralcel OD‐H column, using a mixture of hexane:ethanol:isopropanol (80:15:5, v/v/v) as mobile phase at a flow rate of 1.3 mL/min with a split ratio of 1:5, and quantification was performed by LC‐MS/MS. The limit of quantification was 12.5 ng oxcarbazepine and 31.25 ng of each MHD enantiomer/mL of plasma. The method was applied in the study of kinetic disposition of oxcarbazepine and the MHD enantiomers in the steady state after oral administration of 300 mg/12 h oxcarbazepine in a healthy volunteer. The maximum plasma concentration of oxcarbazepine was 1.2 µg/mL at 0.75 h. The kinetic disposition of MHD is enantioselective, with a higher proportion of the S‐(+)‐MHD enantiomer compared to R‐(?)‐MHD and an AUC^0‐12 _{S‐(+)/R‐(?)} ratio of 5.44. Chirality 25:897–903, 2013. © 2013 Wiley Periodicals, Inc.     

Enantioselective Pharmacokinetics of Doxazosin and Pharmacokinetic Interaction Between the Isomers in Rats

In this study, the stereoselective pharmacokinetics of doxazosin enantiomers and their pharmacokinetic interaction were studied in rats. Enantiomer concentrations in plasma were measured using chiral high‐pressure liquid chromatography (HPLC) with fluorescence detection after oral or intravenous administration of (–)‐(R)‐doxazosin 3.0 mg/kg, (+)‐(S)‐doxazosin 3.0 mg/kg, and rac‐doxazosin 6.0 mg/kg. AUC values of (+)‐(S)‐doxazosin were always larger than those of (–)‐(R)‐doxazosin, regardless of oral or intravenous administration. The maximum plasma concentration (C_max) value of (–)‐(R)‐doxazosin after oral administration was significantly higher when given alone (110.5 ± 46.4 ng/mL) versus in racemate (53.2 ± 19.7 ng/mL), whereas the C_max value of (+)‐(S)‐doxazosin did not change significantly. The area under the curve (AUC) and C_max values for (+)‐(S)‐doxazosin after intravenous administration were significantly lower, and its Cl value significantly higher, when given alone versus in racemate. We speculate that (–)‐(R)‐doxazosin increases (+)‐(S)‐doxazosin exposure probably by inhibiting the elimination of (+)‐(S)‐doxazosin, and the enantiomers may be competitively absorbed from the gastrointestinal tract. In conclusion, doxazosin pharmacokinetics are substantially stereospecific and enantiomer–enantiomer interaction occurs after rac‐administration. Chirality 27:738–744, 2015. © 2015 Wiley Periodicals, Inc.     

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.     

Influence of the route of administration on the pharmacokinetics of pirprofen enantiomers in the rat

The pharmacokinetics of the enantiomers of the non-steroidal anti-inflammatory drug pirprofen were studied in male Sprague-Dawley rats after oral and intravenous (iv) doses of the racemate. No significant differences were detected between the enantiomers after oral or iv dosing in t_½, Vd, or ∑Xu. However, the R:S area under the plasma concentration (AUC) ratio after oral doses (0.92 ± 0.13) was slightly but significantly lower than after matching iv doses (1.05 ± 0.036). The absolute bioavailability of the active S-enantiomer (78.5%) after oral doses was higher than the inactive R-enantiomer (69.3%). The plasma protein binding of both enantiomers was saturable over a fivefold range of plasma concentrations. At higher plasma concentrations, the S-enantiomer was less bound than the R-enantiomer. In an in vitro experiment using everted rat jejunum, no chiral inversion was discernible. The dependency of the AUC ratio of the enantiomers on the route of administration may be due to stereoselective first-pass metabolism. © 1993 Wiley-Liss, Inc.     

Influence of Dose and Route of Administration on the Enantioselective Pharmacokinetics of TJ0711 Hydrochloride in Rats

The enantioselective pharmacokinetics of TJ0711 hydrochloride were studied in rats given different doses of rac‐TJ0711 hydrochloride via intravenous and oral routes. R‐ and S‐TJ0711 hydrochloride were both rapidly absorbed, and the average AUC_0‐∞ of R‐TJ0711 hydrochloride was greater than that of S‐TJ0711 hydrochloride after intragastric administration, with an R/S AUC ratio 1.11 and 1.35 for 30 and 50 mg/kg dose group, respectively. In contrast, the average AUC_0‐∞ of R‐TJ0711 hydrochloride was smaller than that of S‐TJ0711 hydrochloride after intravenous injection, with an R/S AUC ratio 0.57 and 0.73 for 10 and 20 mg/kg dose group, respectively. R‐TJ0711 hydrochloride plasma half‐lives were shorter than those of S‐TJ0711 hydrochloride for all groups. AUC_0‐4h and C_max between the two enantiomers were significantly different after oral administration of 50 mg/kg dose of the racemate, while no significant differences between the two enantiomers were found for all the pharmacokinetic parameters of the 30 mg/kg dose group. Significant differences between the two enantiomers were detected for nearly all the pharmacokinetic parameters after intravenous administration, except for the V_Z of 20 mg/kg dose group. This study suggests that dose and route of administration will influence the enantioselectivity in the pharmacokinetics of TJ0711 hydrochloride in rats. Chirality 27:53–57, 2015. © 2014 Wiley Periodicals, Inc.     

Stereoselective metabolism of benalaxyl in liver microsomes from rat and rabbit

Benalaxyl (BX), methyl‐N‐phenylacetyl‐N‐2,6‐xylyl alaninate, is a potent acylanilide fungicide and consist of a pair of enantiomers. The stereoselective metabolism of BX was investigated in rat and rabbit microsomes in vitro. The degradation kinetics and the enantiomer fraction (EF) were determined using normal high‐performance liquid chromatography with diode array detection and a cellulose‐tris‐(3,5‐dimethylphenylcarbamate)‐based chiral stationary phase (CDMPC‐CSP). The t_1/2 of (?)‐R‐BX and (+)‐S‐BX in rat liver microsomes were 22.35 and 10.66 min of rac‐BX and 5.42 and 4.03 of BX enantiomers. However, the t_1/2 of (?)‐R‐BX and (+)‐S‐BX in rabbit liver microsomes were 11.75 and 15.26 min of rac‐BX and 5.66 and 9.63 of BX enantiomers. The consequence was consistent with the stereoselective toxicokinetics of BX in vitro. There was no chiral inversion from the (?)‐R‐BX to (+)‐S‐BX or inversion from (+)‐S‐BX to (?)‐R‐BX in both rabbit and rat microsomes. These results suggested metabolism of BX enantiomers was stereoselective in rat and rabbit liver microsomes. Chirality, 2011. © 2010 Wiley‐Liss, Inc.     

Stereoselective Toxicity and Metabolism of Lactofen in Primary Hepatocytes From Rat

The stereoselective metabolism of lactofen in primary rat hepatocytes was studied using a chiral high‐performance liquid chromatographic (HPLC) method. Rac‐lactofen and its two enantiomers, S‐(+)‐ and R‐(?)‐lactofen, as well as two of its major metabolites, acifluorfen, S‐(+)‐ and R‐(?)‐desethyl lactofen, were used as substrates,. The single and joint cytotoxicity of parent compounds and the metabolites were assessed by coincubation with rat hepatocytes as target cells. Cytotoxicity was determined by the methyl tetrazolium (MTT) assay. In hepatocyte incubations, S‐(+)‐lactofen was degraded more rapidly than R‐(?)‐lactofen, and a stereospecific formation of S‐(+)‐desethyl lactofen was detected. Metabolism of lactofen to desethyl lactofen was processed with the retention of configuration, and the achiral compound, acifluorfen, was the shared metabolite generated from both S‐(+)‐ and R‐(?)‐lactofen. There was no chiral conversion of lactofen or desethyl lactofen enantiomers during the incubation. For the cytotoxicity research, the calculated EC₅₀ values indicated that when being applied individually, the parent compound was less toxic than its metabolites, while the combination with metabolites enhanced its cytotoxic effects. The data presented here would be helpful for a more comprehensive assessment of the ecotoxicological and environmental risks of lactofen. Chirality 25:743–750, 2013. © 2013 Wiley Periodicals, Inc.     

Study on the stereoselective excretion of tetrahydropalmatine enantiomers in rats and identification of in vivo metabolites by liquid chromatography‐tandem mass spectrometry

The objective of this work was to study the stereoselectivity in excretion of tetrahydropalmatine (THP) enantiomers by rats and identify the metabolites of racemic THP (rac‐THP) in rat urine. Urine and bile samples were collected at various time intervals after a single oral dose of rac‐THP. The concentrations of THP enantiomers in rat urine and bile were determined using a modification of an achiral–chiral high‐performance liquid chromatographic (HPLC) method that had been previously published. The cumulative urinary excretion over 96 h of (?)‐THP and (+)‐THP was found to be 55.49 ± 36.9 μg and 18.33 ± 9.7 μg, respectively. The cumulative biliary excretion over 24 h of (?)‐THP and (+)‐THP was 19.19 ± 14.6 μg and 12.53 ± 10.4 μg, respectively. The enantiomeric (?/+) concentration ratios of THP changed from 2.80 to 5.15 in urine, and from 1.36 to 1.80 in bile. The mean cumulative amount of (?)‐THP was significantly higher than that of (+)‐THP both in urine and bile samples. However, the enantiomeric (?/+) concentration ratios in rat urine and bile were significantly lower than those ratios in rat plasma. These findings suggested the excretion of THP enantiomers was stereoselective rather than a reflection of chiral pharmacokinetic aspects in plasma and (?)‐THP was preferentially excreted in rat urine and bile. Three O‐demethylation metabolites and the parent drug rac‐THP were detected by liquid chromatography‐tandem mass spectrometry in rat urine. One metabolite was obtained by preparative HPLC and identified as 10‐O‐demethyl‐THP. Chirality, 2010. © 2009 Wiley‐Liss, Inc.     

Stereoselective Degradation of alpha‐Cypermethrin and Its Enantiomers in Rat Liver Microsomes

Alpha‐cypermethrin (α‐CP), [(RS)‐a‐cyano‐3‐phenoxy benzyl (1RS)‐cis‐3‐(2, 2‐dichlorovinyl)‐2, 2‐dimethylcyclopropanecarboxylate], comprises a diastereoisomer pair of cypermethrin, which are (+)‐(1R‐cis‐αS)–CP (insecticidal) and (?)‐(1S‐cis‐αR)–CP (inactive). In this experiment, the stereoselective degradation of α‐CP was investigated in rat liver microsomes by high‐performance liquid chromatography (HPLC) with a cellulose‐tris‐ (3, 5‐dimethylphenylcarbamate)‐based chiral stationary phase. The results revealed that the degradation of (?)‐(1S‐cis‐αR)‐CP was much faster than (+)‐(1R‐cis‐αS)‐CP both in enantiomer monomers and rac‐α‐CP. As for the enzyme kinetic parameters, there were some variances between rac‐α‐CP and the enantiomer monomers. In rac‐α‐CP, the V_max and CL_int of (+)‐(1R‐cis‐αS)–CP (5105.22 ± 326.26 nM/min/mg protein and 189.64 mL/min/mg protein) were about one‐half of those of (?)‐(1S‐cis‐αR)–CP (9308.57 ± 772.24 nM/min/mg protein and 352.19 mL/min/mg protein), while the K_m of the two α‐CP enantiomers were similar. However, in the enantiomer monomers of α‐CP, the V_max and K_m of (+)‐(1R‐cis‐αS) ‐CP were 2‐fold and 5‐fold of (?)‐(1S‐cis‐αR)‐CP, respectively, which showed a significant difference with rac‐α‐CP. The CL_int of (+)‐(1R‐cis‐αS)–CP (140.97 mL/min/mg protein) was still about one‐half of (?)‐(1S‐cis‐αR)–CP (325.72 mL/min/mg protein) in enantiomer monomers. The interaction of enantiomers of α‐CP in rat liver microsomes was researched and the results showed that there were different interactions between the IC₅₀ of (?)‐ to (+)‐(1R‐cis‐αS)‐CP and (+)‐ to (?)‐(1S‐cis‐αR)‐CP(IC_50(?)/(+) / IC_50(+)/(?) = 0.61). Chirality 28:58–64, 2016. © 2015 Wiley Periodicals, Inc.     

Stereospecific determination,chiral inversion in vitro and pharmacokinetics in humans of the enantiomers of thalidomide

The purposes of this work were (1) to develop a high performance liquid chromatographic (HPLC) assay for the enantiomers of thalidomide in blood, (2) to study their inversion and degradation in human blood, and (3) to study the pharmacokinetics of (+)-(R)- and (?)-(S)-thalidomide after oral administration of the separate enantiomers or of the racemate to healthy male volunteers. The enantiomers of thalidomide were determined by direct resolution on a tribenzoyl cellulose column. Mean rate constants of chiral inversion of (+)-(R)-thalidomide and (?)-(S)-thalidomide in blood at 37°C were 0.30 and 0.31 h^?1, respectively. Rate constants of degradation were 0.17 and 0.18 h^?1. There was rapid interconversion in vivo in humans, the (+)-(R)-enantiomer predominating at equilibrium. The pharmacokinetics of (+)-(R)- and (?)-(S)-thalidomide could be characterized by means of two one-compartment models connected by rate constants for chiral inversion. Mean rate constants for in vivo inversion were 0.17 h^?1 (R to S) and 0.12 h^?1 (S to R) and for elimination 0.079 h^?1 (R) and 0.24 h^?1 (S), i.e., a considerably faster rate of elimination of the (?)-(S)-enantiomer. Putative differences in therapeutic or adverse effects between (+)-(R)- and (?)-(S)-thalidomide would to a large extent be abolished by rapid interconversion in vivo. © 1995 Wiley-Liss, Inc.     

Pyrrole Derivatives and Diterpene Alkaloids from the South China Sea Sponge Agelas nakamurai

Two pairs of new non‐brominated racematic pyrrole derivatives, (±)‐nakamurine D ( 1 ) and (±)‐nakamurine E ( 2 ), two new diterpene alkaloids, isoagelasine C ( 16 ) and isoagelasidine B ( 21 ), together with 13 known pyrrole derivatives ((±)‐ 3  –  15 ), five known diterpene alkaloids ( 17  –  20 , 22 ) were isolated from the South China Sea sponge Agelas nakamurai . The racemic mixtures, compounds 1  –  4 , were resolved into four pairs of enantiomers, (+)‐ 1 and (?)‐ 1 , (+)‐ 2 and (?)‐ 2 , (+)‐ 3 and (?)‐ 3 , and (+)‐ 4 and (?)‐ 4 , by chiral HPLC . The structures and absolute configurations were elucidated on the basis of comprehensive spectroscopic analyses, quantum chemical calculations, quantitative measurements of molar rotations, application of van't Hoff 's principle of optical superposition, and comparison with the literature data. The NMR and MS data of compound 3 are reported for the first time, as the structure was listed in SciFinder Scholar with no associated reference. These non‐brominated pyrrole derivatives were found in this species for the first time. Compound 18 showed valuable cytotoxicities against HL ‐60, K562, and HCT ‐116 cell lines with IC ₅₀ values of 12.4, 16.0, and 19.8 μm , respectively. Compounds 16  –  19 , 21 , and 22 showed potent antifungal activities against Candida albicans with MIC values ranging from 0.59 to 4.69 μg/ml. Compounds 16  –  19 exhibited moderate antibacterial activities against Proteusbacillus vulgaris (MIC values ranging from 9.38 to 18.75 μg/ml).     

Pharmacokinetics of Terazosin Enantiomers in Healthy Chinese Male Subjects

The purpose of this study was to elucidate the pharmacokinetics of terazosin enantiomers in healthy Chinese male subjects. After a single oral dose of 2‐mg terazosin, the plasma concentrations of terazosin enantiomers were measured over the course of 48 h in 12 healthy subjects. The plasma concentrations of (+)‐(R)‐terazosin at all time points were higher than those of (?)‐(S)‐terazosin. The area under the plasma concentration–time curve (AUC_0–∞) and maximum plasma concentration of (+)‐(R)‐terazosin were significantly greater than those of the (?)‐(S)‐terazosin (P < 0.01, respectively). The R/S ratio of AUC_0–∞ of terazosin was 1.68. For the first time, it was proven that the pharmacokinetics of terazosin was stereoselective in healthy Chinese male subjects. Chirality 24:1047–1050, 2012. © 2012 Wiley Periodicals, Inc.     

Stereoselective Determination of Metoprolol and its Metabolite α‐Hydroxymetoprolol in Plasma by LC‐MS/MS: Application to Pharmacokinetics during Pregnancy

Metoprolol is available for clinical use as a racemic mixture. The S‐(?)‐metoprolol enantiomer is the one expressing higher activity in the blockade of the β₁‐adrenergic receptor. The α‐hydroxymetoprolol metabolite also has activity in the blockade of the β₁‐adrenergic receptor. The present study describes the development and validation of a stereoselective method for sequential analysis of metoprolol and of α‐hydroxymetoprolol in plasma using high‐performance liquid chromatography with tandem mass spectrometry (LC‐MS/MS). 1‐ml aliquots of plasma were extracted with dichloromethane : diisopropyl ether (1:1, v/v). Metoprolol enantiomers and α‐hydroxymetoprolol isomers were separated on a Chiralpak AD column (Daicel Chemical Industries, New York, NY, USA) and quantitated by LC‐MS/MS. The limit of quantitation obtained was 0.2 ng of each metoprolol enantiomer/ml plasma and 0.1 ng/ml of each α‐hydroxymetoprolol isomer/ml plasma. The method was applied to the study of kinetic disposition of metoprolol in plasma samples collected up to 24 h after the administration of a single oral dose of 100‐mg metoprolol tartrate to a hypertensive parturient with a gestational age of 42 weeks. The clinical study showed that the metoprolol pharmakokinetics is enantioselective, with the observation of higher area under the curve (AUC)^0?∞ values for S‐(?)‐metoprolol (AUC_S‐(?)/AUC_R‐(+) = 1.81) and the favoring of the formation of the new chiral center 1′R of α‐hydroxymetoprolol (AUC^0?∞_1′R/1′S = 2.78). Chirality, 25:1–7, 2013. © 2012 Wiley Periodicals, Inc.     

Pharmacokinetics of the enantiomers of verapamil in the dog

The intravenous (0.5 mg/kg) and oral (5 mg/kg) dose kinetics of verapamil were studied in 6 dogs during steady-state oral verapamil dosing (5 mg/kg every 8 h for 3 days). Racemic verapamil and norverapamil, a metabolite of verapamil, were quantitated in plasma by HPLC-fluorescence detection. The verapamil peaks eluting off the column were collected and rechromatographed on an Ultron-OVM column, which resolved the two verapamil enantiomers. After intravenous administration, the systemic clearance and apparent volume of distribution of (?)-(S)-verapamil were nearly twice that of the (+)-(R)-isomer. There was no difference in the elimination half-lives between the two isomers. After oral administration, the oral clearance of (?)-(S)-verapamil was 20 times that of the (+)-(R)-isomer. The apparent bioavailability of (+)-(R)-verapamil was over 14 times that of (?)-(S)-verapamil. The plasma protein binding of the (+)-(R)-isomer was slightly higher by 5% than (?)-(S)-verapamil; however, this effect was not enough to account for the difference between the apparent volume of distribution of the enantiomers, indicating that the tissue binding of (?)-(S)-verapamil was greater than that of the (+)-(R)-isomer. This data on the disposition of the enantiomers of verapamil in the dog is similar to that reported for man and demonstrates that the dog may be an appropriate animal model for man in future studies on the disposition of the enantiomers of verapamil. © 1993 Wiley-Liss, Inc.     

Enantioselective Metabolism and Interference on Tryptophan Metabolism of Myclobutanil in Rat Hepatocytes

Myclobutanil, (RS)‐2‐(4‐chlorophenyl)‐2‐(1H‐1, 2, 4‐triazol‐1‐ylmethyl) hexanenitrile is a widely used triazole fungicide. In this study, enantioselective metabolism and cytotoxicity were investigated in rat hepatocytes by chiral HPLC‐MS/MS and the methyl tetrazolium (MTT) assay, respectively. Furthermore, tryptophan metabolism disturbance in rat hepatocytes after myclobutanil exposure was also evaluated by target metabolomics method. The half‐life (t1/2) of (+)‐myclobutanil was 10.66 h, whereas that for (?)‐myclobutanil was 15.07 h. Such results indicated that the metabolic process of myclobutanil in rat hepatocytes was enantioselective with an enrichment of (?)‐myclobutanil. For the cytotoxicity research, the calculated EC₅₀ (12h) values for rac‐myclobutanil, (+)‐ and (?)‐myclobutanil were 123.65, 150.65 and 152.60 µM, respectively. The results of tryptophan metabolites profiling showed that the levels of kynurenine (KYN) and XA were both up‐regulated compared to the control, suggesting the activation effect of the KYN pathway by myclobutanil and its enantiomers which may provide an important insight into its toxicity mechanism. The data presented here could be useful for the environmental hazard assessment of myclobutanil. Chirality 27:643–649, 2015. © 2015 Wiley Periodicals, Inc.     

Stereoselective Interaction Between Tetrahydropalmatine Enantiomers and CYP Enzymes in Human Liver Microsomes

Tetrahydropalmatine (THP), with one chiral center, is an alkaloid that possesses analgesic and many other pharmacological actives. The aim of the present study is to investigate stereoselective metabolism of THP enantiomers in human liver microsomes (HLM) and elucidate which cytochrome P450 (CYP) isoforms contribute to the stereoselective metabolism in HLM. Additionally, the inhibitions of THP enantiomers on activity of CYP enzymes are also investigated. The results demonstrated that (+)‐THP was preferentially metabolized by HLM. Ketoconazole (inhibitor of CYP3A4/5) inhibited metabolism of (?)‐THP or (+)‐THP at same degree, whereas the inhibition of fluvoxamine (inhibitor of CYP1A2) on metabolism of (+)‐THP was greater than that of (?)‐THP; moreover, the metabolic rate of (+)‐THP was 5.3‐fold of (?)‐THP in recombinant human CYP1A2. Meanwhile, THP enantiomers did not show obvious inhibitory effect on the activity of various CYP isoforms (CYP1A2, 2A6, 2C8, 2C9, 2C19, 2E1, and 3A4/5), whereas (?)‐THP, but not (+)‐THP, significantly inhibited the activity of CYP2D6 with the Ki value of 6.42 ± 0.38 μM. The results suggested that THP enantiomers were predominantly metabolized by CYP3A4/5 and CYP1A2 in HLM, and (+)‐THP was preferentially metabolized by CYP1A2, whereas CYP3A4/5 contributed equally to metabolism of (?)‐THP or (+)‐THP. Besides, the inhibition of CYP2D6 by (?)‐THP may cause drug–drug interaction, which should be considered. Chirality 25:43–47, 2013. © 2012 Wiley Periodicals, Inc.     

Evidence of absorption rate dependency of ibuprofen inversion in the rat

Ibuprofen (IB) is a chiral 2-arylpropionic acid derivative used as a nonsteroidal antiinflammatory drug (NSAID). It undergoes substantial R to S chiral inversion in humans and rats. In addition to systemic inversion, presystemic chiral inversion has been suggested for IB in humans but only after administration of formulations with slow absorption rates. In search for a suitable animal model, the absorption rate dependency of the extent of inversion was examined in male Sprague–Dawley rats given 20 mg/kg of racemic IB in aqueous solution (T_max, 0.6 h), suspension (T_max, 1 h) or as sustained release granules (T_max, 2.3 h). In addition, (R)-IB (5 mg/liter) was incubated in the presence of everted rat gut segments in an organ bath at 37°. After sustained release granules, the S:R AUC ratios (7.3 ± 1.5) were significantly higher than suspension (3.6 ± 1.1) and solution (3.5 ± 0.2). Accordingly, AUC_S and AUC_R, as percent of the total AUC (S + R), significantly increased and decreased, respectively, after administration of the sustained released granules as compared with the solution and suspension. A significant positive linear correlation was found between the S:R AUC ratios and the corresponding T_max for (R)-IB (r = 0.82). In vitro, (R)-IB was inverted by everted jejunum (12.2 ± 1.6%), ileum (14.2 ± 2.0%), and colon (4.4 ± 0.6%) segments. IB was also glucuronidated in the presence of the intestinal segments. Therefore, similar to earlier observations made in humans, in the rat, the S:R AUC ratio was positively and significantly correlated with the absorption rate from the dosage form. Rat small intestine was capable of inverting and conjugating (R)-IB. Hence, rat is a suitable model for studying the chiral inversion of IB. © 1994 Wiley-Liss, Inc.