Enantioselective toxic effects of hexaconazole enantiomers against Scenedesmus obliquus

Enantioselectivity in ecotoxicity of chiral pesticides in the aquatic environment has been a subject of growing interest. In this study, the toxicological impacts of hexaconazole enantiomers were investigated with freshwater algae Scenedesmus obliquus. After 96 h of exposure, the EC₅₀ values for rac‐hexaconazole, (+)‐hexaconazole, and (?)‐hexaconazole were 0.178, 0.355, and 0.065 mg l^?1, respectively. Therefore, the acute toxicities of hexaconazole enantiomers were enantioselective. In addition, the different toxic effects were evaluated when S. obliquus were exposed to 0.2, 0.5, and 1.0 mg l^?1 of rac‐hexaconazole, (+)‐hexaconazole, and (?)‐hexaconazole during 96 h, respectively. The chlorophyll a and chlorophyll b contents of S. obliquus treated by (?)‐hexaconazole were lower than those exposed to (+)‐hexaconazole, whereas the malondialdehyde contents of S. obliquus treated by (?)‐form were higher than those exposed to (+)‐form at higher concentrations. In general, catalase activities were significantly upregulated by exposure to (?)‐enantiomer than (+)‐enantiomer at all three concentrations. However, superoxide dismutase activities exposed to (?)‐hexaconazole were lower than that exposed to (+)‐hexaconazole at 0.2 mg l^?1 and 0.5 mg l^?1. On the basis of these data, the acute toxicity and toxic effects of hexaconazole against S. obliquus were enantioselective, and such enantiomeric differences must be taken into consideration in pesticide risk assessment. Chirality 24:610–614, 2012. © 2012 Wiley Periodicals, 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.     

Acute Toxicity,Bioactivity, and Enantioselective Behavior with Tissue Distribution in Rabbits of Myclobutanil Enantiomers

The enantioselective bioactivity against pathogens (Cercospora arachidicola, Fulvia fulva, and Phytophthora infestans) and acute toxicity to Daphnia magna of the fungicide myclobutanil enantiomers were studied. The (+)‐enantiomer in an antimicrobial activity test was about 1.79–1.96 times more active than the (–)‐enantiomer. In the toxicity assay, the calculated 24‐h LC₅₀ values of the (–)‐form, rac‐form and (+)‐form were 16.88, 13.17, and 11.91 mg/L, and the 48‐h LC₅₀ values were 10.15, 9.24, and 5.48 mg/L, respectively, showing that (+)‐myclobutanil was more toxic. Meanwhile, the enantioselective metabolism of myclobutanil enantiomers following a single intravenous (i.v.) administration was investigated in rabbits. Total plasma clearance value (CL) of the (+)‐enantiomer was 1.68‐fold higher than its antipode. Significant differences in pharmacokinetics parameters between the two enantiomers indicated that the high bioactive (+)‐enantiomer was preferentially metabolized and eliminated in plasma. Consistent consequences were found in the tissues (liver, brain, heart, kidney, fat, and muscle), resulting in a relative enrichment of the low‐activity (–)‐myclobutanil. These systemic assessments of the stereoisomers of myclobutanil cannot be used only to investigate environmental and biological behavior, but also have human health implications because of the long persistence of triazole fungicide and enantiomeric enrichment in mammals and humans. Chirality 26:784–789, 2014. © 2014 Wiley Periodicals, Inc.     

Bioaccumulation and Excretion of Enantiomers of Myclobutanil in Tenebrio molitor Larvae Through Dietary Exposure

The bioaccumulation and excretion of enantiomers of myclobutanil in Tenebrio molitor larvae through dietary exposure under laboratory conditions were investigated using high‐performance liquid chromatography tandem mass spectrometry (HPLC‐MS/MS) based on a ChiralcelOD‐3R [cellulosetris‐tris‐(3, 5‐dichlorophenyl‐carbamate)] column. The wheat bran fed to Tenebrio molitor larvae was spiked with racemic myclobutanil at two dose levels of 20 mg/kg and 2 mg/kg (dry weight). The results showed that there was a significant trend of enantioselective bioaccumulation in the larvae with a preferential accumulation of (?)‐myclobutanil in 20 mg/kg dose exposure, but it was not obviously observed in the 2 mg/kg dose group. A kinetic model considering enantiomerization between the two enantiomers based on first‐order reactions was built and the rate constants were estimated to discuss the kinetic reason for the different concentrations of individual enantiomers in the larvae. The approximations implied an inversion between the two enantiomers with a relatively higher rate of the inversion from (?)‐myclobutanil to (+)‐myclobutanil. Meanwhile, analysis of data of excretion samples suggested the active excretion is probably an important pathway for the insect to eliminate myclobutanil rapidly with nonenantioselectivity as a passive transport process, which was consistent with the low accumulation efficiency of myclobutanil measured by BAF (bioaccumulation factor). Chirality 25:890–896, 2013. © 2013 Wiley Periodicals, 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.     

Enantioselective toxicological response of the green alga Scenedesmus obliquus to isocarbophos

Chiral compounds usually behave enantioselectively in phyto‐biochemical processes. Isocarbophos (ICP) is a chiral pesticide that is widely used. To evaluate the toxicological response of ICP and its enantiomers to Scenedesmus obliquus, algal growth, total chlorophyll, total soluble protein, and the superoxide anion radicals (O₂^?‐) were investigated. The microalgae were treated with ICP and its enantiomers at 0.01–10 mg/l for 96 h. The growth of S. obliquus was stimulated at low levels of ICP and its enantiomers (0.01–1 mg/l), but all were inhibited at high concentrations (10 mg/l). The total soluble protein content and total chlorophyll content of the tested green alga S. obliquus gradually increased, depending on the growth of algal cells in the medium. Meanwhile, the content of O₂^?‐ was decreased. Interestingly, the cell number and content of the chlorophylls and protein decreased with increasing levels of concentration, whereas O₂^?‐ increased. Our results indicated that enantioselectivity was observed in the dose–response of ICP and its enantiomers in S. obliquus. The high O₂^?‐ level might lead to the death of S. obliquus. The stimulation of growth suggests a regulatory mechanism that is related to the capability of the algae to adapt to the O₂^?‐. Chirality 24:481–485, 2012. © 2012 Wiley Periodicals, Inc.     

Separation and toxicity of salithion enantiomers

Enantioseletive toxicities of chiral pesticides have become an environmental concern recently. In this study, we evaluated the enantiomeric separation of salithion on a suite of commercial chiral columns and assessed the toxicity of enantiomers toward butyrylcholinesterase and Daphnia magna. Satisfactory separations of salithion enantiomers could be achieved on all tested columns, that is, Chiralcel OD, Chiralcel OJ, and Chiralpak AD column. However, the Chiralpak AD column offered the best separation and was chosen to prepare micro‐scale of pure salithion enantiomers for subsequent bioassays. The first and second enantiomers eluted on the Chiralpak AD column were further confirmed to be (?)‐S‐salithion and (+)‐R‐salithion, respectively. The half inhibition concentrations to butyrylcholinesterase of racemate, (+)‐R‐salithion, and (?)‐S‐salithion were 33.09, 2.92, and 15.60 mg/l, respectively, showing (+)‐R‐enantiomer being about 5.0 times more potent than its (?)‐S‐form. However, the median lethal concentrations (96 h) of racemate, (+)‐R‐salithion, and (?)‐S‐salithion toward D. magna were 3.54, 1.10, and 0.36 μg/l, respectively, suggesting that (?)‐S‐salithion was about 3.0 times more toxic than (+)‐R‐form. Racemic salithion was less toxic than either of the enantiomers in both bioassays, suggesting that antagonistic interactions might occur between the enantiomers during the toxication action. This work reveals that the toxicity of salithion toward butyrylcholinesterase and D. magna is enantioselective, and this factor should be taken into consideration in the environmental risk assessment of salithion. Chirality 2009. © 2009 Wiley‐Liss, 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 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.     

Studies of Enantiomeric Degradation of the Triazole Fungicide Hexaconazole in Tomato,Cucumber, and Field Soil by Chiral Liquid Chromatography–Tandem Mass Spectrometry

Chiral pesticide enantiomers often show different bioactivity and toxicity; however, this property is usually ignored when evaluating their environmental and public health risks. Hexaconazole is a chiral fungicide used on a variety of crops for the control of many fungal diseases. This use provides opportunities for the pollution of food and soil. In this study, a sensitive and convenient chiral liquid chromatography coupled with tandem mass spectrometry (LC‐MS/MS) method was developed and validated for measuring hexaconazole enantiomers in tomato, cucumber, and soil. Separation was by a reversed‐phase Chiralcel OD‐RH column, under isocratic conditions using a mixture of acetonitrile‐2 mM ammonium acetate in water (60/40, v/v) as the mobile phase at a flow rate of 0.4 mL/min. Parameters including the matrix effect, linearity, precision, accuracy and stability were undertaken. Then the proposed method was successfully applied to investigate the possible enantioselective degradation of rac‐hexaconazole in plants (tomato and cucumber) and soil under field conditions. The degradation of the two enantiomers of hexaconazole proved to be enantioselective and dependent on the media: The (+)‐enantiomer showed a faster degradation in plants, while the (?)‐enantiomer dissipated faster than the (+)‐form in field soil, resulting in relative enrichment of the opposite enantiomer. The results of this work demonstrate that both the environmental media and environmental conditions influenced the direction and rate of enantioselective degradation of hexaconazole. Chirality 25:160–169, 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.     

Toxicokinetics of a single intravenous dose of rac-propranolol versus optically pure propranolol in the rat

Conscious male Wistar SPF Riv:TOX rats were dosed intravenously with 2.5, 5, or 10 mg/kg rac-propranolol·HCl, or with 5 mg/kg of either (-)-(S)- or (+)-(R)-propranolol·HCl. Disposition of (-)-(S)- and (+)-(R)-propranolol after dosing of rac-propranolol was linear in the dose range examined. Total plasma clearance was not changed in animals dosed with the individual enantiomers compared to the animals that were dosed with rac-propranolol. However, for (-)-(S)-propranolol both volume of distribution and elimination half-life decreased, whereas for (+)-(R)-propranolol increases were observed for these characteristics, in animals dosed with the individual enantiomers. Our observations suggest that the (+)-(R)-enantiomer competes with (-)-(S)-propranolol for plasma protein binding sites, resulting in lower plasma protein binding of the (-)-(S)-enantiomer when the racemate is administered. From recent toxicological experiments, it was concluded that rac-propranolol is more toxic than the individual enantiomers in the rat, when dosed iv at the same total mass. It is concluded that the observed potentiation of toxic effects of propranolol enantiomers when administered as a racemate can at least partly be explained by a pharmacokinetic interaction. © 1995 Wiley-Liss, Inc.     

Tissue distribution of bupivacaine enantiomers in sheep

rac-Bupivacaine HCl was infused intravenously to constant arterial blood drug concentrations in sheep using a regimen of 4 mg/min for 15 min followed by 1 mg/min to 24 h. At 24 h, arterial blood was sampled, the animal was killed with a bolus of KCl solution, then rapidly dissected and samples were obtained from heart, brain, lung, kidney, liver, muscle, fat, gut, and rumen. Tissue:blood distribution coefficients for (+)-(R)-bupivacaine exceeded those of (?)-(S)-bupivacaine (P < 0.05) for heart, brain, lung, fat, gut, and rumen by an overall mean of 43%. Blood:plasma distribution coefficients of (?)-(S)-bupivacaine exceeded those of (+)-(R)-bupivacaine by a mean of 29% and this offset the tissue:blood distribution coefficients so that the previously significant enantioselective differences disappeared. It is concluded that although enantioselectivity of bupivacame distribution is shown by the measured tissue:blood distribution coefficients, it is not shown when tissue:plasma water distribution coefficients are calculated, suggesting that there is no intrinsic difference between the bupivacaine enantiomers in tissue affinity. Sheep given fatal intravenous bolus doses of rac-bupivacaine had significantly greater concentrations of (+)-(R)-bupivacaine than (?)-(S)-bupivacaine in brain (P = 0.028) and ventricle (P = 0.036); these could augment the greater myocardial toxicity of this enantiomer found in vitro. © 1993 Wiley-Liss, Inc.     

Stereoselective Accumulation of Propranolol Enantiomers in K562 and K562/ADR Cells

The stereoselective uptake of propranolol enantiomers was investigated by using the K562 and K562 adriamycin‐resistant cell line (K562/ADR) as a model. An enantioselective RP‐HPLC method was applied to determine the accumulation of propranolol (PPL) stereoisomers in K562 and K562/ADR cells. The concentration, time and temperature dependent studies showed that the accumulation of S‐(?)‐PPL was higher than R‐(+)‐PPL in K562 cells and uptake of R‐(+)‐PPL was significantly higher than that of S‐(?)‐PPL in K562/ADR cells. The results indicate the enantioselective accumulation of propranolol enantiomers in K562 and K562 / ADR cells. Chirality 25:361–364, 2013. © 2013 Wiley Periodicals, Inc.     

Chiral Resolution and Absolute Configuration of the Enantiomers of the Psychoactive “Designer Drug” 3,4‐Methylenedioxypyrovalerone

Illicit rac‐MDPV (3,4‐methylenedioxypyrovalerone), manufactured in clandestine labs, has become widely abused for its cocaine‐like stimulant properties. It has recently been found as one of the toxic materials in the so‐called “bath salts,” producing, among other effects, psychosis and tachycardia in humans when introduced by any of the several routes of administration (e.g., intravenous, oral, etc.). The considerable toxicity of this “designer drug” probably resides in one of the enantiomers of the racemate. In order to obtain a sufficient amount of the enantiomers of rac‐MDPV to determine their activity, we improved the known synthesis of rac‐MDPV and found chemical resolving agents, (+)‐ and (–)‐2’‐bromotetranilic acid, that gave the MDPV enantiomers in >96% enantiomeric excess as determined by ¹H nuclear magnetic resonance and chiral high‐performance liquid chromatography. The absolute stereochemistry of these enantiomers was determined by single‐crystal X‐ray diffraction studies. Chirality 27:287‐293, 2015. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.     

Influence of N‐hexane inhalation on the enantioselective pharmacokinetics and metabolism of verapamil in rats

Verapamil (VER) is commercialized as a racemic mixture of the (+)‐(R)‐VER and (?)‐(S)‐VER enantiomers. VER is biotransformed into norverapamil (NOR) and other metabolites through CYP‐dependent pathways. N‐hexane is a solvent that can alter the metabolism of CYP‐dependent drugs. The present study investigated the influence of n‐hexane (nose‐only inhalation exposure chamber at concentrations of 88, 176, and 352 mg/m³) on the kinetic disposition of the (+)‐(R)‐VER, (?)‐(S)‐VER, (R)‐NOR and (S)‐NOR in rats treated with a single dose of racemic VER (10 mg/kg). VER and NOR enantiomers in rat plasma was analyzed by LC‐MS/MS (m/z = 441.3 > 165.5 for the NOR and m/z 455.3 > 165.5 for the VER enantiomers) using a Chiralpak® AD column. Pharmacokinetic analysis was performed using a monocompartmental model. The pharmacokinetics of VER was enantioselective in control rats, with higher plasma proportions of the (?)‐(S)‐VER eutomer (AUC^0?∞ = 250.8 vs. 120.4 ng/ml/h; P ≤ 0.05, Wilcoxon test). The (S)‐NOR metabolite was also found to accumulate in plasma of control animals, with an S/R AUC^0?∞ ratio of 1.5. The pharmacokinetic parameters AUC^0?∞, Cl/F, Vd/F, and t_1/2 obtained for VER and NOR enantiomers were not altered by nose‐only exposure to n‐hexane at concentrations of 88, 176, or 352 mg/m³ (P > 0.05, Kruskal‐Wallis test). However, the verapamil kinetic disposition was not enantioselective for the animals exposed to n‐hexane at concentrations equal to or higher than the TLV‐TWA. This finding is relevant considering that the (?)‐(S)‐VER eutomer is 10–20 times more potent than R‐(+)‐VER in terms of its chronotropic effect on atrioventricular conduction in rats and humans. Chirality 2010. © 2009 Wiley‐Liss, Inc.     

Stereoselective Degradation of Chiral Fungicide Myclobutanil in Rat Liver Microsomes

Myclobutanil, (RS)‐2‐(4‐chlorophenyl)‐2‐(1H‐1, 2, 4‐triazol‐1‐ylmethyl)hexanenitrile is a broad‐spectrum systemic triazole fungicide which consists of a pair of enantiomers. The stereoselective degradation of myclobutanil was investigated in rat liver microsomes. The concentrations of myclobutanil enantiomers were determined by high‐performance liquid chromatography (HPLC) with a cellulose‐tris‐(3,5‐dimethyl‐phenylcarbamate)‐based chiral stationary phase (CDMPC‐CSP) under reversed phase condition. The t_1/2 of (+)‐myclobutanil is 8.49 min, while the t_1/2 of (–)‐myclobutanil is 96.27 min. Such consequences clearly indicated that the degradation of myclobutanil in rat liver microsomes was stereoselective and the degradation rate of (+)‐myclobutanil was much faster than (–)‐myclobutanil. In addition, significant differences between two enantiomers were also observed in enzyme kinetic parameters. The V_max of (+)‐myclobutanil was about 4‐fold of (–)‐myclobutanil and the CL_int of (+)‐myclobutanil was three times as much as (–)‐myclobutanil after incubation in rat liver microsomes. Corresponding consequences may shed light on the environmental and ecological risk assessment for myclobutanil and may improve human health. Chirality 26:51–55, 2013. © 2013 Wiley Periodicals, Inc.     

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.     

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.     

The biological activities of prothioconazole enantiomers and their toxicity assessment on aquatic organisms

Chiral fungicide prothioconazole has a wide range of antifungal spectrum; however, little research has been conducted to evaluate prothioconazole on an enantiomeric level. Five target pathogens and three common aquatic organisms were tested for the enantioselective bioactivity and toxicity of prothioconazole in this work. The antifungal activity of the enantiomers against wheat phytoalexin, rice blast fungus, exserohilum turcicum, Alternaria triticina, and Fusarium avenaceum was determined, and it was found that (?)‐prothioconazole were 85 to 2768 times more active than (+)‐prothioconazole toward these target organisms. In order to reflect the risk to aquatic ecosystem, the acute toxicity of the enantiomers to Daphnia magna, Chlorella pyrenoidosa, and Lemna minor L. was assessed. It was observed that the toxicity of (?)‐prothioconazole to D. magna was 2.2 times higher than (+)‐prothioconazole, but it was lower to C. pyrenoidosa and L. minor L. The toxicities of (+)‐enantiomer and (?)‐enantiomer to D. magna and C. pyrenoidosa were synergy, indicating that the racemate had higher threat to the organisms. It could be concluded that the effects of prothioconazole on target organisms and the acute toxicity to nontarget species were enantioselective with (?)‐enantiomer possessing higher efficiency and lower toxicity. Such enantiomeric differences should be taken into consideration when assessing the performance of prothioconazole.