Tissue Distribution, Accumulation, and Metabolism of Chiral Flufiprole in Loach (Misgurnus anguillicaudatus)

Flufiprole is an insecticide used in the rice field and may pose a potential threat to aquatic organisms including loach. To investigate the transformation products of flufiprole in loach, the accumulation, elimination, and tissue distribution in vivo as well as the metabolism in vitro at the enantiomeric level were studied. Flufiprole enantiomers rapidly accumulated and were metabolized to flufiprole sulfone, fipronil, and flufiprole amide in the tissues. Enantiomeric fractions showed the preferential accumulation and degradation of S-flufiprole. The residue of the chiral metabolite flufiprole amide was also enantioselective. The individual enantiomer treatment indicated that S-flufiprole was preferentially metabolized to flufiprole sulfone and R-flufiprole to fipronil. The metabolites were more persistent than flufiprole with longer half-lives. The metabolism in liver microsomes also reached consistent conclusions. The dietary risk assessment indicated that flufiprole would not cause unacceptable threats to human health. However, the metabolites of flufiprole should be considered in the risk evaluation.

Enantioselective metabolism and enantiomerization of benalaxyl in mice

The enantiomerization and enantioselective metabolism of benalaxyl in mice after a single gavage administration were investigated in the present study. The pharmacokinetic result indicated that elimination of (-)-R-benalaxyl in plasma was slightly faster with the t_1/2 of 26.65 h and 28.88 h for (-)-R- and (+)-S-benalaxyl, respectively. Consistent with this, elimination in tissues and excretion were also enantioselective, with (+)-S-benalaxyl enriched in all tissues, urine and feces. And formation of the metabolites also exhibited an enantioselective manner. Both benalaxyl and most of its metabolites exhibited a potent excretion to feces and urine. In addition, significant enantiomerization of benalaxyl enantiomers was observed in plasma, with a larger extent from R to S than S to R. Thus, this difference in enantiomerization may be one of the reasons to explain the enantioselective enrichment of (+)-S-benalaxyl in mice observed in this study. Data from this study proves that besides metabolic enzymes, enantiomerization could be another important factor that contributes to the enantioselective metabolism of a chiral pesticide. Thus, research at enantiomeric level is necessary for efficient risk assessment of chiral pesticides.

Etoxazole is Metabolized Enantioselectively in Liver Microsomes of Rat and Human in Vitro

Acaricide etoxazole belongs to the ovicides/miticides diphenyloxazole class, affecting adults to lay sterile eggs by inhibiting chitin biosynthesis possibly. The reverse-phase HPLC-MS/MS method was used to determine the etoxazole enantiomers. The enantioselective degradation behavior of rac-etoxazole in liver microsomes of rat and human in vitro with NADPH was dramatically different. The t1/2 of (R)-etoxazole was 15.23 min in rat liver microsomes and 30.54 min in human liver microsomes, while 21.73 and 23.50 min were obtained for (S)-etoxazole, respectively. The Vmax of (R)-etoxazole was almost 5-fold of (S)-etoxazole in liver microsomes of rat in vitro. However, the Vmax of (S)-etoxazole was almost 2-fold of (R)-etoxazole in liver microsomes of human in vitro. The CLint of etoxazole was also shown the enantioselectivity on the contrary in liver microsomes of rat and human. These results indicated that the metabolism of two etoxazole enantiomers was selective in liver microsomes of rat and human in vitro, and enantioselectivity in the two kinds of liver microsomes was in the difference in degradation performance. The reason might be related to the composition and content involved in the enzyme system.

A combined non-targeted and targeted metabolomics approach to study the stereoselective metabolism of benalaxyl enantiomers in mouse hepatic microsomes

Understanding of xenobiotic metabolism is necessary for risk assessment as well as toxicological research. In the present study, nanoLC/LTQ-Orbitrap mass based non-targeted metabolomics method coupled with ultra-performance liquid chromatography (UPLC)/triple quadrupole mass based targeted metabolomics method was carried out to investigate the stereoselective metabolism of benalaxyl in mouse hepatic microsomes. As a result, 7 metabolites of benalaxyl were identified, including 5 previously reported and 2 newly identified metabolites in present work. Hydroxylation, oxidation and esterolysis were major biotransformation reactions of benalaxyl in mouse hepatic microsomes. For stereoselective metabolism study, (-)-R-benalaxyl degraded much faster than its antipode with the t1/2 of 81.24 and 190.38 min for (-)-R- and (+)-S-benalaxyl, respectively. More importantly, stereoselectivity was also observed in the formation of the identified metabolites. In conclusion, the combined use of the mass spectrometry based targeted and non-targeted metabolomics provided a new approach to investigate stereoselective metabolism and identify novel metabolites of chiral pesticides. This study highlights the stereoselective metabolic profile of benalaxyl enantiomers and provides reliable data for benalaxyl toxicological risk assessment in mammal.

Enantioselective Metabolism of Flufiprole in Rat and Human Liver Microsomes

The enantioselective metabolism of flufiprole in rat and human liver microsomes in vitro was investigated in this study. The separation and determination were performed using a liquid chromatography system equipped with a triple-quadrupole mass spectrometer and a Lux Cellulose-2 chiral column. The enantioselective metabolism of rac-flufiprole was dramatically different in rat and human liver microsomes in the presence of the β-nicotinamide adenine dinucleotide phosphate regenerating system. The half-lives (t1/2) of flufiprole in rat and human liver microsomes were 7.22 and 21.00 min, respectively, for R-(+)-flufiprole, whereas the values were 11.75 and 17.75 min, respectively, for S-(-)-flufiprole. In addition, the Vmax of R-(+)-flufiprole was about 3-fold that of S-(-)-flufiprole in rat liver microsomes, whereas its value in the case of S-(-)-flufiprole was about 2-fold that of R-(+)-flufiprole in human liver microsomes. The CLint of rac-flufiprole also showed opposite enantioselectivy in rat and human liver microsomes. The different compositions and contents of metabolizing enzyme in the two liver microsomes might be the reasons for the difference in the metabolic behavior of the two enantiomers.

Stereoselective degradation and toxic effects of benalaxyl on blood and liver of the Chinese lizard Eremias argus

Benalaxyl as a xylem-systemic fungicide is usually direct sprayed on the soil surface, which is potential harm to the animals lived in the soil. However, the stereoselectivity of benalaxyl in reptiles have rarely been studied. In this study, Chinese lizards (Eremias argus) were firstly used to evaluate the stereoselectivity in biodegradation and toxicity of racemate and individual enantiomers of benalaxyl. A method for determining residues of the two enantiomers of benalaxyl in lizard blood and liver by high-performance liquid chromatography coupled with triple quadrupole mass spectrometry (HPLC-MS/MS) was developed. The degradation followed pseudo first-order kinetics and the degradation of the (S)-(+)-benalaxyl was faster than its antipode in blood and liver (Half-time t1/2 of (R)-(-)-benalaxyl and (S)-(+)-benalaxyl were 5.08 h and 3.75 h in blood, 6.21 h and 4.45 h in liver, separately). Moreover, antioxidant defenses consisting of activities of superoxide dismutase (SOD), catalase (CAT), glutathione S-transferase (GST) and lipid peroxide malondialdehyde (MDA) were determined in 24h acute exposure. Enantioselectivity of acute toxicity depended on the concentration and form of benalaxyl. In addition, cellular degeneration, decrease of cell number, clustering phenomena of cell nuclei and preliminary liver fibrosis were observed in pathological detection at the termination of 21-d subchronic exposure (20 mg/kg(-bw) of racemate and individual enantiomers of benalaxyl). The enantiomer fractions (EFs) in racemate and individual enantiomer groups were approached both in blood and liver caused by the chiral conversion. The chiral conversion from (R)-(-)-benalaxyl to (S)-(+)-benalaxyl and (S)-(+)-benalaxyl to (R)-(-)-benalaxyl were the primary cause for no remarkable differences in toxicity between the enantiomers of benalaxyl.

Enantiomerization and enantioselective bioaccumulation of benalaxyl in Tenebrio molitor larvae from wheat bran

The enantiomerization and enatioselecive bioaccumulation of benalaxyl by dietary exposure to Tenebrio molitor larvae under laboratory conditions were studied by HPLC-MS/MS. Exposure of enantiopure R-benalaxyl and S-benalaxyl in T. molitor larvae revealed significant enantiomerization with formation of the R enantiomers from the S enantiomers, and vice versa. Enantiomerization was not observed in wheat bran during the period of 21 days. For the bioaccumulation experiment, the enantiomer fraction in T. molitor larvae was maintained approximately at 0.6, whereas the enantiomer fraction in wheat bran was maintained at 0.5; in other words, the bioaccumulation of benalaxyl was enantioselective in T. molitor larvae. Mathematical models for a process of uptake, degradation, and enantiomerization were developed, and the rates of uptake, degradation, and enantiomerization of R-benealaxyl and S-benealaxyl were estimated, respectively. The results were that the rate of uptake of R-benalaxyl (kRa = 0.052 h(-1)) was slightly lower than that of S-benalaxyl (kSa = 0.061 h(-1)) from wheat bran; the rate of degradation of R-benalaxyl (kRd = 0.285 h(-1)) was higher than that of S-benalaxyl (kSd = 0.114 h(-1)); and the rate of enantiomerization of R-benalaxyl (kRS = 0.126 h(-1)) was higher than that of S-benalaxyl (kSR = 0.116 h(-1)). It was suggested that enantioselectivtiy was caused not only by actual degradation and metabolism but also by enantiomerization, which was an important process in the environmental fate and behavior of chiral pesticides.