Hepatic metabolism of diallyl disulphide in rat and man

1. The metabolism of diallyl disulphide was investigated in vitro with rat and human liver cell subfractions and ex vivo with an isolated perfused rat liver. 2. Diallyl disulphide was oxidized to diallylthiosulphinate by rat liver microsomes with an apparent K(m) = 0.86 +/- 0.1 mM and an apparent V(max) = 0.47 +/- 0.12 nmol min(-1) mg(-1) protein (mean +/- SE). Both cytochrome P450 (CYP) and flavin-containing monooxygenases were involved, with CYP2B1/2 and CYP2E1 being the most active CYP enzymes. 3. In rat and man, microsomal oxidation of allylmethyl sulphide to allylmethyl sulphoxide and allylmethyl sulphone also occurred, although at a low rate. Diallyl disulphide was also metabolized to allylglutathione sulphide and allylmercaptan. In addition, diallylthiosulphinate reacted non-enzymatically with glutathione to form allylglutathione sulphide. 4. When an isolated rat liver was perfused with diallyl disulphide, the metabolites allyl mercaptan, allylmethyl sulphide, allylmethyl sulphoxide, allylmethyl sulphone and allylglutathione sulphide were detected primarily within the liver tissue, with only small amounts of metabolites found in the bile and perfusion medium. The pharmacokinetic parameters for diallyl disulphide were t(1/2) = 6.09 min; AUC(0- infinity ) = 4.77 min mmol l(-1); clearance = 34.22 ml min(-1). 5. A scheme for the metabolism of diallyl disulphide in rat and man is proposed.     

Effects of Ginkgo leaf tablets on the pharmacokinetics of losartan and its metabolite EXP3174 in rats and its mechanism

Context: Ginkgo leaf tablets (GLTs) and losartan are often simultaneously used for the treatment of hypertension in Chinese clinics. However, the herb–drug interaction between GLT and losartan is still unknown.

Objective: This study investigates the effects of GLT on the pharmacokinetics of losartan and its metabolite EXP3174 in rats and its potential mechanism.

Materials and methods: The pharmacokinetic profiles of losartan and EXP3174 of orally administered losartan (10?mg/kg) with or without GLT pretreatment (80?mg/kg/day for 10?days) in Sprague–Dawley rats were determined. In vitro, the effects of GLT on the metabolic stability of losartan were investigated with rat liver microsomes.

Results: The C_max (1.22?±?0.25 vs 1.85?±?0.37?μg/mL) and the AUC_(0–t) (6.99?±?1.05 vs 11.94?±?1.79?mg·h/L) of losartan increased significantly (p?C_max (1.05?±?0.19 vs 0.72?±?0.12?μg/mL) of EXP3174 decreased significantly (p?t_1/2 of losartan was prolonged significantly from 3.94?±?0.62 to 4.75?±?0.52?h (p?Discussion and conclusions: The results indicate that GLT might increase the plasma concentration of losartan and decrease the concentration of EXP3174 through inhibiting the metabolism of losartan.     

Effects of catalpol on the activity of human liver cytochrome P450 enzymes

Abstract

1. Catalpol possesses numerous pharmacological activities, and however, little data available for the effects of catalpol on the activity of human liver cytochrome P450 (CYP) enzymes.

2. This study investigates the inhibitory effects of catalpol on the main human liver CYP isoforms. In this study, the inhibitory effects of catalpol on the eight human liver CYP isoforms 1A2, 2A6, 2E1, 2D6, 2C9, 2C19, 2C8 and 3A4 were investigated in human liver microsomes.

3. The results indicated that catalpol could inhibit the activity of CYP3A4, CYP2E1 and CYP2C9, with IC₅₀ values of 14.27, 22.4 and 14.69?μM, respectively, but those other CYP isoforms were not affected. Enzyme kinetic studies showed that catalpol was not only a noncompetitive inhibitor of CYP3A4, but also a competitive inhibitor of CYP2E1 and CYP2C9, with Ki values of 7.40, 10.75 and 7.37?μM, respectively. In addition, catalpol is a time-dependent inhibitor for CYP3A4, with maximum inactivation (kinact) and 50% maximum inactivation (K_I) values of 0.02?min^?1 and 1.86?μM, respectively.

4. The in vitro studies of catalpol with CYP isoforms suggest that catalpol has the potential to cause pharmacokinetic drug interactions with other co-administered drugs metabolized by CYP3A4, CYP2E1 and CYP2C9. Further in vivo studies are needed in order to evaluate the significance of this interaction.     

Effects of triptolide on pharmacokinetics of amlodipine in rats by using LC–MS/MS

Context: Triptolide and amlodipine are often simultaneously used for reducing urine protein excretion after renal transplantation in China clinics.

Objective: This study investigated the effects of triptolide on the pharmacokinetics of amlodipine in male Sprague–Dawley rats.

Materials and methods: The pharmacokinetics of amlodipine (1?mg/kg) with or without triptolide pre-treatment (2?mg/kg/day for seven?days) were investigated using a sensitive and reliable LC–MS/MS method. Additionally, the inhibitory effects of triptolide on the metabolic stability of amlodipine were investigated using rat liver microsome incubation systems.

Results: The results indicated that when the rats were pre-treated with triptolide, the C_max of amlodipine increased from 13.78?±?3.57 to 19.96?±?4.56?ng/mL (p?T_max increased from 4.04?±?1.15 to 5.89?±?1.64?h (p?AUC_0–t increased by approximately 104% (p?p?Conclusions: In conclusion, these results indicated that triptolide could affect the pharmacokinetics of amlodipine, possibly by inhibiting the metabolism of amlodipine in rat liver when they are co-administered.     

Inhibitory effects of curculigoside on human liver cytochrome P450 enzymes

1.?Curculigoside possesses numerous pharmacological activities, and however, little data available for the effects of curculigoside on the activity of human liver cytochrome P450 (CYP) enzymes.

2.?This study investigates the inhibitory effects of curculigoside on the main human liver CYP isoforms. In this study, the inhibitory effects of curculigoside on the eight human liver CYP isoforms 1A2, 2A6, 2E1, 2D6, 2C9, 2C19, 2C8, and 3A4 were investigated in human liver microsomes.

3.?The results indicated that curculigoside could inhibit the activity of CYP1A2, CYP2C8, and CYP3A4, with IC₅₀ values of 15.26, 11.93, and 9.47?μM, respectively, but that other CYP isoforms were not affected. Enzyme kinetic studies showed that curculigoside was not only a noncompetitive inhibitor of CYP1A2, but also a competitive inhibitor of CYP2C8 and CYP3A4, with Ki values of 5.43, 3.54, and 3.35?μM, respectively. In addition, curculigoside is a time-dependent inhibitor for CYP1A2, with kinact/K_I values of 0.056/6.15?μM^?1?min^?1.

4.?The in vitro studies of curculigoside with CYP isoforms suggest that curculigoside has the potential to cause pharmacokinetic drug interactions with other coadministered drugs metabolized by CYP1A2, CYP2C8, and CYP3A4. Further in vivo studies are needed in order to evaluate the significance of this interaction.     

Coadministration of gemfibrozil and itraconazole has only a minor effect on the pharmacokinetics of the CYP2C9 and CYP3A4 substrate nateglinide

BACKGROUND AND AIMS: Gemfibrozil, and particularly its combination with itraconazole, greatly increases the area under the plasma concentration-time curve [AUC(0, infinity)] and response to the cytochrome P450 (CYP) 2C8 and 3A4 substrate repaglinide. In vitro, gemfibrozil is a more potent inhibitor of CYP2C9 than of CYP2C8. Our aim was to investigate the effects of the gemfibrozil-itraconazole combination on the pharmacokinetics and pharmacodynamics of another meglitinide analogue, nateglinide, which is metabolized by CYP2C9 and CYP3A4. METHODS: In a randomized crossover study with two phases, nine healthy subjects took 600 mg gemfibrozil and 100 mg itraconazole (first dose 200 mg) twice daily or placebo for 3 days. On day 3, they ingested a single 30-mg dose of nateglinide. Plasma nateglinide and blood glucose concentrations were measured for up to 12 h. RESULTS: During the gemfibrozil-itraconazole phase, the AUC(0, infinity) and C(max) of nateglinide were 47% (range 23-74%; P < 0.0001) and 30% (range - 8% to 104%; P = 0.0146) higher than during the placebo phase, respectively, but the t(max) and t1/2 of nateglinide remained unchanged. The combination of gemfibrozil and itraconazole had no effect on the formation of the M7 metabolite of nateglinide but impaired its elimination. The blood glucose response to nateglinide was not significantly changed by coadministration of gemfibrozil and itraconazole. CONCLUSIONS: The combination of gemfibrozil and itraconazole has only a limited influence on the pharmacokinetics of nateglinide. This is in marked contrast to the substantial effect of this combination on the pharmacokinetics of repaglinide. The findings suggest that in vivo gemfibrozil, probably due to its metabolites, is a much more potent inhibitor of CYP2C8 than of CYP2C9.     

In vitro inhibitory effects of Friedelin on human liver cytochrome P450 enzymes

Context: Friedelin is a triterpenoid with several biological activities. However, the affects of Friedelin on the activity of human liver cytochrome P450 (CYP) enzymes remains unclear.

Objective: This study investigates the inhibitory effects of Friedelin on the major human liver CYP isoforms (CYP3A4, 1A2, 2A6, 2E1, 2D6, 2C9, 2C19 and 2C8).

Materials and methods: First, the inhibitory effects of Friedelin (100?μM) on the eight human liver CYP isoforms were investigated in vitro using human liver microsomes (HLMs), and then enzyme inhibition, kinetic studies, and time-dependent inhibition studies were conducted to investigate the IC₅₀, K_i and K_inact/K_I values of Friedelin.

Results: The results indicate that Friedelin inhibited the activity of CYP3A4 and 2E1, with the IC₅₀ values of 10.79 and 22.54?μM, respectively, but other CYP isoforms were not affected. Enzyme kinetic studies showed that Friedelin is not only a noncompetitive inhibitor of CYP3A4, but also a competitive inhibitor of CYP2E1, with K_i values of 6.16 and 18.02?μM, respectively. In addition, Friedelin is a time-dependent inhibitor of CYP3A4 with K_inact/K_i value of 4.84?nM/min.

Discussion and conclusion: The in vitro studies of Friedelin with CYP isoforms suggested that Friedelin has the potential to cause pharmacokinetic drug interactions with other co-administered drugs metabolized by CYP3A4 and 2E1. Further clinical studies are needed to evaluate the significance of this interaction.     

Pharmacokinetics and metabolism of the anti-oestrogen droloxifene in female human subjects

1. Single oral doses of a solution formulation of 14 C-droloxifene citrate (141?mg) appeared to be rapidly and well absorbed in four post-menopausal female subjects. Peak plasma concentrations (C max) of total 14 C (1260 ng eq. ml -1), droloxifene (196 ng ml -1) and the major metabolite droloxifene glucuronide (851ng eq. ml -1) occurred at 0.9-1.1h (T max) and declined bi-exponentially with terminal half-lives of 45.0, 31.6 and 32.0h respectively. The mean AUCs of droloxifene and the major metabolite were 21 and 37% respectively that of total 14 C. 2. Total 14 C was excreted slowly, mainly in the faeces. Mean totals of 6.6 and 90.3% of the dose were excreted in the urine and faeces respectively during 11 days. The data were consistent with biliary excretion and enterohepatic circulation of the major metabolite, droloxifene glucuronide. 3. GC-MS showed that the major 14 C-components in 0-24-h urine were droloxifene (mean 0.4% dose) and its glucuronide (2.3% dose), and in faeces were droloxifene (60.2% dose) and N- desmethyldroloxifene (4.2% dose). Other components in faeces corresponded chromatographically to reference standards, droloxifene N -oxide (1.9% dose), side-chain hydroxylated droloxifene (dimethylamine moiety of droloxifene side-chain replaced by hydroxyl, 1.3% dose) and droloxifene glucuronide (10.7% dose). The latter was resistant to enzymic hydrolysis by the β -glucuronidase used. 4. Intersubject variability in the pharmacokinetics of droloxifene in this study was relatively low (CV < 20% for AUC and half-life).     

Influence of ethanol on the metabolism of alprazolam.

Background: Alprazolam is a commonly used benzodiazepine in clinical practice, and when coingested with ethanol, alprazolam can increase behavioral irritability and aggression. However, the mechanism of its interaction with ethanol remains unknown.

Research design and methods: The pharmacokinetics of alprazolam was studied in vivo in rat experiments involving the simultaneous administration of alprazolam and ethanol, and the interactions between ethanol and alprazolam were investigated in vitro in human liver microsomes. In silico molecular docking was applied to analyze the change in the CYP3A4–alprazolam-binding conformation when ethanol was coadministered with alprazolam.

Results: Compared with alprazolam administered alone (2 mg/kg), the C_max of alprazolam increased when ethanol was simultaneously administered at 3 g/kg. The concentrations of alprazolam significantly increased by 39%, 17%, 105%, and 642% at 5, 10, 30, and 120 min intervals in the brain when coadministered with ethanol, respectively. Molecular docking results suggested that the conformation of CYP3A4 with alprazolam changed when ethanol was bound to the SER119 residue, which seems critical in the process of CYP3A4–alprazolam binding.

Conclusions: Ethanol might increase the toxicity of alprazolam by inhibiting the activity of CYP3A4, although other pharmacokinetic processes may be affected. Ethanol could change the conformation of CYP3A4 and affect alprazolam binding.     

Identification of human liver cytochrome P450 enzymes involved in the metabolism of SCH 351125, a CCR5 antagonist

The identification and relative contribution of human cytochrome P450 enzyme(s) involved in the metabolism of SCH 351125 were investigated. In human liver microsomes, O-deethylation was the major metabolic pathway, whereas aromatization of a piperidine ring to pyridine and the reduction of the N-oxide moiety were minor routes. Recombinant human CYP3A4 and CYP2C9 both exhibited catalytic activity with respect to the formation of rotameric O-deethylated metabolites (M12, M13), the metabolites resulting from aromatization (M22/M24) and N-oxide reduction (M31). Using the relative activity factor (RAF) approach, the relative contributions of CYP3A4 and CYP2C9 to M13 formation were estimated to be 76 and 24%, respectively. There was a high correlation (r?>?0.96) between the rate of formation of M12 and M13 and 6β-hydroxylation of testosterone catalysed by CYP3A4/5. Ketoconazole (2?µM) and CYP3A4/5-specific inhibitory monoclonal antibody inhibited the formation of M12 and M13 from human liver microsomes by approximately 60 and 71%, respectively. The results demonstrate that the in vitro metabolism of SCH 351125 is mediated primarily via CYP3A4 and that CYP2C9 plays a minor role. Clinical study designs should encompass these enzymology data to address any potential drug interactions.     

The effects of triptolide on the pharmacokinetics of sorafenib in rats and its potential mechanism

Context: Combining sorafenib with triptolide could inhibit tumour growth with greater efficacy than single-agent treatment. However, their herb–drug interaction remains unknown.

Objective: This study investigates the herb–drug interaction between triptolide and sorafenib.

Materials and methods: The effects of triptolide (10?mg/kg) on the pharmacokinetics of different doses of sorafenib (20, 50 and 100?mg/kg) in rats, and blood samples were collected within 48?h and evaluated using LC-MS/MS. The effects of triptolide on the absorption and metabolism of sorafenib were also investigated using Caco-2 cell monolayer model and rat liver microsome incubation systems.

Results: The results showed that the C_max (low dose: 72.38?±?8.76 versus 49.15?±?5.46?ng/mL; medium dose: 178.65?±?21.05 versus 109.31?±?14.17?ng/mL; high dose: 332.81?±?29.38 versus 230.86?±?9.68?ng/mL) of sorafenib at different doses increased significantly with the pretreatment of triptolide, and while the oral clearance rate of sorafenib decreased. The t_1/2 of sorafenib increased significant (p?Discussion and conclusions: These results indicated that triptolide could change the pharmacokinetic profiles of sorafenib in rats; these effects might be exerted via decreasing the intrinsic clearance rate of sorafenib in rat liver.     

Ticlopidine decreases the in vivo activity of CYP2C19 as measured by omeprazole metabolism

AIMS: To examine the effect of ticlopidine administration on the activities CYP2C19 and CYP3 A in vivo using omeprazole as a model substrate. METHODS: A single dose of 40 mg omeprazole was administered orally with or without ticlopidine (300 mg daily for 6 days) to six Japanese extensive metabolisers with respect to CYP2C19. Blood samples were taken for the measurement of plasma concentrations of omeprazole, 5-hydroxyomeprazole and omeprazole sulphone. RESULTS: Ticlopidine administration increased omeprazole Cmax (1978+/-859/ 3442+/-569 (control phase/ticlopidine phase, nm )) and decreased the oral clearance of omeprazole (CL/F; 25.70+/-16. 17/10.76+/-1.16 (control phase/ticlopidine phase, l h-1 )) significantly. The 5-hydroxyomeprazole to omeprazole AUC ratio (0. 817+/-0.448/0.236+/-0.053 (control phase/ticlopidine phase)) and the 5-hydroxyomeprazole to omeprazole sulphone AUC ratio (1.114+/-0. 782/0.256+/-0.051 (control phase/ticlopidine phase)) were decreased significantly after ticlopidine administration. The decrease in omeprazole CL/F and the 5-hydroxyomeprazole to omeprazole AUC ratio correlated significantly with their respective absolute values when the drug was given alone. The decrease in CL/F following ticlopidine administration correlated with that in the 5-hydroxyomeprazole to omeprazole AUC ratio. CONCLUSIONS: These findings suggest that ticlopidine inhibited the in vivo activity of CYP2C19, but not, or to a lesser extent CYP3 A4, and that the magnitude of inhibition by ticlopidine is related to the in vivo activity of CYP2C19 before inhibition.     

Characterization of cytochrome P450s mediating ipriflavone metabolism in human liver microsomes

Ipriflavone, a synthetic flavonoid for the prevention and treatment of osteoporosis, has been reported to be extensively metabolized in man to seven metabolites (M1–M7). This study was performed to characterize the human liver cytochrome P450s (CYP) responsible for the metabolism of ipriflavone. Hydroxylation at the β-ring to M3, O-dealkylation to M1 and oxidation at isopropyl group to M4 and M5 are major pathways for ipriflavone metabolism in three different human liver microsome preparations. The specific CYPs responsible for ipriflavone oxidation to the active metabolites, M1, M3, M4 and M5 were identified using a combination of correlation analysis, immuno-inhibition, chemical inhibition in human liver microsomes and metabolism by expressed recombinant CYP enzymes. The inhibitory potencies of ipriflavone and its five metabolites, M1–M5 on seven clinically important CYPs were investigated in human liver microsomes. Our results demonstrate that CYP3A4 plays the major role in O-dealkylation of ipriflavone to M1 and CYP1A2 plays a dominant role in the formation of M3, M4 and M5. Ipriflavone and/or its five metabolites were found to inhibit potently the metabolism of CYPs 1A2, 2C8, 2C9 and 2C19 substrates.     

Preclinical evaluation of the potential for cytochrome P450 inhibition and induction of the selective ALK inhibitor,alectinib

1.?A novel selective anaplastic lymphoma kinase (ALK) inhibitor, alectinib, has shown remarkable efficacy and safety in patients with ALK-positive non-small-cell lung cancer (NSCLC). The purpose of this study was to evaluate in vitro the potential to inhibit and induce cytochrome P450 (CYP) isoforms for alectinib and its major metabolite M4.

2.?Alectinib and M4 did not show the meaningful direct inhibition of six major CYP isoforms (CYP1A2, 2B6, 2C9, 2C19, 2D6 and 3A4) in human liver microsomes (HLM). Alectinib, but not M4, competitively inhibited CYP2C8, by which few marketed drugs are exclusively metabolized, with an inhibition constant of 1.98?μM.

3.?Out of the seven CYP isoforms in HLM, alectinib and M4 showed time-dependent inhibition (TDI) of only CYP3A4, which suggests low TDI potential due to low inactivation efficiency.

4.?Alectinib exhibited quite smaller induction of mRNA expression of CYP1A2, 2B6 and 3A4 genes in human hepatocytes compared to the respective positive controls, suggesting a low potential of enzyme induction.

5.?In summary, the risk of alectinib causing drug-drug interactions with coadministered drugs is expected to be low due to the weak potential of CYP inhibition and induction estimated in the preclinical studies.     

Screening of drug metabolizing enzymes for fusidic acid and its interactions with isoform-selective substrates in vitro

1.?Fusidic acid (FA) is widely used for the treatment of infections of sensitive osteomyelitis or skin and soft tissue caused by bacteria. However, the role of cytochrome P450s (CYPs) in the metabolism of FA is unclear. In the present study, we screened the main CYPs for the metabolism of FA and studied its interactions with isoform-selective substrates in vitro.

2.?The main CYP450s were screened according to the inhibitory effect of specific inhibitors on the metabolism of FA in human liver microsomes (HLMs) or recombinant CYP isoforms. Enzyme kinetic parameters including K_i, K_i′, V_max, and IC₅₀ were calculated to determine the potential of FA to affect CYP-mediated metabolism of isoform-selective substrates.

3.?FA metabolism rate was inhibited by 49.8% and 83.1% under CYP2D6, CYP3A4 selective inhibitors in HLMs. In recombinant experiment, the inhibitory effects on FA metabolism were 83.3% for CYP2D6 and 58.9% for CYP3A4, respectively. FA showed inhibition on CYP2D6 and CYP3A4 with K_is of 13.9 and 38.6?μM, respectively. Other CYP isoforms including CYP1A2, CYP2A6, CYP2C9, CYP2E1, and CYP2C19 showed minimal or no effect on the metabolism of FA.

4.?FA was primarily metabolized by CYP2D6 and CYP3A4 and showed a noncompetitive inhibition on CYP2D6 and a mixed competitive inhibition on CYP3A4. Drug–drug interactions between FA and other chemicals, especially with substrates of CYP2D6 and CYP3A4, are phenomena that clinicians need to be aware of and cautious about.     

Poor Correlation Between Intestinal and Hepatic Metabolic Rates of CYP3A4 Substrates in Rats

Purpose. To clarify the contribution of the intestinal first-pass metabolism to the drug bioavailability, the correlation between the intestinal and hepatic metabolism of human CYP3A4 substrates was investigated in rats. Methods. The metabolic rates of four compounds (lidocaine, quinidine, nifedidpine, and rifabutin) were examined with excised intestinal tissues and liver microsomes. The intestinal and hepatic expression of CYP3A1/23 and CYP3A2 was evaluated by Western blot analysis. Results. Rifabutin was metabolized fastest, and lidocaine was metabolized slowest in excised intestinal tissues. By contrast, lidocaine was metabolized fastest and rifabutin was the slowest in liver microsomes. The hepatic metabolism of lidocaine was inhibited by a CYP2D6 substrate desipramine, not by a CYP3A4 inhibitor ketoconazole. In addition, members of the CYP3A subfamily expressed in the intestine were different from those expressed in the liver. Conclusions. Poor correlation between the intestinal and hepatic metabolism of human CYP3A4 substrates in rats may be caused by the contribution of the CYP2D subfamily to the drug metabolisms in the liver and also by the unique expression of the CYP3A subfamily in the intestine.     

Database analyses for the prediction of in vivo drug-drug interactions from in vitro data

AIMS: In theory, the magnitude of an in vivo drug-drug interaction arising from the inhibition of metabolic clearance can be predicted using the ratio of inhibitor concentration ([I]) to inhibition constant (K(i)). The aim of this study was to construct a database for the prediction of drug-drug interactions from in vitro data and to evaluate the use of the various estimates for the inhibitor concentrations in the term [I]/K(i). METHODS: One hundred and ninety-three in vivo drug-drug interaction studies involving inhibition of CYP3A4, CYP2D6 or CYP2C9 were collated from the literature together with in vitro K(i) values and pharmacokinetic parameters for inhibitors, to allow calculation of average/maximum systemic plasma concentration during the dosing interval and maximum hepatic input plasma concentration (both total and unbound concentration). The observed increase in AUC (decreased clearance) was plotted against the estimated [I]/K(i) ratio for qualitative zoning of the predictions. RESULTS: The incidence of false negative predictions (AUC ratio > 2, [I]/K(i) < 1) was largest using the average unbound plasma concentration and smallest using the hepatic input total plasma concentration of inhibitor for each of the CYP enzymes. Excluding mechanism-based inhibition, the use of total hepatic input concentration resulted in essentially no false negative predictions, though several false positive predictions (AUC ratio < 2, [I]/K(i) > 1) were found. The incidence of true positive predictions (AUC ratio > 2, [I]/K(i) > 1) was also highest using the total hepatic input concentration. CONCLUSIONS: The use of the total hepatic input concentration of inhibitor together with in vitro K(i) values was the most successful method for the categorization of putative CYP inhibitors and for identifying negative drug-drug interactions. However this approach should be considered as an initial discriminating screen, as it is empirical and requires subsequent mechanistic studies to provide a comprehensive evaluation of a positive result.     

Investigation of the metabolism of 14C/13C-practolol in rat using directly coupled radio-HPLC-NMR-MS

1. The metabolic fate of ¹⁴C ¹³C-practolol was investigated using on-line HPLCNMR-MS following oral administration to rat. The major route of elimination for the radiolabel was via the urine with the principal biotransformation products confirmed as the 2-hydroxy- and 2-hydroxyglucronide metabolites. 2. In addition, futile deacetylation, determined by the replacement of ¹³C-labelled acetyl groups with endogenous ¹²C-acetyls accounted for ~7-10% of the urinary metabolites, corresponding to ~5% of the dose undergoing N-deacetylation. 3. Evidence for chiral metabolism was sought via NMR of isolated metabolites using beta cyclodextrin as a chiral shift agent. Practolol was excreted as a racemate. However, some enantioselective metabolism excretion had occurred as the hydroxy- and hydroxyglucuronide were not excreted as racemic mixtures. 4. Directly coupled radio-HPLC-NMR-MS is extremely effective for the identification of the metabolites of radiolabelled xenobiotics in urine samples.     

银杏叶提取物对细胞色素P450影响的研究进展

银杏叶提取物(Ginkgo biloba extract,GBE)是目前使用最为广泛的中草药之一。GBE与其他药物合用是否产生药物相互作用得到关注。GBE与其他药物合用,可能通过影响肝药酶的活性发生药物相互作用,其中GBE对肝药酶的诱导或抑制作用的研究较为深入。本综述回顾了近年来报道的关于GBE对细胞色素P450 CYP3A4、CYP2C9、CYP1A2、CYP2C19的影响。     

In vitro inhibitory effects of dihydromyricetin on human liver cytochrome P450 enzymes

Context: Dihydromyricetin (DHM) is the most abundant and active flavonoid component isolated from Ampelopsis grossedentata (Hand-Mazz) W.T. Wang (Vitaceae) and it possesses numerous pharmacological activities. However, whether DHM affects the activity of human liver cytochrome P450 (CYP) enzymes remains unclear.

Materials and methods: The inhibitory effects of DHM on eight human liver CYP isoforms (i.e., 1A2, 3A4, 2A6, 2E1, 2D6, 2C9, 2C19 and 2C8) were investigated in vitro using human liver microsomes (HLMs).

Results: The results showed that DHM could inhibit the activity of CYP3A4, CYP2E1 and CYP2D6, with IC₅₀ values of 14.75, 25.74 and 22.69?μM, respectively, but that other CYP isoforms were not affected. Enzyme kinetic studies showed that DHM was not only a non-competitive inhibitor of CYP3A4 but also a competitive inhibitor of CYP2E1 and CYP2D6, with Ki values of 6.06, 9.24 and 10.52?μM, respectively. In addition, DHM is a time-dependent inhibitor for CYP3A4 with K_I/K_inact value of 12.17/0.057?min^?1?μM^?1.

Discussion and conclusion: The in vitro studies of DHM with CYP isoforms indicate that DHM has the potential to cause pharmacokinetic drug interactions with other co-administered drugs metabolized by CYP3A4, CYP2E1 and CYP2D6. Further clinical studies are needed to evaluate the significance of this interaction.