Kinetics of diazepam metabolism in rat hepatic microsomes and hepatocytes and their use in predicting in vivo hepatic clearance

Prediction of Drug Clearance from Enzyme and Transporter Kinetics

Accurate estimation of in vivo clearance in human is pivotal to determine the dose and dosing regimen for drug development. In vitro-in vivo extrapolation (IVIVE) has been performed to predict drug clearance using empirical and physiological scalars. Multiple in vitro systems and mathematical modeling techniques have been employed to estimate in vivo clearance. The models for predicting clearance have significantly improved and have evolved to become more complex by integrating multiple processes such as drug metabolism and transport as well as passive diffusion. This chapter covers the use of conventional as well as recently developed methods to predict metabolic and transporter-mediated clearance along with the advantages and disadvantages of using these methods and the associated experimental considerations. The general approaches to improve IVIVE by use of appropriate scalars, incorporation of extrahepatic metabolism and transport and application of physiologically based pharmacokinetic (PBPK) models with proteomics data are also discussed. The chapter also provides an overview of the advantages of using such dynamic mechanistic models over static models for clearance predictions to improve IVIVE.

The Constraints, Construction, and Verification of a Strain-Specific Physiologically Based Pharmacokinetic Rat Model

The use of in vitro-in vivo extrapolation (IVIVE) techniques, mechanistically incorporated within physiologically based pharmacokinetic (PBPK) models, can harness in vitro drug data and enhance understanding of in vivo pharmacokinetics. This study's objective was to develop a user-friendly rat (250 g, male Sprague-Dawley) IVIVE-linked PBPK model. A 13-compartment PBPK model including mechanistic absorption models was developed, with required system data (anatomical, physiological, and relevant IVIVE scaling factors) collated from literature and analyzed. Overall, 178 system parameter values for the model are provided. This study also highlights gaps in available system data required for strain-specific rat PBPK model development. The model's functionality and performance were assessed using previous literature-sourced in vitro properties for diazepam, metoprolol, and midazolam. The results of simulations were compared against observed pharmacokinetic rat data. Predicted and observed concentration profiles in 10 tissues for diazepam after a single intravenous (i.v.) dose making use of either observed i.v. clearance (CL_iv) or in vitro hepatocyte intrinsic clearance (CL_int) for simulations generally led to good predictions in various tissue compartments. Overall, all i.v. plasma concentration profiles were successfully predicted. However, there were challenges in predicting oral plasma concentration profiles for metoprolol and midazolam, and the potential reasons and according solutions are discussed.

Interaction of six protoberberine alkaloids with human organic cation transporters 1, 2 and 3

1. Organic cation transporters (OCTs) play an important role in drug safety and efficacy. Protoberberine alkaloids are ubiquitous organic cations or weak bases with remarkable biological actives. This study was to elucidate the potential interaction of alkaloids (coptisine, jatrorrhizine, epiberberine, berberrubine, palmatine and corydaline) with OCTs using Madin-Darby canine kidney (MDCK) cells stably expressing human OCT1, OCT2 and OCT3. 2. All the tested alkaloids significantly inhibited the uptake of MPP(+), a model OCT substrate, in MDCK-hOCTs cells with the IC50 of 0.931-9.65 μM. Additionally, coptisine, jatrorrhizine and epiberberine were substrates of all the hOCTs with the Km of 0.273-5.80 μM, whereas berberrubine was a substrate for hOCT1 and hOCT2, but not for hOCT3, the Km values were 1.27 and 1.66 μM, respectively. The transport capacity of coptisine in MDCK cells expressing the variants of hOCT1-P341L or hOCT2-A270S was significantly higher than that in wild-type (WT) cells with the Clint (Vmax/Km) of 379 ± 7.4 and 433 ± 5.7 μl/mg protein/min, respectively. 3. The above data indicate that the tested alkaloids are potent inhibitors, and coptisine, jatrorrhizine, epiberberine and berberrubine are substrates of hOCT1, hOCT2 and/or hOCT3 with high affinity. In addition, the variants (OCT1-P341L and OCT2-A270S) possess higher transport capacity to coptisine than WT hOCTs.

Biosynthesis of human diazepam and clonazepam metabolites

A screening of fungal and microbial strains allowed to select the best microorganisms to produce in high yields some of the human metabolites of two benzodiazepine drugs, diazepam and clonazepam, in order to study new pharmacological activities and for chemical standard proposes. Among the microorganisms tested, Cunninghamella echinulata ATCC 9244 and Rhizopus arrhizus ATCC 11145 strains, were the most active producers of the mains metabolites of diazepam which included demethylated, hydroxylated derivatives. Beauveria bassiana ATCC 7159 and Chaetomium indicum LCP 984200 produced the 7 amino-clonazepam metabolite and a product of acid hydrolysis of this benzodiazepine.

Preparation of hepatocytes

This unit describes a process for isolation of intact, viable hepatocytes for use in integrated drug metabolism studies. Isolated hepatocytes are increasingly being used as a biological system for studying the in vitro metabolism of xenobiotics. The isolation of hepatocytes from laboratory animals and nontransplantable human livers donated for research activities involves a two-step enzymatic digestion of the liver tissue. Two methods for the perfusion of liver tissue are included: in situ perfusion and isolation of hepatocytes and perfusion of excised tissue. The basic protocol also includes suggestions for designing drug metabolism experiments using hepatocytes. The final protocol describes cryopreservation and long-term storage of isolated hepatocytes.

Prediction of human pharmacokinetics—evaluation of methods for prediction of hepatic metabolic clearance

Methods for prediction of hepatic clearance (CLH) in man have been evaluated. A physiologically-based in-vitro to in-vivo (PB-IVIV) method with human unbound fraction in blood (fu,bl) and hepatocyte intrinsic clearance (CLint)-data has a good rationale and appears to give the best predictions (maximum ∼2-fold errors; < 25% errors for half of CL-predictions; appropriate ranking). Inclusion of an empirical scaling factor is, however, needed, and reasons include the use of cryopreserved hepatocytes with low activity, and inappropriate CLint- and fu,bl-estimation methods. Thus, an improvement of this methodology is possible and required. Neglect of fu,bl or incorporation of incubation binding does not seem appropriate. When microsome CLint-data are used with this approach, the CLH is underpredicted by 5- to 9-fold on average, and a 106-fold underprediction (attrition potential) has been observed. The poor performance could probably be related to permeation, binding and low metabolic activity. Inclusion of scaling factors and neglect of fu,bl for basic and neutral compounds improve microsome predictions. The performance is, however, still not satisfactory. Allometry incorrectly assumes that the determinants for CLH relate to body weight and overpredicts human liver blood flow rate. Consequently, allometric methods have poor predictability. Simple allometry has an average overprediction potential, > 2-fold errors for ∼1/3 of predictions, and 140-fold underprediction to 5800-fold overprediction (potential safety risk) range. In-silico methodologies are available, but these need further development. Acceptable prediction errors for compounds with low and high CLH should be ∼50 and ∼10%, respectively. In conclusion, it is recommended that PB-IVIV with human hepatocyte CLint and fu,bl is applied and improved, limits for acceptable errors are decreased, and that animal CLH-studies and allometry are avoided.

Prediction of hepatic clearance in human from in vitro data for successful drug development

The in vivo metabolic clearance in human has been successfully predicted by using in vitro data of metabolic stability in cryopreserved preparations of human hepatocytes. In the predictions by human hepatocytes, the systematic underpredictions of in vivo clearance have been commonly observed among different datasets. The regression-based scaling factor for the in vitro-to-in vivo extrapolation has mitigated discrepancy between in vitro prediction and in vivo observation. In addition to the elimination by metabolic degradation, the important roles of transporter-mediated hepatic uptake and canalicular excretion have been increasingly recognized as a rate-determining step in the hepatic clearance. It has been, therefore, proposed that the in vitro assessment should allow the evaluation of clearances for both transporter(s)-mediated uptake/excretion and metabolic degradation. This review first outlines the limited ability of subcellular fractions such as liver microsomes to predict hepatic clearance in vivo. It highlights the advantages of cryopreserved human hepatocytes as one of the versatile in vitro systems for the prediction of in vivo metabolic clearance in human at the early development stage. The following section discusses the mechanisms underlying the systematic underprediction of in vivo intrinsic clearance by hepatocytes. It leads to the proposal for the assessment of hepatic uptake clearance as one of the kinetically important determinants for accurate predictions of hepatic clearance in human. The judicious combination of advanced technologies and understandings for the drug disposition allows us to rationally optimize new chemical entities to the drug candidate with higher probability of success during the clinical development.

Impact of end-product inhibition on the determination ofin vitrometabolic clearance

End-product inhibition was explored as a mechanism for the lower clearance determination obtained from microsomes compared with hepatocytes. Triazolam, diazepam and phenytoin microsomal substrate depletion was reduced by 23, 34 and 39%, respectively, when incubated with their primary metabolites. Ki values of 28+/-6 and 11+/-1 microM were obtained when 4'-hydroxydiazepam and p-hydroxyphenytoin where incubated with diazepam and phenytoin, respectively. Alamethicin (a glucuronidation activator) was unsuccessful in alleviating these effects. IC50 values of 17, 32 and 18 microM for phenytoin and 83, 110 and 97 microM for diazepam were observed with salicylamide- (a glucuronidation inhibitor) treated hepatocytes, control hepatocytes and microsomes, respectively, when incubated with their primary metabolites. These differences suggest that metabolite concentrations in the vicinity of the enzyme are lower in hepatocytes compared with microsomes, reducing the likelihood of end-product inhibition in the former system. In conclusion, end-product inhibition may be more prominent in microsomes (in particular for substrate depletion assays where metabolism tends to be more extensive); results suggest that this phenomenon may contribute to the observed variations in metabolism characteristics and intrinsic clearance (CLint) between hepatocytes and microsomes.

Substrate depletion approach for determining in vitro metabolic clearance: time dependencies in hepatocyte and microsomal incubations

The substrate depletion method is a popular approach used for the measurement of in vitro intrinsic clearance (CL(int)). However, the incubation conditions used in these studies can vary, the consequences of which have not been systematically explored. Initial substrate depletion incubations using rat microsomes and hepatocytes were performed for eight benzodiazepines: alprazolam, clobazam, clonazepam, chlordiazepoxide, diazepam, flunitrazepam, midazolam, and triazolam. Subsequent predictions of in vivo CL(int) (ranging from 3 to 200 ml/min) and hepatic clearance (CL(H)) (ranging from 0.3 to 15 ml/min) demonstrated that the general predictive ability of this approach was similar to that of the traditional metabolite formation method. A more detailed study of the substrate depletion profiles and CL(int) estimates indicated that the concentration of enzyme used is of particular importance. The metabolism of triazolam, clonazepam, and diazepam was monoexponential at all cell densities using hepatocytes; however, with microsomes, biphasic depletion was apparent, particularly at higher microsomal protein concentrations (2-5 mg/ml). Enzyme activity studies indicated that enzyme loss was more pronounced in the microsomal system (ranged from 8 to 65% activity after a 1-h incubation) compared with the hepatocyte system (approximately 100% activity after a 1-h incubation). For clonazepam (a low clearance substrate), these biphasic profiles could be explained by loss of enzyme activity. To ensure accurate predictions of in vivo CL(int) and CL(H) when using the substrate depletion approach, based on the results obtained for this class of drugs, it is recommended that low enzyme concentrations and short incubation times are used whenever possible.

Fuzzy simulation of pharmacokinetic models: case study of whole body physiologically based model of diazepam

The aim of the present study is to develop and implement a methodology that accounts for parameter variability and uncertainty in the presence of qualitative and semi-quantitative information (fuzzy simulations) as well as when some parameters are better quantitatively defined than others (fuzzy-probabilistic approach). The fuzzy simulations method consists of (i) representing parameter uncertainty and variability by fuzzy numbers and (ii) simulating predictions by solving the pharmacokinetic model. The fuzzy-probabilistic approach includes an additional transformation between fuzzy numbers and probability density functions. To illustrate the proposed method a diazepam WBPBPK model was used where the information for hepatic intrinsic clearance determined by in vitro-in vivo scaling was semi-quantitative. The predicted concentration time profiles were compared with those resulting from a Monte Carlo simulation. Fuzzy simulations can be used as an alternative to Monte Carlo simulation.

QUANTITATIVE PREDICTION OF THE IN VIVO INHIBITION OF DIAZEPAM METABOLISM BY OMEPRAZOLE USING RAT LIVER MICROSOMES AND HEPATOCYTES

The diazepam (DZ)-omeprazole (OMP) interaction has been selected as a prototype for an important drug-drug interaction involving cytochrome P450 inhibition. The availability of an in vivo K(i) value (unbound K(i), 21 microM) obtained from a series of steady-state inhibitor infusion studies allowed assessment of several in vitro-derived predictions of this inhibition interaction. Studies monitoring substrate depletion with time were used to obtain in vitro K(i) values that were evaluated against the more traditional metabolite formation approach using microsomes and hepatocytes. OMP inhibited the metabolism of DZ to its primary metabolites 4'-hydroxydiazepam, 3-hydroxydiazepam, and nordiazepam to different extents over a range of concentrations (0.3-150 microM), and a competitive inhibition model best fitted the data. The K(i) values observed using the substrate depletion approach (16 +/- 3 microM and 7 +/- 2 microM in microsomes and hepatocytes, respectively) were in good agreement with the overall weighted K(i) values obtained using the standard metabolite formation approach (12 +/- 2 microM and 16 +/- 5 microM in microsomes and hepatocytes, respectively). In vitro binding and cell uptake studies as well as human serum albumin studies in hepatocytes confirmed the importance of both intracellular and extracellular unbound concentrations of inhibitor when considering inhibition predictions. Both kinetic approaches and both in vitro systems predicted the in vivo interaction well and provide a good example of the ability of in vitro inhibition studies to quantitatively predict an in vivo drug-drug interaction successfully.

Effects of the aqueous extract from Salvia miltiorrhiza Bge on the pharmacokinetics of diazepam and on liver microsomal cytochrome P450 enzyme activity in rats

The aim of this study was to determine the effects of the aqueous extract of Salvia miltiorrhiza Bge (danshen in Chinese) on the pharmacokinetics of diazepam and on liver microsomal cytochrome P450 enzyme activity in rats. Rats (n = 5) were pretreated with danshen extract (100 mg kg(-1) per day, p.o.) for 15 consecutive days. Control rats (n = 5) received saline at the same time. Each rat was then administered a single oral dose of 15 mg kg(-1) diazepam. The pharmacokinetic parameters of diazepam were significantly different between the two groups. In the danshen pretreated group, the maximum concentration of diazepam and the area under the plasma concentration-time curve were reduced to about 72.7% and 44.4%, respectively, while the total body clearance was markedly increased by 2-fold. To help explain the results, liver microsomal suspensions were obtained from rats that were randomly divided into the control group (n = 10), and the low- (20 mg kg(-1) for 15 days, p.o., n = 10) and high-dose groups (100 mg kg(-1) for 15 days, p.o., n = 10) pretreated with danshen extract. Compared with the control rats, the microsomal protein content, cytochrome P450 enzyme level and erythromycin N-demethylase activity of pretreated rats were significantly increased. These results indicate that danshen extract can stimulate the activity of cytochrome P450 isoforms, and changes in the pharmacokinetics of diazepam resulting from danshen extract are related to an increase in metabolic activity of cytochrome P450.

Reassessing models of hepatic extraction

The aim of this investigation is to compare different mathematical models of the liver in the context of in vitro-in vivo correlation. We reanalyze drugs from the Houston reviews [1, 2], and compare the mathematical models. For the well-stirred model, a particular form of the distributed tubes model, and the dispersion model, fits are done to in vitro and in vivo intrinsic clearance data from microsomal and hepatocyte experiments. The distributed and dispersion models have decreased residuals as compared to the well-stirred model, but neither is to be clearly preferred over theother. It seems likely that drug-specific factors have a major impact on the quality of IVIVC correlations. While new experiments are needed to validate IVIVC models, our results indicate that improved correlation of in vitroand in vivo data is possible for high clearance drugs by using either a dispersion or distributed tube model rather than a well-stirred model.

A commentary on the use of hepatocytes in drug metabolism studies during drug discovery and development

Isolated hepatocytes and liver slices, in short-term suspension or longer-term culture, offer the prospect of providing qualitative metabolic information and quantitative pharmacokinetic parameters from key animal species and man at early stages of the drug discovery-development continuum. The propensity for changes in the fidelity of drug metabolism after removal of hepatocytes from the organ has long been recognized. The many and varied approaches which have been undertaken in an attempt to compensate for physiological shortcomings of in vitro hepatocyte systems are reviewed. In this respect, short-term suspension culture may provide a baseline against which to measure the success of extended culture methods, but it should be remembered that even freshly isolated hepatocyte preparations have deficiencies and liabilities that may affect the nature of information gathered. This article discusses the current advances and shortcomings of hepatocyte suspensions and cultures, along with liver slice technology, at both quantitative and qualitative levels.

Utility of hepatocytes to model species differences in the metabolism of loxtidine and to predict pharmacokinetic parameters in rat, dog and man

1. The metabolism of loxtidine (1-methyl-5-[3-[3-[(1-piperidinyl) methyl] phenoxy] propyl] amino-1H-1,2,4-triazole-3-methanol) was studied in freshly isolated rat, dog and human hepatocytes. Metabolism in vitro was comparable with previously available in vivo data in all three species with the marked species differences observed in vivo being reproduced in the hepatocyte model. 2. The major route for the metabolism of loxtidine by rat hepatocytes was N-dealkylation to form the propionic acid and hydroxymethyl triazole metabolites. A minor metabolic route was the oxidation of loxtidine to a carboxylic acid metabolite. The major route of metabolism for loxtidine in dog hepatocytes was glucuronidation with oxidation to the carboxylic acid metabolite being of minor importance. Incubation of loxtidine with human hepatocytes resulted in the drug remaining largely unchanged but with the carboxylic acid metabolite being produced in minor amounts. 3. In vitro studies were undertaken with rat, dog and human hepatocytes to determine the Michaelis-Menten parameters Vmax and Km for the sum of all the metabolic pathways. These kinetic parameters were used to calculate the intrinsic clearance of loxtidine. Using appropriate scaling factors, the predicted in vivo hepatic clearance was then calculated. The predicted intrinsic clearances were 51.4 +/- 12.4, 8.0 +/- 0.8 and 1.0 +/- 0.6 ml/min/kg for rat, dog and human hepatocytes respectively. These data were then used to calculate hepatic clearances of 24.5, 3.1 and 0.2 ml/min/kg for rat, dog and man respectively. 4. In vivo hepatic and intrinsic clearances for loxtidine were determined in rat, dog and human volunteers. The hepatic clearances of loxtidine were 26.6, 6.6 and 0.4 ml/min/kg in rat, dog and man respectively and intrinsic clearances were 58.5, 18.6 and 2.0 ml/min/kg in rat, dog and man respectively. 5. The present studies demonstrate that the hepatocyte model may be a valuable in vitro tool for predicting both qualitative and quantitative aspects of the metabolism of a drug in animals and man at an early stage of the drug development process.

Application of microdialysis to study the in vitro metabolism of drugs in liver microsomes

Current methods for studying in vitro drug metabolism involve add-incubate-separate-measure approach. Separation of the desired analytes requires removal of protein which is typically accomplished by precipitation and centrifugation and extraction of the analytes into an organic phase. The analysis scheme then becomes more complex resulting in a decrease in precision and an increase in assay time. Microdialysis sampling circumvents these problems by allowing researchers to sample the reaction mixture periodically and obtain the complete metabolic profile. In the present study, microdialysis sampling was used to investigate Phase I metabolism of salicylic acid, diazepam and ibuprofen in rat liver microsomes. The major metabolites of these drugs were profiled by LC. Michaelis-Menten enzyme kinetic parameters, Km and Vmax were obtained for the formation of diazepam metabolites by both microdialysis and conventional microsomal incubations and were in good agreement with the values reported in the literature. This study shows that microdialysis has considerable promise as a sampling technique for in vitro drug metabolism studies. By making minor modifications to the instruments, microdialysis can be applied to other in vitro systems such as isolated hepatocytes to study the Phase II metabolism or tissue slices to study drug distribution.

Integration of in vitro data into allometric scaling to predict hepatic metabolic clearance in man: application to 10 extensively metabolized drugs

In this study, we investigated rational and reliable methods of using animal data to predict in humans the clearance of drugs which are mainly eliminated through hepatic metabolism. For 10 extensively metabolized compounds, adjusting the in vivo clearance in the different animal species for the relative rates of metabolism in vitro dramatically improved the predictions of human clearance compared to the approach in which clearance is directly extrapolated using body weight. Using hepatocyte data to normalize the in vivo clearances led to lower median deviations between the observed and predicted clearances in man compared to the approach normalizing data with brain weight (30-40% vs 60-80%, respectively). In addition, the approach integrating in vitro data appeared to be superior with respect to the range of deviations: approximately 2-fold underestimation, in the worst case, was observed by using in vitro data, whereas normalizing data by brain weight led to up to 10-fold underestimation of clearance in man. In addition, the integration of in vitro data provides a more rational basis to predict the metabolic clearance in man and may be applicable to compounds undergoing phase I and phase II metabolism as well.

The use of human hepatocytes to select compounds based on their expected hepatic extraction ratios in humans

PURPOSE

The present investigation retrospectively evaluates the use of human hepatocytes to classify compounds into low, intermediate or high hepatic extraction ratio in man.

METHODS

A simple approach was used to correlate the in vivo hepatic extraction ratio of a number of compounds in man (literature and in-house data) with the corresponding in vitro clearance which was determined in human hepatocytes. The present approach assumes that, for compounds eliminated mainly through liver metabolism, intrinsic clearance is the major determinant for their in vivo hepatic extraction ratio and subsequently their bioavailability in man. The test compounds were selected to represent a broad range of extraction ratios and a variety of metabolic pathways.

RESULTS

The present data show that in vitro clearances in human hepatocytes are predictive for the hepatic extraction ratios in vivo in man. Most of the test compounds (n = 19) were successfully classified based upon human hepatocyte data into low, intermediate or high hepatic extraction compounds, i.e. compounds with potential for high, intermediate or low bioavailabilities in humans.

CONCLUSIONS

The present approach, validated so far with 19 test compounds, appears to be a valuable tool to screen for compounds with respect to liver first-pass metabolism at an early phase of drug discovery.

Prediction of the pharmacokinetic parameters of reduced-dolasetron in man using in vitro-in vivo and interspecies allometric scaling

1. Dolasetron (Anzemet) is a potent and selective 5-HT3 receptor antagonist which is rapidly and extensively reduced to yield its major pharmacologically active metabolite, reduced dolasetron (RD). RD is further metabolized by CYP450 enzymes as well as undergoing renal excretion. As both in vitro and in vivo data on RD were available from animals and man, two approaches to predict the human pharmacokinetic parameters of RD were assessed. 2. First, in vitro studies, using liver microsomes from animal species and man, were undertaken to measure Vmax and K(m) and to assess the intrinsic clearance (CLint). With appropriate liver weight and liver blood flow scaling factors the predicted in vivo metabolic clearance (CLm-pred) was calculated. Human CLm-pred was underestimated by a factor of 5 when it was calculated using the above scaling factors. As, in a prospective study, the observed human in vivo metabolic clearance (CLm-obs) is unknown, CLm-pred was substituted into the least-squares correlation equation obtained from a plot of CLm-pred against CLm-obs' using animal data. The estimate of human CLm-obs was improved as it was only underestimated by a factor of 1.5. 3. Second, allometric scaling of in vivo animal pharmacokinetic data, using body weight, was performed to predict pharmacokinetic parameters in man. Good predictions of human pharmacokinetic parameters of RD were obtained for plasma clearance (1.7 l/min predicted versus 1.61/min observed), half-life (6.0 h predicted versus 5.6 h observed), and volume of distribution (860.91 predicted versus 770.41 observed). 4. The integration of in vitro metabolic data from microsomes gave similar results to conventional allometric scaling, whereas the normalization of clearance by brain weight resulted in an approximately three-fold underestimation of human clearance. 5. For RD, a drug that is eliminated by both renal and metabolic clearance, retrospective conventional allometric scaling allowed accurate prediction of pharmacokinetic parameters in man, whereas in vitro-in vivo scaling resulted in an underestimation of in vivo CLm. Although these results are somewhat at variance, ideally both scaling methods should be applied to improve the prediction of human pharmacokinetic parameters.

Selectivity of omeprazole inhibition towards rat liver cytochromes P450

1. The potency and selectivity of omeprazole as an inhibitor of cytochrome P450-mediated drug oxidations has been assessed in hepatic microsomes from the untreated, phenobarbitone-treated, beta-naphthoflavone-treated and dexamethasone-treated rat. Using the marker substrates diazepam, ethoxycoumarin, ethoxyresofurin and ethylmorphine in the above microsomal preparations, inhibitory activity against CYP1A, 2B, 2C and 3A members of the cytochrome P450 superfamily were determined. 2. In each situation studied the kinetics of inhibition by omeprazole were competitive in nature with Ki's ranging from 25 to > 1000 microM. Marker activities for the 3A family in microsomes from the dexamethasone-treated and phenobarbitone-treated rat (3-hydroxylation of diazepam and N-demethylation of ethylmorphine) were most susceptible to omeprazole inhibition (Km/Ki ratios greater than unity) compared with marker activities for the CYP1A, 2B and 2C sub-families (Km/Ki ratios < or = unity). 3. Omeprazole sulphoxide showed similar potency and selectivity of inhibition to its parent drug. Analogous studies with the same marker activities using ketoconazole indicated that both omeprazole and its sulphoxide metabolite are less potent as an inhibitor of cytochrome P4503A in rat than this well characterised prototype.

Metabolite kinetics of ondansetron in rat. Comparison of hepatic microsomes, isolated hepatocytes and liver slices, with in vivo disposition

1. The kinetics of hydroxylation and N-demethylation of ondansetron have been determined in freshly isolated hepatocytes, hepatic microsomes and precision-cut liver slices from the male Sprague-Dawley rat. In vivo studies have also been carried out to characterize the pharmacokinetics of ondansetron and in vitro data have been assessed for their value as predictors of hepatic clearance. 2. In the three in vitro systems, the formation of hydroxylated and demethylated metabolites were characterized as a function of substrate concentration by a high-affinity, low-capacity site and a low-affinity, high-capacity site which was not saturated over the concentration range studied (2.5-500 microM). Slices gave consistently higher Km's (20 and 30 microM for hydroxylation and demethylation respectively) than hepatocytes (3 and 13 microM respectively) and microsomes (2 and 5 microM respectively.) The rank order of Vmax and CL(int) was the same for each system; hydroxylation rates exceeding demethylation rates. Although two hydroxylations (7- and 8-hydroxy metabolites) occurred exclusively in microsomes, these are believed to originate from a common precursor. 3. The high CL(int) of ondansetron (150 ml/min/SRW, where SRW is a standard rat weight of 250g) is well predicted by scaling either microsomal clearance for microsomal protein recovery or hepatocyte clearance for hepatocellularity (212 and 135 ml/min/SRW respectively). In contrast, the use of liver slice data scaled to a whole liver substantially underestimates CL(int) (9 ml/min/SRW).

Effect of omeprazole on diazepam disposition in the rat: in vitro and in vivo studies

PURPOSE

The inhibitory effects of omeprazole on diazepam metabolism in vitro and in vivo are compared in the rat.

METHODS

3-hydroxylation and N-demethylation of diazepam was investigated in the presence of a range of omeprazole concentrations (2-500 microM) in hepatic microsomes and hepatocytes. Zero order infusions together with matched bolus doses of omeprazole were used to achieve a range of steady state plasma concentrations (10-50mg/L) and to study the diazepam-omeprazole interaction in vivo.

RESULTS

The 3-hydroxlation pathway was more prone to inhibition (KIs 108 +/- 30 and 28 +/- 11 microM in microsomes and hepatocytes, respectively) than the demethylation pathway (KIs of 226 +/- 76 and 59 +/- 27 microM in microsomes and hepatocytes, respectively). In both in vitro systems, the mechanism of inhibition was competitive with Km/KI ratios larger than 1 for the 3HDZ pathway and smaller than 1 for the NDZ pathway. There was an omeprazole concentration dependent decrease in diazepam clearance in vivo which could be modelled using a simple inhibition equation with a KI of 57 microM (19.8mg/L). In contrast there was no statistically significant change in the steady state volume of distribution for diazepam in the presence of omeprazole.

CONCLUSIONS

The in vivo KI for the omeprazole: diazepam inhibition interaction shows closer agreement with the KI values obtained in hepatocytes than with those observed in microsomes.