Impact of incubation conditions on bufuralol human clearance predictions: enzyme lability and nonspecific binding

Time-dependent enzyme inactivation: Numerical analyses of in vitro data and prediction of drug-drug interactions

Cytochrome P450 (CYP) enzyme kinetics often do not conform to Michaelis-Menten assumptions, and time-dependent inactivation (TDI) of CYPs displays complexities such as multiple substrate binding, partial inactivation, quasi-irreversible inactivation, and sequential metabolism. Additionally, in vitro experimental issues such as lipid partitioning, enzyme concentrations, and inactivator depletion can further complicate the parameterization of in vitro TDI. The traditional replot method used to analyze in vitro TDI datasets is unable to handle complexities in CYP kinetics, and numerical approaches using ordinary differential equations of the kinetic schemes offer several advantages. Improvement in the parameterization of CYP in vitro kinetics has the potential to improve prediction of clinical drug-drug interactions (DDIs). This manuscript discusses various complexities in TDI kinetics of CYPs, and numerical approaches to model these complexities. The extrapolation of CYP in vitro TDI parameters to predict in vivo DDIs with static and dynamic modeling is discussed, along with a discussion on current gaps in knowledge and future directions to improve the prediction of DDI with in vitro data for CYP catalyzed drug metabolism.

Influence of incubation conditions on microsomal metabolism of xanthine-derived A1 adenosine receptor ligands

INTRODUCTION

In vitro metabolism models such as liver microsomes represent an important tool for the development of novel radioligands. Comparability and physiological relevance of in vitro metabolism data critically depend on the careful evaluation and optimization of assay protocols. We therefore investigated the influence of incubation conditions on the microsomal stability of xanthine-derived A₁ adenosine receptor (A₁AR) ligands which have been developed for positron emission tomography (PET).

METHODS

Substrate depletion assays using rat liver microsomes (RLM) were performed for three analogous compounds which differ with regard to the metabolically vulnerable substituent at the xanthine C8 position. Incubation conditions were varied systematically. Additionally, the stability of the cofactor NADPH during incubation was investigated.

RESULTS

Microsomal metabolism was strongly influenced by buffer pH, organic solvents and preincubation time. Substrate depletion values varied up to 5-fold depending on incubation matrix composition, however, the rank order of metabolic stability remained unchanged. Prolonged incubation periods led to drastic loss in enzyme activity which could not be prevented by addition of metal chelators or antioxidants. Cofactor NADPH was rapidly oxidized in microsomal matrix, even in the absence of cytochrome P450 substrates.

DISCUSSION

In summary, short incubation times, precise pH control and minimal concentrations of organic solvents are mandatory to obtain reliable microsomal stability data. Furthermore, in vitro metabolic stability of the tested A₁AR ligands varied largely depending on the particular C8 substituent. Consequently, structural modifications at the xanthine C8 position appear to be a promising strategy for the improvement of A₁AR PET radioligands.

Assessment of the Biotransformation of Low-Turnover Drugs in the HµREL Human Hepatocyte Coculture Model

Metabolic profiles of four drugs possessing diverse metabolic pathways (timolol, meloxicam, linezolid, and XK469) were compared following incubations in both suspended cryopreserved human hepatocytes and the HμREL hepatocyte coculture model. In general, minimal metabolism was observed following 4-hour incubations in both suspended hepatocytes and the HμREL model, whereas incubations conducted up to 7 days in the HμREL coculture model resulted in more robust metabolic turnover. In the case of timolol, in vivo human data suggest that 22% of the dose is transformed via multistep oxidative opening of the morpholine moiety. Only the first-step oxidation was detected in suspended hepatocytes, whereas the relevant downstream metabolites were produced in the HµREL model. For meloxicam, both the hydroxymethyl and subsequent carboxylic acid metabolites were abundant following incubation in the HμREL model, while only a trace amount of the hydroxymethyl metabolite was observed in suspension. Similar to timolol, linezolid generated substantially higher levels of morpholine ring-opened carboxylic acid metabolites in the HμREL model. Finally, while the major aldehyde oxidase-mediated mono-oxidative metabolite of XK469 was minimally produced in hepatocyte suspension, the HμREL model robustly produced this metabolite, consistent with a pathway reported to account for 54% of the total urinary excretion in human. In addition, low-level taurine and glycine conjugates were identified in the HµREL model. In summary, continuous metabolite production was observed for up to 7 days of incubation in the HµREL model, covering cytochrome P450, aldehyde oxidase, and numerous conjugative pathways, while predominant metabolites correlated with relevant metabolites reported in human in vivo studies.

Advances in high-resolution MS and hepatocyte models solve a long-standing metabolism challenge: the loratadine story

BACKGROUND

Loratadine (LOR, Claritin(®)) is a long-acting antihistamine used to treat allergic rhinitis. The major active human metabolite, desloratadine (DL, Clarinex(®)), is extensively metabolized to 3-hydroxydesloratadine (3-OH-DL) (M40) and subsequently glucuronidated before elimination. This study revealed the ability of a novel, long-term hepatocyte micropatterned co-culture (MPCC) model to generate in vivo metabolites. Metabolites were detected and characterized using non-targeted MS/MS(ALL) with SWATH™ acquisition by a UHPLC-Q-TOF system. Results & methodology: Human MPCCs extensively metabolized LOR and formed 3-OH-DL-glucuronide (M13). Cross-species comparisons revealed monkey- and rat-specific metabolites with gender-specific DL-pyridine-N-oxide formation in male rats. These results demonstrate a first for an in vitro hepatocyte model to generate circulating metabolites of LOR and detect species-specific differences. Early focus on human metabolites could have spared characterization of nonhuman metabolites in preclinical species.

Identification of Tazarotenic Acid as the First Xenobiotic Substrate of Human Retinoic Acid Hydroxylase CYP26A1 and CYP26B1

Cytochrome P450 (CYP) 26A1 and 26B1 are heme-containing enzymes responsible for metabolizing all-trans retinoic acid (at-RA). No crystal structures have been solved, and therefore homology models that provide structural information are extremely valuable for the development of inhibitors of cytochrome P450 family 26 (CYP26). The objectives of this study were to use homology models of CYP26A1 and CYP26B1 to characterize substrate binding characteristics, to compare structural aspects of their active sites, and to support the role of CYP26 in the metabolism of xenobiotics. Each model was verified by dockingat-RA in the active site and comparing the results to known metabolic profiles ofat-RA. The models were then used to predict the metabolic sites of tazarotenic acid with results verified by in vitro metabolite identification experiments. The CYP26A1 and CYP26B1 homology models predicted that the benzothiopyranyl moiety of tazarotenic acid would be oriented toward the heme of each enzyme and suggested that tazarotenic acid would be a substrate of CYP26A1 and CYP26B1. Metabolite identification experiments indicated that CYP26A1 and CYP26B1 oxidatively metabolized tazarotenic acid on the predicted moiety, with in vitro rates of metabolite formation by CYP26A1 and CYP26B1 being the highest across a panel of enzymes. Molecular analysis of the active sites estimated the active-site volumes of CYP26A1 and CYP26B1 to be 918 Å(3)and 977 Å(3), respectively. Overall, the homology models presented herein describe the enzyme characteristics leading to the metabolism of tazarotenic acid by CYP26A1 and CYP26B1 and support a potential role for the CYP26 enzymes in the metabolism of xenobiotics.

Application of a Micropatterned Cocultured Hepatocyte System To Predict Preclinical and Human-Specific Drug Metabolism

Laboratory animal models are the industry standard for preclinical risk assessment of drug candidates. Thus, it is important that these species possess profiles of drug metabolites that are similar to those anticipated in human, since metabolites also could be responsible for biologic activities or unanticipated toxicity. Under most circumstances, preclinical species reflect human in vivo metabolites well; however, there have been several notable exceptions, and understanding and predicting these exceptions with an in vitro system would be very useful. Human micropatterned cocultured (MPCC) hepatocytes have been shown to recapitulate human in vivo qualitative metabolic profiles, but the same demonstration has not been performed yet for laboratory animal species. In this study, we investigated several compounds that are known to produce human-unique metabolites through CYP2C9, UGT1A4, aldehyde oxidase (AO), or N-acetyltransferase that were poorly covered or not detected at all in the selected preclinical species. To perform our investigation we used 24-well MPCC hepatocyte plates having three individual human donors and a single donor each of monkey, dog, and rat to study drug metabolism at four time points per species. Through the use of the multispecies MPCC hepatocyte system, the metabolite profiles of the selected compounds in human donors effectively captured the qualitative in vivo metabolite profile with respect to the human metabolite of interest. Human-unique metabolites that were not detected in vivo in certain preclinical species (normally dog and rat) were also not generated in the corresponding species in vitro, confirming that the MPCC hepatocytes can provide an assessment of preclinical species metabolism. From these results, we conclude that multispecies MPCC hepatocyte plates could be used as an effective in vitro tool for preclinical understanding of species metabolism relative to humans and aid in the choice of appropriate preclinical models.

Low-Turnover Drug Molecules: A Current Challenge for Drug Metabolism Scientists

In vitro assays using liver subcellular fractions or suspended hepatocytes for characterizing the metabolism of drug candidates play an integral role in the optimization strategy employed by medicinal chemists. However, conventional in vitro assays have limitations in their ability to predict clearance and generate metabolites for low-turnover (slowly metabolized) drug molecules. Due to a rapid loss in the activity of the drug-metabolizing enzymes, in vitro incubations are typically performed for a maximum of 1 hour with liver microsomes to 4 hours with suspended hepatocytes. Such incubations are insufficient to generate a robust metabolic response for compounds that are slowly metabolized. Thus, the challenge of accurately estimating low human clearance with confidence has emerged to be among the top challenges that drug metabolism scientists are confronted with today. In response, investigators have evaluated novel methodologies to extend incubation times and more sufficiently measure metabolism of low-turnover drugs. These methods include plated human hepatocytes in monoculture, and a novel in vitro methodology using a relay of sequential incubations with suspended cryopreserved hepatocytes. In addition, more complex in vitro cellular models, such as HepatoPac (Hepregen, Medford, MA), a micropatterned hepatocyte-fibroblast coculture system, and the HµREL (Beverley Hills, CA) hepatic coculture system, have been developed and characterized that demonstrate prolonged enzyme activity. In this review, the advantages and disadvantages of each of these in vitro methodologies as it relates to the prediction of clearance and metabolite identification will be described in an effort to provide drug metabolism scientists with the most up-to-date experimental options for dealing with the complex issue of low-turnover drug candidates.

Physiology-based IVIVE predictions of tramadol from in vitro metabolism data

PURPOSE

To predict the tramadol in vivo pharmacokinetics in adults by using in vitro metabolism data and an in vitro-in vivo extrapolation (IVIVE)-linked physiologically-based pharmacokinetic (PBPK) modeling and simulation approach (Simcyp®).

METHODS

Tramadol metabolism data was gathered using metabolite formation in human liver microsomes (HLM) and recombinant enzyme systems (rCYP). Hepatic intrinsic clearance (CLintH) was (i) estimated from HLM corrected for specific CYP450 contributions from a chemical inhibition assay (model 1); (ii) obtained in rCYP and corrected for specific CYP450 contributions by study-specific intersystem extrapolation factor (ISEF) values (model 2); and (iii) scaled back from in vivo observed clearance values (model 3). The model-predicted clearances of these three models were evaluated against observed clearance values in terms of relative difference of their geometric means, the fold difference of their coefficients of variation, and relative CYP2D6 contribution.

RESULTS

Model 1 underpredicted, while model 2 overpredicted the total tramadol clearance by -27 and +22%, respectively. The CYP2D6 contribution was underestimated in both models 1 and 2. Also, the variability on the clearance of those models was slightly underpredicted. Additionally, blood-to-plasma ratio and hepatic uptake factor were identified as most influential factors in the prediction of the hepatic clearance using a sensitivity analysis.

CONCLUSION

IVIVE-PBPK proved to be a useful tool in combining tramadol's low turnover in vitro metabolism data with system-specific physiological information to come up with reliable PK predictions in adults.

Time-dependent inhibition and estimation of CYP3A clinical pharmacokinetic drug-drug interactions using plated human cell systems

The current studies assessed the utility of freshly plated hepatocytes, cryopreserved plated hepatocytes, and cryopreserved plated HepaRG cells for the estimation of inactivation parameters k(inact) and K(I) for CYP3A. This was achieved using a subset of CYP3A time-dependent inhibitors (fluoxetine, verapamil, clarithromycin, troleandomycin, and mibefradil) representing a range of potencies. The estimated k(inact) and K(I) values for each time-dependent inhibitor were compared with those obtained using human liver microsomes and used to estimate the magnitude of clinical pharmacokinetic drug-drug interaction (DDI). The inactivation kinetic parameter, k(inact), was most consistent across systems tested for clarithromycin, verapamil, and troleandomycin, with a high k(inact) of 0.91 min(-1) observed for mibefradil in HepaRG cells. The apparent K(I) estimates derived from the various systems displayed a range of variability from 3-fold for clarithromycin (5.4-17.7 μM) to 6-fold for verapamil (1.9-12.6 μM). In general, the inactivation kinetic parameters derived from the cell systems tested fairly replicated what was observed in time-dependent inhibition studies using human liver microsomes. Despite some of the observed differences in inactivation kinetic parameters, the estimated DDIs derived from each of the tested systems generally agreed with the clinically reported DDI within approximately 2-fold. In addition, a plated cell approach offered the ability to conduct longer primary incubations (greater than 30 min), which afforded improved ability to identify the weak time-dependent inhibitor fluoxetine. Overall, results from these studies suggest that in vitro inactivation parameters generated from plated cell systems may be a practical approach for identifying time-dependent inhibitors and for estimating the magnitude of clinical DDIs.

Human liver mitochondrial cytochrome P450 2D6--individual variations and implications in drug metabolism

Constitutively expressed human cytochrome P450 2D6 (CYP2D6; EC 1.14.14.1) is responsible for the metabolism of approximately 25% of drugs in common clinical use. It is widely accepted that CYP2D6 is localized in the endoplasmic reticulum of cells; however, we have identified this enzyme in the mitochondria of human liver samples and found that extensive inter-individual variability exists with respect to the level of the mitochondrial enzyme. Metabolic assays using 7-methoxy-4-aminomethylcoumarin as a substrate show that the human liver mitochondrial enzyme is capable of oxidizing this substrate and that the catalytic activity is supported by mitochondrial electron transfer proteins. In the present study, we show that CYP2D6 contains an N-terminal chimeric signal that mediates its bimodal targeting to the endoplasmic reticulum and mitochondria. In vitro mitochondrial import studies using both N-terminal deletions and point mutations suggest that the mitochondrial targeting signal is localized between residues 23-33 and that the positively-charged residues at positions 24, 25, 26, 28 and 32 are required for mitochondrial targeting. The importance of the positively-charged residues was confirmed by transient transfection of a CYP2D6 mitochondrial targeting signal mutant in COS-7 cells. Both the mitochondria and the microsomes from a CYP2D6 stable expression cell line contain the enzyme and both fractions exhibit bufuralol 1'-hydroxylation activity, which is completely inhibited by CYP2D6 inhibitory antibody. Overall, these results suggest that the targeting of CYP2D6 to mitochondria could be an important physiological process that has significance in xenobiotic metabolism.

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.

In vivo use of the P450 inactivator 1-aminobenzotriazole in the rat: varied dosing route to elucidate gut and liver contributions to first-pass and systemic clearance

The small intestine is regarded as an absorptive organ in the uptake of orally administered drugs, but also has the ability to metabolize drugs by both phase 1 and phase 2 reactions. The amount of drug that reaches the systemic circulation can be reduced by both intestinal and hepatic metabolism. 1-Aminobenzotriazole (ABT) is an irreversible inhibitor of cytochrome P450s. Through in vivo and in vitro studies, ABT has been evaluated for its utility in studying intestinal metabolism in rats. Rats have been exposed to ABT through varied routes of administration followed by p.o. and i.v. administration of midazolam (MDZ), a CYP3A substrate. The MDZ bioavailablity in rats dosed orally and in rats dosed intravenously with ABT is 58.5% and 0.7%, respectively (%F = 2.3% w/o ABT). The approximately 80-fold difference between the two groups suggests the majority of the extraction occurs in the intestine following an oral dose. To further study the utility of ABT, the antihistamine fexofenadine (Fex), which is not significantly metabolized and is a substrate for the uptake and efflux transporters, OATP and P-gp, was tested in rat. There was no change in oral or systemic exposure of Fex when animals were predosed with ABT, suggesting that ABT does not affect these transporters. These findings may lead to a better understanding of the interdependent role of absorption and metabolism and the specificity of ABT. This method should have utility in drug discovery for the identification of factors limiting oral bioavailability.

Minimizing Polymorphic Metabolism in Drug Discovery: Evaluation of the Utility of in Vitro Methods for Predicting Pharmacokinetic Consequences Associated with CYP2D6 Metabolism

Minimizing interindividual variability in drug exposure is an important goal for drug discovery. The reliability of the selective CYP2D6 inhibitor quinidine was evaluated in a retrospective analysis using a standardized approach that avoids laboratory-to-laboratory variation. The goal was to evaluate the reliability of in vitro metabolism studies for predicting extensive metabolizer (EM)/poor metabolizer (PM) exposure differences. Using available literature, 18 CYP2D6 substrates were selected for further analysis. In vitro microsomal studies were conducted at 1 microM substrate and 0.5 microM P450 to monitor substrate depletion. An estimate of the fraction metabolized by CYP2D6 in microsomes was derived from the rate constant determined with and without 1 microM quinidine for 11 substrates. Clearance in EM and PM subjects and fractional recovery of metabolites were taken from the literature. A nonlinear relationship between the contribution of CYP2D6 and decreased oral clearance for PMs relative to EMs was evident. For drugs having <60% CYP2D6 involvement in vivo, a modest difference between EM and PM exposure was observed (<2.5-fold). For major CYP2D6 substrates (>60%), more dramatic exposure differences were observed (3.5- to 53-fold). For compounds primarily eliminated by hepatic P450 and with sufficient turnover to be evaluated in vitro, the fraction metabolized by CYP2D6 in vitro compared favorably with the in vivo data. The in vitro estimation of fraction metabolized using quinidine as a specific inhibitor provided an excellent predictive tool. Results from microsomal substrate depletion experiments can be used with confidence to select compounds in drug discovery using a cutoff of >60% metabolism by CYP2D6.

Predictions of the In Vivo Clearance of Drugs from Rate of Loss Using Human Liver Microsomes for Phase I and Phase II Biotransformations

PURPOSE

The utility of in vitro metabolism to accurately predict the clearance of hepatically metabolized drugs was evaluated. Three major goals were: (1) to optimize substrate concentration for the accurate prediction of clearance by comparing to Km value, (2) to prove that clearance of drugs by both oxidation and glucuronidation may be predicted by this method, and (3) to determine the effects of nonspecific microsomal binding and plasma protein binding.

METHODS

The apparent Km values for five compounds along with scaled intrinsic clearances and predicted hepatic clearances for eight compounds were determined using a substrate loss method. Nonspecific binding to both plasma and microsomal matrices were also examined in the clearance calculations.

RESULTS

The Km values were well within the 2-fold variability expected for between laboratory comparisons. Using both phase I and/or phase II glucuronidation incubation conditions, the predictions of in vivo clearance using the substrate loss method were shown to correlate with published human clearance values. Of particular interest, for highly bound drugs (>95% plasma protein bound), the addition of a plasma protein binding term increased the accuracy of the prediction of in vivo clearance.

CONCLUSIONS

The substrate loss method may be used to accurately predict hepatic clearance of drugs.

A streamlined method to predict hepatic clearance using human liver microsomes in the presence of human plasma

INTRODUCTION

Human liver microsomal incubations are often used to predict the metabolic lability of new chemical entities. The clearance values are scaled-up from in vitro data and mathematically corrected for plasma protein binding, or in some cases the free fraction ratio of plasma to microsomes, using well-established scaling methods such as the well-stirred model. This can be time consuming for multiple compounds since it requires separate experiments to determine in vitro lability, and free fraction.

METHODS

We attempted to streamline clearance predictions by combining experiments into one. Firstly, we combined the free fraction experiments into one free fraction ratio by measuring the partitioning of compound between plasma and microsomes, and by applying this experimental ratio to clearance predictions found that it performed at least as well as free fractions determined separately. We also incubated compounds with plasma added to the incubation mixture and compared the predicted clearances to values determined using traditional mathematical protein binding corrections.

RESULTS

Consistently, incubations with added plasma resulted in CL predictions closer to literature values than incubations only mathematically corrected for protein binding. For example, incorporating plasma into a ketamine incubation resulted in a CL value of 15.1 mL/min/kg, compared with a value of 10.2 using mathematical binding corrections. The literature value is 16.4 mL/min/kg.

DISCUSSION

This work characterizes this new method and compares it to the traditional microsomal incubation method using several literature compounds, and suggests that streamlining the methods may generate quality data faster and with less resource investment.

ALTERED AZT (3′-AZIDO-3′-DEOXYTHYMIDINE) GLUCURONIDATION KINETICS IN LIVER MICROSOMES AS AN EXPLANATION FOR UNDERPREDICTION OF IN VIVO CLEARANCE: COMPARISON TO HEPATOCYTES AND EFFECT OF INCUBATION ENVIRONMENT

Human liver microsomes are a reagent commonly used to predict human hepatic clearance of new chemical entities via phase 1 metabolism. Another common metabolic pathway, glucuronidation, can also be observed in human liver microsomes, although the scalability of this process has not been validated. In fact, several groups have demonstrated that clearance estimated from liver microsomes with UDP-glucuronic acid typically underpredicts the actual in vivo clearance more than 10-fold for compounds that are predominantly glucuronidated. In contrast, clearance predicted using human hepatocytes, for these same compounds, provides a more accurate assessment of in vivo clearance. We sought to characterize the kinetics of glucuronidation of the selective UGT2B7 substrate AZT (3'-azido-3'-deoxythymidine), a selective UGT2B7 substrate, in human liver microsomes (HLMs), recombinant UGT2B7, and human hepatocytes. Apparent Km values in these three preparations were 760, 490, and 87 microM with apparent Vmax values highest in hepatocytes. The IC50 for ibuprofen against AZT glucuronidation, when run at its Km concentration in HLMs and hepatocytes, was 975 and 170 microM respectively. Since incubation conditions have been shown to modulate glucuronidation rates, AZT glucuronidation was performed in various physiological and nonphysiological buffer systems, namely Tris, phosphate, sulfate, carbonate, acetate, human plasma, deproteinized human liver cytosol, and Williams E medium. The data showed that carbonate and Williams E medium, more physiologically relevant buffers, yielded the highest rates of AZT glucuronidation. Km observed in HLM/carbonate was 240 microM closer to that found in hepatocytes, suggesting that matrix differences might cause the kinetic differences observed between liver preparations. Caution should be exercised when extrapolating metabolic lability via glucuronidation or inhibition of UGT enzymes from human liver microsomes, since this system appears to underpredict the degree of lability or inhibition, respectively, due in part to an apparent decrease in substrate affinity.

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.

Effect of Nonspecific Binding to Microsomes and Metabolic Elimination of Buprenorphine on the Inhibition of Cytochrome P4502D6

Recently, the pharmaceutical industry has employed the high-throughput method for the evaluation of cytochrome P450 (CYP) inhibition, using a combination of the heterologously expressed enzyme and a fluoregenic substrate. When buprenorphine (BN), a potent mixed agonist-antagonist analgesic, was evaluated by this method, it exhibited potent inhibition of CYP2D6 with an IC50 value of 0.25 microM in recombinant CYP2D6-expressing insect cell microsomes (rCYP2D6 microsomes). In contrast, the IC50 value was 22.7 microM in human liver microsomes (HLM) using a classical method. Although the substrate concentrations in each study were set to near the Km values, there was a large discrepancy in IC50 values. When we investigated the effect of nonspecific binding to microsomes on the inhibitory potency, with a view to clarifying this discrepancy, the unbound fraction in microsomes (fu,mic) was 0.06-0.21 and 0.99 in HLM and rCYP2D6 microsomes, respectively. The corrected IC50 value (1.74 microM) using free BN concentrations was much smaller than the uncorrected value. On the other hand, it was observed that the concentration of BN in HLM decreased rapidly due to metabolism by CYP3A4 while that in rCYP2D6 microsomes decreased only slightly. We then investigated the effect of incubation time on the inhibitory potency, since the rapid elimination of BN in HLM could have been a cause of the discrepancy. The IC50 value for BN was observed to decrease slightly from 22.7 to 17.1 microM, following the shortening of the incubation time from 10 to 2 min in HLM. We conclude that nonspecific binding to microsomes of the inhibitor could affect the inhibitory potency against CYP2D6. If this factor is considered, a more precise estimate of the risk of adverse drug interaction could be achieved.

Metabolic screening in vitro: metabolic stability, CYP inhibition and induction

Automated, miniaturised assays are now commonplace within Discovery DMPK (drug metabolism and pharmacokinetics). These have evolved considerably since their inception around a decade ago, in-line with both technology and the desire to provide quality data comparable to more traditional analyses. Several formats exist for routine screening of metabolic stability and cyp (cytochrome P450; see Glossary) inhibition and induction, with the major focus being on in vitro systems using human-derived material. Data from other species remain valuable in assessing in vitro-in vivo projections and are pivotal to support safety assessment studies.: