Effect of albumin on phenytoin and tolbutamide metabolism in human liver microsomes: an impact more than protein binding

Does Addition of Protein to Hepatocyte or Microsomal In Vitro Incubations Provide a Useful Improvement in In Vitro-In Vivo Extrapolation Predictability?

Accurate prediction of in vivo hepatic clearance is an essential part of successful and efficient drug development; however, many investigators have recognized that there are significant limitations in the predictability of clearance with a tendency for underprediction for primarily metabolized drugs. Here, we examine the impact of adding serum or albumin into hepatocyte and microsomal incubations on the predictability of in vivo hepatic clearance. The addition of protein into hepatocyte incubations has been reported to improve the predictability for high clearance (extraction ratio) drugs and highly protein-bound drugs. Analyzing published data for 60 different drugs and 97 experimental comparisons (with 17 drugs being investigated from two to seven) we confirmed the marked underprediction of clearance. However, we could not validate any relevant improved predictability within twofold by the addition of serum to hepatocyte incubations or albumin to microsomal incubations. This was the case when investigating all measurements, or when subdividing analyses by extraction ratio, degree of protein binding, Biopharmaceutics Drug Disposition Classification System class, examining Extended Clearance Classification System class 1B drugs only, or drug charge. Manipulating characteristics of small data sets of like compounds and adding scaling factors can appear to yield good predictability, but the carryover of these methods to alternate drug classes and different laboratories is not evident. Improvement in predictability of poorly soluble compounds is greater than that for soluble compounds, but not to a meaningful extent. Overall, we cannot confirm that protein addition improves in vitro-in vivo extrapolation predictability to any clinically meaningful degree when considering all drugs and different subsets. SIGNIFICANCE STATEMENT: The addition of protein into microsomal or hepatocyte incubations has been widely proposed to improve hepatic clearance predictions. To date, studies examining this phenomenon have not included appropriate negative controls where predictability is achieved without protein addition and have been conducted with small data sets of similar compounds that don't apply to alternate drug classes. Here, an extensive analysis of published data for 60 drugs and 97 experimental comparisons couldn't validate any relevant clinically improved clearance predictability with protein addition.

In vitro metabolism, reaction phenotyping, enzyme kinetics, CYP inhibition and induction potential of ataluren

Ataluren promotes ribosomal readthrough of premature termination codons in mRNA which result from nonsense mutations. In vitro studies were performed to characterize the metabolism and enzyme kinetics of ataluren and its interaction potential with CYP enzymes. Incubation of [¹⁴C]‐ataluren with human liver microsomes indicated that the major metabolic pathway for ataluren is via direct glucuronidation and that the drug is not metabolized via cytochrome P450 (CYP). Glucuronidation was also observed in the incubation in human intestinal and kidney microsomes, but not in human pulmonary microsomes. UGT1A9 was found to be the major uridine diphosphate glucuronosyltransferase (UGT) responsible for ataluren glucuronidation in the liver and kidney microsomes. Enzyme kinetic analysis of the formation of ataluren acyl glucuronide, performed in human liver, kidney, and intestinal microsomes and recombinant human UGT1A9, found that increasing bovine serum albumin (BSA) levels enhanced the glucuronidation Michaelis‐Menten constant (K_m) and ataluren protein binding but had a minimal effect on maximum velocity (V_max) of glucuronidation. Due to the decreased unbound Michaelis‐Menten constant (K_m,u), the ataluren unbound intrinsic clearance (CL_int,u) increased for all experimental systems and BSA concentrations. Human kidney microsomes were about 3.7‐fold more active than human liver microsomes, in terms of CL_int,u/mg protein, indicating that the kidney is also a key organ for the metabolism and disposition of ataluren in humans. Ataluren showed no or little potential to inhibit or induce most of the CYP enzymes.

Investigating the Impact of Albumin on the Liver Uptake of Pitavastatin and Warfarin in Nagase Analbuminemic Rats

Albumin has been suggested to enhance the hepatic uptake of organic anion-transporting polypeptide (Oatp) substrates in various in vitro as well as liver perfusion models. However, it is not known whether the interplay between albumin and Oatp substrates is an experimental artifact or if this interaction occurs in vivo. The objective of this work was to investigate the hepatic uptake of warfarin and pitavastatin, which are both extensively bound to albumin but only pitavastatin being an Oatp substrate. Experiments were conducted in Nagase analbuminemic rats (NAR) which exhibit reduced albumin levels compared with F344 (wild type, WT). The fraction unbound (f _u) was 140- and 10-fold greater in NAR plasma for warfarin and pitavastatin, respectively, whereas no meaningful differences were observed with tissue binding. In vitro, pitavastatin uptake into hepatocytes reconstituted in WT plasma was 17- and 3-fold greater than when reconstituted in buffer or NAR plasma, respectively. In vivo, the free tissue-to-free plasma ratios (K _p,u,u) from brain and liver in intact WT and NAR were not significantly different for warfarin. Contrarily, liver K _p,u,u of pitavastatin was 6-fold higher in WT animals, which corresponded to a 2.3-fold reduction in free plasma and 2.6-fold increase in free liver exposure. These results suggest that the enhanced hepatic uptake by albumin is not necessarily an experimental artifact but is also a relevant phenomenon in vivo. This work raises the possibility that other plasma proteins may also effect the function of additional drug transporters, and that modulating plasma protein binding may exhibit meaningful clinical relevance in the disposition of drugs. SIGNIFICANCE STATEMENT: The interplay between albumin and Oatp substrates has been reported in hepatocytes and in liver perfusion studies, but the in vivo relevance of this interaction has yet to be elucidated. Using NAR and its corresponding WT animal, this study demonstrates that albumin may indeed enhance the hepatic uptake of pitavastatin in intact animals. In vivo demonstration of this interplay not only provides further justification for continued investigation into this particular mechanism but also raises the possibility that other plasma proteins may affect additional drug transporters and that modulating plasma protein binding may exhibit meaningful clinical relevance in the disposition of drugs.

Hepatic clearance concepts and misconceptions: Why the well-stirred model is still used even though it is not physiologic reality?

The liver is the most important drug metabolizing organ, endowed with a plethora of metabolizing enzymes and transporters to facilitate drug entry and removal via metabolism and/or biliary excretion. For this reason, much focus surrounds the development of clearance concepts, which are based on normalizing the rate of removal to the input or arterial concentration. By so doing, some authors have recently claimed that it implies one specific model of hepatic elimination, namely, the widely used well-stirred or venous equilibration model (WSM). This commentary challenges this claim and aims to provide a comprehensive discussion of not only the WSM but other currently applied hepatic clearance models - the parallel tube model (PTM), the dispersion model (DM), the zonal liver model (ZLM), and the heterogeneous capillary transit time model of Goresky and co-workers (GM). The WSM, PTM, and DM differ in the patterns of internal blood flow, assuming bulk, plug, and dispersive flows, respectively, which render different degrees of mixing within the liver that are characterized by the magnitudes of the dispersion number (D_N), resulting in different implications concerning the (unbound) substrate concentration in liver (Cu_H). Early models assumed perfusion rate-limited distribution, which have since been modified to include membrane-limited transport. The recent developments associated with the misconceptions and the sensitivity of the models are hereby addressed. Since the WSM has been and will likely remain widely used, the pros and cons of this model relative to physiological reality are further discussed.

Atypical Kinetics and Albumin Effect of Glucuronidation of 5-n-Butyl-4-{4-[2-(1H-tetrazole-5- yl)-1H-pyrrol-1-yl]phenylmethyl}-2,4-dihydro-2-(2,6- dichlorophenyl)-3H-1,2,4-triazol-3-one, a Novel Nonpeptide Angiotensin Type 1 Receptor Antagonist, in Liver Microsomes and UDP-Glucuronosyl-transferase

Ib is a new nonpeptide AT1 receptor antagonist, which plays an active role in cardiovascular protection. Ib monoglucuronide has been identified as its main metabolite. A detailed study of Ib glucuronidation is important for predicting potential DDI. Besides, the elucidation of the “BSA effect” in Ib glucuronidation would make obtained kinetic parameters more predictive in IVIVE. “BSA effect” means that there is a significant change in in vitro kinetic parameters when generated from incubations performed in the presence of bovine serum albumin (BSA). Five UGTs (UGT1A3, UGT2B4, UGT2B7, UGT1A9 and UGT1A8) were identified that produced abundant Ib monoglucuronide, especially UGT1A3. We investigated Ib glucuronidation in liver microsomes from different species (rat, dog, human) and in five identified major human UGTs. Ib glucuronidation in liver microsomes and recombinant human UGTs all showed substrate inhibition kinetics. DLM showed the strongest affinity and activity, HLM showed the lowest affinity, and RLM showed the weakest activity. The addition of BSA did not alter the enzyme kinetics, but significantly altered enzyme kinetic parameters resulting in a reduction in K_m value and an increase in CL_int value. However, high concentrations of BSA could significantly attenuate this positive effect on enzyme affinity and activity, and the effect of BSA on the V_max of Ib glucuronidation was opposite in different enzyme sources. In conclusion, this study demonstrated the substrate inhibition kinetics of Ib glucuronidation in the liver metabolism and the effect of BSA on its kinetic parameters, in order to provide more accurate in vitro data for in vivo prediction.

The Role of Extracellular Binding Proteins in the Cellular Uptake of Drugs: Impact on Quantitative In Vitro-to-In Vivo Extrapolations of Toxicity and Efficacy in Physiologically Based Pharmacokinetic-Pharmacodynamic Research

A critical component in the development of physiologically based pharmacokinetic-pharmacodynamic (PBPK/PD) models for estimating target organ dosimetry in pharmacology and toxicology studies is the understanding of the uptake kinetics and accumulation of drugs and chemicals at the cellular level. Therefore, predicting free drug concentrations in intracellular fluid will contribute to our understanding of concentrations at the site of action in cells in PBPK/PD research. Some investigators believe that uptake of drugs in cells is solely driven by the unbound fraction; conversely, others argue that the protein-bound fraction contributes a significant portion of the total amount delivered to cells. Accordingly, the current literature suggests the existence of a so-called albumin-mediated uptake mechanism(s) for the protein-bound fraction (i.e., extracellular protein-facilitated uptake mechanisms) at least in hepatocytes and cardiac myocytes; however, such mechanism(s) and cells from other organs deserve further exploration. Therefore, the main objective of this present study was to discuss further the implication of potential protein-facilitated uptake mechanism(s) on drug distribution in cells under in vivo conditions. The interplay between the protein-facilitated uptake mechanism(s) and the effects of a pH gradient, metabolism, transport, and permeation limitation potentially occurring in cells was also discussed, as this should violate the basic assumption on similar free drug concentration in cells and plasma. This was made because the published equations used to calculate drug concentrations in cells in a PBPK/PD model did not consider potential protein-facilitated uptake mechanism(s). Consequently, we corrected some published equations for calculating the free drug concentrations in cells compared with plasma in PBPK/PD modeling studies, and we proposed a refined strategy for potentially performing more accurate quantitative in vitro-to-in vivo extrapolations (IVIVEs) of toxicity (efficacy) at the cellular level from data generated in cell assays. Overall, this present study may help to optimize the human dose prediction in preclinical and clinical studies, while prescribing drugs with narrow therapeutic windows that are highly bound to extracellular proteins and/or highly ionized at the physiological pH. This may facilitate building a more accurate safety (efficacy) profile for such drugs.

In vitro and in vivo characterization of 13 CYP2C9 allelic variants found in Chinese Han population

Our previous study detected totally 35 CYP2C9 allelic variants in 2127 Chinese subjects, of whom 21 novel alleles were reported for the first time in Chinese populations. The aim of the present study was to characterize the 13 CYP2C9 allelic variants both in vitro and in vivo. Different types of CYP2C9 variants were highly expressed in COS-7 cells, and 50 μM tolbutamide was added as the probing substrate to evaluate their metabolic abilities in vitro. Subsequently, the concentrations of tolbutamide and its metabolite in the plasma and urine within individuals with different types of genotypes were determined by HPLC to evaluate the catalytic activity of the 13 mutant CYP2C9 proteins in vivo. Our results showed that compared with *1/*1 wild-type subjects, subjects with *1/*40 genotype showed increased oral clearance (CL/F), whereas individuals with *1/*3, *1/*13, *3/*3, *3/*13, *1/*16, *1/*19, *1/*34, *1/*42, *1/*45, *1/*46, and *1/*48 genotype exhibited significantly decreased CL/F, and those with *1/*27, *1/*29, *1/*40, and *1/*41 genotype presented similar CL/F value. When expressed in COS-7 cells, the CYP2C9 variants showed similar pattern to the results in clinical study. The study suggests that, besides two typical defective alleles, *3 and *13, seven CYP2C9 allelic variants (*16, *19, *34, *42, *45, *46, and *48) cause defective effects on the enzymatic activities both in vitro and in vivo. In clinic, patients with these defective alleles should be paid close attention to.

Discovery and preclinical characterization of the cyclopropylindolobenzazepine BMS-791325, a potent allosteric inhibitor of the hepatitis C virus NS5B polymerase

Described herein are structure-activity relationship studies that resulted in the optimization of the activity of members of a class of cyclopropyl-fused indolobenzazepine HCV NS5B polymerase inhibitors. Subsequent iterations of analogue design and syntheses successfully addressed off-target activities, most notably human pregnane X receptor (hPXR) transactivation, and led to significant improvements in the physicochemical properties of lead compounds. Those analogues exhibiting improved solubility and membrane permeability were shown to have notably enhanced pharmacokinetic profiles. Additionally, a series of alkyl bridged piperazine carboxamides was identified as being of particular interest, and from which the compound BMS-791325 (2) was found to have distinguishing antiviral, safety, and pharmacokinetic properties that resulted in its selection for clinical evaluation.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

The importance of plasma protein binding in drug discovery

Plasma protein binding of drugs is a well-recognised phenomena, but it is only recently that the implications for drug action in vivo have been fully appreciated. Plasma proteins, by virtue of their high concentration, control the free drug concentration in plasma and in compartments in equilibrium with plasma, thereby, effectively attenuating drug potency in vivo. The historical background and thermodynamic basis for the 'Free Drug Principle' is presented, along with special considerations for intracellular targets, deep compartments and α1-acid glycoprotein binding. Real and apparent exceptions to the principle are discussed along with a survey of citations from the recent medicinal chemistry literature.

In vitro-in vivo extrapolation of clearance: modeling hepatic metabolic clearance of highly bound drugs and comparative assessment with existing calculation methods

In vitro-in vivo extrapolation (IVIVE) is an important method for estimating the hepatic metabolic clearance (CL) of drugs. This study highlights a problematic area observed when using microsomal data to predict in vivo CL of drugs that are highly bound to plasma proteins, and further explores mechanisms for human CL predictions by associating additional processes to IVIVE disconnect. Therefore, this study attempts to develop a novel IVIVE calculation method, which consists of adjusting the binding terms in a well-stirred liver model. A comparative assessment between the IVIVE method proposed here and previously published methods of Obach (1999. Drug Metab Dispos 27:1350-1359) and Berezhkovskiy (2010. J Pharm Sci 100:1167-1783) was also performed. The assessment was confined by the availability of measured in vitro and in vivo data in humans for 25 drugs highly bound to plasma proteins, for which it can be assumed that metabolism is the major route of elimination. Here, we argue that a difference in drug ionization and binding proteins such as albumin (AL) and alpha-1-acid glycoprotein (AAG) in plasma and liver also needs to be considered in IVIVE based on mechanistic studies. Therefore, converting unbound fraction in plasma to liver essentially increased the predicted CL values, which resulted in much more accurate estimates of in vivo CL as compared with the other IVIVE methods tested. The impact on CL estimate was more apparent for drugs binding to AL than to AAG. This is a mechanistic rational for explaining a considerable proportion of the divergence between previously estimated and observed CL values. Human CL was predicted within 1.5-fold, twofold, and threefold of the observed CL for 84%, 96%, and 100% of the compounds, respectively. Overall, this study demonstrates a significant improvement in the mechanism-based prediction of metabolic CL for these 25 highly bound drugs from in vitro data determined with microsomes, which should facilitate the application of physiologically based pharmacokinetic (PBPK) models in drug discovery and development.

The inhibition study of human UDP-glucuronosyltransferases with cytochrome P450 selective substrates and inhibitors

Human uridine-5'-diphosphoglucuronosyltransferases (UGTs) are the major phase II metabolizing enzymes. In the present study, five human UGTs (UGT1A1, 1A4, 1A6, 2B7, and 2B10) were individually expressed and used to examine the inhibition IC(50) values of 20 selective substrates and inhibitors of major cytochromes P450 (CYPs). The inhibition kinetics of UGT1A1 was also analyzed. The results showed that some compounds like α-naphthoflavone, paclitaxel, midazolam, cyclosporine A, and ketoconazole displayed strong inhibitions on UGT activities with their IC(50) values in a range of 4.1-26 µM. Especially, the IC(50) values were 4.1 ± 0.8 µM for ketoconazole in inhibiting UGT1A1-mediated β-estradiol-3-glucuronidation, and 4.9 ± 0.3 µM for paclitaxel towards UGT1A4-mediated midazolam-N-glucuronidation. Additionally, the IC(50) values of bupropion, tolbutamide, and testosterone in inhibiting UGT-mediated metabolisms were similar with the K(m) values of respective CYPs. Some kinetic behaviours of UGTs were following Michaelis-Menten kinetics, while some were not.

In vitro inhibitory effects of Wen-pi-tang-Hab-Wu-ling-san on human cytochrome P450 isoforms

WHAT IS KNOWN AND OBJECTIVE

Although Wen-pi-tang-Hab-Wu-ling-san (WHW), an oriental herbal medicine, has been prescribed for the treatment of chronic renal failure (CRF) in Korean clinics, no studies regarding WHW-drug interactions had been reported. The purpose of this study was to evaluate the possibility that WHW inhibits the catalytic activities of major cytochrome P450 (CYP) isoforms.

METHODS

The abilities of various WHW extracts to inhibit phenacetin O-de-ethylation (CYP1A2), tolbutamide 4-methylhydroxylation (CYP2C9), omeprazole 4'-hydroxylation (CYP2C19), dextromethorphan O-demethylation (CYP2D6), chlorzoxazone 6-hydroxylation (CYP2E1) and midazolam 1-hydroxylation (CYP3A4) were assessed using human liver microsomes.

RESULTS AND DISCUSSION

WHW extract at concentrations up to 100 μm showed negligible inhibition of the six CYP isoforms tested (CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4), with apparent IC(50) values (concentration of the inhibitor causing 50% inhibition of the original enzyme activity) of 817.5, 601.6, 521.7, 310.2, 342.8 and 487.0 μg/mL, respectively.

WHAT IS NEW AND CONCLUSION

Our in vitro findings suggest that WHW extract at concentrations corresponding to a clinically recommended dosage range has no notable inhibitory effects on CYP isoforms. Therefore, we believe that WHW extract may be free of drug-herb interactions when co-administered with other medicines. However, in vivo human studies are needed to confirm these results.

Studying cytochrome P450 kinetics in drug metabolism

BACKGROUND

Determination of cytochrome P450 enzyme-mediated kinetics in vitro can be useful for predicting drug dosing and clearance in humans. Expressed P450s, human liver microsomes, human hepatocytes (both fresh and cryopreserved), and human liver slices are used to estimate K(m) and V(max) values for determination of intrinsic clearance of the drug for scale-up to predict in vivo clearance.

OBJECTIVE

To describe the advantages and disadvantages of the various in vitro systems used to estimate kinetic parameters for disposition of drugs and the various kinetic profiles that can be observed.

METHODS

A review of the literature was conducted to evaluate the utility of the various in vitro preparations, the methods for determining kinetic parameters and the types of kinetic profiles that may be observed.

RESULTS/CONCLUSIONS

The choice of in vitro system for determining kinetic parameters will depend on the objective of the studies, as each system has advantages and disadvantages. Kinetic parameter determinations must be carefully assessed to assure that the correct kinetic model is applied and the most accurate kinetic parameters are determined.

Enzyme kinetics of GTI-2040, a phosphorothioate oligonucleotide targeting ribonucleotide reductase

Enzyme kinetics of GTI-2040 (5'-GGC TAA ATC GCT CCA CCA AG-3'), a phosphorothioate ribonucleotide reductase antisense, were investigated for the first time in 3' exonuclease solution and human liver microsomes (HLMs), using the ion-pair high-performance liquid chromatogram method for quantification of the parent drug and two major 3'N-1 and 3'N-2 metabolites. Enzyme kinetics of GTI-2040 in 3'-exonuclease solution were found to be well characterized by the Michaelis-Menten model, using the sum of formation rates of 3'N-1 and 3'N-2 (approximately total metabolism) because of sequential metabolism. In HLMs, a biphasic binding was observed for GTI-2040 with high- and low-affinity constants (K(d)s) of 0.03 and 3.8 microM, respectively. Enzyme kinetics of GTI-2040 in HLMs were found to deviate from Michaelis-Menten kinetics when the total GTI-2040 substrate was used. However, after correction for the unbound fractions, the formation rate of total metabolites could be described by Michaelis-Menten kinetics. Using the free substrate fraction, the K(m) and V(max) of GTI-2040 were determined to be 6.33 +/- 3.2 microM and 16.5 +/- 8.4 nmol/mg/h, respectively. Using these values, in vitro hepatic intrinsic clearance (CL(int)) in HLM was estimated to be 2.61 +/- 0.56 ml/h. The CL(int) was then used to predict GTI-2040's in vivo intrinsic clearance in humans by a microsomal protein scaling factor, which gave a mean value of 182.7 l/h, representing 24.1% of the observed in vivo mean scaled hepatic intrinsic clearance of 758.7 l/h in patients with acute myeloid leukemia. We concluded that the saturable nonspecific binding of GTI-2040 in HLMs complicated the interpretation of its enzyme kinetics, and scaled intrinsic clearance from HLMs only partially predicted the in vivo intrinsic clearance.

Changes in maternal liver Cyp2c and Cyp2d expression and activity during rat pregnancy

During human pregnancy, CYP2C9, CYP2C19, and CYP2D6 activities are altered. The aim of the current study was to determine if this phenomenon can be replicated in the rat, and to evaluate the mechanisms that contribute to the changes in Cyp2c and Cyp2d activity during pregnancy. The intrinsic clearance of dextromethorphan O-demethylation, a measure of Cyp2d2 activity, was decreased 80% at both days 9 and 19 of gestation when compared to non-pregnant controls. The decreased intrinsic clearance was a result of both decreased V(max) and increased K(m)-values at both days of gestation. Quantitative RT-PCR revealed that transcripts of Cyp2d2 and Cyp2d4 were significantly decreased at day 19 of pregnancy (p<0.05) when compared to day 9 and non-pregnant controls. The decrease in Cyp2d mRNA levels correlated with a decrease in several nuclear receptor mRNA levels (RARalpha, RXRalpha, HNF1 and HNF3beta) but not with the mRNA levels of nuclear receptors usually associated with regulation of P450 enzymes (PXR, CAR and HNF4alpha). In contrast, Cyp2c12 and Cyp2c6 transcription and protein expression were not significantly altered during rat pregnancy although the intrinsic clearance of Cyp2c6 mediated diclofenac 4'-hydroxylation was increased 2-fold on day 19 of gestation when compared to non-pregnant controls. The increase in intrinsic clearance was due to a decrease in the K(m)-value for 4'-hydroxydiclofenac formation. These data show that pregnancy significantly alters the expression and activity of drug metabolizing enzymes in an enzyme and gestational stage specific manner. These changes are likely to have toxicological and therapeutic implications.

Binding of Inhibitory Fatty Acids Is Responsible for the Enhancement of UDP-Glucuronosyltransferase 2B7 Activity by Albumin: Implications for in Vitro-in Vivo Extrapolation

Studies were performed to elucidate the mechanism responsible for the reduction in Km values of UDP-glucuronosyltransferase 2B7 (UGT2B7) substrates observed for incubations conducted in the presence of albumin. Addition of bovine serum albumin (BSA) and fatty acid-free human serum albumin (HSA-FAF), but not "crude" HSA, resulted in an approximate 90% reduction in the Km values for the glucuronidation of zidovudine (AZT) by human liver microsomes (HLM) and UGT2B7 and a 50 to 75% reduction in the S50 for 4-methylumbelliferone (4MU) glucuronidation by UGT2B7, without affecting Vmax. Oleic, linoleic, and arachidonic acids were shown to be the most abundant unsaturated long-chain fatty acids present in crude HSA and in the membranes of HLM and human embryonic kidney (HEK)293 cells, and it was demonstrated that these and other unsaturated long-chain fatty acids were UGT2B7 substrates. Glucuronides with Rf (retention factor) values corresponding to the glucuronides of linoleic and arachidonic acid were detected when HLM and HEK293 cell lysates were incubated with radiolabeled cofactor, and the intensity of the bands was modulated by the presence of crude HSA (increased) and BSA or HSA-FAF (decreased). Oleic, linoleic, and arachidonic acid inhibited AZT and 4MU glucuronidation by HLM and/or UGT2B7, due to an increase in Km/S50 without a change in Vmax. Addition of BSA and HSA-FAF reversed the inhibition. Likewise, coexpression of UGT2B7 and HSA in HEK293 cells reduced the Km/S50 values of these substrates. It is postulated that BSA and HSA-FAF sequester inhibitory fatty acids released during incubations, and the apparent high Km values observed for UGT2B7 substrates arise from the presence of these endogenous inhibitors.

Methods for Predicting Human Drug Metabolism

Drug metabolism information is a necessary component of drug discovery and development. The key issues in drug metabolism include identifying: the enzyme(s) involved, the site(s) of metabolism, the resulting metabolite(s), and the rate of metabolism. Methods for predicting human drug metabolism from in vitro and computational methodologies and determining relationships between the structure and metabolic activity of molecules are also critically important for understanding potential drug interactions and toxicity. There are numerous experimental and computational approaches that have been developed in order to predict human metabolism which have their own limitations. It is apparent that few of the computational tools for metabolism prediction alone provide the major integrated functions needed to assist in drug discovery. Similarly the different in vitro methods for human drug metabolism themselves have implicit limitations. The utilization of these methods for pharmaceutical and other applications as well as their integration is discussed as it is likely that hybrid methods will provide the most success.

The pivotal role of hepatocytes in drug discovery

This review promotes the value of isolated hepatocytes in modern Drug Discovery programmes and outlines how increased understanding, particularly in the area of in vitro-in vivo extrapolation (IVIVE), has led to more widespread use. The importance of in vitro metabolic intrinsic clearance data for predicting in vivo clearance has been acknowledged for several years and the greater utility of hepatocytes, compared with hepatic microsomes and liver slices, for this application is discussed. The application of hepatocytes in predicting drug-drug interactions (DDIs) resulting from reversible and irreversible (time-dependent) inhibition is relatively novel but affords the potential to study both phase I and phase II processes together with any impact of drug efflux and/or uptake (cellular accumulation). Progress in this area is reviewed along with current opinions on the comparative use of primary hepatocytes and higher throughput reporter gene-based systems for studying cytochrome P450 (CYP) induction. The appreciation of the role of transporter proteins in drug disposition continues to evolve. The study of hepatic uptake using isolated hepatocytes and the interplay between drug transport and metabolism with respect to both clearance and DDIs and subsequent IVIVE is also considered.

Rapid identification of P-glycoprotein substrates and inhibitors

Identifying molecules that interact with P-glycoprotein (P-gp) is important for drug discovery but is also generally reliant on time-consuming in vitro and in vivo studies. As an alternative approach, the current study applied pharmacophore models and database screening to rapidly retrieve molecules that bind as substrates or inhibitors for P-gp from commercial databases and then confirmed their affinity as inhibitors in vitro. Seven molecules (acitretin, cholecalciferol, misoprostol, nafcillin, repaglinide, salmeterol, and telmisartan) with no published details for P-gp affinity, one positive control inhibitor (miconazole), and two negative control molecules (phenelzine and zonisamide) were selected for testing. The MDCK-MDR1 in vitro cell model was used to confirm their inhibitory effect on [3H]digoxin transport, and the ATPase assay was used as an additional in vitro tool to indicate P-gp activation. All seven test drugs were confirmed to have P-gp affinity. Additionally, our experimental results provided plausible explanations for the published pharmacokinetic profiles of the tested drugs and their classification according to the biopharmaceutics and drug disposition classification system. In this study, we showed the successful application of pharmacophore models to accurately predict P-gp binding, which holds promise to anticipate drug-drug interactions from screening drug databases and a priori prediction of novel P-gp inhibitors or substrates.

Quantitative prediction of in vivo inhibitory interactions involving glucuronidated drugs from in vitro data: the effect of fluconazole on zidovudine glucuronidation

AIMS

Using the fluconazole-zidovudine (AZT) interaction as a model, to determine whether inhibition of UDP-glucuronosyltransferase (UGT) catalysed drug metabolism in vivo could be predicted quantitatively from in vitro kinetic data generated in the presence and absence bovine serum albumin (BSA).

METHODS

Kinetic constants for AZT glucuronidation were generated using human liver microsomes (HLM) and recombinant UGT2B7, the principal enzyme responsible for AZT glucuronidation, as the enzyme sources with and without fluconazole. K(i) values were used to estimate the decrease in AZT clearance in vivo.

RESULTS

Addition of BSA (2%) to incubations decreased the K(m) values for AZT glucuronidation by 85-90% for the HLM (923 +/- 357 to 91 +/- 9 microm) and UGT2B7 (478-70 microm) catalysed reactions, with little effect on V(max). Fluconazole, which was shown to be a selective inhibitor of UGT2B7, competitively inhibited AZT glucuronidation by HLM and UGT2B7. Like the K(m), BSA caused an 87% reduction in the K(i) for fluconazole inhibition of AZT glucuronidation by HLM (1133 +/- 403 to 145 +/- 36 microm) and UGT2B7 (529 to 73 microm). K(i) values determined for fluconazole using HLM and UGT2B7 in the presence (but not absence) of BSA predicted an interaction in vivo. The predicted magnitude of the interaction ranged from 41% to 217% of the reported AUC increase in patients, depending on the value of the in vivo fluconazole concentration employed in calculations.

CONCLUSIONS

K(i) values determined under certain experimental conditions may quantitatively predict inhibition of UGT catalysed drug glucuronidation in vivo.

In vitro-in vivo extrapolation of quantitative hepatic biotransformation data for fish. I. A review of methods, and strategies for incorporating intrinsic clearance estimates into chemical kinetic models

Scientists studying mammals have developed a stepwise approach to predict in vivo hepatic clearance from measurements of in vitro hepatic biotransformation. The resulting clearance estimates have been used to screen drug candidates, investigate idiosyncratic drug responses, and support chemical risk assessments. In this report, we review these methods, discuss their potential application to studies with fish, and describe how extrapolated values could be incorporated into well-known compartmental kinetic models. Empirical equations that relate extrapolation factors to chemical log K(ow) are given to facilitate the incorporation of metabolism data into bioconcentration and bioaccumulation models. Because they explicitly incorporate the concept of clearance, compartmental clearance-volume models are particularly well suited for incorporating hepatic clearance estimates. The manner in which these clearance values are incorporated into a given model depends, however, on the measurement frame of reference. Procedures for the incorporation of in vitro biotransformation data into physiologically based toxicokinetic (PBTK) models are also described. Unlike most compartmental models, PBTK models are developed to describe the effects of metabolism in the tissue where it occurs. In addition, PBTK models are well suited to modeling metabolism in more than one tissue.

In vitro–in vivo correlation for drugs and other compounds eliminated by glucuronidation in humans: Pitfalls and promises

Enzymes of the UDP-glucuronosyltransferase (UGT) superfamily are responsible for the metabolism of many drugs, environmental chemicals and endogenous compounds. Identification of the UGT(s) involved in the metabolism of a given compound ('reaction phenotyping') currently relies on multiple confirmatory approaches, which may be confounded by the dependence of UGT activity on enzyme source, incubation conditions, and the occurrence of atypical glucuronidation kinetics. However, the increasing availability of substrate and inhibitor 'probes' for the individual UGTs provides the prospect for reliable phenotyping of glucuronidation reactions using human liver microsomes or hepatocytes, thereby providing data directly relevant to drug metabolism in humans. While the feasibility of computational prediction of UGT substrate selectivity has been demonstrated, the development of easily interpretable and generalisable models requires further improvement in the datasets available for analysis. Quantitative prediction of the hepatic clearance of glucuronidated drugs and the magnitude of inhibitory interactions based on in vitro kinetic data is more problematic. Intrinsic clearance (CL(int)) values generated using human liver microsomes under-predict in vivo hepatic clearance, typically by an order of magnitude. In vivo clearances of glucuronidated drugs are also generally under-predicted by CL(int) values from human hepatocytes, but to a lesser extent than observed with the microsomal model. While it is anticipated that systematic analysis of the potential causes of under-prediction may provide more reliable in vitro-in vivo scaling strategies, mechanistic interpretation of in vitro-in vivo correlation more broadly awaits further advances in our understanding of the structural and cellular determinants of UGT activity.

IN VITRO CHARACTERIZATION OF LAMOTRIGINEN2-GLUCURONIDATION AND THE LAMOTRIGINE-VALPROIC ACID INTERACTION

Studies were performed to investigate the UDP-glucuronosyltransferase enzyme(s) responsible for the human liver microsomal N2-glucuronidation of the anticonvulsant drug lamotrigine (LTG) and the mechanistic basis for the LTG-valproic acid (VPA) interaction in vivo. LTG N2-glucuronidation by microsomes from five livers exhibited atypical kinetics, best described by a model comprising the expressions for the Hill (1869 +/- 1286 microM, n = 0.65 +/- 0.16) and Michaelis-Menten (Km 2234 +/- 774 microM) equations. The UGT1A4 inhibitor hecogenin abolished the Michaelis-Menten component, without affecting the Hill component. LTG N2-glucuronidation by recombinant UGT1A4 exhibited Michaelis-Menten kinetics, with a Km of 1558 microM. Although recombinant UGT2B7 exhibited only low activity toward LTG, inhibition by zidovudine and fluconazole and activation by bovine serum albumin (BSA) (2%) strongly suggested that this enzyme was responsible for the Hill component of microsomal LTG N2-glucuronidation. VPA (10 mM) abolished the Hill component of microsomal LTG N2-glucuronidation, without affecting the Michaelis-Menten component or UGT1A4-catalyzed LTG metabolism. Ki values for inhibition of the Hill component of LTG N2-glucuronidation by VPA were 2465 +/- 370 microM and 387 +/- 12 microM in the absence and presence, respectively, of BSA (2%). Consistent with published data for the effect of fluconazole on zidovudine glucuronidation by human liver microsomal UGT2B7, the Ki value generated in the presence of BSA predicted the magnitude of the LTG-VPA interaction reported in vivo. These data indicate that UGT2B7 and UGT1A4 are responsible for the Hill and Michaelis-Menten components, respectively, of microsomal LTG N2-glucuronidation, and the LTG-VPA interaction in vivo arises from inhibition of UGT2B7.

Endogenous Uremic Substances are not Involved in the Reduced Hepatic Extraction of Metoprolol in Bilateral Ureter-Ligated Rats†

The hepatic extraction of metoprolol is reduced in rats with bilateral ureter ligation (BUL)-induced renal failure. The aim of the present study was to evaluate the effect of uremic substances on the hepatic metabolism of metoprolol in rats with BUL. The metabolic rate in the liver microsomes of BUL rats was similar to that in sham rats, and there was no significant difference between sham and BUL rats in the effect of the supernatant of liver homogenates on the metabolism. The rate of metabolism in the liver microsomes in the presence of the plasma of BUL rats was also similar to that in the presence of the plasma of sham rats. These findings indicated that uremic substances which accumulate in BUL rats do not directly inhibit the activity of CYP2D2, which is responsible for the metabolism of metoprolol in the rat liver.

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.

The relationship between the pharmacology of antiepileptic drugs and human gene variation: an overview

Individual differences in clinical responsiveness to antiepileptic drugs are due to a complex interaction between environmental factors and genetic variation. Considerable interest has arisen in exploiting advances in molecular genetics to improve drug therapy for epilepsy and many other diseases; however, practical application of pharmacogenetics has been difficult to realize. Attempts to define gene variants that are associated with therapeutic (or adverse) effects of antiepileptic drugs rely currently on the prior identification of candidate genes and the subsequent evaluation of the distribution of allelic variants between individuals who have a "good" versus a "poor" clinical response. Many factors can adversely affect interpretation of such data, and careful consideration must be given to the design of genetic association studies involving candidate genes. Candidate genes may be identified in a number of ways; however, for studies of drugs, application of knowledge derived from basic pharmacology can suggest focused and testable hypotheses that are based on the fundamental principles of drug action. Thus, studies of genetic variation as they relate to proteins involved in antiepileptic drug kinetics and dynamics will identify key polymorphisms in endogenous molecules that determine degrees of drug efficacy and toxicity. Delineation of these effects in the coming years will promote enhanced success in the treatment of epilepsy.

CYP2D6 CATALYZES 5-HYDROXYLATION OF 1-(2-PYRIMIDINYL)-PIPERAZINE, AN ACTIVE METABOLITE OF SEVERAL PSYCHOACTIVE DRUGS, IN HUMAN LIVER MICROSOMES

1-(2-Pyrimidinyl)-piperazine (1-PP) is an active metabolite of several psychoactive drugs including buspirone. 1-PP is also the major metabolite in the human circulation and in rat brains following oral administration of buspirone. This study was conducted to identify the enzyme responsible for the metabolic conversion of 1-PP to 5-hydroxy-1-(2-pyrimidinyl)-piperazine (HO-1-PP) in human liver microsomes (HLMs). The product HO-1-PP was quantified by a validated liquid chromatography-tandem mass spectrometry method. In the presence of NADPH, 1-PP (100 microM) was incubated separately with human cDNA-expressed cytochrome P450 isozymes (including CYP2D6, 3A4, 1A2, 2A6, 2C9, 2C19, 2E1, and 2B6) at 37 degrees C. CYP2D6 catalyzed the formation of HO-1-PP from 1-PP. This catalytic activity was >95% inhibited by quinidine, a CYP2D6 inhibitor. HO-1-PP formation rates correlated well with the bufuralol 1-hydroxylase (CYP2D6) activities of individual HLMs. The formation of HO-1-PP followed a Michaelis-Menten kinetics with a K(m) of 171 microM and V(max) of 313 pmol/min x mg protein in HLMs. Collectively, these results indicate that polymorphic CYP2D6 is responsible for the conversion of 1-PP to HO-1-PP.

Impact of serum on clearance predictions obtained from suspensions and primary cultures of rat hepatocytes

The objective of the present study was to compare two configurations of the hepatocyte model namely suspensions (SH) and conventional primary cultures (CPC) for their ability to predict the hepatic clearance in vivo in the rat and, to investigate the impact of serum on the prediction accuracy. The metabolic competences of several cytochrome P450 isoenzymes were investigated both in CPC and SH in the presence or absence of serum. Under the same conditions, the in vitro intrinsic clearance of six test compounds metabolised by a variety of phase I and phase II enzymes (antipyrine, RO-X, mibefradil, midazolam, naloxone and oxazepam) were derived from Vmax/Km scaled up to the corresponding in vivo hepatic metabolic clearance. CYP activities were shown to be stable in both CPC and SH for up to 6 h of incubation, except for the CYP 3A1 activity that decreased in CPC even in the presence of serum. Moreover, the clearances predicted from SH in the presence of serum were closer to the in vivo values than those obtained from CPC. SH represent a convenient model to assess the hepatic metabolism of xenobiotics, the presence of serum in the incubation medium significantly improved in several instances the quality of the predictions.

The comparative interaction of human and bovine serum albumins with CYP2C9 in human liver microsomes

The effect of human serum albumin (HSA), in its endogenous, free fatty acid free (FAF) and globulin free (GF) form, on the activity of CYP2C9 was studied in human liver microsomes using tolbutamide as the substrate. The widely used BSA was included to assess the differential effect of BSA and HSA. CYP2C9 activity was expressed as CLint (Vmax/Km). HSA(FAF) and BSA showed a concentration-dependent and biphasic (activation and inhibition) interaction with CYP2C9 activity. HSA(GF) and HSA exhibited an inhibitory effect, with an inhibition constant, Ki, of 19.9 microM (0.13% albumin) and 42.2 microM (0.35% albumin), respectively. Enzyme-kinetics revealed that the activation is accompanied by a decrease in Km values, while with inhibition Km values increased. A simplified method to calculate clearance, utilizing a single slope (V/S) determination based on V over the lowest linear range of [S] (designated as CLone) was assessed. Virtually identical values were obtained for CLint and CLone. The free-drug hypothesis was tested by comparing ratios of relative CLint/unbound fraction (FDH Test ratio). The FDH Test ratio for HSA was about 1, indicating that HSA binding of tolbutamide reduced the CYP2C9 activity in accord with the free-drug hypothesis. The FDH Test ratios for BSA and HSA(FAF) were 3.7 and 3.0, revealing a monophasic activation of CYP2C9. For 2%HSA(GF) the ratio of 0.3 confirmed inhibition. As revealed by their removal, free fatty acids and globulins, significantly alter the interaction of HSA with CYP2C9. In addition, HSA and BSA showed different effects on the oxidation of tolbutamide by CYP2C9.

VALIDATED ASSAYS FOR HUMAN CYTOCHROME P450 ACTIVITIES

The measurement of the effect of new chemical entities on human cytochrome P450 marker activities using in vitro experimentation represents an important experimental approach in drug development. In vitro drug interaction data can be used in guiding the design of clinical drug interaction studies, or, when no effect is observed in vitro, the data can be used in place of an in vivo study to claim that no interaction will occur in vivo. To make such a claim, it must be assured that the in vitro experiments are performed with absolute confidence in the methods used and data obtained. To meet this need, 12 semiautomated assays for human P450 marker substrate activities have been developed and validated using approaches described in the GLP (good laboratory practices) as per the code of U.S. Federal Regulations. The assays that were validated are: phenacetin O-deethylase (CYP1A2), coumarin 7-hydroxylase (CYP2A6), bupropion hydroxylase (CYP2B6), amodiaquine N-deethylase (CYP2C8), diclofenac 4'-hydroxylase and tolbutamide methylhydroxylase (CYP2C9), (S)-mephenytoin 4'-hydroxylase (CYP2C19), dextromethorphan O-demethylase (CYP2D6), chlorzoxazone 6-hydroxylase (CYP2E1), felodipine dehydrogenase, testosterone 6 beta-hydroxylase, and midazolam 1'-hydroxylase (CYP3A4 and CYP3A5). High-pressure liquid chromatography-tandem mass spectrometry, using stable isotope-labeled internal standards, has been applied as the analytical method. This analytical approach, through its high sensitivity and selectivity, has permitted the use of very low incubation concentrations of microsomal protein (0.01-0.2 mg/ml). Analytical assay accuracy and precision values were excellent. Enzyme kinetic and inhibition parameters obtained using these methods demonstrated high precision and were within the range of values previously reported in the scientific literature. These methods should prove useful in the routine assessments of the potential for new drug candidates to elicit pharmacokinetic drug interactions via inhibition of cytochrome P450 activities.

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.

Kinetic Impact of Presystemic Intestinal Metabolism on Drug Absorption: Experiment and Data Analysis for the Prediction of In Vivo Absorption from In Vitro Data

Orally administered drugs suffer from attack by metabolic enzymes not only in the liver, but also in the gastrointestine during the absorption process across the intestinal tissue. Although kinetic study on hepatic metabolism has been done well, the intestinal metabolism has not been well focused on compared with hepatic metabolism. In order to emphasize the role of intestinal metabolism in drug absorption and bioavailability, I have reviewed the experimental methods for intestinal absorption and metabolism, and the data analysis. Since Klippert et al. reported the prediction of intestinal first-pass effect of phenacetin in the rat from enzyme kinetic data in 1982, several reports have showed a good prediction, but others have not. Although intestinal absorption is an integrated process of transport (transporters) and metabolism (metabolic enzymes), most of the researchers missed the pathway of intestinal drug absorption and applied the kinetic model effective on only systemic metabolism to presystemic intestinal metabolism for their analysis of intestinal metabolism of orally administered drugs. A kinetic model, which incorporated factors of membrane transport, metabolic activity and protein binding, was structured to compare the equations in the reported models. In conclusion, we need more studies including kinetic modeling and experiments to understand the impact of intestinal metabolism on drug absorption. That knowledge must lead to the construction of ADME in silico (e-ADME).