Pleiotropy of Progesterone Receptor Membrane Component 1 in Modulation of Cytochrome P450 Activity

Progesterone receptor membrane component 1 (PGRMC1) is one of few proteins that have been recently described as direct modulators of the activity of human cytochrome P450 enzymes (CYP)s. These enzymes form a superfamily of membrane-bound hemoproteins that metabolize a wide variety of physiological, dietary, environmental, and pharmacological compounds. Modulation of CYP activity impacts the detoxification of xenobiotics as well as endogenous pathways such as steroid and fatty acid metabolism, thus playing a central role in homeostasis. This review is focused on nine main topics that include the most relevant aspects of past and current PGRMC1 research, focusing on its role in CYP-mediated drug metabolism. Firstly, a general overview of the main aspects of xenobiotic metabolism is presented (I), followed by an overview of the role of the CYP enzymatic complex (IIa), a section on human disorders associated with defects in CYP enzyme complex activity (IIb), and a brief account of cytochrome b5 (cyt b5)'s effect on CYP activity (IIc). Subsequently, we present a background overview of the history of the molecular characterization of PGRMC1 (III), regarding its structure, expression, and intracellular location (IIIa), and its heme-binding capability and dimerization (IIIb). The next section reflects the different effects PGRMC1 may have on CYP activity (IV), presenting a description of studies on the direct effects on CYP activity (IVa), and a summary of pathways in which PGRMC1's involvement may indirectly affect CYP activity (IVb). The last section of the review is focused on the current challenges of research on the effect of PGRMC1 on CYP activity (V), presenting some future perspectives of research in the field (VI).

Effects of Crocodile Oil (Crocodylus siamensis) on Liver Enzymes: Cytochrome P450 and Glutathione S-Transferase Activities in High-fat DietFed Rats

Crocodile oil is a highly effective treatment for ailments ranging from skin conditions to cancer. However, the effects of the oil on liver detoxification pathways are not well studied. This study aimed to investigate the effects of crocodile oil on the detoxification enzyme activities and the mRNA expressions of cytochrome P450 1A2 (CYP1A2), cytochrome P450 2E1 (CYP2E1), and glutathione S-transferase (GST) in rats. The rats were divided into four groups (n = 7/group): rats received a standard diet (C), a high-fat diet or HFD (H), and HFD with 1 ml (HCO1) and 3 ml (HCO3) of the oil per kg body weight. Interestingly, the oil yields from this study presented alpha-linolenic acid (0.96%) at similar levels compared with fish oil. The results revealed that HFD significantly increased the activity and relative gene expression of CYP1A2 in the H group (P < 0.05), whereas 3% crocodile oil normalized the enzyme activities compared to the C group. This suggested inhibiting the HFD-induced expression of CYP1A2 mediated by the omega-3 fatty acids found in the oil. Also, crocodile oil supplementation did not reduce the activities of GST. However, the relative gene expression of GSTA1 was significantly decreased (P < 0.05) in the HCO1 and HCO3 groups compared to the H group, which might be attributed to the lower lipid peroxidation that occurred in the liver tissues. Therefore, it could be suggested that using crocodile oil could help in liver detoxification through the CYP1A2 even when offered with a HFD.

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.

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.

Enzyme Kinetics of Uridine Diphosphate Glucuronosyltransferases (UGTs)

Glucuronidation, catalyzed by uridine diphosphate glucuronosyltransferases (UGTs), is an important process for the metabolism and clearance of many lipophilic chemicals, including drugs, environmental chemicals, and endogenous compounds. Glucuronidation is a bisubstrate reaction that requires the aglycone and the cofactor, UDP-GlcUA. Accumulating evidence suggests that the bisubstrate reaction follows a compulsory-order ternary mechanism. To simplify the kinetic modeling of glucuronidation reactions in vitro, UDP-GlcUA is usually added to incubations in large excess. Many factors have been shown to influence UGT activity and kinetics in vitro, and these must be accounted for during experimental design and data interpretation. While the assessment of drug-drug interactions resulting from UGT inhibition has been challenging in the past, the increasing availability of UGT enzyme-selective substrate and inhibitor "probes" provides the prospect for more reliable reaction phenotyping and assessment of drug-drug interaction potential. Although extrapolation of the in vitro intrinsic clearance of a glucuronidated drug often underpredicts in vivo clearance, careful selection of in vitro experimental conditions and inclusion of extrahepatic glucuronidation may improve the predictivity of in vitro-in vivo extrapolation. Physiologically based pharmacokinetic (PBPK) modeling has also shown to be of value for predicting PK of drugs eliminated by glucuronidation.

Evidence-based strategies for the characterisation of human drug and chemical glucuronidation in vitro and UDP-glucuronosyltransferase reaction phenotyping

Enzymes of the UDP-glucuronosyltransferase (UGT) superfamily contribute to the elimination of drugs from almost all therapeutic classes. Awareness of the importance of glucuronidation as a drug clearance mechanism along with increased knowledge of the enzymology of drug and chemical metabolism has stimulated interest in the development and application of approaches for the characterisation of human drug glucuronidation in vitro, in particular reaction phenotyping (the fractional contribution of the individual UGT enzymes responsible for the glucuronidation of a given drug), assessment of metabolic stability, and UGT enzyme inhibition by drugs and other xenobiotics. In turn, this has permitted the implementation of in vitro - in vivo extrapolation approaches for the prediction of drug metabolic clearance, intestinal availability, and drug-drug interaction liability, all of which are of considerable importance in pre-clinical drug development. Indeed, regulatory agencies (FDA and EMA) require UGT reaction phenotyping for new chemical entities if glucuronidation accounts for ≥25% of total metabolism. In vitro studies are most commonly performed with recombinant UGT enzymes and human liver microsomes (HLM) as the enzyme sources. Despite the widespread use of in vitro approaches for the characterisation of drug and chemical glucuronidation by HLM and recombinant enzymes, evidence-based guidelines relating to experimental approaches are lacking. Here we present evidence-based strategies for the characterisation of drug and chemical glucuronidation in vitro, and for UGT reaction phenotyping. We anticipate that the strategies will inform practice, encourage development of standardised experimental procedures where feasible, and guide ongoing research in the field.

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.

Suitable albumin concentrations for enhanced drug oxidation activities mediated by human liver microsomal cytochrome P450 2C9 and other forms predicted with unbound fractions and partition/distribution coefficients of model substrates

Albumin has reportedly enhanced cytochrome P450 (P450)-mediated drug oxidation rates in human liver microsomes. Consequently, measurements of clearances and fractions metabolized could vary depending on the experimental albumin concentrations used. In this study, the oxidation rates of diclofenac and warfarin by human liver microsomes were significantly enhanced in the presence of 0.10% (w/v) bovine serum albumin, whereas those of tolbutamide and phenytoin required 1.0% and 2.0% of albumin for significant enhancement. Values of the fractions metabolized by P450 2C9 for four substrates did not markedly change in the presence of albumin at the above-mentioned concentrations. The oxidation rates of bupropion, omeprazole, chlorzoxazone and phenacetin in human liver microsomes were reportedly enhanced by 0.5%, 1%, 2% and 2% of albumin, respectively. Analysis of reported intrinsic clearance values and suitable albumin concentrations for the currently analyzed substrates and the reported substrates revealed an inverse correlation, with warfarin as an outlier. Suitable albumin concentrations were multivariately correlated with physicochemical properties, that is, the plasma unbound fractions, octanol-water partition coefficient and acid dissociation constant (r = 0.98, p<.0001, n = 10). Therefore, multiple physicochemical properties may be determinants of suitable albumin concentrations for substrate oxidations in human liver microsomes.

Enantioselective capillary electrophoresis provides insight into the phase II metabolism of ketamine and its metabolites in vivo and in vitro

Glucuronidation catalyzed by uridine-5'-diphospho-glucuronosyl-transferases (UGTs) is the most important reaction in phase II metabolism of drugs and other compounds. O-glucuronidation is more common than N-glucuronidation. The anesthetic, analgesic and antidepressive drug ketamine is metabolized in phase I by cytochrome P450 enzymes to norketamine, hydroxynorketamine (HNK) diastereomers and dehydronorketamine (DHNK). Equine urine samples collected two hours after ketamine injection were treated with β-glucuronidase and analyzed with three enantioselective capillary electrophoresis assays. Concentrations of HNK diastereomers and norketamine were significantly higher in comparison to untreated urine and an increase of ketamine and DHNK levels was found in selected but not all samples. This suggests that O-glucuronides of HNK and N-glucuronides of the other compounds are formed in equines. N-glucuronidation of norketamine was studied in vitro with liver microsomes of different species and the single human enzyme UGT1A4. With equine liver microsomes (ELM) a stereoselective N-glucuronidation of norketamine was found that compares well to the results obtained with urines collected after ketamine administration. No reaction was observed with canine liver microsomes, human liver microsomes and UGT1A4. Incubation of ketamine and DHNK with ELM did not reveal any glucuronidation. Enantioselective CE is suitable to provide insight into the phase II metabolism of ketamine and its metabolites.

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.

Inhibition of Organic Anion Transporting Polypeptide 1B1 and 1B3 by Betulinic Acid: Effects of Preincubation and Albumin in the Media

Betulinic acid (BA), a plant-derived pentacyclic triterpenoid, may interact with the members of the organic anion transporting polypeptide 1B subfamily. Here, we investigated the interactions of BA and its analogs with OATP1B1/3 and rat Oatp1b2 in vitro and in vivo. BA inhibited the activity of OATP1B1/3 and rat Oatp1b2 in vitro. Systemic exposure of atorvastatin was substantially altered with the intravenous co-administration of BA (20 mg/kg). Preincubation (incubation with inhibitors, followed by washout) with BA led to a sustained inhibition of OATP1B3, which recovered rapidly in the media containing 10% fetal bovine serum. The addition of albumin to the media decreased intracellular concentrations of BA and expedited the recovery of OATP1B3 activity following preincubation. For asunaprevir and cyclosporin A (previously known to inhibit OATP1B3 upon preincubation), the addition of albumin to the media shortened recovery time with asunaprevir, but not with cyclosporin A. Overall, our results showed that BA inhibits OATP1B transporters in vitro and may incur hepatic transporter-mediated drug interactions in vivo. Our results identify BA as another OATP1B3 inhibitor with preincubation effect and suggest that the preincubation effect and its duration is impacted by altered equilibrium of inhibitors between intracellular and extracellular space (e.g., albumin in the media).

Impaired dacarbazine activation and 7-ethoxyresorufin deethylation in vitro by polymorphic variants of CYP1A1 and CYP1A2: implications for cancer therapy

OBJECTIVES

To extend our understanding of how interindividual variability mediates the efficacy of cancer treatment.

MATERIALS AND METHODS

The kinetics of dacarbazine (DTIC) N-demethylation by the most frequent polymorphic variants of CYP1A1 (T461N, I462V) and CYP1A2 (F186L, D348N, I386F, R431W, R456H) were characterized, along with kinetic parameters for the O-deethylation of the prototypic CYP1A substrate 7-ethoxyresorufin, using recombinant protein expression and high-performance liquid chromatographic techniques.

RESULTS

A reduction of ∼30% in the catalytic efficiencies (measured as in-vitro intrinsic clearance, CLint) was observed for DTIC N-demethylation by the two CYP1A1 variants relative to wild type. Although a modest increase in the CLint value for DTIC N-demethylation was observed for the CYP1A2 D348N variant relative to the wild type, the CLint for the F186L variant was reduced and the I386F, R431W, and R456H variants all showed loss of catalytic function.

CONCLUSION

Comparison of the kinetic data for DTIC N-demethylation and 7-ethoxyresorufin O-deethylation indicated that alterations in the kinetic parameters (Km, Vmax, CLint) observed with each of the CYP1A1 and CYP1A2 polymorphic variants were substrate dependent. These data indicate that cancer patients treated with DTIC who possess any of the CYP1A1-T461N and I462V variants or the CYP1A2-F186L, D348N, I386F, R431W, and R456H variants are likely to have decreased prodrug activation, and hence may respond less favorably to DTIC treatment compared with individuals with wild-type CYP1A alleles.

Human carbonyl reductase 1 participating in intestinal first-pass drug metabolism is inhibited by fatty acids and acyl-CoAs

Human carbonyl reductase 1 (CBR1), a member of the short-chain dehydrogenase/reductase (SDR) superfamily, reduces a variety of carbonyl compounds including endogenous isatin, prostaglandin E₂ and 4-oxo-2-nonenal. It is also a major non-cytochrome P450 enzyme in the phase I metabolism of carbonyl-containing drugs, and is highly expressed in the intestine. In this study, we found that long-chain fatty acids and their CoA ester derivatives inhibit CBR1. Among saturated fatty acids, myristic, palmitic and stearic acids were inhibitory, and stearic acid was the most potent (IC₅₀ 9µM). Unsaturated fatty acids (oleic, elaidic, γ-linolenic and docosahexaenoic acids) and acyl-CoAs (palmitoyl-, stearoyl- and oleoyl-CoAs) were more potent inhibitors (IC₅₀ 1.0-2.5µM), and showed high inhibitory selectivity to CBR1 over its isozyme CBR3 and other SDR superfamily enzymes (DCXR and DHRS4) with CBR activity. The inhibition by these fatty acids and acyl-CoAs was competitive with respect to the substrate, showing the K_i values of 0.49-1.2µM. Site-directed mutagenesis of the substrate-binding residues of CBR1 suggested that the interactions between the fatty acyl chain and the enzyme's Met141 and Trp229 are important for the inhibitory selectivity. We also examined CBR1 inhibition by oleic acid in cellular levels: The fatty acid effectively inhibited CBR1-mediated 4-oxo-2-nonenal metabolism in colon cancer DLD1 cells and increased sensitivity to doxorubicin in the drug-resistant gastric cancer MKN45 cells that highly express CBR1. The results suggest a possible new food-drug interaction through inhibition of CBR1-mediated intestinal first-pass drug metabolism by dietary fatty acids.

Inhibition of cytochrome P450 enzymes by saturated and unsaturated fatty acids in human liver microsomes, characterization of enzyme kinetics in the presence of bovine serum albumin (0.1 and 1.0% w/v) and in vitro - in vivo extrapolation of hepatic clearance

The objective of the study was to determine the effect of fatty acids on CYP enzymes and the effect of BSA on intrinsic clearance of probe substrates. The inhibitory effect of thirteen fatty acids including saturated, mono-unsaturated and polyunsaturated fatty acids on CYP enzymes, kinetic parameters and intrinsic clearance values of nine CYP marker probe substrate reactions in the absence and presence of BSA (0.1 and 1.0% w/v) were characterized in human liver microsomes. The results demonstrate that most of the unsaturated fatty acids showed marked inhibition towards CYP2C8 mediated amodiaquine N-deethylation followed by inhibition of CYP2C9 and CYP2B6 mediated activities. The addition of 0.1% BSA in the incubation markedly improved the unbound intrinsic clearance values of probe substrates by reducing the K_m values with little or no effect on maximal velocity. The addition of BSA (0.1 and 1.0% w/v) did not influence the unbound intrinsic clearance of marker reactions for CYP2A6, and CYP3A4 enzymes. The addition of 0.1% w/v BSA is sufficient to determine the intrinsic clearance of marker probe reactions by metabolite formation approach. The predicted hepatic clearance values for the substrates using the well-stirred model, in the presence of BSA (0.1% BSA), are comparable to the in vivo hepatic clearance values.

Optimized Cocktail Phenotyping Study Protocol Using Physiological Based Pharmacokinetic Modeling and In silico Assessment of Metabolic Drug-Drug Interactions Involving Modafinil

In vivo cocktail pathway phenotyping (ICPP) is routinely used to assess the metabolic drug-drug interaction (mDDI) potential of new drug candidates (NDC) during drug development. However, there are a number of potential limitations to this approach and the use of validated drug cocktails and study protocols is essential. Typically ICPP mDDI studies assess only the impact of interactions following multiple postulated perpetrator doses and hence the emphasis in terms of validation of these studies has been ensuring that there are no interactions between probe substrates. Studies assessing the comparative impact of single and multiple doses of the postulated perpetrator have the potential to provide richer information regarding both the clinical impact and mechanism of mDDIs. Using modafinil as a model compound, we sought to develop an optimized ICPP mDDI study protocol to evaluate the potential magnitude and clinical relevance of mDDIs using a physiologically based pharmacokinetic modeling approach.

The Effect of Albumin on MRP2 and BCRP in the Vesicular Transport Assay

The ABC transporters multidrug resistance associated protein 2 (MRP2) and breast cancer resistance protein (BCRP) are of interest in drug development, since they affect the pharmacokinetics of several drugs. Membrane vesicle transport assays are widely used to study interactions with these proteins. Since albumin has been found to affect the kinetics of metabolic enzymes in similar membrane preparations, we investigated whether albumin affects the kinetic parameters of efflux transport. We found that albumin increased the Vmax of 5(6)-carboxy-2',7'-dichlorofluorescein (CDCF) and estradiol-17-β-D-glucuronide uptake into MRP2 vesicles in the presence of 0.1% bovine serum albumin (BSA) by 2 and 1.5-fold, respectively, while BSA increased Lucifer yellow uptake by 30% in BCRP vesicles. Km values increased slightly, but the change was not statistically significant. The effect of BSA on substrate uptake was dependent on the vesicle amount, while increasing BSA concentration did not significantly improve substrate uptake. These results indicate a minor effect of albumin on MRP2 and BCRP, but it should be considered if albumin is added to transporter assays for example as a solubilizer, since the effect may be substrate or transporter specific.

How to Choose In Vitro Systems to Predict In Vivo Drug Clearance: A System Pharmacology Perspective

The use of in vitro metabolism data to predict human clearance has become more significant in the current prediction of large scale drug clearance for all the drugs. The relevant information (in vitro metabolism data and in vivo human clearance values) of thirty-five drugs that satisfied the entry criteria of probe drugs was collated from the literature. Then the performance of different in vitro systems including Escherichia coli system, yeast system, lymphoblastoid system and baculovirus system is compared after in vitro-in vivo extrapolation. Baculovirus system, which can provide most of the data, has almost equal accuracy as the other systems in predicting clearance. And in most cases, baculovirus system has the smaller CV in scaling factors. Therefore, the baculovirus system can be recognized as the suitable system for the large scale drug clearance prediction.

Enzyme kinetics of uridine diphosphate glucuronosyltransferases (UGTs)

Glucuronidation, catalyzed by uridine diphosphate glucuronosyltransferases (UGTs), is an important process for the metabolism and clearance of many lipophilic chemicals, including drugs, environmental chemicals, and endogenous compounds. Glucuronidation is a bi-substrate reaction that requires the aglycone and a cofactor, UDPGA. Accumulating evidence suggests that the bi-substrate reaction follows a compulsory-order ternary mechanism. To simplify the kinetic modelling of glucuronidation reactions in vitro, UDPGA is usually added to incubations in large excess. Many factors have been shown to influence UGT activity and kinetics in vitro, and these must be accounted for in experimental design and data interpretation. Assessing drug-drug interactions (DDIs) involving UGT inhibition remains challenging. However, the increasing availability of UGT enzyme-specific substrate and inhibitor "probes" provides the prospect for more reliable reaction phenotyping and assessment of DDI potential. Although extrapolation of the in vitro intrinsic clearance of a glucuronidated drug often under-predicts in vivo clearance, careful selection of in vitro experimental conditions and inclusion of extrahepatic glucuronidation may improve the predictivity of in vitro-in vivo extrapolation (IVIVE).

Role of protein-protein interactions in cytochrome P450-mediated drug metabolism and toxicity

Through their unique oxidative chemistry, cytochrome P450 monooxygenases (CYPs) catalyze the elimination of most drugs and toxins from the human body. Protein–protein interactions play a critical role in this process. Historically, the study of CYP–protein interactions has focused on their electron transfer partners and allosteric mediators, cytochrome P450 reductase and cytochrome b5. However, CYPs can bind other proteins that also affect CYP function. Some examples include the progesterone receptor membrane component 1, damage resistance protein 1, human and bovine serum albumin, and intestinal fatty acid binding protein, in addition to other CYP isoforms. Furthermore, disruption of these interactions can lead to altered paths of metabolism and the production of toxic metabolites. In this review, we summarize the available evidence for CYP protein–protein interactions from the literature and offer a discussion of the potential impact of future studies aimed at characterizing noncanonical protein–protein interactions with CYP enzymes.

Prediction of hepatic plasma clearance in vivo from gel-entrapped rat and human hepatocytes

This paper aimed to evaluate the applicability of gel-entrapped rat and human hepatocytes in the prediction of hepatic plasma clearance (CLh,plasma) in vivo. The in vitro intrinsic clearances (CLint,in vitro) for the selected compounds were determined from the substrate disappearance rate, and further used to predict CLh,plasma using 3 classical mathematical models (well-stirred, parallel-tube, and dispersion) and disregarding drug binding. As a result, the predicted values from gel-entrapped rat hepatocytes were mostly within 2 SE of the literature data with a high correlation coefficient (R2) of 0.88–0.91. The predicted data with human hepatocytes also fitted well with the clinical data, indicating a high accuracy in prediction of in-vivo clearance. With respect to the mathematical model for predicting CLh,plasma, the parallel-tube and dispersion models produced a better prediction than the well-stirred model, and we suggest using the parallel-tube model because it is less complex mathematically. In conclusion, gel-entrapped hepatocytes predicted the drug clearance well and seemed to be a useful tool in the process of drug discovery.

Albumin stimulates the activity of the human UDP-glucuronosyltransferases 1A7, 1A8, 1A10, 2A1 and 2B15, but the effects are enzyme and substrate dependent

Human UDP-glucuronosyltransferases (UGTs) are important enzymes in metabolic elimination of endo- and xenobiotics. It was recently shown that addition of fatty acid free bovine serum albumin (BSA) significantly enhances in vitro activities of UGTs, a limiting factor in in vitro–in vivo extrapolation. Nevertheless, since only few human UGT enzymes were tested for this phenomenon, we have now performed detailed enzyme kinetic analysis on the BSA effects in six previously untested UGTs, using 2–4 suitable substrates for each enzyme. We also examined some of the previously tested UGTs, but using additional substrates and a lower BSA concentration, only 0.1%. The latter concentration allows the use of important but more lipophilic substrates, such as estradiol and 17-epiestradiol. In five newly tested UGTs, 1A7, 1A8, 1A10, 2A1, and 2B15, the addition of BSA enhanced, to a different degree, the in vitro activity by either decreasing reaction’s K_m, increasing its V_max, or both. In contrast, the activities of UGT2B17, another previously untested enzyme, were almost unaffected. The results of the assays with the previously tested UGTs, 1A1, 1A6, 2B4, and 2B7, were similar to the published BSA only as far as the BSA effects on the reactions’ K_m are concerned. In the cases of V_max values, however, our results differ significantly from the previously published ones, at least with some of the substrates. Hence, the magnitude of the BSA effects appears to be substrate dependent, especially with respect to V_max increases. Additionally, the BSA effects may be UGT subfamily dependent since K_m decreases were observed in members of subfamilies 1A, 2A and 2B, whereas large V_max increases were only found in several UGT1A members. The results shed new light on the complexity of the BSA effects on the activity and enzyme kinetics of the human UGTs.