Comparison of CYP2D6 content and metoprolol oxidation between microsomes isolated from human livers and small intestines

PBPK Modeling Approach to Predict the Behavior of Drugs Cleared by Metabolism in Pregnant Subjects and Fetuses

This study aimed to develop a physiologically based pharmacokinetic (PBPK) model that simulates metabolically cleared compounds' pharmacokinetics (PK) in pregnant subjects and fetuses. This model accounts for the differences in tissue sizes, blood flow rates, enzyme expression levels, plasma protein binding, and other physiological factors affecting the drugs' PK in both the pregnant woman and the fetus. The PBPKPlus™ module in GastroPlus® was used to model the PK of metoprolol, midazolam, and metronidazole for both non-pregnant and pregnant groups. For each of the three compounds, the model was first developed and validated against PK data in healthy non-pregnant volunteers and then applied to predict the PK in the pregnant groups. The model accurately described the PK in both the non-pregnant and pregnant groups and explained well the differences in the plasma concentration due to pregnancy. When available, the fetal plasma concentration, placenta, and fetal tissue concentrations were also predicted reasonably well at different stages of pregnancy. The work described the use of a PBPK approach for drug development and demonstrates the ability to predict differences in PK in pregnant subjects and fetal exposure for metabolically cleared compounds.

Physiologically-based Pharmacokinetic Modeling to Assess the Impact of CYP2D6-Mediated Drug-Drug Interactions on Tramadol and O-Desmethyltramadol Exposures via Allosteric and Competitive Inhibition

Tramadol is an opioid medication used to treat moderately severe pain. Cytochrome P450 (CYP) 2D6 inhibition could be important for tramadol, as it decreases the formation of its pharmacologically active metabolite, O‐desmethyltramadol, potentially resulting in increased opioid use and misuse. The objective of this study was to evaluate the impact of allosteric and competitive CYP2D6 inhibition on tramadol and O‐desmethyltramadol pharmacokinetics using quinidine and metoprolol as prototypical perpetrator drugs. A physiologically based pharmacokinetic model for tramadol and O‐desmethyltramadol was developed and verified in PK‐Sim version 8 and linked to respective models of quinidine and metoprolol to evaluate the impact of allosteric and competitive CYP2D6 inhibition on tramadol and O‐desmethyltramadol exposure. Our results show that there is a differentiated impact of CYP2D6 inhibitors on tramadol and O‐desmethyltramadol based on their mechanisms of inhibition. Following allosteric inhibition by a single dose of quinidine, the exposure of both tramadol (51% increase) and O‐desmethyltramadol (52% decrease) was predicted to be significantly altered after concomitant administration of a single dose of tramadol. Following multiple‐dose administration of tramadol and a single‐dose or multiple‐dose administration of quinidine, the inhibitory effect of quinidine was predicted to be long (≈42 hours) and to alter exposure of tramadol and O‐desmethyltramadol by up to 60%, suggesting that coadministration of quinidine and tramadol should be avoided clinically. In comparison, there is no predicted significant impact of metoprolol on tramadol and O‐desmethyltramadol exposure. In fact, tramadol is predicted to act as a CYP2D6 perpetrator and increase metoprolol exposure, which may necessitate the need for dose separation.

A Physiologically Based Pharmacokinetic Model for Pregnant Women to Predict the Pharmacokinetics of Drugs Metabolized Via Several Enzymatic Pathways

BACKGROUND

Physiologically based pharmacokinetic modeling is considered a valuable tool for predicting pharmacokinetic changes in pregnancy to subsequently guide in-vivo pharmacokinetic trials in pregnant women. The objective of this study was to extend and verify a previously developed physiologically based pharmacokinetic model for pregnant women for the prediction of pharmacokinetics of drugs metabolized via several cytochrome P450 enzymes.

METHODS

Quantitative information on gestation-specific changes in enzyme activity available in the literature was incorporated in a pregnancy physiologically based pharmacokinetic model and the pharmacokinetics of eight drugs metabolized via one or multiple cytochrome P450 enzymes was predicted. The tested drugs were caffeine, midazolam, nifedipine, metoprolol, ondansetron, granisetron, diazepam, and metronidazole. Pharmacokinetic predictions were evaluated by comparison with in-vivo pharmacokinetic data obtained from the literature.

RESULTS

The pregnancy physiologically based pharmacokinetic model successfully predicted the pharmacokinetics of all tested drugs. The observed pregnancy-induced pharmacokinetic changes were qualitatively and quantitatively reasonably well predicted for all drugs. Ninety-seven percent of the mean plasma concentrations predicted in pregnant women fell within a twofold error range and 63% within a 1.25-fold error range. For all drugs, the predicted area under the concentration-time curve was within a 1.25-fold error range.

CONCLUSION

The presented pregnancy physiologically based pharmacokinetic model can quantitatively predict the pharmacokinetics of drugs that are metabolized via one or multiple cytochrome P450 enzymes by integrating prior knowledge of the pregnancy-related effect on these enzymes. This pregnancy physiologically based pharmacokinetic model may thus be used to identify potential exposure changes in pregnant women a priori and to eventually support informed decision making when clinical trials are designed in this special population.

A Novel PBPK Modeling Approach to Assess Cytochrome P450 Mediated Drug-Drug Interaction Potential of the Cytotoxic Prodrug Evofosfamide

Evofosfamide is a cytotoxic small‐molecule prodrug preferentially activated under hypoxic conditions. The cytotoxicity of evofosfamide impacted the generation of in vitro drug‐drug interaction (DDI) data, especially in vitro induction results. Therefore, a novel physiologically based pharmacokinetic (PBPK) approach was used, which involved available in vitro and clinical data of evofosfamide and combined it with induction data from the prototypical cytochrome P450 (CYP)3A inducer rifampicin. The area under the concentration‐time curve (AUC) ratios of midazolam were above 0.80, indicating that induction of CYP3A by evofosfamide administered weekly is unlikely to occur in humans. Moreover, static and PBPK modeling showed no clinically relevant inhibition via CYP2B6, CYP2D6, and CYP3A4. In conclusion, PBPK models were used to supplement in vitro information of a cytotoxic compound. This approach may set a precedent for future studies of cytotoxic drugs, potentially reducing the need for clinical DDI studies and providing more confidence in the clinical use of approved cytotoxic compounds for which DDI information is sparse.

A UHPLC-MS/MS method coupled with liquid-liquid extraction for the quantitation of phenacetin, omeprazole, metoprolol, midazolam and their metabolites in rat plasma and its application to the study of four CYP450 activities

Drug-drug interactions (DDIs) are thought to be associated with the inhibition of cytochrome P450 activities. The cocktail method with analysis of the metabolism of two or more probe drugs is used to determine CYP450 activities. In this study, we established a UHPLC-MS/MS method for simultaneous quantitation of four CYP450 probe drugs (phenacetin, omeprazole, metoprolol and midazolam) and their metabolites (acetaminophen, 5'-hydroxy omeprazole, α-hydroxy metoprolol and 1'-hydroxy midazolam) in rat plasma. Sample preparation by plasma protein precipitation was combined with a liquid-liquid extraction method. The separation was carried out on a ZORBAX Eclipse Plus C18 Rapid Resolution High Definition column with a gradient elution, using water containing 0.1% formic acid (A) and acetonitrile (B) in a run time of only 3.0 min. Detection was conducted with a 6420 series triple-quadrupole tandem mass spectrometer, using ESI in positive ion mode with multiple reaction monitoring (MRM). The calibration curves were linear over the concentration range 10-5000 ng/mL for phenacetin, omeprazole, metoprolol and midazolam, and 1-500 ng/mL for their metabolites. Intra- and inter-day precisions were within 15%, and the accuracies were in the range of 87-112%. The method was successfully applied to the pharmacokinetic study of probe drugs/metabolites and DDIs with 3-n-butylphthalide (NBP) after administration of a single oral dose of phenacetin, omeprazole, metoprolol and midazolam in rats.

Detoxification of benzo[a]pyrene primarily depends on cytochrome P450, while bioactivation involves additional oxidoreductases including 5-lipoxygenase, cyclooxygenase, and aldo-keto reductase in the liver

Cytochrome P450s are involved in detoxification and activation of benzo[a]pyrene (BaP) with unclear balance and unknown contribution of other oxidoreductases. Here, we investigated the BaP and BaP-induced mutagenicity in hepatic and extra-hepatic tissues using hepatic P450 reductase null (HRN) gpt mice. After 2-week treatment (50 mg/kg, i.p. 4 days), BaP in the liver and lung of HRN-gpt mice were increased. BaP promoted gpt mutant frequency (MF) in HRN-gpt mice liver. MF of gpt in the lung and Pig-a in hematopoietic cells induced by BaP in HRN-gpt mice were increased than in gpt mice. BaP-7,8-diol-9,10-epoxide (BPDE)-DNA adducts in vitro was analyzed for enzymes detection in BaP bioactivation. Specific inhibitors of 5-lipoxygenase, cyclooxygenase-1&2, and aldo-keto reductase resulted in more than 80% inhibition rate in the DNA adduct formation, further confirmed by Macaca fascicularis hepatic S9 system. Our results suggested the detoxification of BaP primarily depends on cytochrome P450, while the bioactivation involves additional oxidoreductases.

Unequivocal evidence supporting the segregated flow intestinal model that discriminates intestine versus liver first-pass removal with PBPK modeling

Merits of the segregated flow model (SFM), highlighting the intestine as inert serosa and active enterocyte regions, with a smaller fractional (f_Q < 0.3) intestinal flow (Q_I ) perfusing the enterocyte region, are described. Less drug in the circulation reaches the enterocytes due to the lower flow (f_Q Q_I ) in comparison with drug administered into the gut lumen, fostering the idea of route-dependent intestinal removal. The SFM has been found superior to the traditional model (TM), which views the serosa and enterocytes totally as a well-mixed tissue perfused by 100% of the intestinal flow, Q_I . The SFM model is able to explain the lower extents of intestinal metabolism of enalapril, morphine and midazolam with i.v. vs. p.o. dosing. For morphine, the urine/bile ratio of the metabolite, morphine glucuronide MGurineMGbile for p.o. was 2.6× that of i.v. This was due to the higher proportion of intestinally formed morphine glucuronide, appearing more in urine than in bile due to its low permeability and greater extent of intestinal formation with p.o. administration. By contrast, the TM predicted the same MGurineMGbile for p.o. vs. i.v. The TM predicted that the contributions of the intestine:liver to first-pass removal were 46%:54% for both p.o. and i.v. The SFM predicted same 46%:54% (intestine:liver) for p.o., but 9%:91% for i.v. By contrast, the kinetics of codeine, the precursor of morphine, was described equally well by the SFM- and TM-PBPK models, a trend suggesting that intestinal metabolism of codeine is negligible. Fits to these PBPK models further provide insightful information towards metabolite formation: available fractions and the fractions of hepatic and total clearances that form the metabolite in question. The SFM-PBPK model is useful to identify not only the presence of intestinal metabolism but the contributions of the intestine and liver for metabolite formation. Copyright © 2016 John Wiley & Sons, Ltd.

Marmoset cytochrome P450 2D8 in livers and small intestines metabolizes typical human P450 2D6 substrates, metoprolol, bufuralol and dextromethorphan

1. Although the New World non-human primate, the common marmoset (Callithrix jacchus), is a potentially useful animal model, comprehensive understanding of drug metabolizing enzymes is insufficient. 2. A cDNA encoding a novel cytochrome P450 (P450) 2D8 was identified in marmosets. The amino acid sequence deduced from P450 2D8 cDNA showed a high sequence identity (83-86%) with other primate P450 2Ds. Phylogenetic analysis showed that marmoset P450 2D8 was closely clustered with human P450 2D6, unlike P450 2Ds of miniature pig, dog, rabbit, guinea pig, mouse or rat. 3. Marmoset P450 2D8 mRNA was predominantly expressed in the liver and small intestine among the tissues types analyzed, whereas marmoset P450 2D6 mRNA was expressed predominantly in the liver where P450 2D protein was detected by immunoblotting. 4. By metabolic assays using marmoset P450 2D8 protein heterologously expressed in Escherichia coli, although P450 2D8 exhibits lower catalytic efficiency compared to marmoset and human P450 2D6 enzymes, P450 2D8 mediated O-demethylations of metoprolol and dextromethorphan and bufuralol 1'-hydroxylation. 5. These results suggest that marmoset P450 2D8 (also expressed in the extrahepatic tissues) has potential roles in drug metabolism in a similar manner to those of human and marmoset P450 2D6.

Investigation of cytochrome P450 inhibitory properties of maslinic acid, a bioactive compound from Olea europaea L., and its structure-activity relationship

Maslinic acid (MA), the main pentacyclic triterpene of Olea europaea L. fruit, possesses a variety of pharmacological actions, including hypoglycemic, antioxidant, cardioprotective and antitumoral activities. Despite its importance, little is known about its effects on the cytochrome P450 (CYP) activity in both humans and animals. Therefore, the aim of this study was to investigate the effects of MA on the CYP 1A2, 2C9/11, 2D1/6, 2E1 and 3A2/4 activities by human and rat liver microsomes and specific CYP isoforms. In humans, MA only weakly inhibited CYP3A4 activity in human liver microsomes and specific CYP3A4 isoform with IC50 value at 46.1 and 62.3µM, respectively. In rats, MA also exhibited weak inhibition on CYP2C11, CYP2E1 and CYP3A2 activities with IC50 values more than 100µM. Enzyme kinetic studies showed that the MA was not only a competitive inhibitor of CYP3A4 in humans, but also a competitive inhibitor of CYP2C11 and 3A2 in rats, with Ki of 18.4, 98.7 and 66.3µM, respectively. Moreover, the presence of hydroxyl group at C-2 position of triterpenic acid in MA compared with oleanolic acid could magnify its competitive inhibition on human CYP3A4 activity. The relatively high Ki values of MA would have a low potential to cause the possible toxicity and drug interactions involving CYP enzymes, thus suggesting a sufficient safety for its putative use as a nutraceutical taken together with drugs.

Nanocomposites of graphene and cytochrome P450 2D6 isozyme for electrochemical-driven tramadol metabolism

Cytochrome P450 enzymes (cyt P450s) with an active center of iron protoheme are involved in most clinical drugs metabolism process. Herein, an electrochemical platform for the investigation of drug metabolism in vitro was constructed by immobilizing cytochrome P450 2D6 (CYP2D6) with cyt P450 reductase (CPR) on graphene modified glass carbon electrode. Direct and reversible electron transfer of the immobilized CYP2D6 with the direct electron transfer constant of 0.47 s(-1) and midpoint potential of -0.483 V was obtained. In the presence of substrate tramadol, the electrochemical-driven CYP2D6 mediated catalytic behavior toward the conversion of tramadol to o-demethyl-tramadol was confirmed. The Michaelis-Menten constant (Km(app)) and heterogeneous reaction rate constant during the metabolism of tramadol were calculated to be 23.85 μM and 1.96 cm s(-1), respectively. The inhibition effect of quinidine on CYP2D6 catalyze-cycle was also investigated. Furthermore, this system was applied to studying the metabolism of other drugs.

Recent examples on the clinical relevance of the CYP2D6 polymorphism and endogenous functionality of CYP2D6

The cytochrome P450 2D6 (CYP2D6) belongs to a group of CYPs considered of utmost importance in the metabolism of xenobiotics. Despite being of only minor abundance in the liver, it is involved in the clearance of >25% of marketed drugs. Accordingly, CYP2D6 can be very efficiently inhibited by a couple of commonly used drugs such as some antidepressants, although induction by any drug has not been observed thus far. CYP2D6 was also one of the first enzymes for which a highly polymorphic expression could be shown leading to a widespread range of functionality, from a complete lack of a functional enzyme to overexpression due to multiplication of active alleles. A clear relationship between the CYP2D6 genotype and adverse events during treatment with CNS-active drugs such as codeine, antidepressants, or antipsychotics could be demonstrated. More recently, some new aspects emerged about the potential endogenous function of CYP2D6 in terms of behavior and brain disorders.

Influence of Chronic Renal Failure on Stereoselective Metoprolol Metabolism in Hypertensive Patients

The influence of chronic renal failure on the stereoselective metabolism of rac-metoprolol was investigated in 15 hypertensive patients, 7 of them with chronic renal failure and 8 with normal renal function. They were treated with rac-metoprolol (200 mg) for 7 days. The patients of both groups presented stereoselectivity in metoprolol metabolism, favoring the formation of 1'R-alpha-hydroxymetoprolol (AUC(1(')R/1(')S)(0-24) approximately 2.5) and (R)-metoprolol acidic metabolite (AUC((S)/(R))(0-24) = 0.8), the latter resulting in the plasma accumulation of (S)-metoprolol (AUC((S)/(R))(0-24) = 1.2). Patients with chronic renal failure presented plasma accumulation of the 4 alpha-hydroxymetoprolol isomers and of both metoprolol acidic metabolite enantiomers. A 50% reduction in Cl(R) does not explain the 3- to 4-fold plasma accumulation of metoprolol acidic metabolite in this group, suggesting that other pathways of metoprolol elimination are affected in chronic renal failure in addition to renal excretion. Chronic renal failure does not change the stereoselective kinetic disposition of metoprolol but modifies its stereoselective metabolism, inducing some of the CYP enzymes involved in the formation of the metoprolol acid metabolite.

Modeling human cytochrome P450 2D6 metabolism and drug-drug interaction by a novel panel of knockout and humanized mouse lines

The highly polymorphic human cytochrome P450 2D6 enzyme is involved in the metabolism of up to 25% of all marketed drugs and accounts for significant individual differences in response to CYP2D6 substrates. Because of the differences in the multiplicity and substrate specificity of CYP2D family members among species, it is difficult to predict pathways of human CYP2D6-dependent drug metabolism on the basis of animal studies. To create animal models that reflect the human situation more closely and that allow an in vivo assessment of the consequences of differential CYP2D6 drug metabolism, we have developed a novel straightforward approach to delete the entire murine Cyp2d gene cluster and replace it with allelic variants of human CYP2D6. By using this approach, we have generated mouse lines expressing the two frequent human protein isoforms CYP2D6.1 and CYP2D6.2 and an as yet undescribed variant of this enzyme, as well as a Cyp2d cluster knockout mouse. We demonstrate that the various transgenic mouse lines cover a wide spectrum of different human CYP2D6 metabolizer phenotypes. The novel humanization strategy described here provides a robust approach for the expression of different CYP2D6 allelic variants in transgenic mice and thus can help to evaluate potential CYP2D6-dependent interindividual differences in drug response in the context of personalized medicine.

Prediction of human pharmacokinetics—gut-wall metabolism

Intestinal mucosal cells operate with different metabolic and transport activity, and not all of them are involved in drug absorption and metabolism. The fraction of these cells involved is dependent on the absorption characteristics of compounds and is difficult to predict (it is probably small). The cells also appear comparably impermeable. This shows a limited applicability of microsome intrinsic clearance (CLint)-data for prediction of gut-wall metabolism, and the difficulty to predict the gut-wall CL (CLGW) and extraction ratio (EGW). The objectives of this review were to evaluate determinants and methods for prediction of first-pass and systemic EGW and CLGW in man, and if required and possible, develop new simple prediction methodology. Animal gut-wall metabolism data do not appear reliable for scaling to man. In general, the systemic CLGW is low compared with the hepatic CL. For a moderately extracted CYP3A4-substrate with high permeability, midazolam, the gut-wall/hepatic CL-ratio is only 1/35. This suggests (as a general rule) that systemic CLGW can be neglected when predicting the total CL. First-pass EGW could be of importance, especially for substrates of CYP3A4 and conjugating enzymes. For several reasons, including those presented above and that blood flow based models are not applicable in the absorptive direction, it seems poorly predicted with available methodology. Prediction errors are large (several-fold on average; maximum-15-fold). A new simple first-pass EGW-prediction method that compensates for regional and local differences in absorption and metabolic activity has been developed. It has been based on human cell in-vitro CLint and fractional absorption from the small intestine for reference (including verapamil) and test substances, and in-vivo first-pass EGW-data for reference substances. First-pass EGW-values for CYP3A4-substrates with various degrees of gastrointestinal uptake and CLint and a CYP2D6-substrate were well-predicted (negligible errors). More high quality in-vitro CLint- and in-vivo EGW-data are required for further validation of the method.

Does the well-stirred model assess the intestinal first-pass effect well?

The pre-systemic intestinal extraction ratio (Eg) has been estimated by an equation based on the well-stirred model, which does not have a term of membrane transport. In this report, we have identified the application limitations of the well-stirred model equation to assess the pre-systemic intestinal extraction ratio. The Eg of metoprolol (CYP2D6 substrate) was assessed by three methods. Intrinsic clearances for metoprolol metabolism in hepatic and gastrointestinal microsomes were from a published report. Method 1 (model-independent method): the Eg of 0.228 was obtained according to the equation, F = Ff × (1 — Eg) × Fh, where F, Ff and Fh were the bioavailability, the fraction entering the intestinal tissue and the hepatic availability, respectively. Method 2: the Eg of 0.0071 was calculated according to the well-stirred model equation, and was much lower than the value of 0.228. Method 3: the Eg of 0.213 was obtained by the transport-metabolism-flow (TMF) model equation, and was much closer to the value of 0.228 obtained by the model-independent method than the Eg of 0.0071 calculated by the well-stirred model equation. Therefore, we propose that the factor of membrane transport process be incorporated into the pharmacokinetic model for the assessment of the pre-systemic intestinal extraction ratio.

Cytochrome P450-mediated metabolism in the human gut wall

Objective

Although the human small intestine serves primarily as an absorptive organ for nutrients and water, it also has the ability to metabolise drugs. Interest in the small intestine as a drug-metabolising organ has been increasing since the realisation that it is probably the most important extrahepatic site of drug biotransformation.

Key findings

Among the metabolising enzymes present in the small intestinal mucosa, the cytochromes P450 (CYPs) are of particular importance, being responsible for the majority of phase I drug metabolism reactions. Many drug interactions involving induction or inhibition of CYP enzymes, in particular CYP3A, have been proposed to occur substantially at the level of the intestine rather than exclusively within the liver, as originally thought. CYP3A and CYP2C represent the major intestinal CYPs, accounting for approximately 80% and 18%, respectively, of total immunoquantified CYPs. CYP2J2 is also consistently expressed in the human gut wall. In the case of CYP1A1, large interindividual variation in the expression levels has been reported. Data for the intestinal expression of the polymorphic CYP2D6 are conflicting. Several other CYPs, including the common hepatic isoform CYP2E1, are expressed in the human small intestine to only a very low extent, if at all. The distribution of most CYP enzymes is not uniform along the human gastrointestinal tract, being generally higher in the proximal regions of the small intestine.

Summary

This article reviews the current state of knowledge of CYP enzyme expression in human small intestine, the role of the gut wall in CYP-mediated metabolism, and how this metabolism limits the bioavailability of orally administered drugs. Possible interactions between drugs and CYP activity in the small intestine are also discussed.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Prediction of modified release pharmacokinetics and pharmacodynamics from in vitro, immediate release, and intravenous data

The aim of this study was to demonstrate the value of mechanistic simulations in gaining insight into the behaviors of modified release (MR) formulations in vivo and to use the properly calibrated models for prediction of pharmacokinetics (PK) and pharmacodynamics (PD). GastroPlus (Simulations Plus, Inc.) was used to fit mechanistic models for adinazolam and metoprolol that describe the absorption, PK, and PD after intravenous (i.v.) and immediate release (IR) oral (p.o.) administration. The fitted model for adinazolam was then used to predict the PD profile for a MR formulation and to design a new formulation with desired onset and duration of action. The fitted metoprolol model was used to gain insight and to explain the in vivo behaviors of MR formulations. For each drug, a single absorption/PK model was fitted that provided simulated plasma concentration-time profiles closely matching observed in vivo profiles across several different i.v. and p.o doses. Sedation score profiles of adinazolam were fitted with an indirect PD model. For metoprolol, the fitted absorption/PK model for IR p.o. doses was used to select in vitro dissolution conditions that best matched the in vivo release of MR doses. This model also explained differences in exposure after administration of MR formulations with different release rates. Mechanistic absorption/PK models allow for detailed descriptions of all processes affecting the two drugs' bioavailability, including release/dissolution, absorption, and intestinal and hepatic first pass extraction. The insights gained can be used to design formulations that more effectively overcome identified problems.

Potential role of intestinal first-pass metabolism in the prediction of drug-drug interactions

BACKGROUND

The contribution of intestine to the magnitude of drug-drug interactions (DDI) may be significant, considering high levels of inhibitors in the gut lumen achieved during absorption and the abundance of metabolic enzymes in the mature enterocytes. Intestinal inhibition is incorporated in the DDI prediction models as the ratio of the intestinal wall availability in the presence and absence of the inhibitor (F(G)(') and F(G), respectively).

OBJECTIVE

This review will focus on the ability of the current approaches to estimate the extent of intestinal DDI accurately, addressing predominantly the most abundant intestinal P450 enzyme, CYP3A4.

METHODS

Considering the sensitivity of the DDI prediction models to the accuracy of the F(G) estimates, the current study focuses on 3 different in vitro and in vivo approaches to assess this parameter.

RESULTS/CONCLUSION

The advantages and limitations of each of F(G) methods are outlined. Accurate assessment of this parameter is essential for the prediction of human drug clearance and drug-drug interaction potential.

The in vitro Inhibitory Potential of Trade Herbal Products on Human CYP2D6-Mediated Metabolism and the Influence of Ethanol

The six commonly used trade herbal products, St. John's wort, common valerian, common sage, Ginkgo biloba, Echinacea purpurea and horse chestnut, and ethanol, were investigated for their in vitro inhibitory potential of cytochrome P450 2D6 (CYP2D6)-mediated metabolism. Herbal components were extracted from commercially available products in a way that ensured the same composition of constituents in the extract as in the original trade products. c-DNA baculovirus expressed CYP2D6 was used with dextromethorphan as substrate. Quinidine was included as a positive control inhibitor. A validated high performance liquid chromatography methodology was used to quantify the formation of dextrorphan (product of dextromethorphan O-demethylation). Ethanol showed a biphasic effect on CYP2D6 metabolism, increasing initially the CYP2D6 activity with 175% of control up to a concentration of 1.1%, where after ethanol linearly inhibited the CYP2D6 activity. All the investigated herbs inhibited CYP2D6 activity to some extent, but only St. John's wort, common sage and common valerian were considered possible candidates for in vivo clinically significant effects. They showed IC50 values of 0.07 +/- 7 x 10(-3) mg/ml, 0.8 +/- 0.05 mg/ml and 1.6 +/- 0.2 mg/ml, respectively. St. John's wort inhibited CYP2D6-mediated metabolism in an uncompetitive manner, while common valerian and common sage in a non-competitive manner demonstrated interherb differences in inhibition patterns and differences when compared to the more homogenous competitive inhibitor quinidine. Common valerian was the only herb that showed a mechanistic inhibition of CYP2D6 activity and attention should be paid to a possible toxicity of this herb.

Species differences between mouse, rat, dog, monkey and human CYP-mediated drug metabolism, inhibition and induction

Animal models are commonly used in the preclinical development of new drugs to predict the metabolic behaviour of new compounds in humans. It is, however, important to realise that humans differ from animals with regards to isoform composition, expression and catalytic activities of drug-metabolising enzymes. In this review the authors describe similarities and differences in this respect among the different species, including man. This may be helpful for drug researchers to choose the most relevant animal species in which the metabolism of a compound can be studied for extrapolating the results to humans. The authors focus on CYPs, which are the main enzymes involved in numerous oxidative reactions and often play a critical role in the metabolism and pharmacokinetics of xenobiotics. In addition, induction and inhibition of CYPs are compared among species. The authors conclude that CYP2E1 shows no large differences between species, and extrapolation between species appears to hold quite well. In contrast, the species-specific isoforms of CYP1A, -2C, -2D and -3A show appreciable interspecies differences in terms of catalytic activity and some caution should be applied when extrapolating metabolism data from animal models to humans.

Absolute oral bioavailability of traxoprodil in cytochrome P450 2D6 extensive and poor metabolisers

BACKGROUND

Traxoprodil, a substituted 4-phenylpiperidine, is an N-methyl-D-aspartate (NMDA) receptor antagonist that is selective for receptors containing the NR2B subunit. In vivo and in vitro studies examining the disposition of traxoprodil have demonstrated that it is mainly metabolised by cytochrome P450 (CYP) 2D6, a major drug-metabolising enzyme that exhibits a genetic polymorphism.

OBJECTIVE

To assess the single-dose absolute oral bioavailability of traxoprodil in healthy male volunteers phenotyped as either CYP2D6 extensive or poor metabolisers.

METHODS

This was an open-label, three-way crossover study. Traxoprodil was administered as a single dose orally in solution of 50, 100 and 300mg and intravenously as a constant rate 2-hour infusion of 50 and 100mg. CYP2D6 phenotype was assigned following single-dose dextromethorphan administration.

RESULTS

In poor metabolisers (n = 6), oral bioavailability was approximately 80% and was consistent with a liver extraction ratio of approximately 20% (plasma clearance of approximately 4 mL/min/kg) indicating near complete absorption. Following intravenous administration, the mean volume of distribution at steady state (V(ss)) was moderate (approximately 6.5 L/kg) and the mean elimination half-life (t((1/2))) was approximately 20 hours. Following oral administration the mean maximum plasma concentration (C(max)) and area under the plasma concentration-time curve from time zero to infinity (AUC(infinity)) increased approximately proportionally with dose. In extensive metabolisers (n = 11), oral bioavailability was dose-dependent and nonlinear. At the 100mg dose, the absolute oral bioavailability was approximately 39.5%. Overall, the oral bioavailability ranged from 22.8% to 62.1% and its estimation was confounded by large differences in plasma concentrations at oral doses without equivalent intravenous doses. Following intravenous administration, plasma clearance was high (approximately 27 mL/min/kg), the V(ss) was moderate (approximately 4 L/Kg) and the t((1/2)) was approximately 2-4 hours. Following oral administration the C(max) and AUC(infinity) increased more than proportionally with dose. Apparent oral clearance decreased with increasing oral dose. However, t((1/2)) was approximately the same at all doses (approximately 4 hours).

CONCLUSION

The pharmacokinetics of traxoprodil were quite different in the two phenotypes. In extensive metabolisers, the oral bioavailability was nonlinear and dose-dependent, while in poor metabolisers, oral bioavailability appeared to be linear and dose-independent. Based on the pharmacokinetics in extensive and poor metabolisers, the nonlinear oral bioavailability in extensive metabolisers may be attributed to saturation of hepatic first-pass CYP2D6 metabolism. Thus, at a high oral dose, the impact of CYP2D6 metabolism on traxoprodil pharmacokinetics is minimal.

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

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

Intestinal and Hepatic Metabolic Activity of Five Cytochrome P450 Enzymes: Impact on Prediction of First-Pass Metabolism

The contribution of the gut is not routinely incorporated into in vitro-in vivo predictions of either clearance or drug-drug interactions, and this omission may partially explain the general underprediction trend often observed. In the current study, the metabolic ability of hepatic and intestinal pooled microsomes was compared for eight CYP3A substrates (midazolam, triazolam, diazepam, alprazolam, flunitrazepam, nifedipine, testosterone, and quinidine) and paclitaxel, tolbutamide, S-mephenytoin, and bufuralol as CYP2C8, CYP2C9, CYP2C19, and CYP2D6 probes, respectively. A general agreement in the type of kinetics was observed between the two systems for the substrates investigated. Of the 16 pathways investigated, 75% of K(m) (S(50)) values obtained in intestinal microsomes (5.9-769 microM) were within 2-fold of hepatic estimates. Irrespective of the cytochrome P450 (P450) investigated and normalization of V(max) values for the P450 abundance, clearance was 4.5- to 50-fold lower in intestinal microsomes (0.0005-0.51 microl/min/P450) compared with the hepatic estimates (0.002-5.8 microl/min/P450), whereas the rank order was consistent between the systems. Assessment of two enterocyte isolation methods (mucosal scraping or enterocyte elution) was performed at the substrate concentrations corresponding to the determined V(max) conditions for 11 pathways. The activity difference between the methods (3-29-fold) was P450-related in the following rank order: CYP2C19 > CYP3A4 > CYP2C9 approximately CYP2D6. After correction for the loss of activity between the methods, the intrinsic activities of hepatic and intestinal CYP3A4, CYP2C9, CYP2C19, and CYP2D6 were comparable for the 16 pathways. The implications of these findings on the prediction of intestinal first-pass metabolism are discussed.

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

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

The human intestinal cytochrome P450 "pie"

Cytochromes P450 (P450s) 3A, 2C, and 1A2 constitute the major "pieces" of the human liver P450 "pie" and account, on average, for 40, 25, and 18%, respectively, of total immunoquantified P450s (J Pharmacol Exp Ther 270:414-423, 1994). The P450 profile in the human small intestine has not been fully characterized. Therefore, microsomes prepared from mucosal scrapings from the duodenal/jejunal portion of 31 human donor small intestines were analyzed by Western blot using selective P450 antibodies. P450s 3A4, 2C9, 2C19, and 2J2 were detected in all individuals and ranged from 8.8 to 150, 2.9 to 27, <0.6 to 3.9, and <0.2 to 3.1 pmol/mg, respectively. CYP2D6 was detected in 29 individuals and ranged from <0.2 to 1.4 pmol/mg. CYP3A5 was detected readily in 11 individuals, with a range (average) of 4.9 to 25 (16) pmol/mg that represented from 3 to 50% of total CYP3A (CYP3A4 + CYP3A5) content. CYP1A1 was detected readily in three individuals, with a range (average) of 3.6 to 7.7 (5.6) pmol/mg. P450s 1A2, 2A6, 2B6, 2C8, and 2E1 were not or only faintly detected. As anticipated, average CYP3A content (50 pmol/mg) was the highest. Excluding CYP1A1, the remaining enzymes had the following rank order: 2C9 > 2C19 > 2J2 > 2D6 (8.4, 1.1, 0.9, and 0.5 pmol/mg, respectively). Analysis of a pooled preparation of the 31 donor specimens substantiated these results. In summary, as in the liver, large interindividual variation exists in the expression levels of individual P450s. On average, CYP3A and CYP2C9 represents the major pieces of the intestinal P450 pie, accounting for 80 and 15%, respectively, of total immunoquantified P450s.

Application of substrate depletion assay for early prediction of nonlinear pharmacokinetics in drug discovery: Assessment of nonlinearity of metoprolol, timolol, and propranolol

The purpose of this study was to investigate the advantages of the substrate depletion assay for evaluating linearity of pharmacokinetics compared with the metabolite formation assay. For propranolol, metoprolol, and nisoldipine with multiple and/or sequential metabolisms, the Michaelis constant (Km) and maximum metabolic intrinsic clearance obtained from the depletion assay using rat and human liver microsomes showed a good correlation with relevant parameters with the formation assay. In vitro kinetics and in vivo pharmacokinetic profiles after oral administration of timolol, metoprolol, and propranolol, were investigated in rats using the depletion assay. The same rank order was found between nonlinearities based on dose-normalized areas under the plasma concentration curve (AUC/Dose) and Km values. Using the kinetic parameters of these compounds, AUC was predicted based on a physiological based pharmacokinetic model incorporated saturable metabolism. The AUCs predicted for propranolol and metoprolol had a good relationship with those observed in the in vivo studies, implying that the depletion assay could be useful for assessing linearity of pharmacokinetics.

HUMAN CYP2D6 AND MOUSE CYP2DS: ORGAN DISTRIBUTION IN A HUMANIZED MOUSE MODEL

Polymorphic cytochrome P450 (P450) 2D6 (CYP2D6) metabolizes several classes of therapeutic drugs, endogenous neurochemicals, and toxins. A CYP2D6-humanized transgenic mouse line was previously developed to model CYP2D6-poor and -extensive metabolizer phenotypes. Human CYP2D6 was detected in the liver, kidney, and intestine of these animals. In this study, we investigated further the cellular expression and relative tissue levels of human CYP2D6 in these transgenic mice in liver, intestine, kidney, and brain. In addition, we compared this with the expression of mouse CYP2D enzymes in these organs. In humans, these organs are of interest with respect to P450-mediated drug metabolism, toxicity, and disease. The expression of human CYP2D6 and mouse CYP2D enzymes in humanized and wild-type mice was quantified by immunoblotting and detected at the cellular level by immunocytochemistry. The cell-specific expression of human CYP2D6 in liver, kidney, and intestine in humanized mice was similar to that reported in humans. The expression patterns of mouse CYP2D proteins were similar to those in humans in liver and kidney but substantially different in intestine. Human CYP2D6 was not detected in brain of transgenic mice. Mouse CYP2D proteins were detected in brain, allowing, for the first time, a direct comparison of CYP2D expression among mouse, rat, and human brain. This transgenic mouse model is useful for investigating CYP2D6-mediated metabolism in liver, kidney, and especially the intestine, where expression patterns demonstrated substantial species differences.

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

Human liver microsomes (HLMs) are frequently utilized in drug discovery to predict the human clearance of a compound. The extent to which the incubation conditions affect the accuracy of a human clearance prediction was determined for bufuralol. HLMs were preincubated at 37 degrees C for varying times (5-120 min) with and without NADPH, and the remaining enzyme activity was determined by incubating compounds that have been characterized to be selective for individual cytochromes P450 or flavin-containing monooxygenase 3. CYP2D6, the high-affinity component of bufuralol metabolism, was shown to be the least stable of the isoforms studied. The loss of CYP2D6 activity was further examined by determining the kinetics of 1'-hydroxybufuralol formation after different preincubation time periods, by using reactive oxygen species (ROS) scavengers, and by utilizing Western blotting techniques. A 3-fold decrease in Vmax was observed over 2 h, whereas the Km remained constant. ROS scavengers were able to block enzyme lability, and Western blots revealed no apparent loss of immunoreactive enzyme. The protein binding of bufuralol was determined in HLMs, recombinant CYP2D6, and human plasma. A prediction of theoretical bufuralol concentrations over a 120-min incubation that incorporated enzyme lability was performed and shown to be closer to actual data than if enzyme lability were ignored. Finally, a similar prediction using literature bufuralol data, coupled with the observed protein binding data, was used to illustrate that the most accurate predictions of bufuralol clearance are obtained when the amount of protein in the incubation is kept to a minimum and the overall incubation time is less than 20 min.

Predicting drug clearance from recombinantly expressed CYPs: intersystem extrapolation factors

1. Recombinantly expressed human cytochromes P450 (rhCYPs) have been underused for the prediction of human drug clearance (CL). 2. Differences in intrinsic activity (per unit CYP) between rhCYP and human liver enzymes complicate the issue and these discrepancies have not been investigated systematically. We define intersystem extrapolation factors (ISEFs) that allow the use of rhCYP data for the in vitro-in vivo extrapolation of human drug CL and the variance that is associated with interindividual variation of CYP abundance due to genetic and environmental effects. 3. A large database (n = 451) of metabolic stability data has been compiled and used to derive ISEFs for the most commonly used expression systems and CYP enzymes. 4. Statistical models were constructed for the ISEFs to determine major covariates in order to optimize experimental design to increase prediction accuracy. 5. Suggestions have been made for the conduct of future studies using rhCYP to predict human drug clearance.

QUANTITATIVE CONTRIBUTION OF CYP2D6 AND CYP3A TO OXYCODONE METABOLISM IN HUMAN LIVER AND INTESTINAL MICROSOMES

Oxycodone undergoes N-demethylation to noroxycodone and O-demethylation to oxymorphone. The cytochrome P450 (P450) isoforms capable of mediating the oxidation of oxycodone to oxymorphone and noroxycodone were identified using a panel of recombinant human P450s. CYP3A4 and CYP3A5 displayed the highest activity for oxycodone N-demethylation; intrinsic clearance for CYP3A5 was slightly higher than that for CYP3A4. CYP2D6 had the highest activity for O-demethylation. Multienzyme, Michaelis-Menten kinetics were observed for both oxidative reactions in microsomes prepared from five human livers. Inhibition with ketoconazole showed that CYP3A is the high affinity enzyme for oxycodone N-demethylation; ketoconazole inhibited >90% of noroxycodone formation at low substrate concentrations. CYP3A-mediated noroxycodone formation exhibited a mean K(m) of 600 +/- 119 microM and a V(max) that ranged from 716 to 14523 pmol/mg/min. Contribution from the low affinity enzyme(s) did not exceed 8% of total intrinsic clearance for N-demethylation. Quinidine inhibition showed that CYP2D6 is the high affinity enzyme for O-demethylation with a mean K(m) of 130 +/- 33 microM and a V(max) that ranged from 89 to 356 pmol/mg/min. Activity of the low affinity enzyme(s) accounted for 10 to 26% of total intrinsic clearance for O-demethylation. On average, the total intrinsic clearance for noroxycodone formation was 8 times greater than that for oxymorphone formation across the five liver microsomal preparations (10.5 microl/min/mg versus 1.5 microl/min/mg). Experiments with human intestinal mucosal microsomes indicated lower N-demethylation activity (20-50%) compared with liver microsomes and negligible O-demethylation activity, which predict a minimal contribution of intestinal mucosa in the first-pass oxidative metabolism of oxycodone.

INVOLVEMENT OF HUMAN HEPATIC UGT1A1, UGT2B4, AND UGT2B7 IN THE GLUCURONIDATION OF CARVEDILOL

Carvedilol ((+/-)-1-carbazol-4-yloxy)-3-[[2-(o-methoxyphenoxy)ethyl]amino]-2-propanol) is metabolized primarily into glucuronide conjugates. In the present study, we identified the human UDP-glucuronosyltransferase (UGT) isoforms involved in the glucuronidation of carvedilol by thin-layer chromatography using microsomes from human liver or insect cells expressing recombinant UGT isoforms. We observed two forms of carvedilol glucuronides, namely G1 and G2, in hepatic microsomes. The glucuronidation of carvedilol was catalyzed by at least three recombinant UGT isoforms: UGT1A1, UGT2B4, and UGT2B7. UGT2B4 formed both G1 and G2, whereas UGT1A1 and UGT2B7 were responsible for the formation of glucuronide G2 and G1, respectively. The enzyme kinetics for carvedilol glucuronidation by UGT1A1, UGT2B4, and UGT2B7 in addition to human liver microsomes were examined by Lineweaver-Burk analysis. The values of Km and Vmax for human liver microsomes were 26.6 microM and 106 pmol/min/mg protein for G1, and 46.0 microM and 44.5 pmol/min/mg protein for G2, respectively. The Km values for UGT1A1, UGT2B4, and UGT2B7 for G1 and G2 (22.1-55.1 microM) were comparable to those of the liver microsomes, whereas the Vmax values were in the range of 3.33 to 7.88 pmol/min/mg protein. The Km and Vmax/Km values for UGT2B4 and UGT2B7 for G1 were similar, whereas UGT2B4 had lower Km and higher Vmax/Km values for G2 compared with those of UGT1A1. These results suggest that G1 formation is catalyzed by UGT2B4 and UGT2B7, whereas G2 is formed by UGT2B4 and UGT1A1. These three hepatic UGT isoforms may have important roles in carvedilol metabolism.

Cytochrome P450 2D6: overview and update on pharmacology, genetics, biochemistry

Of about one dozen human P450 s that catalyze biotransformations of xenobiotics, CYP2D6 is one of the more important ones based on the number of its drug substrates. It shows a very high degree of interindividual variability, which is primarily due to the extensive genetic polymorphism that influences expression and function. This so-called debrisoquine/sparteine oxidation polymorphism has been extensively studied in many different populations and over 80 alleles and allele variants have been described. CYP2D6 protein and enzymatic activity is completely absent in less than 1% of Asian people and in up to 10% of Caucasians with two null alleles, which do not encode a functional P450 protein product. The resulting "poor metabolizer" (PM) phenotype is characterized by the inability to use CYP2D6-dependent metabolic pathways for drug elimination, which affect up to 20% of all clinically used drugs. The consequences are increased risk of adverse drug reactions or lack of therapeutic response. Today, genetic testing predicts the PM phenotype with over 99% certainty. At the other extreme, the "Ultrarapid Metabolizer" (UM) phenotype can be caused by alleles carrying multiple gene copies. "Intermediate Metabolizers" (IM) are severely deficient in their metabolism capacity compared to normal "Extensive Metabolizers" (EM), but in contrast to PMs they express a low amount of residual activity due to the presence of at least one partially deficient allele. Whereas the intricate genetics of the CYP2D6 polymorphism is becoming apparent at ever greater detail, applications in clinical practice are still rare. More clinical studies are needed to show where patients benefit from drug dose adjustment based on their genotype. Computational approaches are used to predict and rationalize substrate specificity and enzymatic properties of CYP2D6. Pharmacophore modeling of ligands and protein homology modeling are two complementary approaches that have been applied with some success. CYP2D6 is not only expressed in liver but also in the gut and in brain neurons, where endogenous substrates with high-turnover have been found. Whether and how brain functions may be influenced by polymorphic expression are interesting questions for the future.

CHARACTERIZATION OF MOUSE SMALL INTESTINAL CYTOCHROME P450 EXPRESSION

The expression of biotransformation enzymes in mouse small intestine is poorly characterized, which limits the utility of transgenic or knockout mouse models for first-pass drug metabolism studies. In response, we have systematically examined the composition and inducibility of cytochrome P450 (P450) protein and mRNA in mouse small intestinal epithelial cells (enterocytes). RNA-PCR was conducted to confirm the expression and identity of CYP1A1, 1B1, 2B10, 2B19, 2B20, 2C29, 2C38, 2C40, 2E1, 3A11, 3A13, 3A16, 3A25, and 3A44 in the enterocytes of untreated mice, but CYP1A2, 2A4/5, 2A12, 2C37, 2C39, and 2F2 were not detected. The inducibility of CYP2B, 2C, and 3A subfamily forms was determined by real-time quantitative RNA-PCR. All five CYP3A forms were induced, in a range from 1.7- to 4.5-fold, by dexamethasone (DEX). Phenobarbital (PB) induced CYP2B9, CYP2B10, and CYP2B20 mRNAs and suppressed CYP2B19 mRNA levels. PB also induced CYP2C29 and CYP2C40, but not CYP2C38 mRNA. At the protein level, CYP1A1, CYP1B1, CYP2B, CYP2C, CYP2E1, and CYP3A were detected in enterocytes from untreated mice by immunoblot analysis. CYP1A1 was inducible by beta-naphthoflavone (BNF), CYP2B and CYP2C by PB, and CYP3A by DEX. CYP2B, 2C, and 3A proteins were all expressed at high levels proximally, and decreased distally. The inducibility of CYP1A1 followed a similar pattern. Intestinal P450 expression was compared between C57BL/6 (B6) and 129/sv (129) mice, strains commonly used in the preparation of transgenic and knockout mouse models. There was no significant strain difference in constitutive levels or induction patterns for CYP2B, 2C, and 3A protein. However, CYP1A1 was induced to a high level by BNF in B6 mice, but was not induced in the 129 mice.

Pharmacokinetics in the newborn

In addition to differences in the pharmacodynamic response in the infant, the dose and the pharmacokinetic processes acting upon that dose principally determine the efficacy and/or safety of a therapeutic or inadvertent exposure. At a given dose, significant differences in therapeutic efficacy and toxicant susceptibility exist between the newborn and adult. Immature pharmacokinetic processes in the newborn predominantly explain such differences. With infant development, the physiological and biochemical processes that govern absorption, distribution, metabolism, and excretion undergo significant growth and maturational changes. Therefore, any assessment of the safety associated with an exposure must consider the impact of these maturational changes on drug pharmacokinetics and response in the developing infant. This paper reviews the current data concerning the growth and maturation of the physiological and biochemical factors governing absorption, distribution, metabolism, and excretion. The review also provides some insight into how these developmental changes alter the efficiency of pharmacokinetics in the infant. Such information may help clarify why dynamic changes in therapeutic efficacy and toxicant susceptibility occur through infancy.

The relative contribution of monoamine oxidase and cytochrome p450 isozymes to the metabolic deamination of the trace amine tryptamine

Tryptamine is a trace amine in mammalian central nervous system that interacts with the trace amine TA(2) receptor and is now thought to function as a neurotransmitter or neuromodulator. It had been reported that deamination of tryptamine to tryptophol was mediated by CYP2D6, a cytochrome P450 that is expressed in human brain, suggesting that tryptamine may be an endogenous substrate for this polymorphic enzyme. We were unable to confirm this report and have reinvestigated tryptamine metabolism in human liver microsomes (HLM) and in microsomes expressing recombinant human cytochrome P450 and monoamine oxidase (MAO) isozymes. Tryptamine was oxidized to indole-3-acetaldehyde by HLM and recombinant human MAO-A in the absence of NADPH, and indole-3-acetaldehyde was further reduced to tryptophol by aldehyde reductase in HLM in the presence of NADPH. Steady-state kinetic parameters were estimated for each reaction step by HLM and MAO-A. The CYP2D6 substrates bufuralol and debrisoquine showed strong inhibition of both tryptophol production from tryptamine in HLM and the formation of indole-3-acetaldehyde from tryptamine catalyzed by recombinant MAO-A. Anti-CYP2D6 monoclonal antibody did not inhibit these reactions. Pargyline, a nonselective MAO inhibitor, did not show cross inhibition to debrisoquine 4-hydroxylation and dextromethorphan O-demethylation by HLM and recombinant CYP2D6 enzyme. This is the first unequivocal report of the selective conversion of tryptamine to tryptophol by MAO-A. CYP2D6 does not contribute to this reaction.