Determination of in vivo absorption, metabolism, and transport of drugs by the human intestinal wall and liver with a novel perfusion technique

Next-Generation Intestinal Toxicity Model of Human Embryonic Stem Cell-Derived Enterocyte-Like Cells

The gastrointestinal tract is the most common exposure route of xenobiotics, and intestinal toxicity can result in systemic toxicity in most cases. It is important to develop intestinal toxicity assays mimicking the human system; thus, stem cells are rapidly being developed as new paradigms of toxicity assessment. In this study, we established human embryonic stem cell (hESC)-derived enterocyte-like cells (ELCs) and compared them to existing in vivo and in vitro models. We found that hESC-ELCs and the in vivo model showed transcriptomically similar expression patterns of a total of 10,020 genes than the commercialized cell lines. Besides, we treated the hESC-ELCs, in vivo rats, Caco-2 cells, and Hutu-80 cells with quarter log units of lethal dose 50 or lethal concentration 50 of eight drugs—chloramphenicol, cycloheximide, cytarabine, diclofenac, fluorouracil, indomethacin, methotrexate, and oxytetracycline—and then subsequently analyzed the biomolecular markers and morphological changes. While the four models showed similar tendencies in general toxicological reaction, hESC-ELCs showed a stronger correlation with the in vivo model than the immortalized cell lines. These results indicate that hESC-ELCs can serve as a next-generation intestinal toxicity model.

Using naso- and oro-intestinal catheters in physiological research for intestinal delivery and sampling in vivo: practical and technical aspects to be considered

Intestinal catheters have been used for decades in human nutrition, physiology, pharmacokinetics, and gut microbiome research, facilitating the delivery of compounds directly into the intestinal lumen or the aspiration of intestinal fluids in human subjects. Such research provides insights about (local) dynamic metabolic and other intestinal luminal processes, but working with catheters might pose challenges to biomedical researchers and clinicians. Here, we provide an overview of practical and technical aspects of applying naso- and oro-intestinal catheters for delivery of compounds and sampling luminal fluids from the jejunum, ileum, and colon in vivo. The recent literature was extensively reviewed, and combined with experiences and insights we gained through our own clinical trials. We included 60 studies that involved a total of 720 healthy subjects and 42 patients. Most of the studies investigated multiple intestinal regions (24 studies), followed by studies investigating only the jejunum (21 studies), ileum (13 studies), or colon (2 studies). The ileum and colon used to be relatively inaccessible regions in vivo. Custom-made state-of-the-art catheters are available with numerous options for the design, such as multiple lumina, side holes, and inflatable balloons for catheter progression or isolation of intestinal segments. These allow for multiple controlled sampling and compound delivery options in different intestinal regions. Intestinal catheters were often used for delivery (23 studies), sampling (10 studies), or both (27 studies). Sampling speed decreased with increasing distance from the sampling syringe to the specific intestinal segment (i.e., speed highest in duodenum, lowest in ileum/colon). No serious adverse events were reported in the literature, and a dropout rate of around 10% was found for these types of studies. This review is highly relevant for researchers who are active in various research areas and want to expand their research with the use of intestinal catheters in humans in vivo.

Current challenges and future perspectives in oral absorption research: An opinion of the UNGAP network

Although oral drug delivery is the preferred administration route and has been used for centuries, modern drug discovery and development pipelines challenge conventional formulation approaches and highlight the insufficient mechanistic understanding of processes critical to oral drug absorption. This review presents the opinion of UNGAP scientists on four key themes across the oral absorption landscape: (1) specific patient populations, (2) regional differences in the gastrointestinal tract, (3) advanced formulations and (4) food-drug interactions. The differences of oral absorption in pediatric and geriatric populations, the specific issues in colonic absorption, the formulation approaches for poorly water-soluble (small molecules) and poorly permeable (peptides, RNA etc.) drugs, as well as the vast realm of food effects, are some of the topics discussed in detail. The identified controversies and gaps in the current understanding of gastrointestinal absorption-related processes are used to create a roadmap for the future of oral drug absorption research.

Absorption and Intestinal Metabolic Profile of Oleocanthal in Rats

Oleocanthal (OLC), a phenolic compound of extra virgin olive oil (EVOO), has emerged as a potential therapeutic agent against a variety of diseases due to its anti-inflammatory activity. The aim of the present study is to explore its in vivo intestinal absorption and metabolism. An in situ perfusion technique in rats was used, involving simultaneous sampling from the luminal perfusate and mesenteric blood. Samples were analysed by UHPLC-MS-MS for the presence of oleocanthal (OLC) and its metabolites. OLC was mostly metabolized by phase I metabolism, undergoing hydration, hydrogenation and hydroxylation. Phase II reactions (glucuronidation of hydrogenated OLC and hydrated metabolites) were observed in plasma samples. OLC was poorly absorbed in the intestine, as indicated by the low effective permeability coefficient (2.23 ± 3.16 × 10^-5 cm/s) and apparent permeability coefficient (4.12 ± 2.33 × 10^-6 cm/s) obtained relative to the values of the highly permeable reference compound levofloxacin (LEV). The extent of OLC absorption reflected by the area under the mesenteric blood-time curve normalized by the inlet concentration (AUC) was also lower than that of LEV (0.25 ± 0.04 vs. 0.64 ± 0.03, respectively). These results, together with the observed intestinal metabolism, suggest that OLC has a moderate-to-low oral absorption; but higher levels of OLC are expected to reach human plasma vs. rat plasma.

Simultaneously predict pharmacokinetic interaction of rifampicin with oral versus intravenous substrates of cytochrome P450 3A/P‑glycoprotein to healthy human using a semi-physiologically based pharmacokinetic model involving both enzyme and transporter turnover

Several reports demonstrated that rifampicin affected pharmacokinetics of victim drugs following oral more than intravenous administration. We aimed to establish a semi-physiologically based pharmacokinetic (semi-PBPK) model involving both enzyme and transporter turnover to simultaneously predict pharmacokinetic interaction of rifampicin with oral versus intravenous substrates of cytochrome P450 (CYP) 3A4/P‑glycoprotein (P-GP) in human. Rifampicin was chosen as the CYP3A /P-GP inducer. Thirteen victim drugs including P-GP substrates (digoxin and talinolol), CYP3A substrates (alfentanil, midazolam, nifedipine, ondansetron and oxycodone), dual substrates of CYP3A/P-GP (quinidine, cyclosporine A, tacrolimus and verapamil) and complex substrates (S-ketamine and tramadol) were chosen to investigate drug-drug interactions (DDIs) with rifampicin. Corresponding parameters were cited from literatures. Before and after multi-dose of oral rifampicin, the pharmacokinetic profiles of victim drugs for oral or intravenous administration to human were predicted using the semi-PBPK model and compared with the observed values. Contribution of both CYP3A and P-GP induction in intestine and liver by rifampicin to pharmacokinetic profiles of victim drugs was investigated. The predicted pharmacokinetic profiles of drugs before and after rifampicin administration accorded with the observations. The predicted pharmacokinetic parameters and DDIs were successful, whose fold-errors were within 2. It was consistent with observations that the DDIs of rifampicin with oral victim drugs were larger than those with intravenous victim drugs. DDIs of rifampicin with CYP3A or P-GP substrates following oral versus intravenous administration to human were successfully predicted using the developed semi-PBPK model.

Regional specificity of the gut-incretin response to small intestinal glucose infusion in healthy older subjects

The importance of the region, as opposed to the length, of small intestine exposed to glucose in determining the secretion of the incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) remains unclear. We sought to compare the glycemic, insulinemic and incretin responses to glucose administered to the proximal (12-60cm beyond the pylorus), or more distal (>70cm beyond the pylorus) small intestine, or both. 10 healthy subjects (9M,1F; aged 70.3±1.4years) underwent infusion of glucose via a catheter into the proximal (glucose proximally; GP), or distal (glucose distally; GD) small intestine, or both (GPD), on three separate days in a randomised fashion. Blood glucose, serum insulin and plasma GLP-1, GIP and CCK responses were assessed. The iAUC for blood glucose was greater in response to GPD than GP (P<0.05), with no difference between GD and GP. GP was associated with minimal GLP-1 response (P=0.05), but substantial increases in GIP, CCK and insulin (P<0.001 for all). GPD and GD both stimulated GLP-1, GIP, CCK and insulin (P<0.001 for all). Compared to GP, GPD induced greater GLP-1, GIP and CCK responses (P<0.05 for all). Compared with GPD, GD was associated with greater GLP-1 (P<0.05), but reduced GIP and CCK (P<0.05 for both), responses. We conclude that exposure of glucose to the distal small intestine appears necessary for substantial GLP-1 secretion, while exposure of both the proximal and distal small intestine result in substantial secretion of GIP.

Quantitative Prediction of Drug-Drug Interactions Involving Inhibitory Metabolites in Drug Development: How Can Physiologically Based Pharmacokinetic Modeling Help?

This subteam under the Drug Metabolism Leadership Group (Innovation and Quality Consortium) investigated the quantitative role of circulating inhibitory metabolites in drug-drug interactions using physiologically based pharmacokinetic (PBPK) modeling. Three drugs with major circulating inhibitory metabolites (amiodarone, gemfibrozil, and sertraline) were systematically evaluated in addition to the literature review of recent examples. The application of PBPK modeling in drug interactions by inhibitory parent-metabolite pairs is described and guidance on strategic application is provided.

Predicting Drug Extraction in the Human Gut Wall: Assessing Contributions from Drug Metabolizing Enzymes and Transporter Proteins using Preclinical Models

Intestinal metabolism can limit oral bioavailability of drugs and increase the risk of drug interactions. It is therefore important to be able to predict and quantify it in drug discovery and early development. In recent years, a plethora of models-in vivo, in situ and in vitro-have been discussed in the literature. The primary objective of this review is to summarize the current knowledge in the quantitative prediction of gut-wall metabolism. As well as discussing the successes of current models for intestinal metabolism, the challenges in the establishment of good preclinical models are highlighted, including species differences in the isoforms; regional abundances and activities of drug metabolizing enzymes; the interplay of enzyme-transporter proteins; and lack of knowledge on enzyme abundances and availability of empirical scaling factors. Due to its broad specificity and high abundance in the intestine, CYP3A is the enzyme that is frequently implicated in human gut metabolism and is therefore the major focus of this review. A strategy to assess the impact of gut wall metabolism on oral bioavailability during drug discovery and early development phases is presented. Current gaps in the mechanistic understanding and the prediction of gut metabolism are highlighted, with suggestions on how they can be overcome in the future.

Paediatric pharmacokinetics: key considerations

A number of anatomical and physiological factors determine the pharmacokinetic profile of a drug. Differences in physiology in paediatric populations compared with adults can influence the concentration of drug within the plasma or tissue. Healthcare professionals need to be aware of anatomical and physiological changes that affect pharmacokinetic profiles of drugs to understand consequences of dose adjustments in infants and children. Pharmacokinetic clinical trials in children are complicated owing to the limitations on blood sample volumes and perception of pain in children resulting from blood sampling. There are alternative sampling techniques that can minimize the invasive nature of such trials. Population based models can also limit the sampling required from each individual by increasing the overall sample size to generate robust pharmacokinetic data. This review details key considerations in the design and development of paediatric pharmacokinetic clinical trials.

Paediatric oral biopharmaceutics: key considerations and current challenges

The complex process of oral drug absorption is influenced by a host of drug and formulation properties as well as their interaction with the gastrointestinal environment in terms of drug solubility, dissolution, permeability and pre-systemic metabolism. For adult dosage forms the use of biopharmaceutical tools to aid in the design and development of medicinal products is well documented. This review considers current literature evidence to guide development of bespoke paediatric biopharmaceutics tools and reviews current understanding surrounding extrapolation of adult methodology into a paediatric population. Clinical testing and the use of in silico models were also reviewed. The results demonstrate that further work is required to adequately characterise the paediatric gastrointestinal tract to ensure that biopharmaceutics tools are appropriate to predict performance within this population. The most vulnerable group was found to be neonates and infants up to 6 months where differences from adults were greatest.

Human in vivo regional intestinal permeability: importance for pharmaceutical drug development

Both the development and regulation of pharmaceutical dosage forms have undergone significant improvements and development over the past 25 years, due primarily to the extensive application of the biopharmaceutical classification system (BCS). The Biopharmaceutics Drug Disposition Classification System, which was published in 2005, has also been a useful resource for predicting the influence of transporters in several pharmacokinetic processes. However, there remains a need for the pharmaceutical industry to develop reliable in vitro/in vivo correlations and in silico methods for predicting the rate and extent of complex gastrointestinal (GI) absorption, the bioavailability, and the plasma concentration-time curves for orally administered drug products. Accordingly, a more rational approach is required, one in which high quality in vitro or in silico characterizations of active pharmaceutical ingredients and formulations are integrated into physiologically based in silico biopharmaceutics models to capture the full complexity of GI drug absorption. The need for better understanding of the in vivo GI process has recently become evident after an unsuccessful attempt to predict the GI absorption of BCS class II and IV drugs. Reliable data on the in vivo permeability of the human intestine (Peff) from various intestinal regions is recognized as one of the key biopharmaceutical requirements when developing in silico GI biopharmaceutics models with improved predictive accuracy. The Peff values for human jejunum and ileum, based on historical open, single-pass, perfusion studies are presented in this review. The main objective of this review is to summarize and discuss the relevance and current status of these human in vivo regional intestinal permeability values.

Intestinal absorption, metabolism, and excretion of (-)-epicatechin in healthy humans assessed by using an intestinal perfusion technique

BACKGROUND

(-)-Epicatechin is a dietary flavonoid present in many foods that affects vascular function, but its action is limited by incomplete absorption, conjugation, and metabolism. Factors that influence this activity may be attributed to instability in the gastrointestinal lumen, low permeability across the intestinal wall, or active efflux from enterocytes and extensive conjugation.

OBJECTIVE

With the use of a multilumen perfusion catheter, we investigated the jejunal absorption, systemic availability, metabolism, and intestinal, biliary, and urinary excretion of (-)-epicatechin in humans.

DESIGN

In a single-center, randomized, open, controlled study in 8 healthy volunteers, 50 mg purified (-)-epicatechin was perfused into an isolated jejunal segment together with antipyrine as a marker for absorption. (-)-Epicatechin and conjugates were measured in intestinal perfusates, bile, plasma, and urine.

RESULTS

Forty-six percent of the dose was recovered in the perfusate either as unchanged (-)-epicatechin (22 mg) or conjugates (0.8 mg); with stability taken into account, this result indicates that ∼46% of the dose had apparently been absorbed. The conjugates were predominantly sulfates, which indicated conjugation by sulfotransferases followed by efflux from the enterocytes. In contrast, epicatechin glucuronides were dominant in plasma, bile, and urine.

CONCLUSIONS

Almost one-half of the (-)-epicatechin is apparently absorbed in the jejunum but with substantial interindividual differences in the extent of absorption. The data suggest that the nature and substitution position of (-)-epicatechin conjugation are major determinants of the metabolic fate in the body, influencing whether the compound is effluxed into the lumen or absorbed into the blood and subsequently excreted.

Regional intestinal drug permeation: biopharmaceutics and drug development

Over the last 25 years, profound changes have been seen in both the development and regulation of pharmaceutical dosage forms, due primarily to the extensive use of the biopharmaceutical classification system (BCS) in both academia and industry. The BCS and the FDA scale-up and post-approval change guidelines were both developed during the 1990s and both are currently widely used to claim biowaivers. The development of the BCS and its wide acceptance were important steps in pharmaceutical science that contributed to the more rational development of oral dosage forms. The effective permeation (Peff) of drugs through the intestine often depends on the combined outcomes of passive diffusion and multiple parallel transport processes. Site-specific jejunal Peff cannot reflect the permeability of the whole intestinal tract, since this varies along the length of the intestine, but is a useful approximation of the fraction of the oral dose that is absorbed. It appears that drugs with a jejunal Peff>1.5×10(-4)cm/s will be completely absorbed no matter which transport mechanisms are utilized. In this paper, historical clinical data originating from earlier open, single-pass perfusion studies have been used to calculate the Peff of different substances from sites in the jejunum and ileum. More exploratory in vivo studies are required in order to obtain reliable data on regional intestinal drug absorption. The development of experimental and theoretical methods of assessing drug absorption from both small intestine and various sites in the colon is encouraged. Some of the existing human in vivo data are discussed in relation to commonly used cell culture models. It is crucial to accurately determine the input parameters, such as the regional intestinal Peff, as these will form the basis for the expected increase in modeling and simulation of all the processes involved in GI drug absorption, thus facilitating successful pharmaceutical development in the future. It is suggested that it would be feasible to use open, single-pass perfusion studies for the in vivo estimation of regional intestinal Peff, but that care should be taken in the study design to optimize the absorption conditions.

Effects of taurocholic acid on glycemic, glucagon-like peptide-1, and insulin responses to small intestinal glucose infusion in healthy humans

CONTEXT

In vitro and animal studies suggest that bile acids have the capacity to reduce blood glucose by stimulating glucagon-like peptide-1 (GLP-1) and, thereby, insulin.

OBJECTIVE

This study evaluated the effects of intrajejunal taurocholic acid (TCA) on blood glucose, GLP-1, and insulin responses to jejunal glucose infusion in healthy men.

PARTICIPANTS AND DESIGN

Ten healthy men were each studied on 2 days in a double-blind, randomized order. After the subjects fasted overnight, a jejunal catheter was positioned and a balloon inflated 30 cm beyond the pylorus with aspiration of endogenous bile. Two grams TCA in saline, or saline control, was infused beyond the balloon over 30 minutes, followed by 2 g TCA or control, together with 60 g glucose, over the next 120 minutes. Blood was sampled frequently for the measurements of blood glucose, total GLP-1, insulin, C-peptide, and glucagon.

RESULTS

Intrajejunal infusion of TCA alone (t = -30 to 0 minutes) had no effect on blood glucose, GLP-1, insulin, C-peptide, or glucagon concentrations. During intrajejunal glucose infusion (t = 0 to 120 minutes), blood glucose concentrations were lower (P < .001), and plasma GLP-1 (P < .001) and the C-peptide/glucose ratio (P = .008) were both greater, whereas plasma insulin, C-peptide, and glucagon levels were not significantly different after TCA than after control.

CONCLUSIONS

In healthy humans, small intestinal infusion of TCA potently reduces the glycemic response to small intestinal glucose, associated with an increase in GLP-1 and C-peptide/glucose ratio. These observations indicate the potential for bile acid-based therapy in type 2 diabetes.

Sucralose, a synthetic organochlorine sweetener: overview of biological issues

Sucralose is a synthetic organochlorine sweetener (OC) that is a common ingredient in the world's food supply. Sucralose interacts with chemosensors in the alimentary tract that play a role in sweet taste sensation and hormone secretion. In rats, sucralose ingestion was shown to increase the expression of the efflux transporter P-glycoprotein (P-gp) and two cytochrome P-450 (CYP) isozymes in the intestine. P-gp and CYP are key components of the presystemic detoxification system involved in first-pass drug metabolism. The effect of sucralose on first-pass drug metabolism in humans, however, has not yet been determined. In rats, sucralose alters the microbial composition in the gastrointestinal tract (GIT), with relatively greater reduction in beneficial bacteria. Although early studies asserted that sucralose passes through the GIT unchanged, subsequent analysis suggested that some of the ingested sweetener is metabolized in the GIT, as indicated by multiple peaks found in thin-layer radiochromatographic profiles of methanolic fecal extracts after oral sucralose administration. The identity and safety profile of these putative sucralose metabolites are not known at this time. Sucralose and one of its hydrolysis products were found to be mutagenic at elevated concentrations in several testing methods. Cooking with sucralose at high temperatures was reported to generate chloropropanols, a potentially toxic class of compounds. Both human and rodent studies demonstrated that sucralose may alter glucose, insulin, and glucagon-like peptide 1 (GLP-1) levels. Taken together, these findings indicate that sucralose is not a biologically inert compound.

In vitro and in vivo small intestinal metabolism of CYP3A and UGT substrates in preclinical animals species and humans: species differences

Intestinal first-pass metabolism has a great impact on the bioavailability of cytochrome P450 3A4 (CYP3A) and/or uridine 5'-diphosphate (UDP)-glucoronosyltranferase (UGT) substrates in humans. In vitro and in vivo intestinal metabolism studies are essential for clarifying pharmacokinetics in animal species and for predicting the effects of human intestinal metabolism. We review species differences in intestinal metabolism both in vitro and in vivo. Based on mRNA expression levels, the major intestinal CYP3A isoform is CYP3A4 for humans, CYP3A4 (3A8) for monkeys, CYP3A9 for rats, cyp3a13 for mice, and CYP3A12 for dogs. Additionally, the intestinal-specific UGT would be UGT1A10 for humans, UGT1A8 for monkeys, and UGT1A7 for rats. In vitro and in vivo intestinal metabolism of CYP3A substrates were larger in monkeys than in humans, although a correlation in intestinal availability between monkeys and humans has been reported. Little information is available regarding species differences in in vitro and in vivo UGT activities; however, UGT-mediated in vivo intestinal metabolism has been demonstrated for raloxifene in humans and for baicalein in rats. Further assessment of intestinal metabolism, particularly for UGT substrates, is required to clarify the entire picture of species differences.

Preparation and characterization of rodent intestinal microsomes: comparative assessment of two methods

Small intestine plays an important role in the first-pass metabolism of orally ingested xenobiotics as a result of expression of both Phase I and Phase II metabolic enzymes, together with associated transporters. Intestinal microsomes thus can be used to study susceptibility of compounds to metabolism in vitro. The present study was undertaken to have a comparative assessment between different methods of preparation of rodent intestinal microsomes. Mouse and rat intestinal microsomes were prepared by two methods, in method A intestines were homogenized, while in method B mucosal cells were scrapped followed by homogenization. Further, microsomes were prepared by centrifugation (10000xg) followed by ultra centrifugation (100000×g) of the homogenates. The prepared microsomes were characterized for protein concentration using Bradford's method and CYP450 content using carbon monoxide bubbling method. The protein concentration and CYP450 content in microsomes prepared by method B was significantly higher than method A. In conclusion, superior quality intestinal microsomes can be obtained from rodents by using scrapped intestinal mucosal cells as compared to the intestinal homogenates.

Regional distribution of cytochrome p450 mRNA expression in the liver and small intestine of cynomolgus monkeys

The cynomolgus monkey is used to study drug metabolism because of its evolutionary closeness to humans. Despite their importance, regional distribution of cytochrome P450 (CYP) enzymes including CYP3As in the liver and small intestine, the major sites of drug metabolism, has not been fully investigated in cynomolgus monkeys. In this study, we measured mRNA expression levels of 14 CYPs in the CYP1, 2, and 3 subfamilies, including orthologs of human CYP3A4 and CYP3A5, in the liver and small intestine of cynomolgus monkeys. Expression levels of each CYP mRNA in various regions of the liver were quantified and comparisons were made between the right lobe, quadrate lobe, left medial lobe, left lateral lobe, and caudate lobe and with four different sections of the right lobe. In the small intestine, the same mRNAs were measured in the duodenum and six different sections from the proximal jejunum to the distal ileum. Expression levels of the CYP mRNAs were not substantially different between liver samples, but varied between the different sections of the small intestine, including CYP3A4. These results suggest that analysis of distinct sections is required for a better understanding of cynomolgus monkey CYPs in the small intestine.

Gefitinib-phenytoin interaction is not correlated with the C-erythromycin breath test in healthy male volunteers

AIMS

We aimed to describe the pharmacokinetic interaction between phenytoin, a potent CYP3A4 and P-glycoprotein (P-gp) (ABCB1) inducer, and gefitinib, a CYP3A4, CYP2D6 and P-gp substrate.

METHODS

An open-label, randomized, two-phase crossover study was conducted. Eighteen healthy male volunteers (nine homozygous CC and nine homozygous TT as determined by their ABCB1 C3435T polymorphism in exon 26) received a single oral dose of 250 mg gefitinib alone or after 5 days treatment with phenytoin (5 mg kg(-1) daily). Gefitinib plasma concentrations were determined by high-performance liquid chromatography. Hepatic CYP3A4 activity was evaluated by the (14)C-erythromycin breath test (ERMBT) and the ABCB1 and CYP2D6 genetic polymorphisms were determined by the TaqMan allelic discrimination assay and long polymerase chain reaction, respectively.

RESULTS

Following treatment with phenytoin, mean gefitinib C(max) and AUC(0-infinity) decreased by 26 +/- 44% [95% confidence interval (CI) for the difference 5-48%, P= 0.005] and 47 +/- 26% (95% CI for the difference 34-60%, P= 0.001), respectively, and apparent oral clearance increased by 126 +/- 93% (95% CI for the difference 80-172%, P= 0.004). Concomitantly, phenytoin increased the mean ERMBT by 91 +/- 44% (95% CI 75-105%, P < 0.001) from baseline, but the extent of liver CYP3A4 induction was not correlated to the extent of interaction. Furthermore, this interaction was independent of ABCB1 genetic polymorphism. The CYP2D6 genotype was slightly but significantly related to gefitinib clearance (P= 0.04) during the control phase.

CONCLUSIONS

The significant interaction between gefitinib and phenytoin was not correlated with the erythromycin breath test and was independent of ABCB1 polymorphism, but may involve presystemic CYP3A-mediated intestinal first-pass.

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.

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

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

The use of biopharmaceutic classification of drugs in drug discovery and development: current status and future extension

Bioavailability (BA) and bioequivalence (BE) play a central role in pharmaceutical product development and BE studies are presently being conducted for New Drug Applications (NDAs) of new compounds, in supplementary NDAs for new medical indications and product line extensions, in Abbreviated New Drug Applications (ANDAs) of generic products and in applications for scale-up and post-approval changes. The Biopharmaceutics Classification System (BCS) has been developed to provide a scientific approach for classifying drug compounds based on solubility as related to dose and intestinal permeability in combination with the dissolution properties of the oral immediate-release (IR) dosage form. The aim of the BCS is to provide a regulatory tool for replacing certain BE studies by accurate in-vitro dissolution tests. The aim of this review is to present the status of the BCS and discuss its future application in pharmaceutical product development. The future application of the BCS is most likely increasingly important when the present framework gains increased recognition, which will probably be the case if the BCS borders for certain class II and III drugs are extended. The future revision of the BCS guidelines by the regulatory agencies in communication with academic and industrial scientists is exciting and will hopefully result in an increased applicability in drug development. Finally, we emphasize the great use of the BCS as a simple tool in early drug development to determine the rate-limiting step in the oral absorption process, which has facilitated the information between different experts involved in the overall drug development process. This increased awareness of a proper biopharmaceutical characterization of new drugs may in the future result in drug molecules with a sufficiently high permeability, solubility and dissolution rate, and that will automatically increase the importance of the BCS as a regulatory tool over time.

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.

Population-based mechanistic prediction of oral drug absorption

The bioavailability of drugs from oral formulations is influenced by many physiological factors including gastrointestinal fluid composition, pH and dynamics, transit and motility, and metabolism and transport, each of which may vary with age, gender, race, food, and disease. Therefore, oral bioavailability, particularly of poorly soluble and/or poorly permeable compounds and those that are extensively metabolized, often exhibits a high degree of inter- and intra-individual variability. While several models and algorithms have been developed to predict bioavailability in an average person, efforts to accommodate intrinsic variability in the component processes are less common. An approach that incorporates such variability for human populations within a mechanistic framework is described together with examples of its application to drug and formulation development.

Identification of intestinal loss of a drug through physiologically based pharmacokinetic simulation of plasma concentration-time profiles

BACKGROUND AND OBJECTIVE

Despite recent advances in understanding of the role of the gut as a metabolizing organ, recognition of gut wall metabolism and/or other factors contributing to intestinal loss of a compound has been a challenging task due to the lack of well characterized methods to distinguish it from first-pass hepatic extraction. The implications of identifying intestinal loss of a compound in drug discovery and development can be enormous. Physiologically based pharmacokinetic (PBPK) simulations of pharmacokinetic profiles provide a simple, reliable and cost-effective way to understand the mechanisms underlying pharmacokinetic processes. The purpose of this article is to demonstrate the application of PBPK simulations in bringing to light intestinal loss of orally administered drugs, using two example compounds: verapamil and an in-house compound that is no longer in development (referred to as compound A in this article).

METHODS

A generic PBPK model, built in-house using MATLAB software and incorporating absorption, metabolism, distribution, biliary and renal elimination models, was employed for simulation of concentration-time profiles. Modulation of intrinsic hepatic clearance and tissue distribution parameters in the generic PBPK model was done to achieve a good fit to the observed intravenous pharmacokinetic profiles of the compounds studied. These optimized clearance and distribution parameters are expected to be invariant across different routes of administration, as long as the kinetics are linear, and were therefore employed to simulate the oral profiles of the compounds. For compounds with reasonably good solubility and permeability, an area under the concentration-time curve for the simulated oral profile that far exceeded the observed would indicate some kind of loss in the intestine.

RESULTS

PBPK simulations applied to compound A showed substantial loss of the compound in the gastrointestinal tract in humans but not in rats. This accounted for the lower bioavailability of the compound in humans than in rats. PBPK simulations of verapamil identified gut wall metabolism, well established in the literature, and showed large interspecies differences with respect to both gut wall metabolism and drug-induced delays in gastric emptying.

CONCLUSIONS

Mechanistic insights provided by PBPK simulations can be very valuable in answering vital questions in drug discovery and development. However, such applications of PBPK models are limited by the lack of accurate inputs for clearance and distribution. This article demonstrates a successful application of PBPK simulations to identify and quantify intestinal loss of two model compounds in rats and humans. The limitation of inaccurate inputs for the clearance and distribution parameters was overcome by optimizing these parameters through fitting intravenous profiles. The study also demonstrated that the large interspecies differences associated with gut wall metabolism and gastric emptying, evident for the compounds studied, make animal model extrapolations to humans unreliable. It is therefore important to do PBPK simulations of human pharmacokinetic profiles to understand the relevance of intestinal loss of a compound in humans.

Intestinal permeability and its relevance for absorption and elimination

Human jejunal permeability (P(eff)) is determined in the intestinal region with the highest expression of carrier proteins and largest surface area. Intestinal P(eff) are often based on multiple parallel transport processes. Site-specific jejunal P(eff) cannot reflect the permeability along the intestinal tract, but they are useful for approximating the fraction oral dose absorbed. It seems like drugs with a jejunal P(eff) > 1.5 x 10(-4) cm s(-1) will be completely absorbed no matter which transport mechanism(s) are utilized. Many drugs that are significantly effluxed in vitro have a rapid and complete intestinal absorption (i.e. >85%) mediated by passive transcellular diffusion. The determined jejunal P(eff) for drugs transported mainly by absorptive carriers (such as peptide and amino acid transporters) will accurately predict the fraction of the dose absorbed as a consequence of the regional expression. The data also show that: (1) the human intestinal epithelium has a large resistance towards large and hydrophilic compounds; and (2) the paracellular route has a low contribution for compounds larger than approximately molecular weight 200. There is a need for more exploratory in vivo studies to clarify drug absorption and first-pass extraction along the intestine. One is encouraged to develop in vivo perfusion techniques for more distal parts of the gastrointestinal tract in humans. This would stimulate the development of more relevant and complex in vitro absorption models and form the basis for an accurate physiologically based pharmacokinetic modelling of oral drug absorption.

The release of GLP-1 and ghrelin, but not GIP and CCK, by glucose is dependent upon the length of small intestine exposed

Previous observations suggest that glucagon-like peptide-1 (GLP-1) is released into the bloodstream only when dietary carbohydrate enters the duodenum at rates that exceed the absorptive capacity of the proximal small intestine to contact GLP-1 bearing mucosa in more distal bowel. The aims of this study were to determine the effects of modifying the length of small intestine exposed to glucose on plasma concentrations of GLP-1 and also glucose-dependent insulinotropic peptide (GIP), insulin, cholecystokinin (CCK) and ghrelin, and antropyloric pressures. Glucose was infused at 3.5 kcal/min into the duodenum of eight healthy males (age 18-59 yr) over 60 min on the first day into an isolated 60-cm segment of the proximal small intestine ("short-segment infusion"); on the second day, the same amount of glucose was infused with access to the entire small intestine ("long-segment infusion"). Plasma GLP-1 increased and ghrelin decreased (P < 0.05 for both) during the long-, but not the short-, segment infusion. By contrast, increases in plasma CCK and GIP did not differ between days. The rises in blood glucose and plasma insulin were greater during the long- than during the short-segment infusion (P < 0.05). During the long- but not the short-segment infusion, antral pressure waves (PWs) were suppressed (P < 0.05). Isolated pyloric PWs and basal pyloric pressure were stimulated on both days. In conclusion, the release of GLP-1 and ghrelin, but not CCK and GIP, is dependent upon >60 cm of the intestine being exposed to glucose.

Innovative methods to study human intestinal drug metabolism in vitro: precision-cut slices compared with ussing chamber preparations

Predictive in vitro methods to investigate drug metabolism in the human intestine using intact tissue are of high importance. Therefore, we studied the metabolic activity of human small intestinal and colon slices and compared it with the metabolic activity of the same human intestinal segments using the Ussing chamber technique. The metabolic activity was evaluated using substrates to both phase I and phase II reactions: testosterone, 7-hydroxycoumarin (7HC), and a mixture of cytochrome P450 (P450) substrates (midazolam, diclofenac, coumarin, and bufuralol). In slices of human proximal jejunum, the metabolic activity of several P450-mediated and conjugation reactions remained constant up to4hof incubation. In the colon slices, conjugation rates were virtually equal to those in small intestine, whereas P450-mediated conversions occurred much slower. In both organs, morphological evaluation and ATP content implied tissue integrity within this period. P450 conversions using the Ussing chamber technique showed that the metabolic rate (sum of metabolites measured in apical, basolateral, and tissue compartments) was constant up to 3 h. For 7HC conjugations, the metabolic rate remained constant up to 4 h. The distribution of the metabolites in the compartments differed between the substrates. Overall, metabolic rates were surprisingly similar in both techniques and appear similar to or even higher than in liver. In conclusion, this study shows that both human intestinal precision-cut slices and Ussing chamber preparations provide useful tools for in vitro biotransformation studies.

Methods to evaluate biliary excretion of drugs in humans: an updated review

Determining the biliary clearance of drugs in humans is very challenging because bile is not readily accessible due to the anatomy of the hepatobiliary tract. The collection of bile usually is limited to postsurgical patients with underlying hepatobiliary disease. In healthy subjects, feces typically are used as a surrogate to quantify the amount of drug excreted via nonurinary pathways. Nevertheless, it is very important to characterize hepatobiliary elimination because this is a potential site of drug interactions that might result in significant alterations in systemic or hepatic exposure. In addition to the determination of in vivo biliary clearance values of drugs, the availability of in vitro models that can predict the extent of biliary excretion of drugs in humans may be a powerful tool in the preclinical stages of drug development. In this review, recent advances in the most commonly used in vivo methods to estimate biliary excretion of drugs in humans are outlined. Additionally, in vitro models that can be employed to investigate the molecular processes involved in biliary excretion are discussed to present an updated picture of the new tools and techniques that are available to study the complex processes involved in hepatic drug transport.

FIRST-PASS EFFECTS OF VERAPAMIL ON THE INTESTINAL ABSORPTION AND LIVER DISPOSITION OF FEXOFENADINE IN THE PORCINE MODEL

The aim of this study in pigs was to investigate the local pharmacokinetics of fexofenadine in the intestine and liver by using the pig as a model for drug transport in the entero-hepatobiliary system. A parallel group design included seven pigs (10-12 weeks, 22.2-29.5 kg) in three groups (G1, G2, G3), and a jejunal single-pass perfusion combined with sampling from the bile duct and the portal, hepatic, and superior caval veins was performed. Fexofenadine was perfused through the jejunal segment alone (G1: 120 mg/l, total dose 24 mg) or with two different verapamil doses (G2: 175 mg/l, total dose 35 mg; and G3: 1000 mg/l, total dose 200 mg). The animals were fully anesthetized and monitored throughout the experiment. Fexofenadine had a low liver extraction (E(H); mean +/- S.E.M.), and the given doses of verapamil did not affect the E(H) (0.13 +/- 0.04, 0.16 +/- 0.03, and 0.12 +/- 0.02 for G1, G2, and G3, respectively) or biliary clearance. The E(H) for verapamil and antipyrine agreed well with human in vivo data. Verapamil did not increase the intestinal absorption of fexofenadine, even though the jejunal permeability of fexofenadine, verapamil, and antipyrine showed a tendency to increase in G2. This combined perfusion and hepatobiliary sampling method showed that verapamil did not affect the transport of fexofenadine in the intestine or liver. In this model the E(H) values for both verapamil and antipyrine were similar to the corresponding values in vivo in humans.