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Intestinal Epithelium and Drug Transporters
Abstract
This chapter focuses on factors influencing regional epithelial permeation of drugs related especially to transporters. It highlights the important key messages as regard to modified‐release formulations. Drug absorption for ionizable drugs is determined by the pH in the region of intended release. Uptake and efflux transporters expressed in the gastrointestinal tract modulate the absorption of several drugs. Uptake transporters are more likely to act in synergy with other enzymes whose regional distribution is similar to CYP3A4, such as glucuronosyltransferases, sulfotransferases, and glutathione‐S‐transferases. The impact of the interplay between drug transport and metabolism on regional absorption can also be studied with perfusion techniques using cannulated in situ or isolated ex vivo intestinal regions in the rat. Drug–drug interactions can lead to changed systemic exposure, resulting in variations in drug response of the coadministered drug/s as well as safety concerns especially for low therapeutic index drugs.
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The release and absorption profile of an oral medication is influenced by the physicochemical properties of the drug and its formulation, as well as by the anatomy and physiology of the gastrointestinal (GI) tract. During drug development the bioavailability of a new drug is typically assessed in early clinical studies in a healthy adult population. However, many disease conditions are associated with an alteration of the anatomy and/or physiology of the GI tract. The same holds true for some subpopulations, such as paediatric or elderly patients, or populations with different ethnicity. The variation in GI tract conditions compared to healthy adults can directly affect the kinetics of drug absorption, and thus, safety and efficacy of an oral medication.
This review provides an overview of GI tract properties in special populations compared to healthy adults and discusses how drug absorption is affected by these conditions. Particular focus is directed towards non-disease dependent conditions (age, sex, ethnicity, genetic factors, obesity, pregnancy), GI diseases (ulcerative colitis and Crohn's disease, celiac disease, cancer in the GI tract, Roux-en-Y gastric bypass, lactose intolerance, Helicobacter pylori infection, and infectious diseases of the GI tract), as well as systemic diseases that change the GI tract conditions (cystic fibrosis, diabetes, Parkinson's disease, HIV enteropathy, and critical illness).
The current knowledge about GI conditions in special populations and their impact on drug absorption is still limited. Further research is required to improve confidence in pharmacokinetic predictions and dosing recommendations in the targeted patient population, and thus to ensure safe and effective drug therapies.
The levels of drug metabolizing enzymes (DMEs) and transporter proteins in the human intestine are pertinent to determine oral drug bioavailability. Despite the paucity of reports on such measurements, it is well recognised that these values are essential for translating in vitro data on drug metabolism and transport to predict drug disposition in gut wall. In the current study, clinically relevant DMEs (cytochrome P450 (CYP) and uridine 5'-diphospho-glucuronosyltransferase (UGT)) and drug transporters were quantified in crude membrane preparations from the human jejunum (n = 4) and ileum (n = 12) using QconCAT-based targeted proteomics. In contrast to previous reports, UGT2B15 and OATP1A2 were quantifiable in all our samples. Overall, no significant disparities in protein expression were observed between jejunum and ileum. Relative mRNA expression for drug transporters did not correlate with the abundance of their cognate protein except for P-gp and OST-α, highlighting the limitations of RNA as a surrogate for protein expression in dynamic tissues with high turnover. Inter-correlations were found within CYP (2C9-2C19 (p = 0.002, R2 = 0.63), 2C9-2J2 (p = 0.004, R2 = 0.40), 2D6-2J2 (p = 0.002, R2 = 0.50)) and UGT (1A1-2B7 (p = 0.02, R2 = 0.87)) family of enzymes. There were also correlations between P-gp and several other proteins (OST-α (p < 0.0001, R2 = 0.77), UGT1A6 (p = 0.009, R2 = 0.38) and CYP3A4 (p = 0.007, R2 = 0.30). Incorporating such correlations into building virtual populations is crucial for obtaining plausible characteristics of simulated individuals. SIGNIFICANCE STATEMENT: A number of drug transporters were quantified for the first time in this study. Several inter-correlations of protein abundance were reported. mRNA expression levels proved to be a poor reflection of differences between individuals regarding the level of protein expression in gut. The reported abundance of DMEs and transporters and their inter-correlations will contribute to better predictions of oral drug bioavailability and drugdrug interactions by linking in vitro observations to potential outcomes through physiologically-based pharmacokinetic models.
Targeted nanocarriers have shown great promise in drug delivery because of optimized drug behavior and improved therapeutic efficacy. How to improve the targeting efficiency of nanocarriers for the maximum possible drug delivery is a critical issue. Here we developed L-carnitine-conjugated nanoparticles targeting the carnitine transporter OCTN2 on enterocytes for improved oral absorption. As a variable, we introduced various lengths of the polyethylene glycol linker (0, 500, 1000, and 2000) between the nanoparticle surface and the ligand (CNP, C5NP, C10NP and C20NP) to improve the ligand flexibility, and consequently for more efficient interaction with the transporter, to enhance the oral delivery of the cargo load into cells. An increased absorption was observed in cellular uptake in vitro and in intestinal perfusion assay in situ when the polyethylene glycol was introduced to link L-carnitine to the nanoparticles; the highest absorption was achieved with C10NP. In contrast, the linker decreased the absorption efficiency in vivo. As the presence or absence of the mucus layer was the primary difference between in vitro/in situ versus in vivo, the presence of this layer was the likely reason for this differential effect. In summary, the size of the polyethylene glycol linker improved the absorption in vitro and in situ, but interfered with the absorption in vivo. Even though this strategy of increasing the ligand flexibility with the variable size of the polyethylene glycol failed to increase oral absorption in vivo, this approach is likely to be useful for enhanced cellular uptake following intravenous administration of the nanocarriers.
Objective:
Gastrointestinal adverse effects occur in 20-30% of patients with metformin-treated type 2 diabetes, leading to premature discontinuation in 5-10% of the cases. Gastrointestinal intolerance may reflect localized high concentrations of metformin in the gut. We hypothesized that reduced transport of metformin via the plasma membrane monoamine transporter (PMAT) and organic cation transporter 1 (OCT1) could increase the risk of severe gastrointestinal adverse effects.
Research design and methods:
The study included 286 severe metformin-intolerant and 1,128 metformin-tolerant individuals from the IMI DIRECT (Innovative Medicines Initiative: DIabetes REsearCh on patient straTification) consortium. We assessed the association of patient characteristics, concomitant medication, and the burden of mutations in the SLC29A4 and SLC22A1 genes on odds of intolerance.
Results:
Women (P < 0.001) and older people (P < 0.001) were more likely to develop metformin intolerance. Concomitant use of transporter-inhibiting drugs increased the odds of intolerance (odds ratio [OR] 1.72, P < 0.001). In an adjusted logistic regression model, the G allele at rs3889348 (SLC29A4) was associated with gastrointestinal intolerance (OR 1.34, P = 0.005). rs3889348 is the top cis-expression quantitative trait locus for SLC29A4 in gut tissue where carriers of the G allele had reduced expression. Homozygous carriers of the G allele treated with transporter-inhibiting drugs had more than three times higher odds of intolerance compared with carriers of no G allele and not treated with inhibiting drugs (OR 3.23, P < 0.001). Use of a genetic risk score derived from rs3889348 and SLC22A1 variants found that the odds of intolerance were more than twice as high in individuals who carry three or more risk alleles compared with those carrying none (OR 2.15, P = 0.01).
Conclusions:
These results suggest that intestinal metformin transporters and concomitant medications play an important role in the gastrointestinal adverse effects of metformin.
Microfluidic organ-on-a-chip models of human intestine have been developed and used to study intestinal physiology and pathophysiology. In this article, we review this field and describe how microfluidic Intestine Chips offer new capabilities not possible with conventional culture systems or organoid cultures, including the ability to analyze contributions of individual cellular, chemical, and physical control parameters one-at-a-time; to coculture human intestinal cells with commensal microbiome for extended times; and to create human-relevant disease models. We also discuss potential future applications of human Intestine Chips, including how they might be used for drug development and personalized medicine.
The human intestinal mucosa is a critical site for absorption, distribution, metabolism, and excretion (ADME)/Tox studies in drug development and is difficult to recapitulate in vitro. Using bioprinting, we generated three-dimensional (3D) intestinal tissue composed of human primary intestinal epithelial cells and myofibroblasts with architecture and function to model the native intestine. The 3D intestinal tissue demonstrates a polarized epithelium with tight junctions and specialized epithelial cell types and expresses functional and inducible CYP450 enzymes. The 3D intestinal tissues develop physiological barrier function, distinguish between high- and low-permeability compounds, and have functional P-gp and BCRP transporters. Biochemical and histological characterization demonstrate that 3D intestinal tissues can generate an injury response to compound-induced toxicity and inflammation. This model is compatible with existing preclinical assays and may be implemented as an additional bridge to clinical trials by enhancing safety and efficacy prediction in drug development.
Quantifying the multiple processes which control and modulate the extent of oral bioavailability for drug candidates is critical to accurate projection of human pharmacokinetics (PK). Understanding how gut wall metabolism and hepatic elimination factor into first-pass clearance of drugs has improved enormously. Typically, the cytochrome P450s, uridine 5′-diphosphate-glucuronosyltransferases and sulfotransferases, are the main enzyme classes responsible for drug metabolism. Knowledge of the isoforms functionally expressed within organs of first-pass clearance, their anatomical topology (e.g. zonal distribution), protein homology and relative abundances and how these differ across species is important for building models of human metabolic extraction. The focus of this manuscript is to explore the parameters influencing bioavailability and to consider how well these are predicted in human from animal models or from in vitro to in vivo extrapolation. A unique retrospective analysis of three AstraZeneca molecules progressed to first in human PK studies is used to highlight the impact that species differences in gut wall metabolism can have on predicted human PK. Compared to the liver, pharmaceutical research has further to go in terms of adopting a common approach for characterisation and quantitative prediction of intestinal metabolism. A broad strategy is needed to integrate assessment of intestinal metabolism in the context of typical DMPK activities ongoing within drug discovery programmes up until candidate drug nomination.
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.
Plasma membrane monoamine transporter (PMAT) is a major uptake2 monoamine transporter that shares extensive substrate and inhibitor overlap with organic cation transporters 1-3 (OCT1-3). Currently, there are no PMAT-specific inhibitors available that can be used in in vitro and in vivo studies to differentiate between PMAT and OCT activities. In this study, we showed that IDT307, a fluorescent analog of 1-methyl-4-phenylpyridinium (MPP+), is a transportable substrate for PMAT; and that IDT307-based fluorescence assay can be used to rapidly identify and characterize PMAT inhibitors. Using the fluorescent substrate-based assays, we analyzed the interactions of eight HIV protease inhibitors (PIs) with human PMAT and OCT1-3 in HEK293 cells stably transfected with individual transporters. Our data revealed that PMAT and OCTs exhibit distinct sensitivity and inhibition patterns towards HIV PIs. PMAT is most sensitive to PI inhibition whereas OCT2 and OCT3 are resistant. OCT1 showed an intermediate sensitivity and a distinct inhibition profile from PMAT. Importantly, lopinavir is a potent PMAT inhibitor and exhibited >120 fold selectivity towards PMAT (IC50 = 1.4 ± 0.2 μM) over OCT1 (IC50 = 174 ± 40 μM). Lopinavir has no inhibitory effect on OCT2 and OCT3 at maximal tested concentrations. Lopinavir also exhibited no or much weaker interactions with uptake1 monoamine transporters. Together, our results revealed that PMAT and OCTs have distinct specificity exemplified by their differential interaction with HIV PIs. Further, we demonstrate that lopinavir can be used as a selective PMAT inhibitor to differentiate PMAT-mediated monoamine and organic cation transport from those mediated by OCT1-3.
The American Society for Pharmacology and Experimental Therapeutics.
Recent studies in laboratory rodents have revealed that circadian oscillation in the physiological functions affecting drug disposition underlies the dosing time dependent change in the pharmacokinetics. However, it is difficult to predict the circadian change in the drug pharmacokinetics in diurnal human by using the data collected from nocturnal rodents. In this study, we used cynomolgus monkeys, a diurnal active animal, to evaluate the relevance of intestinal expression of P-glycoprotein (P-gp) to the dosing time-dependency of the pharmacokinetics of its substrates. The rhythmic phases of circadian gene expression in the suprachiasmatic nuclei (SCN; the mammalian circadian pacemaker) of cynomolgus monkeys were similar to those reported in nocturnal rodents. On the other hand, the mRNA expression of circadian clock genes and pharmacokinetic-related factors in the intestinal epithelial cells of monkeys oscillated at opposite phases in rodents. The oscillation in the intestinal expression of P-gp in monkeys was delayed by approximately 12 h relative to its mRNA rhythm. Furthermore, the intestinal absorption of P-gp substrates (quinidine and etoposide) was substantially suppressed by administering the drugs at the times of day when P-gp levels were abundant. By contrast, there was no significant dosing time-dependent difference in the absorption of the non-P-gp substrate (acetaminophen). The oscillation in the intestinal expression of P-gp appears to affect the pharmacokinetics of its substrates. Identification of circadian factors affecting the drug disposition in laboratory monkeys may improve the predictive accuracy of pharmacokinetics in humans.
The American Society for Pharmacology and Experimental Therapeutics.
Background and objective:
Accumulating evidence has shown that diabetes mellitus may affect the pharmacokinetics of some drugs, leading to alteration of pharmacodynamics and/or toxic effects. The aim of this study was to develop a novel physiologically based pharmacokinetic (PBPK) model for predicting drug pharmacokinetics in patients with type 2 diabetes mellitus quantitatively.
Methods:
Contributions of diabetes-induced alteration of physiological parameters including gastric emptying rates, intestinal transit time, drug metabolism in liver and kidney functions were incorporated into the model. Plasma concentration-time profiles and pharmacokinetic parameters of seven drugs (antipyrine, nisoldipine, repaglinide, glibenclamide, glimepiride, chlorzoxazone, and metformin) in non-diabetic and diabetic patients were predicted using the developed model. The PBPK model coupled with a Monte-Carlo simulation was also used to predict the means and variability of pharmacokinetic parameters.
Results:
The predicted area under the plasma concentration-time curve (AUC) and maximum (peak) concentration (C max) were reasonably consistent (<2-fold errors) with the reported values. Sensitivity analysis showed that gut transit time, hepatic enzyme activity, and renal function affected the pharmacokinetic characteristics of these drugs. Shortened gut transit time only decreased the AUC of controlled-released drugs and drugs with low absorption rates. Impairment of renal function markedly altered pharmacokinetics of drugs mainly eliminated via the kidneys.
Conclusion:
All of these results indicate that the developed PBPK model can quantitatively predict pharmacokinetic alterations induced by diabetes.
Typically, colonic absorption of a drug is mandatory for a sustained release formulation to hold the drug's plasma level for more than 12 or 24 h above the minimum therapeutic plasma concentration (efficacy). According to Drugs@FDA, only 7.4% of the oral drugs are extended release forms probably showing colonic absorption. Therefore an early determination of a drug's colonic absorption using the IntelliCap® in animals or humans will provide the mandatory information to initiate or stop a SR form development. Diltiazem (60 mg) is used in the oral swallowable IntelliCap® and the marketed SR form from Mylan (coated beads). A human study with 14 healthy volunteers compared the Mylan formulation with the IntelliCap® device that releases the drug identical to the in-vitro dissolution of the Mylan product. The plasma profiles of IntelliCap® and Mylan formulation are highly similar. The mean AUC (bioequivalence fulfilled) and mean Cmax of IntelliCap® shows only a difference of +15% and -12%, respectively. But the PK profile of the Mylan formulation shows a broader peak around Cmax. About 81.8% diltiazem was absorbed in the colon (IntelliCap®) comparable to former publications. The Mylan is a SR diffusion coated beads form whereas the IntelliCap® is a monolithic capsule. The beads are transported in the gut and spread which results in a longer Tmax and a broader Cmax peak. The IntelliCap® device can quantitatively measure the colonic absorption of a drug in excellent accordance to a standard oral SR dosage form.
Background & Aims
The circadian clock drives daily rhythms in behavior and physiology. A recent study suggests that intestinal permeability is also under control of the circadian clock. However, the precise mechanisms remain largely unknown. Because intestinal permeability depends on tight junction (TJ) that regulates the epithelial paracellular pathway, this study investigated whether the circadian clock regulates the expression levels of TJ proteins in the intestine.
Methods
The expression levels of TJ proteins in the large intestinal epithelium and colonic permeability were analyzed every 4, 6, or 12 hours between wild-type mice and mice with a mutation of a key clock gene Period2 (Per2; mPer2m/m). In addition, the susceptibility to dextran sodium sulfate (DSS)-induced colitis was compared between wild-type mice and mPer2m/m mice.
Results
The mRNA and protein expression levels of Occludin and Claudin-1 exhibited daily variations in the colonic epithelium in wild-type mice, whereas they were constitutively high in mPer2m/m mice. Colonic permeability in wild-type mice exhibited daily variations, which was inversely associated with the expression levels of Occludin and Claudin-1 proteins, whereas it was constitutively low in mPer2m/m mice. mPer2m/m mice were more resistant to the colonic injury induced by DSS than wild-type mice.
Conclusions
Occludin and Claudin-1 expressions in the large intestine are under the circadian control, which is associated with temporal regulation of colonic permeability and also susceptibility to colitis.
Many drugs prescribed to children are drug transporter substrates. Drug transporters are membrane-bound proteins that mediate the cellular uptake or efflux of drugs, and which are important to drug absorption and elimination. Very limited data are available on the effect of age on transporter expression. The aim of this study was to assess age-related gene expression of hepatic and intestinal drug transporters. MRP2, OATP1B1 and OATP1B3 expression was determined in postmortem liver samples (fetal n=6, neonatal n=19; infant n=7; child n=2; adult n=11) and MDR1 expression in 61 pediatric liver samples. Intestinal expression of MDR1, MRP2 and OATP2B1 was determined in surgical small bowel samples (neonates n=15, infants n=3, adults n=14). Using real time RT-PCR, fetal and pediatric gene expression was determined relative to 18S rRNA (liver) and villin (intestines), and compared to adults using the 2(-ΔΔCt) method. Hepatic expression of MRP2, OATP1B1 and OATP1B3 in all pediatric age groups was significantly lower than in adults. Hepatic MDR1 mRNA expression in fetuses, neonates and infants was significantly lower than in adults. Neonatal intestinal expressions of MDR1 and MRP2 were comparable to those in adults. Intestinal OATP2B1 expression in neonates was significantly higher than in adults. We provide new data that show organ- and transporter-dependent differences in hepatic and intestinal drug transporter expression in an age-dependent fashion. This suggests that substrate drug absorption mediated by these transporters may be subject to age-related variation in a transporter dependent pattern.
This study was conducted to compare the oral bioavailability of an itopride extended-release (ER) formulation with that of the reference immediate-release (IR) formulation in the fasting state. The effect of food on the bioavailability of itopride ER was also assessed.
A single-center, open-label, randomized, multiple-dose, three-treatment, three-sequence, crossover study was performed in 24 healthy male subjects, aged 22-48 years, who randomly received one of the following treatments for 4 days in each period: itopride 150 mg ER once daily under fasting or fed conditions, or itopride 50 mg IR three times daily in the fasting state. Steady-state pharmacokinetic parameters of itopride, including peak plasma concentration (Cmax) and area under the plasma concentration versus time curve over 24 hours after dosing (AUC0-24h), were determined by noncompartmental analysis. The geometric mean ratio of the pharmacokinetic parameters was derived using an analysis of variance model.
A total of 24 healthy Korean subjects participated, 23 of whom completed the study. The geometric mean ratio and its 90% confidence interval of once-daily ER itopride versus IR itopride three times a day for AUC0-24h were contained within the conventional bioequivalence range of 0.80-1.25 (0.94 [0.88-1.01]), although Cmax was reached more slowly and was lower for itopride ER than for the IR formulation. Food delayed the time taken to reach Cmax for itopride ER, but AUC0-24h was not affected. There were no serious adverse events and both formulations were generally well tolerated.
At steady state, once-daily itopride ER at 150 mg has a bioavailability comparable with that of itopride IR at 50 mg given three times a day under fasting conditions. Food delayed the absorption of itopride ER, with no marked change in its oral bioavailability.
Intestinal transporter proteins affect the oral bioavailability of many drugs in a significant manner. In order to estimate or predict their impact on oral drug absorption, data on their intestinal expression levels are needed. So far, predominantly mRNA expression data are available which are not necessarily correlated with the respective protein content. All available protein data were assessed by immunoblotting techniques such as Western blotting which both possess a number of limitations for reliable protein quantification. In contrast to this, mass spectrometry-based targeted proteomics may represent a promising alternative method to provide comprehensive protein expression data. In this review, we will summarize so far available intestinal mRNA and protein expression data for relevant human multidrug transporters. Moreover, recently observed mass spectrometry-based targeted proteomic data will be presented and discussed with respect to potential functional consequences. Associated to this, we will provide a short tutorial how to set up these methods and emphasize critical aspects in method development. Finally, potential limitations and pitfalls of this emerging technique will be discussed. From our perspective, LC-MS/MS-based targeted proteomics represents a valuable new method to comprehensively analyse the intestinal expression of transporter proteins. The resulting expression data are expected to improve our understanding about the intestinal processing of drugs.
Oral drug administration is one of the most preferred and simplest routes among both patients and formulation scientists. Nevertheless, orally delivery of some of the most widely used therapeutic agents (e.g., anticancer drugs, peptides, proteins and vaccines) is still a major challenge due to the limited oral bioavailability associated with them. The poor oral bioavailability of such drugs is attributed to one or many factors, such as poor aqueous solubility, poor permeability, and enzymatic degradation. Various technological strategies (such as permeation enhancers, prodrugs and nanocarriers) have been developed to enhance the bioavailability of these drugs after oral administration. Among the different approaches, advanced and innovative drug delivery systems, especially targeting-based strategies, have garnered tremendous attention. Furthermore, the presence of numerous types of cells and solute carrier transporters throughout the gastrointestinal tract represents numerous potential targeting sites for successful oral delivery that have not yet been exploited for their full potential. This review describes different targeting strategies towards different targeting sites in the gastrointestinal tract. Additionally, exciting improvements in oral drug delivery systems with different targeting strategies (e.g., M cells for oral vaccination and L cells for type 2 diabetes mellitus) are also discussed.
Introduction: After administration, a drug undergoes absorption, distribution, metabolism, and elimination (ADME) before exerting its effect on the body. The combination of these process yields the pharmacokinetic (PK) and pharmacodynamic (PD) profiles of a drug. Although accurate prediction of PK and PD profiles are essential for drug development, conventional in vitro models are limited by their lack of physiological relevance. Recently, microtechnology-based in vitro model systems, termed “organ-on-a-chip,” have emerged as a potential solution.
Areas covered: Orally administered drugs are absorbed through the intestinal wall and transported to the liver before entering systemic circulation, which plays important role in the PK and PD profiles. Recently developed, chip-based in vitro models can be useful models for simulating such processes and will be covered in this paper.
Expert opinion: The potential of intestine-on-a-chip models combined with conventional PK-PD modeling has been demonstrated with promising preliminary results. However, there are several challenges to overcome. Development of the intestinal wall, integration of the gut microbiome, and the provision of an intestine-specific environment must be achieved to realize in vivo-like intestinal model and enhance the efficiency of drug development.
The aim of this study was to derive region-specific transporter expression data suitable for in vitro-to-in vivo extrapolation (IVIVE) within a physiologically based pharmacokinetic (PBPK) modeling framework. A meta-analysis was performed whereby literary sources reporting region-specific transporter expression obtained via absolute and relative quantification approaches were considered in healthy adult Caucasian individuals. Furthermore, intestinal total membrane protein yield was calculated to enable mechanistic IVIVE via absolute transporter abundances. Where required, authors were contacted for additional information. A refined database was constructed where samples were excluded based on quantification in, non-Caucasian subjects, disease tissue, subjects <18 years old, duplicated samples, non-total membrane matrix, pooled matrices, or cDNA. Demographic data were collected where available. The weighted and geometric mean, coefficient of variation, and between-study homogeneity was calculated in each of eight gut segments (duodenum, two jejunum, four ileum, and colon) for 16 transporters. Expression data were normalized to that in the proximal jejunum. From a total of 47 articles, the final database consisted of 2238 measurements for 16 transporters. The solute carrier peptide transporter 1 (PepT1) showed the highest jejunal abundance, while multidrug resistance-associated protein (MRP) 2 was the highest abundance ATP-binding cassette transporter. Transporters displaying significant region-specific expression included the ileal bile acid transporter, which showed 18-fold greater terminal ileum expression compared with the proximal jejunum, while MRP3, organic cation transporter type 1 (OCTN1), and OCT1 showed >2-fold higher expression in other regions compared with the proximal jejunum. This is the first systematic analysis incorporating absolute quantification methodology to determine region-specific intestinal transporter expression. It is expected to be beneficial for mechanistic transporter IVIVE in healthy adult Caucasians. SIGNIFICANCE STATEMENT: Given the burgeoning reports of absolute transporter abundances in the human intestine, the incorporation of such information into mechanistic IVIVE-PBPK models could offer a distinct advantage in facilitating the robust assessment of the impact of gut transporters on drug disposition. The systematic and formal assessment via a literature meta-analysis described herein, enables assignment of the regional-specific expression, absolute transporter abundances, interindividual variability, and other associated scaling factors to healthy Caucasian populations within PBPK models. The resulting values are available to incorporate into PBPK models, and offer a verifiable account describing intestinal transporter expression within PBPK models for persons wishing to utilize them. Furthermore, these data facilitate the development of appropriate IVIVE scaling strategies using absolute transporter abundances.
This study aimed to prepare a self-microemulsifying drug delivery system loaded with myricitrin (M-SMEDDS) to improve the low oral bioavailability of myricitrin. Prepared M-SMEDDS consisted of an oil phase (ethyl oleate), a surfactant (Cremophor EL35) and a co-surfactant (dimethyl carbinol). M-SMEDDS was characterized using the particle size, zeta potential and encapsulation efficiency, while the pharmacokinetics studies were also investigated. The prepared M-SMEDDS exhibited stable physicochemical properties with a small average droplet size (21.68 ± 0.15 nm), negative zeta potential (−23.17 ± 1.03 mV) and high encapsulation efficiency (92.73%). The in vitro release study showed that myricitrin was significantly released from M-SMEDDS compared with the free myricitrin. The relative oral bioavailability of M-SMEDDS was 2.47-fold higher than that of the free drug. These results indicated that the preparation, in vitro and in vivo studies of M-SMEDDS could obviously improve the solubility and oral bioavailability of myricitrin. This study therefore provides preliminary evidence for further clinical research and application of M-SMEDDS.
Bioavailability of orally administered drugs is partly determined by function of drug transporters in the liver and intestine. We therefore explored ABC and SLC family transporters expression (qPCR) and protein abundance (LC‐MS/MS) in human liver and duodenum, jejunum, ileum and colon in paired tissue specimens from 9 organ donors. The transporter proteins were detected in the liver (P‐gp, MRP2, MRP3, BCRP, OATP1B1, OATP1B3, OATP2B1, OCT1, OCT3, OAT2, NTCP, MCT1, MATE1) and the intestine (P‐gp, MRP2, MRP3, MRP4, BCRP, OATP2B1, OCT1, ASBT (only ileum), MCT1, PEPT1). Significantly higher hepatic gene expression and protein abundance of ABCC2/MRP2, SLC22A1/OCT1 and SLCO2B1/OATP2B1 were found, as compared to all intestinal segments. No correlations between hepatic and small intestinal protein levels were observed. These observations provide a description of drug transporters distribution without the impact of inter‐individual variability bias, and may help in construction of superior PBPK and humanized animal models. This article is protected by copyright. All rights reserved.
There is a renewed interest from the pharmaceutical field to develop oral formulations of compounds such as peptides, oligonucleotides, and polar drugs. However, these often suffer from insufficient absorption across the intestinal mucosal barrier. One approach to circumvent this problem is the use of absorption modifying excipient(s) (AME). This study determined the absorption enhancing effect of four AMEs (sodium dodecyl sulfate, caprate, chitosan, N-acetylcysteine) on five model compounds in a rat jejunal perfusion model. The aim was to correlate the model compound absorption to the blood-to-lumen clearance of the mucosal marker for barrier integrity, ⁵¹Cr-EDTA. Sodium dodecyl sulfate and chitosan increased the absorption of the low permeation compounds but had no effect on the high permeation compound, ketoprofen. Caprate and N-acetylcysteine did not affect the absorption of any of the model compounds. The increase in absorption of the model compounds was highly correlated to an increased blood-to-lumen clearance of ⁵¹Cr-EDTA, independent of the AME. Thus, ⁵¹Cr-EDTA could be used as a general, sensitive, and validated marker molecule for absorption enhancement when developing novel formulations.
The Ussing chamber is an old but still powerful technique originally designed to study the vectorial transport of ions through frog skin. This technique is also used to investigate the transport of chemical agents through the intestinal barrier as well as drug metabolism in enterocytes, both of which are key determinants for the bioavailability of orally administered drugs. More contemporary model systems, such as Caco-2 cell monolayers or stably transfected cells, are more limited in their use compared to the Ussing chamber because of differences in expression rates of transporter proteins and/or metabolizing enzymes. While there are limitations to the Ussing chamber assay, the use of human intestinal tissue remains the best laboratory test for characterizing the transport and metabolism of compounds following oral administration. Detailed in this unit is a step-by-step protocol for preparing human intestinal tissue, for designing Ussing chamber experiments, and for analyzing and interpreting the findings.
The intestinal epithelium is the most rapidly renewed tissue in adult mammals, and its renewal is strictly controlled by intestinal stem cells. Extensive studies using genetic models of intestinal epithelium have revealed the mechanisms underlying the self-renewal of intestinal stem cells. Exploiting this knowledge, we developed a novel 3D culture system that enables the outgrowth of intestinal Lgr5+ stem cells derived from mouse and human tissues into ever-expanding crypt–villus mini-guts, known as intestinal epithelial organoids. These organoids are maintained by the self-renewal of stem cells and give rise to all differentiated cell types of the intestinal epithelium. Once established, organoids can be cryopreserved and thawed when needed. This culture system has been widely used for studying stem cell behavior and gene function and for disease modeling.
In recent years, an increasing number of clinical drug-drug interactions (DDIs) have been attributed to inhibition of intestinal organic anion transporting polypeptides (OATPs); however, only a few of these DDI results were reflected in drug labels. This review aims to provide a thorough analysis of intestinal OATP-mediated pharmacokinetic-based DDIs, using both in vitro and clinical investigations, highlighting the main mechanistic findings and discussing their clinical relevance. On the basis of pharmacogenetic and clinical DDI results, a total of 12 drugs were identified as possible clinical substrates of OATP2B1 and/or OATP1A2. Among them, three drugs, namely atenolol, celiprolol, and fexofenadine have emerged as the most sensitive substrates to evaluate clinical OATP-mediated intestinal DDIs when interactions with P-glycoprotein (P-gp) by the test compound can be ruled out. With regard to perpetrators, eight dietary or natural products and one investigational drug, ronacaleret (now terminated), showed clinical intestinal inhibition attributable to OATPs, producing ≥ 20% decreases in AUC of the co-administered drug. Common juices such as apple juice, grapefruit juice, and orange juice are considered potent inhibitors of intestinal OATP2B1 and/or OATP1A2 (decreasing exposure of the co-administered substrate by approximately 85%), and may be adequate prototype inhibitors to investigate intestinal DDIs mediated by OATPs.
Orally administered drugs must pass through the intestinal wall and then through the liver before reaching systemic circulation. During this process drugs are subjected to different processes that may determine the therapeutic value. The intestinal barrier with active drug metabolizing enzymes and drug transporters in enterocytes plays an important role in the determination of drug bioavailability. Accumulating information demonstrates variable distribution of drug metabolizing enzymes and transporters along the human gastrointestinal tract (GI), that creates specific barrier characteristics in different segments of the GI. In this review, expression of drug metabolizing enzymes and transporters in the healthy and diseased human GI as well as their regulatory aspects: genetic, miRNA, DNA methylation are outlined. The knowledge of unique interplay between drug metabolizing enzymes and transporters in specific segments of the GI tract allows more precise definition of drug release sites within the GI in order to assure more complete bioavailability and prediction of drug interactions.
Although oral drug administration is nowadays the favourable route of administration, intestinal drug absorption is challenged by several highly variable and poorly predictable processes such as gastrointestinal motility, intestinal drug solubility and intestinal metabolism. One further determinant that have been identified and characterized during the last two decades is the intestinal drug transport that is mediated by several transmembrane proteins such as P-gp, BCRP, PEPT1 and OATP2B1. It is so far well-established that intestinal transporters can affect oral absorption of many drugs in a significant manner either by facilitating their cellular uptake or by pumping them back to gut lumen which limits their oral bioavailability. Their functional relevance becomes even more apparent in cases of unwanted drug-drug interactions when concomitantly given drugs cause transporter induction or inhibition which in turn leads to increased or decreased drug exposure. The longitudinal expression of several intestinal transporters is not homogeneous along the human intestine which may has functional implications on the preferable site of intestinal drug absorption. Besides the knowledge about the expression of pharmacologically relevant transporters in human intestinal tissue, also their exact localization on the apical or basolateral membrane of enterocytes is of interest but in several cases debatable. Finally, there is obviously a coordinative interplay of intestinal transporters (apical - basolateral), intestinal enzymes and transporter as well as intestinal and hepatic transporters. This review aims to give an updated overview about the expression, localization, regulation and function of clinically relevant transporter proteins in the human intestine.
Although carnitine is present in a variety of foods, the mechanism of its absorption has not been previously studied in humans. We investigated the absorption of carnitine by studying uptake into human intestinal mucosal biopsy specimens. We found evidence of active transport in the duodenum and ileum, but not in the colon. We demonstrated that intracellular concentrations exceeded concentrations in the incubation media at steady states and that uptake against a concentration gradient was abolished by anoxia and by replacement of sodium ion with potassium. Studies of initial rate of uptake over a range of concentrations revealed a curve consistent with a two-component system: a saturable system with a kt of 558 μM and a linear component probably representing passive diffusion. Addition of d-carnitine and l-acetylcarnitine resulted in diminished uptake of l-carnitine, suggesting that these substrates utilize the same transport mechanism. These studies demonstrate the presence of an active intestinal transport system for l-carnitine in human intestinal mucosa.
Over the last 5 years the quantification of transporter protein absolute abundances has dramatically increased in parallel to the expanded use of in vitro - in vivo extrapolation (IVIVE) and physiologically-based pharmacokinetics (PBPK) linked models, for decision making in pharmaceutical company drug development pipelines and regulatory submissions. Although several research groups have developed laboratory-specific proteomic workflows, it is unclear if the large range of reported variability is founded on true inter-individual variability or experimental variability, due to sample preparation, or the proteomic methodology used. To assess the potential for methodological bias on end-point abundance quantification, two independent laboratories, the University of Manchester (UoM) and Bertin Pharma (BPh), employing different proteomic workflows, quantified the absolute abundances of Na/K-ATPase, P-gp and BCRP in the same set of biological samples from human intestinal and Caco-2 cell membranes. Across all samples, P-gp abundances were significantly correlated (p = 0.04, rs = 0.72) with a 2.4-fold higher abundance (p = 0.001) generated at the UoM compared to BPh. There was a systematically higher BCRP abundance in Caco-2 cell samples quantified by BPh compared to UoM, but not in human intestinal samples. Consequently, a similar intestinal relative expression factor (REF), based on distal jejunum and Caco-2 monolayer samples, between laboratories was found for P-gp. However, a 2-fold higher intestinal REF was generated by the UoM (2.22) versus BPh (1.11). We demonstrate that differences in absolute protein abundance are evident between laboratories and those are likely to be founded on laboratory-specific methodologies relating to peptide choice.
The aim of the present study was to investigate the influence of molecular mass and thiol group content of poly(acrylic acid)-cysteine conjugates on the permeation of sulforhodamine 101 and penicillin G. acting as substrates for multidrug resistance-associated protein 2 efflux pump. Poly(acrylic acids) of 2 kDa, 100 kDa, 250 kDa, 450 kDa and 3000 kDa were conjugated with cysteine. The thiol group content of all these polymers was in the range of 343.3±48.4μmol/g to 450.3±76.1μmol/g. Transport studies were performed on rat small intestine mounted in Ussing-type chambers. Since 250 kDa poly(acrylic acid) showed the highest permeation enhancing effect, additionally thiolated 250 kDa polyacrylates displaying 157.2μmol/g, 223.0±18.1 and 355.9μmol/g thiol groups were synthesized in order to investigate the influence of thiol group content on the permeation enhancement. The permeation of sulforhodamine was 3.93- and 3.85- fold improved using 250 kDa poly(acrylic acid)-cysteine conjugate exhibiting 355.9±39.5μmol/g and 223.0±18.1μmol/g thiol groups. Using the same conjugates the permeation of penicilin G was 1.70- and 1.59- fold improved, respectively. The study demonstrates that thiolated poly(acrylic acid) inhibits Mrp2 mediated transport and that the extent of inhibition depends on the molecular mass and degree of thiolation of the polymer.
Copyright © 2015 Elsevier B.V. All rights reserved.
Introduction:
The incidence of pediatric celiac disease has risen and many of these children will receive medications at some time in their life. However, the absorption of drugs in pediatric patients with celiac disease has never been studied. The few studies that do exist have only been performed in adults and indicate that drug concentrations can be altered for some drugs. It is also noteworthy that few researchers have conducted studies to determine if the distribution, metabolism, and excretion of drugs are altered in celiac disease. Areas covered: The pharmacokinetics of drugs greatly differ between children and adults. Combined with the pathophysiological changes known to occur with celiac disease, there is compelling evidence to support that drug exposure in pediatric celiac disease may be altered. Relevant characteristics of celiac disease that may affect drug disposition include intestinal atrophy, hypoalbuminemia, reduced CYP3A enzymes, and thyroid dysfunction. Expert opinion: The safety and efficacy of drug dosing in children with celiac disease can be enhanced with additional pharmacokinetic studies of commonly prescribed drugs in this population. Ideally, these studies should include drugs that have high bioavailability, are highly protein bound, undergo extensive CYP3A enzyme metabolism, and/or have a narrow therapeutic range.
Our understanding of dysfunction of the gastrointestinal system in patients with Parkinson's disease has increased substantially in the past decade. The entire gastrointestinal tract is affected in these patients, causing complications that range from oral issues, including drooling and swallowing problems, to delays in gastric emptying and constipation. Additionally, small intestinal bacterial overgrowth and Helicobacter pylori infection affect motor fluctuations by interfering with the absorption of antiparkinsonian drugs. The multifaceted role of the gastrointestinal system in Parkinson's disease necessitates a specific and detailed assessment and treatment plan. The presence of pervasive α-synuclein deposition in the gastrointestinal tract strongly implicates this system in the pathogenesis of Parkinson's disease. Future studies elucidating the role of the gastrointestinal tract in the pathological progression of Parkinson's disease might hold potential for early disease detection and development of neuroprotective approaches.
Copyright © 2015 Elsevier Ltd. All rights reserved.
One of the major factors affecting oral drug bioavailability is the activity of intestinal transport proteins, particularly for the drugs that undergo absorption by active transport mechanism. Many of the active pharmacological agents and the excipients used in their formulation are reported to modulate the activity of these transporters thereby either enhancing or decreasing the drug absorption and its systemic availability. These excipients are considered pharmacologically "inert" and have been used since years in pharmaceutical formulations. Appreciable interest is developing on the data demonstrating the role of excipients in altering the drug absorption across the intestine. Careful selection of the excipients thus is very important. A correctly chosen excipient can enhance the drug bioavailability and thus its therapeutic efficacy without increasing its dose. For locally acting drugs having systemic side effects, a proper excipient could lead to a decrease in its systemic absorption, thus reducing its side effects. This review focuses on the current findings of the excipients identified to modulate the activity of transporters, their mechanism of modulating the transporter′s activity and various formulation strategies using these excipients to enhance drug absorption.
The abundance and function of transporter proteins at the plasma membrane is likely to be crucial in drug responsiveness. The detection of Human Concentrative Nucleoside Transporters (hCNTs) function is of interest to predict drug sensitivity due to their ability to transport most nucleoside-derived drugs. In the present study two fluorescent nucleoside analogues, Uridine-furan and Etheno-cytidine, were evaluated as tools to study in vivo nucleoside transporter-related functions. These two molecules showed high affinity interactions with hCNT1 and hCNT3 being also hCNT substrates. Both fluorescent microscopy and flux cytometry experiments showed that Uridine-furan uptake was better suited for distinguishing cells that express or not hCNT1 or hCNT3. These data highlight the usefulness of fluorescent nucleoside derivatives as long as they fulfil the requirements of confocal microscopy and flow cytometry for in vivo analysis of hCNT-related function.
The purpose of the present study was to evaluate and compare the absolute protein expression levels of 28 drug-related transporters in Caco-2 cell monolayers cultured for 2, 3, and 4 weeks. Plasma membrane fractions of Caco-2 cells cultured on Transwell inserts for 2, 3 and 4 weeks were prepared and digested with trypsin, and then simultaneous absolute quantification of 28 transporters and Na(+)/K(+)-ATPase was conducted using our established quantitative targeted absolute proteomic technique. Nine transporters and Na(+)/K(+)-ATPase were detected. MDR1, BCRP, PEPT1, OSTα and OSTβ were highly expressed (greater than 1 fmol/μg protein), while MRP2, MRP4, OATP2B1 and MCT1 were moderately expressed (0.328-0.871 fmol/μg protein). No significant difference was observed in the protein expression levels of these transporters or Na(+)/K(+)-ATPase among the 2-, 3- and 4-week cultures. The other 19 transporters, including MRP1, MRP3, OATP1A2, OATP3A1, OATP4A1, and OATP1B3, were not detected. This information about the rank order of transporter protein expression will be useful to predict what transporter(s) are likely or unlikely to influence the permeability of various compounds across monolayers of Caco-2 cells, which are widely used in drug development studies.
Copyright © 2014 The Japanese Society for the Study of Xenobiotics. Published by Elsevier Ltd. All rights reserved.
The efflux transporter, P-glycoprotein (P-gp), located in the apical membranes of intestinal absorptive cells, can reduce the bioavailability of a wide range of orally administered drugs. A number of surfactants/excipients have been shown to inhibit P-gp, and thus potentially enhance drug absorption. In this study, the improved everted gut sac technique was used to screen excipients for their ability to enhance the absorption of digoxin and celiprolol in vitro. The most effective excipients with digoxin were (at 0.5%, w/v): Labrasol > Imwitor 742 > Acconon E = Softigen 767 > Cremophor EL > Miglyol > Solutol HS 15 > Sucrose monolaurate > Polysorbate 20 > TPGS > Polysorbate 80. With celiprolol, Cremophor EL and Acconon E had no effect, but transport was enhanced by Softigen 767 > TPGS > Imwitor 742. In vivo, the excipients changed the pharmacokinetic profile of orally administered digoxin or celiprolol, but without increasing the overall AUC. The most consistent change was an early peak of absorption, probably due to the higher concentration of excipient in the proximal intestine where the expression of P-gp is lower. These studies show that many excipients/surfactants can modify the pharmacokinetics of orally administered drugs that are P-gp substrates.
Intestinal transporters are crucial determinants in the oral absorption of drugs. We therefore studied mRNA expression (N=33) and absolute protein content (N=10) of clinically relevant transporters in healthy epithelium of the duodenum, proximal and distal jejunum and ileum and the ascending, transversal, descending and sigmoidal colon of six organ donors (24-54 years). In the small intestine, the abundance of nearly all studied proteins ranged between 0.2-1.6 pmol/mg with the exception of OCT3 (<0.1 pmol/mg) and PEPT1 (2.6-4.9 pmol/mg) which accounted for ~50% of all measured transporters. OATP1A2 was not detected in any intestinal segment. ABCB1, ABCG2, PEPT1 and ASBT were significantly higher abundant in jejunum/ileum than in colon. In contrast to this, ABCC2, ABCC3 and OCT3 expression was found to be highest in colon. Site-dependent expression was confirmed for ABCB1 and ASBT and significant correlations between mRNA and protein levels for ABCG2, ASBT, OCT3 and PEPT1 in the small intestine. Our data provide further physiological pieces in the puzzle required to predict intestinal drug absorption in man.
This review summarizes the current knowledge on anatomy and physiology of the human gastrointestinal tract in comparison with that of common laboratory animals (dog, pig, rat and mouse) with emphasis on in vivo methods for testing and prediction of oral dosage form performance. A wide range of factors and methods are considered in addition, such as imaging methods, perfusion models, models for predicting segmental/regional absorption, in vitro in vivo correlations as well as models to investigate the effects of excipients and the role of food on drug absorption. One goal of the authors was to clearly identify the gaps in todays knowledge in order to stimulate further work on refining the existing in vivo models and demonstrate their usefulness in drug formulation and product performance testing.
Drug absorption from the gastrointestinal (GI) tract is a highly complex process dependent upon numerous factors including the physicochemical properties of the drug, characteristics of the formulation and interplay with the underlying physiological properties of the GI tract. The ability to accurately predict oral drug absorption during drug product development is becoming more relevant given the current challenges facing the pharmaceutical industry. Physiologically-based pharmacokinetic (PBPK) modeling provides an approach that enables the plasma concentration-time profiles to be predicted from preclinical in vitro and in vivo data and can thus provide a valuable resource to support decisions at various stages of the drug development process. Whilst there have been quite a few successes with PBPK models identifying key issues in the development of new drugs in vivo, there are still many aspects that need to be addressed in order to maximize the utility of the PBPK models to predict drug absorption, including improving our understanding of conditions in the lower small intestine and colon, taking the influence of disease on GI physiology into account and further exploring the reasons behind population variability. Importantly, there is also a need to create more appropriate in vitro models for testing dosage form performance and to streamline data input from these into the PBPK models. As part of the Oral Biopharmaceutical Tools (OrBiTo) project, this review provides a summary of the current status of PBPK models available. The current challenges in PBPK set-ups for oral drug absorption including the composition of GI luminal contents, transit and hydrodynamics, permeability and intestinal wall metabolism are discussed in detail. Further, the challenges regarding the appropriate integration of results from in vitro models, such as consideration of appropriate integration/estimation of solubility and the complexity of the in vitro release and precipitation data, are also highlighted as important steps to advancing the application of PBPK models in drug development. It is expected that the "innovative" integration of in vitro data from more appropriate in vitro models and the enhancement of the GI physiology component of PBPK models, arising from the OrBiTo project, will lead to a significant enhancement in the ability of PBPK models to successfully predict oral drug absorption and advance their role in preclinical and clinical development, as well as for regulatory applications.
Sarpogrelate is a selective 5-hydroxytryptamine receptor subtype 2A antagonist that inhibits platelet aggregation and vasoconstriction. The aim of this study was to compare the pharmacokinetics of a sarpogrelate controlled-release formulation (CR) with those of the immediate-release formulation (IR). The effect of food on the pharmacokinetics of CR sarpogrelate was also evaluated.
A randomized, open-label, 3-period, 3-treatment crossover study was conducted in 50 healthy male subjects. Subjects were allocated into one of six sequence groups. In one period, a 100-mg IR formulation was administered three times at 6-h intervals, and in the other two periods, a 300-mg CR formulation was administered once to fasting and once to fed subjects. Each period was separated by a 7-day washout period. Serial blood samples were collected up to 24 h after the first drug administration in each period. The plasma concentrations of sarpogrelate were analysed by liquid chromatography-tandem mass spectrometry. Pharmacokinetic parameters were calculated by non-compartmental methods.
After the administration of the IR formulation, the plasma concentration reached a peak at 0·48 h and the drug was eliminated with a half-life (t1/2 ) of 0·7 h. After administration of the CR formulation, the plasma concentration reached a peak at 0·5 h and the drug was eliminated with a t1/2 of 3·23 h. The geometric mean ratios (CR/IR) for sarpogrelate area under the plasma concentration-time curve (AUC) and the maximum plasma drug concentration (Cmax) were 1·2040 (90% confidence interval (CI): 1·0992-1·3188) and 0·9462 (90% CI: 0·8504-1·0529). When CR was administered to fed subjects, the time to peak concentration was prolonged to 3·97 h and t1/2 was shortened to 1·45 h. The geometric mean ratios (fasting/fed) for sarpogrelate AUC and Cmax were 0·8573 (90% CI: 0·7687-0·9561) and 0·6452 (90% CI: 0·5671-0·7341).
After the administration of CR and IR formulations of the same daily dose of sarpogrelate hydrochloride, the overall systemic exposure was slightly higher for the CR than for the IR formulation, whereas peak concentration was comparable between the two formulations. Food reduced the bioavailability of sarpogrelate CR.
Published evidence on established and theorized effects of celiac disease on drug absorption and pharmacokinetics is reviewed.
Patients with celiac disease develop a variety of gastric disorders requiring oral medications, but the impact of damage to intestinal villi and other celiac disease sequelae on drug absorption remains poorly understood. A review of the pertinent literature (English-language articles on research in adults published during the period 1970-August 2012) identified several reports of altered drug absorption mechanisms in patients with celiac disease, including accelerated or delayed gastric emptying, increased permeability of jejunal mucosa, changes in intraluminal pH, decreased intestinal surface area, and reduced intestinal cytochrome P-450 enzymes. A small number of published studies suggest that celiac disease may be associated with altered drug absorption, resulting in higher serum concentrations of propranolol, lower peak concentrations of acetaminophen and practolol, higher dosing requirements with levothyroxine, impaired or delayed absorption of certain antibiotics, and other pharmacokinetic effects with a potential impact on medication efficacy and toxicity. However, these studies involved very small patient samples and were poorly controlled, with some yielding contradictory results. More and larger pharmacokinetic studies in patients with celiac disease-especially studies of drugs that are dosed empirically or are not amenable to dosage adjustment according to vital signs or laboratory values-are needed.
Given the sometimes conflicting data on drug absorption in the context of celiac disease, cautious medication selection, dosage adjustment, and monitoring for efficacy and potential adverse effects are advised.
In this review, the gross physiology of the gastrointestinal tract of dogs is compared with that of humans, particularly as it pertains to drug absorption and dosage-form performance. Gastrointestinal (GI) motility and pH are the main parameters considered. Although similar motility patterns and pH profiles prevail in the two species for the most part, there are some differences that could affect the time profile and extent of drug absorption. These include slower gastric emptying in the fed state, faster small intestine transit, and higher and more variable intestinal pH in dogs compared with humans. An attempt is made to identify drug and dosage-form properties that would lead to differences in drug absorption in the two species, e.g., drug physicochemical properties, dosage-form size, and pH dependency of dosage-form release characteristics.
The focus of this review is on the pharmaceutical relevance of the intestinal peptide transporter PepT1. The review is limited to the progress made in the field over the past two years. Much of this progress is being driven by the prevailing view that PepT1 can be used for drug delivery purposes. Studies have indeed shown that several drugs, prodrugs and drug candidates gain entry into the systemic circulation via PepT1. Very recent examples are prodrugs of zanamivir, oseltamivir and didanosine.
Resveratrol exhibits a variety of biological and pharmacological activities despite its extensive metabolism to sulfates and glucuronides in the intestine and liver. The metabolism of resveratrol is cell specific and strongly correlates with enzyme expression levels. However, a high rate of biotransformation, in concert with the action of the efflux transporters MRP2, MRP3, and ABCG2, reduces intracellular resveratrol concentrations, and may thereby decrease its pharmacological activity. Interestingly, biotransformation is also dependent on disease status. For example, significantly greater sulfation of resveratrol occurs in human breast tumor tissue than in adjacent nonmalignant tissue. The observed differences, however, do not correlate with the expression of sulfotransferases responsible for catalyzing resveratrol sulfation, but rather with significantly higher steroid sulfatase mRNA levels. The in vitro activity of resveratrol sulfates may not necessarily reflect their in vivo function, given the fact that ubiquitously existing human sulfatases can convert the metabolites back to active resveratrol in humans.
The rate-limiting step to absorption of drugs from the gastrointestinal (GI) tract is often dissolution from the dosage form. Consideration of the Noyes-Whitney dissolution model shows that drug diffusivity, solubility in the gastrointestinal contents, the surface area of the solid wetted by the lumenal fluids and the GI hydrodynamics all play a role in determining the in vivo dissolution rate. Solubility in the GI contents is determined by aqueous solubility, crystalline form, drug lipophilicity, solubilization by native surfactants and co-ingested foodstuffs, and pKa in relation to the GI pH profile. Compounds with aqueous solubilities lower than 100 μg/ml often present dissolution limitations to absorption. The dose:solubility ratio of the drug provides an estimate of the volume of fluids required to dissolve an individual dose, and when this volume exceeds 1 l, dissolution is often problematic. The surface area of a drug available for dissolution depends on the particle size of the solid and its ability to be wetted by lumenal fluids. Other physiological factors that can play a role in dissolution include the viscosity of the lumenal contents, through its effect on the diffusivity, and mixing and flow patterns within the gut. In order to better predict in vivo dissolution of drugs, dissolution tests which more adequately simulate the physiological conditions are needed.