Dog Colonoscopy Model for Predicting Human Colon Absorption

Development of a canine artificial colonic mucus model for drug diffusion studies

Colonic mucus is a key factor in the colonic environment because it may affect drug absorption. Due to the similarity of human and canine gastrointestinal physiology, dogs are an established preclinical species for the assessment of controlled release formulations. Here we report the development of an artificial colonic mucus model to mimic the native canine one. In vitro models of the canine colonic environment can provide insights for early stages of drug development and contribute to the implementation of the 3Rs (refinement, reduction, and replacement) of animal usage in the drug development process. Our artificial colonic mucus could predict diffusion trends observed in native mucus and was successfully implemented in microscopic and macroscopic assays to study macromolecular permeation through the mucus. The traditional Transwell set up was optimized with the addition of a nylon filter to ensure homogenous representation of the mucus barrier in vitro. In conclusion, the canine artificial colonic mucus can be used to study drug permeation across the mucus and its flexibility allows its use in various set ups depending on the nature of the compound under investigation and equipment availability.

Challenges in Permeability Assessment for Oral Drug Product Development

Drug permeation across the intestinal epithelium is a prerequisite for successful oral drug delivery. The increased interest in oral administration of peptides, as well as poorly soluble and poorly permeable compounds such as drugs for targeted protein degradation, have made permeability a key parameter in oral drug product development. This review describes the various in vitro, in silico and in vivo methodologies that are applied to determine drug permeability in the human gastrointestinal tract and identifies how they are applied in the different stages of drug development. The various methods used to predict, estimate or measure permeability values, ranging from in silico and in vitro methods all the way to studies in animals and humans, are discussed with regard to their advantages, limitations and applications. A special focus is put on novel techniques such as computational approaches, gut-on-chip models and human tissue-based models, where significant progress has been made in the last few years. In addition, the impact of permeability estimations on PK predictions in PBPK modeling, the degree to which excipients can affect drug permeability in clinical studies and the requirements for colonic drug absorption are addressed.

An overview of in vitro, ex vivo and in vivo models for studying the transport of drugs across intestinal barriers

Oral administration is the most commonly used route for drug delivery owing to its cost-effectiveness, ease of administration, and high patient compliance. However, the absorption of orally delivered compounds is a complex process that greatly depends on the interplay between the characteristics of the drug/formulation and the gastrointestinal tract. In this contribution, we review the different preclinical models (in vitro, ex vivo and in vivo) from their development to application for studying the transport of drugs across intestinal barriers. This review also discusses the advantages and disadvantages of each model. Furthermore, the authors have reviewed the selection and validation of these models and how the limitations of the models can be addressed in future investigations. The correlation and predictability of the intestinal transport data from the preclinical models and human data are also explored. With the increasing popularity and prevalence of orally delivered drugs/formulations, sophisticated preclinical models with higher predictive capacity for absorption of oral formulations used in clinical studies will be needed.

Physiologically Based Biopharmaceutics Modeling of Regional and Colon Absorption in Dogs

Colon absorption is a key determinant for the successful development of modified-release (MR) formulations, and the risk that colon absorption may limit the in vivo performance of an MR product can be assessed early by various in vitro tests or by preclinical in vivo regional absorption studies in dogs. Mechanistic physiologically based biopharmaceutics modeling (PBBM) is becoming increasingly accepted to predict in vivo performance and guide formulation development; however, no evaluation of the ability to predict colon absorption has been performed. The purpose of this study was to investigate if regional and colon absorption of drugs in dogs could be predicted with sufficient accuracy using PBBM to enable the replacement of in vivo dog studies in the early assessment of colon absorption limitation risks. This was done by predicting the regional and colon absorption and plasma exposure of 14 drugs after administration to the dog colon according to an a priori approach using the in silico absorption models GI-Sim and GastroPlus. Predictive performance was primarily assessed by comparing observed and predicted plasma concentration-time profiles, AUC_0-t, and the relative bioavailability in the colon (F_rel,colon) as compared to an oral/duodenal reference. Trends in dependency of prediction performance on predicted fraction absorbed, permeability, and solubility/dissolution rate were also investigated. For GI-Sim, the absolute average fold error (AAFE) values for AUC_0-t and F_rel,colon were within a 2-fold prediction error for both solutions (1.88 and 1.51, respectively) and suspensions (1.58 and 1.99, respectively). For GastroPlus, the AAFE values for AUC_0-t and F_rel,colon were outside the set 2-fold prediction error limit for accurate predictions for both solutions (3.63 and 2.98, respectively) and suspensions (2.94 and 2.09, respectively). No trends for over- or underprediction were observed for GI-Sim, whereas GastroPlus showed a slight trend for underprediction of both AUC_0-t and F_rel,colon for compounds with low permeability. In addition, regional differences in the plasma profiles were qualitatively predicted in the majority of cases for both software. Despite the differences in prediction performance, both models can be considered to predict regional differences in absorption as well as AUC_0-t and F_rel,colon with acceptable accuracy in an early development setting. The results of this study indicate that it is acceptable to replace in vivo regional absorption studies in dogs with the evaluated models as a method for the early assessment of the risk for colon absorption limitation of MR drug product candidates.

Regional Intestinal Drug Permeability and Effects of Permeation Enhancers in Rat

Sufficient colonic absorption is necessary for all systemically acting drugs in dosage forms that release the drug in the large intestine. Preclinically, colonic absorption is often investigated using the rat single-pass intestinal perfusion model. This model can determine intestinal permeability based on luminal drug disappearance, as well as the effect of permeation enhancers on drug permeability. However, it is uncertain how accurate the rat single-pass intestinal perfusion model predicts regional intestinal permeability and absorption in human. There is also a shortage of systematic in vivo investigations of the direct effect of permeation enhancers in the small and large intestine. In this rat single-pass intestinal perfusion study, the jejunal and colonic permeability of two low permeability drugs (atenolol and enalaprilat) and two high-permeability ones (ketoprofen and metoprolol) was determined based on plasma appearance. These values were compared to already available corresponding human data from a study conducted in our lab. The colonic effect of four permeation enhancers—sodium dodecyl sulfate, chitosan, ethylenediaminetetraacetic acid (EDTA), and caprate—on drug permeability and transport of chromium EDTA (an established clinical marker for intestinal barrier integrity) was determined. There was no difference in jejunal and colonic permeability determined from plasma appearance data of any of the four model drugs. This questions the validity of the rat single-pass intestinal perfusion model for predicting human regional intestinal permeability. It was also shown that the effect of permeation enhancers on drug permeability in the colon was similar to previously reported data from the rat jejunum, whereas the transport of chromium EDTA was significantly higher (p < 0.05) in the colon than in jejunum. Therefore, the use of permeation enhancers for increasing colonic drug permeability has greater risks than potential medical rewards, as indicated by the higher permeation of chromium EDTA compared to the drugs.

Enterohepatic circulation of glucuronide metabolites of drugs in dog

The enterohepatic circulation (EHC) of drugs is often the result of the direct glucuronidation, excretion of the metabolite into bile, followed by hydrolysis to the aglycone by the gut microbiome and finally reabsorption of drug into the systemic circulation. The aim of present study to identify key factors in determining the EHC in dog for canagliflozin and DPTQ, two compounds cleared by UDP-glucuronosyltransferase (UGT) mediated O-alkyl glucuronidation and cytochrome P450 (P450) mediated oxidation. The pharmacokinetic profiles of the drugs were compared between bile duct cannulated (BDC) and intact beagle dogs after a single intravenous administration. A long terminal elimination phase was observed for DPTQ but not for canagliflozin in intact dogs, while this long terminal half-life was not seen in BDC animals, suggesting the EHC of DPTQ. Quantification of parent drugs and glucuronide metabolites in bile, urine and feces indicated low recovery of parent in bile and urine and low recovery of conjugated metabolites in urine for both drugs, while biliary excretion of these glucuronide metabolites in BDC dog were low for canagliflozin but much higher for DPTQ. The increased fecal recovery of parent drug in intact dog and the lack of glucuronide metabolites suggested the hydrolysis of DPTQ-glucuronides by gut microbiome. Subsequent characterization of in vitro hepatic metabolism and permeability properties indicated the hepatic fraction metabolized by UGT, hydrolysis of metabolites, and reabsorption of the aglycone were key factors in determining the EHC of DPTQ.

Comparison of Deconvolution-Based and Absorption Modeling IVIVC for Extended Release Formulations of a BCS III Drug Development Candidate

In vitro-in vivo correlations (IVIVC) are predictive mathematical models describing the relationship between dissolution and plasma concentration for a given drug compound. The traditional deconvolution/convolution-based approach is the most common methodology to establish a level A IVIVC that provides point to point relationship between the in vitro dissolution and the in vivo input rate. The increasing application of absorption physiologically based pharmacokinetic model (PBPK) has provided an alternative IVIVC approach. The current work established and compared two IVIVC models, via the traditional deconvolution/convolution method and via absorption PBPK modeling, for two types of modified release (MR) formulations (matrix and multi-particulate tablets) of MK-0941, a BCS III drug development candidate. Three batches with distinct release rates were studied for each formulation technology. A two-stage linear regression model was used for the deconvolution/convolution approach while optimization of the absorption scaling factors (a model parameter that relates permeability and input rate) in Gastroplus(TM) Advanced Compartmental Absorption and Transit model was used for the absorption PBPK approach. For both types of IVIVC models established, and for either the matrix or the multiparticulate formulations, the average absolute prediction errors for AUC and C max were below 10% and 15%, respectively. Both the traditional deconvolution/convolution-based and the absorption/PBPK-based level A IVIVC model adequately described the compound pharmacokinetics to guide future formulation development. This case study highlights the potential utility of absorption PBPK model to complement the traditional IVIVC approaches for MR products.

Two-Stage Single-Compartment Models to Evaluate Dissolution in the Lower Intestine

The purpose was to propose two-stage single-compartment models for evaluating dissolution characteristics in distal ileum and ascending colon, under conditions simulating the bioavailability and bioequivalence studies in fasted and fed state by using the mini-paddle and the compendial flow-through apparatus (closed-loop mode). Immediate release products of two highly dosed active pharmaceutical ingredients (APIs), sulfasalazine and L-870,810, and one mesalamine colon targeting product were used for evaluating their usefulness. Change of medium composition simulating the conditions in distal ileum (SIFileum ) to a medium simulating the conditions in ascending colon in fasted state and in fed state was achieved by adding an appropriate solution in SIFileum . Data with immediate release products suggest that dissolution in lower intestine is substantially different than in upper intestine and is affected by regional pH differences > type/intensity of fluid convection > differences in concentration of other luminal components. Asacol® (400 mg/tab) was more sensitive to type/intensity of fluid convection. In all the cases, data were in line with available human data. Two-stage single-compartment models may be useful for the evaluation of dissolution in lower intestine. The impact of type/intensity of fluid convection and viscosity of media on luminal performance of other APIs and drug products requires further exploration.

The biopharmaceutics of successful controlled release drug product: Segmental-dependent permeability of glipizide vs. metoprolol throughout the intestinal tract

The purpose of this work was to study the challenges and prospects of regional-dependent absorption in a controlled-release scenario, through the oral biopharmaceutics of the sulfonylurea antidiabetic drug glipizide. The BCS solubility class of glipizide was determined, and its physicochemical properties and intestinal permeability were thoroughly investigated, both in-vitro (PAMPA and Caco-2) and in-vivo in rats. Metoprolol was used as the low/high permeability class boundary marker. Glipizide was found to be a low-solubility compound. All intestinal permeability experimental methods revealed similar trend; a mirror image small intestinal permeability with opposite regional/pH-dependency was obtained, a downward trend for glipizide, and an upward trend for metoprolol. Yet the lowest permeability of glipizide (terminal Ileum) was comparable to the lowest permeability of metoprolol (proximal jejunum). At the colon, similar permeability was evident for glipizide and metoprolol, that was higher than metoprolol's jejunal permeability. We present an analysis that identifies metoprolol's jejunal permeability as the low/high permeability class benchmark anywhere throughout the intestinal tract; we show that the permeability of both glipizide and metoprolol matches/exceeds this threshold throughout the entire intestinal tract, accounting for their success as controlled-release dosage form. This represents a key biopharmaceutical characteristic for a successful controlled-release dosage form.

Design of prodrugs to enhance colonic absorption by increasing lipophilicity and blocking ionization

Prodrugs are chemistry-enabled drug delivery modifications of active molecules designed to enhance their pharmacokinetic, pharmacodynamic and/or biopharmaceutical properties. Ideally, prodrugs are efficiently converted in vivo, through chemical or enzymatic transformations, to the active parent molecule. The goal of this work is to enhance the colonic absorption of a drug molecule with a short half-life via a prodrug approach to deliver sustained plasma exposure and enable once daily (QD) dosing. The compound has poor absorption in the colon and by the addition of a promoiety to block the ionization of the molecule as well as increase lipophilicity, the relative colonic absorption increased from 9% to 40% in the retrograde dog colonic model. A combination of acceptable solubility and stability in the gastrointestinal tract (GI) as well as permeability was used to select suitable prodrugs to optimize colonic absorption.

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.

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.

The use of gastrointestinal intubation studies for controlled release development

AIMS

This review describes clinical results of gastrointestinal intubation studies of eight controlled release (CR) candidates under development during the 1990s and offers suggestions for determining why, when and how to conduct human intubation studies.

METHODS

Experience with the administration of the following eight compounds to various regions of the gastrointestinal tract is described: CJ-13,610, CP-195,543, CP-331,684, CP-409,092, CP-424,391, azithromycin, sertraline, and trovafloxacin. Also included are human pharmacokinetic studies with prototype CR dosage forms for CJ-13,610 and CP-424,391.

RESULTS

Intubation studies, while appearing invasive, are safe and not unpleasant procedures that have been found to be valuable in the development of CR formulations.

CONCLUSIONS

The following recommendations are made regarding intubation studies: (i) no intubation study is recommended for compounds with high permeability, since these compounds are likely to be well absorbed from the colon; (ii) compounds with moderate permeability may require an intubation study if the dog colon and in silico models predict a marginally acceptable CR concentration-time profile; (iii) use a dose that approximates 1 h of the intended CR delivery rate; (iv) use the smallest volume possible; (v) define and record tubing placement; (vi) use a thermodynamically stable solution or/and suspension.

Formulation design and pharmaceutical development of a novel controlled release form of azithromycin for single-dose therapy

BACKGROUND

Azithromycin's long serum half-life (approximately 68 hours) allows for a short 5-day, 3-day, and now 1-day course therapy with a large 2-g dose. Although the single-dose, 1-day therapy offers the advantage of 100% patient compliance, tolerance of such large dose becomes an issue.

METHODS

The dosage form discussed in this article employed a melt-congealing process to produce matrix microspheres with a 3-hour, first-order release. The vehicle blend included alkalizing agents to minimize GI side effects, minimize loss of bioavailability, and mask the bitter taste of azithromycin.

RESULTS

Azithromycin microspheres are small (approximately 200 microm) with a narrow particle size distribution. Drug release was optimized by controlling the amount of dissolution enhancer in the microspheres and by the addition of proper amount of alkalizing agents in the vehicle blend. The final formulation was selected based on a balance between bioavailability and tolerability.

CONCLUSIONS

Drug release from the microspheres was shown to occur via diffusion through the larger pores formed by dissolution of azithromycin crystals and the smaller interconnected pores formed by dissolution of poloxamer. Several clinical studies have been conducted with the formulation to evaluate its pharmacokinetics and to demonstrate its safety and efficacy. The combined suspension formulation for a 2-g dose of azithromycin provided taste-masking and good tolerability.

Toward an increased understanding of the barriers to colonic drug absorption in humans: implications for early controlled release candidate assessment

The purpose of this study was to increase the understanding of in vivo colonic drug absorption in humans by summarizing and evaluating all regional in vivo human absorption data with focus on the interpretation of the colonic absorption data in relation to intestinal permeability and solubility. In addition, the usefulness of the Biopharmaceutics Classification System (BCS) in early assessment of the in vivo colonic absorption potential of controlled release drug candidates was investigated. Clinical regional absorption data (Cmax, Tmax, and AUC) of 42 drugs were collected from journal articles, abstracts, and internal reports, and the relative bioavailability in the colon (Frel(colon)) was obtained directly or calculated. Bioavailability, fraction dose absorbed, and information if the compounds were substrates for P-glycoprotein (P-gp) or cytochrome P450 3A (CYP3A) were also obtained. The BCS I drugs were well absorbed in the colon (Frel(colon) > 70%), although some drugs had lower values due to bacterial degradation in the colon. The low permeability drugs (BCS III/IV) had a lower degree of absorption in the colon (Frel(colon) < 50%). There was a clear correlation between in vitro Caco-2 permeability and Frel(colon), and atenolol and metoprolol may function as permeability markers for low and high colonic absorption, respectively. No obvious effect of P-gp on the colonic absorption of the drugs in this study was detected. There was insufficient data available to fully assess the impact of low solubility and slow dissolution rate. The estimated in vivo fractions dissolved of the only two compounds administered to the colon as both a solution and as solid particles were 55% and 92%, respectively. In conclusion, permeability and solubility are important barriers to colonic absorption in humans, and in vitro testing of these properties is recommended in early assessment of colonic absorption potential.