Development and Validation of a Physiology-based Model for the Prediction of Oral Absorption in Monkeys

Purpose The development and validation of a physiology-based absorption model for orally administered drugs in monkeys is described. Materials and Methods Physiological parameters affecting intestinal transit and absorption of an orally administered drug in monkeys have been collected from the literature and implemented in a physiological model for passive absorption previously developed for rats and humans. Predicted fractions of dose absorbed have been compared to experimentally observed values for a set of N = 37 chemically diverse drugs. A sensitivity analysis was performed to assess the influence of various physiological model parameters on the predicted fraction dose absorbed. Results A Pearson’s correlation coefficient of 0.94 (95% confidence interval: [0.88, 0.97]; p < 0.0001) between the predicted and observed fraction dose absorbed in monkeys was obtained for compounds undergoing non-solubility limited passive absorption (N = 29). The sensitivity analysis revealed that the predictions of fractions dose absorbed in monkeys are very sensitive with respect to inter-individual variations of the small intestinal transit time. Conclusions The model is well suited to predict the fraction dose absorbed of passively absorbed compounds after oral administration and to assess the influence of inter-individual physiological variability on oral absorption in monkeys.     

Predicting Oral Absorption of Drugs: A Case Study with a Novel Class of Antimicrobial Agents

Purpose. The purpose of this work was to evaluate an oral absorption prediction model, maximum absorbable dose (MAD), which predicts a theoretical dose of drug that could be absorbed across rat intestine based on consideration of intestinal permeability, solute solubility, intestinal volume, and residence time. Methods. In the present study, Caco-2 cell permeability, as a surrogate for rat intestinal permeability, and aqueous solubility were measured for 27 oxazolidinones. The oxazolidinones are a novel class of potential antibacterial agents currently under investigation. These values were used to estimate MAD for each of the compounds. Finally, these predicted values were compared to previously measured bioavailability data in the rat in order to estimate oral absorption properties. Results. A reasonably good correlation between predicted dose absorbed and bioavailability was observed for most of the compounds. In a few cases involving relatively insoluble compounds, absorption was underestimated. For these compounds while aqueous solubility was low, solubility in 5% polysorbate 80 was significantly higher, a solvent possibly more representative of the small intestinal lumen. Conclusions. These results suggest that MAD may be useful for prioritizing early discovery candidates with respect to oral absorption potential. In the case of compounds with poor aqueous solubility, additional factors may have to be considered such as solubility in the intestinal lumen.     

A compartmental absorption and transit model for estimating oral drug absorption.

This report describes a compartmental absorption and transit model to estimate the fraction of dose absorbed and the rate of drug absorption for passively transported drugs in immediate release products. The model considers simultaneous small intestinal transit flow and drug absorption. Both analytical and numerical methods were utilized to solve the model equations. It was found that the fraction of dose absorbed can be estimated by F(a) = 1-(1+0.54 P(eff))(-7), where P(eff) is the human effective permeability in cm/h. A good correlation was found between the fraction of dose absorbed and the effective permeability for ten drugs covering a wide range of absorption characteristics. The model was able to explain the oral plasma concentration profiles of atenolol.     

An Integrated Model for Determining Causes of Poor Oral Drug Absorption

Purpose. To develop an integrated absorption model for estimating the fraction of dose absorbed and determining the causes of poor oral drug absorption. Methods. Both analytical and numerical methods were used to estimate the fraction of dose absorbed. Results. An integrated absorption model was developed by considering transit flow, dissolution, and permeation processes, simultaneously. A framework was proposed to determine permeability-, dissolution-, and solubility-limited absorption. Digoxin, griseofulvin, and panadiplon were employed to illustrate the applications of the integrated model in identifying the causes of poor absorption and guiding formulation development. Conclusions. The integrated absorption model was successfully applied to digoxin, griseofulvin, and panadiplon to estimate the fraction dose absorbed and to roughly determine the causes of poor oral drug absorption.     

Development of an in Vitro Rat Intestine Segmental Perfusion Model to Investigate Permeability and Predict Oral Fraction Absorbed

Purpose The aims of the study are to develop and evaluate an in vitro rat intestine segmental perfusion model for the prediction of the oral fraction absorbed of compounds and to assess the ability of the model to study intestinal metabolism. Methods The system consisted of a perfusion cell with a rat intestinal segment and three perfusion circulations (donor, receiver, and rinsing circulation). Lucifer yellow (LY) was applied as internal standard together with test compounds in the donor circulation. To validate the model, the permeability of eight noncongeneric passively absorbed drugs was determined. Intestinal N-demethylation of verapamil into norverapamil was followed in the donor and receiver circulations by high-performance liquid chromatography analysis. Results The in vitro model allowed ranking of the tested compounds according to their in vivo absorption potential. The Spearman's correlation coefficient between the oral fraction absorbed in humans and the ratio of permeation coefficient of test compound to the permeation coefficient of LY within the same experiment was 0.98 (P < 0.01). Moreover, intestinal N-demethylation of verapamil, its permeation, and the permeation of its metabolite norverapamil could be assessed in parallel. Conclusions Up to six permeation kinetics can be obtained per rat, and the method has shown to be a valuable tool to estimate human oral absorption.     

Predicting human intestinal permeability using single-pass intestinal perfusion in rat.

PURPOSE: The aim of the study was the prediction of human intestinal permeability and fraction absorbed of oral dose using single-pass intestinal perfusion technique (SPIP) in rats. METHODS: Permeability coefficients in anaesthetized rats were determined for 14 compounds. Drug solution in phosphate buffered saline (PBS) was perfused through a ingle-pass intestinal perfusion (SPIP) with flow rate of 0.21 ml/min and samples were taken from outlet tubing at different time points up to 90 min. Phenol red was used as a non-absorbable marker to correct water flux through the segment. Drug concentrations in samples were determined using HPLC and permeability coefficients (Peff) were calculated. RESULTS: The examined compounds demonstrated approximately 12.5 fold difference in magnitude for rat permeability coefficients among themselves. These values were compared with published data for human intestinal permeability, and a strong correlation was found between Peff (rat) and Peff (human); (Peff (human) = 11.04 Peff (rat) - 0.0003; R2= 0.93, P<0.0001). Subsequently the fraction dose absorbed in human (Fa) was estimated and predicted after oral dosing considering Fa(human)=1-e - 38450Peff(rat) (R2= 0.91, P<0.0001). CONCLUSIONS: Considering the high correlation of rat Peff values with those of human we conclude that the SPIP could be utilized with precision to predict the human intestinal permeability. It may also be used as a reliable technique to predict the fraction of dose absorbed following oral administration of drug in solution or regular release dosage form in human.     

DEMONS—A NEW DECONVOLUTION METHOD FOR ESTIMATING DRUG ABSORBED AT DIFFERENT TIME INTERVALS AND/OR DRUG DISPOSITION MODEL PARAMETERS USING A MONOTONIC CUBIC SPLINE

DeMonS—a new numerical deconvolution method for estimating the amount of drug absorbed at different time intervals and/or drug disposition model parameters—is presented here. In DeMonS, the amount of drug absorbed at different time intervals and/or drug disposition model parameters are the unknown parameters to be calculated. The Fritsch–Butland non-decreasing cubic spline was constructed from the cumulative amount of drug absorbed–time data directly derived from the calculated amount of drug absorbed at different time intervals. The drug absorption rate, which is the derivative of this non-decreasing cubic spline, is therefore represented by a piecewise non-negative quadratic function. The drug concentrations were obtained by convoluting the drug absorption rate quadratic function with the drug disposition model function. The nonlinear optimization method with simple parameter bounds was used to estimate the optimal set of unknown parameters by minimizing the sum of squares of residuals between the observed and predicted drug concentrations. DeMonS has been applied to (i) the griseofulvin data for estimating drug absorbed at different time intervals when the drug disposition model parameters were determined separately from intravenous data, (ii) veralipride double-peak phenomenon data to estimate simultaneously the percentage of cumulative veralipride absorbed and the veralipride disposition model parameters without reference intravenous data, (iii) a comparative bioequivalence study of gastrointestinal therapeutic system (GITS) pseudoephedrine HCI (PeHCI) controlled-release oral dosage forms when the drug disposition model parameters were not available, and (iv) estimation of both drug disposition model parameters and the absorption rate of drug from Testoderm^® (testosterone transdermal system) in the presence of endogenous testosterone production. DeMonS was implemented using MATLAB^® and NAG^® MATLAB Toolbox, and is available for Windows 3.1. © 1997 John Wiley & Sons, Ltd.     

Estimating Human Oral Fraction Dose Absorbed: A Correlation Using Rat Intestinal Membrane Permeability for Passive and Carrier-Mediated Compounds

Based on a simple tube model for drug absorption, the key parameters controlling drug absorption are shown to be the dimensionless effective permeability, P _eff ^*, and the Graetz number, Gz, when metabolism or solubility/dissolution is not rate controlling. Estimating the Graetz number in humans and assuming that P _aq ^* is not rate controlling gives the following equation for fraction dose absorbed: F = 1– e ^{–2 P*w}. The correlation between fraction dose absorbed in humans and P _w ^* determined from steady-state perfused rat intestinal segments gives an excellent correlation. It is of particular significance that the correlation includes drugs that are absorbed by passive and carrier-mediated processes. This indicates that P _w ^* is one of the key variables controlling oral drug absorption and that the correlation may be useful for estimating oral drug absorption in humans regardless of the mechanism of absorption.     

Estimating the Fraction Dose Absorbed from Suspensions of Poorly Soluble Compounds in Humans: A Mathematical Model

A microscopic mass balance approach has been developed to predict the fraction dose absorbed of suspensions of poorly soluble compounds. The mathematical model includes four fundamental di-mensionless parameters to estimate the fraction dose absorbed: initial saturation (Is), absorption number (An), dose number (Do), and dissolution number (Dn). The fraction dose absorbed (F) increases with increasing Is, An, and Dn and with decreasing Do. At higher Dn and lower Do, the fraction dose absorbed reaches the maximal F, which depends only on An. The dissolution number limit on F can appear at both lower Do and lower Dn. Likewise, at higher Do and Dn, the fraction dose absorbed reaches a Do limit. Initial saturation makes a significant difference in F at lower Do and Dn. It is shown that the extent of drug absorption is expected to be highly variable when Dn and Do are approximately one. Furthermore, by calculating these dimensionless groups for a given compound, a formulation scientist can estimate not only the extent of drug absorption but also the effect, if any, of particle size reduction on the extent of drug absorption.     

Evolution of a detailed physiological model to simulate the gastrointestinal transit and absorption process in humans, part 1: oral solutions

To enable more precise prediction of oral drug absorption, an existing physiologically based absorption model was revised. The revised model reflects detailed knowledge of human gastrointestinal (GI) physiology including fluid secretion and absorption, and comprises an elaborate representation of the intestinal mucosa. The alimentary canal from the stomach to the rectum was divided into 12 compartments. A mucosal compartment was added to each luminal segment of the intestine. A training set of 111 passively absorbed drugs with reported fractions of dose absorbed was used to optimize the semiempirical equation, which calculates intestinal permeability coefficients. The model was subsequently integrated into an established physiologically based pharmacokinetic software and validated by prediction of plasma concentration-time profiles of eight test compounds with diverse physicochemical properties. A good correlation between the simulated and experimental fractions of dose absorbed was established for the 111 compounds in the training set. Subsequently, the concentration-time profiles of six out of eight test compounds were predicted with high accuracy. The detailed model for GI transit and absorption presented in this study can help to understand the complex processes of oral absorption better and will be useful during the drug development process.     

Polar Molecular Surface Properties Predict the Intestinal Absorption of Drugs in Humans

Purpose. A theoretical method has been devised for prediction of drug absorption after oral administration to humans. Methods. Twenty structurally diverse model drugs, ranging from 0.3 to 100% absorbed, were investigated. The compounds also displayed diversity in physicochemical properties such as lipophilicity, hydrogen bonding potential and molecular size. The dynamic molecular surface properties of the compounds were calculated, taking into account their three-dimensional shape and flexibility. Results. An excellent sigmoidal relationship was established between the absorbed fraction after oral administration to humans (FA) and the dynamic polar molecular surface area (PSA_d) (r² = 0.94). The relationship was stronger than those obtained for more established predictors of drug absorption. Drugs that are completely absorbed (FA > 90%) had a PSA_d 60 ² while drugs that are < 10% absorbed had a PSA_d > 140 ². Conclusions. The results indicate that PS Ad can be used to differentiate poorly absorbed drugs at an early stage of the drug discovery process.     

Effects of Intestinal CYP3A4 and P-Glycoprotein on Oral Drug Absorption—Theoretical Approach

Purpose. To evaluate the effects of gut metabolism and efflux on drug absorption by simulation studies using a pharmacokinetic model involving diffusion in epithelial cells. Methods. A pharmacokinetic model for drug absorption was constructed including metabolism by CYP3A4 inside the epithelial cells, P-gp-mediated efflux into the lumen, intracellular diffusion from the luminal side to the basal side, and subsequent permeation through the basal membrane. Partial differential equations were solved to yield an equation for the fraction absorbed from gut to the blood. Effects of inhibition of CYP3A4 and/or P-gp on the fraction absorbed were simulated for a hypothetical substrate for both CYP3A4 and P-gp. Results. The fraction absorbed after oral administration was shown to increase following inhibition of P-gp. This increase was more marked when the efflux clearance of the drug was greater than the sum of the metabolic and absorption clearances and when the intracellular diffusion constant was small. Furthermore, it was demonstrated that the fraction absorbed was synergistically elevated by simultaneous inhibition of both CYP3A4 and P-gp. Conclusions. The analysis using our present diffusion model is expected to allow the prediction of in vivo intestinal drug absorption and related drug interactions from in vitro studies using human intestinal microsomes, gut epithelial cells, CYP3A4-expressed Caco-2 cells, etc.     

Pancreatoduodenectomy as a source of human small intestine for Ussing chamber investigations and comparative studies with rat tissue

A clear understanding of oral drug absorption is an important aspect of the drug development process. The permeability of drug compounds across intact sections of small intestine from numerous species, including man, has often been investigated using modified Ussing chambers. The maintenance of viable, intact tissue is critical to the success of this technique. This study therefore aimed to assess the viability and integrity of tissue from patients undergoing pancreatoduodenectomy, for use in cross-species Ussing chamber studies. Electrical parameters (potential difference, mV; short-circuit current, μA.cm(-2) ; resistance, Ω.cm(2) ) were monitored over the duration of each experiment, as was the permeability of the paracellular marker atenolol. The permeability values (Papp; cm/s × 10(-6) ) for a training-set of compounds, displaying a broad range of physicochemical properties and known human fraction absorbed values, were determined in both rat and human jejunum, as well as Caco-2 cell monolayers. The results indicate that human jejunum sourced from pancreatoduodenectomy remained viable and intact for the duration of experiments. Permeability values generated in rat and human jejunum correlate well (R(2) = 0.86), however the relationship between permeability in human tissue and Caco-2 cells was comparatively weak (R(2) = 0.58). Relating permeability to known human fraction absorbed (hFabs) values results in a remarkably similar relationship to both rat and human jejunum Papp values. It can be concluded that human jejunum sourced from pancreatoduodenectomy is a suitable source of tissue for Ussing chamber permeability investigations. The relationship between permeability and hFabs is comparable to results reported using alternative test compounds.     

In vitro dissolution/permeation system to predict the oral absorption of poorly water-soluble drugs: effect of food and dose strength on it

The aim of the present work was to confirm the usefulness of the dissolution/permeation system (D/P system) in the estimation of human oral absorption of poorly water-soluble drugs. The D/P system, which can simultaneously evaluate drug absorption processes, dissolution and permeation, can predict the oral absorption of poorly water-soluble drugs in fasted and fed humans, with a correlation between in vivo oral absorption (% of absorbed) and in vitro permeated amount (% of dose/2 h) in the D/P system. The oral absorption (fraction of absorbed dose, %) of poorly water-soluble drugs in the fasted and fed states was predicted using the D/P system. The effect of food on the oral absorption of various drugs estimated by the D/P system significantly correlated with clinical data (correlation coefficient: r(2)=0.924). Moreover, the proportion of oral absorption of cilostazol was predicted to decrease with an increase in its dose strength, which significantly correlated with in vivo human absorption. Consequently, the D/P system was demonstrated to be a useful in vitro system for prediction of the oral absorption of poorly water-soluble drugs.     

A physiological model for the estimation of the fraction dose absorbed in humans

A physiologically based model for gastrointestinal transit and absorption in humans is presented. The model can be used to study the dependency of the fraction dose absorbed (F(abs)) of both neutral and ionizable compounds on the two main physicochemical input parameters (the intestinal permeability coefficient (P(int)) and the solubility in the intestinal fluids (S(int))) as well as physiological parameters such as the gastric emptying time and the intestinal transit time. For permeability-limited compounds, the model produces the established sigmoidal dependence between F(abs) and P(int). In case of solubility-limited absorption, the model enables calculation of the critical mass-solubility ratio, which defines the onset of nonlinearity in the response of fraction absorbed to dose. In addition, an analytical equation to calculate the intestinal permeability coefficient based on the compound's membrane affinity and molecular weight was used successfully in combination with the physiologically based pharmacokinetic (PB-PK) model to predict the human fraction dose absorbed of compounds with permeability-limited absorption. Cross-validation demonstrated a root-mean-square prediction error of 7% for passively absorbed compounds.     

Integrating drug permeability with dissolution profile to develop IVIVC

In this review article, three different approaches to predict in vivo oral absorption based on the in vitro data of drug permeability, solubility and dissolution were introduced. At the drug discovery stage, the absorption potential of each candidate is most important to select better compounds for further development. The concept of maximum absorbable dose is applied widely, not only to evaluate the absorption potential but also to elucidate the rate-limiting process of oral absorption that helps us to understand the cause of poor absorption. To integrate the permeability of the drug with its dissolution profile, two different approaches, in vitro dissolution/permeation system (D/P system) and in silico model and simulation method, are proposed. In the D/P system, by mimicking the in vivo process of drug absorption, the permeated amount of drugs, that is the total output of dissolution and permeation processes, are correlated with the fraction absorbed in human (F(a) ). This system is powerful for evaluating the improved absorption by various formulations and the effect of food intake. On the other hand, in the model and simulation approach, an intrinsic dissolution parameter of drug particle, z, was extracted from the small scale in vitro test and the process of intestinal absorption was re-constructed in silico by incorporating the physiological parameters in human. The effective use of these approaches for the development of oral drug products is discussed through various case studies. Copyright ? 2012 John Wiley & Sons, Ltd.     

Estimating unbound volume of distribution and tissue binding by in vitro HPLC-based human serum albumin and immobilised artificial membrane-binding measurements

The in vivo unbound volume of distribution (V(du)) can be used to estimate the free steady-state plasma concentration with a given dose of a drug administered intravenously. We have demonstrated that the calibrated HPLC retention times obtained on biomimetic stationary phases, such as immobilised human serum albumin and phosphatidyl-choline, can be used to estimate compounds' in vivo behaviour. The mechanistic models are based on the assumption that the sum of the albumin and phospholipid binding has the most significant impact on reducing compounds' free concentration both in plasma and in tissues. The model equations were obtained using the literature human volume of distribution and fraction unbound in plasma values of 135 known drug molecules and have been tested on a further 300 in-house compounds. The model can be used to design compounds with low V(du) values and high fraction unbound in tissues which will minimise the required dose to achieve the efficacious free concentration at the target organ (excluding possible active transport processes).     

Evaluation of a single-pass intestinal-perfusion method in rat for the prediction of absorption in man

Prediction of the fraction of dose absorbed from the intestine (Fa) in man is essential in the early drug discovery stage. In-vitro assays in Caco-2 and MDCK cells are routinely used for that purpose, and their predictive value has been reported. However, in-situ techniques might provide a more accurate estimation of Fa. In this study, we evaluated a single-pass intestinal-perfusion (SPIP) method in the rat for its use in the prediction of absorption in man and compared it with a previous report using cell-based assays. Effective permeability coefficients (Peff) were determined in rats for 14 compounds, and ranged from 0.043x 10(-4) cm s(-1) to 1.67 x 10(-4) cm s(-1). These values strongly correlated (r2 = 0.88) with reported Peff values for man. In addition, the Spearman rank correlation coefficient calculated for in-situ-derived Peff and absorption in man was 0.92 while for the previously tested in-vitro Caco-2 and MDCK systems vs absorption in man, the correlation coefficients were 0.61 and 0.59, respectively. SPIP provided a better prediction of human absorption than the cell-based assays. This method, although time consuming, could be used as a secondary test for studying the mechanisms governing the absorption of new compounds, and for predicting more accurately the fraction absorbed in man.     

Prediction Accuracy of Mechanism-Based Oral Absorption Model for Dogs

The purpose of the present study was to evaluate the prediction accuracy of a mechanism-based oral absorption model for the fraction of a dose absorbed (Fa) in dogs, focusing on poorly soluble drugs. As an open mechanism-based model, the gastrointestinal unified theoretical framework was used in this study. The prediction accuracy of the gastrointestinal unified theoretical framework was evaluated using Fa data in dogs (63 data sets for marketed drugs and proprietary compounds). For neutral compounds, Fa was accurately predicted, suggesting that the physiological parameters of dogs were appropriate except for gastrointestinal pH. An extensive literature survey on the small intestinal pH of dogs was then conducted. The result suggested that the pH value ranged between 6.5 and 7.5, with the midst value of 7.0, but there was a great variation among the literature. To confirm the appropriateness of this pH value, the Fa of free acid compounds was predicted by setting the small intestinal pH to 6.5, 7.0, and 7.5. The proportions of compounds with <2-fold error were 57%, 90%, and 76%, respectively. The results of the present study would enable an appropriate use of a mechanism-based model for drug discovery and development.