In Vivo Predictive Dissolution (IPD) and Biopharmaceutical Modeling and Simulation: Future Use of Modern Approaches and Methodologies in a Regulatory Context.

The overall objective of OrBiTo, a project within Innovative Medicines Initiative (IMI), is to streamline and optimize the development of orally administered drug products through the creation and efficient application of biopharmaceutics tools. This toolkit will include both experimental and computational models developed on improved understanding of the highly dynamic gastrointestinal (GI) physiology relevant to the GI absorption of drug products in both fasted and fed states. A part of the annual OrBiTo meeting in 2015 was dedicated to the presentation of the most recent progress in the development of the regulatory use of PBPK in silico modeling, in vivo predictive dissolution (IPD) tests, and their application to biowaivers. There are still several areas for improvement of in vitro dissolution testing by means of generating results relevant for the intraluminal conditions in the GI tract. The major opportunity is probably in combining IPD testing and physiologically based in silico models where the in vitro data provide input to the absorption predictions. The OrBiTo project and other current research projects include definition of test media representative for the more distal parts of the GI tract, models capturing supersaturation and precipitation phenomena, and influence of motility waves on shear and other forces of hydrodynamic origin, addressing the interindividual variability in composition and characteristics of GI fluids, food effects, definition of biorelevant buffer systems, and intestinal water volumes. In conclusion, there is currently a mismatch between the extensive industrial usage of modern in vivo predictive tools and very limited inclusion of such data in regulatory files. However, there is a great interest among all stakeholders to introduce recent progresses in prediction of in vivo GI drug absorption into regulatory context.

Drug solubility in luminal fluids from different regions of the small and large intestine of humans.

The purpose of this work was to study the solubility of two drugs with different physicochemical properties in luminal fluids obtained from various regions of the human gastrointestinal (GI) tract and to determine the most important luminal parameters influencing their solubility. Jejunal fluids were aspirated from healthy volunteers via an oral intubation tube. Ileal and colonic fluids were obtained from patients undergoing GI surgery. Stoma fluids were also retrieved from patients. pH and buffer capacity of all fluids were determined. Saturation solubility of prednisolone (unionisable) and mesalamine (5-aminosalicylic acid) (zwitterionic) was measured. Mean solubility of prednisolone in the different luminal fluids was 0.50 mg/mL (±0.05) and did not vary significantly between the different regions of the GI tract (ANOVA, p > 0.05). No correlation between prednisolone solubility and jejunal bile salt content was found. Mesalamine solubility increased down the GI tract: 1.97 (±0.25), 3.26 (±0.08), 6.24 (±1.13) and 7.95 (±0.21) mg/mL in jejunal, ileal, ascending and transverse/descending colonic fluids respectively. Buffer capacity also increased and in one patient was observed to range from 6.4 to 28.6 reaching 44.4 mM/L/pH unit in ileal, ascending and transverse/descending colon fluids respectively. Mesalamine solubility was found to be dependent on both buffer capacity and pH, with buffer capacity being the most important (standardized coefficient ß = 0.849, p < 0.0001) compared to pH (ß = 0.219, p < 0.05). For drugs delivered as modified release formulations it is important to consider solubility in different regions of the GI tract as significant differences can arise which will ultimately influence drug bioavailability.

Six years of progress in the oral biopharmaceutics area - A summary from the IMI OrBiTo project.

OrBiTo was a precompetitive collaboration focused on the development of the next generation of Oral Biopharmaceutics Tools. The consortium included world leading scientists from nine universities, one regulatory agency, one non-profit research organisation, three small/medium sized specialist technology companies together with thirteen pharmaceutical companies. The goal of the OrBiTo project was to deliver a framework for rational application of predictive biopharmaceutics tools for oral drug delivery. This goal was achieved through novel prospective investigations to define new methodologies or refinement of existing tools. Extensive validation has been performed of novel and existing biopharmaceutics tools using historical datasets supplied by industry partners as well as laboratory ring studies. A combination of high quality in vitro and in vivo characterizations of active drugs and formulations have been integrated into physiologically based in silico biopharmaceutics models capturing the full complexity of gastrointestinal drug absorption and some of the best practices has been highlighted. This approach has given an unparalleled opportunity to deliver transformational change in European industrial research and development towards model based pharmaceutical product development in accordance with the vision of model-informed drug development.

Improved understanding of the effect of food on drug absorption and bioavailability for lipophilic compounds using an intestinal pig perfusion model.

The purpose of this study was to investigate the relative importance of mechanisms behind the effect of food on the intestinal absorption and bioavailability for low solubility compounds by applying a porcine single-pass perfusion model. Nanoparticle suspensions of the model compounds, danazol and cyclosporine were perfused through the jejunum in isotonic fluid alone (control) and isotonic fluid with a P-glycoprotein (P-gp) inhibitor (verapamil) or dietary and endogenous lipids added. The drugs were also administered as saturated solutions in the isotonic fluid containing lipids. Administration of cyclosporine together with verapamil increased the absorption compared to the control (1.6 times) suggesting an effect on jejunal permeability. However, addition of dietary lipids to the media led to a 50% reduction in the absorption of cyclosporine indicating lack of major effects by P-gp inhibition by lipids in vivo. The absorption of danazol was increased (2.6 times) when administered as a nanosuspension in lipid containing media compared to the control, but decreased (60%) when administered as a solution in the same media. This shows how important dissolution of the drug nanoparticles is in drug absorption. The difference in the effect of lipids in the absorption of cyclosporine and danazol when administered as nanosuspensions may be due to different distribution to the colloidal structures present in the media, thereby rendering the drugs' different diffusion rates in the perfused segment. In conclusion, solubilisation seems to be a more important factor than P-gp inhibition as an explanation for the food-drug interaction observed for several low solubility drugs. In addition, the partition into different colloidal structures seems to play a major role in the dissolution and absorption of poorly soluble drugs.

Introduction to the OrBiTo decision tree to select the most appropriate in vitro methodology for release testing of solid oral dosage forms during development.

The EU research initiative OrBiTo (oral biopharmaceutics tools) involving partners from academia, pharmaceutical industry, small medium enterprises and a regulatory agency was launched with the goal of improving tools to predict the absorption of drugs in humans and thereby accelerating the formulation development process. The OrBiTo project was divided into four work packages (WP), with WP2 focusing on characterization of drug formulations. The present work introduces the OrBiTo WP2 Decision Tree, which is designed to assist the investigator in choosing the most appropriate in vitro methods for optimizing the oral formulation design and development process. The WP2 Decision Tree consists of four stages to guide the investigator. At the first stage, the investigator is asked to choose the formulation type of interest. At the second stage, the investigator is asked to identify which type of equipment (compendial/modified/noncompendial) is preferred/available. At the third stage, characteristics of the active pharmaceutical ingredient (API) are evaluated and in the fourth stage of the decision tree, suitable experimental protocols are recommended. A link to the living Decision Tree document is provided, and we now invite the pharmaceutical sciences community to apply it to current research and development projects and offer suggestions for improvement and expansion.

Rapid development of isolate-specific neutralizing antibodies after primary HIV-1 infection and consequent emergence of virus variants which resist neutralization by autologous sera.

The kinetics of appearance and specificity of HIV-1 neutralizing antibodies was studied in four individuals. HIV-1 was isolated during symptomatic primary HIV-1 infection and repeatedly thereafter, and tested against autologous sera collected in parallel. Our patients developed isolate-specific low-titer neutralizing antibodies within 2-4 weeks, and the titers to the first isolates increased with time. We documented the emergence of virus variants with reduced sensitivity to neutralization by autologous, but not heterologous, sera in three patients. These virus variants were not, however, resistant to neutralization per se, since they were readily neutralized by the positive control serum. Our patients did not develop antibodies capable of neutralizing the new virus variants during the observation period. This suggests either a failure of the immune system to respond to the new virus variants or a mechanism by which the virus evades detection by the immune system. The emergence of neutralization-resistant virus variants was not directly correlated with disease progression since two patients have remained asymptomatic after the emergence of such virus variants. It is, however, likely that the emergence of virus variants which the patient fails to neutralize in the long run contributes to disease progression.

Jejunal permeability and hepatic extraction of fluvastatin in humans.

OBJECTIVES: The primary objective was to investigate the effective permeability and the hepatic extraction of fluvastatin, a new 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor, during a jejunal perfusion in humans. The secondary objective was to investigate the relationship between human jejunal effective permeability values and physicochemical properties for four different drugs. METHODS: Nine healthy male volunteers were included in the study, which consisted of two sequential study parts. In the first part, the jejunal effective permeability of fluvastatin, antipyrine, metoprolol, and atenolol was assessed with use of the regional jejunal perfusion approach (150 minutes, 2.0 ml/min). After a washout period of at least 5 days, the same subjects received an intravenous infusion of fluvastatin (20 minutes, 2.0 mg). Plasma samples were taken in both parts of the study and were analyzed for the content of fluvastatin. RESULTS: The mean hepatic extraction of fluvastatin was 67% after the jejunal perfusion and 73% after the intravenous infusion. The half-life of fluvastatin was approximately 60 minutes after both administration routes. The jejunal effective permeability and the fraction absorbed both correlated (r2 = 0.968, p < 0.05; and r2 = 0.994, p < 0.05) with the partition coefficient (log D, pH 6.5) but not with the molecular size or the hydrogen bond number. CONCLUSION: Fluvastatin is extracted by the liver to a large extent (about 70%) and has a short half-life after both oral and intravenous administration. In this study, the human jejunal effective permeability and the fraction absorbed for these four drugs were better predicted by log D (pH 6.5) than both the molecular size and the hydrogen bond number.

Pharmacokinetic and biopharmaceutic aspects of once daily treatment with metoprolol CR/ZOK: a review article.

In the development of a new controlled release preparation and its subsequent assessment there are a number of factors that need to be considered both related to the drug itself and to the pharmaceutical preparation. This review describes the biopharmaceutical and pharmacokinetic properties of metoprolol CR/ZOK, a recently introduced formulation of a widely used beta 1-selective adrenoceptor antagonist intended for once daily usage. The metoprolol CR/ZOK preparation provides reproducible dissolution and absorption properties resulting in stable plasma concentrations with minimum fluctuations over a 24-hour dosage interval. This has been shown in extensive studies comprising over 200 healthy volunteers. Considerations for a new drug preparation such as bioavailability and variability in relation to a standard treatment and the clinical significance of taking the drug with food and in increasing doses, are potential concerns that do not seem to be a problem for the therapeutic use of metoprolol CR/ZOK. Furthermore, the pharmacokinetic data achieved in young healthy subjects appear to be relevant for the treatment of patients, as shown by the consistent plasma concentration profiles obtained in elderly and in hypertensive patients.

The relationship between metoprolol plasma concentration and beta 1-blockade in healthy subjects: a study on conventional metoprolol and metoprolol CR/ZOK formulations.

Four studies using different daily doses (100 mg, 200 mg, 300 mg and 400 mg) have examined the bioequivalence of the once daily formulation metoprolol CR/ZOK and conventional metoprolol tablets (CT). These studies showed metoprolol CR/ZOK to have a similar beta 1-blocking activity to metoprolol CT but that the bioavailability of the new formulation was lower. This paper presents the analysis of data from all four studies, looking at the relationship between plasma concentrations and beta 1-blockade measured as reduction of exercise induced tachycardia. The log linear pharmacodynamic model was used for each of the four doses to compare the two formulations. Mean slopes and intercepts in the linear regression analysis of log plasma concentration of metoprolol versus beta 1-blockade did not differ significantly between CR/ZOK and CT in any of the four studies. After having shown lack of influence of the absorption rate of the plasma concentration-effect relationship, data for CR/ZOK and CT formulations from the 200 mg study were pooled for each individual subject and fitted to an Emax model. The maximal beta 1-blocking effect (Emax) was 28% (95% confidence interval: 25-31%) and the plasma concentration for obtaining 50% of Emax (C50) was 105 nmol/L (95% confidence interval: 74-135 nmol/L). The plasma concentration-effect data from the 100 mg, 300 mg and 400 mg studies were reasonably well within the 95% prediction interval based on the 200 mg study, which showed the validity of the obtained relationship.(ABSTRACT TRUNCATED AT 250 WORDS)

Drug absorption from nifedipine hydrophilic matrix extended-release (ER) tablet-comparison with an osmotic pump tablet and effect of food.

The aim of the present study was to compare the bioavailability of nifedipine when administered as a hydrophilic matrix tablet (ER) and a push-pull osmotic pump tablet (XL) administrated after fasting, and to evaluate the effect of food for the hydrophilic matrix tablet. For this purpose, three separate studies were performed on healthy volunteers (n = 58) including gammascintigraphic monitoring of tablet erosion and localisation in the gastrointestinal tract for ER in one study. Both ER and XL provided almost constant drug delivery over 24 h, after administration under fasting conditions, and bioequivalence was obtained according to 90% confidence intervals of the difference between formulations within 80-125% for Cmax and AUC. Food significantly increased AUC for ER but no significant difference was obtained between ER and XL with food with respect to extent of bioavailability. The rate of absorption was increased to a higher degree for ER than for XL, as indicated by a Cmax which was almost twice as high for ER compared with XL. This finding was shown to be related to an increased tablet-erosion rate for ER, leading to more rapid drug release.

In vitro and in vivo erosion of two different hydrophilic gel matrix tablets.

The aim of the present work was to establish in vivo predictive in vitro tests for the tablet erosion of two different compositions (A and B) of hydrophilic matrix tablets based on hydroxypropyl methylcellulose. The tablet erosion was studied in a modified USP II apparatus at different agitation intensities and ionic strengths according to 2(2) factorial design. The in vivo tablet erosion was studied in 8 healthy human volunteers by gamma scintigraphy after administration of the tablets together with breakfast. In vitro agitation intensity increased the erosion rate for both tablets whereas increased ionic strength caused a slower rate for tablet A and a faster rate for tablet B. The choice of in vitro testing conditions proved to be critical for the attainment of in vivo predictive results. The best in vitro/in vivo correlation for the two formulations was obtained at a paddle stirring rate of 140 rpm and a ionic strength of 0.14 obtained by addition of sodium chloride.

Oral biopharmaceutics tools - time for a new initiative - an introduction to the IMI project OrBiTo.

OrBiTo is a new European project within the IMI programme in the area of oral biopharmaceutics tools that includes world leading scientists from nine European universities, one regulatory agency, one non-profit research organization, four SMEs together with scientists from twelve pharmaceutical companies. The OrBiTo project will address key gaps in our knowledge of gastrointestinal (GI) drug absorption and deliver a framework for rational application of predictive biopharmaceutics tools for oral drug delivery. This will be achieved through novel prospective investigations to define new methodologies as well as refinement of existing tools. Extensive validation of novel and existing biopharmaceutics tools will be performed using active pharmaceutical ingredient (API), formulations and supporting datasets from industry partners. A combination of high quality in vitro or in silico characterizations of API and formulations will be integrated into physiologically based in silico biopharmaceutics models capturing the full complexity of GI drug absorption. This approach gives an unparalleled opportunity to initiate a transformational change in industrial research and development to achieve model-based pharmaceutical product development in accordance with the Quality by Design concept. Benefits include an accelerated and more efficient drug candidate selection, formulation development process, particularly for challenging projects such as low solubility molecules (BCS II and IV), enhanced and modified-release formulations, as well as allowing optimization of clinical product performance for patient benefit. In addition, the tools emerging from OrBiTo are expected to significantly reduce demand for animal experiments in the future as well as reducing the number of human bioequivalence studies required to bridge formulations after manufacturing or composition changes.

Assessment of enzymatic prodrug stability in human, dog and simulated intestinal fluids.

The aim of this study was to determine the stability of three ester prodrugs, chloramphenicol succinate, enalapril and candesartan cilexetil, in human proximal small intestinal fluid (HIF), dog proximal small intestinal fluids (DIF) and simulated intestinal fluid (FaSSIF), with the addition of pancreatin. The total protein content in the proximal jejunal fluids was determined in HIF and DIF, respectively. Candesartan cilexetil was significantly degraded in HIF (initial t(1/2(0-5 min))=5.4 ± 0.5 min) and in DIF (initial t(1/2(0-5 min))=5.7 ± 0.1 min), while chloramphenicol succinate and enalapril were stable in both media. The degradation of candesartan cilexetil was shown to be mediated by enzymes following Michaelis-Menten enzyme kinetics and was inhibited by addition of esterase inhibitors. The enzymatic capacity reflected by V(max) was 4-fold higher in DIF than in HIF and correlated to its 2-fold higher protein concentration. The degradation of candesartan cilexetil in the FaSSIF-pancreatin solution was slower (t(1/2)=207 ± 34 min) than the degradation in both HIF and DIF. Changing the pH to the enzyme optima or increasing the amount of pancreatin, increased the degradation rate of candesartan cilexetil, but not in the magnitude as in HIF. As a result, two in vitro models, based on in vivo intestinal fluids, were developed using candesartan cilexetil as a model drug. The DIF seems to be a reasonably good model for HIF, although the degradation capacity seems to be somewhat higher, possibly due to the higher enzyme concentration in DIF. Future investigations will develop novel enzymatic based in vitro models for rapid assessment and biopharmaceutical screening tools for prodrugs.

Simulation of gastric lipolysis and prediction of felodipine release from a matrix tablet in the fed stomach.

The importance of intragastric lipolysis to felodipine release from a hydrophilic, extended release tablet in the fed stomach was assessed in USP II apparatus with the tablet fixed on a steel wire above the paddle. The release medium, homogenized long-life milk, was gradually digested with shots of acidic solutions of pepsin over the course of the experiment in absence and in presence of biorelevant concentrations of a lipase that was similar to human gastric lipase. Percentage tablet erosion at specific times in the same media was measured in separate experiments. The data were compared to published data for intragastric release in fed healthy adults. In all cases, felodipine release occurred under sink conditions. Lipase facilitated felodipine release from the eroded polymer, bringing the release profile closer to the in vivo data. Likewise, the relationship between tablet erosion and amount of released felodipine reflected the in vivo data only when lipase was added to the medium. It was concluded that modelling intragastric lipolysis is necessary in order to simulate felodipine release from the extended release tablets in the fed stomach.

Investigation of prandial effects on hydrophilic matrix tablets.

The prolonged release of drug from hydrophilic matrix tablets can be greatly affected by administration in connection with the intake of food. Changes of the tablet erosion are one of the main components of this effect. The aim of the present study was to identify the postprandial factors responsible for changes in tablet erosion and to develop predictive in vitro tests. Two formulations, one sensitive and the other robust to prandial effects in vivo, were investigated in vitro (a) in a complex physiological media simulating fasting and fed conditions; (b) according to a factorial experimental design that included agitation and pH concentrations of salt, surface-active agent, and nonionic solute as factors; and (c) at varying agitation intensities in three different sets of dissolution apparatus. Of the studied factors, only increased agitation enhanced the erosion of tablets in accordance with the in vivo effects of a meal. The other factors retarded erosion or had only minor effects. The hydrodynamic mechanical stress was thus considered to be the main factor responsible for postprandial effects on tablet erosion. The influence of changes in agitation and the opportunity to discriminate between sensitive and robust formulations differed among the three sets of dissolution apparatus. The modified USP II apparatus, operated at speeds of 50 and 100 rpm, is proposed as a discriminatory test.

Evaluation of solubilizers in the drug release testing of hydrophilic matrix extended-release tablets of felodipine.

The drug release of felodipine, a water-insoluble drug, was tested by using sodium lauryl sulphate (SLS), polyoxyethylene 20 sorbitan monooleate (Tween) or cetyltrimethylammonium bromide (CTAB) in the test medium as solubilizers. Three slightly different felodipine extended-release (ER) tablets 10 mg based on the gel matrix principle were evaluated under different solubilizer concentrations, agitation intensities and pH. These tablets were also tested in a bioavailability study together with an oral solution. All three solubilizers substantially enhanced the drug solubility and sink conditions were obtained. The choice of solubilizer affected the drug release rate. This is most probably due to physico-chemical interactions between the gel-forming agent and the solubilizers. All in vitro test conditions provided a good correlation (r2 = 0.94-0.97) to in vivo dissolution, as determined by moment analysis. However, a much steeper in vitro/in vivo relationship was obtained for SLS compared to Tween and CTAB reflecting an inferior discrimination between the tablets by use of this anionic solubilizer.

Design and pharmacokinetics of Logimax, a new extended-release combination tablet of felodipine and metoprolol.

A new, once-daily combination tablet containing felodipine and metoprolol has been developed, using extended-release techniques to obtain even plasma concentrations throughout the dosing interval. The tablet consists of a hydrophilic matrix containing felodipine in which many small membrane-coated metoprolol pellets are embedded. On contact with gastrointestinal fluids, felodipine is released at an almost constant rate by erosion of the hydrophilic matrix. The release of metoprolol also follows near zero-order kinetics, and is mainly controlled by diffusion through the membrane covering each individual metoprolol bead. Smooth plasma concentration profiles are obtained for both drugs of the combination, similar to those found with the corresponding single-drug formulations: felodipine extended-release tablets and metoprolol controlled-release tablets. The new fixed combination tablet also consistently provides even plasma concentrations of felodipine and metoprolol after administration together with food and in elderly hypertensive patients. The convenience of one tablet per day for effective antihypertensive treatment with a combination of felodipine and metoprolol should improve patient compliance with the prescribed regimen.

The pharmacokinetics and pharmacodynamics of metoprolol after conventional and controlled-release administration in combination with hydrochlorothiazide in healthy volunteers.

We have studied a controlled-release formulation containing metoprolol 100 mg and hydrochlorothiazide 12.5 mg. We compared the pharmacokinetics of both substances and the pharmacodynamics of metoprolol with those of a conventional combination tablet. The controlled-release formulation gave less variable plasma metoprolol concentrations, Cmax 138 nmol.l-1 and Cmin 74 nmol.l-1, whereas for the conventional formulation the mean Cmax of metoprolol was 629 nmol.l-1 and the Cmin 20 nmol.l-1. Despite lower relative systemic availability (68%) for metoprolol from the controlled-release formulation and a smaller AUC, metoprolol from the controlled-release formulation produced a greater total effect, calculated as the area under the curve of the effect on exercise heart rate vs. time (303 vs. 259%.h; P < 0.05). Hydrochlorothiazide was rapidly absorbed from both formulations and the plasma concentration profiles were almost superimposable. Controlled-release metoprolol with hydrochlorothiazide combines effective beta 1-adrenoceptor blockade for 24 h without affecting the pharmacokinetics of hydrochlorothiazide.

A novel evaluation of subcutaneous formulations by in vivo magnetic resonance imaging (MRI).

The applicability of magnetic resonance imaging (MRI) for non-invasive in vitro studies of parenteral vehicles without use of marker substances was investigated. A wide range of extended release (ER) formulations such as oils, a lipid emulsion, and water solutions of a cyclodextrin and block co-polymers were visualized in vitro and in vivo by a (1)H-MRI technique. The study included measurements in vitro in a beaker and by injections in pig flesh. In vivo studies were carried out in rats. The contrast of the vehicles vs the background material could be visualized and quantifications of vehicle dispersion and disappearance were performed on obtained in vivo data. A wide range of different vehicles suitable for s.c. ER delivery were tested, such as different oils, a lipid emulsion, and water solutions of a cyclodextrin and block co-polymers. The vehicle volume expansion in vivo was possible to follow. However, this was not generally applicable for all kinds of vehicle component. The tested co-polymers, Poloxamers, were one type of vehicle component that provides an excellent MRI signal. The in vitro tests predicted the suitability of this vehicle for in vivo MRI studies. In the in vivo study of the block co-polymer formulations the apparent vehicle volume increased to a peak value from an initial value close to the injected volume. Thereafter the volume diminished and no vehicle could be detected after 29 h after injection. MRI could be applied for measurements of the dispersion and disappearance of some vehicles at the site of injection after s.c. administration without use of contrast agents.

Steady-state bioavailability and day-to-day variability of a multiple-unit (CR/ZOK) and a single-unit (OROS) delivery system of metoprolol after once-daily dosing.

Steady-state bioavailability and day-to-day variability of plasma levels were evaluated in 18 healthy male subjects in a crossover study of multiple once-daily administration of two novel oral drug delivery systems of metoprolol and an immediate-release tablet (100 mg metoprolol tartrate). Data were collected over two consecutive 24-hr dosing intervals on treatment days 6 and 7. The two extended-release formulations investigated were metoprolol CR/ZOK (95 mg metoprolol succinate), a multiple-unit system consisting of several hundred membrane-coated delivery units, and metoprolol OROS (95 mg metoprolol fumarate), a single-unit osmotic delivery system. The extended drug release and absorption observed after administration of metoprolol CR/ZOK and metoprolol OROS resulted in similar steady-state plasma concentrations after once-daily dosing. Compared to the immediate-release tablet, they produced considerably lower plasma peaks, three- to fourfold higher trough concentrations, 8-9 hr longer mean residence times, and 20% lower relative bioavailability. Moreover, the two once-daily metoprolol products were found bioequivalent in Cmax and AUC based on 90% confidence intervals for the mean ratio CR/OROS. Repeated plasma concentration measurements on two consecutive 24-hr periods suggested that all three metoprolol treatments produced reproducible and consistent plasma concentrations from day to day at steady state. Assessment of day-to-day variability, however, resulted in significantly lower variation in AUC for the multiple-unit CR/ZOK formulation compared to the single-unit OROS tablet. These results imply that there may be formulation-related differences in the in vivo behavior of the two products despite their being bioequivalent in extent and rate of absorption.