Invitro Invivo Research Articles

Evaluation of Solubility-Limited Absorption as a Surrogate to Predicting Positive Food Effect of BCS II/IV Drugs.

Physiologically based pharmacokinetic (PBPK) models are increasingly used to predict food effect (FE) but model parameterization is challenged by in vitro-in vivo (IVIV) disconnect and/or parameter nonidentifiability. To overcome these issues, we propose a simplified PBPK model, in which all solubility-driven processes are lumped into a single parameter, solubility, which is optimized against observed concentration-time data. A set of commercially available biopharmaceutical classification system (BCS) II/IV compounds was selected to measure the solubility in a fasted state simulated intestinal fluid (FaSSIF) medium. The compounds were ranked from the lowest to the highest dose-adjusted FaSSIF solubility (FaSSIF/D) value and subdivided into three areas based on an upper and a lower limit: drugs with FaSSIF/D > upper limit having no FE, drugs with FaSSIF/D < lower limit having FE, and drugs between the limits said to be in the sensitivity range (SR), for which we tested the hypothesis that solubility-limited absorption (SLA) identified by simplified PBPK model can reliably predict positive FE if their exposures are not impacted by gut efflux or gut metabolism. We demonstrate, using a subset of drugs within SR for which PBPK models were available, that drugs with SLA exhibited a positive FE, while those with no SLA did not show FE. This proposal allows for a reliable binary prediction of FE to enable timely decisions on the need for pilot FE studies as well as the timing of pivotal FE studies.

IN VITRO-IN VIVO CORRELATION (IVIVC): TAMOXIFEN NANOSPHERES

The primary goal of this study was to establish an in vitro-in vivo correlation (IVIVC) for tamoxifen (TAM)- loaded polycaprolactone-chitosan nanospheres. The plasma attention of TAM and time records have been used to calculate the pharmacokinetic parameters. Cmax for the nano spheres stayed determined to be 459.20 ng mL-1, compared to 442.20 ng mL-1 for natural TAM. Furthermore, the Wagner-Nelson technique was employed to compare the drug release profile by determining the in vivo fixation time data. In line with this, a level between the intestinal absorption in rats and the dissolution % for nanospheres loaded with TAM was created. Plotting the share absorbed in vivo vs the share launched in vitro simultaneously is the best way to illustrate a relationship.

Conjunction of semi-mechanistic in vitro-in vivo modeling and population pharmacokinetics as a tool for virtual bioequivalence analysis - a case study for a BCS class II drug

Virtual bioequivalence trial (VBE) simulations based on (semi)mechanistic in vitro-in vivo (IVIV) modeling have gained a huge interest in the pharmaceutical industry. Sophisticated commercially available software allows modeling variable drug fates in the gastrointestinal tract (GIT). Surprisingly, the between-subject and inter-occasion variability (IOV) of the distribution volumes and clearances are ignored or simplified, despite substantially contributing to varied plasma drug concentrations. The paper describes a novel approach for IVIV-based VBE by using population pharmacokinetics (popPK). The data from two bioequivalence trials with a poorly soluble BCS class II drug were analyzed retrospectively. In the first trial, the test drug product (biobatch 1) did not meet the bioequivalence criteria, but after a reformulation, the second trial succeeded (biobatch 2). The popPK model was developed in the Monolix software (Lixoft SAS, Simulation Plus) based on the originator’s plasma concentrations. The modified Noyes-Whitney model was fitted to the results of discriminative biorelevant dissolution tests of the two biobatches and seven other reformulations. Then, the IVIV model was constructed by joining the popPK model with fixed drug disposition parameters, the drug dissolution model, and mechanistic approximation of the GIT transit. It was used to simulate the drug concentrations at different IOV levels of the primary pharmacokinetic parameters and perform the VBE. Estimated VBE success rates for both biobatches well reflected the outcomes of the bioequivalence trials. The predicted 90% confidence intervals for the area under the time-concentration curves were comparable with the observed values, and the 10% IOV allowed the closest approximation to the clinical results. Simulations confirmed that a significantly lower maximum drug concentration for biobatch 1 was responsible for the first clinical trial’s failure. In conclusion, the proposed workflow might aid formulation screening in generic drug development.

In Vitro-In Vivo Correlation in Dermal Delivery: The Role of Excipients.

The composition of topical and transdermal formulations is known to determine the rate and the extent of drug delivery to and through the skin. However, to date, the role of excipients in these formulations on skin delivery of actives has received little attention from scientists in the field. Monitoring skin absorption of both drug and vehicle may provide insights into the mechanism by which excipients promote permeation and may facilitate the design of effective and safer products. Previously, we have investigated the use of quantitative Confocal Raman Spectroscopy (CRS) to investigate the delivery of an active to the skin, and we also reported the first fully quantitative study that compared this method with the well-established in vitro permeation test (IVPT) model. To further explore the potential of quantitative CRS in assessing topical delivery, the present work investigated the effects of commonly used excipients on the percutaneous absorption of a model drug, ibuprofen (IBU). Permeation of IBU and selected solvents following finite dose applications to human skin was determined in vitro and in vivo by Franz diffusion studies and quantitative CRS, respectively. The solvents used were propylene glycol (PG), dipropylene glycol (DPG), tripropylene glycol (TPG), and polyethylene glycol 300 (PEG 300). Overall, the cumulative amounts of IBU that permeated at 24 h in vitro were similar for PG, DPG, and TPG (p > 0.05). These three vehicles outperformed PEG 300 (p < 0.05) in terms of drug delivery. Concerning the vehicles, the rank order for in vitro skin permeation was DPG ≥ PG > TPG, while PEG 300 did not permeate the skin. A linear relationship between maximum vehicle and IBU flux in vitro was found, with a correlation coefficient (R2) of 0.95. When comparing in vitro with in vivo data, a positive in vitro–in vivo (IVIV) correlation between the cumulative permeation of IBU in vitro and the total amount of IBU that penetrated the stratum corneum (SC) in vivo was observed, with a Pearson correlation coefficient (R2) of 0.90. A strong IVIV correlation, R2 = 0.82, was found following the linear regression of the cumulative number of solvents permeated in vitro and the corresponding skin uptake in vivo measured with CRS. This is the first study to correlate in vivo permeation of solvents measured by CRS with data obtained by in vitro diffusion studies. The IVIV correlations suggest that CRS is a powerful tool for profiling drug and vehicle delivery from dermal formulations. Future studies will examine additional excipients with varying physicochemical properties. Ultimately, these findings are expected to lead to new approaches for the design, evaluation, and optimization of formulations that target actives to and through the skin.

In Vitro Biotransformation Assays Using Liver S9 Fractions and Hepatocytes from Rainbow Trout (Oncorhynchus mykiss): Overcoming Challenges with Difficult to Test Fragrance Chemicals.

In vitro metabolic stability assays using rainbow trout (Oncorhynchus mykiss) isolated hepatocytes (RT-HEP) or hepatic S9 fractions (RT-S9) were introduced to provide biotransformation rate data for the assessment of chemical bioaccumulation in fish. The present study explored the suitability of the RT-HEP and RT-S9 assays for difficult test chemicals, and the in vitro-based predictions were compared to in silico-based predictions and in vivo-measured bioconcentration factors (BCFs). The results show that volatile or reactive chemicals can be tested with minor modifications of the in vitro protocols. For hydrophobic chemicals, a passive dosing technique was developed. Finally, a design-of-experiment approach was used to identify optimal in vitro assay conditions. The modified assay protocols were applied to 10 fragrances with diverse physicochemical properties. The in vitro intrinsic clearance rates were higher in the S9 than in the hepatocyte assay, but the in vitro-in vivo (IVIV) predictions were comparable between the 2 assays. The IVIV predictions classified the test chemicals as nonbioaccumulative (BCF < 2000), which was in agreement with the in vivo data but in contrast to the in silico-based predictions. The findings from the present study provide strong evidence that the RT-HEP and RT-S9 assays can provide reliable estimates of in vivo biotransformation rates for test chemicals with difficult physicochemical properties. Environ Toxicol Chem 2020;39:2396-2408. © 2020 SETAC.

Adding a Gastric Step to the Intestinal In Vitro Digestion Model Improves the Prediction of Pharmacokinetic Data in Beagle Dogs of Two Lipid-Based Drug Delivery Systems.

Drug release from a lipid-based drug delivery system (LbDDS) is typically studied in vitro using a one-step intestinal digestion model. However, lately the importance of incorporating gastric digestion has been stressed. The aim of the present study was to compare a two-step gastro-intestinal (GI) in vitro digestion model to the commonly used one-step intestinal digestion model. The models were evaluated by studying release of the model drug A1260 from two LbDDSs (F-I and F-II), for which in vivo pharmacokinetic data from oral administration to beagle dogs were available. The amount of A1260 recovered in the aqueous phases during and after the GI digestion of F-I and F-II was related to the Cmax and AUC0-48h of the plasma concentration-time profiles of each formulation and produced a rank order in vitro-in vivo (IVIV) relation. In comparison, a similar IVIV rank ordering was obtained when relating the amount of A1260 recovered in the aqueous phase prior (t = 0 min), and following 15 min of intestinal digestion, to the plasma concentration-time profiles. However, after 60 min of intestinal digestion, the LbDDSs performed equally in the one-step in vitro digestion model, contrary to what was observed in the two-step digestion model, and in vivo. As the GI digestion model produced a clearer distinction in terms of LbDDS rank ordering of the two LbDDSs, compared to the intestinal digestion model, it was found to be a promising in vitro model to study and estimate the LbDDS behavior in vivo.

Okra-Thioglycolic Acid Conjugate—Synthesis, Characterization, and Evaluation as a Mucoadhesive Polymer

The success of mucoadhesive drug delivery systems relies on the type of polymer used, which becomes adhesive naturally upon hydration. Intended polymers should be able to maintain prolonged contact with biological membranes, and to protect or cater the drug to a prolonged period. Most of the hydro polymers form weak non-covalent bonds, that hinder localization of dosage forms at specific sites resulting in therapeutic inefficiency. This can be overcome by the thiol functionalization of natural polymers. In the present study, natural okra gum (OG) was extracted, followed by thiolation (TOG) and evaluated for mucoadhesion property and its role in enhancing the efficacy of repaglinide as a model drug (short-acting Type II antidiabetic drug). The thiol functionalization of OG (TOG) was confirmed by a Fourier-transform infrared spectroscopy (FTIR) study that showed a polyhedral to a spherical shape that had a rougher surface. Differential scanning calorimetry (DSC) and X-Ray Diffraction (XRD) studies of TOG indicated a decline in endothermic transition temperature and high crystallinity, respectively, in comparison to OG. CSFR (Crushing Strength: Friability Ratio), weight and thickness variations of repaglinidetablets formulated using TOG were &gt;80% and &lt;2.5% respectively. The highest swelling index (107.89 ± 1.99%) and strong mucoadhesion due to high disulfide bonds were observed for repaglinide TOG tablets in comparison to RG OG tablets. In-vitro release studies indicated a controlled drug release from thiolated formulations proportional to the concentration of thiomers that have a good correlation with in-vivo studies. Pharmacokinetic studies indicated higher AUC (area under the curve), longer t1/2 with thiomers. and Level A IVIV (in vitro in vivo) correlation was established from the bioavailability and dissolution data. Consequently, all the obtained results suggest that thiomers based formulations can be promising drug delivery systems, even in targeting onerous mucosal surfaces like nasal, ocular or vaginal.

Mechanistic Assessment of the Effect of Omeprazole on the In Vivo Pharmacokinetics of Itraconazole in Healthy Volunteers.

SUBA-itraconazole and Sporanox are two oral formulations of itraconazole. Drug-drug interactions with omeprazole have been previously reported; however, mechanistic understanding of the pharmacological and physiological interactions of omeprazole with orally administered itraconazole within a population modeling paradigm is lacking. The objective of this analysis was to mechanistically describe and quantify the effect of omeprazole on the pharmacokinetics of itraconazole and its major metabolite, hydroxyitraconazole from the SUBA itraconazole and Sporanox formulations. An in vitro-in vivo (IVIV) pharmacokinetic model of itraconazole and hydroxyitraconazole was developed including data from an omeprazole interaction study with SUBA itraconazole. Meta-models of gastric pH for healthy subjects and subjects receiving omeprazole were integrated into the IVIV model to capture omeprazole-mediated gastric pH changes on itraconazole dissolution and absorption. Omeprazole influenced the kinetics of itraconazole through altering the dissolution and absorption due to the pH-dependent solubility of itraconazole, inhibition of efflux transporters, and inhibiting the metabolism of itraconazole and hydroxyitraconazole. The model-predicted population effects of omeprazole on itraconazole from SUBA-itraconazole were to increase the area under the concentration-time curve (AUC0-24) and maximum concentration (Cmax) by 35 and 31%, respectively, and to decrease AUC0-24 and Cmax from Sporanox by 68 and 76%, respectively. Unlike SUBA itraconazole, which requires basic pH for itraconazole release, the omeprazole-induced pH-mediated reduction in Sporanox dissolution overrides any increased exposure from the drug-drug interaction at hepatic metabolizing enzymes or efflux transporters. The model presented here is the most complete quantitative description of the pharmacokinetics of itraconazole and hydroxyitraconazole currently available.

Organic Anion Transporter 2-Mediated Hepatic Uptake Contributes to the Clearance of High-Permeability-Low-Molecular-Weight Acid and Zwitterion Drugs: Evaluation Using 25 Drugs.

High-permeability-low-molecular-weight acids/zwitterions [i.e., extended clearance classification system class 1A (ECCS 1A) drugs] are considered to be cleared by metabolism with a minimal role of membrane transporters in their hepatic clearance. However, a marked disconnect in the in vitro-in vivo (IVIV) translation of hepatic clearance is often noted for these drugs. Metabolic rates measured using human liver microsomes and primary hepatocytes tend to underpredict. Here, we evaluated the role of organic anion transporter 2 (OAT2)-mediated hepatic uptake in the clearance of ECCS 1A drugs. For a set of 25 ECCS 1A drugs, in vitro transport activity was assessed using transporter-transfected cells and primary human hepatocytes. All but two drugs showed substrate affinity to OAT2, whereas four (bromfenac, entacapone, fluorescein, and nateglinide) also showed OATP1B1 activity in transfected cells. Most of these drugs (21 of 25) showed active uptake by plated human hepatocytes, with rifamycin SV (pan-transporter inhibitor) reducing the uptake by about 25%-95%. Metabolic turnover was estimated for 19 drugs after a few showed no measurable substrate depletion in liver microsomal incubations. IVIV extrapolation using in vitro data was evaluated to project human hepatic clearance of OAT2-alone substrates considering 1) uptake transport only, 2) metabolism only, and 3) transporter-enzyme interplay (extended clearance model). The transporter-enzyme interplay approach achieved improved prediction accuracy (average fold error = 1.9 and bias = 0.93) compared with the other two approaches. In conclusion, this study provides functional evidence for the role of OAT2-mediated hepatic uptake in determining the pharmacokinetics of several clinically important ECCS 1A drugs.

Rate-Determining and Rate-Limiting Steps in the Clearance and Excretion of a Potent and Selective p21-Activated Kinase Inhibitor: A Case Study of Rapid Hepatic Uptake and Slow Elimination in Rat.

Background: Significant under-prediction of in vivo clearance in rat was observed for a potent p21-activated kinase (PAK1) inhibitor, GNE1.Objective: Rate-determining (rapid uptake) and rate-limiting (slow excretion) steps in systemic clearance and elimination of GNE1, respectively, were evaluated to better understand the cause of the in vitro-in vivo (IVIV) disconnect.Methods: A series of in vivo, ex vivo, and in vitro experiments were carried out: 1) the role of organic cation transporters (Oct or Slc22a) was investigated in transporter knock-out and wild-type animals with or without 1-aminobenzotriazole (ABT) pretreatment; 2) the concentration-dependent hepatic extraction ratio was determined in isolated perfused rat liver; and 3) excreta were collected from both bile duct cannulated and non-cannulated rats after intravenous injection.Results: After intravenous dosing, the rate-determining step in clearance was found to be mediated by the active uptake transporter, Oct1. In cannulated rats, biliary and renal clearance of GNE1 accounted for only approximately 14 and 16% of the total clearance, respectively. N-acetylation, an important metabolic pathway, accounted for only about 10% of the total dose. In non-cannulated rats, the majority of the dose was recovered in feces as unchanged parent (up to 91%) overnight following intravenous administration.Conclusion: Because the clearance of GNE1 is mediated through uptake transporters rather than metabolism, the extrahepatic expression of Oct1 in kidney and intestine in rat likely plays an important role in the IVIV disconnect in hepatic clearance prediction. The slow process of intestinal secretion is the rate-limiting step for in vivo clearance of GNE1.

Sex-related in vitro/in vivo and PK/PD correlations after oral single dose furosemide administration

Abstract Background: The goal of this study was to develop an in vivo-in vitro (IVIV) correlation, both in men and women, which allows constructing a model to predict bioequivalence assessments for

Dissolution method for delapril and manidipine combination tablets based on an absorption profile of manidipine

The present study describes the development and validation of a dissolution method for delapril (DEL) and manidipine (MAN) combination tablets, using a simulated absorption profile based on in vivo data for MAN. The suitable in vitro dissolution profile for this formulation was obtained using 900mL of citrate buffer pH 3.2 at 37°C±0.5°C as dissolution medium and USP apparatus 2 (paddle) at 75rpm. All samples were analyzed by a liquid chromatography (LC) method. Under these conditions, a significant linear relationship between the absorbed (calculated by deconvolution approach) and dissolved fractions of MAN was obtained (R=0.997) and an in vivo-in vitro (IVIV) correlation for this particular formulation containing MAN can be established. Validation parameters for dissolution methodology such as the specificity, linearity, accuracy and precision were also evaluated according to the international guidelines, giving results within the acceptable range. Therefore, the proposed dissolution conditions can be applied for the simultaneous release analysis of DEL and MAN from the solid dosage form, contributing to the improvement of the quality control of pharmaceutics and minimizing the number of bioavailability studies.

Model of the distribution of diastolic left ventricular posterior wall thickness in healthy adults and its impact on the behavior of a string of virtual cardiomyocytes.

Correlation of the thickness of the left ventricular posterior wall (LVPWd) with various parameters, including age, gender, weight and height, was investigated in this study using regression models. Multicenter derived database comprised over 4,000 healthy individuals. The developed models were further utilized in the in vitro–in vivo (IVIV) translation of the drug cardiac safety data with use of the mathematical model of human cardiomyocytes operating at the virtual healthy population level. LVPWd was assumed to be equivalent to the length of one-dimensional string of virtual cardiomyocyte cells which was presented, as other physiological factors, to be a parameter influencing the simulated pseudo-ECG (pseudoelectrocardiogram), QTcF and ∆QTcF, both native and modified by exemplar drug (disopyramide) after I Kr current disruption. Simulation results support positive correlation between the LVPWd and QTcF/∆QTc. Developed models allow more detailed description of the virtual population and thus inter-individual variability influence on the drug cardiac safety.Electronic supplementary materialThe online version of this article (doi:10.1007/s12265-014-9558-4) contains supplementary material, which is available to authorized users.

Vergleichende In-vitro- und In-vivo-Studien zur Permeation und Penetration von Ketoprofen und Ibuprofen an humaner Haut

The aim of the present in vitro and in vivo studies was to compare the permeation and penetration of a 2.5% ketoprofen (CAS 22071-15-4) gel [Phardol Schmerz-Gel (Test-D)] with the permeation and penetration of two other ketoprofen gels (Ref-I, Ref-E) and an ibuprofen (CAS 15687-27-1) gel (Ref-D) on excised human skin. Furthermore, in vivo studies were performed. The permeation studies utilizing static Franz diffusion cells allow the determination of the transdermal (systemic) transport, whereas the penetration studies in vitro (according to the Saarbrücker model) and in vivo permit setting up a concentration-depth profile. For this purpose the permeation kinetics of ketoprofen from three different gels (each containing 2.5% ketoprofen) over a period of two days were determined at heat-separated human skin of different donors. The in vitro permeability coefficients for Test-D (6.50 x 10(-7) cm x s(-1)) and Ref-I (5.72 x 10(-7) cm x s(-1)) were comparable and the transport occurred for both by a factor of 8-9 faster than with Ref-E (0.78 x 10(-7) cm x s(-1)). In parallel to the permeation studies with ketoprofen, the permeability coefficient of caffeine from an ointment was assessed using the skin biopsies of the same donors as a quality assurance. In a second part of the studies, the in vitro penetration of ketoprofen from Test-D was determined over a period of 3 h at three different skin biopsies in comparison to a commercially available 5% ibuprofen gel (Ref-D). As a main result a concentration-depth profile for ketoprofen and ibuprofen could be issued. The ketoprofen (37.7 +/- 12.1 microg/cm2) and the ibuprofen (30.1 +/- 6.0 microg/cm2) penetrate to the same order of magnitude into the upper part of the Stratum corneum, whereas ibuprofen stronger accumulates in the deeper layers (ketoprofen: 27.3 +/- 8.5 microg/cm2; ibuprofen: 73.7 +/- 31.1 microg/cm2). An additional in vivo penetration study was performed with Test-D to set up an in vitro-in vivo (IVIV) correlation. Over a period of 3 h, the amount of ketoprofen in the Stratum corneum in vivo was 78.4 +/- 19.1 microg/cm2 being comparable to the in vitro data.

Multifunctional Porous Silicon for Therapeutic Drug Delivery and Imaging

Major challenges in drug formulation are the poor solid state stability of drug molecules, poor dissolution/solubility and/or poor pharmacokinetic properties (bioavailability), which may lead to unreliable in vitro-in vivo (IVIV) correlation. To improve current therapeutical strategies, novel means to deliver poorly water soluble active pharmaceutical ingredients, as well as to target them to specific sites or cells in the body are needed. Biomedical applications of porous silicon (PSi) have been actively investigated during the last 10 years, especially in the areas of drug delivery and imaging, due to the biocompatibility and biodegradability of PSi materials, which makes them a potential candidate for controlled drug release. In addition, the unique pore sizes and easily functionalized surface properties of PSi materials allow high drug payloads and controlled kinetics from the drug release formulations. Modification of the PSi surface properties also facilitates biofunctionalization of the surface and the possibility to attach targeting moieties (e.g., antibodies and peptides), thus enabling effective targeting of the payload. In this review, we briefly address the production methodologies of PSi, and we will mainly present and discuss several examples about the biocompatibility of PSi, the most recent in vitro and in vivo applications of PSi as a carrier in drug/protein/peptide delivery and tissue engineering, as well as PSi as a platform for drug targeting and imaging.

Evaluation of Human Pharmacokinetics, Therapeutic Dose and Exposure Predictions Using Marketed Oral Drugs

In this article approaches to predict human pharmacokinetics (PK) are discussed and the capability of the exemplified methodologies to estimate individual PK parameters and therapeutic dose for a set of marketed oral drugs has been assessed. For a set of 63 drugs where the minimum efficacious concentration (MEC) and human PK were known, the clinical dose was shown to be well predicted or in some cases over-estimated using a simple one-compartment oral PK model. For a subset of these drugs, in vitro potency against the primary human targets was gathered, and compared to the observed MEC. When corrected for plasma protein binding, the MEC of the majority of compounds was < or=3 fold over the respective in vitro target potency value. A series of in vitro and in vivo experiments were conducted to predict the human PK parameters. Metabolic clearance was generally predicted well from human hepatocytes. Interestingly, for this compound set, allometry or glomerular filtration rate (GFR) ratio methods appeared to be applicable for renal CL even where CL(renal) > GFR. For approximately 90% of compounds studied, the predicted CL using in vitro-in vivo (IVIV) extrapolation together with a CL(renal) estimate, where appropriate, was within 2-fold of that observed clinically. Encouragingly volume of distribution at steady state (V(ss)) estimated in preclinical species (rat and dog) when corrected for plasma protein binding, predicted human V(ss) successfully on the majority of occasions--73% of compounds within 2-fold. In this laboratory, absorption estimated from oral rat PK studies was lower than the observed human absorption for most drugs, even when solubility and permeability appeared not to be limiting. Preliminary data indicate absorption in the dog may be more representative of human for compounds absorbed via the transcellular pathway. Using predicted PK and MEC values estimated from in vitro potency assays there was a good correlation between predicted and observed dose. This analysis suggests that for oral therapies, human PK parameters and clinical dose can be estimated from a consideration of data obtained from in vitro screens using human derived material and in vivo animal studies. The benefits and limitations of this holistic approach to PK and dose prediction within the drug discovery process are exemplified and discussed.

Influence of dissolution medium buffer composition on ketoprofen release from ER products and in vitro–in vivo correlation

The purpose of this work was to investigate the influence of dissolution medium composition on the in vitro release of ketoprofen from a series of ER products and the impact of the different buffer media on the in vivo–in vitro (IVIV) relationship. The products investigated were coated micro bead preparations having increasing levels of coating to retard drug release. Four common dissolution media; USP phosphate buffers of pH 7.2 and 6.8, phosphate (modified isotonic) buffer pH 6.8 and a fasted state simulated intestinal fluid without lipid components (FaSSIFLF) of pH 6.5, were employed in the USP 2 apparatus. Release profiles were compared to the corresponding in vivo release profiles, obtained following deconvolution of the plasma level versus time profiles obtained from a 10-subject five-period cross-over study. Despite the relative similarity in composition of the media employed, significant differences in release profiles were observed reflecting media differences in buffer capacity, ionic strength and pH. As a consequence, the quality and shape of the IVIV relationship changed significantly, the only apparent IVIVC incorporating all four ER products, which was non-linear, was obtained using the phosphate (modified isotonic) buffer of pH 6.8. This data was fitted, using a non-linear least squares method, by the equation of Polli et al. [J. Pharm. Sci. 85 (1996) 753] and gave an alpha parameter estimate of 2, consistent with initial dissolution being more rapid in vitro than in vivo. The systematic shift in profiles, particularly with buffer capacity, underlines the sensitivity of IVIV relationship to medium composition and hence the current difficulties in making a rational choice of an appropriate single dissolution medium.