Biopharmaceutics classification of selected beta-blockers: solubility and permeability class membership

The purpose of this study was to determine the permeability and solubility of seven beta-blockers (acebutolol, atenolol, labetalol, metoprolol, nadolol, sotalol, and timolol) and to classify them according to the Biopharmaceutics Classification System (BCS). Apparent permeability coefficients (Papp) were measured using the Caco-2 cell line, and the solubility was determined at 37 degrees C over a pH range of 1.0-7.5. The permeability coefficients ranged from 1.0x10(-7) to 4.8x10(-5) cm/s. On the basis of the in vitro permeability and solubility data observed in the study, labetolol, metoprolol, and timolol can be categorized as BCS Class I drugs, whereas acebutolol, atenolol, and nadolol belong to BCS Class III. The permeability coefficients in Caco-2 cells were consistent with the reported extent of intestinal absorption in humans for all drugs except sotalol. Sotalol displayed low permeability in the Caco-2 cell line, but the extent of intestinal absorption in humans is over 90%. The low permeability through the Caco-2 monolayers might be largely related to its low lipophilicity. In addition, the difference between the tightness of the intercellular junction in vivo and in vitro may partially contribute to this disparity in the sotalol permeability of in vivo and in vitro.     

Estimation of the Relative Contribution of the Transcellular and Paracellular Pathway to the Transport of Passively Absorbed Drugs in the Caco-2 Cell Culture Model

Purpose. The objective of this investigation was to determine, using the Caco-2 cell culture model, the extent to which the paracellular and transcellular routes contributed to the transport of passively absorbed drugs. An effort was also made to determine the controlling factors in this process. Methods. We selected a heterologous series of drugs with varying physicochemical parameters for the investigation. Effective permeability coefficients of the model drugs (naproxen, phenytoin, salicylic acid, chlorothiazide, furosemide, propranolol, diltiazem, ephedrine, and cimetidine), at pH 7.2 and pH 5.4, were estimated using confluent monolayers of Caco-2 cells. The biophysical model approach, based on molecular size restricted diffusion within an electrostatic field of force, used by Adson et al. (1 ,2), was employed to estimate the permeability coefficients of the ionized and unionized forms of the drugs for the paracellular and transcellular route. Results and Conclusions. The permeability coefficients of the acidic drugs was greater at pH 5.4, whereas that of the basic drugs was greater at pH 7.2 and the transcellular pathway was the favored pathway for most drugs, probably due to its larger accessible surface area. The paracellular permeability of the drugs was size and charge dependent. The permeability of the drugs through the tight junctions decreased with increasing molecular size. Further, the pathway also appeared to be cation-selective, with the positively charged cations of weak bases permeating the aqueous pores of the paracellular pathway at a faster rate than the negatively charged anions of weak acids. Thus, the extent to which the paracellular and transcellular routes are utilized in drug transport is influenced by the fraction of ionized and unionized species (which in turn depends upon the pKa of the drug and the pH of the solution), the intrinsic partition coefficient of the drug, the size of the molecule and its charge.     

UV-spectrophotometry study of membrane transport processes with a novel diffusion cell

A new system for prediction of drug absorption that takes into account drug dissolution and pH change in the gastro-intestinal tract was developed. In this new system, a drug (solid form) is added into a drug-dissolving vessel (pH 1.0) and the dissolved drug is transferred to a pH adjustment vessel (pH 6.0). Then the drug solution is transferred to the apical surface of Caco-2 cells, and the permeation rate of the drug across a Caco-2 monolayer is determined. This system was able to predict the oral absorption ratios of ten water-soluble drugs in humans. Using this system, it was predicted that drugs that permeated Caco-2 at a rate of more than 0.1% of the dose in 200 min would be almost completely absorbed after oral administration in humans. For a drug whose permeation ratio was less than 0.03%, the absorption ratio was predicted to be less than 30%. This system also enabled prediction of the absorption rate and variability in the absorption of albendazole, a drug with poor water solubility. It also enabled assessment of the improvement in absorption using a solid dispersion of albendazole-polymers that improved the water solubility. The results suggest that this system is useful for oral absorption screening of new drugs and pharmaceutical products.     

Development and evaluation of an in vitro method for prediction of human drug absorption II. Demonstration of the method suitability.

The aim of the present work was to assess the actual suitability and general applicability of a new in vitro permeation method based on an appositely developed artificial membrane to evaluate and predict drug absorption potential. The proposed method was employed to evaluate the apparent permeability of a set of 21 structurally diverse drugs having very different solubility and permeability properties, covering the whole range of fraction absorbed in humans (F(a) from 13 to 100%); 13 of the drugs in this study were part of the list suggested by FDA for validation of in vitro permeation methods. An excellent linear correlation (R(2)=0.957) was obtained between artificial membrane apparent permeability and human absorption data in the whole range of F(a) values examined (including all the drugs belonging to the above FDA list), indicating the good predictive ability of the proposed method not only for highly absorbed hydrophobic compounds but also, differently from other in vitro permeation methods, for poorly or middling permeable drugs. The predictive ability of the new method was greater than those obtained for the same set of drugs with PAMPA and Caco-2 permeability literature data, probably due to the poor sensitivity of these methods towards hydrophilic drugs. The better performance of our artificial membrane was attributed to the hydrophilic nature of the support that, differently from the commonly used hydrophobic supports, offers less resistance to permeation of hydrophilic compounds. A comparison of permeation data of theophylline, ketoprofen, aciclovir and furosemide (selected, respectively, as models of I-IV BCS classes) obtained using a Caco-2 cell based dynamic method and the developed artificial membrane and the corresponding F(a) values in humans further confirmed the suitability of the proposed permeation method as predictor of the oral absorption of passively absorbed drugs.     

Use of a Caco-2 cell culture model for the characterization of intestinal absorption of antibiotics.

The use of cell culture models, based on human cell lines derived from the intestinal epithelium, is a promising new tool for the in vitro study of oral absorption of drugs. An assay has been developed using the Caco-2 cell line with the aim of studying the in vitro permeability of antibiotics. The reproducibility of the assay conditions have been assessed by means of the transport of two different marker molecules: 3H-mannitol and fluorescein, and transepithelial electrical resistance (TEER) value for cells monolayers. The results show that cells after 21 days of culture give significantly tighter monolayers than those after 15 days with higher reproducibility. Apparent permeability coefficients (Papp) have been measured for 13 antibiotics, known to be absorbed at different rates in humans. Papp values span from 0.18 x 10(-6) cm/s for cephaloridine to 5.79 x 10(-6) cm/s for rifampicin where the corresponding bioavailability values, known from literature, span from < 3 to 98%. A Caco-2 in vitro model appears to be suitable to investigate the transport of drugs across the intestinal epithelium. This model gives no information about the metabolic phase that follows the absorption of a drug but could provide information to investigate its pharmacokinetical behavior.     

An in vitro system for prediction of oral absorption of relatively water-soluble drugs and ester prodrugs

We developed an in vitro system simulating the physiological condition in the gastrointestinal (GI) tract for prediction of oral absorption of relatively water-soluble drugs and ester prodrug pivampicillin. This evaluation system includes a drug-dissolving vessel (DDV, assumed stomach), a pH adjustment vessel (PAV, assumed intestine) and a side-by-side diffusion chamber that is mounted by a Caco-2 monolayer, which is grown on a polycarbonate filter, or by a rat intestine between the donor and receiver compartments. Our proposed system can accommodate large amounts of solid drugs, simulating a drastic pH change process in GI tract, that is, an orally administered solid drug is dissolved in the stomach (pH 1-2) and transferred to the intestine (pH 6), and that dissolution process can also be monitored. The optimal flow rates for our system are 0.35-1.10 ml/min. Using this system, cumulative permeations of eight relatively water-soluble drugs were compared, and these cumulative permeations indicated the ability of drug absorption in humans. Drugs that permeated across a Caco-2 monolayer at cumulative permeation of more than 0.03% or over 0.04% in rat intestine can be almost completely absorbed in humans. If the cumulative permeation across a Caco-2 monolayer is lower than 0.03% or below 0.04% in the rat intestine, there was a good linear correlation between cumulative permeation across a Caco-2 monolayer and oral absorption in humans, or between cumulative permeation across a rat intestine and oral absorption in humans. In the case of relatively water-soluble drugs, a good linear correlation was obtained between cumulative permeation across a Caco-2 monolayer and cumulative permeation across a rat intestine. This result indicates that it is possible to predict the oral absorption of a relatively water-soluble drug in humans based on the cumulative permeation of the drug across a Caco-2 monolayer and/or a rat intestine. The time course of permeation of the ester prodrug pivampicillin, which is metabolized in a Caco-2 monolayer or in a rat intestine, was also evaluated. It stated clearly that it is also possible to predict the oral absorption of pivampicillin in humans based on the cumulative permeation across a Caco-2 monolayer or rat intestine. Our newly developed system enables more kinds of oral preparations and also pH-dependent soluble drugs to be evaluated.     

Quantitative Biopharmaceutics Classification System: The Central Role of Dose/Solubility Ratio

PURPOSE: To develop a quantitative biopharmaceutics drug classification system (QBCS) based on fundamental parameters controlling rate and extent of absorption. METHODS: A simple absorption model that considers transit flow, dissolution, and permeation processes stochastically was used to illustrate the primary importance of dose/solubility ratio and permeability on drug absorption. Simple mean time considerations for dissolution, uptake, and transit were used to identify relationships between the extent of absorption and a drug's dissolution and permeability characteristics. RESULTS: The QBCS developed relies on a (permeability, dose/ solubility ratio) plane with cutoff points 2 x 10(-6)-10(-5) cm/s for the permeability and 0.5-1 (unitless) for the dose/solubility ratio axes. Permeability estimates, P(app) are derived from Caco-2 studies, and a constant intestinal volume content of 250 ml is used to express the dose/solubility ratio as a dimensionless quantity, q. A physiologic range of 250-500 ml was used to account for variability in the intestinal volume. Drugs are classified into the four quadrants of the plane around the cutoff points according to their P(app), q values, establishing four drug categories. i.e., I (P(app) > 10(-5) cm/s, q < or = 0.5), II (P(app) > 10(-5) cm/s, q > 1), III (P(app) < 2 x 10(-6) cm/s. q < or = 0.5), and IV (P(app) < 2 x 10(-6) cm/s, q > 1). A region for borderline drugs (2 x 10(-6) < P(app) < 10(-5) cm/s, 0.5 < q < 1) was defined too. For category I, complete absorption is anticipated, whereas categories II and III exhibit dose/ solubility ratio-limited and permeability-limited absorption, respectively. For category IV, both permeability and dose/solubility ratio are controlling drug absorption. Semiquantitative predictions of the extent of absorption were pointed out on the basis of mean time considerations for dissolution, uptake, and transit in conjunction with drug's dose/solubility ratio and permeability characteristics. A set of 42 drugs were classified into the four categories, and the predictions of intestinal drug absorption were in accord with the experimental observations. CONCLUSIONS: The QBCS provides a basis for compound classification into four explicitly defined drug categories using the fundamental biopharmaceutical properties, permeability, and dose/solubility ratio. Semiquantitative predictions for the extent of absorption are essentially based on these drug properties, which either determine or are strongly related to the in vivo kinetics of drug dissolution and intestinal wall permeation.     

Fraction of a dose absorbed estimation for structurally diverse low solubility compounds

The purpose of the present study was to investigate the prediction accuracy of the fully mechanistic gastrointestinal unified theoretical (GUT) framework for in vivo oral absorption of low solubility drugs. Solubility in biorelevant media, molecular weight, logP(oct), pK(a), Caco-2 permeability, dose and particle size were used as the input parameters. To neglect the effect of the low stomach pH on dissolution of a drug, the fraction of a dose absorbed (Fa%) of undissociable and free acids were used. In addition, Fa% of free base drugs with the high pH stomach was also included to increase the number of model drugs. In total twenty nine structurally diverse compounds were used as the model drugs. Fa% data at several doses and particle sizes in humans and dogs were collated from the literature (total 110 Fa% data). In approximately 80% cases, the prediction error was within 2 fold, suggesting that the GUT framework has practical predictability for drug discovery, but not for drug development. The GUT framework appropriately captured the dose and particle size dependency of Fa% as the particle drifting effect was taken into account. It should be noted that the present validation results cannot be applied for salt form cases and other special formulations such as solid dispersions and emulsion formulations.     

The “High Solubility” Definition of the Current FDA Guidance on Biopharmaceutical Classification System May Be Too Strict for Acidic Drugs

PURPOSE: The purpose of this study was to assess if the definition of high solubility as proposed in the FDA Guidance on Biopharmaceutical Classification System (BCS) is too strict for highly permeable acidic drugs. METHODS: The solubility and permeability values of 20 (18 acidic and 2 non-acidic) nonsteroidal anti-inflammatory drugs (NSAID) were determined. The NSAIDs were grouped into three different sets having acetic acid, propionic acid, or other acidic moieties such as fenamate, oxicam, and salicylate. Two nonacidic NSAIDs (celecoxib and rofecoxib) were also included for comparison purposes. Equilibrium solubility values were determined at pH 1.2, 5.0, 7.4, and in biorelevant media simulating fed intestinal fluid at pH 5.0. For a select number of acids, we also measured solubility values in media simulating gastric and fasted intestinal fluids. Permeability classification was established relative to that of reference drugs in the Caco-2 cell permeability model. Permeability coefficients for all drugs were measured at concentrations corresponding to the lowest and highest marketed dose strengths dissolved in 250 ml volume, and their potential interaction with cellular efflux pumps was investigated. RESULTS: All NSAIDs with different acidic functional groups were classified as highly permeable based on their Caco-2 cell permeability. Only ketorolac appeared to have a potential for interaction with cellular efflux pumps. Solubility classification was based on comparison of equilibrium solubility at pH 1.2, 5.0. and 7.4 relative to marketed dose strengths in 250 ml. The pKa values for the acidic NSAIDs studied were between 3.5 and 5.1. and, as expected, their solubility increased dramatically at pH 7.4 compared to pH 1.2. Only three NSAIDs, ketorolac, ketoprofen. and acetyl salicylic acid, meet the current criteria for high solubility over the entire pH range. However, with the exception of ibuprofen, oxaprozin, and mefenamic acid, the remaining compounds can be classified as Class I drugs (high solubility-high permeability) relative to solubility at pH 7.4. The use of bio-relevant media simulating gastric and intestinal milieu for solubility measurements or increasing the dose volume to 500 ml did not provide for a better boundary for solubility classification. CONCLUSIONS: Based on the current definition of solubility, 15 of the 18 acidic NSAIDs in this study will be classified as Class II compounds as the solubility criteria applies to the entire pH range of 1.2 to 7.4, although the low solubility criteria does not hold true over the entire pH range. Whence, of the 18 acidic drugs, 15 can be classified as Class I based on the pH 7.4 solubility alone. This finding is intriguing because these drugs exhibit Class I behavior as their absorption does not seem to be dissolution or solubility limited. It could then be argued that for acidic drugs, the boundaries for solubility are too restrictive. Solubility at pH > 5 (pH in duodenum) may be more appropriate because most compounds are mainly absorbed in the intestinal region. Consideration for an intermediate solubility classification for highly permeable ionizable compounds that reflects physiological conditions seems warranted.     

Effect of simulated intestinal fluid on drug permeability estimation across Caco-2 monolayers

Presently, the Caco-2 cell culture model is widely used during drug discovery and development as a predictive tool for the oral absorption of drug candidates. For transport experiments in the Caco-2 system, HBSS-like buffered salt solutions are commonly used, although different shortcomings have been associated with the use of these buffers. In this paper, we investigated the effect of using fasted state simulated intestinal fluid (FaSSIF) as potential biorelevant medium for the drug permeability estimation across Caco-2 monolayers. The transport characteristics of 19 model compounds were determined in the Caco-2 cell culture model in the presence of FaSSIF as compared to classic transport medium. A sigmoidal relation was obtained when the estimated P(app), s of the apical to basolateral transport were plotted versus the reported values of the fraction absorbed in man. Although no effect of FaSSIF as compared to classic transport medium (TM) was observed on the total predictability of the model, an impact was demonstrated (1) on the bi-directional transport of actively transported drugs (including talinolol, digoxin and doxorubicin), (2) on recovery and (3) on the solubility and permeability estimation of poorly water-soluble drugs. The observed differences may be attributed to a P-gp inhibitory effect of sodium taurocholate (NaTC), micellar encapsulation by the NaTC/lecithin mixed micelles and/or an increase of the solubility of lipophilic drugs. As the experimental conditions should mimic the physiological in vivo conditions, the use of FaSSIF as medium during Caco-2 experiments may improve the biorelevance of the model.     

Absorption, distribution, metabolism, and excretion considerations in selection of orally active indole-containing endothelin antagonist.

A series of potent indole-containing endothelin antagonists were evaluated in rat pharmacokinetic studies as part of a rational drug design program. Early compounds in this series were found to show poor gastrointestinal absorption, limiting their utility as oral agents. Structural modifications and pharmacokinetic studies indicated that reducing the overall H-bonding potential, through a reduction in the number of H-bond donors and acceptors, could increase absorption of the molecules. There was a correlation between calculated H-bonding capacity and rate of permeability across Caco-2 monolayers for this series of compounds. Caco-2 permeability was also shown to be indicative of the estimated extent of absorption in rats. Balancing the requirements of absorption and systemic clearance lead to the selection of an alcohol-containing compound, compound 7a (single enantiomer of compound 7) that was moderately absorbed after oral administration and converted to an active acid metabolite, which itself was of low intrinsic clearance. Species differences were observed between the absorption of compound 7a in rat and dog and also in the extent of conversion to the acid metabolite. Absorption was estimated at 30% in rat and 100% in dog. Approximately 30% of the absorbed drug was converted to systemically available acid metabolite in rat, compared with only 3% in dog.     

In vitro testing of drug absorption for drug 'developability' assessment: forming an interface between in vitro preclinical data and clinical outcome

Drug 'developability' assessment has become an increasingly important addition to traditional drug efficacy and toxicity evaluations, as pharmaceutical scientists strive to accelerate drug discovery and development processes in a time- and cost-effective manner. The fraction of drug absorbed and the maximum absorbable dose (MAD) can be estimated from in vivo clinical pharmacokinetics, mass balance studies or in vivo drug permeability in humans by different calculation methods. Unfortunately, in vivo data are usually unavailable at the early stages of drug discovery and development, and in vitro screening for the permeability, solubility, activity and toxicity of a drug has become a routine measurement in drug discovery and development. These in vitro data could be used to predict drug 'developability' with different calculation methods before selecting candidates for clinical evaluation. The fraction of drug absorbed in human could be predicted by in vivo human permeability or in vitro Caco2 permeability. For example, if drug permeability in Caco2 cells reaches 13.3 to 18.1 x 10(-6) cm/s, its predicted in vivo permeability in humans would reach 2 x 10(-4) cm/s, and its predicted fraction of drug absorbed would be > 90%, which is defined as highly permeable. The MAD could also be predicted with in vitro permeability, or calculated absorption rate constant. In addition, in vitro solubility and permeability data can also be used for the biopharmaceutics classification system (BCS) and, subsequently, to direct formulation optimization strategies. If drug 'developability' becomes an obstacle for drug delivery based on these in vitro data and predictions at the early stages of drug discovery and development, options such as prodrug approaches could be explored to enhance drug 'developability', in addition to different formulation methods. Therefore, in vitro absorption testing is a highly valuable tool in the decision-making process to select candidates for in vivo clinical studies at early-stage drug discovery and development.     

Optimized conditions for prediction of intestinal drug permeability using Caco-2 cells.

The effects of various experimental conditions on in vitro drug permeability to Caco-2 monolayers were investigated to determine the optimized conditions for the prediction of intestinal drug absorption. Concerning the pH of the transport medium in the Caco-2 study, two different pH values, 6.0 and 7.4, were tested for the apical medium with the pH of the basolateral medium fixed to 7.4. The change in the apical pH showed pronounced effects on the permeability of both passively and actively transported drugs. It was found that the transport study under the condition of an apical pH value of 6.0 showed a better prediction of in vivo drug absorption in human. The appropriate conditions for determining the permeability of poorly soluble drugs were also examined. First, the effects of bile acids, surfactant and some agents used for solubilizing drugs on the permeability and transepithelial electrical resistance (TEER) of Caco-2 monolayers were investigated. Taurocholic and cholic acid showed no effects on the permeability of 3H-Dexamethasone (DEX) and TEER at 10 mM concentration, suggesting the possibility of use in the Caco-2 study. Polyethyleneglycol-400 and dimethylsulfoxide reduced the permeability of DEX concentration dependently, whereas ethanol induced no significant changes in the permeability. Furthermore, it was demonstrated that the addition of plasma protein (bovine serum albumin) to the basolateral medium apparently facilitated the transport of poorly soluble drugs with high lipophilicity across Caco-2 monolayers. These findings clearly suggest the importance of considering the physiological conditions of in vivo drug absorption in optimizing the in vitro experimental conditions for transport study using Caco-2 cells, in order to obtain a satisfactory in vitro-in vivo correlation.     

The composite solubility versus pH profile and its role in intestinal absorption prediction

The purpose of this study was to examine absorption of basic drugs as a function of the composite solubility curve and intestinally relevant pH by using a gastrointestinal tract (GIT) absorption simulation based on the advanced compartmental absorption and transit model. Absorption simulations were carried out for virtual monobasic drugs having a range of pKa, log D, and dose values as a function of presumed solubility and permeability. Results were normally expressed as the combination that resulted in 25% absorption. Absorption of basic drugs was found to be a function of the whole solubility/pH relationship rather than a single solubility value at pH 7. In addition, the parameter spaces of greatest sensitivity were identified. We compared 3 theoretical scenarios: the GIT pH range overlapping (1) only the salt solubility curve, (2) the salt and base solubility curves, or (3) only the base curve. Experimental solubilities of 32 compounds were determined at pHs of 2.2 and 7.4, and they nearly all fitted into 2 of the postulated scenarios. Typically, base solubilities can be simulated in silico, but salt solubilities at low pH can only be measured. We concluded that quality absorption simulations of candidate drugs in most cases require experimental solubility determination at 2 pHs, to permit calculation of the whole solubility/pH profile.     

pH-Dependent solubility and permeability criteria for provisional biopharmaceutics classification (BCS and BDDCS) in early drug discovery

The Biopharmaceutics Classification System (BCS) is a scientific framework that provides a basis for predicting the oral absorption of drugs. These concepts have been extended in the Biopharmaceutics Drug Disposition Classification System (BDDCS) to explain the potential mechanism of drug clearance and understand the effects of uptake and efflux transporters on absorption, distribution, metabolism, and elimination. The objective of present work is to establish criteria for provisional biopharmaceutics classification using pH-dependent passive permeability and aqueous solubility data generated from high throughput screening methodologies in drug discovery settings. The apparent permeability across monolayers of clonal cell line of Madin-Darby canine kidney cells, selected for low endogenous efflux transporter expression, was measured for a set of 105 drugs, with known BCS and BDDCS class. The permeability at apical pH 6.5 for acidic drugs and at pH 7.4 for nonacidic drugs showed a good correlation with the fraction absorbed in human (Fa). Receiver operating characteristic (ROC) curve analysis was utilized to define the permeability class boundary. At permeability ≥ 5 × 10(-6) cm/s, the accuracy of predicting Fa of ≥ 0.90 was 87%. Also, this cutoff showed more than 80% sensitivity and specificity in predicting the literature permeability classes (BCS), and the metabolism classes (BDDCS). The equilibrium solubility of a subset of 49 drugs was measured in pH 1.2 medium, pH 6.5 phosphate buffer, and in FaSSIF medium (pH 6.5). Although dose was not considered, good concordance of the measured solubility with BCS and BDDCS solubility class was achieved, when solubility at pH 1.2 was used for acidic compounds and FaSSIF solubility was used for basic, neutral, and zwitterionic compounds. Using a cutoff of 200 μg/mL, the data set suggested a 93% sensitivity and 86% specificity in predicting both the BCS and BDDCS solubility classes. In conclusion, this study identified pH-dependent permeability and solubility criteria that can be used to assign provisional biopharmaceutics class at early stage of the drug discovery process. Additionally, such a classification system will enable discovery scientists to assess the potential limiting factors to oral absorption, as well as help predict the drug disposition mechanisms and potential drug-drug interactions.     

Use of simulated intestinal fluid for Caco-2 permeability assay of lipophilic drugs

The most commonly used method to assess intestinal permeability is the measurement of compound flux across a Caco-2 cells monolayer by using Hanks balanced salt solution (HBSS)-like buffers. Nevertheless, lipophilic acid drugs are poorly or not at all soluble in these types of buffers and their adsorption on the transwell plate is commonly observed. To reduce adsorption and increase solubility, permeability assays need to be developed in conditions other than classic conditions for lipophilic compounds. The best model to increase recovery of lipophilic compounds was determined as fasted state simulated intestinal fluid (FaSSIF) in the apical compartment and HBSS with 1% bovine serum albumin (BSA) in basolateral compartment. This model allows a correlation between absorption on Caco-2 cells and absorbed fraction in humans. For 35 compounds, only 2 outliers were observed in the Caco-2 assay using the FaSSIF model. These two outliers were the same outlier compounds as those observed with a classic Caco-2 method. Furthermore, a permeability assay of Pgp substrates evidenced efflux transport in both models and addition of a Pgp inhibitor suppressed Pgp efflux transport. FaSSIF in the apical compartment and HBSS with 1% BSA in the basolateral compartment is the model of choice to predict in vivo absorption for lipophilic acid drugs.     

A Theoretical Basis for a Biopharmaceutic Drug Classification: The Correlation of in Vitro Drug Product Dissolution and in Vivo Bioavailability

A biopharmaceutics drug classification scheme for correlating in vitro drug product dissolution and in vivo bioavailability is proposed based on recognizing that drug dissolution and gastrointestinal permeability are the fundamental parameters controlling rate and extent of drug absorption. This analysis uses a transport model and human permeability results for estimating in vivo drug absorption to illustrate the primary importance of solubility and permeability on drug absorption. The fundamental parameters which define oral drug absorption in humans resulting from this analysis are discussed and used as a basis for this classification scheme. These Biopharmaceutic Drug Classes are defined as: Case 1. High solubility-high permeability drugs, Case 2. Low solubility-high permeability drugs, Case 3. High solubility-low permeability drugs, and Case 4. Low solubility-low permeability drugs. Based on this classification scheme, suggestions are made for setting standards for in vitro drug dissolution testing methodology which will correlate with the in vivo process. This methodology must be based on the physiological and physical chemical properties controlling drug absorption. This analysis points out conditions under which no in vitro-in vivo correlation may be expected e.g. rapidly dissolving low permeability drugs. Furthermore, it is suggested for example that for very rapidly dissolving high solubility drugs, e.g. 85% dissolution in less than 15 minutes, a simple one point dissolution test, is all that may be needed to insure bioavailability. For slowly dissolving drugs a dissolution profile is required with multiple time points in systems which would include low pH, physiological pH, and surfactants and the in vitro conditions should mimic the in vivo processes. This classification scheme provides a basis for establishing in vitro-in vivo correlations and for estimating the absorption of drugs based on the fundamental dissolution and permeability properties of physiologic importance.     

The solubility of drugs in molten suppository bases

Solubility of 18 drugs used in rectal therapy in molten suppository base, (Massa suppositoriorum 15, Pharmacopeia of the G.D.R. Rosupol U, 37 degrees C) is determined. Separation of drug-saturated base from drug cristals is carried out in an air-heated centrifuge followed by a partition step between petroleum ether and an aqueous medium and photometrical determination of drug concentration. The procedure leads to results with relative standard deviation of 2% (medium value of 18 drugs). The investigation gave solubility values in the range between 0.002% (theobromine) and 26% (lidocaine), whereas benzoate, phenobarbital sodium and procaine hydrochloride are completely insoluble. Sodium salicylate has an unexpected high solubility (0.48%). The solubility is temperature depended and thus the solubility enthalpy can be determined. Relations between structure and solubility are discussed for pyrazolin-5-ones and purines. A correlation between solubilities in the base and in aqueous buffer (phosphate, pH 7.4) does not exist in the series studied.     

Effect of buffer media composition on the solubility and effective permeability coefficient of ibuprofen

The effect of perfusion medium composition on the two important biopharmaceutical parameters drug solubility and permeability was determined for ibuprofen. Eight commonly used buffers were examined. Equilibrium solubility, buffer capacity profiles and permeability coefficients, using the in situ rat gut perfusion model, were determined for each medium at 37 degrees C. The solubility of ibuprofen differed sixfold over the range of buffer systems studied. The differences in solubility were associated with different pHs of the buffers when saturated with drug and also the presence of micelles and divalent ions. The solubility of ibuprofen in FeSSIF was significantly higher than predicted from the pH due to micellisation, while that in Krebs was significantly lower due to ibuprofen-calcium salt formation. Buffer capacities varied over a 40-fold range. The pK(a) values of the buffer components were determined from the buffer capacity versus pH profiles and were in good agreement with the thermodynamic values when corrected for temperature and ionic strength. Smaller, but statistically significant differences in P(app) values for ibuprofen were also observed between some of the buffers. During perfusion, pHs of the perfusate samples gradually changed over time towards a median value of approximately 6.5. HBSS gave a P(app) approximately 50% greater than that observed in PBS 7.4. Physicochemical factors such as medium pH, buffer capacity and osmolarity should be considered when determining the P(app) values of ionisable compounds. Care needs to be exercised when comparing P(app) values from different laboratories as buffer composition can have a significant effect on both solubility and permeability of a drug, whose ionisation is substantially changed over the pH range of the buffers. Despite the high amount ionised, ibuprofen appears to be well absorbed and it can be classified as a highly permeable drug.     

Prediction of absorbability of poorly water-soluble drugs based on permeability obtained through modified in vitro chamber method

Permeability is an underlying parameter to control drug absorption. For highly water-soluble drugs, the high correlation between their permeability and fraction absorbed in humans is reported. In the present study, to predict the absorbability of poorly water-soluble drugs in humans, a new experimental method of the permeation study was proposed and subjected to examination. Firstly, using the in vitro chamber method modified to contain 5% (final concentration) dimethyl sulufoxide (DMSO) in both compartments of the chamber (DMSO-MS), the effect of DMSO on membrane integrity was evaluated. Secondly, the correlation between the apparent permeability coefficients (Papp) obtained through DMSO-M or DMSO-MS and fractions absorbed in humans were investigated using 7 poorly water-soluble drugs. Membrane integrity of the rat intestinal tissues was maintained after using DMSO-MS, as with that after using the conventional in vitro chamber method. Papp of two paracellular markers obtained through DMSO-MS was not different from that obtained through the conventional chamber method. In the permeation study of the P-glycoprotein substrate, Papp from both mucosal to serosal and serosal to mucosal sides obtained through DMSO-MS was not significantly different from that obtained through the conventional chamber method. The correlation between Papp obtained through DMSO-MS and Fa which was expressed by the equation of Fa=1-exp (-Pappx1.38x10(5)) (r2=0.980), was more favorable than the correlation between Papp obtained through DMSO-M and Fa which was expressed by the equation of Fa=1-exp (-Pappx2.12x10(5)) (r2=0.875). These results showed that DMSO-MS was a useful method for predicting the absorbability of poorly water-soluble drugs.