Comparative Analysis of Chemical Descriptors by Machine Learning Reveals Atomistic Insights into Solute-Lipid Interactions.

This study explores the research area of drug solubility in lipid excipients, an area persistently complex despite recent advancements in understanding and predicting solubility based on molecular structure. To this end, this research investigated novel descriptor sets, employing machine learning techniques to understand the determinants governing interactions between solutes and medium-chain triglycerides (MCTs). Quantitative structure-property relationships (QSPR) were constructed on an extended solubility data set comprising 182 experimental values of structurally diverse drug molecules, including both development and marketed drugs to extract meaningful property relationships. Four classes of molecular descriptors, ranging from traditional representations to complex geometrical descriptions, were assessed and compared in terms of their predictive accuracy and interpretability. These include two-dimensional (2D) and three-dimensional (3D) descriptors, Abraham solvation parameters, extended connectivity fingerprints (ECFPs), and the smooth overlap of atomic position (SOAP) descriptor. Through testing three distinct regularized regression algorithms alongside various preprocessing schemes, the SOAP descriptor enabled the construction of a superior performing model in terms of interpretability and accuracy. Its atom-centered characteristics allowed contributions to be estimated at the atomic level, thereby enabling the ranking of prevalent molecular motifs and their influence on drug solubility in MCTs. The performance on a separate test set demonstrated high predictive accuracy (RMSE = 0.50) for 2D and 3D, SOAP, and Abraham Solvation descriptors. The model trained on ECFP4 descriptors resulted in inferior predictive accuracy. Lastly, uncertainty estimations for each model were introduced to assess their applicability domains and provide information on where the models may extrapolate in chemical space and, thus, where more data may be necessary to refine a data-driven approach to predict solubility in MCTs. Overall, the presented approaches further enable computationally informed formulation development by introducing a novel in silico approach for rational drug development and prediction of dose loading in lipids.

Precellys® Evolution Homogenizer - a versatile instrument for milling, mixing, and amorphization of drugs in preformulation.

The aim of this work was the evaluation and introduction of the Bertin Precellys® Evolution homogenizer with Cryolys® as a valuable and versatile tool for the improvement of workflows in the preformulation phase of drug development. The presented pilot experiments indicate that the instrument can be applied for (1) screening of appropriate vehicles for the generation of micro- and nano suspensions, (2) small-scale manufacturing of suspension formulations for preclinical animal studies, (3) drug amorphization and identification of appropriate excipients for amorphous systems, and (4) preparation of homogenous powder blends. The instrument allows the rapid, parallel, and compound-sparing screening of formulation approaches and small-scale formulation manufacturing, in particular for low solubility compounds. For the characterization of generated formulations, miniaturized methods are introduced such as a screening tool for suspension sedimentation and redispersion and a non-sink dissolution model in biorelevant media in microtiter plates. This work summarizes exploratory, proof-of-concept studies and opens up new opportunities for more extended studies with this instrument in various application areas.

From Quantum Chemistry to Prediction of Drug Solubility in Glycerides.

Lipid-based delivery is a key technology for dealing with the challenges of poorly soluble drugs. Therefore, prediction of drug solubility in lipid-based excipients and their mixtures is an important research goal in computational pharmaceutics. This study is based on the conductor-like screening model for real solvents (COSMO-RS), which combines quantum chemical surface calculations with fluid phase thermodynamics. An experimental dataset of 51 drugs was collected with measured thermochemical data and solubility results in medium and long-chain tri- and monoglycerides. For the theoretical model, the excipients were represented by a single structure in a simplified glyceride approach. COSMO-RS was able to capture the solubility trends in the different excipients. Only a few compounds showed rather poor predictions and these outliers were often comparatively larger molecules. The present study also evaluated the effects of individual fatty acid hydrolysis on glycerides' solubilization capability. In conclusion, the application of COSMO-RS modeling for drug solubility prediction in lipid-based formulations is highly promising, in particular for rank-ordering excipients in an early development phase. In future, this in silico approach may also address solubilization effects of minor components in excipients or in excipient mixtures, which is interesting from a product quality perspective so that it can further advance this field of molecular pharmaceutics.

Risdiplam distributes and increases SMN protein in both the central nervous system and peripheral organs.

Spinal muscular atrophy (SMA) is a rare, inherited neuromuscular disease caused by deletion and/or mutation of the Survival of Motor Neuron 1 (SMN1) gene. A second gene, SMN2, produces low levels of functional SMN protein that are insufficient to fully compensate for the lack of SMN1. Risdiplam (RG7916; RO7034067) is an orally administered, small-molecule SMN2 pre-mRNA splicing modifier that distributes into the central nervous system (CNS) and peripheral tissues. To further explore risdiplam distribution, we assessed in vitro characteristics and in vivo drug levels and effect of risdiplam on SMN protein expression in different tissues in animal models. Total drug levels were similar in plasma, muscle, and brain of mice (n = 90), rats (n = 148), and monkeys (n = 24). As expected mechanistically based on its high passive permeability and not being a human multidrug resistance protein 1 substrate, risdiplam CSF levels reflected free compound concentration in plasma in monkeys. Tissue distribution remained unchanged when monkeys received risdiplam once daily for 39 weeks. A parallel dose-dependent increase in SMN protein levels was seen in CNS and peripheral tissues in two SMA mouse models dosed with risdiplam. These in vitro and in vivo preclinical data strongly suggest that functional SMN protein increases seen in patients' blood following risdiplam treatment should reflect similar increases in functional SMN protein in the CNS, muscle, and other peripheral tissues.

Interactions of dimethylaminoethyl methacrylate copolymer with non-acidic drugs demonstrated high solubilization in vitro and pronounced sustained release in vivo.

Recent work demonstrated remarkable solubilization effects of methacrylate-copolymer Eudragit EPO (EPO) not only with acidic drugs but interestingly also with poorly soluble basic compounds. The current work studied EPO-mediated solubilization effects first in vitro using felodipine (FLP) and tamoxifen (TMX) as model compounds. EPO-containing solutions were subsequently compared in a rat pharmacokinetic study against reference solutions and suspensions. Surprisingly, solution formulations with EPO did not result in an increased relative oral bioavailability. Exposure was reduced for both drugs and plasma-profiles of the EPO solutions showed a delayed and lower maximum plasma concentration compared to the reference formulations. This sustained in vivo release was likely due to combined effects of strong drug-polymer interactions and pH-dependent precipitation of the polymer in the rat intestine. Remarkable was that in vitro drug-polymer coprecipitates did not reveal crystalline drug by polarized light microscopy. Thus, such a formulation approach provides a rather simple opportunity to modify drug release in vivo. However, this may be rather an approach for preclinical formulations, if high peak-to-trough ratios of plasma levels are problematic regarding adverse effects related to Cmax or if plasma concentrations drop too fast below required pharmacological concentrations.

Unexpected Solubility Enhancement of Drug Bases in the Presence of a Dimethylaminoethyl Methacrylate Copolymer.

The methacrylate copolymer Eudragit EPO (EPO) has previously shown to greatly enhance solubilization of acidic drugs via ionic interactions and by multiple hydrophobic contacts with polymeric side chains. The latter type of interaction could also play a role for solubilization of other compounds than acids. The aim of this study was therefore to investigate the solubility of six poorly soluble bases in presence and absence of EPO by quantitative ultrapressure liquid chromatography with concomitant X-ray powder diffraction analysis of the solid state. For a better mechanistic understanding, spectra and diffusion data were obtained by 1H nuclear magnetic resonance (NMR) spectroscopy. Unexpected high solubility enhancement (up to 360-fold) was evidenced in the presence of EPO despite the fact that bases and polymer were both carrying positive charges. This exceptional and unexpected solubilization was not due to a change in the crystalline solid state. NMR spectra and measured diffusion coefficients indicated both strong drug-polymer interactions in the bulk solution, and diffusion data suggested conformational changes of the polymer in solution. Such conformational changes may have increased the accessibility and extent of hydrophobic contacts thereby leading to increased overall molecular interactions. These initially surprising solubilization results demonstrate that excipient selection should not be based solely on simple considerations of, for example, opposite charges of drug and excipient, but it requires a more refined molecular view. Different solution NMR techniques are especially promising tools to gain such mechanistic insights.

Can we estimate the critical micelle concentration of amphiphilic drug bases from molecular connectivity indices?

Self-aggregation of drugs is since many years an important topic in the pharmaceutical sciences. Drugs can aggregate similar to surfactants and thereby exhibit a critical micelle concentration (CMC). The present work focused on amphiphilic drug bases and it was aimed to predict log(CMC) based on chemical structure alone. A dataset of 35 compounds was gathered mostly form the literature and complemented with own measurements based on ultrasonic resonator technology. The hydrophilic-lipophilic balance (HLB) values of the protonated bases were calculated and provided a range of 22.9-27.4. Based on a hypothesis from surfactant sciences, it was tried to predict log(CMC) with connectivity and shape indices as well as molecular dipole moment. A fairly good model was obtained using the Randix index (RI), 3 D Wiener number (WN) and molecular dipole moment (DM) (R2 = 0.824). Interestingly, a simple linear regression of log(CMC) with the Randic index alone, resulted in an acceptable model (R2 = 0.755). The present work should help with early identification of drug bases that exhibit surfactant-like behavior and an estimation of log(CMC) values is proposed. An improved understanding of drug aggregation and prediction of log(CMC) helps to better cope with physical consequences like, for example, "anomalous" drug solubility in drug discovery and development.

The quest for exceptional drug solubilization in diluted surfactant solutions and consideration of residual solid state.

Solubility screening in different surfactant solutions is an important part of pharmaceutical profiling. A particular interest is in low surfactant concentrations that mimic the dilution of an oral dosage form. Despite of intensive previous research on solubilization in micelles, there is only limited data available at low surfactant concentrations and generally missing is a physical state analysis of the residual solid. The present work therefore studied 13 model drugs in 6 different oral surfactant solutions (0.5%, w/w) by concomitant X-ray diffraction (XRPD) analysis to consider effects on solvent-mediated phase transformations. A particular aspect was potential occurrence of exceptionally high drug solubilization. As a result, general solubilization correlations were observed especially between surfactants that share chemical similarity. Exceptional solubility enhancement of several hundred-fold was evidenced in case of sodium dodecyl sulfate solutions with dipyridamole and progesterone. Furthermore, carbamazepine and testosterone showed surfactant-type dependent hydrate formation. The present results are of practical relevance for an optimization of surfactant screenings in preformulation and early development and provide a basis for mechanistic modeling of surfactant effects on solubilization and solid state modifications.

A Systematic Study of Molecular Interactions of Anionic Drugs with a Dimethylaminoethyl Methacrylate Copolymer Regarding Solubility Enhancement.

The methacrylate-copolymer Eudragit EPO (EPO) has raised interest in solubility enhancement of anionic drugs. Effects on aqueous drug solubility at rather low polymer concentrations are barely known despite their importance upon dissolution and dilution of oral dosage forms. We provide evidence for substantial enhancement (factor 4-230) of aqueous solubility of poorly water-soluble anionic drugs induced by low (0.1-5% (w/w)) concentration of EPO for a panel of seven acidic crystalline drugs. Diffusion data (determined by 1H nuclear magnetic resonance spectroscopy) indicate that the solubility increasing effect monitored by quantitative ultraperformance liquid chromatography was caused primarily by molecular API polymer interactions in the bulk liquid phase. Residual solid API remained unaltered as tested by X-ray powder diffraction. The solubility enhancement (SE) revealed a significant rank correlation (rSpearman = -0.83) with rDiffAPI, where SE and rDiffAPI are defined ratios of solubility and diffusion coefficient in the presence and absence of EPO. SE decreased in the order of indomethacin, mefenamic acid, warfarin, piroxicam, furosemide, bezafibrate, and tolbutamide. The solubilizing effect was attributed to both ionic and hydrophobic interactions between drugs and EPO. The excellent solubilizing properties of EPO are highly promising for pharmaceutical development, and the data set provides first steps toward an understanding of drug-excipient interaction mechanisms.

Miniaturized X-ray powder diffraction assay (MixRay) for quantitative kinetic analysis of solvent-mediated phase transformations in pharmaceutics.

Many pharmaceutical compounds exhibit polymorphism, which may result in solvent-mediated phase transformations. Since the polymorphic form has an essential influence on physicochemical characteristics such as solubility or dissolution rate, it is crucial to know the exact polymorphic composition of a drug throughout pharmaceutical development. This study addressed the need to perform quantitative X-ray analysis of polymorphic mixtures on a 96-well scale (MixRay). A calibration of polymorphic mixtures (anhydrate and hydrate) was performed with three model drugs, caffeine, piroxicam, and testosterone, and linear correlations were obtained for all compounds. The MixRay approach for piroxicam was applied to a solubility and residual solid screening assay (SORESOS) to quantify the amount of hydrate and anhydrate corresponding to kinetic bulk concentrations. Changes in these drug concentrations correlated well with the kinetic changes in the residual solid. The influence of excipients on the solid state and kinetic concentrations of piroxicam was also studied. Excipients strongly affected polymorphic transformation kinetics of piroxicam and concentrations after 24h depended on the excipient used. The new calibration X-ray method combined with bulk concentration analysis provides a valuable tool for both pharmaceutical profiling and early formulation development.

Theoretical Considerations of the Prigogine-Defay Ratio with Regard to the Glass-Forming Ability of Drugs from Undercooled Melts.

Drug behavior in undercooled melts is highly important for pharmaceutics with regard to amorphous solid dispersions, and therefore, categories were recently introduced that differentiate glass formers (GFs) from other drugs that are nonglass formers (nGFs). The present study is based on the assumption that molecular properties relevant for the so-called Prigogine-Defay (PD) ratio would be indicative of a drug's glass-forming ability. The PD ratio depends in theory on the entropy of fusion and molar volume. Experimental data were gathered from a broad set of pharmaceutical compounds (n = 54) using differential scanning calorimetry. The obtained entropy of fusion and molar volume were indeed found to significantly discriminate GFs from nGFs. In a next step, the entropy of fusion was predicted by different in silico methods. A first group contribution method provided rather unreliable estimates for the entropy of fusion, while an alternative in silico approach seemed more promising for drug categorization. Thus, a significant discrimination model employed molar volume, a so-called effective hydrogen bond number, and effective number of torsional bonds (or torsional units) to categorize GFs and nGFs (p ≤ 0.0000). The results led to new insights into drug vitrification and to practical rules of thumb. The latter may serve as guidance in pharmaceutical profiling and early formulation development with respect to amorphous drug formulations.

Miniaturized INtrinsic DISsolution Screening (MINDISS) assay for preformulation.

This study describes a novel Miniaturized INtrinsic DISsolution Screening (MINDISS) assay for measuring disk intrinsic dissolution rates (DIDR). In MINDISS, compacted mini disks of drugs (2-5mg/disk) are prepared in custom made holders with a surface area of 3mm(2). Disks are immersed, pellet side down, into 0.35ml of appropriate dissolution media per well in 96-well microtiter plates, media are stirred and disk-holders are transferred to new wells after defined periods of time. After filtration, drug concentration in dissolution media is quantified by Ultra Performance Liquid Chromatography (UPLC) and solid state property of the disk is characterized by Raman spectroscopy. MINDISS was identified as an easy-to-use tool for rapid, parallel determination of DIDR of compounds that requires only small amounts of compound and of dissolution medium. Results obtained with marketed drugs in MINDISS correlate well with large scale DIDR methods and indicate that MINDISS can be used for (1) rank-ordering of compounds by intrinsic dissolution in late phase discovery and early development, (2) comparison of polymorphic forms and salts, (3) screening and selection of appropriate dissolution media, and (4) characterization of the intestinal release behavior of compounds along the gastro intestinal tract by changing biorelevant media during experiments.

Atomic force microscopy-based screening of drug-excipient miscibility and stability of solid dispersions.

PURPOSE: Development of a method to assess the drug/polymer miscibility and stability of solid dispersions using a melt-based mixing method. METHODS: Amorphous fractured films are prepared and characterized with Raman Microscopy in combination with Atomic Force Microscopy to discriminate between homogenously and heterogeneously mixed drug/polymer combinations. The homogenous combinations are analyzed further for physical stability under stress conditions, such as increased humidity or temperature. RESULTS: Combinations that have the potential to form a molecular disperse mixture are identified. Their potential to phase separate is determined through imaging at molecular length scales, which results in short observation time. De-mixing is quantified by phase separation analysis, and the drug/polymer combinations are ranked to identify the most stable combinations. CONCLUSIONS: The presented results demonstrate that drug/polymer miscibility and stability of solid dispersions, with many mechanistic details, can be analyzed with Atomic Force Microscopy. The assay allows to identify well-miscible and stable combinations within hours or a few days.

Benzodioxoles: novel cannabinoid-1 receptor inverse agonists for the treatment of obesity.

The application of the evolutionary fragment-based de novo design tool TOPology Assigning System (TOPAS), starting from a known CB1R (CB-1 receptor) ligand, followed by further refinement principles, including pharmacophore compliance, chemical tractability, and drug likeness, allowed the identification of benzodioxoles as a novel CB1R inverse agonist series. Extensive multidimensional optimization was rewarded by the identification of promising lead compounds, showing in vivo activity. These compounds reversed the CP-55940-induced hypothermia in Naval Medical Research Institute (NMRI) mice and reduced body-weight gain, as well as fat mass, in diet-induced obese Sprague-Dawley rats. Herein, we disclose the tools and strategies that were employed for rapid hit identification, synthesis and generation of structure-activity relationships, ultimately leading to the identification of (+)-[( R)-2-(2,4-dichloride-phenyl)-6-fluoro-2-(4-fluoro-phenyl)-benzo[1,3]dioxol-5-yl]-morpholin-4-yl-methanone ( R)-14g . Biochemical, pharmacokinetic, and pharmacodynamic characteristics of ( R)-14g are discussed.

High throughput solubility measurement in drug discovery and development.

Measurement of drug solubility in various solvents is one of the key elements of compound characterization during the whole discovery and development process. This review summarizes current experimental approaches and addresses recent advances in the experimental methods used to determine drug solubility in drug discovery and early development. This paper focuses on high throughput methods designed to determine kinetic and thermodynamic (equilibrium) solubility but traditional methods are also presented. The focus, positioning, experimental setup, pros and cons, and limitations of individual assays are discussed and differences in solubility studies in discovery and development environments are highlighted. Finally, future needs and trends in solubility assay development designed to overcome current bottlenecks and trade-offs between speed and quality/quantity of measurements are addressed.

Miniaturized assay for solubility and residual solid screening (SORESOS) in early drug development.

PURPOSE: The aim was to develop a miniaturized method for solubility and residual solid screening of drug compounds in aqueous and non-aqueous vehicles in early drug development. METHODS: Different crystal modifications of caffeine, carbamazepine, and piroxicam were added into 96-well filter plates and solubility was determined in 100 microl of 17 pharmaceutical vehicles. After filtration, drug concentration was determined by Ultra Performance Liquid Chromatography (UPLC). Residual solid drug in the filter plates was analyzed by high-throughput (HT) transmission X-ray Powder Diffraction (XRPD). RESULTS: HT XRPD analysis revealed solid form conversions of all compounds during solubility determination, e.g., formation of hydrates in aqueous vehicles (caffeine, carbamazepine, piroxicam) or conversion of a metastable crystal form to the stable form (caffeine). Drug solubility was strongly dependent on the crystal modifications formed during the solubility assay. CONCLUSIONS: The new assay allows the simultaneous, small scale screening of drug solubility in various pharmaceutical vehicles and identification of changes in solid form. It is useful for the identification of formulations and formulation options in non-clinical and clinical development.

Development of a partially automated solubility screening (PASS) assay for early drug development.

A medium-throughput, compound-saving, thermodynamic solubility assay for early drug development was developed. Solid compound suspended in heptane was used for simple, time-saving, and flexible compound distribution into 96-well plates, with minor risk to generate new physical forms during dispensing. Low volume, well-stirred incubation vessels were generated by using a combination of V-shaped wells, well caps, and vertically inserted stir bars. This allowed solubility determination up to 100 mg/mL in 40-80 microL volumes in aqueous and nonaqueous, low- and high-viscosity solvents. After removal of residual solid through syringe filters mounted on microtiter plates, the filtrate was quantified by ultra performance liquid chromatography (UPLC) using a 1.2 min gradient. Combined with a robotic liquid handling system, throughput was 45 samples per hour and >600 solubility measurements per week. Results from the partially automated solubility screening (PASS) assay correlated well with reported solubility values (r2 = 0.882). The PASS assay is useful for compound-saving, thermodynamic solubility measurement at the discovery-development interface where maximal solubility in many commonly used solvents needs to be determined. PASS results provide a basis for the identification of formulation strategies, the selection of appropriate excipients, and for the prediction of the potential in vivo behavior of compounds.

Drug-excipient compatibility testing using a high-throughput approach and statistical design.

The aim of our research was to develop a miniaturized high throughput drug-excipient compatibility test. Experiments were planned and evaluated using statistical experimental design. Binary mixtures of a drug, acetylsalicylic acid, or fluoxetine hydrochloride, and of excipients commonly used in solid dosage forms were prepared at a ratio of approximately 1:100 in 96-well microtiter plates. Samples were exposed to different temperature (40 degrees C/ 50 degrees C) and humidity (10%/75%) for different time (1 week/4 weeks), and chemical drug degradation was analyzed using a fast gradient high pressure liquid chromatography (HPLC). Categorical statistical design was applied to identify the effects and interactions of time, temperature, humidity, and excipient on drug degradation. Acetylsalicylic acid was least stable in the presence of magnesium stearate, dibasic calcium phosphate, or sodium starch glycolate. Fluoxetine hydrochloride exhibited a marked degradation only with lactose. Factor-interaction plots revealed that the relative humidity had the strongest effect on the drug excipient blends tested. In conclusion, the developed technique enables fast drug-excipient compatibility testing and identification of interactions. Since only 0.1 mg of drug is needed per data point, fast rational preselection of the pharmaceutical additives can be performed early in solid dosage form development.

The critical concentration of C1-esterase inhibitor (C1-INH) in human serum preventing auto-activation of the first component of complement (C1).

C1-esterase inhibitor (C1-INH) was depleted from normal human serum (NHS) at 4 degrees C by affinity chromatography with a monoclonal anti-C1-INH antibody (mAb 13 E1) coupled to CNBr-activated Sepharose 4B. The C1-INH-depleted serum (C1-INH-depl-HS) had normal levels of C1, C4, and CH 50 and C1-INH concentration was less than 10% of normal (15 microg/ml in C1-INH-depl-HS compared to 230 microg/ml in NHS). C1-auto-activation in C1-INH-depl-HS was followed by measuring C4-consumption in a haemolytic assay and by detection of activated C1s in a C1s-ELISA. After a lag phase of 10-20 min, C1-auto-activation in C1-INH depl-HS occurred and reached its maximum after 40 min at 37 degrees C. In contrast, neutralization of C1-INH activity in NHS by addition of monoclonal antibodies directed against its C1s-binding site, resulted in an immediate, spontaneous C1-activation without a lag-phase and reached its maximum already after 20 min (mAb 140) and 25 min (mAb 88G2). Addition of highly purified C1-INH or NHS as source of C1-INH to C1-INH-depleted serum to a final concentration of 55 microg/ml (22% of normal C1-INH concentration in HS) was sufficient to control spontaneous C1-activation.

P-glycoprotein and surfactants: effect on intestinal talinolol absorption.

BACKGROUND AND OBJECTIVE: Surfactants used in pharmaceutical formulations can modulate drug absorption by multiple mechanisms including inhibition of intestinal P-glycoprotein (P-gp). Our objective was to analyze the effect of 2 surfactants with different affinity for P-gp in vitro on the intestinal absorption and bioavailability of the P-gp substrate talinolol in humans. METHODS: In vitro, the influence of surfactants on talinolol permeability was studied in Caco-2 cells. In vivo, an open-label 3-way crossover study with 9 healthy male volunteers was performed. Subjects were intubated with a 1-lumen nasogastrointestinal tube. The study solution, containing either talinolol (50 mg), talinolol and D-alpha-tocopheryl polyethylene glycol 1000 succinate (TPGS) (0.04%), or talinolol and Poloxamer 188 (0.8%), was administered through the tube. RESULTS: TPGS, but not Poloxamer 188, inhibited the P-gp-mediated talinolol transport in Caco-2 cells. In healthy volunteers TPGS increased the area under the plasma concentration-time curve with extrapolation to infinity (AUC 0-infinity ) of talinolol by 39% (90% confidence interval, 1.10-1.75) and the maximum plasma concentration (C max) by 100% (90% confidence interval, 1.39-2.88). Poloxamer 188 did not significantly alter the AUC 0-infinity or C max of talinolol. CONCLUSIONS: This in vivo intraduodenal perfusion study showed that low concentrations of TPGS, close to the concentrations that showed P-gp inhibition in vitro, significantly increased the bioavailability of talinolol. The study design excluded modulation of solubility by TPGS and unspecific surfactant-related effects. The latter was supported by the absence of modulation of the talinolol pharmacokinetics by Poloxamer 188, which does not modulate P-gp. Therefore we consider intestinal P-gp inhibition by TPGS as the major underlying mechanism for the increase in talinolol bioavailability.