Shifting the Focus from Dissolution to Permeation: Introducing the Meso-fluidic Chip for Permeability Assessment (MCPA).

In response to the growing ethical and environmental concerns associated with animal testing, numerous in vitro tools of varying complexity and biorelevance have been developed and adopted in pharmaceutical research and development. In this work, we present one of these tools, i.e., the Meso-fluidic Chip for Permeability Assessment (MCPA), for the first time. The MCPA combines an artificial barrier (PermeaPad®) with an organ-on-chip device (MIVO®) and real-time automated concentration measurements, to yield a sustainable, yet effortless method for permeation testing. The system offers three major physiological aspects, i.e., a biomimetic membrane, an optimal membrane interfacial area-to-donor-volume-ratio (A/V) and a physiological flow on the acceptor/basolateral side, which makes the MPCA an ideal candidate for mechanistic studies and excellent in vivo bioavailability predictions. We validated the method with a handful of assorted drug compounds in unstirred and stirred donor conditions, before exploring its applicability as a tool for dissolution/permeation testing on a BCS class III/I drug (pyrazinamide) crystalline adducts and BCS class II/IV (hydrocortisone) amorphous solid dispersions. The results were highly reproducible and clearly displayed the method's potential for evaluating the performance of enabling formulations, and possibly even predicting in vivo performance. We believe that, upon further development, the MCPA will serve as a useful in vitro tool that could push sustainability into pharmaceutics by refining, reducing and replacing animal testing in early-stage drug development.

Interpreting permeability as a function of free drug fraction: The case studies of cyclodextrins and liposomes.

In order to solubilize poorly soluble active pharmaceutical ingredients, various strategies have been implemented over the years, including the use of nanocarriers, such as cyclodextrins and liposomes. However, improving a drug's apparent solubility does not always translate to enhanced bioavailability. This work aimed to investigate to which extent complexation with cyclodextrins and incorporation into liposomes influence drug in vitro permeability and to find a mechanistic description of the permeation process. For this purpose, we investigated hydroxypropyl-ß-cyclodextrin (HP-ß-CD) and phosphatidylcholine liposomes formulations of three chemically diverse compounds (atenolol, ketoprofen and hydrocortisone). We studied drug diffusion of the formulations by UV-localized spectroscopy and advanced data fitting to extract parameters such as diffusivity and bound-/free drug fractions. We then correlated this information with in vitro drug permeability obtained with the novel PermeaPadⓇ barrier. The results showed that increased concentration of HP-ß-CD leads to increased solubilization of the poorly soluble unionized ketoprofen, as well as hydrocortisone. However, this net increment of apparent solubility was not proportional to the increased flux measured. On the other hand, normalising the flux over the empirical free drug concentration, i.e., the free fraction, gave a meaningful absolute permeability coefficient. The results achieved for the liposomal formulation were consistent with the finding on cyclodextrins. In conclusion, we proved the adequacy and usefulness of our method for calculating free drug fractions in the examined enabling formulations, supporting the validity of the established drug diffusion/permeation theory that the unbounded drug fraction is the main driver for drug permeation across a membrane.

Design and Characterization of an Ethosomal Gel Encapsulating Rosehip Extract.

Rising environmental awareness drives green consumers to purchase sustainable cosmetics based on natural bioactive compounds. The aim of this study was to deliver Rosa canina L. extract as a botanical ingredient in an anti-aging gel using an eco-friendly approach. Rosehip extract was first characterized in terms of its antioxidant activity through a DPPH assay and ROS reduction test and then encapsulated in ethosomal vesicles with different percentages of ethanol. All formulations were characterized in terms of size, polydispersity, zeta potential, and entrapment efficiency. Release and skin penetration/permeation data were obtained through in vitro studies, and cell viability was assessed using an MTT assay on WS1 fibroblasts. Finally, ethosomes were incorporated in hyaluronic gels (1% or 2% w/v) to facilitate skin application, and rheological properties were studied. Rosehip extract (1 mg/mL) revealed a high antioxidant activity and was successfully encapsulated in ethosomes containing 30% ethanol, having small sizes (225.4 ± 7.0 nm), low polydispersity (0.26 ± 0.02), and good entrapment efficiency (93.41 ± 5.30%). This formulation incorporated in a hyaluronic gel 1% w/v showed an optimal pH for skin application (5.6 ± 0.2), good spreadability, and stability over 60 days at 4 °C. Considering sustainable ingredients and eco-friendly manufacturing technology, the ethosomal gel of rosehip extract could be an innovative and green anti-aging skincare product.

Commercially Available Cell-Free Permeability Tests for Industrial Drug Development: Increased Sustainability through Reduction of In Vivo Studies.

Replacing in vivo with in vitro studies can increase sustainability in the development of medicines. This principle has already been applied in the biowaiver approach based on the biopharmaceutical classification system, BCS. A biowaiver is a regulatory process in which a drug is approved based on evidence of in vitro equivalence, i.e., a dissolution test, rather than on in vivo bioequivalence. Currently biowaivers can only be granted for highly water-soluble drugs, i.e., BCS class I/III drugs. When evaluating poorly soluble drugs, i.e., BCS class II/IV drugs, in vitro dissolution testing has proved to be inadequate for predicting in vivo drug performance due to the lack of permeability interpretation. The aim of this review was to provide solid proofs that at least two commercially available cell-free in vitro assays, namely, the parallel artificial membrane permeability assay, PAMPA, and the PermeaPad® assay, PermeaPad, in different formats and set-ups, have the potential to reduce and replace in vivo testing to some extent, thus increasing sustainability in drug development. Based on the literature review presented here, we suggest that these assays should be implemented as alternatives to (1) more energy-intense in vitro methods, e.g., refining/replacing cell-based permeability assays, and (2) in vivo studies, e.g., reducing the number of pharmacokinetic studies conducted on animals and humans. For this to happen, a new and modern legislative framework for drug approval is required.

Validation and testing of a new artificial biomimetic barrier for estimation of transdermal drug absorption.

Human skin remains the most reliable model for studying the transdermal permeation of active compounds. Due to the limited source, porcine skin has been used extensively for performing penetration tests. Performing penetration studies by using human and animal skin, however, would also involve a series of ethical issues and restrictions. For these reasons, new biomimetic artificial barriers are being developed as possible alternatives for transdermal testing. If appropriately optimized, such products can be cost-effective, easily standardized across laboratories, precisely controlled in specific experimental conditions, or even present additional properties compared to the human and animal skin models such as negligible variability between replicates. In this current work we use the skin mimicking barrier (SMB) for drug permeability tests. The aim was to evaluate the suitability of the new barrier for studying the percutaneous absorption of the lipophilic extract of the plant Zingiber officinale Roscoe in vitro and compare its permeability ability with the artificial membrane Permeapad® and porcine skin. Our results showed that the permeability values obtained through the SMB are comparable are comparable to those obtained by using the porcine skin, suggesting that the new barrier may be an acceptable in vitro model for conducting percutaneous penetration experiments.

Drug delivery to the brain: In situ gelling formulation enhances carbamazepine diffusion through nasal mucosa models with mucin.

The objective of this work was to optimize a thermosensitive in situ gelling formulation to improve intranasal and nose-to-brain delivery of the antiepileptic drug carbamazepine (CBZ). A preliminary procedure of vehicles obtained just mixing different fractions of poloxamer 407 (P407) and poloxamer 188 (P188) revealed preparations with phase transition temperatures, times to gelation and pH values suitable for nasal delivery. Subsequently, the mucoadhesive properties of the most promising formulations were tuned by adding hydroxypropylmethylcellulose types of different viscosity grades, and the effect of the adhesive polymers was evaluated by testing in vitro time and strength of mucoadhesion on specimens of sheep nasal mucosa. The formulation that showed the greatest mucoadhesive potential in vitro, with a time and force of mucoadhesion equal to 1746,75 s and 3.66 × 10-4 N, respectively, was that composed of 22% P407, 5% P188 and 0.8% HPMC low-viscous and it was further investigated for its ability to increase drug solubility and to control the release of the drug. Lastly, the capability of the candidate vehicle to ensure drug permeation across the biomimetic membrane Permeapad®, an artificial phospholipid-based barrier with a stratified architecture, and the same barrier enriched with a mucin layer was verified. The final formulation was characterized by a pH value of 6.0, underwent gelation at 32.33°C in 37.85 s, thus showing all the features required by in situ gelling thermosensitive preparations designed for nasal delivery and, more notably, it conserved the ability to favor drug permeation in the presence of mucin. These findings suggest that the optimized gelling system could be a promising and easy to realize strategy to improve CBZ delivery to the brain exploiting both a direct and indirect pathway.

Modelling drug diffusion through unstirred water layers allows real-time quantification of free/loaded drug fractions and release kinetics from colloidal-based formulations.

The correlation between in vivo and in vitro data is yet not sufficiently optimized to allow a significant reduction and replacement of animal testing in pharmaceutical development. One of the main reasons for this lies in the poor mechanistic understanding and interpretation of the physical mechanisms enabling formulation rely on for deploying the drug. One mechanism that still lacks a proper interpretation is the kinetics of drug release from nanocarriers. In this work, we investigate two different types of classical enabling formulations - i) cyclodextrin solutions and ii) liposomal dispersions - by a combination of an experimental method (i.e. UV-Vis localized spectroscopy) and mathematical modelling/numerical data fitting. With this approach, we are able to discriminate precisely between the amount of drug bound to nanocarriers or freely dissolved at any time point; in addition, we can precisely estimate the binding and diffusivity constants of all chemical species (free drug/bound drug). The results obtained should serve as the first milestone for the further development of reliable in vitro/in silico models for the prediction of in vivo drug bioavailability when enabling formulations are used.

Human Lactobacillus Biosurfactants as Natural Excipients for Nasal Drug Delivery of Hydrocortisone.

The inclusion of a chemical permeation enhancer in a dosage form is considered an effective approach to improve absorption across the nasal mucosa. Herein we evaluated the possibility of exploiting biosurfactants (BS) produced by Lactobacillus gasseri BC9 as innovative natural excipients to improve nasal delivery of hydrocortisone (HC). BC9-BS ability to improve HC solubility and the BS mucoadhesive potential were investigated using the surfactant at a concentration below and above the critical micelle concentration (CMC). In vitro diffusion studies through the biomimetic membrane PermeaPad® and the same synthetic barrier functionalized with a mucin layer were assessed to determine BC9-BS absorption enhancing properties in the absence and presence of the mucus layer. Lastly, the diffusion study was performed across the sheep nasal mucosa using BC9-BS at a concentration below the CMC. Results showed that BC9-BS was able to interact with the main component of the nasal mucosa, and that it allowed for a greater solubilization and also permeation of the drug when it was employed at a low concentration. Overall, it seems that BC9-BS could be a promising alternative to chemical surfactants in the nasal drug delivery field.

Towards a better mechanistic comprehension of drug permeation and absorption: Introducing the diffusion-partitioning interplay.

The process of passive drug absorption from the gastrointestinal tract is still poorly understood and modelled. Additionally, the rapidly evolving field of pharmaceutics demands efficient, affordable and reliable in vitro tools for predicting in vivo performance. In this work, we combined established methods for quantifying drug diffusivity (localized UV-spectroscopy) and permeability (Permeapad® plate) in order to gain a better understanding of the role of unstirred water layers (UWLs) in drug absorption. The effect of diffusion/permeability media composition and viscosity on the apparent permeation resistance (Rapp) of model drugs caffeine (CAF) and hydrocortisone (HC) were tested and evaluated by varying the type and concentration of viscosity-enhancing agent - glycerol or a poly(ethylene glycol) (PEG) with different average molecular weights. For all types of media, increased viscosity lead to reduction in diffusivity but could not alone explain the observed effect, which was attributed to intermolecular polymer-drug interactions. Additionally, for both drugs, smaller hydrophilic viscosity-enhancing agents (glycerol and PEG 400) had larger influence than larger ones (PEG 3350 and 6000). The results highlighted the role of UWL as an additive barrier to permeation and indicated that diffusion through UWL is the rate-limiting step to CAF's permeation, whilst HC permeability is a partition-driven process.

Influence of Lactobacillus Biosurfactants on Skin Permeation of Hydrocortisone.

One of the most widely used strategies to improve drug diffusion through the skin is the use of permeation enhancers. The aim of this work was to investigate the effect of two biosurfactants (BS), produced by Lactobacillus crispatus BC1 and Lactobacillus gasseri BC9, on the skin permeation profile of hydrocortisone (HC, model drug). HC aqueous solubility and in vitro diffusion studies through porcine skin were performed in the presence of BC1-BS and BC9-BS at concentrations below and above critical micellar concentrations (CMC). Moreover, skin hydration tests and differential scanning calorimetry (DSC) analysis were performed to further investigate BS interaction with the outermost layer of the skin. Both BS increased HC solubility, especially at concentrations above their CMC. At concentrations below the CMC, drug permeation through the skin was improved, as the result of a dual effect: a) the formation of a superficial lipophilic environment, as confirmed by the reduction in skin hydration and b) the interaction between BS and the stratum corneum (SC), as demonstrated by the DSC curves. From the obtained data, it appears that BC1-BS and BC9-BS could represent new promising green excipients for drug permeation enhancement through the skin.

Impact of Mucin on Drug Diffusion: Development of a Straightforward in Vitro Method for the Determination of Drug Diffusivity in the Presence of Mucin.

Mucosal drug delivery accounts for various administration routes (i.e., oral, vaginal, ocular, pulmonary, etc.) and offers a vast surface for the permeation of drugs. However, the mucus layer which shields and lubricates all mucosal tissues can compromise drugs from reaching the epithelial site, thus affecting their absorption and therapeutic effect. Therefore, the effect of the mucus layer on drug absorption has to be evaluated early in the drug-development phase, prior to in vivo studies. For this reason, we developed a simple, cost-effective and reproducible method employing UV-visible localized spectroscopy for the assessment of the interaction between mucin and drugs with different physicochemical characteristics. The mucin-drug interaction was investigated by measuring the drug relative diffusivity (Drel) in the presence of mucin, and the method was validated by fitting experimental and mathematical data. In vitro permeability studies were also performed using the mucus-covered artificial permeation barrier (mucus-PVPA, Phospholipid Vesicle-based Permeation Assay) for comparison. The obtained results showed that the diffusion of drugs was hampered by the presence of mucin, especially at higher concentrations. This novel method proved to be suitable for the investigation on the extent of mucin-drug interaction and can be successfully used to assess the impact that the mucus layer has on drug absorption.

Interpreting non-linear drug diffusion data: Utilizing Korsmeyer-Peppas model to study drug release from liposomes.

The aim of this work was to clarify the dynamics behind the influence of ionic strength on the changes in drug release from large unilamellar vesicles (LUVs). For this purpose, we have investigated the transport of two different model drugs (caffeine and hydrocortisone) formulated into liposomes through different types of barriers with different retention properties (regenerated cellulose and the newly introduced biomimetic barrier, Permeapad®). Drug release from liposomes was studied utilizing the standard Franz diffusion cells. LUV dispersions were exposed to the isotonic, hypotonic and hypertonic environment (difference of 300 mOsm/kg between the initial LUVs and the environment) and experimental data treated with both linear and non-linear (Korsmeyer-Peppas) regression models. To alter the rigidity of the liposomal membranes, cholesterol was introduced in the liposomal barriers (up to 25% w/w). Korsmeyer-Peppas model was proven to be suited to analyse experimental data throughout the experimental time frame, providing important additive information in comparison to standard linear approximation. The obtained results are highly relevant as they improve the interpretation of drug release kinetics from LUVs under osmotic stress. Moreover, the findings can be utilized in the development of liposomal formulations intended for nose-to-brain targeted drug delivery.

Studying the effect of solubilizing agents on drug diffusion through the unstirred water layer (UWL) by localized spectroscopy.

An experimental/computational approach has been successfully applied in order to study the effect of solubilizing vehicles (cyclodextrins and liposomes) on the passive diffusion of four active pharmaceutical ingredients (API) of different nature (hydrophilic, ionizable and lipophilic) through an unstirred water layer (UWL) model. This approach allowed the measurement of flux changes through the UWL and the computational calculation of different parameters relevant to interpret the interplay within solubilizing vehicles and UWL diffusion. In the case of cyclodextrin, this approach allowed the determination of free drug diffusivity (Df), bound drug diffusivity (Db) and the equilibrium constant (K). In the case of liposomes, the experimental approach allowed the determination of the liposomes/water partition coefficient (Plip/w) as well as relative API diffusivity ((D)¯, i.e. the drug diffusion in the presence of solubilizing agents). This work demonstrates that the presence of solubilizing vehicles hampers the diffusion of API through UWL due to a combination of reduction in relative diffusivity and concentration gradient. These results are highly relevant as they might help to explain why biological performance of API is affected by the presence of solubilizing/complexing agents.

The Hypotonic Environmental Changes Affect Liposomal Formulations for Nose-to-Brain Targeted Drug Delivery.

Systemic administration of drugs is ineffective in the treatment of central nervous system disorders because of the blood-brain barrier. Nasal administration has been suggested as an alternative administration route as drugs absorbed in the olfactory epithelium bypass the blood-brain barrier and reach the brain within minutes. However, the nasal mucosa properties (e.g., tonicity, pH) are not constant because of physiological and environmental factors, and this might limit the therapeutic outcome of nanocarrier-based formulations. To shine light on the impact of environmental ionic strength on nanocarrier-based formulations, we have studied how liposomal formulations respond to the change of tonicity of the external environment. Large unilamellar vesicles loaded with 6 different drugs were exposed to different hypotonic environments, creating an osmotic gradient within the inner core and external environment of the liposomes up to 650 mOsm/kg. Both size and polydispersity of liposomes were significantly affected by tonicity changes. Moreover, the release kinetics of hydrophilic and lipophilic drugs were largely enhanced by hypotonic environments. These results clearly demonstrate that the environmental ionic strength has an impact on liposomal formulation stability and drug release kinetics and it should be considered when liposomal formulations for nose-to-brain targeted drug delivery are designed.

Experimental Determination of Drug Diffusion Coefficients in Unstirred Aqueous Environments by Temporally Resolved Concentration Measurements.

The diffusion coefficient (also known as diffusivity) of an active pharmaceutical ingredient (API) is a fundamental physicochemical parameter that affects passive diffusion through biological barriers and, as a consequence, bioavailability and biodistribution. However, this parameter is often neglected, and it is quite difficult to find diffusion coefficients of small molecules of pharmaceutical relevance in the literature. The available methods to measure diffusion coefficients of drugs all suffer from limitations that range from poor sensitivity to high selectivity of the measurements or the need for dedicated instrumentation. In this work, a simple but reliable method based on time-resolved concentration measurements by UV-visible spectroscopy in an unstirred aqueous environment was developed. This method is based on spectroscopic measurement of the variation of the local concentration of a substance during spontaneous migration of molecules, followed by standard mathematical treatment of the data in order to solve Fick's law of diffusion. This method is extremely sensitive and results in highly reproducible data. The technique was also employed to verify the influence of the environmental characteristics (i.e., ionic strength and presence of complexing agents) on the diffusivity. The method can be employed in any research laboratory equipped with a standard UV-visible spectrophotometer and could become a useful and straightforward tool in order to characterize diffusion coefficients in physiological conditions and help to better understand the drug permeability process.

Influence of the environmental tonicity perturbations on the release of model compounds from large unilamellar vesicles (LUVs): A mechanistic investigation.

In this work, the influence of environmental tonicity perturbations on the size and release kinetics of model markers from liposomes (calcein and rhodamine) was investigated. Large unilamellar vesicles (LUVs) were prepared from a mixture composed of organic solvents containing dissolved phosphatidylcholine and phosphate buffered saline (PBS, pH 7.4). Organic phase was removed by rotary evaporation and the obtained liposomal dispersions were extruded to reduce the liposomal sizes to approx. 400 nm. The LUVs were exposed to PBS of different tonicity to induce water migration, and consequently, generate an osmotic pressure on the vesicle membranes. The markers release kinetics were studied by the dialysis method employing Franz diffusion cells. LUVs appeared to be more susceptible to the osmotic swelling than the shrinking and the size changes were significantly more pronounced for calcein-loaded LUVs in comparison to rhodamine-loaded LUVs. The calcein release from LUVs was highly affected by the water influx/efflux, whereas rhodamine release was less affected by the tonicity perturbations. Mechanistically, it appeared that hydrophilic molecules (calcein) followed the water flux, whereas lipophilic molecules (rhodamine) seemed to be more affected by the changes in LUVs size and consequent alteration of the tightness of the phospholipid bilayer (where the lipophilic marker was imbedded in). These results demonstrate that the different tonicity (within the inner core and external environment of vesicles) can enhance/hamper the diffusion of a marker from LUVs and that osmotically active liposomes could be used as a novel controlled drug delivery system.

Novel in situ gel-forming solid dosage form (gfSDF) prepared by the simple syringe-based moulding: A screening study.

The aim of this study was to prepare and optimize a novel type of in situ gel-forming solid dosage form (gfSDF) to be used in the treatment of mucosal/skin ulcerations. For this purpose, a simple but reliable syringe-based hot melt/moulding method was employed. Chloramphenicol (antibiotic) and ibuprofen (anti-inflammatory) were chosen as model active pharmaceutical ingredients (APIs) to be loaded into the gfSDFs. To optimize the formulations, the gfSDFs of different compositions were studied in terms of APIs release from the matrix, solid-state characteristics, gellification properties and gfSDFs resistance to mechanical stress. Release studies showed that both APIs were released at a constant rate at different pH (pH5 and 7.4, respectively) and the changes in the formulation composition affected the release behaviour. Differential scanning calorimetry (DSC) results evidenced the complete solubilization of both API in the solid matrix. Texture analysis showed that the gfSDFs were capable of swelling once in a contact with aqueous environment and that the textural properties changed extemporaneously from the solid to gel form. The gel formed after hydration exhibited high cohesiveness and adhesiveness, an indication of good mucoadhesion properties. Friability testing confirmed satisfactory physical strength for a solid dosage form.

The Potential of Cyclodextrins as Novel Active Pharmaceutical Ingredients: A Short Overview.

Cyclodextrins (CDs) are cyclic oligosaccharides of natural origin that were discovered more than 100 years ago. The peculiar cone-like conformation of the sugar ring, expressing a lipophilic cavity and a hydrophilic external surface, allows these substances to spontaneously complex poorly soluble compounds in an aqueous environment. For more than 50 years, these substances have found applicability in the pharmaceutical and food industries as solubilizing agents for poorly soluble chemical entities. Nowadays, several research groups all over the world are investigating their potential as active pharmaceutical ingredients (APIs) for the treatment of several illnesses (e.g., hypercholesterolemia, cancer, Niemann-Pick Type C disease). The aim of this review is to briefly retrace cyclodextrins' legacy as complexing agents and describe the current and future prospects of this class of chemical entities in pharmaceutics as new APIs.

ß-CD-dextran polymer for efficient sequestration of cholesterol from phospholipid bilayers: Mechanistic and safe-toxicity investigations.

The aim of this work was to investigate the suitability of ß-cyclodextrin-dextran (BCD-dextran) polymer as cholesterol sequestering agent in vitro. For this purpose, BCD-dextran-cholesterol complexation was studied by phase solubility studies as well as with a specifically designed in vitro model based on giant unilamellar vesicles (GUVs) to evaluate the ability of this polymer to sequestrate cholesterol from phospholipid bilayers. Cholesterol-sequestering ability of BCD-dextran was also investigated on different cell lines relevant for the hematopoietic system and results were correlated to cells toxicity. BCD-dextran polymer was capable of extracting significant amount of cholesterol from phospholipid bilayers and to a higher extent in comparison to available ß-cyclodextrins (BCDs). The ability of BCD-dextran in sequestering cholesterol resulted also very high on cell lines relevant for the hematopoietic system. Moreover, BCD-dextran resulted less toxic on cell cultures due to higher selectivity in sequestering cholesterol in comparison to MBCD (that sequestrated also significant amounts of cholesteryl esters). In conclusion, BCD-dextran resulted an extremely efficient cholesterol-sequestering agent and BCD-dextran resulted more selective to cholesterol extraction in comparison to other BCDs (therefore of lower cytotoxicity). This phenomenon might play a key role to develop an efficient treatment for hypercholesterolemia based on cholesterol segregation.

Permeapad™ for investigation of passive drug permeability: The effect of surfactants, co-solvents and simulated intestinal fluids (FaSSIF and FeSSIF).

The aim of the present work was to investigate the potential of the new and innovative artificial barrier, Permeapad™, when exposed to surfactants and co-solvents, often employed for poorly water soluble compounds. The barrier was in addition also exposed to fasted and fed state simulated intestinal fluids versions 1 and 2 (FaSSIF and FeSSIF), all of which the Permeapad™ barrier was compatible with based upon relative comparison of the permeability of the hydrophilic marker calcein in phosphate buffer. The new barrier therefore holds a huge potential due to its functional stability and robustness. It can be used as a standard tool to investigate permeability of drugs in the presence of different surfactants and co-solvents, from DMSO stock solutions at even high concentrations and for the evaluation of permeability in the presence of biomimetic media (BMM).