Bacterial extracellular vesicles: towards realistic models for bacterial membranes in molecular interaction studies by surface plasmon resonance

One way to mitigate the ongoing antimicrobial resistance crisis is to discover and develop new classes of antibiotics. As all antibiotics at some point need to either cross or just interact with the bacterial membrane, there is a need for representative models of bacterial membranes and efficient methods to characterize the interactions with novel molecules -both to generate new knowledge and to screen compound libraries. Since the bacterial cell envelope is a complex assembly of lipids, lipopolysaccharides, membrane proteins and other components, constructing relevant synthetic liposome-based models of the membrane is both difficult and expensive. We here propose to let the bacteria do the hard work for us. Bacterial extracellular vesicles (bEVs) are naturally secreted by Gram-negative and Gram-positive bacteria, playing a role in communication between bacteria, as virulence factors, molecular transport or being a part of the antimicrobial resistance mechanism. bEVs consist of the bacterial outer membrane and thus inherit many components and properties of the native outer cell envelope. In this work, we have isolated and characterized bEVs from one Escherichia coli mutant and three clinical strains of the ESKAPE pathogens Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa. The bEVs were shown to be representative models for the bacterial membrane in terms of lipid composition with speciesstrain specific variations. The bEVs were further used to probe the interactions between bEV and antimicrobial peptides (AMPs) as model compounds by Surface Plasmon Resonance (SPR) and provide proof-of-principle that bEVs can be used as an easily accessible and highly realistic model for the bacterial surface in interaction studies. This further enables direct monitoring of the effect induced by antibiotics, or the response to host-pathogen interactions.

Tailored anti-biofilm activity - Liposomal delivery for mimic of small antimicrobial peptide

The eradication of bacteria embedded in biofilms is among the most challenging obstacles in the management of chronic wounds. These biofilms are found in most chronic wounds; moreover, the biofilm-embedded bacteria are considerably less susceptible to conventional antimicrobial treatment than the planktonic bacteria. Antimicrobial peptides and their mimics are considered attractive candidates in the pursuit of novel therapeutic options for the treatment of chronic wounds and general bacterial eradication. However, some limitations linked to these membrane-active antimicrobials are making their clinical use challenging. Novel innovative delivery systems addressing these limitations represent a smart solution. We hypothesized that incorporation of a novel synthetic mimic of an antimicrobial peptide in liposomes could improve its anti-biofilm effect as well as the anti-inflammatory activity. The small synthetic mimic of an antimicrobial peptide, 7e-SMAMP, was incorporated into liposomes (~280 nm) tailored for skin wounds and evaluated for its potential activity against both biofilm formation and eradication of pre-formed biofilms. The 7e-SMAMP-liposomes significantly lowered inflammatory response in murine macrophages (~30 % reduction) without affecting the viability of macrophages or keratinocytes. Importantly, the 7e-SMAMP-liposomes completely eradicated biofilms produced by Staphylococcus aureus and Escherichia coli above concentrations of 6.25 μg/mL, whereas in Pseudomonas aeruginosa the eradication reached 75 % at the same concentration. Incorporation of 7e-SMAMP in liposomes improved both the inhibition of biofilm formation as well as biofilm eradication in vitro, as compared to non-formulated antimicrobial, therefore confirming its potential as a novel therapeutic option for bacteria-infected chronic wounds.

Drug loading to mesoporous silica carriers by solvent evaporation: A comparative study of amorphization capacity and release kinetics

The sorption of poorly aqueous soluble active pharmaceutical ingredients (API) to mesoporous silica carriers is an increasingly common formulation strategy for dissolution rate enhancement for this challenging group of substances. However, the success of this approach for a particular API depends on an array of factors including the properties of the porous carrier, the loading method, or the attempted mass fraction of the API. At present, there is no established methodology for the rational selection of these parameters. In the present work, we report a systematic comparison of four well-characterised silica carriers and seven APIs loaded by the same solvent evaporation method. In each case, we find the maximum amorphization capacity by x-ray powder diffraction analysis and measure the in vitro drug release kinetics. For a selected case, we also demonstrate the potential for bioavailability enhancement by a permeation essay.

Current challenges and future perspectives in oral absorption research: An opinion of the UNGAP network

Although oral drug delivery is the preferred administration route and has been used for centuries, modern drug discovery and development pipelines challenge conventional formulation approaches and highlight the insufficient mechanistic understanding of processes critical to oral drug absorption. This review presents the opinion of UNGAP scientists on four key themes across the oral absorption landscape: (1) specific patient populations, (2) regional differences in the gastrointestinal tract, (3) advanced formulations and (4) food-drug interactions. The differences of oral absorption in pediatric and geriatric populations, the specific issues in colonic absorption, the formulation approaches for poorly water-soluble (small molecules) and poorly permeable (peptides, RNA etc.) drugs, as well as the vast realm of food effects, are some of the topics discussed in detail. The identified controversies and gaps in the current understanding of gastrointestinal absorption-related processes are used to create a roadmap for the future of oral drug absorption research.

Predicting Oral Absorption of fenofibrate in Lipid-Based Drug Delivery Systems by Combining In Vitro Lipolysis with the Mucus-PVPA Permeability Model

The aim of this work was to develop a new in vitro lipolysis-permeation model to predict the in vivo absorption of fenofibrate in self-nanoemulsifying drug delivery systems (SNEDDSs). More specifically, the in vitro intestinal lipolysis model was combined with the mucus-PVPA (Phospholipid Vesicle-based Permeation Assay) in vitro permeability model. Biosimilar mucus (BM) was added to the surface of the PVPA barriers to closer simulate the intestinal mucosa. SNEDDSs for which pharmacokinetic data after oral dosing to rats was available in the literature were prepared, and the ability of the SNEDDSs to maintain fenofibrate solubilized during in vitro lipolysis was determined, followed by the assessment of drug permeation across the mucus-PVPA barriers. The amount of drug solubilized over time during in vitro lipolysis did not correlate with the AUC (area under the curve) of the plasma drug concentration curve. However, the AUC of the drug permeated after in vitro lipolysis displayed a good correlation with the in vivo AUC (R² > 0.9). Thus, it was concluded that the in vitro lipolysis-mucus-PVPA permeation model, simulating the physiological digestion and absorption processes, was able to predict in vivo absorption data, exhibiting great potential for further prediction of in vivo performance of SNEDDSs.

The Vaginal-PVPA: A Vaginal Mucosa-Mimicking In Vitro Permeation Tool for Evaluation of Mucoadhesive Formulations

Drug administration to the vaginal site has gained increasing attention in past decades, highlighting the need for reliable in vitro methods to assess the performance of novel formulations. To optimize formulations destined for the vaginal site, it is important to evaluate the drug retention within the vagina as well as its permeation across the mucosa, particularly in the presence of vaginal fluids. Herewith, the vaginal-PVPA (Phospholipid Vesicle-based Permeation Assay) in vitro permeability model was validated as a tool to evaluate the permeation of the anti-inflammatory drug ibuprofen from liposomal formulations (i.e., plain and chitosan-coated liposomes). Drug permeation was assessed in the presence and absence of mucus and simulated vaginal fluid (SVF) at pH conditions mimicking both the healthy vaginal premenopausal conditions and vaginal infection/pre-puberty/post-menopause state. The permeation of ibuprofen proved to depend on the type of formulation (i.e., chitosan-coated liposomes exhibited lower drug permeation), the mucoadhesive formulation properties and pH condition. This study highlights both the importance of mucus and SVF in the vaginal model to better understand and predict the in vivo performance of formulations destined for vaginal administration, and the suitability of the vaginal-PVPA model for such investigations.

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 (D_rel) 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.

Deformable liposomes for skin therapy with human epidermal growth factor: The effect of liposomal surface charge

The topical administration of exogenous human epidermal growth factor (hEGF) is a promising approach for improved chronic wound therapy. To develop therapeutically superior hEGF formulation, we prepared hEGF-containing neutral (NDLs), cationic (CDLs) and anionic (ADLs) deformable liposomes (DLs), respectively, since it is expected that the liposomal surface charge can affect both the liposomal physicochemical properties, their skin penetration potential and therapeutic efficacy of liposome-associated drug. All prepared liposomes were of similar size (300-350 nm) with high hEGF load (~80% entrapment efficacy). Among the studied DLs, ADLs were found to be most promising for sustained release of hEGF, as assessed in vitro using the polyamide membrane. Ex vivo studies revealed that all DLs were excellent systems for skin therapy with hEGF and no penetration of hEGF through the full thickness human skin was detected. ADLs provided a depot exhibiting the highest hEGF retention onto the human skin surface. ADLs also revealed enhanced mitogenic activities in human fibroblasts compared to both NDLs and CDLs after 48 hrs treatment. Moreover, hEGF-containing ADLs significantly enhanced mitogenic activity in fibroblast as compared to activity of hEGF solution (positive control). Similar trends were observed in human keratinocytes after 24 hrs of treatment. We proved that the liposomal surface charge affects the therapeutic potential of hEGF-containing liposomes. hEGF-containing ADLs can be a promising nanosystem-based formulation for localized therapy of chronic wounds.

Drug permeability profiling using cell-free permeation tools: Overview and applications

Cell-free permeation systems are gaining interest in drug discovery and development as tools to obtain a reliable prediction of passive intestinal absorption without the disadvantages associated with cell- or tissue-based permeability profiling. Depending on the composition of the barrier, cell-free permeation systems are classified into two classes including (i) biomimetic barriers which are constructed from (phospho)lipids and (ii) non-biomimetic barriers containing dialysis membranes. This review provides an overview of the currently available cell-free permeation systems including Parallel Artificial Membrane Permeability Assay (PAMPA), Phospholipid Vesicle-based Permeation Assay (PVPA), Permeapad®, and artificial membrane based systems (e.g. the artificial membrane insert system (AMI-system)) in terms of their barrier composition as well as their predictive capacity in relation to well-characterized intestinal permeation systems. Given the potential loss of integrity of cell-based permeation barriers in the presence of food components or pharmaceutical excipients, the superior robustness of cell-free barriers makes them suitable for the combined dissolution/permeation evaluation of formulations. While cell-free permeation systems are mostly applied for exploring intestinal absorption, they can also be used to evaluate non-oral drug delivery by adjusting the composition of the membrane.

Going skin deep: A direct comparison of penetration potential of lipid-based nanovesicles on the isolated perfused human skin flap model

Phospholipid-based nanocarriers are attractive drug carriers for improved local skin therapy. In the present study, the recently developed isolated perfused human skin flap (IPHSF) model was used to directly compare the skin penetration enhancing potential of the three commonly used nanocarriers, namely conventional liposomes (CLs), deformable liposomes (DLs) and solid lipid nanoparticles (SLNs). Two fluorescent markers, calcein (hydrophilic) or rhodamine (lipophilic), were incorporated individually in the three nanosystems. The nanocarrier size ranged between 200 and 300nm; the surface charge and entrapment efficiency for both markers were dependent on the lipid composition and the employed surfactant. Both carrier-associated markers could not penetrate the full thickness human skin, confirming their suitability for dermal drug delivery. CLs exhibited higher retention of both markers on the skin surface compared to DLs and SLNs, indicating a depo formation. DLs and SLNs enabled the deeper penetration of the two markers into the skin layers. In vitro and ex vivo skin penetration studies performed on the cellophane membrane and full thickness pig/human skin, respectively, confirmed the findings. In conclusion, efficient dermal drug delivery can be achieved by optimization of a lipid nanocarrier on the suitable skin-mimicking model to assure system's accumulation in the targeted skin layer.

In vitro models to estimate drug penetration through the compromised stratum corneum barrier

OBJECTIVES

The phospholipid vesicle-based permeation assay (PVPA) is a recently established in vitro stratum corneum model to estimate the permeability of intact and healthy skin. The aim here was to further evolve this model to mimic the stratum corneum in a compromised skin barrier by reducing the barrier functions in a controlled manner.

METHODS

To mimic compromised skin barriers, PVPA barriers were prepared with explicitly defined reduced barrier function and compared with literature data from both human and animal skin with compromised barrier properties. Caffeine, diclofenac sodium, chloramphenicol and the hydrophilic marker calcein were tested to compare the PVPA models with established models.

RESULTS AND DISCUSSIONS

The established PVPA models mimicking the stratum corneum in healthy skin showed good correlation with biological barriers by ranking drugs similar to those ranked by the pig ear skin model and were comparable to literature data on permeation through healthy human skin. The PVPA models provided reproducible and consistent results with a distinction between the barriers mimicking compromised and healthy skin. The trends in increasing drug permeation with an increasing degree of compromised barriers for the model drugs were similar to the literature data from other in vivo and in vitro models.

CONCLUSIONS

The PVPA models have the potential to provide permeation predictions when investigating drugs or cosmeceuticals intended for various compromised skin conditions and can thus possibly reduce the time and cost of testing as well as the use of animal testing in the early development of drug candidates, drugs and cosmeceuticals.

Biomimetic PVPA in vitro model for estimation of the intestinal drug permeability using fasted and fed state simulated intestinal fluids

A prerequisite for successful oral drug therapy is the drug's ability to cross the gastrointestinal barrier. Considering the increasing number of new chemical entities in modern drug discovery, reliable and fast in vitro models are required for early and efficient prediction of intestinal permeability. To mimic the intestinal environment, use of biorelevant media may provide valuable information on in vivo drug permeation. The present study aims at improving the novel biomimetic phospholipid vesicle-based permeation assay's (PVPAbiomimetic) biorelevance by investigating the applicability of the biorelevant media; fasted state simulated intestinal fluid (FaSSIF) and fed state simulated intestinal fluid (FeSSIF). The FaSSIF and FeSSIF's influence on the permeability of the model drugs acyclovir, indomethacin, griseofulvin and nadolol was then assessed. The barriers' robustness in terms of storage stability was also evaluated. The barriers were found to maintain their integrity in presence of FaSSIF and FeSSIF. The model drugs showed changes in permeability in presence of the different simulated intestinal fluids that were in agreement with previous reports. Moreover, the barrier showed improved storage stability by maintaining its integrity for 6months. Altogether, this study moves the PVPAbiomimetic an important step towards a better in vitro permeability model for use in drug development.

Chitosan in mucoadhesive drug delivery: focus on local vaginal therapy

Mucoadhesive drug therapy destined for localized drug treatment is gaining increasing importance in today's drug development. Chitosan, due to its known biodegradability, bioadhesiveness and excellent safety profile offers means to improve mucosal drug therapy. We have used chitosan as mucoadhesive polymer to develop liposomes able to ensure prolonged residence time at vaginal site. Two types of mucoadhesive liposomes, namely the chitosan-coated liposomes and chitosan-containing liposomes, where chitosan is both embedded and surface-available, were made of soy phosphatidylcholine with entrapped fluorescence markers of two molecular weights, FITC-dextran 4000 and 20,000, respectively. Both liposomal types were characterized for their size distribution, zeta potential, entrapment efficiency and the in vitro release profile, and compared to plain liposomes. The proof of chitosan being both surface-available as well as embedded into the liposomes in the chitosan-containing liposomes was found. The capability of the surface-available chitosan to interact with the model porcine mucin was confirmed for both chitosan-containing and chitosan-coated liposomes implying potential mucoadhesive behavior. Chitosan-containing liposomes were shown to be superior in respect to the simplicity of preparation, FITC-dextran load, mucoadhesiveness and in vitro release and are expected to ensure prolonged residence time on the vaginal mucosa providing localized sustained release of entrapped model substances.

Liposomes for (trans)dermal drug delivery: the skin-PVPA as a novel in vitro stratum corneum model in formulation development

Penetration potential of vesicles destined for trans(dermal) administration remains to be of great interests both in respect to drug therapy and cosmetic treatment. This study investigated the applicability of the phospholipid vesicle-based permeation assay (PVPA) as a novel in vitro skin barrier model for screening purposes in preformulation studies. Various classes of liposomes containing hydrophilic model drug were examined, including conventional liposomes (CLs), deformable liposomes (DLs) and propylene glycol liposomes (PGLs). The size, surface charge, membrane deformability and entrapment efficiency were found to be affected by the vesicle lipid concentration, the presence of the surfactant and propylene glycol. All liposomes exhibited prolonged drug release profiles with an initial burst effect followed by a slower release phase. The permeation of the drug from all of the tested liposomes, as assessed with the mimicked stratum corneum--PVPA model, was significantly enhanced as compared to the permeability of the drug in solution form. Although the DLs and the PGLs exhibited almost the same membrane elasticity, the permeability of the drug delivered by PGLs was higher (6.2 × 10⁻⁶ cm/s) than DLs (5.5 × 10⁻⁶ cm/s). Therefore, this study confirmed both the potential of liposomes as vesicles in trans(dermal) delivery and potential of the newly developed skin-PVPA for the screening and optimization of liposomes at the early preformulation stage.

Pectosomes and chitosomes as delivery systems for metronidazole: the one-pot preparation method

Mucoadhesive liposomes offer a potential for improved residence time of liposomal systems targeting contact with mucosal tissues, such as in buccal, oral, colon, and vaginal drug delivery. Most of the currently available methods rely on the coating of preformed liposomes by various mucoadhesive polymers. The aim of this study was to develop novel mucoadhesive system by the one-pot preparation method. The pectin- and chitosan-containing liposomes, namely pectosomes and chitosomes, were prepared by the modified solvent injection method. In order to optimize this novel delivery system, we used pectins and chitosans of both high and low degree of esterification/deacetylation (DE/DD), respectively. Sonication was applied to reduce the original vesicle size. All vesicles were characterized for their size, zeta potential, metronidazole entrapment, and stability. Both pectosomes and chitosomes were found to entrap more metronidazole than conventional plain liposomes. Preliminary data indicate that the polymer is present on the liposomal surface, embedded within inner liposomal bilayers, and entrapped inside the aqueous compartment. The next step in the evaluation of this system is the testing of its mucoadhesiveness.

A synthetic antimicrobial peptidomimetic (LTX 109): stereochemical impact on membrane disruption

LTX 109 is a synthetic antimicrobial peptidomimetic (SAMP) currently in clinical phase II trials for topical treatment of infections of multiresistant bacterial strains. All possible eight stereoisomers of the peptidomimetic have been synthesized and tested for antimicrobial effect, hemolysis, and hydrophobicity, revealing a strong and unusual dependence on the stereochemistry for a molecule proposed to act on a general membrane mechanism. The three-dimensional structures were assessed using nuclear magnetic resonance spectroscopy (NMR) and molecular dynamics (MD) simulations in aqueous solution and in phospholipid bilayers. The solution structures of the most active stereoisomers are perfectly preorganized for insertion into the membrane, whereas the less active isomers need to pay an energy penalty in order to enter the lipid bilayer. This effect is also found to be reinforced by a significantly improved water solubility of the less active isomers due to a guanidyl-π stacking that helps to solvate the hydrophobic surfaces.

In-vitro permeability of poorly water soluble drugs in the phospholipid vesicle-based permeation assay: the influence of nonionic surfactants

Objectives

The aim of this study was to determine the influence of nonionic surfactants on drug permeability using the phospholipid vesicle-based permeation assay (PVPA), which excludes other than trans-membrane diffusion pathways.

Methods

Barrier integrity was monitored both by electrical resistance and permeability measurement of the hydrophilic marker calcein. Permeability of the model drugs ketoprofen and nadolol across the PVPA-barrier was measured by HPLC-UV. Micelle association of the model drugs was determined using ultrafiltration, whereby micelle-bound drug and molecular drug were separated.

Key findings

The nonionic surfactant poloxamer 188 was demonstrated not to affect barrier integrity. Drug permeability was found depressed in the presence of poloxamer 188 in a concentration-dependent manner. Both drugs were found to associate with poloxamer 188 micelles. The extent of the decrease in permeability correlated mostly, but not in all cases, with the fraction of micelle-bound drug.

Conclusions

Micelle association was one important but not the only factor affecting drug permeability across the PVPA-barrier.

In-vitro permeability screening of melt extrudate formulations containing poorly water-soluble drug compounds using the phospholipid vesicle-based barrier

Objectives

The phospholipid vesicle-based barrier has recently been introduced as an in-vitro permeation model mimicking gastro-epithelial barriers in terms of passive diffusion of drugs. The aim of this study was to investigate whether the phospholipid vesicle-based barrier was suitable for permeability screening of complex formulations such as solid dispersions.

Methods

Solid dispersions containing the poorly water-soluble drugs HIV-PI 1 (log P = 6.2, molar mass = 628.80 g/mol) and HIV-PI 2 (log P = 5.3, molar mass = 720.95 g/mol), a hydrophilic polymer and different surfactants were tested with respect to their influence on integrity of the barrier in terms of electrical resistance and permeability for calcein. Furthermore, utilisation of a more biologically relevant medium, Hank's balanced salt solution supplemented with Mg2+- and Ca2+-ions (HBSS (Mg2+, Ca2+)), has been tested.

Key findings

Except for the polyoxyl 40 hydrogenated castor oil-containing solid dispersion, no influence on the phospholipid vesicle-based barrier could be observed from the tested samples. Presence of active pharmaceutical ingredients (APIs) in the solid dispersions led to the same results as the corresponding placebo results. First experiments analysing the passive diffusion of both APIs in HBSS (Mg2+, Ca2+), evaluated as suitable transport medium, have shown promising results regarding the suitability of the phospholipid vesicle-based barrier for investigation of solid dispersions.

Conclusions

The study indicated that the phospholipid vesicle-based barrier was compatible with selected melt extrudate formulations. The model seemed capable to reveal different transport routes in comparison with Caco-2 cell permeability tests.

The Phospholipid Vesicle-Based Drug Permeability Assay: 5. Development toward an Automated Procedure for High-Throughput Permeability Screening

In vitro screening for oral absorption has become an essential part of drug discovery and development. Recently, a new phospholipid vesicle-based permeation assay was developed which has shown to satisfyingly predict passive absorption of drugs in humans. The purpose of the current study was to investigate whether the assay may be further developed into a high-throughput tool by automating its most time-consuming steps. The following challenges were addressed: (1) to design, build, and test a heat-sealing machine for mounting of the desired type of filter support onto both single wells and 24-well titer plate inserts and (2) to transfer the permeability assay to a robotic workstation with attached ultraviolet (UV) reader. The workstation is able to pipette and transport both plates and filter inserts and perform on-line photometric quantification of the amount of drug permeated. To enable the robot to move single (Standard Transwell; Corning Inc, Lowell, MA) filter inserts, an extension of the gripping arm was designed, built, and tested. Furthermore, in an alternative approach 24-well filter plates (Millicell; Millipore, Billerica, MA) were used instead of single filter inserts. The latter turned out to be more suitable in terms of error-free high-throughput robotic handling. The permeability values of drugs gained by the two automated procedures were compared with those measured by manual handling of the assay. Only neglectable differences in permeability values were seen.

In conclusion, the most time-consuming steps of the assay were shown to be eligible for automation. This represents an interesting addition to the toolbox of in vitro permeability screening assays running in a medium- to high-throughput format due to its easiness, its transferability to other laboratories, and its good correlation with in vivo data on fraction absorbed of drugs in humans.

Drug permeability across a phospholipid vesicle based barrier: 3. Characterization of drug–membrane interactions and the effect of agitation on the barrier integrity and on the permeability

Recently, we reported on the development and structural characterization of a phospholipid vesicle based barrier useful for medium throughput screening of passive drug permeability. Here, we investigate the physical and functional integrity of the phospholipid vesicle based barriers to agitation by stirring or shaking, and whether agitation affects drug permeability of sulpiride, metoprolol and testosterone. In addition, three drugs (caffeine, naproxen and sulphasalazine) which were shown in a previous study to affect the electrical resistance of the barriers, were investigated for their influence on the permeability of a simultaneously applied hydrophilic marker (calcein), and on the thermotropic phase transition of the phospholipid bilayers using differential scanning calorimetry (DSC). Electrical resistance measurements indicated that the barriers should withstand shaking speeds up to 150rpm without losing their integrity, but significant release of phospholipids from the membrane barriers to the donor and acceptor chambers was observed under agitation >or=150rpm. When using agitation up to 100rpm no increase in permeability was observed for sulpiride, metoprolol and testosterone. The phospholipid vesicle-based barrier thus differ from other permeability models in that agitation does not lead to an increase in permeability, not even for highly lipophilic drugs such as testosterone. This is explained by the different morphology of the vesicle-based barrier which is containing a 100microm thick layer of mostly aqueous compartments immobilised within a matrix of phospholipids vesicles. Sulphasalazine and naproxen were shown to decrease the electrical resistance and increase the permeability of the hydrophilic marker calcein. The DSC experiments showed that these two drugs probably interact with the head groups of the phospholipids. In contrast, caffeine gave an increase in electrical resistance and a decrease in permeability of calcein. From the DSC experiments no signs of interaction of caffeine with the phospholipid bilayer could be observed.

Drug permeability across a phospholipid vesicle-based barrier

Recently we reported on the development of a phospholipid vesicle-based barrier as a medium throughput method for screening of drug permeability. The aim of this present study is to characterize the barrier structure, including an estimation of the amount of phospholipid within it, its storage stability and its stability over various pH ranges found in different parts of the gastrointestinal tract. The amount of lipid in the barrier was quantified using a colorimetric phospholipase D-based assay. The total amount averaged 3.30mg phospholipid per barrier. The preparation process comprises the consecutive deposition of two types of liposomes on a filter support. We estimated that the smallest liposomes, with a mean diameter of 298nm, would fill the pore volume of the filter when tightly packed. The volume of the bigger liposomes, deposited on top of the filter, was calculated to generate a 0.1mm thick layer. Visualisation of fluorescently labelled liposomes by confocal laser-scanning microscopy confirmed that the pores of the filter were completely filled with liposomes and that there was a liposome layer on top. Small angle X-ray scattering (SAXS) analysis was used to study the lamellarity of the liposomes. The liposomes contained oligo- and/or multilamellar structures before and after deposition. The functionality of the barriers during storage at three different temperatures was examined for a period of up to 4 weeks by measuring the permeability of the hydrophilic marker calcein across them. The conclusion was that the phospholipid vesicle-based barriers could be stored at -80 degrees Celsius for up to 2 weeks without significant changes. The stability of the barriers in a pH range from 2.0 to 8.0 was investigated by performing permeation studies with fluorescein at different pH values. It was found that the phospholipid vesicle-based barrier did not lose its integrity within this range. Thus, the barriers appear suitable for further studies to provide insight into segmental absorption in the human gastrointestinal tract. Furthermore, because the phospholipid vesicle-based barrier can be stored, larger batches can be produced. This makes the phospholipid vesicle-based barrier more appropriate for high throughput screening.

Drug permeability across a phospholipid vesicle based barrier: A novel approach for studying passive diffusion

The aim of this study was to develop a novel predictive medium-throughput screening method for drug permeability, with use of a tight barrier of liposomes on a filter support. To our knowledge no one has succeeded in depositing membrane barriers without the use of an inert solvent such as hexadecane. The first part of the study involved development of a protocol for preparation of these barriers, which were made of liposomes from egg phosphatidylcholin in phosphate buffer pH 7.4 with 10 % (v/v) ethanol. The liposomes were deposited into the pores and onto the surface of a filter support (mixed cellulose ester) by use of centrifugation. Solvent evaporation and freeze-thaw cycling were then used to promote fusion of liposomes. A tight barrier could thus be obtained as shown with calcein permeability and electrical resistance. In the second part of the study the model was validated using 21 drug compounds, which cover a wide range of physicochemical properties and absorption (F(a)) in humans (13-100%). The drug permeation studies were carried out at room temperature with phosphate buffer (pH 7.4) in both acceptor and donor chambers. The apparent permeability coefficients obtained from the phospholipid vesicle based model correlated well with literature data on human absorption in vivo, which suggests that its performance is adequate and that the method is suitable for rapid screening of passive transport of new chemical entities. The results obtained from our model were compared with polar surface area (PSA) and experimental logD and with results obtained by established permeability screening methods such as immobilized liposome chromatography (ILC), the PAMPA models and the Caco-2 model. Our approach seems to model the in vivo absorption better than PSA, experimental logD, the ILC and PAMPA models, when similar conditions are used as in our assay, and equally well as the Caco-2 model and the Double Sink PAMPA (DS-PAMPA) model.

A method to determine the incorporation capacity of camptothecin in liposomes

The purpose of this study was to establish a new experimental approach to determine the maximum amount of camptothecin (CPT) that can be incorporated in liposomes, and to use this method to compare the CPT-incorporation capacity of various liposome formulations. Small, CPT-saturated liposomes were prepared by dispersing freeze-dried blends of lipids and drug in phosphate buffer, and subsequent probe-sonication. Excess precipitated CPT could be separated from the liposomes by ultracentrifugation. The small and homogeneous liposome size obtained gave a good and reproducible recovery of liposomes in the supernatant (>80%), whereas the acidic pH (pH 6.0) kept CPT in its hydrophobic lactone form, which is poorly soluble in the buffer. The maximum CPT-incorporation capacity of 12 different liposome formulations was investigated, using the described method, and was found to vary widely. With liposomes made of neutral and anionic phospholipids, the solubility of CPT in the buffer was improved by approximately a factor of 10 (from 2.7 to 15-50 microg/mL) as compared with buffer. With cationic liposomes containing 1,2-dioleoyl-3-trimethyl-ammonium-propane (DOTAP), a maximum CPT-solubilization of 100-fold, the buffer solubility was reached, probably owing to an electrostatic interaction between the cationic lipids and the carboxylate-CPT isomer. Increasing DOTAP fractions within egg-phosphatidylcholine (EPC)/DOTAP liposomes reached a CPT-incorporation plateau at 20 mol% DOTAP. The presented approach appears suitable to study the incorporation capacity of any drug component within small vesicles as long as the liposome incorporation is high relative to the intrinsic water solubility of the drug.