Accurate potentiometric determination of lipid membrane-water partition coefficients and apparent dissociation constants of ionizable drugs: electrostatic corrections

A novel device for the determination of liposome/water partition coefficients

A novel, cheap and easy-to-construct device and a simple method for partition coefficient determination in liposome/water system based on modified equilibrium dialysis have been developed. The device consists of two vials separated by a semi-permeable membrane, through which the free form of a low molecular weight substance is transported by shaking assisted diffusion. Five test substances, eugenol, carvacrol, thymol, 4-hydroxybenzyl alcohol (4-HBA) and butylparaben were analyzed after equilibration in aqueous phase by three methods, HPLC-UV, GC-MS and DPV with comparable results. This shows the possibility of using the proposed method in any laboratory with any equipment capable of analyzing the substance under study. The liposome/water partition coefficients (log Pl/w) determined for eugenol (2.39), thymol (2.83), carvacrol (2.78) and butylparaben (3.30) are consistent with previously published data. A strong effect of NaCl on the liposome/water partition coefficient was observed. The value of log Pl/w = 1.06 determined for 4-HBA in the presence of 0.15 mol L-1 NaCl in the partitioning liposomal system was considerably lower than in the absence of the salt (log Pl/w = 2.06). The developed method was used to determine the partition coefficient of morphine in liposome/water system without NaCl (log Pl/w = 2.65) under given conditions.

Bioconcentration of Several Series of Cationic Surfactants in Rainbow Trout

Cationic surfactants have a strong affinity to sorb to phospholipid membranes and thus possess an inherent potential to bioaccumulate, but there are few measurements of bioconcentration in fish. We measured the bioconcentration of 10 alkylamines plus two quaternary ammonium compounds in juvenile rainbow trout at pH 7.6, and repeated the measurements at pH 6.2 for 6 of these surfactants. The BCF of the amines with chain lengths ≤ C14 was positively correlated with chain length, increasing ∼0.5 log units per carbon. Their BCF was also pH dependent and approximately proportional to the neutral fraction of the amine in the water. The BCFs of the quaternary ammonium compounds showed no pH dependence and were >2 orders of magnitude less than for amines of the same chain length at pH 7.6. This indicates that systemic uptake of permanently charged cationic surfactants is limited. The behavior of the quaternary ammonium compounds and the two C16 amines studied was consistent with previous observations that these surfactants accumulate primarily to the gills and external surfaces of the fish. At pH 7.6 the BCF exceeded 2000 L kg-1 for 4 amines with chains ≥ C13, showing that bioconcentration can be considerable for some longer chained cationic surfactants.

Membrane-Water Partition Coefficients to Aid Risk Assessment of Perfluoroalkyl Anions and Alkyl Sulfates

This study determined the sorption affinity to artificial phospholipid membranes ( K_MW) for series of perfluorinated carboxylates (PFCAs), perfluorinated sulfonates (PFSAs), alkyl sulfates (C _xSO₄), and 1-alkanesulfonates (C _xSO₃). A sorbent dilution assay with solid supported lipid membranes (SSLM) showed consistent CF₂ unit increments of 0.59, and CH₂ unit increments of 0.53, for the log K_MW of perfluorinated and hydrogenated anions, respectively. PFSAs sorbed 0.90 log units stronger than analogue PFCAs; C _xSO₄ sorbed 0.75 log units stronger than analogue C _xSO₃ anions. The log K_MW values for the octyl analogues increase in the order H(CH₂)₈SO₃^- (1.74) < H(CH₂)₈SO₄^- (2.58) < F(CF₂)₈CO₂^- (PFNA, 4.04) < F(CF₂)₈SO₃^- (PFOS, 4.88). Intrinsic partition ratios determined on a phospholipid coated HPLC column (IAM-HPLC) closely aligned with SSLM K_MW values. COSMO-RS based molecular calculations of K_MW aligned with SSLM K_MW values for hydrogenated anions with C₈-C₁₄ alkyl chains but strongly underestimated CF₂ and CH₂ unit increments for C₄-C₈ based anions. Dividing the critical narcotic membrane burden of 100 mmol/kg by the experimental K_MW predicts lethal baseline toxicity concentrations (LC_50,narc). The LC_50,narc coincides with the lowest reported acute LC₅₀ values for several anionic surfactants but were on average about an order of magnitude lower.

Nanostructures in n-Octanol Equilibrated with Additives and/or Water

Fluid fatty alcohols are believed to be nanostructured but broadly amorphous (i.e., noncrystalline) fluids and solvents, including the most popular fatty tissue mimetic, hydrated n-octanol (i.e., hydro-octanol). To check this premise, we studied dry octanol and hydro-octanol as a model of relatively short fluid n-alkanols with small-angle X-ray scattering (SAXS). We also combined this alkanol with the matching alkane (i.e., octane) and with a common anti-inflammatory pain killer (ketoprofen). This revealed that (hydro-)octanol and arguably any other short fatty alcohol form a mesophase. Its basic structural motif are regularly packed polar nanoclusters, reflected in the inner peak in the SAXS diffractogram of (hydro-)octanol and other fluid n-alkanols. The nanoclusters arguably resemble tiny, (inverse) hydrated bilayer fragments, located on a thermally smeared para-crystalline lattice. Additives to hydro-octanol can change the nanoclusters only moderately, if at all. For example, octane and the drug ketoprofen added to hydro-octanol enlarge the nanoclusters only little because of the mixture's packing frustration. To associate with and to bring more water into hydro-octanol, an additive must hence transform the nanoclusters: it expands them into irregularly distributed aqueous lacunae that form a proto-microemulsion, reflected in the previously unknown Guinier's SAXS signal. A "weak" (i.e., a weakly polar or nonpolar) additive can moreover create only size-limited lacunae. Coexistence of nanoclusters and lacunae as well as size variability of the latter in hydro-octanol subvert the concept of octanol-water partition coefficient, which relies on the studied compartment homogeneity. In turn, it opens new possibilities for interfacial catalysis. Reinterpreting "octanol-water partition coefficient" data in terms of octanol-water association or binding constant(s) could furthermore diminish the variability of molecular lipophilicity description and pave the ground toward a more precise theoretical quantification and prediction of molecular properties.

The effect of membrane softeners on rigidity of lipid vesicle bilayers: Derivation from vesicle size changes

Deformability is not just a fundamentally interesting vesicle characteristic; it is also the key determinant of vesicle ability to cross the skin barrier; i.e. skin penetrability. Development of bilayer vesicles for drug and vaccine delivery across the skin should hence involve optimization of this property, which is controllable by the concentration of bilayer softeners in or near the vesicle bilayers. To this end, we propose a simple method for quantifying the effect of bilayer softeners on deformability of bilayer vesicles. The method derives the bending rigidity of vesicle bilayers from vesicle size dependence on softener concentration. To exemplify the method, we studied mixtures of soybean phosphatidylcholine with anionic sodium deoxycholate, non-ionic polyoxyethylene (20) sorbitan oleyl ester (polysorbate 80), or non-ionic polyoxyethylene (20) oleyl ether (C18:1EO20, Brij® 98). With each of the tested bilayer softeners, the bending rigidity of the resulting mixed-amphipat vesicle bilayers decreased quasi-exponentially as the concentration of the bilayer softener increased, as one would expect on theoretical ground. The bilayer bending rigidity reached low values, near the thermal stability limit, i.e. kBT, before vesicle transformation into non-vesicular aggregates began. For a soybean phosphatidylcholine concentration of 5.0mmolkg-1, the bilayer bending rigidity reached 1.5kBT at the total deoxycholate concentration of 4.1mmolkg-1 and 3.4kBT at the total polysorbate 80 concentration of 2.0mmolkg-1. In the case of C18:1EO20, the bilayer bending rigidity reached 1.5kBT at the bilayer surface occupancy α=0.1. The dependence of vesicle size on bilayer softener concentration thus reveals vesicle transformation into different aggregate structures (such as mixed micelles with poor skin penetrability) and practically valuable information on vesicle deformability. Our results compare favorably with results of literature measurements. We provide practical guidance on using the new analytical method to optimize deformable vesicle formulations.

Development of topical therapeutics for management of onychomycosis and other nail disorders: a pharmaceutical perspective

The human nail plate is a formidable barrier to drug permeation. Development of therapeutics for management of nail diseases thus remains a challenge. This article reviews the current knowledge and recent advances in the field of transungual drug delivery and provides guidance on development of topical/ungual therapeutics for management of nail diseases, with special emphasis on management of onychomycosis, the most common nail disease. Selection of drug candidates, drug delivery approaches, and evaluation of formulations are among the topics discussed. A comprehensive mathematical description for transungual permeation is also introduced.

Spectrofluorimetric determination of terbinafine hydrochloride and linezolid in their dosage forms and human plasma

A highly sensitive, simple and rapid spectrofluorimetric method was developed for the determination of Terbinafine HCl (TRH) and linezolid (LNZ) in their pharmaceutical formulations. The proposed method is based on measuring the native fluorescence of the studied drugs in water at 336 nm after excitation at 275 nm for TRH and 375 nm after excitation at 254 nm for LNZ. The fluorescence-concentration plots were rectilinear over the range of 0.02-0.15 μg/mL for TRH and 0.5-5.0 μg/mL for LNZ. With lower detection limits of 3.0 and 110.0 ng/mL and a lower quantification limit of 9.0 and 320.0 ng/mL for TRH and LNZ, respectively. The method was successfully applied to the analysis of TRH in its commercial tablets, cream, gel and spray formulations and the results were in good agreement with those obtained with the official method. In addition the method was also applied to the analysis of LNZ in its capsule and I.V solution and the results were in good agreement with those obtained with the comparison method. The effect of sensitizers was studied. The method was extended to the determination of the studied drugs in spiked human plasma and the results were satisfactory.

An in vitro spectrometric method for determining the partition coefficients of non-steroidal anti-inflammatory drugs into human erythrocyte ghost membranes

Usefulness of second derivative spectrophotometry for determining the partition coefficients (K(p)s) of four non-steroidal anti-inflammatory drugs (NSAIDs) between human erythrocyte ghost (HEG) membranes and buffer at simulated physiological conditions (pH=7.4, 37 °C) has been adequately emphasized. In the absorption spectra for each of the investigated NSAIDs, λ(max) was red-shifted in presence of HEG membranes, indicating that NSAIDs have the nature of metachromasy between lipid bilayer and water. Further quantitative spectral data for calculating K(p)s could not be obtained from the absorption spectra because of the presence of background signal impacts of HEG lipid bilayers. Second derivative spectra were calculated from absorption spectra and fortunately showed three isosbestic derivative points for each NSAID, indicating without doubt that the background signals were entirely eliminated. From the relation between the derivative intensity change (ΔD) induced by addition of HEG membranes, K(p)s were calculated and obtained with RSD of below 6%. Fractions of partitioned NSAIDs are in well-harmony with that derived from the experimental values. Moreover, validity of the proposed method was confirmed. Conclusively, the second derivative spectrometry has proven to be a facile, reliable and more expeditious method to obtain in vitro K(p)s of drugs to HEG without previous separation.

Rational design of new product candidates: the next generation of highly deformable bilayer vesicles for noninvasive, targeted therapy

Amphipat bilayer vesicles are a subgroup of "fat-and-water" mixtures useful as drug carriers. Scrutinising amphipat aggregation in terms of the popular molecular descriptors (esp. the Israelachvili's form-factor or HLB number) is "too static" to foretell reliably and quantitatively bilayer vesicle formation. A better predictor introduced in this work is the effective area per lipid chain (cross-section of a "tail", A(c)), which also correlates, quasi-exponentially, with the ease of bilayer vesicle formation and bilayer deformability. The latter is highest near an uppermost, bilayer-compatible but nearly headgroup independent, A(c)-value reachable on different paths to bilayer solubilisation. The deformable bilayer vesicles class is thus more diverse than had previously been recognised. It includes phospholipid or phospholipid-surfactant blends (1st generation), synergistic phospholipid-amphipat or drug mixtures (2nd generation), and novel (non-phospholipid) amphipat combinations with appropriate effective tail(s) cross-section (3rd generation). Typically, vesicularisation ability and bilayer adaptability of such preparations is proportional, and arguably depends upon, the dynamic and stress-dependent molecular re-arrangement during aggregate formation and bilayer adaptation. In the previously described formulations such re-arrangement took place within or across the mixed lipid bilayer. This work shows that water-soluble molecules redistribution near a bilayer surface can be similarly effective. The new mechanism for bilayer properties modulation thus potentially avoids using harsher molecules in the adaptable vesicles, and can utilise buffers, microbicides, etc., in their stead. A plethora of amphipats can comprise hyper-adaptable vesicles of the new generation, including some that are more stable than the previously recognised ones. Encompassing well-chosen hydrophilic additive(s) and/or drug(s), such hyper-adaptable vesicles can be blended into fluid or semisolid preparations suitable for non-invasive, and potentially parenteral, applications. Pharmacologically relevant examples include, but are not limited to, the composite adaptable phospholipid-free vesicles loaded with anti-mycosis drugs (such as terbinafine), surfactant-free preparations of non-steroidal anti-inflammatory drugs (such as indomethacin or ketoprofen), etc. Further interesting implementations of the new technology contain hyper-adaptable drug-free vesicles that suppress human skin inflammation after local application better than hydrocortisone and broadly similar to conventional topical NSAIDs. The carriers described in this work thus provide unprecedented options for cutaneous or targeted subcutaneous deposition of drugs and/or for the sustained delivery of the corresponding carrier associated therapeutic agents.

Electrodynamics of lipid membrane interactions in the presence of zwitterionic buffers

Due to thermal motion and molecular polarizability, electrical interactions in biological systems have a dynamic character. Zwitterions are dipolar molecules that typically are highly polarizable and exhibit both a positive and a negative charge depending on the pH of the solution. We use multilamellar structures of common lipids to identify and quantify the effects of zwitterionic buffers that go beyond the control of pH. We use the fact that the repeat spacing of multilamellar lipid bilayers is a sensitive and accurate indicator of the force balance between membranes. We show that common buffers can in fact charge up neutral membranes. However, this electrostatic effect is not immediately recognized because of the concomitant modification of dispersion (van der Waals) forces. We show that although surface charging can be weak, electrostatic forces are significant even at large distances because of reduced ionic screening and reduced van der Waals attraction. The zwitterionic interactions that we identify are expected to be relevant for interfacial biological processes involving lipid bilayers, and for a wide range of biomaterials, including amino acids, detergents, and pharmaceutical drugs. An appreciation of zwitterionic electrodynamic character can lead to a better understanding of molecular interactions in biological systems and in soft materials in general.

Biomembrane models and drug-biomembrane interaction studies: Involvement in drug design and development

Contact with many different biological membranes goes along the destiny of a drug after its systemic administration. From the circulating macrophage cells to the vessel endothelium, to more complex absorption barriers, the interaction of a biomolecule with these membranes largely affects its rate and time of biodistribution in the body and at the target sites. Therefore, investigating the phenomena occurring on the cell membranes, as well as their different interaction with drugs in the physiological or pathological conditions, is important to exploit the molecular basis of many diseases and to identify new potential therapeutic strategies. Of course, the complexity of the structure and functions of biological and cell membranes, has pushed researchers toward the proposition and validation of simpler two- and three-dimensional membrane models, whose utility and drawbacks will be discussed. This review also describes the analytical methods used to look at the interactions among bioactive compounds with biological membrane models, with a particular accent on the calorimetric techniques. These studies can be considered as a powerful tool for medicinal chemistry and pharmaceutical technology, in the steps of designing new drugs and optimizing the activity and safety profile of compounds already used in the therapy.

The vesicle-to-micelle transformation of phospholipid-cholate mixed aggregates: a state of the art analysis including membrane curvature effects

We revisited the vesicle-to-micelle transformation in phosphatidylcholine-cholate mixtures paying special attention to the lipid bilayer curvature effects. For this purpose, we prepared unilamellar vesicles with different starting sizes (2r(v)=45-120nm). We then studied mixtures of the unilamellar vesicles (1-8mmol kg(-1)) and sodium cholate (0-11.75mmolkg(-1)) by static and dynamic light scattering. The transformation generally comprises at least two, largely parallel phenomena; one increases and the other decreases the average mixed aggregate size. In our view, cholate first induces bilayer fluctuations that lead to vesicle asphericity, and then to lipid bilayer poration followed by sealing/reformation (or fusion). The cholate-containing mixed bilayers, whether in vesicular or open form, project thread-like protrusions with surfactant enriched ends even before complete bilayer solubilisation. Increasing cholate concentration promotes detachment of such protrusions (i.e. mixed micelles formation), in parallel to further softening/destabilising of mixed amphipat bilayers over a broad range of concentrations. Vesicles ultimately fragment into mixed thread-like micelles. Higher cholate relative concentrations yield shorter thread-like mixed micelles. Most noteworthy, the cholate-induced bilayer fluctuations, the propensity for large aggregate formation, the transformation kinetics, and the cholate concentration ensuring complete lipid solubilisation all depend on the starting mean vesicle size. The smallest tested vesicles (2r(v)=45nm), with the highest bilayer curvature, require ~30% less cholate for complete solubilisation than the largest tested vesicles (2r(v)=120nm).