Benzodiazepine localisation at the lipid-water interface: effect of membrane composition and drug chemical structure

Repurposing anti-cancer porphyrin derivative drugs to target SARS-CoV-2 envelope

Antiviral medicines to treat COVID-19 are still scarce. Porphyrins and porphyrin derivatives (PDs) usually present broad-spectrum antiviral activity with low risk of resistance development. In fact, some PDs are clinically approved to be used in anti-cancer photodynamic therapy and repurposing clinically approved PDs might be an alternative to treat COVID-19. Here, we characterize the ability of temoporfin, verteporfin, talaporfin and redaporfin to inactivate SARS-CoV-2 infectious particles. PDs light-dependent and -independent effect on SARS-CoV-2 infectivity were evaluated. PDs photoactivation successfully inactivated SARS-CoV-2 with very low concentrations and light dose. However, only temoporfin and verteporfin inactivated SARS-CoV-2 in the dark, being verteporfin the most effective. PDs treatment reduced viral load in infected Caco-2 cells, while not inducing cytotoxicity. Furthermore, light-independent treatment with temoporfin and verteporfin act on early stages of viral infection. Using lipid vehicles as membrane models, we characterized PDs interaction to the viral envelope. Verteporfin presented the lowest IC50 for viral inactivation and the highest partition coefficients (Kp) towards lipid bilayers. Curiously, although temoporfin and redaporfin presented similar Kps, redaporfin did not present light-independent antiviral activity, and only temoporfin and verteporfin caused lipid membrane disorder. In fact, redaporfin is located closer to the bilayer surface, while temoporfin and verteporfin are located closer to the centre. Our results suggest that viral envelope affinity, with penetration and destabilization of the lipid bilayer, seems critical to mediate PDs antiviral activity. Altogether, these findings open new avenues for the off-label application of temoporfin and verteporfin in the systemic treatment of COVID-19.

An eco-friendly cellulose support functionalized with tin titanate nanoparticles for the fast removal of clonazepam drug from the drinking water: adsorption mechanisms

This research studied the performance of tin titanate (SnTiO₃, SnT) and cellulose-based composites for the removal of clonazepam (CZP) drug by physical adsorption. The cellulose was extracted from a plant named tithonia tubaeformis, which is considered as weed in the crop fields of Mexico. The analysis by microscopy revealed that the SnTiO₃ powders are formed by a mixture of coalesced grains and nanotubes with lengths in the range of 97-633 nm. Furthermore, the X-ray diffraction analysis indicated that the SnT powders present a mixture of cassiterite and rutile phases. Experiments for the CZP removal from drinking water were carried out, and several parameters such as initial drug concentration (1-10 mg/L), amount of SnT adsorbent per liter of contaminated solution (10-50 mg/L), and pH (3-10) were varied in order to study their influence on the CZP removal percentage. Essentially, we found that the SnT dosage of 50 mg/L produced the most efficient and fastest CZP removal, since 94.3% of CZP was removed after only 10 min of reaction. Moreover, a piece of cellulose (Cell) was decorated with 50 mg of SnT powder to form the Cell+SnT composite, and this was able to remove a maximum of 80.5% of CZP after 180 min of reaction. If the amount of SnT powder deposited on the Cell+SnT composite is raised up to 100 mg, the composite can remove 95.5% of CZP. The adsorption capacity was also calculated for the SnT powders and Cell+SnT composite and found that it was 6.3 times higher for the SnT powders. Furthermore, the Raman spectra recorded for the Cell+SnT composites demonstrated the presence of surface defects, which acted as adsorption centers for the CZP molecules. The results of this investigation demonstrate that eco-friendly and low-cost floatable composites can be used for the removal of pharmaceutical contaminants, which is an advantage over adsorbent powders.

Combined in-silico and in-vitro experiments support acid-base equilibrium as a tool to estimate the localization depth of 4-nitrophenol within a phospholipid bilayer

The interaction and location of 4-nitrophenol (PNP) in biomembranes are relevant in the bioaccumulation and potentiation of the intensive toxic effects of this persistent organic pollutant. In this work, in-silico analyses predicted that, in a fluid phospholipid bilayer, the minimum energy of the protonated (PNPH) and deprotonated (PNP^-) species is located within the glycerol and choline region, respectively. This was experimentally confirmed by acid-base equilibrium experiments and theory, allowing the estimation of the mean location of PNP within a bilayer region with a dielectric constant D = 50.6 compatible with the phosphate/choline moiety of egg-yolk phosphatidylcholine unilamellar (EPC) vesicles. The comparison with the D = 43.2 value obtained in Triton X-100 micelles allow predicting a mean surface potential of ψ = 25.37 mV for the EPC-water interface. Changes in the chemical shifts and longitudinal relaxation times of EPC hydrogens by ¹H NMR confirm the deeper location of the PNPH within the glycerol region and at the choline region (PNP^-) at higher pH. Intermolecular PNP^-EPC dipolar interactions within the choline region was also demonstrated at pH 10.2 using ROESY experiments. Additional information was obtained trough ³¹P NMR, that detected an increase in the anisotropy at the membrane interface after insertion of PNP which probably act as a bridge between choline moieties rigidizing the crystalline structure at that spot. Concluding, here we provide experimental support to the "pH-piston hypothesis" proposed some decades ago in the pharmaceutical field, and that reinforce the importance of the environmental conditions (e.g. pH) to modulate the bioavailability of this highly toxic pollutant.

Experimental versus theoretical log D7.4 , pKa and plasma protein binding values for benzodiazepines appearing as new psychoactive substances

The misuse of benzodiazepines as new psychoactive substances is an increasing problem around the world. Basic physicochemical and pharmacokinetic data is required on these substances to interpret and predict their effects upon humans. Experimental log D7.4 , pKa and plasma protein binding values were determined for 11 benzodiazepines that have recently appeared as new psychoactive substances (3-hydroxyphenazepam, 4'-chlorodiazepam, desalkylflurazepam, deschloroetizolam, diclazepam, etizolam, flubromazepam, flubromazolam, meclonazepam, phenazepam, and pyrazolam) and compared with values generated by various software packages (ACD/I-lab, MarvinSketch, ADMET Predictor and PreADMET). ACD/I-LAB returned the most accurate values for log D7.4 and plasma protein binding while ADMET Predictor returned the most accurate values for pKa . Large variations in predictive errors were observed between compounds. Experimental values are currently preferable and desirable as they may aid with the future 'training' of predictive models for these new psychoactive substances.

Synaptosomal membrane-based Langmuir-Blodgett films: a platform for studies on γ-aminobutyric acid type A receptor binding properties

In this work we used Langmuir-Blodgett films (LB) as model membranes to study the effect of molecular packing on the flunitrazepam (FNZ) accessibility to the binding sites at the GABAA receptor (GABAA-R). Ligand binding data were correlated with film topography analysis by atomic force microscopy images (AFM) and SDS-PAGE. Langmuir films (LF) were prepared by the spreading of synaptosomal membranes (SM) from bovine brain cortex at the air-water interface. LBs were obtained by the transference, at 15 or 35 mN/m constant surface pressure (π), of one (LB15/1c and LB35/1c) or two (LB35/2c) LFs to a film-free hydrophobic alkylated substrate (CONglass). Transference was performed in a serial manner, which allowed the accumulation of a great number of samples. SDS-PAGE clearly showed a 55 kDa band characteristic of GABAA-R subunits. Detrended fluctuation analysis of topographic data from AFM images exhibited a single slope value (self-similarity parameter α) in CONglass and a discontinuous slope change in the α value at an autocorrelation length of ∼100 nm in all LB samples, supporting the LF transference to the substrate. AFM images of CONglass and LB15/1c exhibited roughness and average heights that were similar between measurements and significantly lower than those of LB35/1c and LB35/2c, suggesting that the substrate coverage in the latter was more stable than in LB15/1c. While [(3)H]FNZ binding in LB15/1c did not reach saturation, in LB35/1c the binding kinetics became sigmoid with a binding affinity lower than in the SM suspension. Our results highlight the π dependence of both binding and topological data and call to mind the receptor mechanosensitivity. Thus, LB films provide a tool for bionanosensing GABAA-R ligand binding as well as GABAA-R activity modulation induced by the environmental supramolecular organization.

Probing the combined effect of flunitrazepam and lidocaine on the stability and organization of bilayer lipid membranes. A differential scanning calorimetry and dynamic light scattering study

Combined effects of flunitrazepam (FNZ) and lidocaine (LDC) were studied on the thermotropic equilibrium of dipalmitoyl phosphatidylcholine (dpPC) bilayers. This adds a thermodynamic dimension to previously reported geometric analysis in the erythrocyte model. LDC decreased the enthalpy and temperature for dpPC pre- and main-transitions (ΔHp, ΔHm, Tp, Tm) and decreased the cooperativity of the main-transition (ΔT(1/2,m)). FNZ decreased ΔHm and, at least up to 59 μM, also decreased ΔHp. In conjunction with LDC, FNZ induced a recovery of ∆T(1/2,m) control values and increased ΔHm even above the control level. The deconvolution of the main-transition peak at high LDC concentrations revealed three components possibly represented by: a self-segregated fraction of pure dpPC, a dpPC-LDC mixture and a phase with a lipid structure of intermediate stability associated with LDC self-aggregation within the lipid phase. Some LDC effects on thermodynamic parameters were reverted at proper LDC/FNZ molar ratios, suggesting that FNZ restricts the maximal availability of the LDC partitioned into the lipid phase. Thus, beyond its complexity, the lipid-LDC mixture can be rationalized as an equilibrium of coexisting phases which gains homogeneity in the presence of FNZ. This work stresses the relevance of nonspecific drug-membrane binding on LDC-FNZ pharmacological interactions and would have pharmaceutical applications in liposomal multidrug-delivery.

StarD7 behaves as a fusogenic protein in model and cell membrane bilayers

StarD7 is a surface active protein, structurally related with the START lipid transport family. So, the present work was aimed at elucidating a potential mechanism of action for StarD7 that could be related to its interaction with a lipid-membrane interface. We applied an assay based on the fluorescence de-quenching of BD-HPC-labeled DMPC-DMPS 4:1 mol/mol SUVs (donor liposomes) induced by the dilution with non-labeled DMPC-DMPS 4:1 mol/mol LUVs (acceptor liposomes). Recombinant StarD7 accelerated the dilution of BD-HPC in a concentration-dependent manner. This result could have been explained by either a bilayer fusion or monomeric transport of the labeled lipid between donor and acceptor liposomes. Further experiments (fluorescence energy transfer between DPH-HPC/BD-HPC, liposome size distribution analysis by dynamic light scattering, and the multinuclear giant cell formation induced by recombinant StarD7) strongly indicated that bilayer fusion was the mechanism responsible for the StarD7-induced lipid dilution. The efficiency of lipid dilution was dependent on StarD7 electrostatic interactions with the lipid-water interface, as shown by the pH- and salt-induced modulation. Moreover, this process was favored by phosphatidylethanolamine which is known to stabilize non-lamellar phases considered as intermediary in the fusion process. Altogether these findings allow postulate StarD7 as a fusogenic protein.

Surface active benzodiazepine-bromo-alkyl conjugate for potential GABAA-receptor purification

A conjugable analogue of the benzodiazepine 5-(2-hydroxyphenyl)-7-nitro-benzo[e][1,4]diazepin-2(3H)-one containing a bromide C(12)-aliphatic chain (BDC) at nitrogen N1 was synthesized. One-pot preparation of this benzodiazepine derivative was achieved using microwave irradiation giving 49% yield of the desired product. BDC inhibited FNZ binding to GABA(A)-R with an inhibition binding constant K(i) = 0.89 μM and expanded a model membrane packed up to 35 mN m(-1) when penetrating in it from the aqueous phase. BDC exhibited surface activity, with a collapse pressure π = 9.8 mN m(-1) and minimal molecular area A(min) = 52 Å(2)/molecule at the closest molecular packing, resulted fully and non-ideally mixed with a phospholipid in a monolayer up to a molar fraction x≅ 0.1. A geometrical-thermodynamic analysis along the π-A phase diagram predicted that at low x(BDC) (<0.1) and at all π, including the equilibrium surface pressures of bilayers, dpPC-BDC mixtures dispersed in water were compatible with the formation of planar-like structures. These findings suggest that, in a potential surface grafted BDC, this compound could be stabilize though London-type interactions within a phospholipidic coating layer and/or through halogen bonding with an electron-donor surface via its terminal bromine atom while GABA(A)-R might be recognized through the CNZ moiety.

Acid–base equilibrium of puerarin in CTAB micelles

Electronic absorption spectra, fluorescence emission spectra, 1HNMR and ab initio quantum calculation are used to study the acid-base equilibrium of puerarin in cetytrimethylammonium bromide (CTAB) micelles with different microstructures and microenvironments. Experiments suggest the microenvironment provided by CTAB micelle cause the acid-base equilibrium of puerarin to move to the deprotonation reaction. The changes in the chemical shifts of the individual groups of protons in CTAB indicate that the location of puerarin changes from the inner to the outer of the CTAB micelles with an increase in puerarin concentration, which enhances the interaction between puerarin and CTAB. The binding of puerarin with CTAB micelle is a spontaneous (deltaG<0) and endothermic process (deltaH<0), and the hydrophobic and electrostatic force is the main driving force for its solubilization.

Natural terpenes: Self-assembly and membrane partitioning

Monoterpenes (MTs) are highly hydrophobic substances present in essential oils. They cover a wide spectrum of biological effects with a membrane interaction as a common point. Here we studied the surface activity of camphor, cineole, thymol, menthol and geraniol, and their ability to reach and incorporate into model membranes affecting some features of their dynamic organization. All the MTs studied self-aggregated in water with critical micellar concentrations (CMC) between 3 and 8 microM. Their octanol-water and membrane-water partition coefficients were correlated with one another. They all penetrated in monomolecular layers of dipalmitoyl-phosphatildylcholine at the air-water interface, even at surface pressures (pi) above the equilibrium lateral pressure of bilayers; thymol exhibited the highest (61.3 mN/m) and camphor the lowest (37 mN/m) pi(cut-off) value. They affected the self-aggregation of Triton X-100, increasing its CMC from 0.16 mM in the absence of MTs up to 0.68 mM (e.g. for geraniol), and the topology of sPC vesicles, increasing its surface curvature, suggesting their location at the polar head group region of the membrane. The latter was supported by their ability to increase differentially the polarity of the membrane environment sensed by two electrochromic dyes. Dipole moment values (between 1.224 and 2.523 D) and solvation areas (between 80 and 97 A(2)) were calculated from their energy-minimized structures. The relative contribution of each experimental, theoretical and structural property to determine MTs' effects on membrane dynamics were evaluated by a principal component analysis.

Hydroxyzine, promethazine and thioridazine interaction with phospholipid monomolecular layers at the air–water interface

In this work the interaction of Hydroxyzine, Promethazine and Thioridazine with Langmuir films of dipalmitoylphosphatidylcholine (dpPC) and dipalmitoylphosphatidic acid (dpPA), is studied. Temporal variations in lateral surface pressure (pi) were measured at different initial pi (pi(i)), subphase pH and drug-concentration. Drugs with the smallest (PRO) and largest (HYD) molecular size exhibited the lowest adsorption (k(a)) and the highest desorption (k(d)) rate constant values, respectively. The affinity binding constants (K(b)) obtained in monolayers followed the same profile (K(b,PRO) < K(b,HYD) < K(b,THI)) of the egg-PC/water partition coefficients (P) determined in bilayers. The drug concentration required to reach the half-maximal Deltapi at pi(i) = 14 mN/m (K(0.5)), was very sensitive to pH. The maximal increment in pi upon drug incorporation into the monolayer (deltapi(max)) will depend on the phospholipid collapse pressure (pi(c)), the monolayers's compressibility and drug's size, shape and charge. The higher pi(c) of dpPC lead to higher pi(cut-off) values (maximal pi allowing drug penetration), if compared with dpPA. In dpPC and dpPA pi(cut-off) decreased as a function of the molecular size of the uncharged drugs. In dpPA, protonated drugs became electrostatically trapped at the monolayer surface hence drug penetration, monolayer deformation and pi increase were impaired and the correlation between pi(cut-off) and drug molecular size was lost.

Combined quantitative and mechanistic study of drug-membrane interactions using a novel 2H NMR approach

Several analytical methods are available for determining the partition coefficients of drug compounds in model phospholipid membranes, but such methods provide little information at the molecular level about how the membrane affinity of drugs relates to their interactions with the lipid molecules. A new (2)H nuclear magnetic resonance (NMR) approach has been developed here that quantifies the affinity of (2)H-labeled small molecules for different phospholipid membranes and, simultaneously, provides information on the mechanism of the drug-membrane interaction. In the example given, (2)H NMR analysis of a weakly basic ion pump inhibitor found that the drug partitioned preferentially into membranes of predominantly unsaturated or short-chain phospholipids. The (2)H NMR analysis also suggested that the membrane specificity of the drug was directly correlated to the ability of its phenyl moiety to penetrate into the interior of the lipid bilayer. The (2)H NMR approach could be of value in guiding medicinal chemistry toward or away from structures promoting interactions with specific types of biological membranes.

Effects of flunitrazepam on the Lα-HII phase transition of phosphatidylethanolamine using merocyanine 540 as a fluorescent indicator

Previous studies of our group demonstrated that flunitrazepam is a lipophilic drug capable of interacting with membranes through a partition equilibrium phenomenon. Its localization at the phospholipid polar head region could explain the decrease in the size of dipalmitoylphosphatidylcholine (dpPC) vesicles, through a mechanism that involves the increment in the relative volume of this region with a subsequent increase in the vesicle's surface curvature. In the present work, we investigated if flunitrazepam can affect the L(alpha)-H(II) phase transition of phosphatidylethanolamine through a similar mechanism. This study was approached by using merocyanine 540, a dye sensitive to the molecular packing of membrane lipids. A detailed analysis of merocyanine absorption and fluorescence emission and excitation spectra was performed. The results indicated that the fluorescence emitted came mainly from the monomeric form of merocyanine and that it resulted a good indicator of this phase transition, as was previously described. Flunitrazepam did not affect significantly the onset of the phase transition but showed a tendency to diminish the dye fluorescence emission intensity, which could involve a lower partition of merocyanine in the vesicles. Moreover, the results suggest that this drug produced a delay in the completeness of the phase transition and a decrement in the cooperativity of this phenomenon.

Dissociation constants of phenothiazine drugs incorporated in phosphatidylcholine bilayer of small unilamellar vesicles as determined by carbon-13 nuclear magnetic resonance spectrometric titration

The dissociation constants (pKms) of the phenothiazine drugs promazine, chlorpromazine, and triflupromazine, incorporated in the phosphatidylcholine (PC) bilayer of small unilamellar vesicles (SUV), were investigated by a 13C nuclear magnetic resonance (NMR) titration method employing their N-13CH3 (ionizable group) labelled derivatives. Use of the labelled drugs enabled direct observations of the ionization equilibrium of the N-dimethyl group. A second derivative spectrophotometric study proved that 95-98% of the phenothiazine species in the sample solutions (200 microM phenothiazine in the presence of 27 mM PC SUV) were incorporated into the PC bilayer, which simplified the calculation of pKm values by allowing that the phenothiazines in the aqueous phase could be neglected. The pKm values were calculated from the chemical shift dependence of the N-dimethyl 13C NMR signal on the pH value of sample solutions. The pKm values obtained were smaller than those measured in aqueous solutions by about one unit. The existence of cholesterol (30 mol%) in the PC bilayer showed little effect on the pKm values, suggesting that cholesterol in the bilayer does not largely affect the interfacial region where the N-dimethyl group of the incorporated phenothiazines is located. The results offered clear evidence for the pKm decrease and provided their precise values.

Surface activity of thymol: implications for an eventual pharmacological activity

In the present work, we studied the ability of thymol to affect the organization of model membranes and the activity of an intrinsic membrane protein, the GABA(A) receptor (GABA(A)-R). In this last aspect, we tried to elucidate if the action mechanism of this terpene at the molecular level, involves its binding to the receptor protein, changes in the organization of the receptor molecular environment, or both. The self-aggregation of thymol in water with a critical micellar concentration approximately = 4 microM and its ability to penetrate in monomolecular layers of soybean phosphatidylcholine (sPC) at the air-water interface, even at surface pressures above the equilibrium, lateral pressure of natural bilayers were demonstrated. Thymol affected the self-aggregation of Triton X-100 and the topology of sPC vesicles. It also increased the polarity of the membrane environment sensed by the electrochromic dye merocyanine. A dipolar moment of 1.341 Debye was calculated from its energy-minimized structure. Its effect on the binding of [3H]-flunitrazepam ([3H]-FNZ) to chick brain synaptosomal membranes changed qualitatively from a tendency to the inhibition to a clear activatory regime, up on changing the phase state of the terpene (from a monomeric to a self-aggregated state). Above its CMC, thymol increased the affinity of the binding of [3H]-FNZ (K(d-control)= 2.9, K(d-thymol)= 1.7 nM) without changing the receptor density (B(max-control)= 910, B(max-thymol)= 895 fmol/mg protein). The activatory effect of thymol on the binding of [ [3H]-FNZ was observed even in the presence of the allosteric activator gamma-aminobutyric acid (GABA) at a concentration of maximal activity, and was blocked by the GABA antagonist bicuculline. Changes in the dipolar arrangement and in the molecular packing of GABA(A)-R environment are discussed as possible mediators of the action mechanism of thymol.

Monoterpenes affect chlorodiazepoxide–micelle interaction through micellar dipole potential modifications

The ability of several natural terpenes to affect benzodiazepine (BZD)-micelle interaction through the membrane dipolar organization was investigated. The acid-base equilibrium of chlorodiazepoxide (CDX) and the spectroscopic behavior of the electrochromic dye merocyanine were tested in the presence and in the absence of Triton X-100 micelles (used to mimic a membrane environment) containing or not cineole, menthol, geraniol or camphor. CDX's apparent pK increased in the environment of terpene-containing micelles compared with pure Triton X-100 micelles. Decrements in electric potentials (between -111 and -128 mV with respect to pure detergent) were calculated from Boltzmann equation. This result suggested, that in the presence of terpenes, the tendency of CDXH(+) to remain in the membrane phase increased. The dielectric constant (D) of the microenvironment sensed by merocyanine within Triton X-100 micelles, determined from lambda(max,2) of merocyanine monomer, was D=9 and increased in the presence of all the terpenes assayed (D congruent with 11). The decrease in merocyanine partitioning (A(peak1)/A(peak2) increased) also reflected an increment in the negative dipole potential. The present results suggest that terpenes contributed to the whole dipolar arrangement of the micelle with a dipole moment vector which had an intense component oriented parallel to the intrinsic dipole of the Triton X-100 molecules in the micelles. This led to a more negative environment of the interface region where CDX was located, and increased the net polarity of the deepest micelle regions sensed by merocyanine.

Effect of phosphatidylserine content on the partition coefficients of diazepam and flurazepam between phosphatidylcholine-phosphatidylserine bilayer of small unilamellar vesicles and water studied by second derivative spectrophotometry

The affinity of the psychotropic benzodiazepine drugs diazepam (DZ) and flurazepam (FZ) to phosphatidylserine (PS) was examined since PS is abundantly contained in brain membranes. The effect of PS content on the partition coefficients (K(p)s) of these drugs between phosphatidylcholine (PC)-PS bilayer membranes of small unilamellar vesicles (SUV) and water was measured using second derivative spectrophotometry. The second derivative spectra of DZ and FZ measured in the solutions containing various amounts of PC-PS SUV clearly showed derivative isosbestic points and a distinct derivative intensity change depending on the amount of the SUV added. The derivative intensity differences (AD) of the drugs before and after addition of the SUV suspension were measured at a specific wavelength. Using the AD values, the Kp values were calculated and obtained with relative standard deviation of below 10%. The Kp values of both drugs increased according to the PS content in the PS-PC bilayer membranes of the SUV proving that both have higher affinity to the PC-PS bilayer membranes than to PC membranes. The effect was much larger for FZ, i.e., the Kp value of FZ at 30 mol% PS content increased to about five times the value for the PC SUV. This can be explained by the fact that at the experimental pH of 7.4, 80% of FZ molecules are in a cationic form (pKa=8.1), so that these molecules are highly accessible to the negatively charged PS molecules. The results support the rapid and high distribution of DZ and FZ in the central nervous system after their administration.

Flunitrazepam-membrane non-specific binding and unbinding: two pathways with different energy barriers

The effect of molecular packing on flunitrazepam's ability to interact with bio-membranes was studied using dipalmitoylphosphatidylcholine monomolecular layers at the air-water interface as a model membrane. Flunitrazepam penetrated from the subphase into monolayers at lateral pressures below 44.8 mN/m and induced their concentration-dependent expansion. As inferred from the values of compressibility modulus, the elasticity of the liquid-condensed phase decreased in the presence of flunitrazepam. Although this drug hardly penetrated into high-packed monolayers, it was easily incorporated in the low-packed ones at an extent sufficient to reach the partition equilibrium. Below a molecular area of 75 A(2), contrary to what would be expected, the drug surface concentration increased as a function of surface pressure, suggesting that after its penetration in disordered phases, it became energetically or physically trapped in newly-formed liquid condensed clusters. The phenomenon of flunitrazepam penetration and release would have different energy barriers depending on the membrane phase-state.

Location of Pinacyanol in Micellar Solutions of N-Alkyl Trimethylammonium Bromide Surfactants

The interaction of pinacyanol (PIN), a cationic dye formed by monomer and dimer species, with three cationic surfactants (DTAB, TTAB, and HTAB) has been studied spectroscopically and by acid-base equilibrium in the micellar concentration range. In the presence of surfactants, the absorption maximum of the two main peaks undergoes bathochromic shifts. The spectral shifts suggest a hydrophobic environment of the chromophore. The presence of micelles favors the monomer species; i.e., it reduces the extent of dimerization. The pK(a) of PIN in micellar medium is similar to the value in pure water. When acid-base equilibrium was considered, the changes in the interfacial pK(a) allowed to us to determine the constant dielectric for the interfacial region (epsilon=69). This led to the conclusion that the dye must be solubilized between the solution and the hydrocarbon chain core, i.e., in the aqueous micellar interface. This location can be explained by a cation-pi interaction between the uncharged ring system of the dye and the cationic headgroups of the surfactants. Copyright 2001 Academic Press.

Flunitrazepam induces geometrical changes at the lipid-water interface

Flunitrazepam (FNTZ) effects on molecular packing and surface curvature in artificial model membranes were investigated. FNTZ, from the subphase under dipalmitoylphosphatidylcholine (dpPC) monolayers at the air-water interface, expanded the surface pressure-area isotherm and induced an increment in the limiting area; in this conditions, the collapse pressure of dpPC decreased, indicating a lowering in the stability of the monolayer. Thermodynamic-geometric correlations based on molecular parameters predicted a decrement in the aggregation number and stability, and an increase in the curvature of the self-aggregated structure of dpPC in aqueous medium in the presence of FNTZ. Accordingly, negative-staining electron microscopy of dpPC aqueous dispersions showed that the mean diameter of dpPC vesicles decreased 2 and 2.87 times in the presence of 10 nM and 50 µM FNTZ, respectively, compared with control samples. The release of a soluble marker entrapped in dpPC liposomes increased slightly respect to the control in the presence of FNTZ. In dpPC-dpPE mixed liposomes 50 µM FNTZ induced a decrement in the amount of the aminophospholipid exposed to the outer monolayer. Concluding, an FNTZ-induced expansion of dpPC-water interface region affected the constraints imposed on the lipid-water system by the molecular geometry, interacting free energies and entropy that determine the shape of a multimolecular structure. In liposomes composed of a pure phospholipid, the bilayer expansion leaded, through a structure instability, to reduce the liposome size; in mixed liposomes, phospholipid molecules translocation could be observed as another compensating mechanism of the initial perturbation. These results may be relevant for understanding benzodiazepines' effects non-mediated by membrane receptors.

Determination by fluorimetric titration of the ionization constants of ciprofloxacin in solution and in the presence of liposomes

A fluorescence titration method was applied for the determination of pKa of ciprofloxacin (CPX) in solution. Values of 6.18 +/- 0.05 and 8.76 +/- 0.03 were obtained for pKa1 and pKa2, respectively. The method was used to determine the ionization constants in the presence of liposomes of dipalmitoylphosphatidylcholine (DPPC) and DPPC with 10 mol% of dipalmitoylphosphatidylglycerol. A dependence on the surface charge of liposomes was found which supported the existence of a basic electrostatic interaction between CPX and the phospholipid bilayer. Both pK values for the N-4 butyl-piperazinyl derivative (BCPX) of the parent compound were also determined in solution and in the presence of liposomes. The competition of both drugs for the same binding site as 1-anilino-8-naphtalene sulfonate demonstrate that the interaction is governed by electrostatic forces.

Flunitrazepam partitioning into natural membranes increases surface curvature and alters cellular morphology

In recent studies, we showed that flunitrazepam (FNTZ) and other benzodiazepines interact with artificial phospholipid membranes locating at the polar head group region, inducing a membrane expansion, reducing the molecular packing and reorganising molecular dipoles. In the present paper we investigated the possibility that those phenomena could be transduced into changes in the curvature of membranes from natural origin. Hence we studied the effect of FNTZ on cellular morphology using human erythrocyte as a natural assay system. Shape changes of erythrocytes were evaluated by light microscopy and expressed as a morphological index (MI). FNTZ induced echinocytosis in a time-dependent manner with MI values significantly higher than those of control (without drug) or DMSO (vehicle) samples. Lidocaine, a local anesthetic known to induce stomatocytosis by incorporating in the inner monolayer, counterbalanced the concentration-dependent FNTZ crenating effects. FNTZ induced protective effects, compared with control and DMSO, against time-dependent hemolysis. Hypotonic-induced hemolysis, was also lowered by FNTZ in a concentration-dependent manner. Both antihemolytic effects suggested a drug-induced membrane expansion allowing a greater increase in cell volume before lysis. In such a complex system like a cell, curvature changes triggered by drug partitioning towards the plasma membrane, might be an indirect effect exerted through modifications of ionic-gradients or by affecting cytoskeleton-membrane linkage. In spite of that, the curvature changes can be interpreted as a mechanism suitable to relieve the tension generated initially by drug incorporation into the bilayer and may be the resultant of the dynamic interactions of many molecular fluxes leading to satisfy the spontaneous membrane curvature.

Surface active drugs: self-association and interaction with membranes and surfactants. Physicochemical and biological aspects

Many pharmacologically active compounds are of amphiphilic (or hydrophobic) nature. As a result, they tend to self-associate and to interact with biological membranes. This review focuses on the self-aggregation properties of drugs, as well as on their interaction with membranes. It is seen that drug-membrane interactions are analogous to the interactions between membranes and classical detergents. Phenomena such as shape changes, vesiculation, membrane disruption, and solubilization have been observed. At the molecular level, these events seem to be modulated by lipid flip-flop and formation of non-bilayer phases. The modulation of physicochemical properties of drugs by self-association and membrane binding is discussed. Pathological consequences of drug-membrane interaction are described. The mechanisms of drug solubilization by surfactants are reviewed from the physicochemical point of view and in relation to drug carrying and absorption by the organism.