Site of action of the local anesthetic tetracaine in a phosphatidylcholine bilayer with incorporated cardiolipin

Anticrystal Engineering of Ketoprofen and Ester Local Anesthetics: Ionic Liquids or Deep Eutectic Mixtures?

Ionic liquids (ILs) and deep eutectic mixtures (DEMs) are potential solutions to the problems of low solubility, polymorphism, and low bioavailability of drugs. The aim of this work was to develop and investigate ketoprofen (KET)-based ILs/DEMs containing an ester local anesthetic (LA): benzocaine (BEN), procaine (PRO) and tetracaine (TET) as the second component. ILs/DEMs were prepared via a mechanosynthetic process that involved the mixing of KET with an LA in a range of molar ratios and applying a thermal treatment. After heating above the melting point and quench cooling, the formation of supercooled liquids with Tgs that were dependent on the composition was observed for all KET-LA mixtures with exception of that containing 95 mol% of BEN. The KET-LA mixtures containing either ≥ 60 mol% BEN or 95 mol% of TET showed crystallization to BEN and TET, respectively, during either cooling or second heating. KET decreased the crystallization tendency of BEN and TET and increased their glass-forming ability. The KET-PRO systems showed good glass-forming ability and did not crystallize either during the cooling or during the second heating cycle irrespective of the composition. Infrared spectroscopy and molecular modeling indicated that KET and LAs formed DEMs, but in the KET-PRO systems small quantities of carboxylate anions were present.

On the structure of prilocaine in aqueous and amphiphilic solutions

The solvation of prilocaine has been investigated in pure water and in amphiphilic solutions using a combination of neutron diffraction and simulations.

Anesthetic diffusion through lipid membranes depends on the protonation rate

Hundreds of substances possess anesthetic action. However, despite decades of research and tests, a golden rule is required to reconcile the diverse hypothesis behind anesthesia. What makes an anesthetic to be local or general in the first place? The specific targets on proteins, the solubility in lipids, the diffusivity, potency, action time? Here we show that there could be a new player equally or even more important to disentangle the riddle: the protonation rate. Indeed, such rate modulates the diffusion speed of anesthetics into lipid membranes; low protonation rates enhance the diffusion for local anesthetics while high ones reduce it. We show also that there is a pH and membrane phase dependence on the local anesthetic diffusion across multiple lipid bilayers. Based on our findings we incorporate a new clue that may advance our understanding of the anesthetic phenomenon.

Sensing small molecule interactions with lipid membranes by local pH modulation

Herein, we utilized a label-free sensing platform based on pH modulation to detect the interactions between tetracaine, a positively charged small molecule used as a local anesthetic, and planar supported lipid bilayers (SLBs). The SLBs were patterned inside a flow cell, allowing for various concentrations of tetracaine to be introduced over the surface in a buffer solution. Studies with membranes containing POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) yielded an equilibrium dissociation constant value of Kd = 180 ± 47 μm for this small molecule-membrane interaction. Adding cholesterol to the SLBs decreased the affinity between tetracaine and the bilayers, while this interaction tightened when POPE (1-hexadecanoyl-2-(9-Z-octadecenoyl)-sn-glycero-3-phosphoethanolamine) was added. Studies were also conducted with three negatively charged membrane lipids, POPG (1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (sodium salt)), POPS (1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (sodium salt)), and ganglioside GM1. All three measurements gave rise to a similar tightening of the apparent Kd value compared with pure POPC membranes. The lack of chemical specificity with the identity of the negatively charged lipid indicated that the tightening was largely electrostatic. Through a direct comparison with ITC measurements, it was found that the pH modulation sensor platform offers a facile, inexpensive, highly sensitive, and rapid method for the detection of interactions between putative drug candidates and lipid bilayers. As such, this technique may potentially be exploited as a screen for drug development and analysis.

New procedure to synthesize silver nanoparticles and their interaction with local anesthetics

Silver nanoparticles (AgNPs) were prepared in aqueous colloid dispersions by the reduction of Ag⁺ with glucose in alkaline medium. Tetraethyl orthosilicate and L-asparagine were added as stabilizers of NPs. The AgNPs were characterized, and their interaction with three local anesthetics (procaine, dibucaine, or tetracaine) was investigated. Optical spectra show the characteristic absorption band of AgNPs, due to surface plasmon resonance. Modifications in the position and shape of this band reflect the self-assembly of metal NPs mediated by anesthetic molecules and the progress in time of the aggregation process. Zeta-potential measuring was applied in order to characterize the electrostatic stability of the NPs. The size and shape of the AgNPs, as well as the features of the assemblies formed by their association in the presence of anesthetics, were evidenced by transmission electron microscopy images. Atomic force microscopy images showed the characteristics of the films of AgNPs deposited on glass support. The effect of the anesthetics could be described in terms of electrostatic forces between the negatively charged AgNPs and the anesthetic molecules, existing also in their cationic form at the working pH. But also hydrophobic and hydrogen bonding interactions between the coated nanoparticles and anesthetics molecular species should be considered.

Stereostructure-based differences in the interactions of cardiotoxic local anesthetics with cholesterol-containing biomimetic membranes

Amide-type pipecoloxylidide local anesthetics, bupivacaine, and ropivacaine, show cardiotoxic effects with the potency depending on stereostructures. Cardiotoxic drugs not only bind to cardiomyocyte membrane channels to block them but also modify the physicochemical property of membrane lipid bilayers in which channels are embedded. The opposite configurations allow enantiomers to be discriminated by their enantiospecific interactions with another chiral molecule in membranes. We compared the interactions of local anesthetic stereoisomers with biomimetic membranes consisting of chiral lipid components, the differences of which might be indicative of the drug design for reducing cardiotoxicity. Fluorescent probe-labeled biomimetic membranes were prepared with cardiolipin and cholesterol of varying compositions and different phospholipids. Local anesthetics were reacted with the membrane preparations at a cardiotoxically relevant concentration of 200 μM. The potencies to interact with biomimetic membranes and change their fluidity were compared by measuring fluorescence polarization. All local anesthetics acted on lipid bilayers to increase membrane fluidity. Chiral cardiolipin was ineffective in discriminating S(-)-enantiomers from their antipodes. On the other hand, cholesterol produced the enantiospecific membrane interactions of bupivacaine and ropivacaine with increasing its composition in membranes. In 40 mol% and more cholesterol-containing membranes, the membrane-interacting potency was S(-)-bupivacaine<racemic bupivacaine<R(+)-bupivacaine, and S(-)-ropivacaine<R(+)-ropivacaine. Ropivacaine (S(-)-enantiomer), levobupivacaine (S(-)-enantiomeric), and bupivacaine (racemic) interacted with biomimetic membranes in increasing order of intensity. The rank order of membrane interactivity agreed with that of known cardiotoxicity. The stereoselective membrane interactions determined by cholesterol with higher chirality appears to be associated with the stereoselective cardiotoxic effects of local anesthetics. The stereostructure and membrane interactivity relationship supports the clinical use and development of S(-)-enantiomers to decrease the adverse effects of pipecoloxylidide local anesthetics on the cardiovascular system.

Bupivacaine, but not lidocaine, disrupts cardiolipin-containing small biomimetic unilamellar liposomes

Inadvertent intravenous administration of bupivacaine, unlike that of lidocaine, is associated with significant cardiotoxicity. However, the mechanism(s) underlying this phenomenon is uncertain. High concentrations of cardiolipin, an anionic phospholipid, are found in the mitochondria membrane of cardiomyocytes. We hypothesized that bupivacaine, but not lidocaine, interacts avidly with cardiolipin in the mitochondria membrane of cardiomyocytes and alters its integrity thereby accounting, in part, for cardiotoxicity. Accordingly, the purpose of this study was to begin to address this issue by determining the effects of bupivacaine and lidocaine on permeability of cardiolipin-containing biomimetic small unilamellar liposomes. We found that bupivacaine, but not lidocaine, elicited a significant, concentration-dependent increase in carboxyfluorescein release from cardiolipin-containing small unilamellar liposomes (size, 165nm) composed of egg yolk phosphatidylcholine and cholesterol (p<0.05). Both drugs had no significant effects on carboxyfluorescein release from liposomes devoid of cardiolipin (p>0.5). Collectively, these data indicate that bupivacaine, but not lidocaine, interacts avidly and selectively with biomimetic small unilamellar liposomes containing cardiolipin and disrupts their integrity. We suggest that these interactions underlie, in part, bupivacaine-induced cardiotoxicity.

Tetracaine-membrane interactions: effects of lipid composition and phase on drug partitioning, location, and ionization

Interactions of the local anesthetic tetracaine with unilamellar vesicles made of dimyristoyl or dipalmitoyl phosphatidylcholine (DMPC or DPPC), the latter without or with cholesterol, were examined by following changes in the drug's fluorescent properties. Tetracaine's location within the membrane (as indicated by the equivalent dielectric constant around the aromatic fluorophore), its membrane:buffer partition coefficients for protonated and base forms, and its apparent pK(a) when adsorbed to the membrane were determined by measuring, respectively, the saturating blue shifts of fluorescence emission at high lipid:tetracaine, the corresponding increases in fluorescence intensity at this lower wavelength with increasing lipid, and the dependence of fluorescence intensity of membrane-bound tetracaine (TTC) on solution pH. Results show that partition coefficients were greater for liquid-crystalline than solid-gel phase membranes, whether the phase was set by temperature or lipid composition, and were decreased by cholesterol; neutral TTC partitioned into membranes more strongly than the protonated species (TTCH(+)). Tetracaine's location in the membrane placed the drug's tertiary amine near the phosphate of the headgroup, its ester bond in the region of the lipids' ester bonds, and associated dipole field and the aromatic moiety near fatty acyl carbons 2-5; importantly, this location was unaffected by cholesterol and was the same for neutral and protonated tetracaine, showing that the dipole-dipole and hydrophobic interactions are the critical determinants of tetracaine's location. Tetracaine's effective pK(a) was reduced by 0.3-0.4 pH units from the solution pK(a) upon adsorption to these neutral bilayers, regardless of physical state or composition. We propose that the partitioning of tetracaine into solid-gel membranes is determined primarily by its steric accommodation between lipids, whereas in the liquid-crystalline membrane, in which the distance between lipid molecules is larger and steric hindrance is less important, hydrophobic and ionic interactions between tetracaine and lipid molecules predominate.

Amphiphilic effects of dibucaine·HCl on rotational mobility of n-(9-anthroyloxy)stearic acid in neuronal and model membranes

We studied dibucaine's effects on specific locations of n-(9-anthroyloxy)palmitic acid or stearic acid (n-AS) within phospholipids of synaptosomal plasma membrane vesicles isolated from bovine cerebral cortex (SPMV) and model membranes. Giant unilamellar vesicles (GUVs) were prepared with total lipids (SPMVTL) and mixture of several phospholipids (SPMVPL) extracted from SPMV. Dibucaine.HCl increased rotational mobility (increased disordering) of hydrocarbon interior, but it decreased mobility (increased ordering) of membrane interface, in both native and model membranes. The degree of rotational mobility in accordance with the carbon atom numbers of phospholipids comprising neuronal and model membranes was in the order at the 16, 12, 9, 6 and 2 position of aliphatic chain present in phospholipids. The sensitivity of increasing or decreasing effect of rotational mobility of hydrocarbon interior or surface region by dibucaine.HCl differed depending on the neuronal and model membranes in the descending order of SPMV, SPMVPL and SPMVTL.

Local anesthetics facilitate ion transport across lipid planar bilayer membranes under an electric field: Dependence on type of lipid bilayer

In order to elucidate the role of structural change of lipid membrane bilayer in the mode of action of local anesthetic, we studied the effects of local anesthetics, charged tetracaine and uncharged benzocaine, on ion permeability across various lipid planar bilayers (PC, mixed PC/PS (4/1, mol/mol); mixed PC/PE (1/1, mol/mol); mixed PC/SM (4/1, mol/mol)) under a constant applied voltage. The membrane conductances increased in the order of PC<<PC/PS<or=PC/SM<<PC/PE. When the constant voltage of -100 or -70 mV was applied through the lipid bilayer membranes in the presence of positively charged tetracaine, the fluctuating current pulses with the large amplitude generated, but not appeared in the absence of tetracaine. The addition of uncharged benzocaine generated the fluctuating currents with the small amplitude. Both charged tetracaine and uncharged benzocaine facilitated electrophoretically the transport of small ions such as KCl in the buffer solution through the fluctuating pores in the lipid bilayer membranes formed by interaction with the local anesthetic under the negative applied membrane potential. The current pulses also contained actual transport of charged tetracaine together with the transport of the small ions. The amplitude and the duration time of the electrical current generated by adding the local anesthetics were dependent on the type of the lipid, the applied voltage and its voltage polarity.

Evidence for heterodimers of 2,4,5-trichlorophenol on planar lipid layers. A FTIR-ATR investigation

Trichlorophenols are weak acids of high hydrophobicity and are able to transport protons across the mitochondrial membrane. Thus the proton motive force is dissipated and the ATP production decreased. In situ Fourier Transform Infrared-Attenuated Total Reflection (FTIR-ATR) experiments with 2,4,5-trichlorophenol (TCP) adsorbed to model membranes resulted in good evidence for the formation of the TCP-heterodimer. Two surfaces were examined: a dipalmitoyl phosphatidic acid (DPPA) monolayer and a planar DPPA/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer. TCP was adsorbed from 1 to 3 mM solutions at pH 6.0 to the lipid layers leading to surface layers at the water/lipid interface. Difference spectra showed an effect on DPPA acyl chains even when it was covered with POPC. Time-resolved measurements revealed two distinct adsorption processes, which were assigned to TCP and its deprotonated anion (phenoxide), respectively. For DPPA/POPC bilayers, the adsorption of TCP was faster than that of its phenoxide, whereas adsorption of both species to DPPA monolayers proceeded with similar velocity. In both cases, phenoxide formation at the membrane was found to be delayed with respect to phenol adsorption. Phenoxide and phenol were retained after replacing the TCP solution with buffer. For the retained species, we estimated a phenol/phenoxide molar ratio of 1 at pH 6.0 (pKa=6.94 for TCP), demonstrating strong evidence for heterodimer formation.

Amphiphilic effects of local anesthetics on rotational mobility in neuronal and model membranes

To provide a basis for studying the molecular mechanism of pharmacological action of local anesthetics, we carried out a study of the membrane actions of tetracaine, bupivacaine, lidocaine, prilocaine and procaine. Fluorescence polarization of 12-(9-anthroyloxy)stearic acid (12-AS) and 2-(9-anthroyloxy)stearic acid (2-AS) were used to examine the effects of local anesthetics on differential rotational mobility between polar region and hydrocarbon interior of synaptosomal plasma membrane vesicles (SPMV) isolated from bovine cerebral cortex, and liposomes of total lipids (SPMVTL) and phospholipids (SPMVPL) extracted from the SPMV. The two membrane components differed with respect to 2 and 12 anthroyloxy stearate (2-AS, 12-AS) probes, indicating that a difference in the membrane fluidity may be present. In a dose-dependent manner, tetracaine, bupivacaine, lidocaine, prilocaine and procaine decreased anisotropy of 12-AS in the hydrocarbon interior of the SPMV, SPMVTL and SPMVPL, but tetracaine, bupivacaine, lidocaine and prilocaine increased anisotropy of 2-AS in the membrane interface. These results indicate that local anesthetics have significant disordering effects on hydrocarbon interior of the SPMV, SPMVTL and SPMVPL, but have significant ordering effects on the membrane interface, and thus they could affect the transport of Na(+) and K(+) in nerve membranes, leading to anesthetic action.

Molecular characteristics of astaxanthin and beta-carotene in the phospholipid monolayer and their distributions in the phospholipid bilayer

The molecular characteristics of the monolayers of astaxanthin with polar group on the beta-ionone ring in the molecule and beta-carotene without polar group and their interactions in mixed carotenoid-phospholipid monolayers and the effects of carotenoids on the phase behavior of the phospholipid bilayers were examined by the monolayer technique and differential scanning calorimetry (DSC). We found from the monolayer study that beta-carotene had an amphiphilic nature. The molecular assembly of astaxanthin in the monolayer at the hydrophobic/hydrophilic interface was more stable than that of beta-carotene. Dimyristoylphosphatidylcholine (DMPC) in the monolayer was miscible with astaxanthin in the range of 0-0.4 mol fractions of astaxanthin, but not fully miscible with beta-carotene even at low concentrations below 0.1 mol fraction of beta-carotene. Surface potential and compression/expansion cycles of beta-carotene monolayer indicated the formation of molecular aggregates by itself. DSC study showed that when small amount of astaxanthin was added, the transition temperature of dipalmitoylphosphatidylcholine (DPPC) was markedly shifted to lower temperatures and that the transition peak was asymmetrically broadened, indicative of a significant depression in cooperativity of the gel to liquid-crystalline transition. The asymmetric DSC endothermic bands of DPPC incorporating small amounts of astaxanthin were well fit by deconvolution into two to three domains containing different concentrations of astaxanthin. On the contrary, the incorporation of beta-carotene resulted in a small depression of the main transition temperature with a slight broadening of the transition peak, suggesting a small miscibility of beta-carotene with the phospholipid bilayer or a formation of aggregates of beta-carotene in the membranes. These results suggest that there would be a high localized concentration in the phase separated membrane for astaxanthin or beta-carotene to function effectively as scavenger.

Mitochondrial Creatine Kinase Binding to Phospholipids Decreases Fluidity of Membranes and Promotes New Lipid-Induced β Structures As Monitored by Red Edge Excitation Shift, Laurdan Fluorescence, and FTIR

Structural modifications induced by the binding of mitochondrial creatine kinase (mtCK) to saturated and unsaturated phospholipids were monitored by using Laurdan, a membrane probe sensitive to the polarity of the environment. The abrupt change characteristic of a phase transition of lipids alone was attenuated by addition of mtCK. Generalized polarization spectra indicated that mtCK surface binding changed the phospholipid liquid-crystalline state to a more rigid state. Infrared spectra of lipids further strengthened these results: upon mtCK binding, the phospholipid methylene chains had a more rigid conformation than that observed without mtCK at the same temperature. After mtCK binding to vesicles of perdeuterated dimyristoylphosphatidylcholine and nondeuterated dimyristoylphosphatidylglycerol, no lateral phase separation was observed, suggesting that both lipids were rigidified. Moreover, mtCK bound to liposomes exhibited an uncommon red edge excitation shift of 19 nm, while that of the soluble enzyme was only 6 nm. These results indicated that the environment of some mtCK tryptophan residues was motionally restricted. Strong stabilization of the enzyme structure against heat denaturation was observed upon lipid binding. In addition, lipids promoted a new reversible protein-protein or protein-lipid interaction, as evidenced by infrared data showing a slight modification of the beta sheet over alpha helix ratio with formation of a new 1632-cm(-)(1) beta sheet instead of the soluble protein 1636-cm(-)(1) one. Such modifications, inducing a decrease in the fluidity of the mitochondrial membranes, may play a role in vesicle aggregation; they could be implicated in the appearance of contact sites between internal and external mitochondrial membranes.

Interaction of local anesthetics with phospholipids in Langmuir monolayers

We have used epifluorescence microscopy to study the interactions of two local anesthetics of the "caine" family (tetracaine and dibucaine), with Langmuir monolayers of the phospholipid dipalmitoylphosphatidylcholine (DPPC). These results show that incorporation of either dibucaine or tetracaine causes significant changes in the domain shapes of the liquid condensed phase in monolayers. In particular, at low pH, where the charged cationic form of the local anesthetics predominates, local anesthetic: DPPC monolayers formed significantly less compact liquid condensed domains with highly ramified shapes, compared to DPPC-only controls. For high pH values at which both local anesthetics are electrically neutral, the liquid condensed domains in mixed monolayers resembled that of DPPC-only controls, indicating that these effects have their origins in electrostatic interactions between the local anesthetics and the phospholipid headgroups. Epifluorescence images obtained using the intrinsic fluorescence of dibucaine indicated that dibucaine partitions into both the liquid condensed and liquid expanded phases.

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.

Do local anesthetics have an influence on the percutaneous penetration of a model corticosteroid? An in vivo study using the vasoconstrictor assay

Local anesthetics may exert nonspecific interactions with membrane components which can affect drug permeability. To investigate pharmacodynamically whether these membrane interactions lead to penetration enhancement of the coadministered model drug betamethasone-17-benzoate through human skin, the vasoconstrictor assay was used. Information on the penetration-enhancing properties of local anesthetic-containing vehicles compared to a plain standard were obtained from activity-response curves, where the enhancement factor was determined from the horizontal distance between the standard and a test in the linear range of the curves. The local anesthetics are able to enhance drug penetration through human skin to a different extent with lidocaine being the most efficient enhancer. An increase in the drug solubility and the diffusion coefficient in the stratum corneum due to membrane fluidization are possible mechanisms of action.

Light-induced reversible local fusions of thylakoid membranes in the presence of dibucaine or tetracaine

The dynamics of structural changes in pea chloroplasts in the presence of 25-50 microM dibucaine or tetracaine has been examined using electron microscopy. The light-induced uptake of anesthetic cations by thylakoids is attended by the appearance of local fusions of stroma-exposed thylakoid membranes. The first membrane protrusions and interthylakoid contacts are observed after 4 s illumination and they become numerous by 10 s. As a result, a network of anastomoses is formed which is maintained during at least 10 min. These effects are reversible in the dark and can be reproduced several times. The formation of membrane fusions is inhibited by the addition of protonophore. It is supposed that the energy-dependent uptake of protonated anesthetics by thylakoids leads to an increase in positive surface charge and thus a lateral pressure on the inner side of the thylakoid membrane. The appearance of membrane protrusions (crinkles) having the positive curvature of their inner surface may be considered as a way of compensating for lateral pressure. Presumably, anastomoses result from the fusion of crinkles to adjacent thylakoids.

Inner sarcolemmal leaflet Ca(2+) binding: its role in cardiac Na/Ca exchange

A recently completed model of Ca concentration and movements in the cardiac cell diadic cleft space predicts that removal or neutralization of inner sarcolemmal (SL) leaflet anionic Ca-binding sites at the sarcolemmal border of this space will greatly diminish Na/Ca exchange-mediated Ca efflux. The present study tests this prediction using the local anesthetic dibucaine as a probe. It is shown, in isolated SL, that dibucaine competitively displaces Ca specifically from anionic phospholipid headgroups. Dibucaine also displaces Ca from the SL when applied to intact cells. It does not affect the content or release of Ca from sarcoplasmic reticulum (SR) in these cells. This eliminates a primary effect on SR Ca as a contributing factor to dibucaine's effect on Na/Ca exchange-mediated Ca efflux. Measurement of this efflux from whole cells shows a highly significant reduction of 58% (p < 0.001) by 0.5 mM dibucaine. The inhibiting effect of dibucaine on Na/Ca exchange-mediated Ca efflux can be significantly reversed by augmentation of Ca release from SR by caffeine at the time of activation of Na/Ca exchange. This supports the contention that the dibucaine-SL interaction is a competitive one vis-a-vis Ca. The results are supportive of the model in which inner SL leaflet Ca-binding sites account for the delay of Ca diffusion from the diadic cleft, thereby prolonging the time for which [Ca] remains elevated in the cleft. The prolonged increased [Ca] significantly enhances the ability of Na/Ca exchange to remove Ca from the cell during the excitation-contraction cycle.