Structure-based prediction of drug distribution across the headgroup and core strata of a phospholipid bilayer using surrogate phases

Solvation of drugs in the core (C) and headgroup (H) strata of phospholipid bilayers affects their physiological transport rates and accumulation. These characteristics, especially a complete drug distribution profile across the bilayer strata, are tedious to obtain experimentally, to the point that even simplified preferred locations are only available for a few dozen compounds. Recently, we showed that the partition coefficient (P) values in the system of hydrated diacetyl phosphatidylcholine (DAcPC) and n-hexadecane (C16), as surrogates of the H- and C-strata of the bilayer composed of the most abundant mammalian phospholipid, PC, agree well with the preferred bilayer location of compounds. High P values are typical for lipophiles accumulating in the core, and low P values are characteristic of cephalophiles preferring the headgroups. This simple pattern does not hold for most compounds, which usually have more even distribution and may also accumulate at the H/C interface. To model complete distribution, the correlates of solvation energies are needed for each drug state in the bilayer: (1) for the H-stratum it is the DAcPC/W P value, calculated as the ratio of the C16/W and C16/DAcPC (W for water) P values; (2) for the C-stratum, the C16/W P value; (3) for the H/C interface, the P values for all plausible molecular poses are characterized using the fragment DAcPC/W and C16/W solvation parameters for the parts of the molecule embedded in the H- and C-strata, respectively. The correlates, each scaled by two Collander coefficients, were used in a nonlinear, mass-balance based model of intrabilayer distribution, which was applied to the easily measurable overall P values of compounds in the DMPC (M = myristoyl) bilayers and monolayers as the dependent variables. The calibrated model for 107 neutral compounds explains 94% of experimental variance, achieves similar cross-validation levels, and agrees well with the nontrivial, experimentally determined bilayer locations for 27 compounds. The resulting structure-based prediction system for intrabilayer distribution will facilitate more realistic modeling of passive transport and drug interactions with those integral membrane proteins, which have the binding sites located in the bilayer, such as some enzymes, influx and efflux transporters, and receptors. If only overall bilayer accumulation is of interest, the 1-octanol/W P values suffice to model the studied set.

Prediction of micelle/water and liposome/water partition coefficients based on molecular dynamics simulations, COSMO-RS, and COSMOmic

Liposomes and micelles find various applications as potential solubilizers in extraction processes or in drug delivery systems. Thermodynamic and transport processes governing the interactions of different kinds of solutes in liposomes or micelles can be analyzed regarding the free energy profiles of the solutes in the system. However, free energy profiles in heterogeneous systems such as micelles are experimentally almost not accessible. Therefore, the development of predictive methods is desirable. Molecular dynamics (MD) simulations reliably simulate the structure and dynamics of lipid membranes and micelles, whereas COSMO-RS accurately reproduces solvation free energies in different solvents. For the first time, free energy profiles in micellar systems, as well as mixed lipid bilayers, are investigated, taking advantage of both methods: MD simulations and COSMO-RS, referred to as COSMOmic (Klamt, A.; Huniar, U.; Spycher, S.; Keldenich, J. COSMOmic: A Mechanistic Approach to the Calculation of Membrane-Water Partition Coefficients and Internal Distributions within Membranes and Micelles. J. Phys. Chem. B 2008, 112, 12148-12157). All-atom molecular dynamics simulations of the system SDS/water and CTAB/water have been applied in order to retrieve representative micelle structures for further analysis with COSMOmic. For the system CTAB/water, different surfactant concentrations were considered, which results in different micelle sizes. Free energy profiles of more than 200 solutes were predicted and validated by means of experimental partition coefficients. To our knowledge, these are the first quantitative predictions of micelle/water partition coefficients, which are based on whole free energy profiles from molecular methods. Further, the partitioning in lipid bilayer systems containing different hydrophobic tail groups (DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine), SOPC (stearoyl-oleoylphosphatidylcholine), DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine), and POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine)) as well as mixed bilayers was calculated. Experimental partition coefficients (log P) were reproduced with a root-mean-square error (RMSE) of 0.62. To determine the influence of cholesterol as an important component of cellular membranes, free energy profiles in the presence of cholesterol were calculated and shown to be in good agreement with experimental data.

Capacities of membrane lipids to accumulate neutral organic chemicals

Lipids have been considered as the predominant components for bioaccumulation of organic chemicals. However, differences in accumulation properties between different types of lipid (e.g., storage and membrane lipids) have rarely been considered. Moreover, in view of toxic effects on organisms, chemical accumulation specifically in biological membranes is of particular importance. In this review article, partition coefficients of 240 neutral organic compounds between liposomes (phospholipid membrane vesicles) and water (K(lipw)), reported in the literature or measured additionally for this work, were evaluated. Values of log K(lipw) and log K(ow) (octanol-water partition coefficients) differ by 0.4 on average. Polyparameter linear free energy relationships (PP-LFERs) can describe the log K(lipw) data even better (standard deviations = 0.28-0.31) than the log K(ow) model. Recent experimental data for highly hydrophobic compounds fit well to the PP-LFERs and do not indicate the existence of a previously postulated "hydrophobicity cutoff". Predictive approaches based only on the molecular structure (KOWWIN, SPARC, COSMOthermX, COSMOmic) were also evaluated for K(lipw) prediction. The PP-LFERs revealed that partition coefficients into membrane lipids can be two log units higher than those into storage lipids for H-bond donor compounds, suggesting that distinguishing between the two lipids is necessary to account for the bioaccumulation of these compounds, and that tissues rich in membrane lipids (e.g., kidneys, liver) instead of fat tissue can be the primary phase for accumulation.

Lipid composition effect on permeability across PAMPA

The parallel artificial membrane permeability assay (PAMPA) system has promise to rapidly screen drug candidate passive permeability, but has been poorly described in terms of its lipid membrane structure and function. The objective was to investigate the role of PAMPA lipid composition on the permeability of five model compounds. PAMPA was used and employed individual phospholipids that varied in phosphate head group and acyl chain unsaturation. Transport of benzoic acid, taurocholic acid, metoprolol, sucrose, and mannitol was measured. Membrane fluidity was assessed by 1,3-diphenylhexatriene fluorescence anisotropy. Results indicate that compound permeability across PAMPA differed in their sensitivity to membrane lipid composition, where compounds with appreciable permeability (i.e. at least 0.2 x 10(-6)cm/s) were possibly sensitive to membrane fluidity and apparent ion pair effects. Benzoic acid permeability ranged 51-fold across membrane types, suggesting acyl chain effect on membrane fluidity. Mannitol, sucrose, and taurocholic acid permeabilities were low and independent of lipid composition. Metoprolol permeability ranged 17-fold and exhibited a markedly high permeability across 1,2-dioleoyl-sn-glycero-3-[phospho-L-serine] due to apparent ion pair-facilitated transport. Compound permeability was lowest across the phosphatidylcholines, which is consistent with phosphatidylcholine exhibiting relatively high membrane rigidity. In contrast to results from phosphatidylethanolamines and phosphatidylserines, acyl chain unsaturation had no effect on permeability across phosphatidylcholines. In conclusion, while much remains unknown about PAMPA structure and subsequent PAMPA permeability, results here from five solutes suggest that, for solutes with appreciable permeability, lipid composition modulated drug permeability through possible membrane fluidity and apparent ion pair influences.

Relationships between structure and high-throughput screening permeability of diverse drugs with artificial membranes: application to prediction of Caco-2 cell permeability

To evaluate the absorption of drugs with diverse structures across a membrane via the transcellular route, their permeability was measured using the parallel artificial membrane permeation assay (PAMPA). The permeability coefficients obtained by PAMPA were analyzed using a classical quantitative structure-activity relationship (QSAR) approach with simple physicochemical parameters and 3D-QSAR, VolSurf. We formulated correlation equations for diverse drugs similar to the equation obtained for peptide-related compounds in our previous study. The hydrogen-bonding ability of molecules, not only the hydrogen-accepting ability but also the hydrogen-donating ability, in addition to hydrophobicity at a particular pH, was significant in determining variations in PAMPA permeability coefficients. Based on this result, an in silico good prediction model for the passive transcellular permeability of diverse structural compounds was obtained. The artificial lipid-membrane permeability coefficients of the drugs, except salicylic acid, were well correlated with the Caco-2 permeability in a previous report suggesting the importance of absorption by the transcellular mechanism for these drugs.

Partitioning of Tetrachlorophenol into Lipid Bilayers and Sarcoplasmic Reticulum: Effect of Length of Acyl Chains, Carbonyl Group of Lipids and Biomembrane Structure

We report results of a partitioning study of 2,3,4,6-tetrachlorophenol (TeCP). In the study we explored (1) the effect of the length of acyl chains of lipids (C16:1 - C24:1) and alkanes (C6-C16), (2) the role of the carbonyl group of lipids, and (3) the effect of molecular structure of the sarcoplasmic reticulum membrane on TeCP partitioning. Mole fraction partition coefficients have been measured using equilibrium dialysis for un-ionized (HA), and ionized (A) species, Kp(x) (HA), Kp(x) (A). Their values are concentration-dependent. Partition coefficients were analyzed in terms of a model that accounts for saturation of membrane associated with the finite area of partition site, and electrostatic interactions of (A-) species with charged membrane. Limiting values of partition coefficients, corresponding to infinite dilution of solute, Kp(x0) (HA), Kp(x0) (A) were obtained. Kp(x0) (HA) and Kp(x0) (A ) measure the strength of solute-membrane interactions. Studies were done with single-layered vesicles of lipids with variable chain length: 1,2-dipalmitoleoyl-sn-glycero-3-phosphocholine (C16:1), 1,2-dioleoyl-sn-glycero-3-phosphocholine (C18:1), 1,2-dierucoyl-sn-glycero-3-phosphocholine (C22:1), and 1 ,2-dinervonoyl-sn-glycero-3-phosphocholine (C24:1), and egg-PC. Kp(x0) for transfer of TeCP from water into lipid membranes was found to be independent of the length of acyl chains, whereas Kp(x0) for transfer from water into alkanes increased with the length of alkane. The effect of the carbonyl CO group of lipids on partitioning was measured using 1,2-di-o-octadecenyl-sn-glycero-3-phosphocholine (CO absent) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (CO present) liposomes. Carbonyl groups, known to change dipolar potential, had no effect on partitioning. Partition coefficients of un-ionized and ionized forms of TeCP were invariant to the presence of proteins and other membrane components of sarcoplasmic reticulum (SR) membrane.

Liposome/water lipophilicity: methods, information content, and pharmaceutical applications

This review discusses liposome/water lipophilicity in terms of the structure of liposomes, experimental methods, and information content. In a first part, the structural properties of the hydrophobic core and polar surface of liposomes are examined in the light of potential interactions with solute molecules. Particular emphasis is placed on the physicochemical properties of polar headgroups of lipids in liposomes. A second part is dedicated to three useful methods to study liposome/water partitioning, namely potentiometry, equilibrium dialysis, and (1)H-NMR relaxation rates. In each case, the principle and limitations of the method are discussed. The next part presents the structural information encoded in liposome/water lipophilicity, in other words the solutes' structural and physicochemical properties that determine their behavior and hence their partitioning in such systems. This presentation is based on a comparison between isotropic (i.e., solvent/water) and anisotropic (e.g., liposome/water) systems. An important factor to be considered is whether the anisotropic lipid phase is ionized or not. Three examples taken from the authors' laboratories are discussed to illustrate the factors or combinations thereof that govern liposome/water lipophilicity, namely (a) hydrophobic interactions alone, (b) hydrophobic and polar interactions, and (c) conformational effects plus hydrophobic and ionic interactions. The next part presents two studies taken from the field of QSAR to exemplify the use of liposome/water lipophilicity in structure-disposition and structure-activity relationships. In the conclusion, we summarize the interests and limitations of this technology and point to promising developments.

Interaction of phenolic uncouplers in binary mixtures: concentration-additive and synergistic effects

The uncoupling activities of 14 binary mixtures of substituted phenols and of 4 binary mixtures of phenols and anisols were investigated at different pH values. Experiments were performed with time-resolved spectroscopy on membrane vesicles (chromatophores) of the photosynthetic bacteria Rhodobacter sphaeroides. Phenols are known to destroy the electrochemical proton gradient in energy-transducing membranes by a protonophoric mechanism. Anisols do not have protonophoric activity but disturb membrane structure and functioning as a nonspecific baseline toxicant. It was postulated in the literature that, for certain substituted phenols, the formation of a dimer between the phenoxide and the neutral phenol may contribute significantly to the overall protonophoric activity. In 13 of 14 mixtures of substituted phenols but in none of the mixtures of phenols with anisols, such a dimer appears to be formed between two different mixture partners. An extended shuttle mechanism of uncoupling, which includes a term for the contribution of such a mixed dimer, provided a good description of all experimental data. Opposite speciation favors interaction and ortho substituents abate interaction, which adds evidence for the dimerformation via a hydrogen bond between the phenol-OH and the phenoxide. These findings are significant not only regarding the mechanism of protonophoric action but also for the risk assessment process of chemical mixtures in the environment. When assessing the effect of mixtures, concentration addition is regarded as a reference X concept to estimate effects of similarly acting compounds. The substituted phenols in this work act according to the same action mechanism of uncoupling. Nevertheless, the overall effect of four of the investigated mixtures, which exhibit stronger dimer formation as compared to the single compounds or for which the resulting dimer is intrinsically more active, exceeded the effect calculated according to concentration addition considerably. In future work, this synergistic effect observed in-vitro has to be validated in-vivo to deduce its implications for the risk assessment process.

pH dependence of the partitioning of triphenyltin and tributyltin between phosphatidylcholine liposomes and water

Triorganotin compounds are very toxic contaminants. The site of their basic mechanism of action of acute toxicity is the biomembrane. Liposome-water distribution ratios of triphenyltin and tributyltin were determined between pH 3 and pH 8 with the equilibrium dialysis method in the micromolar concentration range, which is the concentration range where acute toxicity is observed. In addition, biomembrane-water distribution ratios of tributyltin were determined with chromatophores of Rhodobacter sphaeroides that contain approximately 70% protein intercalated in the lipid bilayer. The liposome-water distribution of both compounds showed only weak pH dependence. For tributyltin, the apparent distribution ratio decreased from 4100 at low pH to 2000 at high pH, while this ratio decreased from 70 000 to 22 000 for TPT. The distribution ratio of the triorganotin cation exceeded that of the neutral hydroxo complex by a factor of 2. The distribution ratio of both the cation and the hydroxo complex of triphenyltin exceeded that of tributyltin by a factor of 10. It is postulated that the sorption of the cation is governed by complex formation with ligands in the phospholipids, presumably the phosphate group. The biomembrane-water distribution ratio of tributyltin was found to be lower than the liposome-water distribution ratio at high pH. The hydroxo complex appears to partition only to the lipid fraction of the biomembrane. Yet, at low pH the biomembrane-water distribution ratio exceeded the liposome-water distribution ratio, which is attributed to complex formation of the cationic species with ligands of the protein fraction.

Hydrophobicity parameter of diazines IV: a new hydrogen-accepting parameter of monosubstituted (di)azines for the relationship of partition coefficients in different solvent systems

We recently proposed a new hydrogen-accepting scale, S(HA), for each member of the substituted (di)azine series on the basis of the heat of formation calculated under various dielectric environments by the COSMO method. In this paper, the S(HA) scale was used to examine relationships between log P(CL) (P(CL): CHCl(3)/H(2)O partition coefficient) and log P(oct) (P(oct): 1-octanol/H(2)O partition coefficient) for each of the 2-substituted pyridine (I), monosubstituted pyrazine (II), and pyrimidine (III) series. This S(HA) parameter worked nicely, representing the hydrogen-accepting effect of the solute molecule. A correlation equation with excellent quality, such as log P(CL) = a log P(oct) + sS(HA) + constant, was obtained for each series. We further defined the parameter S(HA/PY), derived from S(HA) values for the heterocyclic series by shifting the reference points to unsubstituted pyridine, to unify separately derived correlation equations. Thus, the correlation between log P(CL) and log P(oct) for all combined data of three series was derived by using a single equation as log P(CL) = a log P(oct) + sS(HA/PY) + constant. The S(HA) parameters were reasonably considered as being free-energy related, and the rationale for the hydrogen-bond-acceptor scale was presented.

Quantitative structure-hydrophobicity and structure-activity relationships of antibacterial gramicidin S analogs

The structure-hydrophobicity-antibacterial activity relationships of gramicidin S and its analogs retaining a beta-pleated sheet structure were examined quantitatively with physicochemical substituent and molecular parameters using regression analyses. Variations in their apparent hydrophobicity in an octanol/buffer (pH 7) system, log P' (O/W), were analyzed in terms of the "effective" hydrophobicity and steric parameters of side chain substituents of residues at certain positions in the molecule; however, some of the conformational factors have not been fully defined. For the partitioning into liposomes and the growth inhibitory activity against species of Gram-positive bacteria, the log P' (O/W) value simulated the hydrophobic effects of gramicidin S and its analogs better than substituent parameters. The side chain hydrophobicity was assumed to work together with effects attributed to variations in the entire cyclic peptide structure including conformational components undefined in the structure-log P' (O/W) analysis in these activities.

Photocytotoxicity of water-soluble metalloporphyrin derivatives

A new water-soluble porphyrin derivative, 2,4-bis(1-decyloxyethyl)-deuteroporphyrinyl-6,7-bisaspart ic acid (C10-DP), and its metal complexes (Ga, I(n), Zn, Mn, Cu, Ni and Fe) were examined for their physicochemical properties (absorption, fluorescence, triplet lifetime and partition coefficient) and photocytotoxicity on HeLa cells. The five derivatives with longer (> 1 ms) triplet lifetimes (free base, Zn, Ga, I(n) and Sn complexes) exhibited remarkable photocytotoxicity, and the other derivatives (Mn, Cu, Ni and Fe), which had or were deduced to have fairly short (< 0.01 ms) triplet lifetimes, manifested no photocytotoxicity, indicating that the triplet lifetime of these derivatives played a significant role in their photocytotoxicity. Cellular fluorescence due to C10-DP and its gallium complex was observed mainly on the plasma membrane at the concentrations showing significant photocytotoxicity with low (< 32.6%) cytotoxicity in the dark (2-10 microM).

Characterization of distribution behavior of 2-imidazolines into multilamellar liposomes

The distribution of 2-imidazolines in neutral dimyristoylphosphatidylcholine (DMPC) liposomes, in negatively charged liposomes containing dicetylphosphate (DCP) or phosphatidylserine (PS), and in positively charged liposomes containing stearylamine (STA), has been investigated. Electrophoretic mobilities of multilamellar liposomes have also been measured as a function of drug concentration. Apparent equilibrium partition coefficients (log K'm) increased as a function of the DCP or PS concentration in DMPC liposomes whereas log K'm decreased with STA concentration, except for lofexidine and clonidine. Similarly, the electrokinetic parameters increased in DMPC liposomes that exhibited a small, positive surface charge, decreased in DMPC/cholesterol/DCP (7:1:2 mole ratio) liposomes, and increased in DMPC/STA (3:1 mole ratio) liposomes, except for clonidine which showed a decrease, as a function of the 2-imidazoline concentration. Surface potential change (delta psi o) due to drug inclusion in the liposomes obtained from theoretical considerations exhibited a positive linear relationship with log K'm. Values of delta psi o were greater but less sensitive to log K'm in negatively charged than in neutral or positively charged liposomes at 1 mM drug concentration. Likewise, surface charge densities varied in the same order as the surface potentials as a function of log K'm of the 2-imidazolines, except for clonidine and lofexidine. These data indicate the relative importance of the membrane surface characteristics on the partitioning behavior, and also possibly the membrane transport behavior, of the 2-imidazoline drugs.

Differential scanning calorimetry study of the interaction of antidepressant drugs, noradrenaline, and 5-hydroxytryptamine with a membrane model

Differential scanning calorimetry was used to study the interaction of a membrane model with two neuromediators [noradrenaline and 5-hydroxytryptamine (Serotonin)] and four antidepressant drugs (imipramine, indalpine, citalopram, and milnacipran), known to be uptake inhibitors of these neuromediators. The study was carried out on dipalmitoyl phosphatidylcholine liposomes, a bilayer phospholipid system taken as a simplified model of biological membranes. Analysis of the thermograms led us to classify the molecules according to their lipophilic action, as in the conventional measurement of the water-octanol partition coefficient, and also enabled us to precisely determine their location along the phospholipid bilayer. A hypothesis based on this localization is put forward concerning the competitive, or otherwise, character of the blocking of uptake of the neuromediators. The extreme cases of interaction and localization of imipramine and milnacipran, a new antidepressant, relative to the bilayer are also analyzed in terms of side effects.

Quantitative analysis of uncoupling activity of substituted phenols with a physicochemical substituent and molecular parameters

The uncoupling potency of a series of substituted phenols with rat-liver mitochondria was analyzed quantitatively with physicochemical substituent and molecular parameters such as log P, P being the partition coefficient in a phosphatidylcholine liposome/water system, log KA, KA being the acid dissociation constant, and the Taft-Kutter-Hansch steric constant, Es, for ortho-substituents. The potency evaluated from the concentration in the medium required for a defined response was analyzed, showing that the incorporation of compounds in terms of log P, a certain balance between neutral and ionized forms expressible by a parabolic function of log KA and the steric shielding effect of the ortho-substituents on the negatively charged center of ionized form are highly significant factors governing the variations in potency. The potency was also quantitatively separated into the intrinsic potency as the protonophore inside the inner mitochondrial membrane and the incorporation factor in terms of log P. Some phenols found as outliers from the correlations and some others distorting the quality of the correlations were shown to have inhibitory effects on the respiratory chain by specific and non-specific modes of action, respectively, besides uncoupling activity.

Structural requirements of salicylanilides for uncoupling activity in mitochondria: quantitative analysis of structure-uncoupling relationships

The uncoupling activities of more than 20 salicylanilides were measured in rat liver mitochondria. The activities, expressed as the minimum concentrations required for full release of state-4 respiration, ranged over three orders of magnitude. The acid dissociation constant, pKA, and the partition coefficient between octanol and water (Poct) of some of the salicylanilides were determined. These two parameters were found to be well expressed in terms of the Hammett constant, sigma, and the hydrophobic substituent coefficient, II, respectively. The pKA and log Poct values of all the salicylanilides were predicted according to these relationships. Furthermore, the capacity factor, k', on high-performance liquid chromatography was determined on glyceryl-coated-controlled pore glass (gly-CPG). Values of log k' correlated well with those of log Poct. The uncoupling activities of the salicylanilides were analyzed in terms of these three parameters. Both hydrophobic and electron-withdrawing properties were found to be essential for induction of potent uncoupling activity. The correlations using log k' were better than those using log Poct.