The Thermodynamics of Partitioning of Phenothiazines between Phosphate Buffer and the Lipid Phases of Cyclohexane, n-Octanol and DMPC Liposomes

Thermodynamics of selective serotonin reuptake inhibitors partitioning into 1,2-dioleoyl-sn-glycero-3-phosphocholine bilayers

Knowledge of thermodynamics of lipid membrane partitioning of amphiphilic drugs as well as their binding site within the membrane are of great relevance not only for understanding the drugs' pharmacology but also for the development and optimization of more potent drugs. In this study, the interaction between two representatives of selective serotonin reuptake inhibitors, including paroxetine and sertraline, and large unilamellar vesicles (LUVs) composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) was investigated by second derivative spectrophotometry and Fourier transform infrared spectroscopy (FTIR) to determine the driving force of the drug partitioning across lipid membranes. It was found that temperature increase from 25 to 42 °C greatly enhanced the partitioning of paroxetine and sertraline into DOPC LUVs, and sertraline intercalated into the lipid vesicles to a greater extent than paroxetine in the temperature range examined. The partitioning of both drugs into DOPC LUVs was a spontaneous, endothermic and entropy-driven process. FTIR measurements suggested that sertraline could penetrate deeply into the acyl tails of DOPC LUVs as shown by the considerable shifts in the lipid's CH₂ and C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O stretching modes induced by the drug. Paroxetine, however, could reside closer to the head groups of the lipid since its presence caused a larger shift in the PO₂⁻ bands of DOPC LUVs. The findings reported here provide valuable insights into the influence of small molecules' chemical structure on their molecular interaction with the lipid bilayer namely their possible binding sites within the lipid bilayer and their thermodynamics profiles of partitioning, which could benefit rational drug design and drug delivery systems.

Paroxetine and sertraline have the same thermodynamics profile of phospholipid bilayer partitioning but different location within the lipid bilayer.

Thermodynamics of partitioning of phenothiazine drugs between phosphatidylcholine bilayer vesicles and water studied by second-derivative spectrophotometry

The partition coefficients (Kps) of phenothiazine drugs (trifluoperazine, triflupromazine, chlorpromazine and promazine) between phosphatidylcholine (PC) small unilamellar vesicles (SUV) and water were determined over the temperature range of 10-40 degrees C by a second-derivative spectrophotometric method. The second derivative spectra of each drug solution containing various amounts of SUV showed distinct derivative isosbestic points confirming the entire elimination of the residual background signal effects of the SUV. The Kp values were calculated from the derivative intensity change of the drugs induced by the addition of SUV to the drug buffer solutions (pH 7.4) and obtained with the R.S.D. below 10% (n=3). The van't Hoff analysis of the temperature dependence of Kp values revealed negative deltaH(w-->l) and positive deltaS(w-->l), suggesting an enthalpy/entropy driven mechanism for the phenothiazine partitioning. The negative deltaH(w-->l) implies that the electrostatic interaction, positively charged alkyl amino groups of phenothiazine drugs with negatively charged phosphate groups on the surface of PC SUV, partly contributes to the partitioning. The existence of halogen atom(s) on the phenothiazine ring at position C-2 enhanced the Kp value (H<Cl<CF3). This enhancement can be accounted for by an increase in the deltaS(w-->l) value (H<Cl<CF3), and the deltaS(w-->l) increase is considered to be enhancement of disorder in the hydrophobic acyl chain regions of PC SUV membranes derived from the phenothiazine ring insertion and thus depends on the bulkiness of the substituent. The enthalpy-entropy correlation analysis yielding a good linear relationship also suggests that the phenothiazine drugs studied have identically an enthalpy-entropy compensation mechanism for the partitioning.

Thermodynamics of partitioning of benzocaine in some organic solvent/buffer and liposome systems

The thermodynamics of partitioning of benzocaine (BZC) were studied in octanol/buffer (ROH/W), isopropyl myristate/buffer (IPM/W), cyclohexane/buffer (CH/W), and dimyristoyl phosphatidylcholine (DMPC) and dipalmitoyl phosphatidylcholine (DPPC) liposome systems. In all cases the partition coefficients were greater than unity; therefore the free energies of transfer were negative, that is, the processes of transfer of BZC from aqueous media to organic systems were spontaneous. The partition coefficients were approximately three-fold higher in DMPC liposomes compared with the ROH/W system in the 30 degrees -40 degrees C temperature range. The enthalpies of transfer from aqueous media to ROH and IPM were negative, but positive for CH, while this property was negative for DMPC liposomes and positive for DPPC liposomes. The entropies of transfer were positive in almost all cases, except for DMPC. The results presented here confirm the lipophilic nature of BZC.

Study on liposomes by capillary electrophoresis

Liposomes made of mixtures of zwitterionic and anionic lipids were investigated by means of capillary electrophoresis and dynamic light scattering. The influence of the molar lipid ratio and of the buffers, used in the running electrolyte solution, on the physical characteristics of the liposomes were investigated. Data on effective electrophoretic mobilities, total charges as well as sizes of the liposomes are given. In addition, examples on the use of liposomes as carriers in electrokinetic capillary electrophoresis for the separation of benzene derivatives, steroids, and phenols are shown.

Structure-activity studies on monoamine oxidase inhibitors by calorimetric and quantum mechanical calculations

Structure-activity relationship studies were carried out on a new series of hydrazino-thiosemicarbazide derivatives, which inhibit monoamino oxidase (MAO). Fifty-five compounds were synthesized and tested "in vitro" for their inhibitory effects on rat liver mitochondrial MAO. The most efficient MAO inhibitors were the benzylidene derivatives (sequence: see text] where R is the piperonyl radical and ethyl or isopropyl substituents are in R1 position. Correlation of MAO activity with hydrophobic, electronic and steric properties of tested compounds, evaluated by means of Quantum Mechanical calculations and calorimetric analysis (DSC) suggest that electronic and steric parameters give a better fit than hydrophobicity with the biological activity.

Immobilized-liposome chromatographic analysis of drug partitioning into lipid bilayers

The chromatographic retardation of drugs on a gel bed with immobilized liposomes was shown to correlate with the absorption of the drugs through epithelial cell layers, which is related to drug partitioning into the lipid bilayers of cell membranes. The capacity factors were divided by the lipid concentration (mM) in the gel bed to obtain specific capacity factors, Ks. The logarithm of the octanol-water distribution ratios showed a linear correlation with log Ks, whereas the logarithm of the apparent permeability coefficients in epithelial cell monolayers and the absorption of drugs orally administered in humans (data from other laboratories) increased over the interval 0 < log Ks < 1 and attained a saturation level in the interval 1 < log Ks < 3. Immobilized-liposome chromatography may be applicable for prediction of drug uptake through epithelial cell membranes.

Partitioning and thermodynamics of dipyridamole in the n-octanol/buffer and liposome systems

The thermodynamics of partitioning (K) of dipyridamole has been determined in n-octanol/buffer and liposome-buffer systems at pH 7.4. Dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) were used to prepare multilamellar liposomes. Partitioning of dipyridamole did not depend on the amount of n-octanol employed, however, partitioning was dependent upon the quantity of DMPC employed to prepare liposomes. Plots of log K vs 1/T were linear in the n-octanol and liposome systems. Partitioning was generally greater in liposomes than in the n-octanol/buffer system. Among liposomes, the partitioning was greater in DMPC liposomes at all temperatures. The values of enthalpy (delta H) and entropy (delta S) were positive in both the n-octanol and liposome systems. These values were lower in DMPC liposomes and were comparable in the n-octanol and DPPC liposomes. Thus, the interaction of dipyridamole depends on the rigidity of lipid bilayers and liposomes constitute a more selective partitioning system than the n-octanol/buffer system.

The liposome partitioning system for correlating biological activities of imidazolidine derivatives

The partitioning of 10 imidazolidines in various liposome/buffer systems (logK'm) has been determined and compared to partitioning in the n-octanol/buffer system (logP'). The logK'm, which was generally greater than the logP', increased or decreased upon the addition of dicetylphosphate (DCP) or stearylamine (STA), respectively, to dimyristoylphosphatidylcholine (DMPC) liposomes. Quantitative correlations of alpha 2-adrenergic potencies of imidazolidines have been made by regression analyses with logP', logK'm, binding affinity, and intrinsic activity. Both central and peripheral potencies correlated with logK'm but not with logP'. Multiple regressions yielded improved predictable quantification of these potencies. Thus, the liposomal membrane system shows certain advantages over the n-octanol/buffer system for the prediction of biological activities of the imidazolidines.

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.

The liposome as a model membrane in correlations of partitioning with alpha-adrenoceptor agonist activities

The apparent partition coefficients of a group of imidazoline alpha-adrenoceptor agonists in liposome/buffer systems (K'm) and in the n-octanol/buffer system (P') have been compared in quantitative structure-activity relationships (QSAR) employing biological activities and receptor binding affinities. A parabolic relationship between log K'm and log P' was found, and log K'm was greater than log P' for all liposome compositions. In liposomes, log K'm decreased in the order, negatively charged greater than neutral greater than positively charged. Overall, hyper- and hypotensive activities of these drugs correlated better with log K'm than with log P'; however, poor correlations were obtained between partition coefficients and in vitro binding affinities. Linear correlations of log K'm with hypotensive activities were obtained with negatively charged liposomes, whereas correlations with hypertensive activities were obtained using positively charged liposomes. Multiple regressions of biological activities with binding affinities showed positive correlations with hypotensive but not hypertensive activities with or without the inclusion of log K'm or log P'. Thus, the liposome represents a more selective model membrane system than a bulk oil phase for predicting the biological activities of imidazoline alpha-adrenoceptor agonists.

Correlation of partitioning of nitroimidazoles in the n-octanol/saline and liposome systems with pharmacokinetic parameters and quantitative structure-activity relationships (QSAR)

The partitioning of a series of nine nitroimidazole drugs in liposomes (log Km) of various compositions has been determined and compared to their partitioning in the n-octanol/saline system (log K) at 30 degrees C. The log Km ranged from 1.5 to 0.5 and was three- to fourfold greater than the log K; further, the linear correlation coefficient was greatest when cholesterol (CHOL)-free liposomes were used. Functional-group contributions were compared from their hydrophobic substituent constants and, except in the case of RO-07-2044 and iodoazomycin riboside, yielded negative values in all systems. Literature values of four pharmacokinetic parameters obtained in dogs and acute LD50 values of the nitroimidazoles in BALB/c mice were highly correlated with log K or log Km only in CHOL-free liposomes. Comparing the relative sensitizing effect of the nitroimidazoles in murine EMT-6 or Chinese hamster V79 tumor cell cultures and their partition coefficients, the correlation in EMT-6 cells was poor, whereas the correlation in V79 cells was greater than 0.9 when log Km was used but less than 0.6 when log K was used. Thus, the liposome model is a better predictor of nitroimidazole activity than the n-octanol/saline system and, also, it is a more flexible model for selecting the optimum conditions for QSAR studies.