Lipophilicity of morphine microspecies and their contribution to the lipophilicity profile

The complete set of experimental microscopic partition coefficients of morphine was determined for the first time for any compound. The acid-base microequilibria were characterized by combining pH-potentiometry and deductive methods using auxiliary compounds of reduced complexity. The results show around three times as many non-charged than zwitterionic microspecies in aqueous solution. Partition of the individual microspecies was mimicked by model compounds of the closest possible similarity, then correction factors were determined and introduced. Thus the intrinsic partition coefficients of all the microspecies could be quantitated, including the non-charged and the zwitterionic ones. The non-charged microspecies is 1070 times as lipophilic as its zwitterionic protonation isomer. Their contribution ratio to the overall lipophilicity is 3090. The lipophilicity profile of morphine was expressed, calculated and depicted in terms of species-specific lipophilicities over the entire pH range.     

Determination of lipophilicity of two quinolone antibacterials, ciprofloxacin and grepafloxacin, in the protonation equilibrium.

The objective of this study was to compare protonation equilibrium and lipophilicity of two quinolone antibacterials, grepafloxacin (GPFX) and ciprofloxacin (CPFX), in order to give an insight into effects on the physicochemical properties by slight structural motifs. The protonation equilibrium was investigated by a spectrophotometry. Macro- and micro-dissociation constants were simultaneously determined, based on nonlinear regression analysis using the MULTI program, and then microspecies distribution could be described accordingly. Zwitterionic microspecies predominated at isoelectrical point (pI) for both drugs, and the concentration ratio of neutral to zwitterionic forms was near 4-fold greater for GPFX than that for CPFX. The apparent partition coefficient (D(O/B,pH)) versus pH profiles had the shape of a parabolic curve in an n-octanol/buffer system, and reached the maximum around pI for both, respectively. Moreover, two introduced methyl groups in GPFX increased not only intrinsic lipophilicity but also neutral microspecies fraction relative to CPFX, and D(O/B,pH) of GPFX was consequently far higher than that of CPFX. The results emphasized that there were significant differences in protonation equilibrium and lipophilicity between GPFX and CPFX, which conduced to explaining their different behavior in terms of antibacterial activities and pharmacokinetics.     

Comparison between immobilized artificial membrane (IAM) HPLC data and lipophilicity in n-octanol for quinolone antibacterial agents

The membrane phospholipid affinity of ten quinolone antibacterial agents, including both acidic and zwitterionic compounds, was measured by HPLC on two different immobilized artificial membrane (IAM) stationary phases, namely IAM.PC.MG and IAM.PC.DD2; it is expressed as the logarithm of the retention factor measured with (or extrapolated to) 100% aqueous eluent at pH 7.0, .

Quinolones are a class of highly potent, orally active, broad-spectrum antibacterial agents. For these compounds, lipophilicity values in n-octanol found in the literature, either calculated or measured, are not consistent with each other and are too low to be compatible with their pharmacokinetic properties.

The values obtained in this study showed no relation with any of the lipophilicity values in the literature (clog P(a), clog P(b), MLP, log D^7.4). In contrast, they were collinear with a new lipophilicity scale we had previously obtained by an original ion-pair reversed-phase HPLC method set up to estimate the lipophilicity of the neutral forms, log P^N. Moreover, when comparing the retention of quinolones on IAM to the retention of structurally unrelated neutral compounds, we observed that they interact with phospholipids with the same affinity as neutral isolipophilic compounds.

The use of an eluent at pH 5.5, instead of pH 7.0, increased the retention on IAM not only for acidic, but also for zwitterionic congeners, indicating that phospholipid affinity is enhanced in the experimental conditions that depress the ionization of the acidic function, even when the ionization of the amino function increases simultaneously.

To gain an insight into the mechanism of quinolones/serum-protein interactions, we investigated about possible relationships between quinolones affinity data for serum proteins and IAM data. Quinolone affinity for both HSA and AGP was already demonstrated poorly related to n-octanol lipophilicity values, probably due to the occurrence of electrostatic interactions. Only poor relationships were found between IAM and HSA affinity data, whereas quite good relationships were found with AGP affinity data. However, IAM.PC.DD2 data correlated better than those on IAM.PC.MG with quinolone affinity for both serum-proteins, mainly due to the fact that IAM.PC.MG phase is scarcely discriminative for the compounds with the highest retention values.

The results suggest that IAM retention can produce a lipophilicity scale that, unlike solvent/water partition coefficients, is consistent with the pharmacokinetic behaviour of zwitterionic quinolones.     

Retention of quinolones on human serum albumin and alpha1-acid glycoprotein HPLC columns: relationships with different scales of lipophilicity.

The retention of 10 quinolone antibacterial agents on HPLC stationary phases supporting human serum albumin (HSA) or alpha(1)-acid glycoprotein (AGP) was investigated. Among ofloxacine and flumequine, the two chiral compounds in the selected set, only the latter showed a split chromatographic peak and only on HSA but not on AGP, indicating that enantioselective specific sites play only a minor role in the retention. The retention of quinolones, which included four acidic and six zwitterionic congeners, was correlated with various lipophilicity scales: (i) theoretically calculated values, clogP, (ii) values measured at pH 7.4 by the shake-flask method, logD(7.4), and (iii) values extrapolated by retention data measured by ion-pair reversed-phase high performance liquid chromatography (RP-HPLC). We assumed that the latter values, logP(i.p.), were close to the lipophilicity of the neutral forms, logP(N), for both acidic and zwitterionic congeners. No relationship was found between retention on serum proteins and clogP values, whereas a reasonable relationship was found with logD(7.4) values, but only when the two subclasses, acidic and zwitterionic congeners, were considered separately. The relationship between retention data on serum proteins and logP(i.p.) values indicated that the affinity for serum proteins depends on the lipophilicity of the neutral forms only for logP values up to 1.5. Above this value, protein retention does not further increase, becoming almost constant. Based on both the observations above reported and the small values of the slopes of regression equations, we conclude that the interaction of the more lipophilic quinolones, mainly the zwitterions, with serum proteins is not governed uniquely by lipophilicity but also by other mechanisms, probably of electrostatic nature.     

Protonation equilibrium and lipophilicity of moxifloxacin

This study was performed to characterise the protonation equilibrium at the molecular level and pH-dependent lipophilicity of moxifloxacin. After determining macro- and micro-constants, distribution features of four microspecies in aqueous phase were assessed. The apparent partition coefficient versus pH profile of moxifloxacin showed a parabolic curve in n-octanol/buffer system which reached near pI. The true partition coefficient was calculated from the log P(app) and microconstants values.     

Complete resolution of the microscopic protonation equilibria of N-methyl-D-aspartic acid and related compounds

Protonation equilibria of N-methyl-D-aspartate (NMDA, a specific glutamate receptor agonist) and its derivatives are characterized at the macroscopic and microscopic levels. (1)H NMR-pH and pH-potentiometric titrations were carried out to determine the macroconstants. Microconstants were obtained by appropriate combination of acidity and NMR parameters of the parent compound and its three synthetic derivatives. These derivatives were close models of the NMDA minor microspecies, allowing the calculation of all the 12 microconstants, the 8 microspecies concentrations and 3 site interactivity parameters. Reliability of the microconstants was assessed by three independent test methods. It was found that protonation of the secondary amino site decreases the beta- and alpha-carboxylate basicities almost exactly by one and two orders of magnitude, respectively, whereas protonation of one of the carboxylates lessens the basicity of the other one by a factor of 3. NMR-pH profiles, macro- and microscopic protonation schemes and species-specific distribution diagrams are presented.     

Differences in endogenous esterification and retention in the rat trachea between budesonide and ciclesonide active metabolite.

The airway retention of inhaled glucocorticosteroids (GCs) depends largely on their lipophilicity. Inhaled budesonide (BUD) becomes highly lipophilic reversibly by the formation of esters acting as a reservoir of active BUD. Ciclesonide (CIC) was also reported to form esters after hydrolysis to active metabolite (CIC-AM). We have investigated lipophilicity and airway retention of BUD, CIC/CIC-AM, fluticasone propionate (FP), and mometasone furoate (MF), and compared esterification of BUD and CIC-AM and its contribution to GC airway retention. Rat tracheas were preincubated with the esterification inhibitor cyclandelate or vehicle. A (3)H-GC ( approximately 10(-7) M: BUD, CIC, CIC-AM, FP, MF) was added for 20 min. After incubation, one half of the trachea was used for analysis of GC uptake and the other to analyze GC release during 3 h in drug-free medium. GC species in trachea halves were analyzed by radiochromatography. At 20 min, the uptake of BUD was similar to that of CIC/CIC-AM; however, the BUD-ester pool was 9-fold greater (p < 0.01). BUD overall retention in trachea at 3 h was greater than that of other GCs (p < 0.01), and the BUD-ester pool was 3-fold greater than the CIC-AM-ester pool (p < 0.01). Cyclandelate decreased the initial BUD- and CIC-AM-ester pools (p < 0.01), and reduced the overall retention of BUD at 3 h (p < 0.01) but not of CIC-AM. Thus, BUD becomes esterified in the airways more promptly and to a greater extent than CIC-AM, and BUD esterification prolongs BUD airway retention. In contrast, airway retention of CIC-AM and CIC seems to be determined mainly by their lipophilicity, similar to FP and MF, which are not esterified.     

Influence of molecular lipophilicity on the diffusion of arylpropionate non-steroidal anti-inflammatory drugs into the cerebrospinal fluid.

The diffusion of seven arylpropionic acid non-steroidal anti-inflammatory drugs (NSAIDs) into the cerebrospinal fluid (CSF) has been investigated in male Wistar rats by means of quantitative structure-activity relationship (QSAR) study. After intraperitoneal administration of each drug (5 mg/kg), blood and CSF samples were collected at different times (0.5, 1, 3, and 6 h). The fraction bound to plasma proteins (fb) was determined using ultracentrifugation. The total (CT) and free (CF) plasma concentrations and the concentrations in CSF (CCSF) were measured by a reversed-phase high performance liquid chromatographic (RP-HPLC) method. The areas under the curve of the free plasma (AUCF) and CSF (AUCCSF) concentrations were calculated according to the trapezoidal rule. The overall drug transit into CSF was estimated by the ratio RAUC (AUCCSF: AUCF). The lipophilicity of the compounds was expressed as their polycratic capacity factors (log k'w) measured in a RP-HPLC system. The RAUC ranged from 0.24 to 6.58 and fb from 91.4 to 99.8%. The compounds with an intermediate lipophilicity value (3 < logk'w < 3.6) easily entered the CSF (RAUC > 1). A parabolic relationship was found between log k'w and log RAUC, emphasizing the role of molecular lipophilicity in the diffusion into CSF. Considering the fb value of each drug in regard to this non-linear relationship, it can be hypothesized that the diffusion rate of NSAIDs into the CSF depends primarily on the lipophilicity.     

Effect of menthone and related compounds on skin permeation of drugs with different lipophilicity and molecular organization of stratum corneum lipids

The objective of this article was to investigate the enhancing effect of menthone, menthol and pulegone on the transdermal absorption of drugs with different lipophilicity and probe their mechanisms of action at molecular level. Five model drugs, namely osthole, tetramethylpyrazine, ferulic acid, puerarin and geniposide, which were selected based on their lipophilicity denoted by logK_o/w, were tested using in vitro permeation studies in which Franz diffusion cells and rat skin were employed. Infrared spectroscopy and molecular dynamic simulation were used to investigate the effect of these enhancers on the stratum corneum (SC) lipids, respectively. Three compounds could effectively promote the transdermal absorption of drugs with different lipophilicity, and the overall promoting capacities were in the following increasing order: pulegone?<?menthol?<?menthone. The penetration enhancement ratio was roughly in parabolic curve relationships with the drug lipophilicity after treatment with menthol or menthone, while the penetration enhancement effect of pulegone hardly changed with the alteration of the drug lipophilicity. The molecular mechanism studies suggested that menthone and menthol enhanced the skin permeability by disordering the ordered organization of SC lipids and extracted part of SC lipids, while pulegone appeared to promote drug transport across the skin only by extracting part of SC lipids.     

Lipophilicity of zwitterionic sulphate conjugates of tiaramide, propranolol and 4'-hydroxypropranolol

1. Metabolism of basic drugs may result in the formation of zwitterionic sulphate conjugates. The additional ionization introduced by the sulphate group into these compounds compared with the basic parent drug does not produce a corresponding increase in hydrophilic character. 2. Zwitterionic conjugates have constant lipophilicity between their pKa values. The opposite charges on the ionizing functional groups in this pH range appear to cancel the effect of each other on lipophilicity. 3. In the case of propranolol the O-sulphate derivative is more lipophilic than the parent compound at pH values below 7, despite the ionized character of the sulphate function. 4. The decrease in lipophilicity appears to be related to the separation in the molecular structure of the amino and sulphate groups.     

The complete microspeciation of arginine and citrulline

(1)H NMR-pH titrations of arginine, the most basic natural amino acid and citrulline, its neutral counterpart were carried out. Two other closely related auxiliary compounds were also studied. The 8 macroscopic protonation constants were determined. Combining the four datasets in a deductive method, all the 12 microconstants of arginine, the 4 microconstants of citrulline and arginine amide were calculated. An error-propagation analysis and the pH-dependent distribution of the 8 arginine microspecies are provided.     

Mechanisms of Liposomes/Water Partitioning of (p-Methylbenzyl)alkylamines

Purpose. The objective of this study was to compare and interpret the variations in lipophilicity of homologous (p-methylbenzyl)alkylamines (MBAAs) in isotropic (octanol/water) and anisotropic (zwitterionic liposomes/water) system. Methods. Two experimental approaches were used, namely the pH-metric method to measure lipophilicity parameters in octanol/water and liposomes/water systems, and changes in NMR relaxation rates to validate the former method and to gain additional insights into the mechanisms of liposomes/water partitioning. Results. For long-chain homologues (N-butyl to N-heptyl), the octanol/ water and liposomes/water systems mostly expressed hydrophobicity. In contrast, the lipophilicity of the shorter homologues (N-methyl to N-propyl) in the two systems expressed various electrostatic and polar interactions. Conclusions. The study sheds light on the molecular interactions between zwitterionic liposomes and amphiphilic solutes in neutral and cationic form.     

In-vitro and in-vivo studies of cefpirom using bile salts as absorption enhancers

Cephalosporins have to be administered by injection because of the poor intestinal absorption of the orally delivered drugs. Because of the obvious drawbacks of drug delivery by injection, the development of alternatives with enhanced oral bioavailability is receiving much attention in pharmaceutical research. Cefpirom (Cp) is a new semi-synthetic amino-2-thiazolyl-methoxyimino cephalosporin that has been substituted in position 3 with a cyclopenteno-pyridinium group in order to create a zwitterionic compound. It exhibits highly hydrophilic properties, as shown from its extremely low partition coefficient, and therefore its lipophilicity was increased using bile salts. The effect of this on the partition coefficients determined in the n-octanol/buffer system was confirmed using an in-vitro transport model with artificial and biological membranes. The pharmacokinetic properties of Cp were investigated in rabbits after intraduodenal administration with and without bile salts. Furthermore, the physiological compatibility of the bile salts was investigated using active D-glucose transport.     

Lipophilicity and disposition of 1-aryl-piperazines in the rat

Abstract

1. The disposition of eight 1-aryl-piperazines was investigated in rats after i.v. administration. The concentration of 1-aryl-piperazine in body fluids and tissues was determined by?h.p.l.c. or electron-capture g.l.c. Correlations of kinetics and physiological parameters with the lipophilicity of each 1-aryl-piperazine, determined by?h.p.l.c. retention on a reverse-phase C₁₈ column at neutral pH, were investigated.

2. Binding to rat plasma proteins varied within the series, increasing with lipophilicity. For the majority of the derivatives the blood-to-plasma ratio was close to unity, implying an almost equal distribution between erythrocytes and plasma. The most lipophilic 1-aryl-piperazine of the series partitioned more into erythrocytes.

3. The eight compounds differed widely in V_ss and total Cl values and as a general trend both values increased with lipophilicity. The percentage of the dose excreted unchanged in the urine decreased progressively with increasing lipophilicity.

4. 1-Aryl-piperazines were distributed extensively in all the tissues examined, concentrating particularly in the eliminating organs and lung. They easily entered the rat brain, C_max values generally being reached within five minutes of parenteral injection. 1-Aryl-piperazine brain uptake increased with lipophilicity.     

Ionization, lipophilicity and solubility properties of repaglinide

Potentiometric and spectrophotometric titrations were used for the determination of ionization behaviour, lipophilicity and solubility profile of repaglinide. Acid-base equilibria were characterized by means of protonation macro- and microconstants using Target Factor Analysis of spectrophotometric data. Lipophilicity profiles were evaluated by determination of partition coefficients of neutral and ionized forms of repaglinide in biphasic octanol/water system. The intrinsic solubilities of repaglinide were determined from the solubility data and temperature dependence of intrinsic solubilities were evaluated using van't Hoff equation. Repaglinide possesses two protonation sites and in aqueous solutions exhibits ampholitic properties. At isoelectric pH the zwitterionic form of the molecule predominates over the uncharged form with the tautomeric ratio, logKz=1.9. The difference between calculated and measured logP values, as well as the difference between logP values of uncharged form of repaglinide, HR0, and either one of mono-charged forms indicated the significant partition of zwitterion into octanol. Temperature dependence of solubility data revealed exothermic dissolution process with DeltasolH=-36 kJmol-1 and negative entropy of solution of DeltasolS=-0.19 kJK-1mol-1.     

Comparative drug exsorption in the perfused rat intestine.

The factors affecting drug exsorption into the gastrointestinal tract are uncertain. In this study, the intestinal clearance (CLi) of compounds which vary in their lipophilicity, serum protein binding, molecular weight and ionic charge at physiological pH, has been measured. Male Sprague-Dawley rats with ligated bile ducts were infused with the test compounds through the jugular vein. The small intestine was intubated and perfused with Tyrode solution at 20 mL h-1. The CLi of the compounds investigated (urea, polyethylene glycol, inulin, albumin, dextran, barbituric acid, salicylic acid, thiobarbital, thiopental, thioseconal, theophylline, S-disopyramide and quinidine) was determined under anaesthesia by dividing the rate of a component's appearance rate in the perfusate by its carotid arterial concentration. Serum protein binding of the compounds was determined by equilibrium dialysis. The n-octanol-water partition coefficients of the compounds were measured as indices of lipophilicity. The CLi values of dextran, albumin, inulin, polyethylene glycol and urea were 0.56, 1.03, 4.5, 4.8 and 12.0 mL h-1, respectively. The larger the molecular weight of a compound, the smaller its CLi. The molecular weight is apparently one of the major determinants of CLi. Thiobarbital, thiopental and thioseconal are compounds of similar structure with increasing lipophilicity and serum protein binding. The CLi of thiobarbital, thiopental and thioseconal was proportional to the unbound fraction in serum. The unbound clearance (CLui) of three thiobarbiturates were similar (approximately 11 mL h-1). The unbound fraction of drug in serum appears to be a factor determining their CLi. Barbituric acid and salicylic acid, two acidic compounds, showed a low CLi (less than 1 mL h-1).(ABSTRACT TRUNCATED AT 250 WORDS)     

Lipophilicity of zwitterionic sulphate conjugates of tiaramide,propranolol and 4′-hydroxypropranolol

1. Metabolism of basic drugs may result in the formation of zwitterionic sulphate conjugates. The additional ionization introduced by the sulphate group into these compounds compared with the basic parent drug does not produce a corresponding increase in hydrophilic character.

2. Zwitterionic conjugates have constant lipophilicity between their pKa values. The opposite charges on the ionizing functional groups in this pH range appear to cancel the effect of each other on lipophilicity.

3. In the case of propranolol the O-sulphate derivative is more lipophilic than the parent compound at pH values below 7, despite the ionized character of the sulphate function.

4. The decrease in lipophilicity appears to be related to the separation in the molecular structure of the amino and sulphate groups.     

Synthesis and biological evaluation of new thiazolyl/benzothiazolyl-amides, derivatives of 4-phenyl-piperazine

A series of thiazolyl-N-phenyl piperazines has been synthesised and tested for anti-inflammatory activity. Their R(M) values were determined as an expression of their lipophilicity. Theoretical calculation of their lipophilicity, as clog P and logPsk also performed. The effect of the synthesised compounds on inflammation, using the carrageenin induced mouse paw oedema model was studied. In general, the studied compounds were found to be potent anti-inflammatory agents (44-74.1%). Anti-inflammatory activity was influenced by some structural characteristics of the synthesised compounds. An attempt was made to correlate their biological activity with some physicochemical parameters using a quantitative structure-activity relationship approach (QSAR).     

Structure-hepatic disposition relationships for phenolic compounds.

Phenolic compounds are widely used in therapeutic, environmental, and industrial applications. The present work seeks to define the hepatic disposition of 11 phenolic compounds with varying lipophilicities and molecular weights. The hepatic disposition kinetics were studied in a once-through in situ rat liver perfusion preparation in order to avoid extra-hepatic metabolism and recirculation effects. The phenols were administered using the impulse-response technique and the time course of hepatic venous effluent concentration was examined by moments and a two-compartment dispersion model. While the extraction of the phenolic compounds was relatively independent of lipophilicity, the estimated permeability-surface area (PS) product for influx of solutes into the hepatocytes could be related to the compounds' octanol-buffer partition coefficients (log Papp). This log PS-logPapp relationship was consistent with that reported earlier for another series of solutes with a wide range of lipophilicity. The metabolites produced from each of the phenolic compounds used in this study had mean transit times similar to those of their corresponding parent phenols, suggesting that the metabolites were not trapped in the liver as a consequence of their higher polarity. It is concluded that the strong solute lipophilicity-toxicity and lipophilicity-skin penetration relationships often seen for aqueous solutions of phenols are not evident for the hepatic extraction of these compounds. Such a conclusion is consistent with the hepatic extraction of phenolic compounds being mainly determined by a blood flow limitation in delivery of the phenol to the liver, rather than the intrinsic liver metabolic enzyme activities at the doses injected.