Measurement of lipophilicity indices by reversed-phase high-performance liquid chromatography: comparison of two stationary phases and various eluents

LogP determination for highly lipophilic hydrogen-bonding anion receptor molecules

Lipophilicity, usually expressed as octanol-water partition coefficient (logP_o/w), is an important property in biomedical research, drug design and technology. However, high logP_o/w values of complex hydrogen-bonding molecules are not easy to measure or calculate. Exemplary problematic molecules are prospective active components (ionophores) of polymeric sensor membranes - the working elements of ion-selective electrodes. High lipophilicities of the membrane components are crucial for the sensor lifetime. In this work, lipophilicities of a wide range of urea-, carbazole- and indolocarbazole-based anion receptor molecules (some newly synthesized) and two common plasticizers were determined using a chromatography-based approach and/or the COSMO-RS method. Very high logP_o/w values, up to around 20, i.e. far beyond directly experimentally accessible range, were obtained. The agreement between the two approaches ranged from very good to satisfactory. Based on these results, simple fragment-based equations were developed for quick lipophilicity estimation without any specialized software. Membrane-water partition coefficients for the studied compounds were modeled. Limitations and biases of the used methods are discussed.

The Impact of Lipophilicity in Drug Discovery: Rapid Measurements by Means of Reversed-Phase HPLC

Lipophilicity constitutes a vital physicochemical property in drug design as it is connected with pharmacodynamic and pharmacokinetic properties as well as toxicological aspects of candidate drugs. Traditional partitioning experiments to determine n-octanol-water coefficients are laborious and time-consuming, while they cannot be reliably performed for highly lipophilic or compounds undergoing degradation. Alternatively, lipophilicity of candidate drugs can be accurately and reproducibly determined using reversed-phase liquid chromatography. In this chapter, the details of protocols for lipophilicity assessment using reversed-phase HPLC, under conditions which provide the best simulation of n-octanol-water partition coefficients, are described.

Determination of lipophilicity by gradient elution high-performance liquid chromatography

A novel method for the determination of lipophilicity using a simple HPLC protocol based on gradient elution chromatography is presented and compared to the common isocratic log k'(w) procedure. Linear relationships with high correlation coefficients between both methods for biologically active nucleosides and cyclic nucleotides as well as for environmentally relevant aromatic hydrocarbons were found. A mathematical fit to support the empirically determined linear relationship is presented. It is shown that the observed relationship between log k'(w) and the apparent capacity factor (k'(g)) determined by gradient elution is derivable by theoretical considerations as well. Since the gradient method is much less time-consuming compared to other procedures, it represents a convenient alternative for determining lipophilicity data in the future.

Quantitative retention-structure and retention-activity relationship studies of local anesthetics by micellar liquid chromatography

The retention of compounds in micellar liquid chromatography (MLC) is governed by hydrophobic and electrostatic forces. For ionic compounds, both interactions should be considered. The present report offers a novel retention model that includes the hydrophobicity of compounds and the molar fraction of the charged form of compounds and compares it with other previously reported models. High correlations between the logarithm of capacity factors and these structural parameters were obtained for local anesthetics with different degrees of ionization using a nonionic surfactant solution as mobile phase. Modeling the retention of compounds as a function of physicochemical parameters and experimental variables is established by means of multiple linear regression. In addition, a predictive model for estimating the hydrophobicity of local anesthetics is proposed. Finally, quantitative and qualitative retention-activity relationships in MLC are also investigated for these compounds. An excellent correlation between the capacity factors in MLC and the anesthetic potency of local anesthetics was obtained.

Determination of n-octanol/water partition coefficient for DDT-related compounds by RP-HPLC with a novel dual-point retention time correction

n-Octanol/water partition coefficients (P) for DDTs and dicofol were determined by reversed-phase high performance liquid chromatography (RP-HPLC) on a C(18) column using methanol-water mixture as mobile phase. A dual-point retention time correction (DP-RTC) was proposed to rectify chromatographic retention time (t(R)) shift resulted from stationary phase aging. Based on this correction, the relationship between logP and logk(w), the logarithm of the retention factor extrapolated to pure water, was investigated for a set of 12 benzene homologues and DDT-related compounds with reliable experimental P as model compounds. A linear regression logP=(1.10±0.04) logk(w) - (0.60±0.17) was established with correlation coefficient R(2) of 0.988, cross-validated correlation coefficient R(cv)(2) of 0.983 and standard deviation (SD) of 0.156. This model was further validated using four verification compounds, naphthalene, biphenyl, 2,2-bis(4-chlorophenyl)-1,1-dichloroethane (p,p'-DDD) and 2,2-bis(4-chlorophenyl)-1,1-dichloroethene (p,p'-DDE) with similar structure to DDT. The RP-HPLC-determined P values showed good consistency with shake-flask (SFM) or slow-stirring (SSM) results, especially for highly hydrophobic compounds with logP in the range of 4-7. Then, the P values for five DDT-related compounds, 2-(2-chlorophenyl)-2-(4-chlorophenyl)-1,1,1-trichloroethane (o,p'-DDT), 2-(2-chlorophenyl)-2-(4-chlorophenyl)-1,1-dichloroethane (o,p'-DDD), 2-(2-chlorophenyl)-2-(4-chlorophenyl)-1,1-dichloroethene (o,p'-DDE), and 2,2,2-trichloro-1,1-bis(4-chlorophenyl)ethanol (dicofol) and its main degradation product 4,4'-dichlorobenzophenone (p,p'-DBP) were evaluated by the improved RP-HPLC method for the first time. The excellent precision with SD less than 0.03 proved that the novel DP-RTC protocol can significantly increases the determination accuracy and reliability of P by RP-HPLC.

Physicochemical characterization of NPC 1161C, a novel antimalarial 8-aminoquinoline, in solution and solid state

NPC 1161C is a novel antimalarial drug of interest because of its superior curative and prophylactic activity, and favorable toxicity profile against in vivo and in vitro models of malaria, pneumocystis carinii pneumonia, and leishmaniasis. The preformulation studies performed included determination of pK(a)s, aqueous and pH solubility, cosolvent solubility, log P, pH stability, thermal analysis, and preliminary hygroscopicity studies. The mean pK(a1), pK(a2), and pK(a3) were determined to be 10.12, 4.07, and 1.88, respectively. The aqueous solubility was found to be 2.4×10(-4) M having a saturated solution pH of 4.3-5.0 and a low intrinsic solubility of 1.6×10(-6) M. A mathematical model of the pH-solubility profile was derived from pH 2.2 to 8.0. An exponential decrease in solubility was observed with increasing pH. The excess solid phase in equilibrium with the solution in aqueous buffers was determined to be the free-base form of the drug. A significant increase in solubility was observed with all the cosolvents studied, in both unbuffered and buffered systems. Mean log P of the salt and the free base were estimated to be 2.18 and 3.70, respectively. The compound had poor stability at pH 7.0 at 37 °C, with a t (90) of 3.58 days. Thermal analysis of the drug using DSC and TGA revealed that the drug is present as a semi-crystalline powder, which transformed into the amorphous state after melting. The drug was also found to sublime at higher temperatures. Determination of physicochemical properties of NPC 1161C provided useful information for the development of a dosage form and preclinical evaluation.

Alternative measures of lipophilicity: from octanol-water partitioning to IAM retention

This review describes lipophilicity parameters currently used in drug design and QSAR studies. After a short historical overview, the complex nature of lipophilicity as the outcome of polar/nonpolar inter- and intramolecular interactions is analysed and considered as the background for the discussion of the different lipophilicity descriptors. The first part focuses on octanol-water partitioning of neutral and ionisable compounds, evaluates the efficiency of predictions and provides a short description of the experimental methods for the determination of distribution coefficients. A next part is dedicated to reversed-phase chromatographic techniques, HPLC and TLC in lipophilicity assessment. The two methods are evaluated for their efficiency to simulate octanol-water and the progress achieved in the refinement of suitable chromatographic conditions, in particular in the field of HPLC, is outlined. Liposomes as direct models of biological membranes are examined and phospolipophilicity is compared to the traditional lipophilicity concept. Difficulties associated with liposome-water partitioning are discussed. The last part focuses on Immobilised Artificial Membrane (IAM) chromatography as an alternative which combines membrane simulation with rapid measurements. IAM chromatographic retention is compared to octanol-water and liposome-water partitioning as well as to reversed-phase retention and its potential to predict biopartitioning and biological activities is discussed.

Octanol/water partitioning simulation by reversed-phase high performance liquid chromatography for structurally diverse acidic drugs: Effect of n-octanol as mobile phase additive

The role of n-octanol as mobile phase additive for the lipophilicity assessment of 45 structurally diverse acidic drugs both at neutral (pH 2.5) and ionized form (pH 7.4) was investigated. Extrapolated retention factors logk(w) were determined on a BDS C18 column using methanol as organic modifier and different amounts of n-octanol as mobile phase additive. For more polar compounds, the effect of n-octanol in retention was found to decrease as their lipophilicity increased. In the case of carboxylic acids and oxicams, the differentiation in retention, in presence and absence of n-octanol, could be further attributed to the attenuation of polar interactions, concerning mainly hydrogen bonding. At pH 2.5, the use of n-octanol saturated buffer, without further addition of n-octanol in the mobile phase, led to 1:1 correlation with logP. At physiological pH, 1:1 correlation was obtained between logD(7.4) and logk(w)(oct) indices upon addition of 0.25% n-octanol, in the case of weak acids. For strongly ionized compounds, a good correlation was also established under the same conditions. The corresponding equation, however, possessed a large negative intercept and a slope lower than unity.

Counter-current chromatographic estimation of hydrophobicity of Z-(cis) and E-(trans) enalapril and kinetics of cis/trans isomerization

The kinetics of Z-(cis)/E-(trans) isomerization of enalapril was investigated by reversed phase high-performance liquid chromatography (RP-HPLC) using a monolith ODS column under a series of different temperature and pH conditions. At a neutral pH 7, the rate (k(obs)) of Z-(cis)/E-(trans) isomerization of enalapril at 4 degrees C (9.4 x 10(-3)min(-1)) is much lower than at 23 degrees C (1.8 x 10(-1)min(-1)), while the fractional concentration of Z-(cis) isomer is always higher than that of E-(trans) isomer in the pH range 2-7. The fractional concentration of the E-(trans) isomer becomes a maximum (about 40%) in the pH range 3-6, where enalapril exists as a zwitterion. The hydrophobicity (logP(O/W)) of both isomers was estimated by high-speed counter-current chromatography (HSCCC). Normal phase HSCCC separation using a tert-butyl methyl ether-acetonitrile-20mM potassium phosphate buffer (pH 5) two-phase solvent system (2:2:3, v/v/v) at 4 degrees C was effective in partially separating the isomers, and the partition coefficient (K) of each isomer was directly calculated from the retention volume (V(R)). The logP(O/W) values of Z-(cis) and E-(trans) isomers were -0.46 and -0.65, respectively.

Contribution to the standardization of the chromatographic conditions for the lipophilicity assessment of neutral and basic drugs

The chromatographic conditions aiming to a better simulation of n-octanol-water partitioning using a base deactivated silica (BDS) column as stationary phase were investigated for structurally diverse basic and neutral drugs. Extrapolated retention factors log k(w), determined using different methanol fractions as organic modifier, were considered as lipophilicity indices. The effect of n-decylamine and n-octanol as mobile phase additives was examined and the appropriateness of the final retention outcome to reproduce lipophilicity data was evaluated. Moreover, the influence of n-octanol on the linearity of the log k/methanol fraction relationship and on the uniformity of the retention mechanism was investigated. 1:1 correlation between log k(w) values and the logarithm of the distribution coefficient (logD) was established for basic drugs in presence of both n-decylamine and n-octanol as mobile phase additives. However, for neutral drugs n-decylamine proved to be a sufficient and more important factor than n-octanol.

Application of the ion pair concept to the n-octanol-water partitioning of cefepime and cefpirome

The ion pair concept was applied for the assessment of lipophilicity of cefepime and cefpirome. Octanol-water distribution coefficients were determined in presence of different concentrations [X-] of sodium octanesulphonate. The log Dx values within the linear part of the log Dx/[X-] relationships were extrapolated to log Do values corresponding to the partitioning in absence of the counter ion. Measurements were feasible at pH values close to the isoelectric points of the acidic and basic functions. In that pH range the conduction of the experiments in presence of the hydrophobic counter anion facilitated the partitioning of the two cephalosporins to octanol, circumventing the problems arising from their high hydrophilicity. This procedure could not be applied at lower pH, possibly due to a further drastic decrease in the 'intrinsic' lipophilicity or to reduced ion pairing potential of octanesulphonate, and at higher pH due to the disruption of the zwitterionic structure. Extrapolated log Do values were compared to actual log D measurements performed for a reference quinolinium compound and for cefpirome. Extrapolated retention factors log kw close to the isoelectric point were also determined by reversed phase HPLC and compared to the log Do values.

Different retention behavior of structurally diverse basic and neutral drugs in immobilized artificial membrane and reversed-phase high performance liquid chromatography: comparison with octanol-water partitioning

The retention behavior of 43 structurally diverse neutral and basic drugs in immobilized artificial membrane chromatography was investigated and compared to the reversed-phase retention and octanol-water partitioning. IAM chromatography was performed using morpholinepropanesulfonic acid (MOPS) or phosphate buffer saline (PBS) at pH 7.4 as the aqueous component of the mobile phase. The differences in the retention factors were attributed to increased electrostatic interactions in the MOPS environment, dependent on the fraction of charged species. Electrostatic interactions were found to play a key role in the relationships with reversed-phase retention factors determined under two different mobile phase conditions as well as in the relationships with lipophilicity data. IAM retention factors correlated better with octanol-water partition coefficients log P than with log D(7.4), as a result of the contribution of electrostatic forces in IAM retention. With log D(7.4) the relationships were improved when the fraction of charged species was taken into consideration. In any case the regression coefficient of log P or log D(7.4) was considerably lower than 1 reflecting the reduced hydrophobic environment of the IAM stationary phase. The different data sets were submitted to principal component analysis for further exploration of their similarities/dissimilarities.

Relation between retention factors of immunosuppressive drugs in microemulsion electrokinetic chromatography with biosurfactants and octanol–water partition coefficients

Retention (capacity) factors (k' values) of immunosuppressive drugs were determined in microemulsion electrokinetic chromatography (MEEKC) systems as a tool for the indirect estimation of partition coefficients (POW) between 1-octanol and water. The microemulsions were based on phosphatidylcholine (PC) and bile acids (BAs) as biosurfactants and isopropyl myristate (IPM) as oil. Immunosuppressants were azathioprine (AZA), mycophenolate mofetil (MMF), tacrolimus (FK506) and cyclosporine A (CyA). Capacity factors of the analytes were determined from electrophoretic mobilities using an aqueous phosphate buffer (20 mM; pH 7.5) for all the systems. Retention was compared with that in the most commonly used microemulsion based on sodium dodecyl sulphate (SDS). logPOW versus logk' calibration lines were constructed using reference compounds with known POW. In addition, data of logPOW of the immunosuppressants were determined by partitioning between octanol and water, and were calculated by the aid of computer program. A different sequence of logPOW for two analytes was found in the biosurfactant-based systems compared with the SDS-containing one. Excellent agreement was observed between the logPOW values derived from the microemulsions containing deoxycholate compared with the data determined by partitioning between octanol and water. It was concluded that the retention factors in the systems with biosurfactants are good estimators for the partitioning in biological systems.

Method for measuring the logarithm of the octanol-water partition coefficient by using short octadecyl-poly(vinyl alcohol) high-performance liquid chromatography columns

A simple, quick, versatile and inexpensive HPLC method to estimate the logarithm of the octanol-water partition coefficient (log Pow) employing a methanol-water gradient and a short octadecyl-poly(vinyl alcohol) (ODP) column is described. This method is different from published HPLC-based log Pow methods because it uses retention times from a rapid methanol-water gradient to directly generate log Pow estimates, rather than from a series of isocratic mixtures extrapolated to 100% water. These HPLC log Pow values have good precision and correlate well with traditional shake-flask log Pow values. If necessary, the log Pow determination (including replications) can easily be carried out using only a milligram of sample. By suppressing ionization of acids and bases by the use of a buffer in the aqueous phase, the method can measure the log Pow of neutral organic molecules at any pH between 2 and 13. The method can be used with impure material and is rapid, 7 min per run and 4 min equilibration; it lends itself to and has been utilized for high-throughput hydrophobicity determinations (we have now carried out thousands of HPLC log Pow measurements by this method).

Determination of octanol-water partition coefficient for terpenoids using reversed-phase high-performance liquid chromatography

Octanol-water partition coefficients (Kow) for 57 terpenoids were measured using a RP-HPLC method. Sample detection was achieved with standard UV and refractive index detectors and required no special column treatment. Measured log Kow values for the terpenoids ranged from 1.81 to 4.48 with a standard error of between 0.03 and 0.08 over the entire range. Partition coefficients determined by the RP-HPLC method were compared against shake flask, atom/fragment contribution, fragment and atomistic methods. The HPLC values were found to give the best correlation with shake flask results. Log Kow values calculated by the atom/fragment contribution method gave the best correlation with the HPLC values when compared to fragment and atomistic methods.

Measurement and prediction of hydrophobicity parameters for highly lipophilic compounds: application of the HPLC column-switching technique to measurement of log P of diarylpyrazines

In the preparatory stage of structure-activity relationship (QSAR) studies of anti-platelet aggregant pyrazine derivatives, log P values (P: 1-octanol/water partition coefficient) of diarylpyrazines were measured by a newly developed HPLC column-switching technique. The system consists of two processes: (1) adsorption of the sample at the top end of a short precolumn, and then (2) quantifying the enriched analyte by a conventional analytical column. By using the log P values thus obtained, the correction factor for the steric hindrance caused by the vicinal diphenyl groups was estimated. The log k values (k; retention factor) were also measured with methanol-buffer (pH 7.4) eluents and related to log P. The eluent of 50% methanol content (M50) gave a good linear relationship over a wide range of log P (-0.3< log P < 5.2), indicating that log k(M50) parameter is useful for predicting the log P value.

Effect of separation conditions on chromatographic determination of hydrophobicity of acidic xenobiotics

Problems encountered in the chromatographic determination of hydrophobicity of acidic xenobiotics are discussed. First, the definition and meaning of hydrophobicity is briefly presented. Next, the methods of determination of the hydrophobicity parameter by reversed-phase high-performance liquid chromatography are described. The methods of determination of the dead volume are analyzed with regard to calculation of the thermodynamically valid retention parameters. Relationships between retention factors and pH of mobile phase which have been reported in the literature are presented. The effects of ionic strength and buffer composition on the apparent retention parameters are discussed. The reversed-phase stationary phase materials presently employed for hydrophobicity determinations are reviewed. Application of micellar electrokinetic chromatography in the determination of hydrophobicity of ionizable analytes is presented. The ability of chromatography to provide the measures of hydrophobicity of xenobiotics best modelling their biological activity is underlined.

Synthesis, lipophilicity and biological evaluation of indole-containing derivatives of 1,3,4-thiadiazole and 1,2, 4-triazole

3-[(2-Methyl-1H-3-indolyl) methyl]-4-aryl-4, 5-dihydro-1H-1,2,4-triazole-5-thiones 6a-c and their respective N-¿5-[2-methyl-1H-3-indolyl) methyl]-1,3,4-thiadiazol-2-yl¿-N-arylamines 7a,b have been prepared. The antidepressant profile of 6a,c and 7a was studied on mice with respect to that of the analogous 3-(1H-1-indolylmethyl)-4-aryl-4,5-dihydro-1H-1,2,4-triazole-5-thio nes 1a-c and the respective N-¿5-[(2-methyl-1H-3-indolyl) methyl]-1,3,4-thiadiazole-2-yl¿-N-arylamines 2a-c, the synthesis and antimicrobial potency of which we have recently reported. Behavioral effects, induced by the members of both series, in conjunction with their activity in some specific tests (forced swim, pentetrazole convulsions) on mice, show that these derivatives cross the blood-brain barrier and could develop an antidepressant activity comparable to that of imipramine. Blood-brain barrier penetration is also supported by the lipophilicity data obtained for all analogs.

Determination of true octanol-water partition coefficients by means of solvent generated liquid-liquid chromatography

Compared to other methods for the determination of octanol-water partition coefficients chromatography offers a number of advantages: sample purification is unnecessary, the partition coefficients of the components of a mixture can be measured simultaneously and a minimum amount of sample is needed. In the past these determinations were almost exclusively carried out by liquid-solid chromatography (LSC) on alkyl bonded silica as stationary phase (conventionally described as 'reversed-phase liquid chromatography; RPLC). Such systems based on liquid-solid distribution are, however, a poor simulation of liquid-liquid partition. On the other hand liquid-liquid chromatographic columns loaded with high amounts of water-saturated octanol are unstable since they suffer from "column bleeding"--a loss of the stationary liquid octanol phase caused by erosion. It is shown that the technique of solvent generated liquid-liquid chromatography (SGLLC) leads to stable columns with liquid-liquid partition as the predominant retention mechanism, if systematic errors due to specific adsorption effects are avoided by the selection of an "inert" solid support. This is demonstrated by the comparison of LSC and SGLLC data. SGLLC significantly reduces the scattering of the retention data of calibration standards around the calibration line. The use of SGLLC thus significantly improves the accuracy and precision of the resulting octanol-water partition coefficients.

Determination of peptide hydrophobicity parameters by reversed-phase high-performance liquid chromatography

The log kw values of fourteen potential fibrinogen receptor antagonist peptides (RGDX) determined by reversed-phase HPLC were correlated to hydrophobic parameters of the amino acid side-chain log P in position X of the tetrapeptides. Comparing the polymer columns with LiChrosorb RP-8, the correlation coefficient using a polyethylene column is higher (0.94) than that for RP-8 (0.88), which demonstrates the importance of a homogeneous hydrophobic surface and makes this method very suitable for the determination of the overall hydrophobicity of shorter peptides. The hydrophobicity parameters log kw of the RGDX peptides (-1.15 to 2.19) were used to investigate the influence of molecular parameters of X on the potency of RGDX in inhibiting platelet aggregation. The results confirm the importance of hydrophobicity for the contribution of X to the biological activity of RGDX.

Hydrophobicity estimations by reversed-phase liquid chromatography. Implications for biological partitioning processes

Liquid chromatography has long been used for the estimation of "hydrophobicity" of solutes of biological, environmental and agricultural interest. These measurements have taken the form of octanol-water partition coefficient estimation, or less often the more fundamental processes that the octanol-water partition coefficient is intended to model. Here we review both the chromatographic methods used for these estimations, their successes and failures, and discuss pertinent solution thermodynamics of the partitioning of small molecules between bulk phases, such as octanol and water, and between a bulk phase and an interphase, such as partitioning of solute molecules into lipid layers and biological membranes.