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.     

Protonation equilibrium and lipophilicity of olamufloxacin (HSR-903), a newly synthesized fluoroquinolone antibacterial.

This study was performed to characterize the protonation equilibrium at the molecular level and pH-dependent lipophilicity of olamufloxacin. The deprotonation fraction of the carboxyl group as a function of pH was specifically calculated at the critical wavelength 294 nm, where UV pH-dependent absorbance of olamufloxacin was independent of the ionized state of the aminopyrrolidinyl amino group but heavily depended on that of the carboxyl moiety. Accordingly, micro-protonation equilibrium could be described using a nonlinear least-squares regression program MULTI. In contrast, macro-protonation equilibrium was depicted at most wavelengths where olamufloxacin absorbance was influenced by ionized states of both proton-binding groups, results coinciding with the former. Furthermore, distribution features of four microspecies in aqueous phase were assessed. The apparent partition coefficient versus pH profile of olamufloxacin showed a parabolic curve in n-octanol/buffer system which reached peak near pH 8, agreeing with the above determined isoelectric point (pI). Ion-pair effect was observed for olamufloxacin under an acidic condition, eliciting experimental values higher than those theoretically calculated, which was similar to ciprofloxacin but not levofloxacin due to amino group type. Moreover, olamufloxacin was moderately lipophilic in comparison with other quinolones, with an apparent partition coefficient of 1.95 at pH 7.4.     

Distribution characteristics of grepafloxacin, a fluoroquinolone antibiotic, in lung epithelial lining fluid and alveolar macrophage

The purpose of this study was to investigate the distribution of Grepafloxacin (GPFX), a new quinolone antimicrobial agent, in the lung epithelial lining fluid (ELF) and the alveolar macrophage (AM) in rats, which are potential infection sites in respiratory tract infections. We also aimed to clarify the mechanism governing the transferability of GPFX into the alveolus compartment from a kinetic point of view. The AUC ratios of ELF/plasma and AM/plasma after the oral administration of GPFX were 5.69 +/- 1.00 and 352 +/- 57, respectively, which were several-fold greater than those of ciprofloxacin (CPFX). Pharmacokinetic analyses of time profiles of GPFX concentrations in ELF and AM revealed that the influx clearance from plasma to ELF across the alveolar barrier is 5-fold greater than the efflux clearance from ELF. In addition, the permeability of GPFX across the cultured AM cell membrane was 7-fold and 11-fold greater than that of levofloxacin (LVFX) and CPFX, respectively. The extent of intracellular binding to AM cells (expressed as a constant (alpha)) was the greatest for GPFX, followed by CPFX and LVFX. There was a significant correlation between the alpha value and the partitioning to the immobilized artificial membrane (IAM) column, which consists of phospholipid residues covalently bound to silica. These results suggest that GPFX is highly distributed in ELF and AM, and that the high transferability of GPFX into ELF may be attributable to the existence of asymmetrical transport across the alveolar barrier. In addition, it was suggested that both rapid permeability across the AM cell membrane and avid binding to the membrane phospholipids may be responsible for the high accumulation of GPFX in AM.     

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.     

Zwitterions can be predominant in membrane penetration of drugs: experimental proof

The complete set of species-specific partition coefficients was determined, and the predominant contribution of zwitterionic species to the overall lipophilicity was experimentally proven for the first time. The compounds studied were the amphoteric eburnane alkaloid cis- and trans-apovincaminic acids of therapeutic interest. Partition of the individual microspecies was mimicked by model compounds of the closest possible similarity, and then correction factors were determined and introduced. The noncharged microspecies of the cis-epimer is 30?900 times as lipophilic as its zwitterionic protonation isomer, while the analogous ratio for the trans-epimer is around 15?800. Due to the overwhelming dominance of the zwitterionic form, however, its contribution to the overall lipophilicity exceeds 8 and 5 times that of the noncharged form for the two epimers, respectively. The lipophilicity profile of these zwitterionic compounds is expressed, calculated, and depicted in terms of species-specific lipophilicities over the entire pH range.     

Ciprofloxacin Encapsulation Into Giant Unilamellar Vesicles: Membrane Binding and Release

This study aimed at investigating some respects of binding and interaction between water-soluble drugs and liposomal carrier systems depending on their size and lamellarity. As model substance, ciprofloxacin hydrochloride (CPFX) was incorporated into giant unilamellar vesicles (GUVs) to study their CPFX encapsulation/binding capacity. To characterize molecular interactions of various CPFX microspecies with lipid bilayer, zeta potential and electron paramagnetic resonance (EPR) spectroscopy measurements were performed. The increase of the zeta potential at pH 5.4 but no change at pH 7.2 was interpreted in terms of the CPFX microspecies' distribution at the two pH values. EPR observations showed an increased fluidity because of CPFX binding to GUVs. We worked out and applied a three-compartment dialysis model to separately determine the rate of drug diffusion through the liposomal membrane. Equilibrium dialysis showed (a) different permeation of CPFX through the membranes of GUVs and multilamellar vesicles (MLVs), with characteristic half-lives of 54.4 and 18.1 h, respectively; and (b) increased retention of CPFX in case of GUVs with released amounts of 70% compared with about 97% in case of MLVs. Our results may provide further details for efficient design of liposomal formulations incorporating water-soluble drugs. © 2012 Wiley Periodicals, Inc. and the American Pharmacists Association J Pharm Sci 102:694–705, 2013     

在质子解离平衡中测定左氧氟沙星的亲脂性

目的测定左氧氟沙星的质子解离平衡和亲脂性。方法用紫外分光光度法测定质子解离平衡,并解析其水溶液中的离子分布曲线和分子荷电性质。使用正辛醇/水系统测定表观分配系数。结果两性离子在其等电点下的分布达到最大,而且其单个分子平均净电荷量对生理pH值的波动很敏感。在正辛醇/水系统中,左氧氟沙星的表观分配系数-pH值曲线呈现钟形形状,在等电点下达到峰值。结论左氧氟沙星的离子分布、分子荷电性质和表观分配系数均表现出强的pH值依赖性。     

Lipophilic and electrostatic forces encoded in IAM-HPLC indexes of basic drugs: their role in membrane partition and their relationships with BBB passage data

The membrane phospholipid affinity data, log k(w)(IAM), for 14 basic drugs spanning a wide lipophilicity range were measured by HPLC on two different phospholipid stationary phases, i.e. IAM.PC.MG and IAM.PC.DD2. These data related weakly with log P(N) values, the n-octanol/water partition coefficients of the neutral forms; poorer relationships were found with log D(7.0) values, the n-octanol/water partition coefficients of the mixtures of neutral and ionized forms at pH 7.0. The lack of collinearity confirms that, differently from partition in n-octanol/water, partition in phospholipids encodes not only lipophilic/hydrophobic intermolecular recognition forces but also ionic bonds, due to electrostatic interactions between electrically charged species and phospholipids, according to the "pH-piston hypothesis". This component of interaction was parameterized by Δ log k(w)(IAM) values; they are the differences between the log k(w)(IAM) values experimentally measured and the values expected for neutral isolipophilic compounds. Δ log k(w)(IAM) values of the various analytes changed almost linearly from positive to negative values at increasing lipophilicity. This behavior is consistent with an interaction mechanism with membrane phospholipids including two intermolecular interaction forces: (i) lipophilic/hydrophobic interactions, which decrease on ionization proportionally to the lipophilicity of the neutral forms, and (ii) electrostatic interactions, which increase on ionization and are quite constant for all the analytes at a given ionization degree. Since BBB passage of the considered compounds is supposed to be based on passive mechanisms, we investigated the possible relationships between log BB values, i.e. the logarithms of the ratio between brain and blood concentrations, and three physico-chemical parameters, i.e. (i) log P(N) (lipophilic interaction of the neutral form), (ii) log k(w)(IAM) (global interaction with phospholipids), and (iii) Δ log k(w)(IAM) (electrostatic component of interaction with phospholipids). The results suggest that the electrostatic interactions encoded in log k(w)(IAM) values might act as trapping forces in a phospholipid barrier. Actually, we observed an inverse linear dependence of log BB on Δ log k(w)(IAM) values, but only for the compounds showing positive Δ log k(w)(IAM) values. We conclude that the driving force for BBB passage is the lipophilicity of the neutral forms, log P(N), and not the lipophilicity actually displayed at the experimental pH, log D(7.0). Indeed, the latter does not adequately take into account the role played by protonation in the analyte/membrane interactions because protonation, although hindering membrane passage, can either reduce or enhance partition in phospholipids, depending on analyte lipophilicity.     

Carrier-mediated uptake of grepafloxacin, a fluoroquinolone antibiotic, by the isolated rat lung cells

Grepafloxacin (GPFX) is a new quinolone antibiotic (NQ) which is highly distributed to the lung and other tissues. In the present study, to characterize the distribution mechanism of GPFX to the lung, the uptake of GPFX by isolated rat lung cells was examined in vitro. GPFX was rapidly taken up by the cells, and the uptake reached a steady-state within 5 min. The cell-to-medium concentration ratio at equilibrium was 56.8+/-1.9 microL/mg protein, which was much higher than the cellular volume. GPFX uptake consisted of a saturable component (Km: 264+/-181 microM, Vmax: 2.94+/-2.33 nmol/min/mg protein) and a nonsaturable component (Pdif: 7.04+/-2.17 microL/min/mg protein). The uptake of GPFX was reduced in the presence of ATP-depletors (FCCP and Rotenone) and by the replacement of sodium with choline in the medium, suggesting that GPFX uptake is at least partially mediated by an Na+- and energy-dependent process. GPFX uptake tended to be reduced in the presence of other NQs such as levofloxacin, lomefloxacin and sparfloxacin, but was only minimally affected by the substrates of several uptake mechanisms already identified in the liver and kidney such as taurocholate, p-aminohippurate, L-carnitine and tetraethylammonium. These results suggested that GPFX is taken up by the lung partially via carrier-mediated transport system(s), distinct from the identified transporters, and such active transport systems may at least partially account for the efficient distribution of GPFX to the lung.     

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.     

磷脂膜色谱与正辛醇/水系统亲脂性测量尺度的比较

磷脂膜色谱 (immobilizedartificialmembranechromatography,IAMC)是 2 0世纪 90年代初发展起来的一个高通量筛选药物与磷脂膜相互作用和具有适宜体内药代动力学特征候选药物的工具[1～ 4 ] 。近年来 ,研究结果[5,6 ] 表明 ,由于磷脂膜色谱能较好的模拟细胞膜有序磷脂层的空间环境 ,因此 ,它通常要比正辛醇 水系统更能准确的评价药物与生物膜的相互作用和获得更精确的药物亲脂性参数。因为尚未见磷脂膜色谱和正辛醇 水系统亲脂性测量尺度的系统性比较研究 ,所以本文选择了 2 7个不同化学结构类型药物的集合 ,包括酸性、碱性、中性和两性化合物 ,分别使用正辛醇 水系统和磷脂膜色谱两种生物膜相互作用评价系统对这一系列化合物的亲脂性参数进行考察 ,进而比较这两种不同的生物膜模拟系统。材料与方法药品与试剂　格帕沙星、环丙沙星、左氧氟沙星和OPC 1 72 0 3由日本大冢制药株式会社提供 ;HSR 90 3和NR 762由日本北陆制药株式会社提供 ;奎尼丁、普萘洛尔、阿莫西林、对氨基水杨酸、吡哌酸、利多卡因、普鲁卡因酰胺和正辛醇 (SigmaChemCo,StLouis,...     

The inhibitory effects of fluoroquinolones on L-carnitine transport in placental cell line BeWo

L-Carnitine plays an important role in lipid metabolism by facilitating the transport of long-chain fatty acids across the mitochondrial inner membrane followed by fatty acid beta-oxidation. It is known that members of the OCTN family play an important role in L-carnitine transport in the placenta. Investigation of drug-drug or drug-nutrient interaction in the placenta is important for establishment of safety drug medication during pregnancy. The aim of this study was to determine the effects of fluoroquinolones, inhibitors of OCTN2, on L-carnitine transport in the placenta which is known to have a high expression level of OCTN2. We investigated the inhibitory effect of five fluoroquinolones, ciprofloxacin (CPFX), gatifloxacin (GFLX), ofloxacin (OFLX), levofloxacin (LVFX) and grepafloxacin (GPFX), on L-carnitine transport mediated by OCTN2 in placental cell line BeWo cells. We found that all of the fluoroquinolones inhibited L-carnitine transport, GPFX being the strongest inhibitor. We also found that the inhibitory effects of LVFX and GPFX depended on their existence ratio of zwitterionic forms as, we reported previously. Furthermore, we elucidated the LVFX transport mechanism in BeWo cells. LVFX was transported actively by transporters. However, we found that LVFX transport was Na+-independent and l-carnitine had no inhibitory effect on LVFX transport, suggesting that LVFX acts as inhibitor of OCTN2, not as a substrate for OCTN2.     

Microspeciation of amphoteric molecules of unusual acid-base properties

The phisico-chemical properties of bio- and drug molecules greatly influence their interactions in the body and strongly effect the mechanism of drug action. Among these properties, macroscopic and site-specific protonation constants are of crucial importance. Latter one is the tool to calculate the relative concentration of the various microspecies in the compartments of the body at different pH values, and also, it is the versatile parameter to improve the pharmacokinetic properties of a new molecule in a particular family of drugs. In the present thesis work, the microspeciation of three molecules of great pharmaceutical importance and unusual acid-base properties, were carried out. The microconstants of tenoxicam, the non-steroidal anti-inflammatory drug, were described, introducing a novel deductive method using Hammett constants. For this purpose, a total of 8 tenoxicam and piroxicam derivatives were synthesised. To the best of our knowledge, the log k(N)O microconstant of tenoxicam obtained thus is the lowest enolate basicity value, which, however, can be well explained by the effects of the intramolecular environment. The developed evaluation procedure is suitable for microconstant determination of compounds in other molecule families. Besides, prodrug-type compounds and analogues similar to the structures of selective COX-2 isoenzyme inhibitors were synthesised. The other two molecules studied, the 6-aminopenicillanic acid and 7-cephalosporanic acid, the core molecules of the two most important beta-lactam antibiotic-types were derivatised and investigated by 1D and 2D NMR techniques. The NMR-pH titration on the parent compounds and their ester derivatives, combined with in situ pH-measurements allowed the microspeciation of these easily decomposing molecules. One of the protonation constant of 7-ACA (log kN(O) = 4.12), to the best of our knowledge, is the least non-aromatic basic amino-site among the natural compounds.     

Differences in the hepatobiliary transport of two quinolone antibiotics, grepafloxacin and lomefloxacin, in the rat

The biliary excretion of grepafloxacin (GPFX) was compared with that of lomefloxacin (LFLX) in rats. The biliary clearances (Cl(plasma)(bile)) of GPFX was 2.9 times greater than LFLX based on the plasma concentration reached during constant intravenous (i.v.) infusion. The liver-plasma unbound concentration ratio, K(pu), of GPFX (1.7) was also higher than that of LFLX (0.7). The hepatic uptake clearance, assessed from an integration plot analysis, of GPFX was comparable with the hepatic blood flow rate, and 1.5 times that of LFLX, indicating that membrane transport in the uptake process is more efficient for GPFX. This was also supported by the difference between the uptake clearance of GPFX and LFLX in isolated rat hepatocytes. The bile-liver unbound concentration ratio of GPFX and LFLX was approximately 6 and 3, respectively, and the biliary clearance based on the unbound liver concentration of GPFX was 1.8 times that of LFLX. These results suggest that the concentrative transport of GPFX also across the canalicular membrane was more efficient than that of LFLX. Thus, the membrane transport activity via both sinusoidal and canalicular membranes determines the net excretion of each compound.     

Mechanism for the tissue distribution of grepafloxacin, a fluoroquinolone antibiotic, in rats.

This study was carried out to investigate the most important factor(s) governing the tissue distribution of grepafloxacin (GPFX), a fluoroquinolone antibiotic, in rats. The tissue-to-blood concentration ratio (K(p)) of GPFX at steady state during constant infusion was highest in the lung, followed by the pancreas, kidney, and spleen. After bolus injection, GPFX was efficiently taken up by most of the organs examined, the uptake clearance other than the lung being almost blood flow-limited. Approximately 10% of the intravenously injected dose was rapidly trapped by the lung, but GPFX distribution rapidly decreased within 30 s due to the washout by the plasma flow. Thus, the higher distribution of GPFX to the lung compared with the other organs cannot be accounted for by a difference in its uptake or efflux. Subcellular fractionation after the infusion indicated that GPFX is primarily distributed to the organelle fractions in most organs, 60% of lung-associated GPFX being recovered in the nucleus and plasma membrane fraction. Such subcellular distribution in the lung was proportional to the phosphatidylserine (PhS) content of each fraction. The steady-state K(p) value in each tissue in vivo also correlated with the tissue content of PhS. GPFX preferentially binds to PhS, compared with other phospholipids, and this binding was inhibited by weakly basic drugs, such as quinidine, imipramine, and propranolol, that have also been reported to bind to PhS. The association of GPFX with PhS synthase transformants of Chinese hamster ovary (CHO-K1) cells depends on the PhS content of each cell line, this association being also inhibited by basic drugs. These results suggest that binding of GPFX to PhS is the major determinant of the high distribution of GPFX to the lung.     

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.     

The physicochemical properties and the in vivo AChE inhibition of two potential anti-Alzheimer agents, bis(12)-hupyridone and bis(7)-tacrine

The lipophilicity and solubility profiles of bis(12)-hupyridone (B12H) and bis(7)-tacrine (B7T), two novel acetylcholinesterase inhibitors dimerized from huperzine A fragments and tacrine, respectively, were investigated over a broad pH range. Lipophilicity was assessed by both shake flask method with 1-octanol-water system and a reverse-phase HPLC system with methanol-water as mobile phase. The former method was used for determining the lipophilicities of the ionized forms (log D) of the dimers while the latter method was used for that of the neutral forms (log P). The log P values for B12H and B7T were found to be 5.4 and 8.2, respectively, indicating that the two dimers are highly lipophilic. The solubilities of both dimers were found to be affected by pH. The solubility of B12H was >1.41 mg/ml when the pH was <7, but <0.06 mg/ml when the pH was >8. The solubility of B7T was >0.26 mg/ml when the pH was <9, but <0.005 mg/ml when the pH was >12. The ionic strength of a solution could affect the solubilities considerably (11.16 mg/ml for B12H and 12.71 mg/ml for B7T in water; 2.07 mg/ml for B12H and 0.36 mg/ml for B7T in saline). The ionization constants (pK(a)) of the two dimers were determined by UV spectrophotometry. Both dimers were found to have two pK(a) values: 7.5+/-0.1 (pK(a1)) and 10.0+/-0.2 (pK(a2)) for B12H; and 8.7+/-0.1 (pK(a1)) and 10.7+/-0.4 (pK(a2)) for B7T. Furthermore, an in vivo pharmacological assay conducted in mice showed that a maximum AChE inhibition occurred 15 min after the single-dose and intraperitoneal administration of either dimer. This indicates that the two dimers may easily cross the blood-brain barrier. In summary, these physiochemical characteristics suggest that the two dimers may be promising candidates for the development of better drugs for Alzheimer's disease.     

通过喹诺酮抗生素与磷脂膜的相互作用预测肺泡巨噬细胞单层的体外摄取

目的;预测肺泡巨噬细胞单层的体外摄取.方法:以培养的肺泡巨噬细胞单层为体外模型,用磷脂膜色谱,脂质体/水系统评价药物与磷脂膜的相互作用,分别表示为lg k_(1AM),lg D_(L B,7.4),用正辛醇/水系统测定参考的疏水性参数(lg D_(O/B,7.4).结果:lg D_(L B,7.4)(r~2=0.93)比lg D_(O B,7.4)(r~2=0.65)具有与lg k_(1AM)更显著的相关性.lg k_(1AM)和lg D_(L B,7.4)均 比lg D_(O/B,7.4)与细胞内药物的蓄积度有更好的相关性.但对于由5个两性喹酮抗生素和奎尼丁组成的受试集合,三者均与药物进入细胞内的速度具有相近的显著的正相关性.结论:磷脂膜色谱和脂质体/水系统给出相似的亲脂性测量尺度,且均与正辛醇/水系统区别有显著性.与疏水性参数相比,膜亲和性是更有效的肺泡巨噬细胞内药物蓄积和结合的预测参数.     

The influence of nonspecific microsomal binding on apparent intrinsic clearance, and its prediction from physicochemical properties.

The apparent intrinsic clearance of 13 drugs has been determined using rat liver microsomes at three different concentrations of microsomal protein. The kinetics was studied using the in vitro half-life method. The nonspecific binding of these drugs to the microsomes was also studied under the same conditions, except for cofactor removal, using equilibrium dialysis. The intrinsic clearances are shown to be dependent on the microsomal concentration, but are approximately constant when corrected for the extent of nonspecific binding to the microsomes. The large difference between observed intrinsic clearance and unbound intrinsic clearance that exists for some compounds, particularly lipophilic bases, is highlighted. A simple model has been developed for understanding the binding of compounds to microsomes and is demonstrated to accurately predict the extent of microsomal binding at one concentration of microsomes from measurement at another. The binding of a further 25 drugs to rat liver microsomes at a microsomal concentration of 1 mg/ml was also studied, along with measurements of lipophilicity using octanol-water partition coefficients. It is shown that the extent of microsomal binding is correlated with lipophilicity, but that basic compounds show a different behavior to acidic and neutral compounds. Microsomal binding is shown to be best predicted using a model where log P is used for basic compounds, and log D(7.4) is used for acidic and neutral compounds. This model has been developed further so that the extent of binding to microsomes of any given concentration can be estimated purely from a knowledge of lipophilicity and ionization.     

Physicochemical profile of nimesulide. Exploring the interplay of lipophilicity, solubility and ionization

The lipophilicity and solubility profile of nimesulide was investigated over a broad pH range. Lipophilicity was assessed by direct partitioning experiments in the octanol-water system using the shake flask method, as well as by reversed-phase HPLC using methanol as organic modifier with or without addition of n-octanol. In the latter case the extrapolated retention factors logk(w) were considered as lipophilicity indices. The presence of n-octanol in the mobile phase proved to be a crucial factor for the establishment of a logk(w)/pH profile very similar to the logD/pH profile of nimesulide. Solubility was determined by the shake flask method using saturated buffer solutions. Both lipophilicity and solubility--pH profiles of nimesulide showed deviations from the theoretically expected behavior as dictated by the Henderson-Hasselbach equation and the usually recorded difference of 4 log units between the corresponding values of the neutral and ionized species in the case of a weak acid. As a consequence the lipophilicity and solubility profiles were found not to be mirror images of each other. However, the pK(a) value of nimesulide could be accurately calculated using part of both lipophilicity and solubility profiles since deviations affected mostly the values at increased ionization.