Ionization constants by curve fitting: determination of partition and distribution coefficients of acids and bases and their ions

A convenient procedure has been developed for the determination of partition and distribution coefficients. The method involves the potentiometric titration of the compound, first in water and then in a rapidly stirred mixture of water and octanol. An automatic titrator is used, and the data is collected and analyzed by curve fitting on a microcomputer with 64 K of memory. The method is rapid and accurate for compounds with pKa values between 4 and 10. Partition coefficients can be measured for monoprotic and diprotic acids and bases. The partition coefficients of the neutral compound and its ion(s) can be determined by varying the ratio of octanol to water. Distribution coefficients calculated over a wide range of pH values are presented graphically as "distribution profiles". It is shown that subtraction of the titration curve of solvent alone from that of the compound in the solvent offers advantages for pKa determination by curve fitting for compounds of low aqueous solubility.     

Ionization constants and water solubilities of some aminoalkylphenothiazine tranquillizers and related compounds

A simple graphical method is described for deriving the ionization constants of poorly soluble organic bases from the pH dependence of the water solubility. Ionization constants and water solubilities of 13 aminoalkylphenothiazines and related drugs have been determined using this procedure. Potent tranquillizing activity in this group of compounds is shown to be associated with a low water solubility at blood pH.     

芦丁的溶解度及油水分配系数研究

目的研究芦丁在不同介质中的溶解度和油水分配系数,为设计芦丁渗透泵控释制剂提供基础。方法以饱和法测定溶解度,以超声振摇法测定芦丁在正辛醇和水相中的萃取平衡浓度比,计算油水分配系数（P）。结果芦丁在水中的溶解度为194.57mg/L,在2%十二烷基硫酸钠（SDS）溶液中的溶解度为863.17mg/L,在其他不同pH的磷酸盐缓冲溶液中随着pH的升高逐渐升高;油水分配系数随着pH的升高而逐渐降低。结论芦丁为难溶性药物,其溶解度和油水分配系数都与pH有一定的关系;芦丁溶解度和油水分配系数的测定可为进一步研究芦丁渗透泵片提供基础。     

Temperature,medium and structural effects on the acid dissociation constants of certain Schiff bases derived from isatin with some amino acids and aroylhydrazines

The acid dissociatioin constants of certain Schiff bases derived from isatin with p-substituted benzoyl hydrazines and somee amino acids of glycine, β- and α-alanine, valine, β-phenyl-α-alanine and anthranilic acid have been determined potentiometrically at different temperatures in different aquo-organic solvent mixtures (ethanol, dioxane, ddimethyl formamide, methanol, acetone and tetrahydrofurance). The pKa values were demonstrated on the light of the different electronic and steric effects of the substituents and the solvent characteristics. In all the mixed media used, ionization of the compounds decreased by increasing the mole fraction of the organic cosolvent. Theermodynamic parameters (ΔH°, ΔG°, ΔS°) were evaluated. The structural effects of the investigated compounds on these parameters were reported and discussed.     

Reversed-phase high-performance liquid chromatography for evaluating the distribution of pharmaceutical substances in suppository base-phosphate buffer system

The aim of this study was to assess the potency of the reversed-phase high-performance liquid chromatography (RP-HPLC) for in vitro evaluation of the distribution behavior of common drugs between one of the generally used suppository bases Witepsol H₁₅ and the rectal liquid which is imitated by a phosphate buffer, pH 7.2. The distribution coefficients (log K) of nine compounds — paracetamol, caffeine, diclofenac, propyphenazone, indomethacin, codeine base, codeine phosphate, phenobarbital acid and phenobarbital sodium salt were determined by the classical ‘shake-flask’ method followed by RP-HPLC quantitative assay. The capacity factors log k′ of the compounds were determined on reversed-phase C₁₈ column at a number of methanol–5 mM phosphate buffer, pH 7.2 mobile phases containing different percentages of methanol (φ_MeOH). The apparent capacity factors log k′_w^app were derived by extrapolation of the methanol concentration to zero and using the correction for ionization, the real capacity factors log k′_w were calculated. The lipophilicity of the compounds was assessed by the partition coefficients CLOGP and the distribution coefficients CLOGD_7.2, calculated for the n-octanol–water system. Correlations between log k′_w and CLOGP, log k′_w^app and CLOGD_7.2, log k′_w^app and log K were found. The last correlation indicated that the parameter log k′_w^app was suitable for evaluating the distribution behavior of the studied drugs in the examined Witepsol H₁₅-rectal liquid system. The predictive power of this correlation was tested by a set of nine non-congeners. It was shown that the classical ‘shake-flask’ method for determination of the distribution behavior of the studied drugs between the suppository base Witepsol H₁₅ and the phosphate buffer, pH 7.2 might be replaced by the RP-HPLC technique due to its priorities of rapid, stable and reproducible experiments.     

Relationships between octanol-water partition data, chromatographic indices and their dependence on pH in a set of beta-adrenoceptor blocking agents

A thorough investigation on the dependence of octanol/water distribution coefficients (log D) of basic and amphiprotic beta-blocker drugs on pH and pKa values has been carried out. An attempt is made to clarify the factors which govern the chromatographic parameters (log k') in reversed-phase high performance liquid chromatography (RPLC) in terms of chemical structure and of distribution coefficients. The correlations between log D and log k' of different compounds show that the bulkiness and the branching of nitrogen substituent are controlling factors in RPLC measurements. Finally, an ion pair chromatographic system is considered for the prediction of octanol/water partition coefficients.     

Substituent effects on the ionization and partitioning of p-(aminoethyl)phenols and structurally related compounds: Electrostatic effects dependent on conformation

Computational methods to predict pK_a values and partition coefficients of drug molecules based on linear free energy relationships (LFERs) rely largely on the principles of independence and additivity of the functional group contributions in each molecule to the overall free energy. Nonadditivities in functional group contributions are often seen when multiple polar functional groups are in close proximity and in cases where conformational flexibility allows widely separated polar functional groups to interact. The degree to which long-range interactions may alter group contributions in more conformationally constrained molecules such as p-(aminoethyl)phenol and structurally similar analogs is more difficult to predict. In this study, both macroscopic and microscopic ionization constants and decadiene/water partition coefficients as a function of pH at 25°C were obtained for p-(aminoethyl)phenol and six structurally related compounds to explore the reliability of the independence and additivity assumptions necessary in using the LFERs for predictions. A long-range interaction between the phenol and amine groups in the series has been found to affect the pK_a values of both groups and to alter species-specific partition coefficients. Pronounced shifts in microscopic ionization constants involving zwitterion forms are clearly indicative of amplified long-range interactions between ionized substituents. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 98:4534–4544, 2009     

Isoelectric points of some sulfonamides: determination by microelectrophoresis and by calculations involving acid-base strength

The isoelectric points of four very slightly soluble sulfonamides were measured by microelectrophoresis of dilute suspensions as a function of pH. Ionic strength and pH were adjusted with KCl, KOH, and HCl only. The isoelectric points were also calculated from published values of acid and basic ionization constants which had been determined by potentiometric titration, and from changes in ultraviolet absorption spectra and in solubility as a function of pH. Including one sulfonamide whose isoelectric point as measured by microelectrophoresis was published, rather good agreement between the two methods was observed for all but one compound. All values were between 3.5 and 4.6, indicating that the sulfonamides function as weak acids rather than as amphoteric compounds at physiological pH.     

Ionization constants by curve fitting: application to the determination of partition coefficients

A multiparametric curve-fitting technique for pKa calculation has been adapted for use with a programmable calculator or microcomputer. This method provides for the convenient and accurate determination of the ionization constant in aqueous solution and of the apparent ionization constant in the presence of octanol. From these parameters, partition coefficients and apparent partition coefficients are easily calculated and agree with data reported using the shaker technique or HPLC. The curve-fitting method has been applied to the differential titration technique in which the solvent curve is subtracted from the solution curve before calculations are begun. This method has been applied to the potentiometric titration of aqueous solutions of the salts of bases with a very low solubility in water.     

Narcotic and narcotic antagonist pKa''s and partition coefficients and their significance in clinical practice

The pKa's, partition coefficients and drug distribution coefficients (apparent partition coefficients) have been investigated for a number of narcotics and where possible for their congener narcotic antagonist. These studies were carried out by a novel microelectrometric titration technique as a function of temperature and pH. This method enables one to determine not only the dissociation constants to deconvolute overlapping pKa's, but also to determine the solubilities and oil-water distribution of these various drugs. The drug distribution coefficients displayed marked sensitivity to pH at values which span the range of attainable human physiological pH values. This has significant pharmacological implications for proper choice and scaling of drug dosages under various clinical situations among which are cited hyperventilation under a general anesthetic while concomitantly under a narcotic analgesic, obstetetrical analgesia, and medical and anti-abuse usage of narcotic antagonists. The partition coefficients and drug distribution coefficients were noticeably different at 20degreesC (where such measurements are customarily made) from those at 37degreesC (body temperature). Furthermore, various drugs exhibit very non-equivalent increases in drug distribution coefficients with increasing temperature, ranging from 21% for naltrexone. This non-regularity indicates that it will not be valid to extrapolate by any constant factor the measurements made at lower temperatures. Even the true partition coefficients increase with temperature from 20degrees to 37degreesC.     

Determination of piribenzil in urine containing a metabolite and dextropropoxyphene by an ion pair extraction method

Piribenzil, an antispasmodic quaternary ammonium compound, has been determined quantitatively in human urine containing a metabolite of piribenzil and dextropropoxyphene. Ion pair extraction with bromothymol blue as counter ion was used and the extraction conditions were calculated from extraction constants and partition coefficients determined using methylene chloride as organic solvent. The determination was made by photometry in the range 5–50 μg of piribenzil methyl sulphate/ml of urine. The recovery was 97% ± 3%. The co-extraction of urine components such as nicotine, tyramine, tryptamine and choline was investigated by determination of constants for the partition of these compounds.     

盐酸苯环壬酯油水分配系数的测定及pH值对其的影响

目的： 测定盐酸苯环壬酯油水分配系数,研究不同pH值对药物脂水分配系数的影响,为盐酸苯环壬酯新剂型的处方设计提供理论参考。方法： 本实验采用摇瓶-高效液相色谱法测定盐酸苯环壬酯在不同pH值条件下正辛醇-水/缓冲液体系中的表观脂水分配系数。结果： 盐酸苯环壬酯的脂水分配系数lgP值为1.29±0.16;在pH为1.2,3.0,4.5,5.0,5.5,6.0,6.8,7.0,8.0,9.0的环境中,脂水分配系数lgP值分别为0.31,0.72,1.16,1.34,1.99,2.26,2.22,1.61,2.34,3.24。结论： 盐酸苯环壬酯的脂水分配系数与介质的pH值有关,在pH 4.5~8.0生理条件下,盐酸苯环壬酯的脂溶性较好,水溶性差。     

Buccal uptake,urinary excretion,and physicochemical properties of zipeprol and its N-dealkylated products

Buccal uptake, partition characteristics, ionization constants, and urinary excretion were studied for zipeprol and two of its metabolities. Buccal uptake, measured in a single subject, was found to be correlated with n-heptane/water (ph 7.4) partition values of the compounds, and with proportional excreation of the unchanged drug. The importance of the shape of buccal uptake-pH curves in the prediction of absorption and excretion of these compounds is discussed.     

Microelectrometric titration measurement of the pKa's and partition and drug distribution coefficients of narcotics and narcotic antagonists and their pH and temperature dependence.

The pKa's, partition coefficients, and drug distribution coefficients (apparent partition coefficients) have been investigated for a number of narcotics and, where possible, for their congener narcotic antagonists. These studies were carried out by a microelectrometric titration technique as a function of temperature and pH. This method enables one to determine not only the dissociation constants to deconvolute overlapping pKa's, but also to determine the solubilities of oil-water distribution of these various drugs. The drug distribution coefficients displayed marked sensitivity to pH at values which span the range of attainable human physiological pH values. This has significant pharmacological implications for proper choice and scaling of drug dosages under various clinical situations. The partition coefficients and drug distribution coefficients were noticeably different at 20 degrees (where such measurements are customarily made) than at 37 degrees (body temperature). Furthermore, various drugs exhibit very nonequivalent increases in drug distribution coefficients with increasing temperature, ranginf from 21% for morephine to 200% for naltrexone. This nonregularity indicates that it will not be valid to extrapolate by any constant factor the measurements made at lower temperatures. Even the true partition coefficients increase with temperature from 20 degrees to 37 degrees. There is more of a difference in the drug distribution coefficients for naloxone and naltrexone than might have been expected from the similarities in their structures with naltrexone being significantly less lipophilic than naloxone. This would imply that this would lead to naloxone having a more rapid onset for antagonist activity and likewise a shorter duration of action than naltrexone.     

醋酸氯地孕酮平衡溶解度及表观油水分配系数的测定

目的:测定醋酸氯地孕酮(CA)的平衡溶解度及表观油水分配系数,为CA新剂型的体外评价进行处方前研究。方法:采用高效液相色谱法测定CA在水及不同pH(1·2、2·0、3·0、4·5、5·5、6·8、7·4、8·0)介质及常用9种有机溶剂(包括甲醇、乙醇等)中的平衡溶解度;采用摇瓶法测定其在正辛醇与水及各pH介质组成的体系中的表观油水分配系数(P)。结果:37℃时,CA在水及9种pH值介质中的平衡溶解度依次为0·473、0·533、0·423、0·056、0·309、0·428、0·447、0·428、0·448μg·mL-1,在甲醇、乙醇等中的平衡溶解度为7320·61～344248·70μg·mL-1;CA在水中的lgP值为4·05,不同pH条件下的lgP值差别不大(4·11～4·25)。结论:CA不溶于水,易溶于有机溶剂,在弱酸环境下有一定程度的降解;且其lgP值较大,提示在制剂研究中需采用适当的增溶手段。     

Physico-chemical profiling of centrally acting molecules for prediction of pharmacokinetic properties

Physico-chemical profiling is a fundamental tool at the early stage of drug discovery in screening drug-like candidates. Complex physico-chemical profiling, including molecular properties such as solubility, ionization, lipophilicity and permeability, has been found to be of predictive power in ADME (absorption, distribution, metabolism, elimination). In the present thesis work, the physico-chemical properties of centrally acting compounds were investigated. We determined the protonation constants (K), the partition coeffitient in octanol/water (Poct) and cyclohexane/water (Pch) systems of antidepressive sertraline and 15 antipsychotic piperidine and piperazine derivatives and calculated the delta logP (logPoct-logPch) values of the molecules. Due to the poor water solubility of the compounds potentiometry using the "co-solvent" technique was applied for the determination of the protonation constants. The logP values were measured by the dual-phase potentiometric titration in octanol/water system and the traditional shake-flask method was used in cyclohexane/water system. Highly precise physico-chemical data were obtained by these validated methods. The relationship between the structure of the molecules and the physico-chemical data was investigated. The pharmacokinetic properties of the compounds were predicted by the physico-chemical parameters. Linear relationship has been found between the brain penetration characterized by the logBB values and the delta logP values. The validity of the equation was controlled by the delta logP and the logBB values of sertraline.     

Partitioning behaviour of local anesthetic fomocaine derivatives.

The octanol/water partitioning behaviour of local anesthetic fomocaine derivatives has been investigated. The true partition coefficients obtained from log P(app) values reflect well the high lipophilicity of these aminoalkylaryl-ether type compounds. A consequence of this is that the partitioning of the protonated form is not negligible as was proved by the log P(i) values of the ionized molecule measured at pH 4.0. The effect of ionization on partitioning was studied and characterized by the pQ values. This ionization influence on lipophilicity was found quite constant (pQ approximately 4) among the hydroxypiperidine analogs. A parabolic relationship was obtained between the acute toxicity and the log P values of several of the examined compounds.     

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.     

A membrane model of the human oral mucosa as derived from buccal absorption performance and physicochemical properties of the β-blocking drugs atenolol and propranolol

The buccal absorption characteristics and physicochemical properties of the β-adrenoceptor blocking agents propranolol and atenolol have been investigated to evaluate their permeation properties across biological lipid membranes. The dissociation constants, solubilities of free base, and n-heptane partition coefficients show that propranolol in its unionized form is much more lipophilic than atenolol, both drugs being bases with a similar pKa. Buccal absorption was studied under conditions of varying drug concentration, contact time, and pH, and controlled through the use of a non-absorbable marker. The absorption findings are in general agreement with the pH-partition theory. A new compartmental diffusional model that includes membrane storage and a hypothetical ‘aqueous pH-buffering surface system’ allowed a more exhaustive interpretation to be made. A method for the estimation of the intrinsic pH and buffer capacity of the postulated surface system from drug pH-absorption data and partition coefficients alone is described. With human oral mucosa the intrinsic pH was near 6·7, and the buffering capacity of the system (expressed as the ratio ΔpH/ΔpH eff) about 2·86. The method was validated using published absorption data from the rat small intestine. Absorption of unionized drug through pores is shown to be negligible in the buccal absorption situation. The time course of absorption suggests membrane storage of lipophilic compounds; the in vivo partition coefficient of unionized propranolol relative to the mucous membrane could be calculated for the peusdo-steady state of absorption, i.e. the partition equilibrium between mouth content and membrane, to be approximately 776; this value is of the same order as the in vitro partition coefficient for the erythrocyte/plasma system. The lipid biophase of the buccal membrane is estimated semiquantitatively to be of intermediate polarity (? = 3–4).     

Synthesis, biological evaluation, and quantitative structure-activity relationship analysis of new Schiff bases of hydroxysemicarbazide as potential antitumor agents.

Thirty Schiff bases of hydroxysemicarbazide (Ar-CH=NNHCONHOH) have been synthesized and tested against L1210 murine leukemia cells. The IC(50) values were found to be in a range from 2.7 x 10(-6) to 9.4 x 10(-4) M. A total of 17 out of the 30 compounds had higher inhibitory activities than hydroxyurea (an anticancer drug currently used for the treatment of melanoma, leukemia, and ovarian cancer) against L1210 cells. Six compounds with IC(50) values in micromolar range were 11- to 30-fold more potent than hydroxyurea (IC(50) = 8.2 x 10(-5) M). The partition coefficient (log P) and ionization constants (pK(a)) of a model compound [1-(3-trifluoromethylbenzylidene)-4-hydroxysemicarbazide, 1] were measured by the shake-flask method, and the measured log P was used to derive Hansch-Fujita pi constant of -CH=NNHCONHOH. On the basis of the newly derived pi and those of other moieties, the partition coefficients (SlogP) of the other 29 compounds were calculated by the summation of pi values. Quantitative structure-activity relationship (QSAR) analysis showed that, besides the essential pharmacophore (-NHCONHOH), hydrophobicity (SlogP), molecular size/polarizability (calculated molar refractivity), and the presence of an oxygen-containing group at the ortho position (I) were important determinants for the antitumor activities. In conclusion, the results obtained in this study show that several Schiff bases of hydroxysemicarbazide are potent inhibitors of tumor cells and warrant further investigation as cancer chemotherapeutic agents.