Correlations between alkane/water and octan-1-ol/ water distribution coefficients and isocratic reversed-phase liquid chromatographic capacity factors of acids, bases and neutrals

Prediction of Partition Coefficients of Environmental Toxins Using Computational Chemistry Methods

The partitioning of compounds between aqueous and other phases is important for predicting toxicity. Although thousands of octanol-water partition coefficients have been measured, these represent only a small fraction of the anthropogenic compounds present in the environment. The octanol phase is often taken to be a mimic of the inner parts of phospholipid membranes. However, the core of such membranes is typically more hydrophobic than octanol, and other partition coefficients with other compounds may give complementary information. Although a number of (cheap) empirical methods exist to compute octanol-water (log k _OW) and hexadecane-water (log k _HW) partition coefficients, it would be interesting to know whether physics-based models can predict these crucial values more accurately. Here, we have computed log k _OW and log k _HW for 133 compounds from seven different pollutant categories as well as a control group using the solvation model based on electronic density (SMD) protocol based on Hartree-Fock (HF) or density functional theory (DFT) and the COSMO-RS method. For comparison, XlogP3 (log k _OW) values were retrieved from the PubChem database, and KowWin log k _OW values were determined as well. For 24 of these compounds, log k _OW was computed using potential of mean force (PMF) calculations based on classical molecular dynamics simulations. A comparison of the accuracy of the methods shows that COSMO-RS, KowWin, and XlogP3 all have a root-mean-square deviation (rmsd) from the experimental data of ≈0.4 log units, whereas the SMD protocol has an rmsd of 1.0 log units using HF and 0.9 using DFT. PMF calculations yield the poorest accuracy (rmsd = 1.1 log units). Thirty-six out of 133 calculations are for compounds without known log k _OW, and for these, we provide what we consider a robust prediction, in the sense that there are few outliers, by averaging over the methods. The results supplied may be instrumental when developing new methods in computational ecotoxicity. The log k _HW values are found to be strongly correlated to log k _OW for most compounds.

Structural determinants of drug partitioning in n-hexadecane/water system

Surrogate phases have been widely used as correlates for modeling transport and partitioning of drugs in biological systems, taking advantage of chemical similarity between the surrogate and the phospholipid bilayer as the elementary unit of biological phases, which is responsible for most of the transport and partitioning. Solvation in strata of the phospholipid bilayer is an important drug characteristic because it affects the rates of absorption and distribution, as well as the interactions with the membrane proteins having the binding sites located inside the bilayer. The bilayer core can be emulated by n-hexadecane (C16), and the headgroup stratum is often considered a hydrophilic phase because of the high water content. Therefore, we tested the hypothesis that the C16/water partition coefficients (P) can predict the bilayer locations of drugs and other small molecules better than other surrogate systems. Altogether 514 PC16/W values for nonionizable (458) and completely ionized (56) compounds were collected from the literature or measured, when necessary. With the intent to create a fragment-based prediction system, the PC16/W values were factorized into the fragment solvation parameters (f) and correction factors based on the ClogP fragmentation scheme. A script for the PC16/W prediction using the ClogP output is provided. To further expand the prediction system and reveal solvation differences, the fC16/W values were correlated with their more widely available counterparts for the 1-octanol/water system (O/W) using solvatochromic parameters. The analysis for 50 compounds with known bilayer location shows that the available and predicted PC16/W and PO/W values alone or the PC16/O values representing their ratio do not satisfactorily predict the preference for drug accumulation in bilayer strata. These observations indicate that the headgroups stratum, albeit well hydrated, does not have solvation characteristics similar to water and is also poorly described by the O/W partition characteristics.

Novel RPLC stationary phases for lipophilicity measurement: Solvatochromic analysis of retention mechanisms for neutral and basic compounds

An RPLC was developed to rapidly determine lipophilicity of neutral and basic compounds using three base deactivated RPLC stationary phases particularly designed for the analysis of basic compounds, namely, Supelcosil ABZ(+)Plus, Discovery RP Amide C16, and Zorbax Extend C18. The work consisted of three sets of experiments. In the first log kw values of neutral compounds were extrapolated using hydroorganic mobile phases at different compositions. Good correlation between log kw and log Poct indicated that the method was appropriate for these supports, without adding a silanol masking agent. In the second set of experiments, isocratic log k values of neutral and basic compounds were measured with three different mobile phases. The best estimation of lipophilicity was obtained for neutral and basic compounds when the secondary interactions were strongly reduced (i. e., when basic compounds were under their neutral form). In the third set of experiments, isocratic retention factors of basic compounds (in their neutral form) were measured with a high-pH mobile phase, on a chemically stable support (Zorbax Extend C18). Under these chromatographic conditions, correlation between the isocratic retention factors and log Poct (log D10.5) for basic compounds was similar to that for neutral compounds.

Lipophilicity evaluation by RP-HPLC of two homologous series of methotrexate derivatives

A correlation between the structure and lipophilicity has been carried out within the homologous members of two series of aliphatic bis(amides) and lipoamino acid conjugates of the anticancer drug methotrexate. Basing on their reversed-phase HPLC behaviour, the values of the experimental parameter RQ, obtained by using different mobile phases with increasing concentration of acetonitrile, were considered as a parameter of lipophilicity of the tested compounds. A comparison with their calculated log P was also performed.

A study of the relationship between the structure and physicochemical parameters of a homologous series of oxprenolol esters at various pH values and temperatures

A number of beta-adrenergic blockers, including timolol and propranolol, are administered in eyedrops for the treatment of glaucoma, but their therapeutic value is limited by a relatively high incidence of cardiovascular and respiratory side effects. Because of poor ocular bioavailability, many ocular drugs are applied in high concentrations, which give rise to both ocular and systemic side effects. Methods to increase ocular bioavailability include (a) the development of drug delivery devices designed to release drugs at controlled rates, (b) the use of various vehicles that retard precorneal drug loss, and (c) the conversion of drugs to biologically reversible derivatives (prodrugs) with increased corneal penetration properties, from which the active drugs are released by enzymatic hydrolysis. A homologous series of aliphatic esters of oxprenolol were synthesized and investigated as potential prodrugs for ocular use. The stability of each O-acyl derivative was investigated in aqueous solutions over the pH range 2.2-9.0 at 37 degrees C. The observed rate constants (k[obs]), shelf-lives (t90), lipophilicities, and Arrhenius parameters were determined for each ester. A study of the relationship between the structure and physicochemical parameters of the homologous series of oxprenolol esters at various pH values and temperatures was made.

Synthesis, physical properties, toxicological studies and bioavailability of L-pyroglutamic and L-glutamic acid esters of paracetamol as potentially useful prodrugs

Paracetamol ester prodrugs with L-pyroglutamic and L-glutamic acid, biosynthetic precursors of glutathione, have been synthesized to reduce paracetamol hepatotoxicity and improve bioavailability. The toxicological studies of paracetamol esters show that only L-5-oxo-pyrrolidine-2-paracetamol carboxylate reduces toxicity after administration of an overdose. The glutathione hepatic values in mice obtained by intraperitoneal injection of the ester are superimposable on controls and the oral LD50 was found to be greater than 2000 mg kg-1 and the intraperitoneal LD50 was 1900 mg kg-1. These results taken together with hydrolysis and bioavailability data show that ester is a potential candidate as a prodrug of paracetamol.

Lipophilicity influence on conjunctival drug penetration in the pigmented rabbit: a comparison with corneal penetration

The influence of lipophilicity on the conjunctival penetration of beta blockers in the pigmented rabbit was investigated and compared with that on corneal penetration. The beta blockers were hydrophilic sotalol, atenolol, nadolol, pindolol, and acebutolol; lipophilic metoprolol, timolol, oxprenolol, levobunolol, labetalol, and alprenolol; and the very lipophilic propranolol and betaxolol. Drug penetration was evaluated by using the isolated pigmented rabbit conjunctiva and cornea in the modified Ussing chamber and was monitored by reversed phase HPLC. The conjunctiva was more permeable to all the beta blockers than was the cornea. A sigmoidal relationship, rather than the familiar parabolic relationship, best described the influence of lipophilicity on both conjunctival and corneal drug penetration. The ratio of corneal to conjunctival permeability coefficients was most sensitive to changes in log PC within the region of 1.5 and 2.5. Outside of this region, the ratio was relatively independent of changes in lipophilicity. For several beta blockers, their intrinsic sympathomimetic activity may play a minor role in influencing their conjunctival and corneal penetration.

Glycolamide esters as biolabile prodrugs of carboxylic acid agents: synthesis, stability, bioconversion, and physicochemical properties

Benzoic acid esters of various substituted 2-hydroxyacetamides (glycolamides) were found to be hydrolyzed extremely rapidly in human plasma solutions, the half-lives of hydrolysis being less than 5 s in 50% plasma solutions for some N,N-disubstituted glycolamide esters. The rapid rate of hydrolysis could be largely attributed to cholinesterase (also called pseudocholinesterase) present in plasma. From a study of a variety of substituted glycolamide esters and structurally related esters, the most prominent structural requirement needed for a rapid rate of hydrolysis was found to be the glycolamide ester structure combined with the presence of two substituents on the amide nitrogen atom. A structural similarity of such esters with benzoylcholine, a good substrate for cholinesterase, was put forward. Esters of N,N-disubstituted glycolamides are suggested to be a useful biolabile prodrug type for several carboxylic acid agents. The esters combine a high susceptibility to undergo enzymatic hydrolysis in plasma with a high stability in aqueous solution. Furthermore, as demonstrated with the benzoic acid model esters, it is feasible to obtain ester derivatives with almost any desired water solubility or lipophilicity with retainment of marked lability to enzymatic hydrolysis.

Water solubility: a critique of the solvatochromic approach

The reasoning and some of the assumptions behind the solvatochromic approach to water solubility are evaluated, particularly in relation to its claims. It is shown that the contribution of the pi term is uncertain, and that it can be dropped from the correlation without substantially affecting the degree of fit. The solvatochromic method is compared with the long-standing relationship between solubility and the octanol-water partition coefficient, and it is demonstrated that the latter offers a far superior route to solubility estimation.

Correlation between reversed-phase high-performance liquid chromatography and plasma protein binding

As hydrophobic interactions are involved in both reversed-phase liquid chromatography and plasma protein binding, the relationship between retention time and binding was investigated experimentally in two series of compounds. For betaxolol and its O-alkyl analogues, the nature of the O-alkyl group strongly influences the retention time on a Spherisorb CN 5-microns column. With 0.03 M acetate buffer (pH 5.6)-acetonitrile (60:40) as the mobile phase, k' values increase from 1.8 to 8.3 with a concomitant increase in plasma protein binding from 0.5% (R = H) to 88.2% (R = cyclopentylmethyl). The relationship between the free fraction and log k' is sigmoidal. In the second example, structural changes in the propyl side-chain of alpidem (a new anxiolytic) lead to minor variations in the protein binding: 98.9 to 84.9%. This slight decrease with the more polar metabolite is correlated with a sharp decrease in the k' values from 14.7 to 0.94 on a Supelcosil LC 18 DB column. Based on retention times, it should be feasible to predict qualitatively, if not quantitatively in some instances, plasma protein binding in a series of structurally similar compounds.

Determination of hydrophobic parameters by reversed-phase liquid chromatography: theory, experimental techniques, and application in studies on quantitative structure-activity relationships

The use of RPLC retention parameters as descriptors of the hydrophobic nature of bioactive compounds has been evaluated. The relationship between the capacity factor, measured on n-alkyl-bonded stationary phases using binary eluents, and the n-octanol-water partition coefficient has been illustrated experimentally and theoretically. It is suggested that retention parameters, in particular the capacity factor (log kw) obtained by extrapolation of retention data from binary eluents to 100% water, could successfully replace the n-octanol-water partition coefficient in studies on quantitative structure-activity relationship, and that their use may result in a better correlation with biological data.

Pilocarpine prodrugs. II. Synthesis, stability, bioconversion, and physicochemical properties of sequentially labile pilocarpine acid diesters

Various novel diesters of pilocarpic acid were synthesized and evaluated as prodrug forms for pilocarpine with the aim of improving the ocular delivery characteristics of the drug. The pilocarpic acid monoesters previously studied cyclized spontaneously to pilocarpine in aqueous solution and although they showed enhanced corneal permeability when compared with pilocarpine these monoesters suffered from poor solution stability. The present study shows that this problem can be totally overcome by blocking the free hydroxyl group in the monoesters. Diesters of pilocarpic acid were obtained by esterification of this group. Such compounds were found to possess a high stability in aqueous solution (shelf lives of more than 5 years at 20 degrees C were estimated) but at the same time were readily converted to pilocarpine under conditions simulating those occurring in vivo through a sequential process involving enzymatic hydrolysis of the O-acyl bond followed by spontaneous lactonization of the intermediate pilocarpic acid monoester. Rate data are given for the conversion of the diesters in human plasma and in various rabbit eye homogenates. The pH-solubility profile was derived for a diester and lipophilicity parameters were determined for the compounds. All diesters were markedly more lipophilic than pilocarpine and the corresponding pilocarpic acid monoesters. The results suggest that pilocarpic acid diesters may be potentially useful pilocarpine prodrugs as they combine a high solution stability with an adequate rate of conversion to pilocarpine under in vivo conditions.

Prodrugs of 5-fluorouracil. V. 1-Alkoxycarbonyl derivatives as potential prodrug forms for improved rectal or oral delivery of 5-fluorouracil

The hydrolysis and physicochemical properties of seven 1-alkoxycarbonyl derivatives of 5-fluorouracil were studied to assess their potential as prodrugs with the aim of enhancing the delivery characteristics of the parent drug. All derivatives were hydrolyzed to quantitatively yield 5-fluorouracil. The pH-rate profiles for the hydrolysis were measured. The rates of hydrolysis were markedly accelerated in the presence of plasma, affording half-lives of hydrolysis less than 4 min in 80% human plasma. The derivatives were more lipophilic than 5-fluorouracil but the aqueous solubility was only slightly reduced or, for one derivative, even greater than that of 5-fluorouracil. A preliminary absorption study in rabbits showed an absolute bioavailability of 5-fluorouracil of 100% following rectal administration of 1-(butoxycarbonyl)-5-fluorouracil as compared with no absorption following administration of 5-fluorouracil itself. The potential utility of the prodrug derivatives to enhance the oral and/or rectal delivery of the parent drug is suggested.

Pilocarpine prodrugs I. Synthesis, physicochemical properties and kinetics of lactonization of pilocarpic acid esters

Various alkyl and aralkyl esters of pilocarpic acid were synthesized and evaluated as prodrug forms for pilocarpine with the purpose of improving the ocular bioavailability of pilocarpine through increased corneal membrane permeability. The esters were found to undergo a quantitative cyclization to pilocarpine in aqueous solution of pH 3.5-10, the rate of cyclization being a function of the polar and steric effects within the alcohol portion of the esters. The rates of lactonization increased proportionally with the hydroxide ion activity over the pH range studied which is in accord with a reaction mechanism involving intramolecular nucleophilic attack of alkoxide ion on the ester carbonyl moiety. At pH 7.4 and 37 degrees C, half-times of lactonization ranging from 30 min (p-chlorobenzyl ester) to 1105 min (n-hexyl ester) were observed for the various esters. The esters are markedly more lipophilic than pilocarpine. The results suggested that the pilocarpic acid esters may be potentially useful prodrugs, especially when further derivatized to give in vitro stable pilocarpic acid diesters.