Distribution of hydrophobic ionizable xenobiotics between water and lipid membranes: pentachlorophenol and pentachlorophenate. A comparison with octanol-water partition

Evaluation of the EMA log kow trigger for fish BCF testing based on data for several human pharmaceuticals

In the European Medicines Agency (EMA) "Guideline for Environmental Risk Assessment of Medicinal Products for Human Use," a fish bioconcentration factor (BCF) study is triggered in Phase I for pharmaceuticals having log Kow >4.5, to support Persistence, Bioaccumulation and Toxicity (PBT) screening, and in Phase II to assess secondary poisoning and bioaccumulation ('B') potential when log Kow ≥3. The standard sampling schedule outlined in OECD Test Guideline 305 (TG305) may require assessment of approximately 200 fish following exposure to low- and high-test concentrations and a negative control. We report experimental log Kow and BCF values for 64 human pharmaceuticals that were used to evaluate the current BCF testing trigger of log Kow ≥3, and whether a single BCF exposure concentration allows accurate classification of bioaccumulation potential. Our data support raising the BCF testing trigger to log Kow ≥4, and use of a single test concentration. The resulting reduction in the use of fish is consistent with the 3 R s principle and did not adversely affect classification accuracy. An assessment of potential risk of secondary poisoning was also conducted for three drugs classified as either B or vB, and no risks were identified.

Membrane/Water Partitioning and Permeabilities of Perfluoroalkyl Acids and Four of their Alternatives and the Effects on Toxicokinetic Behavior

The search for alternatives to bioaccumulative perfluoroalkyl acids (PFAAs) is ongoing. New, still highly fluorinated alternatives are produced in hopes of reducing bioaccumulation. To better estimate this bioaccumulative behavior, we performed dialysis experiments and determined membrane/water partition coefficients, K_mem/w, of six perfluoroalkyl carboxylic acids (PFCAs), three perfluoroalkanesulfonic acids, and four alternatives. We also investigated how passive permeation might influence the uptake kinetics into cells, measuring the passive anionic membrane permeability P_ion through planar lipid bilayers for six PFAAs and three alternatives. Experimental K_mem/w and P_ion were both predicted well by the COSMO-RS theory (log RMSE 0.61 and 0.46, respectively). K_mem/w values were consistent with the literature data, and alternatives showed similar sorption behavior as PFAAs. Experimental P_ion values were high enough to explain observed cellular uptake by passive diffusion with no need to postulate the existence of active uptake processes. However, predicted pK_a and neutral permeabilities suggest that also the permeation of the neutral species should be significant in case of PFCAs. This can have direct consequences on the steady-state distribution of PFAAs across cell membranes and thus toxicity. Consequently, we propose a model to predict pH-dependent baseline toxicity based on K_mem/w, which considers the permeation of both neutral and anionic species.

Uptake and Metabolism of Human Pharmaceuticals by Fish: A Case Study with the Opioid Analgesic Tramadol

Recent species-extrapolation approaches to the prediction of the potential effects of pharmaceuticals present in the environment on wild fish are based on the assumption that pharmacokinetics and metabolism in humans and fish are comparable. To test this hypothesis, we exposed fathead minnows to the opiate pro-drug tramadol and examined uptake from the water into the blood and brain and the metabolism of the drug into its main metabolites. We found that plasma concentrations could be predicted reasonably accurately based on the lipophilicity of the drug once the pH of the water was taken into account. The concentrations of the drug and its main metabolites were higher in the brain than in the plasma, and the observed brain and plasma concentration ratios were within the range of values reported in mammalian species. This fish species was able to metabolize the pro-drug tramadol into the highly active metabolite O-desmethyl tramadol and the inactive metabolite N-desmethyl tramadol in a similar manner to that of mammals. However, we found that concentration ratios of O-desmethyl tramadol to tramadol were lower in the fish than values in most humans administered the drug. Our pharmacokinetic data of tramadol in fish help bridge the gap between widely available mammalian pharmacological data and potential effects on aquatic organisms and highlight the importance of understanding drug uptake and metabolism in fish to enable the full implementation of predictive toxicology approaches.

Membrane partitioning of ionic liquid cations, anions and ion pairs - Estimating the bioconcentration potential of organic ions

Recent efforts have been directed towards better understanding the persistency and toxicity of ionic liquids (ILs) in the context of the "benign-by-design" approach, but the assessment of their bioaccumulation potential remains neglected. This paper reports the experimental membrane partitioning of IL cations (imidazolium, pyridinium, pyrrolidinium, phosphonium), anions ([C(CN)₃]^-, [B(CN)₄]^-, [FSO₂)₂N]^-, [(C₂F₅)₃PF₃]^-, [(CF₃SO₂)₂N]^-) and their combinations as a measure for estimating the bioconcentration factor (BCF). Both cations and anions can have a strong affinity for phosphatidylcholine bilayers, which is mainly driven by the hydrophobicity of the ions. This affinity is often reflected in the ecotoxicological impact. Our data revealed that the bioconcentration potential of IL cations and anions is much higher than expected from octanol-water-partitioning based estimations that have recently been presented. For some ILs, the membrane-water partition coefficient reached levels corresponding to BCFs that might become relevant in terms of the "B" (bioaccumulation potential) classification under REACH. However, this preliminary estimation need to be confirmed by in vivo bioconcentration studies.

Determination of preservative and antimicrobial compounds in fish from Manila Bay, Philippines using ultra high performance liquid chromatography tandem mass spectrometry, and assessment of human dietary exposure

Ultra high performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) determination of four paraben preservatives (methyl, ethyl, propyl and butyl) and two antimicrobial agents (triclosan and triclocarban) belonging to personal care products (PCPs) in 20 species of fish from Manila Bay (Philippines) was performed. Detection of PCPs with greater frequency indicates the ubiquitous contamination of Manila Bay. Concentrations of total paraben were one order of magnitude higher than the antimicrobials in almost all fish, except in Stolephorus indicus and Leiognathus equulus. A positive correlation was observed between parabens concentration and fish length (r = 0.31-0.49; p<0.05 to <0.001) and fish weight (r = 0.28-0.49; p<0.05 to <0.001), but not for the antimicrobials. The estimated dietary exposure values of the four parabens in the Philippines through fish is four orders of magnitude lower than the acceptable daily intake (ADI) of 10mg/kg/day, but the values of antimicrobials are just half of the ADI of TCS. To our knowledge, this is the first report of PCPs contamination in fish from Philippines.

COSMOmic: a mechanistic approach to the calculation of membrane-water partition coefficients and internal distributions within membranes and micelles

A new approach for the modeling of molecules in micellar systems and especially in biomembranes, COSMOmic, is presented, and its performance is validated on the example of the partitioning of molecules between water and biological membranes. Starting from quantum chemical calculations of the surfactant, solvent, and solute molecules, and being based on the COSMO-RS method for fluid-phase thermodynamic properties, COSMOmic is essentially free of additional adjustable parameters. The inclusion of an elastic energy correction into the COSMOmic model did not turn out to yield any significant improvement. The novel COSMOmic method allows for the efficient prediction of the distribution of molecules in micellar systems.

The evaluation of liposome-water partitioning of 8-hydroxyquinolines and their copper complexes

Bioavailability and toxicity of mixtures are urgent research issues, but usually mixtures of exclusively organic chemicals or exclusively metals are investigated. In our study, we explored the role of combinations of hydrophobic ionogenic organic compounds (HIOCs) with copper (Cu2+)for uptake and bioavailability of metals and hydrophobic metal complexes in an in vitro membrane system. We investigated the influence of the interactions of copper and 8-hydroxyquinolines, both components used in formulations of pesticides, on their partitioning into liposomes, which are model systems for biological membranes and are composed of lipid bilayers made of phosphatidylcholine. The test set of compounds comprised the parent compound 8-hydroxyquinoline and 8-hydroxyquinolines with hydrophobic (e.g., 5-chloro-8-hydroxyquinoline, 5,7-dichloro-8-hydroxyquinoline, 5,7-dibromo-8-hydroxyquinoline) and with hydrophilic (e.g., 8-hydroxyquinoline-5-sulfonic acid) substituents. Hydrophobic 8-hydroxyquinolines facilitate the passive uptake of copper into phospholipid bilayers by complex formation. Not only the neutral species of the ligands and their neutral copper ligand complexes are significantly taken up into the membrane, but also the cationic and anionic species of the ligands and the cationic complexes. The neutral, anionic, and cationic species of 8-hydroxyquinoline and the hydrophobic substituted 8-hydroxyquinolines exhibit linear correlations between their logarithmic liposome-water partitioning coefficients (log Klipw) and the logarithmic octanol-water partitioning coefficients of their neutral species (log Kow, neutral). The neutral species show the strongest partitioning followed by the anionic and cationic species. The associated quantitative structure activity relationships describing the dependency of log Klipw of the various species from log Kow, neutral of the neutral ligand species have slopes between 0.9 and 1. In contrast, the partitioning of the neutral and cationic copper-8-hydroxyquinoline complexes is dependent on several factors including the hydrophobicity of the ligand, the effective molecular size, and the polarization of the complex itself. In consequence, there is no linear relationship between log Klipw of these complexes and log Kow of the neutral species of their ligands. The complexes with very bulky substituents showed a reduced uptake. The Klipw of the nominally neutral complexes, where Cu2+ is coordinated with two ligands, were a factor three to four higher than the Klipw of the positively charged complexes with only one ligand. Although liposome-water partitioning merely describes one element of the uptake process into biological membranes, it is a key process for bioavailability of hydrophobic compounds and, presumably, also plays a crucial role for biological uptake of the described metal organic complexes.

A Quantitative Structure−Activity Relationship Model for the Intrinsic Activity of Uncouplers of Oxidative Phosphorylation

A quantitative structure-activity relationship (QSAR) has been derived for the prediction of the activity of phenols in uncoupling oxidative and photophosphorylation. Twenty-one compounds with experimental data for uncoupling activity as well as for the acid dissociation constant, pKa, and for partitioning constants of the neutral and the charged species into model membranes were analyzed. From these measured data, the effective concentration in the membrane was derived, which allowed the study of the intrinsic activity of uncouplers within the membrane. A linear regression model for the intrinsic activity could be established using the following three descriptors: solvation free energies of the anions, an estimate for heterodimer formation describing transport processes, and pKa values describing the speciation of the phenols. In a next step, the aqueous effect concentrations were modeled by combining the model for the intrinsic uncoupling activity with descriptors accounting for the uptake into membranes. Results obtained with experimental membrane-water partitioning data were compared with the results obtained with experimental octanol-water partition coefficients, log Kow, and with calculated log Kow values. The properties of these different measures of lipophilicity were critically discussed.

Distribution of pentachlorophenol in phospholipid bilayers: a molecular dynamics study

Molecular dynamics computer simulations of pentachlorophenol (PCP) in palmitoyl-oleoyl-phosphatidylethanolamine and palmitoyl-oleoyl-phosphatidylcholine lipid bilayers were carried out to investigate the distribution of PCP and the effects of PCP on the phospholipid bilayer structure. Starting from two extreme starting structures, including PCP molecules outside the lipid bilayer, the PCP distribution converges in simulations of up to 50 ns. PCP preferentially occupies the region between the carbonyl groups and the double bonds in the acyl chains of the lipid molecules in the bilayer. In the presence of PCP, the lipid chain order increases somewhat in both chains, and the average tilt angle of the lipid chains decreases. The increase in the lipid chain order in the presence of PCP was more pronounced in the palmitoyl-oleoyl-phosphatidylcholine bilayer compared to the palmitoyl-oleoyl-phosphatidylethanolamine bilayer. The number of trans conformations of lipid chain dihedrals does not change significantly. PCP aligns parallel to the alkyl chains of the lipid to optimize the packing in the dense ordered chain region of the bilayer. The hydroxyl group of PCP forms hydrogen bonds with both water and lipid oxygen atoms in the water/lipid interface region.

pH dependence of the partitioning of triphenyltin and tributyltin between phosphatidylcholine liposomes and water

Triorganotin compounds are very toxic contaminants. The site of their basic mechanism of action of acute toxicity is the biomembrane. Liposome-water distribution ratios of triphenyltin and tributyltin were determined between pH 3 and pH 8 with the equilibrium dialysis method in the micromolar concentration range, which is the concentration range where acute toxicity is observed. In addition, biomembrane-water distribution ratios of tributyltin were determined with chromatophores of Rhodobacter sphaeroides that contain approximately 70% protein intercalated in the lipid bilayer. The liposome-water distribution of both compounds showed only weak pH dependence. For tributyltin, the apparent distribution ratio decreased from 4100 at low pH to 2000 at high pH, while this ratio decreased from 70 000 to 22 000 for TPT. The distribution ratio of the triorganotin cation exceeded that of the neutral hydroxo complex by a factor of 2. The distribution ratio of both the cation and the hydroxo complex of triphenyltin exceeded that of tributyltin by a factor of 10. It is postulated that the sorption of the cation is governed by complex formation with ligands in the phospholipids, presumably the phosphate group. The biomembrane-water distribution ratio of tributyltin was found to be lower than the liposome-water distribution ratio at high pH. The hydroxo complex appears to partition only to the lipid fraction of the biomembrane. Yet, at low pH the biomembrane-water distribution ratio exceeded the liposome-water distribution ratio, which is attributed to complex formation of the cationic species with ligands of the protein fraction.

Hydrophobicity and sorption of chlorophenolates to lipid membranes

We have studied sorption of ionized species of chlorophenols and pentahalophenols to lipid membranes using egg-phosphatidylcholine (egg-PC) vesicles and measuring their zeta-potential as a function of aqueous concentration of the phenolates. The zeta-potential isotherms can be understood in terms of a sorption model that is a combination of the Gouy-Chapman model of the electrical double layer at the membrane-water interface and the Langmuir model for sorption. Two intrinsic sorption parameters were determined: the linear partition coefficient beta m, which relates the membrane surface density of the phenolates to their aqueous concentration and the area of the adsorption site, Ps. The linear partition coefficient is the measure of the affinity of phenolates to the lipid membrane. It depends strongly on the molecular structure: 2,6-dichlorophenolate beta m = (0.45 +/- 0.08) x 10(-7); m; 3,5-dichlorophenolate beta m = (0.22 +/- 0.02) x 10(-6) m; 2,4,6-trichlorophenolate beta m = (0.63 +/-0.06) x 10(-6) m; 2,4,5-trichlorophenolate beta m = (0.11 +/- 0.01) x 10(-5) m; 2,3,5,6-tetrachlorophenolate beta m = (0.56 +/- 0.07) x 10(-5) m; 2,3,4,5-tetrachlorophenolate beta m = (0.55 +/- 0.06) x 10(-5) m; pentachlorophenolate beta m = (0.34 +/- 0.05) x 10(-4) m; pentafluorophenolate beta m = (1.00 +/- 0.13) x 10(-7) m and pentabromophenolate beta m = (0.19 +/- 0.04) x 10(-3) m. Ps was found to be independent of phenolate structure, Ps = 3.3 +/- 0.1 nm2. The membrane affinity of chlorophenolates was compared with the octanol-water partition coefficients of un-ionized chlorophenols. It was shown that the free energy of transfer of chlorophenolates from water into the lipid membrane can be divided into non-electrostatic and electrostatic contributions. The no-nelectrostatic contribution corresponds to the hydrophobicity parameter alpha = 3.94 +/- 0.0.08 kcal per nm2 of molecular surface area. The electrostatic contribution contains a term inversely proportional to the molecular radius of the phenolate ion which has the physical meaning of the work of transfer of the phenolate ion from water into the membrane. The polarity of the sorption region of egg-PC membranes is given in terms of the dielectric constant and was estimated to be 12.4 (range 10.5-13.4).