Lipophilic and polar interaction forces between acidic drugs and membrane phospholipids encoded in IAM-HPLC indexes: their role in membrane partition and relationships with BBB permeation data.

The membrane phospholipid affinity data, log k(w)(IAM), for 18 acidic and unionized 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 both log P(N) values, the n-octanol/water partition coefficients of the neutral forms, and log D(7.4) values, the n-octanol/water partition coefficients of the mixtures of neutral and ionized forms at pH 7.4. 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". Since, differently from bases, electrostatic interactions between acids and phospholipids take place at the surface of phospholipid layers (choline moieties), and not near their lipophilic core (phosphate moieties), they were parameterized by a new procedure yielding "Δ'log k(w)(IAM)" parameters, i.e. the difference between the IAM retention factors observed for the analytes and those of neutral compounds with the same n-octanol partition values displayed by the analytes at pH 7.4. All acidic analytes, but one, and all unionized analytes, but the unionizable ones, showed positive Δ'log k(w)(IAM) values, indicating that they partition stronger in phospholipids than in n-octanol. Log BB values (capability to pass BBB) weakly related with both lipophilicity and phospholipid affinity values; in contrast they are inversely related with Δ'log k(w)(IAM) values. The relationships between log BB and Δ'log k(w)(IAM) practically overlapped the previously found log BB/Δlog k(w)(IAM) relationships for bases. The excess of polar interaction component between acidic drugs and phospholipids, mainly electrostatic forces, although enhancing partition in phospholipids, hinders membrane passage, analogously to the behavior previously reported for bases. The study suggests that IAM-HPLC is an effective technique to perform simple and fast measurements of the intermolecular recognition forces related to membrane partition and permeation. This can contribute to better understand the mechanisms governing both partition of charged species in cell membranes and passage through them, also allowing the possible optimization of the pharmacokinetic properties of the drugs at the early stages of their development.

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.

Capillary electrochromatography as a new tool to assess drug affinity for membrane phospholipids.

This work proposes a new capillary electrochromatography (CEC) method for determination of drug partition in membrane phospholipids. CEC experiments were carried out in a 100 µm (ID) fused-silica capillary, partially packed with a chromatographic phospholipid stationary phase, so-called Immobilized Artificial Membrane, IAM.PC.DD2. The observed retention values were corrected by both the electro-osmotic and electrophoretic mobility values, measured by capillary electrophoresis (CE) experiments, assuming the values of the logarithms of "chromatographic" affinity factors, log k(CEC) as indexes of affinity for phospholipids. Analogously to biochromatography, all the values were determined with a totally aqueous mobile phase, or extrapolated to 100% aqueous buffer. The analytes were 16 structurally unrelated compounds, of basic, neutral, and acidic nature. To evaluate the effectiveness of CEC data to describe partition in phospholipids, log k(CEC) were related to both log P and log k(w)(IAM) values. log P are the lipophilicity values expressed as the logarithms of n-octanol/water partition coefficients and log k(w)(IAM) are the retention data measured by High Performance Liquid Chromatography (HPLC) on an IAM.PC.DD2 column, assumed as the reference values for phospholipid affinity. Phospholipid affinity scale by CEC related to that achieved by HPLC, but only if two different subclasses were considered separately, i.e. protonated and unprotonated analytes; indeed, all the compounds protonated at the experimental pH value (7.0) were retained stronger in CEC than in HPLC. This discrepancy may be due to the use of different buffers in CEC and HPLC since, to avoid the occurrence of a high current, the eluent in CEC experiments was of different composition and lower ionic strength than in HPLC. CEC analyses were faster and required lower amounts of both solvent and stationary phase than HPLC; moreover, with the exception of only three analytes, all analyses were performed with 100% aqueous eluents avoiding time-consuming and tedious extrapolation procedures.

Enantioselective retention of beta-blocking agents on human serum albumin and alpha 1-acid glycoprotein HPLC columns: relationships with different scales of lipophilicity.

The enantioselective retention of thirteen beta-blockers on HPLC stationary phases supporting human serum albumin (HSA) or alpha(1)-acid glycoprotein (AGP) was investigated. Eight beta-blockers were enantiomerically resolved on the AGP column whereas only four beta-blockers were resolved on the HSA column. Moreover, interactions between beta-blockers and AGP were much stronger than those with HSA. Retention values on both HSA and AGP for less retained enantiomers related well with various lipophilicity parameters, with the best relationships found with log k(w)(IAM) values obtained on HPLC stationary phases supporting phospholipids, i.e. the so-called Immobilized Artificial Membrane (IAM). Differently from n-octanol lipophilicity values, these values encode both lipophilic. Electrostatic intermolecular recognition forces which may be involved in the interaction between ionized analytes, such as beta-blockers, and proteins. However, their effectiveness to describe non-specific interactions with serum-proteins for other classes of drugs needs further investigations. Analyses performed on AGP with eluent containing dimethyloctylamine (DMOA) as the displacer demonstrated that enantioselective sites bind to both (-)-forms and (+)-forms, but the binding to (-)-forms is stronger. The enantiomer competition to bind to a same site may be relevant from a pharmacokinetic point of view when racemic mixtures are administered. Finally, in contrast to previously reported data in the literature, we found that AGP can bind enantioselectively not only the more lipophilic congeners but also the less lipophilic ones.

Measurement of bisphenol A and bisphenol B levels in human blood sera from healthy and endometriotic women.

A sensitive HPLC method with fluorescence detection was developed for the determination of bisphenol A (BPA) and bisphenol B (BPB) in human blood serum. The detection limits of the method were 0.18 and 0.20 ng/mL for BPA and BPB, respectively. A single-step liquid-liquid extraction was used for the pre-treatment of serum samples. The recoveries of BPA and BPB spiked to sera were 85.6 and 87.7%, respectively. The analyses of sera from both healthy and endometriotic women emphasized the absence of bisphenols in all the control cases (11 women), whereas BPA was found in 30 sera (51.7%) and BPB was found in 16 sera (27.6%) in the group of 58 patients with endometriosis; in nine of such sera BPA and BPB were present simultaneously. Only relatively to the sera quantitated, BPA concentrations ranged from 0.79 to 7.12 ng/mL (mean concentration 2.91 +/- 1.74 ng/mL), whereas BPB concentrations ranged from 0.88 to 11.94 ng/mL (mean concentration 5.15 +/- 4.16 ng/mL). Therefore, the presence of at least one of the two bisphenols was verified in a percentage as high as 63.8% in the sera from endometriotic women, suggesting the existence of a relationship between endometriosis and BPA and/or BPB exposure. Indeed, it is well known that bisphenols can work as xenoestrogens, owing to their structural similarity to natural and synthetic estrogens (e.g. estradiol and dietilstilbestrol). However, further studies are necessary to confirm this hypothesis and to assess the actual dose at which exposures to bisphenols are able to increase the sensitivity of the endometriotic cells to estradiol.

Characterization of alkanoyl-10-O-minocyclines in micellar dispersions as potential agents for treatment of human neurodegenerative disorders.

Minocycline is a widely used antibacterial agent. Moreover, it is also demonstrated to be effective in several neurodegenerative disorders, due to its antioxidant and anti-inflammatory activities. However, the last activity is only apparent at very high doses. In fact, minocycline poorly crosses the blood-brain barrier (BBB) due to its low lipophilicity and half-life. The present work details the physicochemical characterization of a series of alkanoyl-10-O-minocycline derivatives (2-6), which are able to produce self-assembled aggregates in aqueous solution. The n-octanol/aqueous phase lipophilicity of minocycline and its derivatives were assessed by theoretical calculation, by shake-flask method, and by reversed-phase HPLC. Moreover, we determined their affinity for membrane phospholipids measuring their HPLC retention on phospholipid-based stationary phases, the so-called "Immobilized Artificial Membranes" (IAMs). Our results indicate high lipophilicity values for the minocycline derivatives (compounds 2-6); these values and the corresponding phospholipid affinities increase with the length of the hydrocarbon moiety substituent. Furthermore, the ability of the investigated alkanoyl-10-O-minocycline derivatives to self-assemble could allow a direct administration by oral and intraperitoneal routes as supramolecular systems. The advantages are an enhancement of drug solubilization, a sustained release, and the consequent less frequent drug administration. Moreover, we can hypothesize the potential solubilization in the micellar core of other poorly water soluble drugs which could improve the therapeutic effects of the pharmaceutical formulation in a combined therapy. Given the high lipophilicity of the title derivatives, they can be supposed to offer higher half-life and a better BBB penetration than minocycline. Since the new derivatives retain the structural features related to the antioxidant and anti-inflammatory effects of minocycline, they can be regarded not only as long-acting antimicrobial agents but also as candidate drugs for a targeted treatment of mental illness.