Relationships between Octanol-Water Partition Data, Chromatographic Indices and Their Dependence on pH in a Set of Nonsteroidal Anti-Inflammatory Drugs

Extraction of Ibuprofen from Natural Waters Using a Covalent Organic Framework

Ibuprofen is one of the most widely used pharmaceuticals, and due to its inefficient removal by conventional wastewater treatment, it can be found in natural surface waters at high concentrations. Recently, we demonstrated that the TpBD-(CF₃)₂ covalent organic framework (COF) can adsorb ibuprofen from ultrapure water with high efficiency. Here, we investigate the performance of the COF for the extraction of ibuprofen from natural water samples from a lake, river, and estuary. In general, the complexity of the natural water matrix induced a reduction in the adsorption efficiency of ibuprofen as compared to ultrapure water. The best performance, with over 70% adsorption efficiency, was found in lake water, the sample which featured the lowest pH. According to the theoretical calculations, ibuprofen more favorably interacts with the COF pores in the protonated form, which could partially account for the enhanced adsorption efficiency found in lake water. In addition, we explored the effect of the presence of competing pharmaceuticals, namely, acetaminophen and phenobarbital, on the ibuprofen adsorption as binary mixtures. Acetaminophen and phenobarbital were adsorbed by TpBD-(CF₃)₂ with low efficiency and their presence led to an increase in ibuprofen adsorption in the binary mixtures. Overall, this study demonstrates that TpBD-(CF₃)₂ is an efficient adsorbent for the extraction of ibuprofen from natural waters as well.

Dependence of norfloxacin diffusion across bilayers on lipid composition

Antibiotic resistance is a growing concern in medicine and raises the need to develop and design new drug molecules that can efficiently inhibit bacterial replication. Spurring the passive uptake of the drug molecules is an obvious solution. However our limited understanding of drug-membrane interactions due to the presence of an overwhelming variety of lipids constituting cellular membranes and the lack of facile tools to probe the bio-physical interactions between drugs and lipids imposes a major challenge towards developing new drug molecules that can enter the cell via passive diffusion. Here, we used a label-free micro-fluidic platform combined with giant unilamellar lipid vesicles to investigate the permeability of membranes containing mixtures of DOPE and DOPG in DOPC, leading to a label-free measurement of passive membrane-permeability of autofluorescent antibiotics. A fluoroquinolone drug, norfloxacin was used as a case study. Our results indicate that the diffusion of norfloxacin is strongly dependent on the lipid composition which is not expected from the traditional octanol-lipid partition co-efficient assay. The anionic lipid, DOPG, slows the diffusion process whereas the diffusion across liposomes containing DOPE increases with higher DOPE concentration. Our findings emphasise the need to investigate drug-membrane interactions with focus on the specificity of drugs to lipids for efficient drug delivery, drug encapsulation and targeted drug-delivery.

Polar interactions drug/phospholipids estimated by IAM-HPLC vs cultured cell line passage data: Their relationships and comparison of their effectiveness in predicting drug human intestinal absorption

The relationships between data of passage through Caco-2 cultured cell lines (log Papp), taken from the literature, for 38 structurally unrelated compounds and both n-octanol lipophilicity parameters (log P(N) and log D(7.4)) and phospholipid affinity indexes were investigated. Phospholipid affinity(log k W(IAM)) was experimentally determined by HPLC on two different phospholipid stationary phases and the polar/electrostatic interaction component drug/phospholipids (Δ log k W(IAM)) was calculated according to a method we previously proposed. Log Papp moderately related to lipophilicity values measured at pH 7.4 (log D(7.4)), according to a parabolic pattern, but poorly related with log k W(IAM). Furthermore, a significant inverse linear relationship with Δ l og k W(IAM) values was only observed for the analytes with m.w. >300 Da, for which paracellular diffusion can be considered a minor transport route in vivo. Indeed, it has been reported that Caco-2 passage data also encode secondary passage mechanisms, which participate in a different extent to the jejunal absorption in vivo and cannot be directly equated to the corresponding human in situ log Peff values, unless a normalization is performed. In an attempt to elucidate this issue, 47 structurally unrelated compounds whose cultured cell line passage data were corrected for the effects of the aqueous boundary layer and paracellular permeability, so as to express transcellular intrinsic permeability, log P 0(Caco-2/MDCK), were also considered. Highly significant inverse linear relationships were observed between log P 0(Caco-2/MDCK) and Δlog k W(IAM) values from both IAM.PC.MG (r(2)=0.765) and IAM.PC.DD2 (r(2)=0.806) stationary phases whereas the relationships with either lipophilicity in n-octanol or log k W(IAM) values were very poor. The results of the present study, in complete agreement with those of our recent study on the relationships between jejunal absorption data measured in situ and Δ log k W(IAM) values, confirm the soundness of Δ log k W(IAM) parameters in the prediction of the intestinal absorption of drugs. From a mechanistic point of view, they suggest that the polar/electrostatic forces between drugs and phospholipids play a major role in the passage through biomembranes.

Determination of n-octanol/water partition coefficients of weak ionizable solutes by RP-HPLC with neutral model compounds

The utilization of neutral compounds as model compounds is put forward for determination of the n-octanol/water partition coefficient (K(ow)) of highly hydrophobic, weak acidic compounds by reversed-phase high performance liquid chromatography (RP-HPLC). It is based on a linear relationship between the logarithm of apparent n-octanol/water partition coefficient (log K(ow)″), expressing hydrophobicity of acidic solutes more accurately, and the logarithm of RP-HPLC retention factor of the solutes corresponding to the neat aqueous fraction of mobile phase (log k(w)). The availability of neutral model compounds was theoretically tested for this novel protocol. Moreover, a high consistency of linear log K(ow)″-log k(w) correlations was demonstrated between a mixed training set of neutral and acidic model compounds, and a training set of neutral model compounds. It is proved in theory that for a certain set of compounds investigated, all derived linear relationships between log K(ow)″ and log k(w) have a unit slope and the same intercept, regardless of mobile phase pH. This model was applied to measure log K(ow) of lipophilic aristolochic acid I (AA I) and aristolochic acid II (AA II). Log K(ow) values for AA I and AA II are 4.45±0.07 and 3.99±0.06, respectively. To the best of our knowledge, this is the first report on experimental log K(ow) data for AAs. The proposed strategy solves the problem of lacking suitable acidic model compounds with reliable experimental K(ow) in determining K(ow) of lipophilic acidic solutes by RP-HPLC.

Extension of the liquid chromatography/quantitative structure-property relationship method to assess the lipophilicity of neutral, acidic, basic and amphotheric drugs

A reported chromatographic method to determine the 1-octanol/water partition coefficient (logP(o/w)) has been used to estimate the lipophilicity of 33 drugs with diverse structures and functionalities, including neutral, acid, basic, and amphoteric compounds. The applicability of the chromatographic method has been extended to the UHPLC technique, and the results obtained were compared to those obtained from conventional HPLC. No significant difference between the results obtained by both techniques is noticed. Thus, the suitability of UHPLC, which involves shorter run times, for lipophilicity assessment is demonstrated. In order to show the consistency of this chromatographic method, the logP(o/w) values of those drugs which have acid-base properties have been also determined by potentiometry, and the final results have been compared with both values derived from the chromatographic method and the ones from the literature.

Energetic Aspects of Diclofenac Acid in Crystal Modifications and in Solutions—Mechanism of Solvation, Partitioning and Distribution

Temperature dependency of saturated vapor pressure and heat capacity for the diclofenac acid (Form II) were measured and thermodynamic functions of sublimation calculated (DeltaG(sub)(298) = 49.3 kJ x mol(-1); DeltaH(sub)(298) = 115.6 +/- 1.3 kJ x mol(-1); DeltaS(sub)(298) = 222 +/- 4 J x mol(-1) x K(-1)). Crystal polymorphic Forms I (P2(1)/c) and II (C2/c) of diclofenac acid have been prepared and characterized by X-ray diffraction experiments. The difference between crystal lattice energies of the two forms were obtained by solution calorimetry: DeltaDeltaH(sol)(I --> II) = 1.6 +/- 0.4 kJ x mol(-1). Temperature dependencies of the solubility in buffers with pH 2.0 and 7.4, n-octanol and n-hexane were measured. The thermodynamic functions of solubility, solvation, and transfer processes were deduced. Specific and non-specific solvation terms were distinguished using the transfer from the "inert" n-hexane to the other solvents. The transfer of diclofenac acid molecules from the buffers to n-octanol (partitioning and distribution) is an entropy driven process.

The pH dependency of the binding of drugs to plasma proteins in man

An analysis of pH-induced changes of drug binding may contribute to the understanding of the mechanisms involved and the clinical relevance. A literature search was performed, and acceptance criteria set up, to select reported data for quantitative evaluation. The relationship between percentage of unbound drug, fu, and pH was analyzed, and the relevance of physicochemical characteristics of the ligand drugs and the importance of hydrogen ion-induced changes in plasma proteins for the pH sensitivity of the binding were evaluated. With all basic and the majority of acidic drugs, fu depended linearly on pH. Basic drugs showed a consistent behavior with fu decreasing with increasing pH. Acidic compounds behaved differently: With some, fu increased, and with others fu decreased, with pH, and with a third group of acids fu was pH independent. Large differences in the pH sensitivity of the plasma protein binding among individual compounds were found. The fu in plasma for some bases and acids increased up to 136% and 95%, respectively, at pH values seen in severe acidemia or alkemia. These changes in fu could be clinically relevant with narrow-therapeutic-range drugs. Physicochemical properties and other characteristics of the ligands affect the pH sensitivity of the interaction with plasma proteins, but there was clear evidence indicating that pH-induced changes in the plasma proteins are also involved in the observed pH-dependent interaction with ligands. It is generally accepted that the unbound, free fraction in whole blood or plasma is an important determinant of the pharmacokinetics and pharmacodynamics of drugs. pH-dependent protein binding and consequent changes in the free fraction have been reported for many drugs. From a basic science point of view, the systematic study of pH-induced perturbations of the drug-protein interaction may provide insight into the mechanism and forces involved in the binding of drugs to plasma proteins. From a clinical viewpoint it may be of interest to know the extent of pH-induced changes in the unbound fraction of drugs under extreme acidemic or alkalemic conditions. Arterial blood pH values compatible with life reportedly range between 6.7 and 8.0. pH values as low as 6.3 have been measured in survivors of drowning accidents. To the best knowledge of the authors, a review and interpretation of pH-associated changes in the protein binding of drugs has not been attempted to date. The goals of this investigation were to (1) review published results of studies that determined the impact of pH changes on the protein binding of drugs in man, (2) select representative data using predetermined criteria, (3) determine relevant factors impacting the pH sensitivity of the drug-protein interaction, and (4) attempt to interpret the results and their clinical relevance.

Method for measuring the logarithm of the octanol-water partition coefficient by using short octadecyl-poly(vinyl alcohol) high-performance liquid chromatography columns

A simple, quick, versatile and inexpensive HPLC method to estimate the logarithm of the octanol-water partition coefficient (log Pow) employing a methanol-water gradient and a short octadecyl-poly(vinyl alcohol) (ODP) column is described. This method is different from published HPLC-based log Pow methods because it uses retention times from a rapid methanol-water gradient to directly generate log Pow estimates, rather than from a series of isocratic mixtures extrapolated to 100% water. These HPLC log Pow values have good precision and correlate well with traditional shake-flask log Pow values. If necessary, the log Pow determination (including replications) can easily be carried out using only a milligram of sample. By suppressing ionization of acids and bases by the use of a buffer in the aqueous phase, the method can measure the log Pow of neutral organic molecules at any pH between 2 and 13. The method can be used with impure material and is rapid, 7 min per run and 4 min equilibration; it lends itself to and has been utilized for high-throughput hydrophobicity determinations (we have now carried out thousands of HPLC log Pow measurements by this method).

Interactions of nonsteroidal antiinflammatory drugs with phospholipids: comparison between octanol/buffer partition coefficients and chromatographic indexes on immobilized artificial membranes

A set of seventeen nonsteroidal antiinflammatory drugs (NSAIDs), consisting of structurally unrelated carboxylic acids and piroxicam, was examined by high-performance liquid chromatography (HPLC) on an immobilized artificial membrane (IAM) column that is a solid-phase model of fluid membranes. The chromatographic capacity factors extrapolated to 100% aqueous phase (log KWIAM) were compared with n-octanol/buffer lipophilicity parameters. The interactions with phospholipids were much better predicted from the intrinsic partition coefficient, log P, than from the apparent partition value, log D7.4, indicating that phospholipids can counteract the influence of electrically charged functions of analytes on lipophilic interactions. The log KWIAM and log P values for both NSAIDs and structurally unrelated neutral compounds result in unique scale if uniquely partition-based mechanisms take place. However, an electrostatic repulsion component was observed for the NSAIDs bearing the carboxylic function directly linked to the aromatic ring, and for ibuprofen. Hence, the IAM-derived scale is distinctive from the one obtained by lipophilic parameters. The IC50 values on cyclooxygenase 2 (COX-2) in intact cells determined by different authors have been successfully correlated with respective IAM parameters, whereas no correlation was found with COX-1 activity data. These results suggest that membrane affinity may represent an important prerequisite for the specific binding NSAIDs/COX-2.

pH-metric log P. 4. Comparison of partition coefficients determined by HPLC and potentiometric methods to literature values

The pKa and log P of 20 compounds, including six substituted phenols, two substituted quinolines, N-methylaniline, five barbiturate derivatives, two phenothiazines, and several other molecules of pharmaceutical interest, were determined by the potentiometric technique at 25 degrees C and ionic strength 0.1 M (KNO3). The log P values were determined also by partition HPLC. Three of the substances were of very low aqueous solubility, and for these the aqueous pKas were determined by extrapolation from methanol-water solutions using the Yasuda-Shedlovsky technique. Values of log P obtained both by potentiometry and by partition HPLC, which ranged from 0.3 to 5.4, were in very good acordance with literature values. The general applicability of the potentiometric technique to ionizable compounds of diversely varied structures was demonstrated by the study.

Hydrophobicity and central nervous system agents: on the principle of minimal hydrophobicity in drug design

The problem of getting drugs across the so-called blood-brain barrier (BBB) has long been under extensive investigation; however, the other side of the problem, that of keeping drugs out of the central nervous system (CNS), has not been studied so intently. As we strive to make more and more refined drugs with fewer side effects, the problem of keeping drugs out of the CNS has possibly become more important than getting them in. The role of lipophilicity has long been recognized as being important in CNS penetration by chemicals, but we believe that not enough attention has been devoted to just exactly what is meant when it is said that "a lipophilic drug is needed for CNS penetration." How lipophilic? Can hydrophilic properties keep drugs out of the CNS? How hydrophilic should they be? There are other reasons for making drugs hydrophilic. Hydrophobic drugs, other factors being equal, are more inhibitory of biochemical systems than hydrophilic congeners. Evidence is beginning to show that cytochrome P450 is induced in direct proportion to hydrophobicity by a variety of compounds, and cytochrome P450 may produce modifications in lipophilic molecules in the body. Hydrophobic drugs are more slowly eliminated. This report discusses these problems in terms of the octanol-water (log P) hydrophobic scale. The principle is proposed that, without convincing evidence to the contrary, drugs should be made as hydrophilic as possible without loss of efficacy. Antihistamines are discussed in terms of what kind of hydrophobic-hydrophilic balance is best to avoid CNS-related problems.