Octanol/water partitioning simulation by reversed-phase high performance liquid chromatography for structurally diverse acidic drugs: Effect of n-octanol as mobile phase additive

Assessment of Lipophilicity Descriptors of Selected NSAIDs Obtained at Different TLC Stationary Phases

Lipophilicity study of selected NSAIDs, the group of the bioactive compounds usually used in humans and animals medicine, with the use of experimental and calculation methods was evaluated. LogP values are proposed and compared as descriptors of the lipophilicity of eleven compounds (from oxicams and coxibs). Obtained data were designated by thin-layer chromatography (TLC) in various chromatographic conditions, with stationary phases with different properties. The mobile phase systems were prepared by mixing the respective amounts of water and organic modifier, methanol and acetone, in the range of 30 to 80% (v/v) in 5% increments. Retention parameters (RF, RM and RM0) were calculated and statistically evaluated to establish correlations. All experimentally determined RM0 values were compared with partition coefficients obtained by computational methods using linear regression analysis. Moreover, in order to extract information about the lipophilicity of compounds from large retention datasets, two chemometric approaches, namely principal component analysis (PCA) and cluster analysis (CA) were carried out. Established models of lipophilicity may have the potential to predict the biological activity of a number of drugs. The presented knowledge may also be of use during drug discovery processes, broadening the knowledge of potential ways to modify the physicochemical properties of chemical compounds.

Application of Co-Amorphous Technology for Improving the Physicochemical Properties of Amorphous Formulations

Co-amorphous technology was recently introduced to stabilize drugs in the amorphous state for drug development. We examined the predictability of the formation of co-amorphous systems and identified two reliable indicators of successful formation: (1) a negative Δ H_mix value and (2) small Δlog P between components. Moreover, we found that the stability of co-amorphous systems was improved when (1) Δ H_mix was negative and (2) amorphous forms of the constituent compounds were stable. Furthermore, we concluded that co-amorphous systems with small (negatively large) Δ H_mix values had lower hygroscopicity. Typically, amorphous solid dispersions exhibit hygroscopicity because polymers exhibit large hygroscopicity. We proved the superiority of co-amorphous technology over amorphous solid dispersion in this respect. Our results provide methods for (1) establishing a screening method and (2) improving hygroscopicity, which may make co-amorphous technology more useful than amorphous solid dispersion technology.

The Impact of Lipophilicity in Drug Discovery: Rapid Measurements by Means of Reversed-Phase HPLC

Lipophilicity constitutes a vital physicochemical property in drug design as it is connected with pharmacodynamic and pharmacokinetic properties as well as toxicological aspects of candidate drugs. Traditional partitioning experiments to determine n-octanol-water coefficients are laborious and time-consuming, while they cannot be reliably performed for highly lipophilic or compounds undergoing degradation. Alternatively, lipophilicity of candidate drugs can be accurately and reproducibly determined using reversed-phase liquid chromatography. In this chapter, the details of protocols for lipophilicity assessment using reversed-phase HPLC, under conditions which provide the best simulation of n-octanol-water partition coefficients, are described.

Removal of pharmaceuticals from secondary effluents by an electro-peroxone process

This study compared the removal of pharmaceuticals from secondary effluents of wastewater treatment plants (WWTPs) by conventional ozonation and the electro-peroxone (E-peroxone) process, which involves electrochemically generating H2O2 in-situ from O2 in sparged O2 and O3 gas mixture (i.e., ozone generator effluent) during ozonation. Several pharmaceuticals with kO3 ranging from <0.1 to 6.8 × 10(5) M(-1) s(-1) were spiked into four secondary effluents collected from different WWTPs, and then treated by ozonation and the E-peroxone process. Results show that both processes can rapidly remove ozone reactive pharmaceuticals (diclofenac and gemfibrozil), while the E-peroxone process can considerably accelerate the removal of ozone-refractory pharmaceuticals (e.g., ibuprofen and clofibric acid) via indirect oxidation with OH generated from the reaction of sparged O3 with electro-generated H2O2. Compared with ozonation, the E-peroxone process enhanced the removal kinetics of ozone-refractory pharmaceuticals in the four secondary effluents by ∼40-170%, and the enhancement was more pronounced in secondary effluents that had relatively lower effluent organic matter (EfOM). Due to its higher efficiency for removing ozone-refractory pharmaceuticals, the E-peroxone process reduced the reaction time and electrical energy consumption required to remove ≥90% of all spiked pharmaceuticals from the secondary effluents as compared to ozonation. These results indicate that the E-peroxone process may provide a simple and effective way to improve existing ozonation system for pharmaceutical removal from secondary effluents.

Insight into the retention mechanism on immobilized artificial membrane chromatography using two stationary phases

The retention behavior of sixty structurally diverse drugs on two immobilized artificial membrane (IAM) columns, IAM.PC.MG and IAM.PC.DD2 types, at two pH values, 7.4 and 5.5, was established. Extrapolated to pure aqueous phase retention factors, logkw(IAM), were determined and the role of acetonitrile as organic modifier was explored, considering the relationships with the slopes, S, of the extrapolation procedure. Good interrelations between retention factors on the two IAM stationary phases were observed, although logkw(IAM.PC.DD2) values are generally higher than logkw(IAM.PC.MG). In order to investigate the underlying retention mechanism, relationships between IAM retention factors and lipophilicity, expressed as logP or logD at pH 7.4 were established. Electrostatic interactions were considered by introducing the positively and negatively charged molecular fractions as additional parameters in the logkw(IAM)/logD relationships. The positive contribution of these fractions supported the involvement of the electrostatic interactions in the retention mechanism. Special attention was given to the retention behavior of zwitterionic compounds and for compounds with special structural characteristics.

Application of Design of Experiment and Simulation Methods to Liquid Chromatography Analysis of Topical HIV Microbicides Stampidine and HI443

This study intended to determine if experimental design and Monte Carlo simulation methods can be utilized to optimize the liquid chromatography (LC) analysis of active molecules. The method was applied for the simultaneous analysis of two topical microbicides, stampidine (STP) and HI443 in bulk and nanoformulations. The Plackett-Burman design was used for screening; whereas, Box-Behnken design was used to evaluate the main and interaction effects of the selected factors on the responses, namely peak area of STP (Y₁), HI443 (Y₂), tailing of STP (Y₃), and HI443 (Y₄). The Monte Carlo simulation was applied to get the minimum defect rate (DR) of the process. The optimized LC conditions were found to be X₁; flow rate: 0.6 mL/min, X₂; injection volume: 18 μL, and X₃; initial gradient acetonitrile ratio: 92% v/v with a minimal DR of 0.077%. The optimized method was applied to determine the percent encapsulation efficiency (%EE) and in vitro release profile of STP and HI443 from solid lipid nanoparticles (SLNs). The %EE of STP and HI443 in SLNs was found to be 30.56 ± 9.44 and 94.80 ± 21.90% w/w, respectively, (n=3). It was observed that the release kinetics of STP followed the first order, whereas, HI443 followed the Peppas kinetic model in SLNs. The LC method was also applied for the estimation of molar extinction coefficients (ε₂₇₀ ) of both drugs for the first time. These values were estimated to be 7,569.03 ± 217.96 and 17,823.67 ± 88.12 L/mol/cm for STP and HI443, respectively, (n=3). The results suggest that experimental design and Monte Carlo simulation can be effectively used to reduce the DR of a process and to optimize the chromatographic conditions for the analysis of bio-active agents as applied in this study.

Influence of n-octanol in mobile phase on QSRRs of lipophilicity and retention mechanism of acidic and basic compounds in RP-HPLC

Effect ofn-octanol on retention behavior, mechanism and QSRRs of ionizable compounds in RP-HPLC.

Formation of sulfonyl aromatic alcohols by electrolysis of a bisazo reactive dye

Five sulfonyl aromatic alcohols, namely 4-((2-hydroxyethyl)sulfonyl)phenol, 4-((2-(2-((4-hydroxyphenyl)sulfonyl)ethoxy)vinyl)sulfonyl)phenol, 4-(ethylsulfonyl)phenol, 4-(vinylsulfonyl)phenol and 5-((4-aminophenyl)sulfonyl)-2-penten-1-ol were identified by LC-ESI-Qq-TOF-MS as products formed by electrolysis of the bisazo reactive dye Reactive Black 5 (RB5). Since electrolyses were performed in an undivided cell equipped with Ni electrodes in alkaline medium, amines like 4-(2-methoxyethylsulfonyl)benzene-amine (MEBA) with m/z 216 were also suspected to be formed due to the plausible chemical reaction in the bulk or the cathodic reduction of RB5 and its oxidation by-products. Aiming to check this hypothesis, a method was used for the preparation of MEBA with 98% purity, via chemical reduction also of the dye RB5. The logP of the synthesized sulfonyl aromatic compounds was calculated and their logk_w values were determined chromatographically. These data were discussed in regard to the relationship between hydrophobicity/lipophilicity and toxicity.

Determination of n-octanol/water partition coefficient for DDT-related compounds by RP-HPLC with a novel dual-point retention time correction

n-Octanol/water partition coefficients (P) for DDTs and dicofol were determined by reversed-phase high performance liquid chromatography (RP-HPLC) on a C(18) column using methanol-water mixture as mobile phase. A dual-point retention time correction (DP-RTC) was proposed to rectify chromatographic retention time (t(R)) shift resulted from stationary phase aging. Based on this correction, the relationship between logP and logk(w), the logarithm of the retention factor extrapolated to pure water, was investigated for a set of 12 benzene homologues and DDT-related compounds with reliable experimental P as model compounds. A linear regression logP=(1.10±0.04) logk(w) - (0.60±0.17) was established with correlation coefficient R(2) of 0.988, cross-validated correlation coefficient R(cv)(2) of 0.983 and standard deviation (SD) of 0.156. This model was further validated using four verification compounds, naphthalene, biphenyl, 2,2-bis(4-chlorophenyl)-1,1-dichloroethane (p,p'-DDD) and 2,2-bis(4-chlorophenyl)-1,1-dichloroethene (p,p'-DDE) with similar structure to DDT. The RP-HPLC-determined P values showed good consistency with shake-flask (SFM) or slow-stirring (SSM) results, especially for highly hydrophobic compounds with logP in the range of 4-7. Then, the P values for five DDT-related compounds, 2-(2-chlorophenyl)-2-(4-chlorophenyl)-1,1,1-trichloroethane (o,p'-DDT), 2-(2-chlorophenyl)-2-(4-chlorophenyl)-1,1-dichloroethane (o,p'-DDD), 2-(2-chlorophenyl)-2-(4-chlorophenyl)-1,1-dichloroethene (o,p'-DDE), and 2,2,2-trichloro-1,1-bis(4-chlorophenyl)ethanol (dicofol) and its main degradation product 4,4'-dichlorobenzophenone (p,p'-DBP) were evaluated by the improved RP-HPLC method for the first time. The excellent precision with SD less than 0.03 proved that the novel DP-RTC protocol can significantly increases the determination accuracy and reliability of P by RP-HPLC.

Prediction of pesticides chromatographic lipophilicity from the computational molecular descriptors

Quantitative structure-property relationship models were developed for the prediction of pesticides and some PAH compounds lipophilicity based on a wide set of computational molecular descriptors and a set of experimental chromatographic data. The chromatographic lipophilicity of pesticides has been evaluated by high-performance liquid chromatography (HPLC) using different chemically bonded (C18, C8, CN and Phenyl HPLC columns) stationary phases and two different organic modifiers (methanol and acetonitrile, respectively) in the mobile phase composition. Through a systematic study, by using the classic multivariate analysis, several quantitative structure-property/lipophilicity multi-dimensional models were established. Multiple linear regression and genetic algorithm for the variable subset selection were used. The internal and external statistical evaluation procedures revealed some appropriate models for the chromatographic lipophilicity prediction of pesticides. Moreover, the statistical parameters of regression and those obtained by applying t-test for the intercept (a(0)) and for the slope (a(1)) in order to evaluate relationship between experimental and predicted octanol-water partition coefficients in case of the test set compounds, revealed two statistically valid models that can be successfully used in lipophilicity prediction of pesticides.

Alternative measures of lipophilicity: from octanol-water partitioning to IAM retention

This review describes lipophilicity parameters currently used in drug design and QSAR studies. After a short historical overview, the complex nature of lipophilicity as the outcome of polar/nonpolar inter- and intramolecular interactions is analysed and considered as the background for the discussion of the different lipophilicity descriptors. The first part focuses on octanol-water partitioning of neutral and ionisable compounds, evaluates the efficiency of predictions and provides a short description of the experimental methods for the determination of distribution coefficients. A next part is dedicated to reversed-phase chromatographic techniques, HPLC and TLC in lipophilicity assessment. The two methods are evaluated for their efficiency to simulate octanol-water and the progress achieved in the refinement of suitable chromatographic conditions, in particular in the field of HPLC, is outlined. Liposomes as direct models of biological membranes are examined and phospolipophilicity is compared to the traditional lipophilicity concept. Difficulties associated with liposome-water partitioning are discussed. The last part focuses on Immobilised Artificial Membrane (IAM) chromatography as an alternative which combines membrane simulation with rapid measurements. IAM chromatographic retention is compared to octanol-water and liposome-water partitioning as well as to reversed-phase retention and its potential to predict biopartitioning and biological activities is discussed.

LogD: lipophilicity for ionisable compounds

The octanol/water partition coefficient (Kow) for organic compounds is widely used in predictive environmental studies. A significant proportion of contaminants of surface and ground water are ionisable (e.g. many pesticides, pharmaceuticals, metabolites). Such compounds may be partially ionised dependent on the pH. Since the neutral and ionic species exhibit different polarities, the Kow value of ionisable pesticides is pH dependent. It is therefore essential to determine Kow values over the full range of pH that occurs in the environment in order to get appropriate predictors. Numerous methods are available to measure lipophilicity but only a few are appropriate for ionisable pesticides (e.g. pH metric and filter probe methods). Parameters such as pH and ionic strength need to be carefully controlled when working with ionisable compounds. Variation of these factors probably explains why literature can yield Kow values that differ by more than one order of magnitude for some compounds. In this article, Kow values obtained for six acidic pesticides with three different methods are compared as well (data from the literature, measured by pH metric method and calculated with five computer programs). The values used in predictive regression equations needs to be either measured with a suitable method or selected from the literature with great care.