Drug-phospholipid interactions. 2. Predicting the sites of drug distribution using n-octanol/water and membrane/water distribution coefficients

Biosorption of process-equipment-related leachables (PERLs) in biomanufacturing: A quantitative approach to study partitioning of PERLs in a cell culture system

The assessment and potential risk of process equipment-related leachables (PERLs) in the production of biopharmaceuticals and cell therapeutics using single-use (SU) equipment has been discussed previously. However, potential interactions of cells with PERLs have not yet been considered. Here, we present a quantitative adsorption study of neutral, organic small-molecule leachable compounds - known for extractables & leachables (E&L) analysis of SU equipment - in aqueous suspensions of CHO and T cells. The solid-water partition coefficient K_d was obtained for all compounds that showed adsorption. The findings implied that hydrophobic interactions are dominant; however, there was no unambiguous correlation between the derived adsorption coefficient K_d and the octanol-water partition coefficient K_ow. Interestingly, a maximum affinity of both cell types to the leachable bis(2,4-di-tert-butylphenyl)phosphate, which is known to be detrimental to cell development, was observed. A comparison of both cell types revealed that they generally interact with the same compounds in most cases but to different extents. Using partition coefficients enables estimation of the concentrations of leachable compounds associated with the biomass phase and in the aqueous suspensions and could be used for risk assessment of SU systems in biopharmaceutical and cell therapy (CT) manufacturing processes.

Machine learning predicts the impact of antibiotic properties on the composition and functioning of bacterial community in aquatic habitats

In the past decades, hundreds of antibiotics have been isolated from microbial metabolites or have been artificially synthesized for protecting humans, animals and crops from microbial infections. Their everlasting usage results in impacts on the microbial community composition and causes well-known collateral damage to the functioning of microbial communities. Nevertheless, the impact of different antibiotic properties on aquatic microbial communities have so far only poorly been disentangled. Here we characterized the environmental risk of 50 main kinds of antibiotics from 9 classes at a concentration of 10 μg/L for aquatic bacterial communities via metadata analysis combined with machine learning. Metadata analysis showed that the alpha diversity of the bacterial community increased only after treatment with aminoglycoside and β-lactam antibiotics, while its structure was changed by almost all tested antibiotics. The antibiotic treatment also disturbed the functions of the bacterial community, especially with regard to metabolic pathways, including amino acids, cofactors, vitamins, xenobiotics and carbohydrate metabolism. The critical characteristics (atom stereocenter count, number of hydrogen atoms in the antibiotic, and the adipose water coefficient) of antibiotics affecting the composition of the bacterial community in aquatic habitats were screened by machine learning. The key characteristics of antibiotics affecting the function bacterial communities were the number of hydrogen atoms, molecular weight and complexity. In summary, by developing machine learning models and by performing metadata analysis, this study provides the relationship between the properties of antibiotics and their adverse impacts on aquatic microbial communities from a macro perspective. The study also provides guidance for the rational design of antibiotics.

The hybrid system successfully to consisting of activated sludge and biofilter process from hospital wastewater: Ecotoxicological study

This article aimed to demonstrate solution hospital wastewater due to more consumption of antibiotics, public concern has been significantly increased for usage, fates and occurrences of these emerging compounds in the environments and biota. Therefore, it does need more discoveries about occurrences and new treatment methods. Since the conventional treatment methods are low efficient on antibiotics, integration and combination of biological systems together or with an additional process has been shown that provided a better result. However, here, the potential of a full scale combined treating system with activated sludge-scoria biofilter (ASSB) was investigated for removal of ceftriaxone (CEF) and amoxicillin (AMX). To determine the potential biodegradability of proposed system, the solid-water distribution coefficient (K_d) was calculated. Overally, 118 samples were collected from three points; wastewater entering, exiting the activated sludge, and exiting the biofilter. To determine the amount of CEF and AMX antibiotics, the samples were analyzed using HPLC-UV. The results showed that the activated sludge system were able to eliminate the AMX and CEF antibiotics about 70.36 and 84.49%, respectively. In compare to activated sludge, the average mean of ASSB system for the removal efficiency were 87.53% (for AMX) and 93.17% (for CEF), respectively. As a result, it can be found that the efficiency of the combined activated sludge-biofilter system in removing of the low levels of antibiotics was more than individual activated system. The result of K_d revealed that AMX (with a K_d about 0.172) has lower tendency to biomass rather than CEF (with a K_d about 0.512). The ecological toxicity assessment guaranteed there is no risk for fish and daphnia when the activated sludge and also ASSB effluents to be discharged into the environment even without any diluting.

The Lysosomotropic Activity of Hydrophobic Weak Base Drugs is Mediated via Their Intercalation into the Lysosomal Membrane

Lipophilic weak base therapeutic agents, termed lysosomotropic drugs (LDs), undergo marked sequestration and concentration within lysosomes, hence altering lysosomal functions. This lysosomal drug entrapment has been described as luminal drug compartmentalization. Consistent with our recent finding that LDs inflict a pH-dependent membrane fluidization, we herein demonstrate that LDs undergo intercalation and concentration within lysosomal membranes. The latter was revealed experimentally and computationally by (a) confocal microscopy of fluorescent compounds and drugs within lysosomal membranes, and (b) molecular dynamics modeling of the pH-dependent membrane insertion and accumulation of an assortment of LDs, including anticancer drugs. Based on the multiple functions of the lysosome as a central nutrient sensory hub and a degradation center, we discuss the molecular mechanisms underlying the alteration of morphology and impairment of lysosomal functions as consequences of LDs’ intercalation into lysosomes. Our findings bear important implications for drug design, drug induced lysosomal damage, diseases and pertaining therapeutics.

Designing safer oral drugs

Designing an oral drug such that its estimated dose to humans is both efficacious and safe is challenging. During the early design stage, where in vitro or preclinical species in vivo safety data are limited, heuristic-based criteria often related to physicochemical properties are used for guidance. The causal relationship between a compound's log P and its human in vivo toxicity is considered. With respect to designing efficacious oral drugs that potentially have reduced toxicity liabilities, an alternative heuristic-based criterion is proposed based on the amount of compound in the body at steady state. In humans, a threshold for the amount of compound in the body at steady state of 0.5 mg kg-1 is suggested. The criterion is based on the minimum toxic blood-plasma concentration that produces clinically relevant side effects or symptoms in humans for 242 oral drugs. It can be used to estimate a therapeutic window against which a compound's estimated in vivo plasma levels for a particular dose size and frequency can be assessed. The relationship between this criterion and acceptable oral dose sizes for different charge types with different in vivo plasma clearances is discussed.

In silico physicochemical parameter predictions

Drug discovery is a complex process with the aim of discovering efficacious molecules where their potency and selectivity are balanced against ADMET properties to set the appropriate dose and dosing interval. The link between physicochemical properties and molecular structure are well established. The subsequent connections between physicochemical properties and a drug's biological behavior provide an indirect link back to structure, facilitating the prediction of a biological property as a consequence of a particular molecular manipulation. Due to this understanding, during early drug discovery in vitro physicochemical property assays are commonly performed to eliminate compounds with properties commensurate with high attrition risks. However, the goal is to accurately predict physicochemical properties to prevent the synthesis of high risk compounds and hence minimize wasted drug discovery efforts. This paper will review the relevance to ADMET behaviors of key physicochemical properties, such as ionization, aqueous solubility, hydrogen bonding strength and hydrophobicity, and the in silico methodology for predicting them.

The effect of lipid composition, bilayer phase and temperature on the uptake of hematoporphyrin by liposomal membranes

In this study we investigated, spectroscopically, the binding of hematoporphyrin (HP) to non-charged lipid vesicles as a function of temperature and the molecular structure of the phospholipid. The temperature dependence of partitioning was employed to evaluate the thermodynamic parameters of the process. We studied the binding of HP to liposomes composed of different phospholipids: natural lecithin and three chemically defined phosphatidylcholines: dimiristoyl-phosphatidylcholine (DMPC), 1-palmitoyl-2-myristoyl-phosphatidylcholine (PMPC) and 1-stearoyl-2-myristoyl-phosphatidylcholine (SMPC), at different temperatures. The last three lipids differ only in the length of the fatty acid on 1 position of the glycerol backbone. Consequently, they have different phase transition temperatures and different order parameters. For SMPC, PMPC and DMPC, we checked the effect of temperatures above and below the phase transition while for lecithin, whose phase transition temperature is well below 0 °C, only temperatures above the phase transition could be tested. A very distinct effect of the phase transition on the binding constant was observed. Below this temperature a dramatic decrease in the binding was observed as the temperature was increased. Above the phase transition, the effect of temperature declined and the changes were minor compared to the changes observed when the bilayers undergo the solid-gel phase transition. Differences in HP binding to the various bilayers were attributed to the differences in the order parameters of DMPC, PMPC, SMPC and lecithin bilayers.

Personalized medicine: changing the paradigm of drug development

Despite an increased investment in research and development, there has been a steady decline in the number of drugs brought to market over the past 40 years. The tools of personalized medicine are refining diseases into molecular categories, and future therapeutics may be dictated by a patient's molecular profile relative to these categories. The adoption of a personalized medicine approach to drug development may improve the success rate by minimizing variability during each phase of the drug development process. This chapter describes the current paradigm of drug development and then discusses how molecular profiling/personalized medicine might be used to improve upon this paradigm.

Database of published retention factors for immobilized artificial membrane HPLC and an assessment of the effect of experimental variability

A database was collated of published experimental logarithmic values for the relative retention factors (log k(IAM)) measured using an immobilized artificial membrane column and high-performance liquid chromatography (IAM HPLC). Log k(IAM) is an alternative measure of hydrophobicity to the octanol/water partition coefficient (log K(OW)). While there are several accepted methods to measure log K(OW), no standardized method exists to determine log k(IAM). The database of collated log k(IAM) values includes 13 key experimental parameters and contains 1,686 values for 555 compounds, which are predominantly polar organic compounds and include drug molecules and surfactants. These compounds are acidic, basic, and neutral and both ionized and un-ionized under the conditions of analysis. The data compiled demonstrated experimental variability for each experimental parameter considered, including column stationary phase, pH, temperature, and mobile phase. Reducing the experimental variability allowed for greater consistency in the datasets.

The retention properties of nucleobases in alkyl C8-/C18- and IAM-chromatographic systems in relation to log Pow

In order to evaluate the differences in the partition properties of 35 structurally congeneric nucleobases of biological interests in octanol-water biphasic, alkyl C(8)/C(18), and IAM systems, a comparative chromatographic study was performed. Comparing with the reversed-phase C(8)/C(18) retention data, most of the purines possessed weaker IAM retention except for those with specific H-bond and/or electrostatic interactions. Quantitative correlations between the experimental log P(ow) literature values and the IAM, C(8), and C(18) log k were evaluated (R(2)=0.943, 0.794, and 0.767, respectively). Although IAM retention correlated significantly better (larger R(2) value) with the log P(ow) values statistically, the latter was revealed apparently behaving more like (slope approaching unity) alkyl C(8)/C(18) retention and hence also has the same shortcoming in under-representing analytes capable of forming short-term H-bond/electrostatic interactions with polar head-groups of phospholipids. A chemically meaningful structure-retention model (q(2)=0.824 and R(2)=0.968) was derived, in which the hydrophobic interaction is identified as the underlying factor for the retention of purines in IAM system modulated non-trivially by H-bond/electrostatic interactions.

Quantitative characterization of binding of small molecules to extracellular matrix

Extracellular matrix (ECM) is a major tissue component that, besides its cell support function, is implicated in cell-cell signaling, wound repair, cell adhesion, and other cell and tissue functions. For small molecules acting in tissues, including chemicals, signaling peptides, effectors, inhibitors, and other man-made and physiological compounds, non-specific binding to ECM is a critical phenomenon affecting their disposition. We describe here a method for a quantitative characterization of the ECM binding, using a solidified ECM layer incubated with medium containing studied small molecules. Working conditions of Matrigel, a commercial basement membrane preparation, were optimized in terms of the protein concentration, surface area, gel layer thickness, solidification time, and mixing speed. The release of proteins from the solidified layer into the buffer was monitored and taken into account. Two major proteins, laminin and collagen IV, dissolve at different rates. The Matrigel stability data, obtained under varying incubation conditions and gentle mixing, can also be useful in other ECM-related research. The experimental binding data, averaged over all binding sites, were analyzed assuming a fast linear binding. The binding constants were determined for 10 small organic molecules for both dissolved proteins and the solidified layer. The binding constants tend to increase with lipophilicity of the compounds, as characterized by the 1-octanol/water partition coefficients.

Use of physicochemical calculation of pKa and CLogP to predict phospholipidosis-inducing potential: a case study with structurally related piperazines

Several cationic amphiphilic compounds are known to induce phospholipidosis, a condition primarily characterized by excessive accumulation of phospholipids in different cell types, giving the affected cells a finely foamy appearance. Excessive storage of lamellar membranous intralysosomal inclusion bodies is the hallmark for phospholipidosis on the electron microscopic level. In case of alveolar phospholipidosis, foamy macrophages accumulate within the alveolar spaces of the lung. Based on such findings in a one-year toxicity study with gepirone in rats, we studied the molecular properties of this compound and compounds suspected of being phospholipidosis inducers by means of physicochemical calculations. Physicochemical molecular calculations of molecular weight, ClogP (partition coefficient octanol/water), logD at pH 7.4, and pKa were performed, for the cationic amphiphilic compounds chlorpromazine, amiodarone, imipramine, propranolol and fluoxetine, and for the structurally related compounds 1-phenylpiperazine (1-PHP), gepirone (and its major metabolites, 3-OH-gepirone and 1-pyrimidinylpiperazine [1-PP]), and buspirone. ClogP and calculated pKa cluster differently for the amphiphilic drugs compared to the chemical series of piperazines. In line with this analysis, lamellar inclusion bodies were found in an in vitro validation experiment in the human monoblastoid cell line U-937, incubated for 96 h at 10 microg/mL with cationic amphiphilic drugs (amiodarone, imipramine, or propranolol). No such lamellar inclusion bodies were seen for any of the compounds from the chemical series of piperazines including gepirone and its metabolites. The data presented support the use of simple physicochemical calculations of ClogP and pKa to discriminate rapidly between compounds suspected of being phospholipidosis inducers. Finally, the discriminative power of these physicochemical ClogP and pKa calculations to predict phospholipidosis-inducing potential was further validated by extension of the set of compounds.

Combined quantitative and mechanistic study of drug-membrane interactions using a novel 2H NMR approach

Several analytical methods are available for determining the partition coefficients of drug compounds in model phospholipid membranes, but such methods provide little information at the molecular level about how the membrane affinity of drugs relates to their interactions with the lipid molecules. A new (2)H nuclear magnetic resonance (NMR) approach has been developed here that quantifies the affinity of (2)H-labeled small molecules for different phospholipid membranes and, simultaneously, provides information on the mechanism of the drug-membrane interaction. In the example given, (2)H NMR analysis of a weakly basic ion pump inhibitor found that the drug partitioned preferentially into membranes of predominantly unsaturated or short-chain phospholipids. The (2)H NMR analysis also suggested that the membrane specificity of the drug was directly correlated to the ability of its phenyl moiety to penetrate into the interior of the lipid bilayer. The (2)H NMR approach could be of value in guiding medicinal chemistry toward or away from structures promoting interactions with specific types of biological membranes.

Prediction of drug-membrane interactions by IAM–HPLC: effects of different phospholipid stationary phases on the partition of bases

The chromatographic capacity factors of 39 neutral and basic compounds were measured on an immobilized artificial membrane-phosphatidylcholine-drug discovery (IAM-PC-DD) HPLC column, and the values compared with both octanol/water partition coefficients and capacity factors previously obtained on an IAM-PC-MG column. These two columns differ in their lipidic phase, since the IAM-PC-MG phase is made of phosphatidylcholine as found in biomembranes, whereas the glycerol linker is absent in the IAM-PC-DD phase. We found that the two phases interact differently with basic compounds at different degrees of ionization. On the IAM-PC-MG column, ionized compounds are as strongly or more strongly retained than isolipophilic neutral compounds. In contrast, their retention on the IAM-PC-DD column is less strong than, or at most as strong as, that of isolipophilic neutral compounds. The IAM-PC-MG data appear as better predictors of the interactions between drugs and biological membranes. Indeed, they correlate better than the IAM-PC-DD data with partitioning in both biological membrane and liposomes; moreover, they are better correlated with biological activities from the literature. These results suggest that even modest modifications in the structure of IAM phospholipids can have a major effect on the retention of basic compounds. We conclude that an acceptable IAM-HPLC estimate of the interactions between biomembranes and basic compounds should rely on stationary phases that reproduce the structure of natural phospholipids.

Physicochemical properties of neuromuscular blocking agents and their impact on the pharmacokinetic-pharmacodynamic relationship

BACKGROUND

Among the factors influencing the onset of action of neuromuscular blocking agents (NMBA), the potency (EC50) and the rate of equilibration between blood and the effect compartment (k(e0)) have been highlighted. Although these descriptors are intrinsically influenced by the physicochemical characteristics of the drug, the impact of lipid solubility, molecular weight and protein binding on pharmacokinetic-pharmacodynamic (PK-PD) descriptors has not been established for most NMBA.

METHODS

The octanol/phosphate buffer distribution coefficients (logD) of various NMBA (vecuronium, rocuronium, mivacurium isomers (cis-cis, cis-trans and trans-trans), doxacurium, cisatracurium, atracurium, succinylcholine) were determined. The free fraction for each drug was measured using an ultrafiltration technique. PK-PD descriptors were obtained from selected clinical studies. Correlations between physicochemical parameters (including molecular weight) and PK-PD descriptors were assessed by linear or multiple linear regression.

RESULTS

A wide range of log D (-4.15 for succinylcholine to 0.75 for vecuronium) and free fraction (from 31% for vecuronium to 80% for succinylcholine) is observed for NMBA. Molecular weight combined with either lipid solubility (r2=0.70; P=0.001) or free fraction (r2=0.84; P<0.001) were highly correlated with potency, while for k(e0) a greater degree of correlation was obtained when both lipid solubility and free fraction (r2=0.74; P=0.002) were included.

CONCLUSIONS

The basic characteristics of NMBAs, namely, molecular weight, lipid solubility and protein binding, are strongly associated with the kinetics of the drug response.

Estrogenicity and acute toxicity of selected anilines using a recombinant yeast assay

Suspected estrogen modulators include industrial organic chemicals (i.e., xenoestrogens), and have been shown to consist of alkylphenols, bisphenols, biphenylols, and some hydroxy-substituted polycyclic aromatic hydrocarbons. The most prominent structural feature identified to be important for estrogenic activity is a polar group capable of donating hydrogen bonds (i.e., hydroxyl) on an aromatic system. The present study was undertaken to explore the estrogenic activity and acute toxicity of chemicals containing a weaker hydrogen bond donor group on aromatic systems, i.e., the amino substituent. There is a great deal of chemical similarity between aromatic amines (anilines) and aromatic alcohols (phenols). The chemicals chosen for the current study contained an amino-substituted benzene ring with hydrophobic constituents varying in size and shape. Thus, 37 substituted aromatic amines were assayed for estrogenic activity EC50 and acute toxicity LC50 using the Saccharomyces cerevisiae recombinant yeast assay. While the EC50 of 17-beta-estradiol occurs at the 10(-10) range, the aniline with the greatest activity had an EC50 of 10(-6) M. Thus, anilines, in general, are capable only of very weak estrogenic activity in this assay. A comparison of estrogenic potency between the present group of anilines and a set of previously tested analogous phenols indicated that anilines are consistently less estrogenic than phenols. A comparison of hazard indices (EC50/LC50) of these chemicals revealed that, for the vast majority of anilines, the EC50 and LC50 were in the same order of magnitude. More specifically, estrogenic activity of para-substituted alkylanilines increases with alkyl group size up to 5 carbons in length, after which the acute toxicity of the larger alkyl-substituents precluded the ability of the compound to induce the estrogenic response.

QSAR and the rational design of long-acting dual D2-receptor/beta 2-adrenoceptor agonists

This paper describes the development of a QSAR model for the rational control of functional duration of topical long-acting dual D(2)-receptor/beta(2)-adrenoceptor agonists for the treatment of chronic obstructive pulmonary disease. A QSAR model highlighted the importance of lipophilicity and ionization in controlling beta(2) duration. It was found that design rules logD(7.4) > 2, secondary amine pK(a) > 8.0, yielded ultra-long duration compounds. This model was used successfully to guide the design of long- and ultra-long-acting compounds. The QSAR model is discussed in terms of the exosite model, and the plasmalemma diffusion microkinetic hypothesis, for the control of beta(2) duration. Data presented strongly suggests that beta(2) duration is primarily controlled by the membrane affinity of these compounds.

A comparison of physiochemical property profiles of development and marketed oral drugs

The process of drug discovery applies rigorous selection pressures. Marketed oral drugs will generally possess favorable physiochemical properties with respect to absorption, metabolism, distribution, and clearance. This paper describes a study in which the distributions of physiochemical properties of oral drugs in different phases of clinical development are compared to those already marketed. The aim is to identify the trends in physiochemical properties that favor a drug's successful passage through clinical development and on to the market. Two libraries were created, one of current development oral drugs and one of marketed oral drugs. Statistical analysis of the two showed that the mean molecular weight of orally administered drugs in development decreases on passing through each of the different clinical phases and gradually converges toward the mean molecular weight of marketed oral drugs. It is also clear that the most lipophilic compounds are being discontinued from development.

Solid-supported lipid membranes as a tool for determination of membrane affinity: high-throughput screening of a physicochemical parameter

Quantification of membrane affinity is an important early screening step in modern drug design. However, current approaches using different lipid membrane models usually are time-consuming or show severe experimental drawbacks. In this paper we describe the use of solid-supported lipid membranes (TRANSIL) as a new tool for the determination of membrane affinity. Eighteen pharmaceuticals (neutrals, acids, and bases) have been analyzed for their lipophilicity at physiological pH in an automated setup; phase separation of lipid and aqueous phase can be achieved simply by a short low-speed centrifugation or filtration. The membrane affinity is then calculated by quantification of the total drug concentration and the amount of drug remaining in the aqueous phase after incubation with solid-supported lipid membranes. Lipophilicity parameters relying on solid-supported lipid membranes correlate well with octanol-water partition coefficients K(ow) for neutral organic compounds (range of log K(ow) = 1.5-5, n = 7, r = 0.93). Data acquisition with this lipid membrane model system is highly re-producible. Even in the case of ionizable drugs, where K(ow) tends to underestimate membrane affinity, the latter can be correctly quantified using solid-supported lipid membranes: data comparison shows good agreement of the presented approach with established but time-consuming standardized lipid/buffer systems. Solid-supported lipid membranes allow a fast and reliable quantification of membrane affinity, enabling high-throughput screening of this physicochemical parameter.

Rapid-gradient HPLC method for measuring drug interactions with immobilized artificial membrane: comparison with other lipophilicity measures

A fast-gradient high-performance liquid chromatographic (HPLC) method has been suggested to characterize the interactions of drugs with an immobilized artificial membrane (IAM). With a set of standards, the gradient retention times can be converted to Chromatographic Hydrophobicity Index values referring to IAM chromatography (CHI(IAM)) that approximates an acetonitrile concentration with which the equal distribution of compound can be achieved between the mobile phase and IAM. The CHI(IAM) values are more suitable for interlaboratory comparison and for high throughput screening of new molecular entities than the log k(IAM) values (isocratic retention factor on IAM). The fast-gradient method has been validated against the isocratic log k(IAM) values using the linear free energy relationship solvation equations based on the data from 48 compounds. The compound set was selected to provide a wide range and the least cross-correlation between the molecular descriptors in the solvation equation: (2) where SP is a solute property (e.g., logarithm of partition coefficients, reversed-phase (RP)-HPLC retention parameters, such as log k, log k(w), etc.) and the explanatory variables are solute descriptors as follows: R(2) is an excess molar refraction that can be obtained from the measured refractive index of a compound, pi(2)(H) is the solute dipolarity/polarizability, summation operatoralpha(2)(H) and summation operatorbeta(2)(0) are the solute overall or effective hydrogen-bond acidity and basicity, respectively, and V(x) is the McGowan characteristic volume (in cm(3)/100 mol) that can be calculated for any solute simply from molecular structure using a table of atomic constants. It was found that the relative constants of the solvation equation were very similar for the CHI(IAM) and for the log k(IAM). The IAM lipophilicity scale was quite similar to the octanol/water lipophilicity scale for neutral compounds. The effect of charge on the interaction with IAM was studied by varying the mobile phase pH.

Evaluation and prediction of drug permeation

A major challenge confronting the pharmaceutical scientist is to optimize the selective and efficient delivery of new active entities and drug candidates. Successful drug development requires not only optimization of specific and potent pharmacodynamic activity, but also efficient delivery to the target site. Following advances in rational drug design, combinatorial chemistry and high-throughput screening techniques, the number of newly discovered and promising active compounds has increased dramatically in recent years, often making delivery problems the rate-limiting step in drug research. To overcome these problems, a good knowledge of the pharmacokinetic barriers encountered by bioactive compounds is required. This review gives an overview of the properties of relevant physiological barriers and presents some important biological models for evaluation of drug permeation and transport. Physicochemical determinants in drug permeation and the relevance of quantitative and qualitative approaches to the prediction and evaluation of passive drug absorption are also discussed.

Cubic phases for studies of drug partition into lipid bilayers

Drug partition into lipid bilayers in a cubic liquid-crystalline phase was investigated. Glyceryl monooleate was used to form the lipid bilayer in a reversed bicontinuous cubic liquid-crystalline phase. The reason for using the cubic phase is that it may coexist with an external aqueous phase, and that the phase boundary (cubic phase/aqueous bulk) is well-defined due to the stiffness of the cubic phase. This makes the cubic phase a potential candidate for high throughput screening (HTS) of the lipophilicity and the dissociation constant (if any) of drug compounds. Clomethiazole (CMZ), lidocaine, prilocaine and 4-phenylbutylamine (4-PBA) were chosen as model drug compounds. It was shown that it is possible to determine a pH-dependent apparent partition coefficient, Kbl/w, of a drug compound using a lipid bilayer expressed as a cubic liquid-crystalline structure. Good agreement was found when the resulting Kbl/w vs. pH curves for CMZ, lidocaine and prilocaine were fitted to a mathematical expression. This included the bilayer/water partition coefficient for the unionised and ionised drug respectively and the pKa of the drug. The effect of different experimental conditions; such as amount of cubic phase, temperature, agitation, sample preparation and interfacial area between the cubic phase and the aqueous bulk on the partition kinetics were investigated as well. The studies reveal that the time needed to reach partition equilibrium was, as expected, substantially reduced (from days to hours) by decreasing the amount of cubic phase, increasing the interfacial area between the cubic phase and the aqueous phase, and increasing the temperature and the agitation of the sample. It was also shown that the bilayer affinity of 4-PBA was increased when a zwitterionic lipid (i.e. dioleoyl phosphatidylcholine, DOPC) was incorporated in the bilayer.

Determination of liposome partitioning of ionizable drugs by titration

Drug partitioning to liposomes has been suggested as a model for partitioning to biomembranes but has been lacking a rapid analytical assay useful for drug screening. A fast pH-metric titration method for the determination of liposome partitioning of ionizable drugs using small unilamellar phosphatidylcholine vesicles prepared by sonic homogenization has been successfully developed, enabling the use of high lipid-to-drug ratios. Liposome-water partition coefficients of diclofenac and propranolol were determined to study the impact of varying titration parameters, temperature, equilibration time, lipid, and liposome types on the partitioning. To validate this method, the results were compared to literature values generated with different techniques and to pH-metric titration results with large unilamellar vesicles. The rapid pH-metric assay gave liposome partitioning data for the two model compounds which were consistent with other analytical techniques and liposome types.

The membrane lipid environment modulates drug interactions with the P-glycoprotein multidrug transporter

The P-glycoprotein multidrug transporter functions as an ATP-driven efflux pump for a large number of structurally unrelated hydrophobic compounds. Substrates are believed to gain access to the transporter after partitioning into the membrane, rather than from the extracellular aqueous phase. The binding of drug substrates to P-glycoprotein may thus be modulated by the properties of the lipid bilayer. The interactions with P-glycoprotein of two drugs (vinblastine and daunorubicin) and a chemosensitizer (verapamil) were characterized by quenching of purified fluorescently labeled protein in the presence of various phospholipids. Biphasic quench curves were observed for vinblastine and verapamil, suggesting that more than one molecule of these compounds may bind to the transporter simultaneously. All three drugs bound to P-glycoprotein with substantially higher affinity in egg phosphatidylcholine (PC), compared to brain phosphatidylserine (PS) and egg phosphatidylethanolamine (PE). The nature of the lipid acyl chains also modulated binding, with affinity decreasing in the order egg PC > dimyristoyl-PC (DMPC) > dipalmitoyl-PC (DPPC). Following reconstitution of the transporter into DMPC, all three compounds bound to P-glycoprotein with 2-4-fold higher affinity in gel phase lipid relative to liquid-crystalline phase lipid. The P-glycoprotein ATPase stimulation/inhibition profiles for the drugs were also altered in different lipids, in a manner consistent with the observed changes in binding affinity. The ability of the drugs to partition into bilayers of phosphatidylcholines was determined. All of the drugs partitioned much better into egg PC relative to DMPC and DPPC. The binding affinity increased (i.e., the value of Kd decreased) as the drug-lipid partition coefficient increased, supporting the proposal that the effective concentration of the drug substrate in the membrane is important for interaction with the transporter. These results provide support for the vacuum cleaner model of P-glycoprotein action.

Ionization constants and distribution coefficients of phenothiazines and calcium channel antagonists determined by a pH-metric method and correlation with calculated partition coefficients

The pH-metric technique was used to determine the ionization constants and distribution coefficients of 10 phenothiazines and five ionizable calcium channel antagonists. Because the studied compounds were poorly water soluble and quite lipophilic with partition coefficients in the range of 3.5 to 5.5, organic cosolvents had to be added for the determination of the ionization constants to avoid precipitation of the free bases. The effect of the cosolvents dioxane and methanol on the extrapolation to pure water was compared. For both cosolvents a very good agreement with accessible published ionization constants was obtained, however the slope of the regression line was much smaller for dioxane, yielding more reliable estimates according to the standard deviation of the extrapolated values. Thus, dioxane might be preferable to methanol as a cosolvent for the determination of ionization constants of sparingly water soluble bases. Also the n-octanol/water partition coefficients were determined and compared with published data and values calculated with the ClogP, ACD, and HINT programs. Although the obtained values were approximate in conformity with the published data, the calculated partition coefficients differed from the experimental ones considerably for the majority of the investigated compounds. Furthermore, the ion pair partitioning and the distribution coefficients at physiological pH 7.4 were determined. The pH-dependent distribution profiles showed the strong influence of the ionization constants and of the distribution of the ion pairs on the overall distribution. This result strongly suggests that greater use should be made of measured distribution coefficients in quantitative structure-activity relationship studies. The potentiometric method is a convenient way to determine the distribution properties of drug molecules at pH values relevant for the biological system under investigation.

Quantitative structure-pharmacokinetics relationships: II. A mechanistically based model to evaluate the relationship between tissue distribution parameters and compound lipophilicity

The tissue-to-unbound plasma distribution coefficients (Kpus) of 14 rat tissues after i.v. administration of nine 5-n-alkyl-5-ethyl barbituric acids, determined in a previous study, were used to identify a model of the relationship between tissue distribution and lipophilicity of the homologs, expressed in terms of their octanol to water partition ratio, P. Based on mechanistic considerations and assumptions, the parameter model was expressed as Kpu tau = fw.tau [1 + a tau (nPt.tau)Pb tau], where fw.tau is the tissue water content. (nPt. tau) is the binding capacity of the tissue, n is the number of the binding sites, a tau and b tau are the parameters of the relationship Ka tau = a tau Pb tau; and Ka tau is the binding association constant of each tissue. The parameter model was linearized and fitted to the predetermined Kpu values, yielding correlation coefficients ranging between .940 and .997. The predictive performance of the parameter model was evaluated using a leave-one-out procedure with subsequent computation of the mean prediction error (ME = measurement of the prediction bias) and the square root of the mean squared prediction error (RMSE = measurement of the prediction accuracy). The ME varied between -22.48 and 61.14%, indicating a slight tendency for overpredicting. The RMSE was between 24.73 and 102% for the individual tissues across the different homologs; and between 28.33 and 85.2% for the individual homologs across the different tissues. The apparently high Kpu prediction errors, when translated through the low sensitivity of the barbiturate whole-body physiologically based pharmacokinetic model, established previously, leads to predicted tissue concentration-time profiles within 5 to 20% of the original ones. Therefore, it is concluded, that the identified mechanistically based model is a good predictor of the tissue-to-unbound Kpus in the rat tissues.

Chromatographic indexes on immobilized artificial membranes for the prediction of transdermal transport of drugs

A set of 12 drugs, consisting of structurally unrelated neutral, basic, acidic and amphoteric compounds, was examined by high performance liquid chromatrography (HPLC) on a model of fluid membrane bilayers, the immobilized artificial membrane (IAM) column. The logarithms of chromatographic capacity factors extrapolated to 100% aqueous phase at pH 5.5 (log kw) were measured and compared to the n-octanol/water partition coefficients (log P). The scale derived from the IAM system was different from the lipophilicity scale expressed by the log P, due to the peculiar capability of phospholipids to well accommodate the ionized form of some molecules and show additive or repulsive extra-interactions when particular structural motifs on the molecule are present. The relationship between log P and log kw previously obtained for compounds interacting on IAM phase by a uniquely lipophilicity-based mechanism, allowed us to calculate, from log P, the values of log kw expected for the drugs considered. These values were subtracted from the log kw experimentally determined and the differences were assumed to quantify the amount of extra-interactions (hydrogen bond and electrostatic interactions) with phospholipids (delta log kw). The coefficients of permeability through the human skin (Kp) for the compounds considered did not correlate with either log kw or log P values. However, the Kp values correlated well with the delta log kw values indicating that the higher the ability of a molecule to cross the skin barrier, the lower its component of interaction with phospholipids not accounted for by lipophilicity-based interactions.

Disposition of diphenyl sulphoxide in rat

1. Radiolabelled diphenyl sulphoxide (U-14C- or 35S-) was administered by gavage (1.0 mmol/kg body weight) to the adult male Wistar rat following an overnight fast. 2. For both labelled forms faeces was the major route of excretion of radioactivity (50%) with substantial amounts still being voided during the third and fourth days (13%). Urinary elimination (42%) was similar during the first (20%) and second (17%) days and a small amount of radioactivity (7%) was found within the carcass after 4 days. 3. Plasma data showed a peak concentration at 40 min (tmax), a distribution half-life of 2 h (t1/2 alpha) and an elimination half-life of 22.5 h (t1/2 beta). Biliary studies revealed that 16% of the dose traversed the bile duct during the first day with nearly half of this being excreted in the first 8 h. 4. From urinary data, metabolism occurred via ring hydroxylation with subsequent conjugate formation. Oxidation of the sulphur to form the sulphone also took place. No evidence for sulphoxide reduction, cleavage of the ring structures or exclusion of the sulphur was obtained.

Structure-toxicity relationships for aminoalkanols: a comparison with alkanols and alkanamines

The relative toxicity (log IGC50(-1)) of 49 selected aliphatic amines and aminoalkanols was evaluated in the static Tetrahymena pyriformis population growth impairment assay. Excess toxicity, indicated by potency greater than predicted for non-polar narcotic alkanols, was associated with both classes of test chemicals. Moreover, the aminoalkanols were found to be more toxic than the corresponding alkanamines. A high quality 1-octanol/water partition coefficient (log K(ow)) dependent quantitative structure-activity relationship (QSAR), logIGC50(-1) = 0.78 (log K(ow)) - 1.42; r2 = 0.934, was developed for alkanamines. This QSAR represented the amine narcosis mechanism of toxic action. No quality QSAR was developed for the aminoalkanols. However, several structure-toxicity features were observed for this class of chemicals. Two-amino-1-hydroxy derivatives being more toxic than the corresponding derivatives, where the amino and hydroxy moieties were separated by methylene groups. Hydrocarbon branching next to the amino moiety resulted in decreased toxicity. Aminoalkanol alters lipid metabolism in T. pyriformis.