The linear free-energy relationship between partition coefficients and the binding and conformational perturbation of macromolecules by small organic compounds

Boronic acid complexes with amino phenolic N,O-ligands and their use for non-covalent protein fluorescence labeling

Phenylboronic acid (PBA) forms neutral tetrahedral N,O-coordinated 6-membered cyclic complexes with stability constants reaching the values as large as 1.3 × 10⁴ M^-1 at pH 7.4 in water with amino phenolic compounds including 2-(2'-hydroxyphenyl)-1H-benzimidazole (HPBI) often used for protein probing and labeling. The crystal structures of isolated complexes demonstrate unusually high for boronate adducts degree of the tetrahedral character of the boron atom with short B-N bonds in agreement with their high solution stability. The complexation of PBA with HPBI, causes a strong enhancement of the fluorescence of the "enol" form of the ligand, increases the affinity of the dye to a protein (bovine serum albumin) and makes more pronounced the shift in emission maximum induced by the protein binding. Similar, but larger effects are observed with an amino HPBI derivative and with a stronger boronic acid benzoxaborole. Thus, the binding constant to the protein about 2 × 10⁴ M^-1 for free HPBI increases to 1.2 × 10⁶ M^-1 for the complex of 5-amino-HPBI with benzoxaborole making it suitable for an efficient non-covalent protein labeling or bioconjugation.

An 18F-Labeled Poly(ADP-ribose) Polymerase Positron Emission Tomography Imaging Agent

Poly(ADP-ribose) polymerase (PARP) is involved in repair of DNA breaks and is over-expressed in a wide variety of tumors, making PARP an attractive biomarker for positron emission tomography (PET) and single photon emission computed tomography imaging. Consequently, over the past decade, there has been a drive to develop nuclear imaging agents targeting PARP. Here, we report the discovery of a PET tracer that is based on the potent PARP inhibitor olaparib (1). Our lead PET tracer candidate, [18F]20, was synthesized and evaluated as a potential PARP PET radiotracer in mice bearing subcutaneous glioblastoma xenografts using ex vivo biodistribution and PET-magnetic resonance imaging techniques. Results showed that [18F]20 could be produced in a good radioactivity yield and exhibited specific PARP binding allowing visualization of tumors over-expressing PARP. [18F]20 is therefore a potential candidate radiotracer for in vivo PARP PET imaging.

Development and evaluation of predictive model for bovine serum albumin-water partition coefficients of neutral organic chemicals

The binding of organic chemicals to serum albumin can significantly reduce their unbound concentration in blood and affect their biological reactions. In this study, we developed a new QSAR model for bovine serum albumin (BSA) - water partition coefficients (K_BSA/W) of neutral organic chemicals with large structural variance, logK_BSA/W values covering 3.5 orders of magnitude (1.19-4.76). All chemical geometries were optimized by semi-empirical PM6 algorithm. Several quantum chemical parameters that reflect various intermolecular interactions as well as hydrophobicity were selected to develop QSAR model. The result indicates the regression model derived from logK_ow, the most positive net atomic charges on an atom, Connolly solvent excluded volume, polarizability, and Abraham acidity could explain the partitioning mechanism of organic chemicals between BSA and water. The simulated external validation and cross validation verifies the developed model has good statistical robustness and predictive ability, thus can be used to estimate the logK_BSA/W values for chemicals in application domain, accordingly to provide basic data for the toxicity assessment of the chemicals.

Modeling biophysical and biological properties from the characteristics of the molecular electron density, electron localization and delocalization matrices, and the electrostatic potential

The electron density and the electrostatic potential are fundamentally related to the molecular hamiltonian, and hence are the ultimate source of all properties in the ground- and excited-states. The advantages of using molecular descriptors derived from these fundamental scalar fields, both accessible from theory and from experiment, in the formulation of quantitative structure-to-activity and structure-to-property relationships, collectively abbreviated as QSAR, are discussed. A few such descriptors encode for a wide variety of properties including, for example, electronic transition energies, pK(a)'s, rates of ester hydrolysis, NMR chemical shifts, DNA dimers binding energies, π-stacking energies, toxicological indices, cytotoxicities, hepatotoxicities, carcinogenicities, partial molar volumes, partition coefficients (log P), hydrogen bond donor capacities, enzyme-substrate complementarities, bioisosterism, and regularities in the genetic code. Electronic fingerprinting from the topological analysis of the electron density is shown to be comparable and possibly superior to Hammett constants and can be used in conjunction with traditional bulk and liposolubility descriptors to accurately predict biological activities. A new class of descriptors obtained from the quantum theory of atoms in molecules' (QTAIM) localization and delocalization indices and bond properties, cast in matrix format, is shown to quantify transferability and molecular similarity meaningfully. Properties such as "interacting quantum atoms (IQA)" energies which are expressible into an interaction matrix of two body terms (and diagonal one body "self" terms, as IQA energies) can be used in the same manner. The proposed QSAR-type studies based on similarity distances derived from such matrix representatives of molecular structure necessitate extensive investigation before their utility is unequivocally established.

Electron-density descriptors as predictors in quantitative structure--activity/property relationships and drug design

The use of electron density-based molecular descriptors in drug research, particularly in quantitative structure--activity relationships/quantitative structure--property relationships studies, is reviewed. The exposition starts by a discussion of molecular similarity and transferability in terms of the underlying electron density, which leads to a qualitative introduction to the quantum theory of atoms in molecules (QTAIM). The starting point of QTAIM is the topological analysis of the molecular electron-density distributions to extract atomic and bond properties that characterize every atom and bond in the molecule. These atomic and bond properties have considerable potential as bases for the construction of robust quantitative structure--activity/property relationships models as shown by selected examples in this review. QTAIM is applicable to the electron density calculated from quantum-chemical calculations and/or that obtained from ultra-high resolution x-ray diffraction experiments followed by nonspherical refinement. Atomic and bond properties are introduced followed by examples of application of each of these two families of descriptors. The review ends with a study whereby the molecular electrostatic potential, uniquely determined by the density, is used in conjunction with atomic properties to elucidate the reasons for the biological similarity of bioisosteres.

Investigation of the Interaction Mode of Phenothiazine Neuroleptics with α1-Acid Glycoprotein

The interaction of phenothiazine neuroleptics with α1-acid glycoprotein (AGP) and desialylated AGP (asialoAGP) has been investigated by fluorescence, circular dichroism spectroscopy and by equilibrium dialysis. The binding parameters of phenothiazines obtained from fluorescence agreed closely with those obtained from circular dichroism and equilibrium dialysis. The binding affinities (nK) to AGP were slightly higher than binding affinities to asialoAGP. Attempts to correlate binding affinities with partition coefficients suggested that hydrophobic forces were mainly involved in the binding of phenothiazine neuroleptics to AGP and asialoAGP. However, electrostatic interaction was also found to be involved as suggested by experimental data obtained from the influence of oleic acid and caesium chloride on the drug binding to the two proteins.

Taxane analogues against breast cancer: a quantitative structure-activity relationship study

Breast cancer is the second leading cause of cancer death among women in the United States. Two taxane analogues, taxol and taxotere, are the most important antimitotic drugs currently in clinical use for the treatment of breast cancers. However, recent reports have indicated that the use of these drugs often results in various undesired side effects as well as multi-drug resistance. These limitations have led to the development of new taxane derivatives with fewer side effects, superior pharmacological properties, and improved anticancer activity to maximize the induced benefits for breast cancer patients. Herein, four series of taxane derivatives were used to correlate their inhibitory activities against breast cancer cells with their hydrophobic and steric properties in order to understand their chemical-biological interactions. The resulting QSARs show that the inhibitory activities of taxane analogues against breast cancers are mainly dependent either on their hydrophobicity or the hydrophobic/molar refractivity descriptor of their substituents. A parabolic correlation with MR(Y) is the most encouraging example, in which the optimum value of this parameter is well defined. We believe this correlation may prove to be an adequate predictive model that can help provide guidance in design and synthesis and subsequently yield highly specific compounds that may have high anti-breast-cancer activity with fewer side effects and superior pharmacological properties. On the basis of this QSAR model, five compounds are suggested as potential synthetic targets. Internal (cross-validation (LOO-q(2) and LMO-q(2)), quality factor (Q), Fischer statistics (F), and Y-randomization) and external validation tests have validated all the QSAR models.

Complex formation between dodecylpyridinium chloride and multicharged anionic planar substances

The complex formation between dodecylpyridinium chloride (DPC) and multicharged anionic planar substances, 14 azo dyes and 3 benzene- or naphthalenesulfonates, has been studied by the potentiometric titration using a surfactant selective electrode. The agreement between the observed maximum binding number and the number of anionic charges (n) on dye molecules showed n:1 complex formation. The binding isotherms were found to be composed of two types of binding; one is the noncooperative binding observed at low surfactant concentrations and the other is the cooperative binding at the higher concentrations. The microscopic binding constant for the noncooperative binding was found to take the values in the range of 50-200 mol(-)(1) dm(3) for many of the substances, but, takes more large values up to 2500 mol(-)(1) dm(3) for the substances which have a large hydrophobic part or the structure of separate hydrophobic and hydrophilic regions. A multiple regression analysis showed that the data of the corresponding standard free energy change of binding were well interpreted by the equation (in unit of kJ mol(-)(1)) DeltaG degrees = - 5.85 log P(S) - 1.68 log P(D) - 2.12z + 28.4, where P(S) and P(D) are the partition coefficients of the surfactants and planar substances in the 1-octanol/water system and z is the number of anionic charges on the planar molecules. At the beginning of the cooperative binding, precipitate formation was observed for almost all of the present systems. Among these, some of the dyes having the structure of separate hydrophobic and hydrophilic regions formed a needlelike crystal, which was accompanied by a hysteresis phenomenon in the binding isotherm. The stable complex formation by both the hydrophobic and electrostatic interactions between the surfactant and the planar substances was found to be important for the crystal formation. Depending on the manner of arrangement of the charged groups on the planar substances, the origin of the binding cooperativity was ascribed to the interactions between surfactants bound to one planar-substance molecule or to the association of the complexes. It was also found that the present small binding systems are useful as the model of ligand binding to protein local structures.

DNA renaturation at the water-phenol interface

We study the renaturation of complementary single-stranded DNAs in a water-phenol two-phase system, with or without shaking. In very dilute solutions, each single-stranded DNA is strongly adsorbed at the interface at high salt concentrations. The adsorption of the single-stranded DNA is specific to phenol and relies on stacking and hydrogen bonding. We establish the interfacial nature of DNA renaturation at high salt, either with vigorous shaking (in which case the reaction is known as the Phenol Emulsion Reassociation Technique or PERT) or without. In the absence of shaking, the renaturation involves a surface diffusion of the single-stranded DNA chains. A comparison of PERT with other known renaturation reactions shows that PERT is the most efficient one and reveals similarities between PERT and the renaturation performed by single-stranded nucleic acid binding proteins. The most efficient renaturation reactions (either with PERT or in the presence of condensing agents) occur in heterogeneous systems, in contrast with standard thermal renaturation, which takes place in the bulk of a homogeneous phase. This work highlights the importance of aromaticity in molecular biology. Our results lead to a better understanding of the partitioning of nucleic acids, and should help to design improved extraction procedures for damaged nucleic acids. We present arguments in favor of interfacial scenarios involving phenol in prebiotic chemistry.

Valence Topological Charge-Transfer Indices for Dipole Moments

New valence topological charge-transfer indices are applied to the calculation of dipole moments. The algebraic and vector semisum charge-transfer indices are defined. The combination of the charge-transfer indices allows the estimation of the dipole moments. The model is generalized for molecules with heteroatoms. The ability of the indices for the description of the molecular charge distribution is established by comparing them with the dipole moments of a homologous series of phenyl alcohols. Linear and non-linear correlation models are obtained. The new charge-transfer indices improve the multivariable non-linear regression equations for the dipole moment. When comparing with previous results, the variance decreases 92%. No superposition of the corresponding G_k–J_k and – pairs is observed. This diminishes the risk of co-linearity. Inclusion of the oxygen atom in the π-electron system is beneficial for the description of the dipole moment, owing to either the role of the additional p orbitals provided by the heteroatom or the role of steric factors in the π-electron conjugation. Linear and non-linear correlations between the fractal dimension and various descriptors point not only to a homogeneous molecular structure but also to the ability to predict and tailor drug properties.

Asymmetric sulfoxidations mediated by alpha-chymotrypsin

The oxidation of aryl alkyl sulfides with H(2)O(2) in aqueous solution is a reasonably facile reaction producing racemic sulfoxides. We show that in the presence of the hydrolytic enzyme alpha-chymotrypsin such a sulfoxidation is accelerated and, more importantly, becomes stereoselective. With phenyl isobutyl sulfide as a model, the chymotrypsin-mediated, highly asymmetric oxidation is shown to occur in the hydrophobic binding pocket of the enzyme active site. The stereoselectivity of the chymotrypsin-mediated sulfoxidations is correctly explained by means of structure-based molecular modeling of the enzyme-sulfide complexes.

Characterization of (111)In(3+) complexes of DTPA amide derivatives: biodistribution and clearance studied by gamma imaging

A large series of structurally related diethylenetriaminepentaacetic acid amide derivatives with different structures and lipophilic properties were synthesized and radiolabeled with (111)In(3+). Two of the more hydrophobic compounds studied ([(111)In]L(9) and [(111)In]L(10)) showed high affinity for human serum albumin (HSA). The biodistribution and clearance properties shown by all complexes upon injection in Wistar rats were followed by gamma imaging. The blood retention time of the chelates correlates better with their binding to HSA than with their hydrophilic/lipophilic ratio. Hydrophilic and negatively charged complexes undergo renal retention, while the majority of the lipophilic complexes are retained in the blood for a longer period of time and are cleared through the liver.

Strategies for capillary electrophoresis: method development and validation for pharmaceutical and biological applications

This review is in support of the development of selective, reproducible and validated capillary electrophoretis (CE) methods. Focusing on pharmaceutical and biological applications, the successful use of CE is demonstrated by more than 800 references, mainly from 1994 until 1998. Approximately 80 recent reviews have been catalogued. These articles sum up the existing strategies for method development in CE, especially in the search for generally accepted concepts, but also looking for new, promising reagents and ideas. General strategies for method development were derived not only with regard to selectivity and efficiency, but also with regard to precision, short analysis time, limit of detection, sample pretreatment requirements and validation. Standard buffer recipes, surfactants used in micellar electrokinetic capillary chromatography (MEKC), chiral selectors, useful buffer additives, polymeric separation media, electroosmotic flow (EOF) modifiers, dynamic and permanent coatings, actions to deal with complex matrices and aspects of validation are collected in 20 tables. Detailed schemes for the development of MEKC methods and chiral separations, for optimizing separation efficiency, means of troubleshooting, and other important information for key decisions during method development are given in 19 diagrams. Method development for peptide and protein separations, possibilities to influence the EOF and how to stabilize it, as well as indirect detection are considered in special sections.

Characterizing cavities in model inclusion molecules: a comparative study

We have selected fullerene-60 and -70 cavities as model systems in order to test several methods for characterizing inclusion molecules. The methods are based on different technical foundations such as a square and triangular tessellation of the molecule taken as a unitary sphere, spherical tessellation of the molecular surface, numerical integration of the atomic volumes and surfaces, triangular tessellation of the molecular surface, and a cubic lattice approach to a molecular space. Accurate measures of the molecular volume and surface area have been performed with the pseudo-random Monte Carlo (MCVS) and uniform Monte Carlo (UMCVS) methods. These calculations serve as a reference for the rest of the methods. The SURMO2 and MS methods have not recognized the cavities and may not be convenient for intercalation compounds. The programs that have detected the cavities never exceed 5% deviation relative to the reference values for molecular volume and surface area. The GEPOL algorithm, alone or combined with TOPO, shows results in good agreement with those of the UMCVS reference. The uniform random number generator provides the fastest convergence for UMCVS and a correct estimate of the standard deviations. The effect of the internal cavity on the accessible surfaces has been calculated.

Traditional receptor theory and its application to neuroreceptor measurements in functional imaging

The mathematical, biological discipline of receptor pharmacology and the graphical methods of describing receptor behaviour evolved over a century of painstaking observation and model testing. Laws regarding in vitro theories are classically considered inoperative in vivo. Nevertheless, functional imaging techniques have rapidly evolved to allow receptor measurement and rules of thumb have been developed which clearly prove valid receptor parameters can be derived from functional imaging studies. The field is evolving so rapidly now that nuclear medicine researchers are in danger in applying these techniques without recourse to an understanding of the orthodox discipline of receptor pharmacology. This review attempts to document the basis of receptor pharmacology and to give an account of the theoretical and practical basis on which this can be applied in vivo. The review is targeted towards single-photon emission tomography because of the rapid growth in the area, but many parts draw on the literature relating to positron emission tompgraphy since the first translation of in vitro to in vivo measurement was performed with this technique.

Assessment of impact of physico-chemical drug properties on monitoring drug levels by micellar electrokinetic capillary chromatography with direct serum injection

The impact of physico-chemical properties of 25 compounds, including antiepileptic, anti-inflammatory and beta-blocking drugs, on their determination by micellar electrokinetic capillary chromatography (MECC) with direct serum injection (DSI) is discussed. Having a pH 9.2 buffer containing 75 mM sodium dodecyl sulfate (SDS), elution is dependent on hydrophobicity, the order of emergence being basically according to increasing octanol/water partition coefficients (logP values). Peak shape is determined by the dissociation behavior (expressed by pKa) and plasma protein binding (PPB). Sharp peaks are produced by compounds having low PPB and, independently of PPB, by drugs with pKa values which are similar to the buffer pH. Broad or double peaks are established by drugs of low pKa values and significant (> about 40%) PPB. In order to evaluate the effective amount of a protein-bound drug measured by MECC-DSI, serum levels of drugs with different PPB, namely ethosuximide (no PPB), phenobarbital (PPB of about 50%) and naproxen (PPB > 99%) have been determined by both MECC-DSI and MECC with extract injection (MECC-EXI). In each case, with more than 40 sera, there is good agreement between the two sets of data. Thus, employing MECC-DSI, total amounts of drugs are determined, i.e. a complete release of the drugs from the proteins is effected by the impact of dodecyl sulfate on the sampled proteins.

Binding of suprofen to human serum albumin. Role of the suprofen carboxyl group

The binding of suprofen (SP), a non-steroidal anti-inflammatory drug of the arylpropionic acid class, and its methyl ester derivative (SPM) to human serum albumin (HSA) was studied by dialysis and spectroscopic techniques. In spite of the remarkable differences in the physicochemical properties of SP and SPM, the binding of each molecule to HSA was quantitatively very similar. Thermodynamic analysis suggests that the interaction of SP with HSA may be caused by electrostatic as well as hydrophobic forces, whereas the interactions with SPM may be explained by hydrophobic and van der Waals forces. Similarities in the difference UV absorption spectra between ligand-detergent micelle and -HSA systems indicate that the SP and SPM molecules are inserted into a hydrophobic crevice on HSA. The same studies suggest that the carboxyl group of SP interacts with a cationic sub-site which is closely associated with the SP binding site. Proton relaxation rate measurements indicate that the thiophen ring and propanoate portion of the SP molecule is the major binding site for HSA. The locations of SP and SPM binding sites were identified by using fluorescence probes which bind to a known site on HSA. The displacement data implied that SP primarily binds to Site II, while the high affinity site of SPM as well as low affinity site of SP are at the warfarin binding site in the Site I area. From binding data with chemically modified HSA derivatives, it is likely that highly reactive tyrosine (Tyr) and lysine (Lys) residues, which may be Tyr-411 and Lys-195, are specifically involved in SP binding. In contrast, these two residues are clearly separated from the SPM binding site. The binding of SP and SPM is independent of conformational changes on HSA that accompany N-B transition. There is evidence that the carboxyl group may play a crucial role in the high affinity binding processes of SP to HSA.

Fundamental studies in reversed-phase liquid-solid extraction of basic drugs; III: Sample matrix effects

Using a set of 13 basic solutes (including a quarternary ammonium compound) with controlled pKa, log P and plasma protein binding, the factors which influence the extraction of these compounds from plasma by reversed-phase liquid-solid extraction have been studied. Direct analysis of plasma, avoiding deproteination, etc., is possible providing the sample is applied to the cartridge under the appropriate conditions. These conditions are dictated by the degree of plasma protein binding and are controlled and predicted from the lipophilicity of the compound. Plasma samples containing lipophilic compounds (log D(pH 7.4) > 2.03) require acidification and samples containing more polar compounds (log D(pH 7.4) < 0.9) should be applied untreated. Failure to use the appropriate conditions can result in significant losses from the cartridge on application or washing. In the intermediate lipophilicity range the optimal application conditions cannot be predicted. The effect of various aqueous-organic wash solvents have been investigated, and it was found that the cationic but not the proteinaceous components of plasma also affect the extraction process. This phenomenon which is probably due to an attenuation of the secondary cation exchange mechanism, results in more facile elution than when compounds are applied in water.

Preparation and evaluation of 17-ethynyl-substituted 16 alpha-[18F]fluoroestradiols: selective receptor-based PET imaging agents

We have prepared and studied six new analogs of 16 alpha-fluoroestradiol (FES): 17 alpha- and 17 beta-ethynyl-FES (7 [FEES] and 7a), and the 11 beta-ethyl (8 and 8a) and 11 beta-methoxy (9 and 9a) derivatives, novel estrogen receptor-based PET imaging agents. The relative binding affinity (RBA) for the estrogen receptor (ER) versus FES is increased for 7, 9 and 9a but decreased for 7a, 8 and 8a. All six analogs have been labeled in the 16 alpha position with 18F by the nucleophilic displacement of the corresponding 16 beta-trifluoromethanesulfonate with nBu4N18F. Subsequent ethynylation with lithium trimethylsilylacetylide yielded the FEES analogs (total synthesis time: 120 min; effective specific activity: 200-2400 Ci/mmol). Selective uptake in the uterus was high for [18F]7, [18F]8, [18F]9 and [18F]9a (% ID/g values at 1 h: 11.2, 12.9, 9.9 and 8.3, respectively), while uptake was effectively blocked by coinjection of an excess of unlabeled estradiol. The FEES analogs, [18F]7, [18F]8 and [18F]9, exhibited the highest selectivity, in terms of target (uterus)-to-blood ratios, ever seen amongst estrogen radiopharmaceuticals, 154, 145 and 169, respectively. The analogs [18F]7a and [18F]8a displayed no uptake in the uterus, consistent with their low RBAs. Metabolism studies revealed that most of the uterine activity is unmetabolized while the blood exhibits a rapid and subsequently sustained mixture of metabolites. The muscle shows a metabolic profile intermediate to the uterus and blood. These analogs provide an array of desirable characteristics for the optimal PET imaging of ER-rich target tissues.

Comparative Study of Interaction Mode of Diazepines with Human Serum Albumin and α1-Acid Glycoprotein

The binding of nine diazepines to human serum albumin (HSA) and to alpha 1-acid glycoprotein (AGP) was investigated by means of fluorescence and circular dichroism (CD) spectroscopies. The binding parameters of diazepam obtained from fluorescence agreed with those obtained from CD measurements. Diazepines have one tight binding site on both HSA and AGP. The binding parameters (nK) of the diazepine: HSA systems are slightly higher than those of diazepine:AGP systems. The relationship between the binding parameters for these two serum proteins and the physicochemical parameters of diazepines was studied by multiple regression analysis to elucidate the binding mode. Moreover, the effects of long-chain fatty acids and cesium chloride on the binding of diazepines to HSA and AGP were also studied. The driving force for the binding of diazepines to both proteins appears to be hydrophobic interaction. In addition, steric effects and electrostatic interactions may also contribute to the binding of diazepines to HSA and AGP, respectively.

Equilibrium and kinetic studies of the binding of tri- and tetra-anionic ligands to bovine serum albumin

The binding of tri- and tetra-anionic azo dyes (Amaranth, Ponceau 4R, and Ponceau 6R) to bovine serum albumin (BSA) at pH = 7.0 and 25 degrees C has been studied by equilibrium dialysis, spectrophotometry, and by stopped-flow and temperature-jump methods. Equilibrium dialysis revealed that BSA has one primary binding site and about two secondary sites for each dye. The values of the binding constant for the primary site show that the stability of the complex at the primary site progressively increases with an increase in the number and the density of anionic charges on ligand. Kinetic data have been found to be consistent with a scheme in which a rapid bimolecular binding is followed by two isomerizations of the complex (in the case of Amaranth) or by one isomerization (in the cases of Ponceau 4R and Ponceau 6R). Equilibrium and rate constants for each step of the scheme were determined. From the results it was found that the increment of the number and the density of anionic charges on ligand accelerates the forward process of the final isomerization step but retards the backward one of it, resulting in the enhancement of the stability of the complex at the primary site. On the basis of these results and the structure of the ligands, the detailed binding mechanism has been discussed in the light of the electrostatic interaction between the ligands and the binding site on BSA.

A quantitative structure-activity relationship approach to the minimization of albumin binding

The binding of 2,6-disubstituted xanthones to human serum albumin (HSA) has been investigated using an ultrafiltration technique. A set of 26 compounds was chosen for study using a selection procedure aimed at minimizing the interparameter correlations, while ensuring that the physicochemical properties covered the maximum possible range of values. The magnitude of binding has been expressed as the compound concentration required to produce a specified bound concentration, in preference to equilibrium constants and number of albumin binding sites. Albumin binding was found to have a nonlinear dependence on the octanol-water partition coefficient (log P) and has been rationalized in terms of a simple binding model.

Quinacrine inhibits the calcium-induced calcium release in heavy sarcoplasmic reticulum vesicles

Quinacrine is a fluorescence probe useful for studying the effect of local anesthetics. The interaction of quinacrine and sarcoplasmic reticulum membranes measured by fluorescence spectroscopy indicates the presence of a saturable binding site. Typical local anesthetics are able to displace quinacrine bound to heavy sarcoplasmic reticulum membranes. The effectiveness of that displacement decreases in the order dibucaine greater than tetracaine greater than benzocaine greater than lidocaine greater than procaine greater than procainamide, indicating that the size and hydrophobicity of quinacrine are major determinants in the binding process. The use of radioactive tracer and a rapid filtration technique reveals that quinacrine interacts, at lower concentrations, with sarcoplasmic reticulum membranes by blocking the Ca2+-induced Ca2+ release. Higher quinacrine concentrations also affect the Ca2+-pump activity.

Environment-selective synergism using self-assembling cytotoxic and antimicrobial agents

Environment-selective synergistic toxicity using combinations of aldehydes and hydrazine derivatives was demonstrated in two different model systems in vitro. Combinations of 5-nitro-2-furaldehyde with semi-carbazide and of 2-hydrazinopyridine with pyridine-2-carboxaldehyde, which can react in situ to form antimicrobial hydrazones, demonstrated greater degrees of synergism against the intracellular pathogen, Salmonella typhimurium, at pH 5 relative to pH 7.4. Combinations are more selectively toxic at pH 5 (vs pH 7.4) than individual precursors and preformed hydrazone products because acid catalysis of hydrazone formation plays a role only for the combinations. A combination of decanal and N-amino, N'-octylguanidine (AOG) exhibited more pronounced synergistic cytolytic activity against erythrocytes in 0% serum than in 1% serum. Serum protein binding of decanal inhibited the formation of the more cytotoxic hydrazone, N-decylidenimino,N'-1-octylguanidine (DIOG), from the less cytotoxic AOG and decanal, and serum protein binding of DIOG prevented this cytotoxin from reaching the cell membrane. Because decanal binding cannot play a role in the cytotoxicity of preformed DIOG, it was less selective for cells in 0% serum than the combination of AOG and decanal. The pH 5 and 0% serum environments represent very simple models for macrophage phagolysosomal compartments and poorly vascularized solid tumor interiors respectively. If environment-selective synergism can be used as a basis for target-selective synergism in other in vitro model systems and in vivo, self-assembling combinations could provide a basis for rational introduction of target-selective synergism into chemotherapeutic drug design.

Some advantages of calculating octanol-water partition coefficients

Reliable values for the octanol-water partition coefficients of small solutes used in enzyme binding/inhibition studies are desirable if a hydrophobic effect is being examined. Many investigators will grant that measuring each solute by shake-flask is the ideal, but reversed-phase HPLC data is becoming more commonplace since it is easier to obtain. This note examines one study where HPLC hydrophobicity, from an apparently valid procedure, obscured an obvious hydrophobic relationship. Another example is cited where probable errors in shake-flask measurement also lead to an erroneous conclusion. In both instances, values obtained from a simple method of calculation of log P(o/w) values could have provided a warning.