Correlation of serum binding of penicillins with partition coefficients

Diffusion of Drugs into Prostatic Fluid and Milk

THE PROSTATIC FLUID/PLASMA CONCENTRATION RATIO of various sulfonamides, antibiotics and antibacterial agents, and the milk/plasma ratio of sulfonamides and basic drugs have been quantitatively correlated with the degree of dissociation, as represented by log U/D, and the partition coefficient (log P). Because of the lower pH values of the prostatic fluid (6.6) and milk (6.8) as compared with the plasma pH (7.4), the degree of dissociation appears to be the most important factor in determining the distribution of these weak acids or bases. Partition coefficient also plays a secondary role. The log Po for maximum diffusion into milk is lower than that for maximum gastrointestinal, buccal or percutaneous absorption. This may be attributed to plasma protein binding which prevents diffusion of the drug from the blood circulation into the milk since high lipophilicity favors protein binding. The clinical implications of the correlations obtained are discussed.

Synthesis and structure-activity relationship studies of 4-arylthiosemicarbazides as topoisomerase IV inhibitors with Gram-positive antibacterial activity. Search for molecular basis of antibacterial activity of thiosemicarbazides

1-(indol-2-carbonyl)-4-(4-nitrophenyl)-thiosemicarbazide was synthesized and antibacterial and type IIA topoisomerases (DNA gyrase and topoisomerase IV) activity was evaluated. It was found that it shows activity against Gram-positive bacteria with MICs of 50 μg/mL and inhibitory action against topoisomerase IV with an IC(50) of 14 μM. Although modification of its structure resulted in molecules with a lower biological profile, our observations strongly implicate that thiosemicarbazide derivatives participate in at least two different mechanisms of antibacterial activity; one is connected with the inhibition of topoisomerase IV, while the nature of the other cannot be elucidated from the limited data collected thus far. The differences in bioactivity further investigated by the molecular modeling approach and docking studies suggest that inhibitory activity of 4-arylthiosemicarbazides is connected with electronic structure rather than the geometry of the molecule.

The interaction of penicillins with phospholipids

The interaction of the antibiotics, penicillin G and ampicillin, with sonicated sols of phosphatidylcholine, lysophosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and phosphatidylserine has been examined by Sephadex gel nitration and dynamic dialysis. Nuclear magnetic resonance spectroscopy provided evidence of a predominantly hydrophobic interaction between the antibiotics and the phospholipids, phosphatidylserine and phosphatidylcholine. Confirmation of hydrophobic interaction was provided by a rheological investigation of the effects of urea and guanidine hydrochloride on the antibiotic-phospholipid complex. Penicillin G was found to interact to a greater degree than ampicillin, a result which is of interest in the light of present knowledge of in vivo activity of these antibiotics.

4D-QSAR analysis and pharmacophore modeling: electron conformational-genetic algorithm approach for penicillins

4D-QSAR studies were performed on a series of 87 penicillin analogues using the electron conformational-genetic algorithm (EC-GA) method. In this EC-based method, each conformation of the molecular system is described by a matrix (ECMC) with both electron structural parameters and interatomic distances as matrix elements. Multiple comparisons of these matrices within given tolerances for high active and low active penicillin compounds allow one to separate a smaller number of matrix elements (ECSA) which represent the pharmacophore groups. The effect of conformations was investigated building model 1 and 2 based on ensemble of conformers and single conformer, respectively. GA was used to select the most important descriptors and to predict the theoretical activity of the training (74 compounds) and test (13 compounds, commercial penicillins) sets. The model 1 for training and test sets obtained by optimum 12 parameters gave more satisfactory results (R(training)(2)=0.861, SE(training)=0.044, R(test)(2)=0.892, SE(test)=0.099, q(2)=0.702, q(ext1)(2)=0.777 and q(ext2)(2)=0.733) than model 2 (R(training)(2)=0.774, SE(training)=0.056, R(test)(2)=0.840, SE(test)=0.121, q(2)=0.514, q(ext1)(2)=0.641 and q(ext2)(2)=0.570). To estimate the individual influence of each of the molecular descriptors on biological activity, the E statistics technique was applied to the derived EC-GA model.

Preparation of novel anthranilic acids as antibacterial agents: Extensive evaluation of structural and physical properties on antibacterial activity and human serum albumin affinity

In the past few years a significant effort has been devoted by Pharmacia toward the discovery of novel antibiotics. We have recently described the identification of an anthranilic acid lead 1 and the optimization resulting in the advanced lead 2. In this report, we describe the preparation of several selected analogs to probe the dependency of this template for antibacterial activity and the affinity these compounds have for human serum albumin (HSA). These analogs illustrate that decreased affinity for HSA can be achieved while retaining relevant antibacterial activity. The most important factor for reduced HSA affinity is decrease in logP rather than a structural change.

QSAR modeling of beta-lactam binding to human serum proteins

The binding of beta-lactams to human serum proteins was modeled with topological descriptors of molecular structure. Experimental data was the concentration of protein-bound drug expressed as a percent of the total plasma concentration (percent fraction bound, PFB) for 87 penicillins and for 115 beta-lactams. The electrotopological state indices (E-State) and the molecular connectivity chi indices were found to be the basis of two satisfactory models. A data set of 74 penicillins from a drug design series was successfully modeled with statistics: r2 = 0.80, s = 12.1, q2 = 0.76, spress = 13.4. This model was then used to predict protein binding (PFB) for 13 commercial penicillins, resulting in a very good mean absolute error, MAE = 12.7 and correlation coefficient, q2 = 0.84. A group of 28 cephalosporins were combined with the penicillin data to create a dataset of 115 beta-lactams that was successfully modeled: r2 = 0.82, s = 12.7, q2 = 0.78, spress = 13.7. A ten-fold 10% leave-group-out (LGO) cross-validation procedure was implemented, leading to very good statistics: MAE = 10.9, spress = 14.0, q2 (or r2press) = 0.78. The models indicate a combination of general and specific structure features that are important for estimating protein binding in this class of antibiotics. For the beta-lactams, significant factors that increase binding are presence and electron accessibility of aromatic rings, halogens, methylene groups, and =N- atoms. Significant negative influence on binding comes from amine groups and carbonyl oxygen atoms.

Prediction of adipose tissue: plasma partition coefficients for structurally unrelated drugs

Tissue:plasma (P(t:p)) partition coefficients (PCs) are important parameters describing tissue distribution of drugs. The ultimate goal in early drug discovery is to develop and validate in silico methods for predicting a priori the P(t:p) for each new drug candidate. In this context, tissue composition-based equations have recently been developed and validated for predicting a priori the non-adipose and adipose P(t:p) for neutral organic solvents and pollutants. For ionizable drugs that bind to different degrees to common plasma proteins, only their non-adipose P(t:p) values have been predicted with these equations. The only compound-dependent input parameters for these equations are the lipophilicity parameter, such as olive oil-water PC (K(vo:w)) or n-octanol-water PC (P(o:w)), and/or unbound fraction in plasma (fu(p)) determined under in vitro conditions. Tissue composition-based equations could potentially also be used to predict adipose tissue-plasma PCs (P(at:p)) for ionized drugs. The main objective of the present study was to modify these equations for predicting in vivo P(at:p) (white fat) for 14 structurally unrelated ionized drugs that bind substantially to plasma macromolecules in rats, rabbits, or humans. The second objective was to verify whether K(vo:w) or P(o:w) provides more accurate predictions of in vivo P(at:p) (i.e., to verify whether olive oil or n-octanol is the better surrogate for lipids in adipose tissue). The second objective was supported by comparing in vitro data on P(at:p) with those on olive oil-plasma PC (K(vo:p)) for five drugs. Furthermore, in vivo P(at:p) was not only predicted from K(vo:w) and P(o:w) of the non-ionized species, but also from K*(vo:w) and P*(o:w), taking into account the ionized species in addition. The P(at:p) predicted from K*(vo:w), P*(o:w), and P(o:w) differ from the in vivo P(at:p) by an average factor of 1.17 (SD = 0.44, r = 0.95), 15.0 (SD = 15.7, r = 0.59), and 40.7 (SD = 57.2, r = 0.33), respectively. The in vitro values of K(vo:p) differ from those of P(at:p) by an average factor of 0.86 (SD = 0.16, r = 0.99, n = 5). The results demonstrate that (i) the equation using only data on fu(p) as input and olive oil as lipophilicity surrogate is able to provide accurate predictions of in vivo P(at:p), and (ii) olive oil is a better surrogate of the adipose tissue lipids than n-octanol. The present study is an innovative method for predicting in vivo fat partitioning of drugs in mammals.

Algal toxicity of antibacterial agents used in intensive farming

The growth inhibiting effects of eight antibiotics used either therapeutically or as growth promoters in intensive farming on two species of micro algae, Microcystis aeruginosa (freshwater cyanobacteria) and Selenastrum capricornutum (green algae) were investigated. The effects of the antibiotics benzylpenicillin (penicillin G) (BP), chlortetracycline (CTC), olaquindox (O), spiramycin (SP), streptomycin (ST), tetracycline (TC), tiamulin (TI) and tylosin (TY) were tested in accordance with the ISO 8692 (1989) standard protocol. Algal growth was measured as increase in chlorophyll concentration by extraction with ethanol followed by measurement of fluorescence. Results were quantified in terms of growth rates using the Weibull equation to describe the concentration response relationship. The toxicity (EC50 value, mg/l) in alphabetic order were BP (0.006); CTC (0.05); O (5.1); SP (0.005); ST (0.007); TC (0.09); TI (0.003) and TY (0.034) for M. aeruginosa. BP (NOEC = 100); CTC (3.1); O (40); SP (2.3); ST (0.133); TC (2.2); TI (0.165) and TY (1.38) for S. capricornutum. In this investigation M. aeruginosa is found to be about two orders of magnitude more sensitive than S. capricornutum. It was observed that most of the compounds were unstable during the test period due to hydrolysis and photolysis.

Interaction of substituted phenoxazine chemosensitizers with bovine serum albumin

The binding of 10-[3'-[N-bis(hydroxyethyl)amino]propyl]phenoxazine [BPP], 10-[3'-[N-bis(hydroxyethyl)amino]propyl]-2-chlorophenoxazine [BPCP], 10-[3'-[N-bis-(hydroxyethyl)amino]propyl]-2-trifluoromethylphenoxazin e [BPFP], 10-(3'-N-pyrrolidino propyl)-2-chlorophenoxazine [PPCP] or 10-(3'-N-pyrrolidinopropyl)-2-trifluoromethylphenoxazine [PPFP] to bovine serum albumin (BSA) has been measured by gel filtration and equilibrium dialysis methods. The binding of these modulators to bovine serum albumin based on dialysis experiments has been characterized by the following parameters: percentage (beta) of bound drug, the association constant 'K1', the apparent binding constant 'k' and the free energy deltaFdegrees. The binding of phenoxazine derivatives to bovine serum albumin is correlated with their octanol-water partition coefficient, log10P. In addition, the displacing activity of hydroxyzine and acetylsalicylic acid on the binding of phenoxazines to albumin has been studied. The results of the displacing experiments showed that the phenoxazine benzene rings and the tertiary amines attached to the side chain of the phenoxazine moiety are bound to a hydrophobic area on the albumin molecule.

QSAR analysis of skin permeability of various drugs in man as compared to in vivo and in vitro studies in rodents

A general mathematical model involving partition coefficient, molecular weight and hydrogen bonding has been formulated for correlating the structures and skin permeability of a wide range of compounds through human skin and through hairless mouse skin. The correlations obtained are dependent not only on the biological system but also on the vehicle used. Without the use of lipophilic vehicle, the ideal lipophilicity for maximum permeability through human skin as measured by log Po(oct/w) ranges from 2.5 to 6 (extrapolated value). When a lipophilic vehicle was used in hairless mouse skin study, the log Po(oct/w) was lowered to around 0.4 approximately 0.6. While increased M.W. always has a negative effect on the permeability, increased H-bond can have either a slight positive or a slight negative effect, depending on the experiments (absorption vs. permeability constant). Cross validations with previously unanalyzed data as well as other biological systems support the usefulness of the general model developed for passive diffusion.

Hydrophobicity of beta-lactam antibiotics. Explanation and prediction of their behaviour in various partitioning solvent systems and reversed-phase chromatography

beta-Lactam antibiotics tend to undergo self-association in hydrophilic organic solvents, which leads to a strong dependence of their experimentally observable log P values on the partitioning conditions. As a result, most of the earlier obtained log P values for beta-lactam antibiotics cannot be applied as a common hydrophobicity measure, but they proved to be linearly related to each other and to a large body of reversed-phase chromatographic data. The retention of cephalosporins on reversed-phase liquid chromatographic columns is complicated by silanophilic interactions. However, under elution conditions that eliminate these silanophilic interactions, good correlations with log P data are observed, and a unified hydrophobicity scale for 90 penicillin and cephalosporin compounds could be evaluated. The Hansch and Leo additive scheme was shown to be valid for the calculation of hydrophobicities for penicillin and cephalosporin C-6(7) substituents, but it failed when applied to the prediction of cephalosporin C-3-substituent hydrophobicities. The hydrophobic increments for the sixteen most common cephalosporin C-3-substituents were empirically evaluated from literature data, and a simple equation was derived for an overall beta-lactam antibiotic hydrophobicity calculation. The proposed scale is valid for predicting the partitioning of most beta-lactam antibiotics in both hydrophilic and lipophilic organic-water systems, although it should be used with caution when applied to antibiotics containing additionally charged side-chains.

Structure-pharmacokinetic relationships among the N1,N3-alkylxanthines in rats

The pharmacokinetics of four N3-alkylxanthine and four N1-methyl-N3-alkylxanthine derivatives has been investigated in rats after intravenous administration of the individual alkylxanthines. The concentration of N1,N3-alkylxanthine in plasma and urine was determined by HPLC. A one-compartment model adequately described the plasma concentration time data. The steady-state volume of distribution (Vss) was calculated using model-independent methods. The relation between Vss and unbound drug fraction in plasma (fu) was significantly correlated (Vss = 0.844fu + 0.119; r = 0.999, P less than 0.01), indicating that the differences in fu among these xanthine derivatives is mainly responsible for differences in Vss. The decrease in Vss and increase in plasma protein binding with lipophilicity reflected a relatively constant tissue affinity. The total body clearance increased with lipophilicity with the exception of the first three lower congeners which were almost completely excreted unchanged in urine, mainly via active tubular secretion. Renal elimination was markedly reduced by the presence of a methyl group at the N1-position. Renal clearance decreased with increasing lipophilicity, due to increased tubular reabsorption whereas non-renal (hepatic) clearance increased with increasing lipophilicity.

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.

Synthesis of N-substituted 3'-amino-3'-deoxythymidines and their biological evaluation against HIV

Treatment of 3'-amino-3'-deoxythymidine (1) with carboxylic acid anhydrides afforded the corresponding acylamino derivatives 2a-f. Reaction of 1 with a variety of isothiocyanates led to the corresponding thioureido derivatives 3a-i. Also, conversion of 1 into 3'-carbylamino-3'-deoxythymidine (7) is reported. The compounds 2, 3, and 8 were evaluated for their anti-HIV activity in MT-4 cells, but did not show sufficient efficacy.

Quantitative structure--pharmacokinetic relationship of a series of sulfonamides in the rat

The pharmacokinetics of a series of sulfonamides were investigated after intravenous administration of a 7 mg/kg dose of individual sulfonamides to cannulated female lean Zucker rats. The concentrations of the sulfonamides in blood were determined by colorimetry. The blood concentration-time curves were fitted to a biexponential equation. The partition coefficient, log P, and pKa values of the sulfonamides were taken from the literature, log P and pKa values differed markedly across the series. The extent of protein binding varied enormously, increasing with partition coefficient. There was no significant relationship between the volume of distribution and partition coefficient. However, when the influence of protein binding on volume of distribution was eliminated, a significant linear relationship emerged. Total clearance formed a relatively complex nonlinear relationship with partition coefficient. The relationship of elimination half-life and partition coefficient was inverse of that between clearance and partition coefficient because of a lack of significant relationship between volume of distribution and partition coefficient.

Drug binding and solubility in milk

The binding and solubility of nitrofurantoin, piroxicam, indomethacin, prednisolone, diazepam, dicumarol, and griseofulvin in milk were determined at 15, 25, and 37 degrees C in bovine milk samples with fat contents of 0.75 and 3.50%. Drug binding to milk components was independent of drug concentration over the drug concentration studied, and the fat content of milk strongly affected binding values of most of the listed drugs. Further, drug binding increased with decreasing temperatures for most of the drugs examined. The solubility of all drugs is greatly enhanced in milk compared to their aqueous solubility (pH 6.5 phosphate buffer). The high solubility cannot be accounted for solely on the basis of drug binding to milk components. An attempt is made to correlate the binding and solubility data with physicochemical properties of the drugs (logP, pKa, aqueous solubility). The potential significance of these findings is discussed with regard to preparation and in vivo delivery of drugs from drug-milk formulations.

The binding of thioureylene compounds to human serum albumin

The binding interactions of some thioureylene compounds to human serum albumin were studied in vitro by ultraviolet spectroscopy and equilibrium dialysis. Binding of 6-n-propyl-2-thiouracil, 6-n-benzyl-2-thiouracil and 2-thiouracil to human serum albumin results in a red shift of the ultraviolet absorption maximum, suggesting that the binding site is a hydrophobic area of the protein. Bindings of 6-n-propyl-2-thiouracil and 6-n-benzyl-2-thiouracil to human serum albumin are characterized by two classes of sites while 6-n-propyl-uracil and 2-thiouracil bind to one low-affinity binding site. In addition, an identification of those sites was performed by measuring the displacement of these drugs. The data show that the moderate-affinity site is common with the warfarin site while the low-affinity site is likely to be shared by benzodiazepines. It is concluded that the binding is enhanced by the hydrophobicity of the substituent in the thioureylene compounds, and it is further shown that thiol-group substitutions in the thioureylene ring will weaken the binding.

Prediction of hepatic first-pass metabolism and plasma levels following intravenous and oral administration of barbiturates in the rabbit based on quantitative structure-pharmacokinetic relationships

Based on the concept of physiological pharmacokinetics, the hepatic first-pass metabolism and plasma levels following intravenous and oral administration of barbiturates in the rabbit was predicted based on the relationships between principle kinetic parameters and lipophilicity (chloroform-water partition coefficient). Good log-log linear relationships between kinetic parameters and lipophilicity were obtained for the seven barbiturates examined. The values of correlation coefficient were improved slightly by using the corrected values for partition coefficients of nonionic molecules in the cases of principle parameters such as drug-protein and drug-blood cell affinity, intrinsic hepatic clearance, and unbound volume of distribution. There was also a good linear relationship between absorption rate constant (mean absorption time) and lipophilicity. The mean hepatic transit time was negligible for the most lipophilic drug (hexobarbital) examined; this suggests that the mean absorption time for these barbiturates does reflect the absorption process. The available fraction in relation to hepatic first-pass metabolism was well predicted from the lipophilicity by both well-stirred and parallel-tube models, and the difference in the values predicted by both models was minimal. There were good relationships between predicted and observed values for plasma levels after intravenous and oral administration, except for two (cyclobarbital and phenobarbital) of the seven drugs used. The great difference between predicted and observed values for these two drugs was considered due to substituent effects in liver metabolism.

Simple spectrophotometric assay for measuring protein binding of penem antibiotics to human serum

The binding of antibiotics to plasma (serum) proteins through hydrogen bonding can significantly influence the biological characteristics of these drugs. A rapid spectrophotometric assay has been developed that measures the level of free (unbound) penem antibiotic in serum ultrafiltrates. Whole human serum was adjusted to a standard concentration of antibiotic and then filtered by centrifugation through a Centrifree (Amicon Corp., Lexington, Mass.) filter that retained greater than 99.9% of serum protein. The degree of penem protein binding was determined spectrophotometrically by measuring the level of unbound drug in the ultrafiltrate at 322 nm. At this wavelength, no interfering absorption from residual protein was detected in the ultrafiltrate, and penem absorption was linear over a wide concentration range. The method gave protein-binding values comparable to those obtained by a high-pressure liquid chromatography assay but was more rapid, since it did not require solvent extraction and high-pressure liquid chromatography calibration procedures. The spectrophotometric assay has been used to assay over 100 penems to determine the structure-activity relationships that are involved with the high serum protein binding of these agents. As with penicillins and some cephalosporins, the nonpolar nature of the penem side chain at the C-2 position strongly influenced the degree of penem binding to serum proteins.

Interaction of povidone with aromatic compounds. VI: Use of partition coefficients (log Kd) to correlate with log P values and apparent Kd values to express the binding as a function of pH and pKa

The binding of neutral aromatic compounds onto povidone was studied. It was found that the binding depends on the lipophilic character of the compound and that a linear free relationship exists between the logarithm of the partition coefficients of the macromolecular pseudo-two-phase-aqueous phase, and the log partition coefficient of n-octanol-water (log P). For the ionized ligand molecules it was shown that the binding could be expressed in terms of the acid dissociation constant of the solute, Ka, and in terms of two partition coefficients, K1 = HAPVP/HAwater and K2 = APVP/Awater for the nondissociated and dissociated forms, respectively, resulting in the apparent partition coefficient Kdapp = (HAPVP + APVP)/(HAwater + Awater). An expression was derived, permitting one to determine K1 and K2 from measurable quantities. The apparent partition coefficients (Kdapp) were independent of both drug and povidone concentrations, indicating that the modes of povidone compound interactions were essentially invariant over the ranges of systematic variables studied. The method provides a simple means of evaluating ligand-macromolecule interaction as a function of pH of the solvent and pKa of the ligand.

Relationship between the solubility parameter and the binding of drugs by plasma proteins

An equation, based on regular solution theory, was used to relate the solubility parameter to the binding of drugs by plasma proteins. The equation was tested on a homologous series and a good correlation was found. Sulfonamides showed maximum binding when their solubility parameters were similar to the solubility parameters of the amino acids situated in a sequence with one tryptophan residue. This observation supports the assumption that this sequence is the primary binding site for the sulfonamides. Binding peaks were also found at solubility parameters in other drug series corresponding to the solubility parameters of human serum albumin (HSA) or bovine serum albumin (BSA) amino acids. It is suggested that the solubility parameter could be used to predict the binding of drugs to plasma proteins.

QSAR analysis of drug excretion into human breast milk

Breast feeding has increased by approximately 25% in the United States during the past decade and this trend appears to be continuing. The number of drugs available to lactating women is also growing at a rapid pace. The excretion of drugs into breast-milk presents a potential danger to infants. In spite of this, little is known about the excretion of drugs into breast-milk. The ability to predict which drugs are potential hazards would be very useful in the clinical setting. This study quantitatively correlates the human milk to plasma concentration ratio of various basic and acidic drugs (log M/P) with the square root of the molecular weight, the partition coefficient (log P) and the degree of dissociation (log U/D). For basic drugs there is a negative-dependence on both log P and log U/D. High lipophilicity favours protein binding and reduces the amount of drug available for diffusion into milk. Therefore, as log P increases, the log M/P decreases. The negative-dependence on log U/D indicates that the higher the degree of dissociation of the base in plasma, the greater the log M/P will be. This fits well with the concept of ion-trapping. A strong base is more likely to be transferred and then trapped in milk which has a lower pH than plasma. For acidic drugs there is a negative-dependence on both square root (MW) and log P. The negative-dependence on square root (MW) suggests that large molecules are less likely to be able to diffuse into the milk. A negative-dependence on log P appears to hold true for bases and acids. Log M/P decreases as log P increases. This is probably due to increased protein binding by lipophilic drugs through non-specific hydrophobic interaction with plasma protein.

Specific and non-specific ligand binding to serum albumin

The applicability of a model of specific and non-specific binding sites for the binding of organic ligands to serum albumin was examined statistically. It is proposed in this model that there are a limited number of a single class of high affinity sites and an unlimited number of low affinity sites. Ligands bind to the former sites independently, as in the Scatchard model for a single class of binding sites, and to the latter sites in a partition-like manner, the amounts bound increasing linearly with the concentration of free ligands. In this study, data on the binding of warfarin to human serum albumin reported by Wilting et al. [J. Wilting et al., J. biol. Chem. 255, 3032 (1980)] and on the bindings of indomethacin and salicylic acid to human serum albumin reported by Hultmark et al. [D. Hultmark et al., Acta pharm. suecica 12, 259 (1975)] were used. These binding data were analyzed according to the model of specific and non-specific sites as well as the Scatchard model of two classes of binding sites. Statistical analyses showed that the model of specific and non-specific binding sites fitted the data for binding of all compounds very well, indicating the applicability of this model. An index of the upper limit of ligand concentrations necessary for accurate analysis of the binding data was also discussed.

Interaction of povidone with aromatic compounds V: Relationship of binding tendency in a macromolecular solution treated as a pseudo two phase and a monophase

The pseudo-two-phase model is proposed to correlate complex formation of ligand molecules with povidone with partition coefficients (log P or II constants). The conditions which permit the use of the pseudo-two-phase model for binding of ligand onto macromolecules are determined. This model seems to be a more rational choice than the frequently used complex formation model (monophase). This is demonstrated theoretically and confirmed experimentally. The advantages of the use of such a model are also discussed.

The binding of aryl carboxylic acid derivatives to human serum albumin--a structure-activity study

The binding interactions of some aryl carboxylic acid derivatives have been examined by circular dichroism and fluorescence spectroscopy. With specific probes, we have shown that the seven ligands under study bind primarily to the benzodiazepine site on HSA. Their association constants are in the range of 10(5)-10(6) M-1 as found by spectropolarimetric titration, and appear to be closely related to some chemical features. It is concluded that the binding is enhanced by the lengthening of the carbon chain substituent with a terminating carboxyl moiety and by halogen substitution in the aromatic rings. It is further shown that hydrophilic substitutions such as hydroxyl or ketone groups in the carbon chain substituent will decrease the binding.

Chain length dependency of fatty acid and carbamate binding to serum albumin

The binding interactions of bovine serum albumin (BSA) with the unbranched fatty acids (FA) pentanoate (five-carbon chain length: C5) up to nonanoate (C9), and the carbamates n-methyl carbamate (equivalent to C3) up to n-hexyl carbamate (equivalent to C8) were examined using an ultrafiltration technique. A single, high-affinity site was observed for each of the FA, with an increasing number of secondary sites with increasing chain length. From binding affinity and competition data, there appear to be distinct albumin sites for the short-chain (less than or equal to C7) and the medium-chain (greater than or equal to C8) FA. Published data suggest that the medium-chain FA site is one of the major drug-binding sites on human serum albumin (HSA) or BSA, the indole/benzodiazepine site. Competition between the FA and warfarin for BSA or HSA binding was studied by ultrafiltration and fluorescence methods and suggests that the short-chain FA site may lie in the same region as a second major drug-binding site, the large warfarin-binding area. Thermodynamic parameters of the FA-BSA interactions are suggestive of primary binding being a combination of electrostatic and hydrophobic binding and secondary binding being purely hydrophobic in nature. Carbamate interactions with BSA show several primary sites and also suggest a disparity between the binding of ligands of less than or equal to 7 and greater than or equal to 8 in total length, but there was no evidence of competition between FA and carbamates. A model is proposed to explain these observations, which includes the suggestion that several classes of hydrophobic binding areas exist, each of which is specific for ligands of a restricted range of chain lengths.

Plasma protein binding of fentanyl

Whole plasma of 15 normal volunteers bound 79·16% ± 0·16 s.e. (n = 45) of fentanyl, at a concentration of 0·6 ng ml−1. Measurement was by equilibrium dialysis at 37°C, pH 7·4. The largest contribution to binding appears to be due to serum albumin, since 45·52% ± 0·40 s.e. (n = 3) of fentanyl was bound to a solution of purified albumin at a concentration of 46 g litre−1 in buffer. Physiological concentrations of isolated very low density, low density and high density lipoprotein fractions in buffer bound 18·39% ± 0·65 s.e.; 39·14% ± 0·42 and 21·18% ± 0·51 (n = 15) respectively. A significant correlation was found between percent binding and serum albumin concentration (r = 0·745, P = 0·0022) and oestrogen and progestagen therapy (r = 0·766, P = 0·0014). There was no significant correlation with fasting serum cholesterol, triglyceride, age, sex or the concentration of total protein minus albumin. Binding to fibrinogen and α1-acid glycoprotein did not occur. Binding increased with increasing pH, temperature and ionic strength of the buffer. The results were compatible with hydrophobic bond formation between fentanyl and proteins. Fentanyl concentration did not affect the percent bound to whole plasma or the protein fractions over a range of 0·6 ng-10 mg ml−1. Dilution of plasma with buffer gave a linear relation of percent bound or free to log plasma dilution. The binding of fentanyl to pooled plasma of the normal subjects was not affected by a wide variety of anionic, cationic and uncharged drugs when these were tested at a concentration of 20 μg ml−1. At higher concentrations, aspirin, phenylbutazone and quinidine caused inhibition of fentanyl binding. A linear relation was found between percent bound and concentration of the inhibitory ligand. For aspirin, r = 0·873, P <0·01; for phenylbutazone, r = 0·81, P <0·05; and for quinidine, r = 0·982, P <0·01. Aspirin and phenylbutazone inhibited binding of fentanyl to albumin, while quinidine caused inhibition of binding to lipoproteins of all three densities, but not to albumin. Changes in the concentrations of the common ions of plasma (except H+), of free fatty acids and of creatinine did not affect fentanyl binding to whole plasma. 8 M urea reduced binding by 25% of the normal value.

Tissue distribution of beta-lactam antibiotics. Experimental studies in rabbits

Tissue distribution of ampicillin, dicloxacillin and cefuroxime was studied in rabbits. Different methods allowing frequent samples to be obtained from the same animal were used. Tissue fluid was obtained from subcutaneously implanted steel net cages and by implantation of thin cotton threads under the muscle fascia. A microtechnique was developed for determination of antibiotic concentrations in small muscle samples. The concentration time course in serum and tissue was followed after a single intravenous or intramuscular injection of the antibiotic. In tissue cage fluid, levels of ampicillin and dicloxacillin were low as compared to the serum concentrations. Also, maximum levels occurred later than in serum and elimination was slower. In contrast, rapid peak levels were achieved in muscle tissue with all three investigated antibiotics and the muscle concentrations declined parallel to the serum levels. The results also indicated that beta-lactam antibiotics are not homogeneously distributed in muscle tissue but mainly confined to the extracellular fluid volume. Whole tissue levels could therefore underestimate the actual antibiotic concentrations in the tissue fluid where the bacteria causing soft tissue infections are most likely to be found. Serum levels, however, seemed to be a good indicator for the concentration time course in tissue fluid.

Correlation of hydrophobicity with protein binding for clorobiocin analogs

A new method of equilibrium dialysis was used to measure the binding of analogs of clorobiocin (18631 R.P.) to human serum albumin. Binding constants and numbers of binding sites on human serum albumin were calculated from the binding data and were used to calculate the percentage of compounds free in equilibrium with 4% albumin. Partition coefficients between n-octanol and phosphate buffer (0.05 M, pH 7.4) also were measured. A positive linear correlation (r = 0.918, s = 0.240, and n = 10) was obtained between log (bound/free compound) and log partition coefficient.

Binding of drugs to human muscle

Binding of 22 drugs to human muscle tissue has been determined by ultrafiltration. All drugs tested were bound, the bound fraction ranging from 13% (aminophenazone) to greater than 98% (desipramine). Both linear and nonlinear binding was observed. For chemically related substances, binding to muscle tissue correlated with plasma binding and lipid solubility. There were significant differences in binding to muscle from different individuals. With respect to pharmacokinetics of drugs, it is suggested that binding to muscle tissue may be at least as important as plasma binding.