Binding of drugs to human serum albumin. I. Circular dichroism studies on the binding of some analgesics, sedatives and anti-depressive agents

Anticancer and antitrypanosomal activities of trinuclear ruthenium compounds with orthometalated phenazine ligands

Trinuclear ruthenium complexes with orthometalated phenazines of general formula [Ru3(μ3-O)(μ2-OAc)5(L)(py)2]PF6 (L = dppn, benzo[i]dipyrido[3,2-a:2',3'-c]phenazine, 1; dppz, dipyrido[3,2-a:2',3'-c]phenazine, 2; CH3-dppz, 7-methyldipyrido[3,2-a:2',3'-c]phenazine, 3; Cl-dppz, 7-chlorodipyrido[3,2-a:2',3'-c]phenazine, 4) were investigated for their cytotoxic activity toward the B16F10 murine melanoma and the L929 non-cancer cell lines and against Trypanosoma cruzi (2-4). This study also reports a multi-technique investigation into how complexes 1-4 interact with DNA and human serum albumin, HSA. At concentrations ranging from 2 to 50 μM, all the complexes reduced B16F10 murine melanoma cell viability by over 50%. Complex 4 had the highest cytotoxic effect in the series, diminishing B16F10 cell viability to 38% at 2 μM, with an overall order for anticancer activity of 4 > 2 > 3 > 1. Complexes 2-4 showed remarkable activity in inhibiting epimastigote and amastigote forms of T. cruzi. Complex 2 showed better antitrypanosomal activity than the reference drug (IC50 = 1.19 μM and IC50 = 0.25 μM for epimastigote and amastigotes forms, respectivily). Ethidium bromide (EB) displacement assays showed that DNA intercalation progressively increases with the extension of the π-conjugation of the cyclometalating ligand and the presence of substituents in the phenazinic portion (1 > 4-3 > 2), showing that complex 1 is a stronger intercalator than EB itself (Kapp > 107 M-1). Viscosity measurements followed the same trend. Cytotoxicity against cancer cells and antitrypanosomal activity follow the same order, which is different to the tendency of DNA intercalation, suggesting DNA is not the main target of these complexes. Compound 1-4 showed very high affinity with HSA (Kb ∼109 M-1). Circular dichroism results also showed that the complexes alter significantly the secondary structure of the HSA, lowering the α-helix % from 86.2 (pure protein) to less than 5% for compounds 1, 2 and 4 at 2.8 μM. These findings demonstrated the important role of phenazines for the biological activity of triruthenium compounds.

Fluorescence detection of serum albumin with a turnover-based sensor utilizing Kemp elimination reaction

The Kemp elimination reaction is a well-known chemical reaction that is facilitated on a protein surface microenvironment, and in particular is highly accelerated in a unique binding pocket of serum albumin. We have designed and synthesized a fluorescently activatable coumarin derivative with a benzisoxazole scaffold to enable monitoring of the Kemp elimination reaction in terms of fluorescence change for the first time. We show that this fluorescent sensor can sensitively and selectively quantitate serum albumin in blood samples. It also works in a dry-chemistry format.

Evaluation of alternatives to warfarin as probes for Sudlow site I of human serum albumin: characterization by high-performance affinity chromatography

Warfarin is often used as a site-specific probe for examining the binding of drugs and other solutes to Sudlow site I of human serum albumin (HSA). However, warfarin has strong binding to HSA and the two chiral forms of warfarin have slightly different binding affinities for this protein. Warfarin also undergoes a slow change in structure when present in common buffers used for binding studies. This report examined the use of four related, achiral compounds (i.e., coumarin, 7-hydroxycoumarin, 7-hydroxy-4-methylcoumarin, and 4-hydroxycoumarin) as possible alternative probes for Sudlow site I in drug binding studies. High-performance affinity chromatography and immobilized HSA columns were used to compare and evaluate the binding properties of these probe candidates. Binding for each of the tested probe candidates to HSA was found to give a good fit to a two-site model. The first group of sites had moderate-to-high affinities for the probe candidates with association equilibrium constants that ranged from 6.4 x 10(3)M(-1) (coumarin) to 5.5 x 10(4)M(-1) (4-hydroxycoumarin) at pH 7.4 and 37 degrees C. The second group of weaker, and probably non-specific, binding regions, had association equilibrium constants that ranged from 3.8 x 10(1)M(-1) (7-hydroxy-4-methylcoumarin) to 7.3 x 10(2)M(-1) (coumarin). Competition experiments based on zonal elution indicated that all of these probe candidates competed with warfarin at their high affinity regions. Warfarin also showed competition with coumarin, 7-hydroxycoumarin and 7-hydroxy-4-methycoumarin for their weak affinity sites but appeared to not bind and/or compete for all of the weak sites of 4-hydroxycoumarin. It was found from this group that 4-hydroxycoumarin was the best alternative to warfarin for examining the interactions of drugs at Sudlow site I on HSA. These results also provided information on how the major structural components of warfarin contribute to the binding of this drug at Sudlow site I.

Protein binding of piroxicam studied by means of affinity chromatography and circular dichroism

The protein binding of piroxicam, a widely used non-steroidal anti-inflammatory drug has been investigated by high-performance liquid affinity chromatography, with phenylbutazone and diazepam used as markers for binding-site characterization, and by circular dichroism titration. It was found that piroxicam binds to high-affinity phenylbutazone-binding sites and to high-affinity diazepam-binding sites. No binding to the low-affinity sites of either marker was established. High values of the primary (high-affinity) binding constants corresponding to both types of binding site were obtained by means of a mathematical method cited in the literature. The circular dichroic spectra of piroxicam were studied at a given albumin concentration and various drug concentrations. A new Cotton effect was observed and was ascribed to the binding of piroxicam to the protein molecule. The values of differential molar ellipticity (delta theta) were treated by a new mathematical procedure for analysis of the data obtained. A high affinity constant was calculated for one class of binding site. Its value is in good agreement with the values obtained by affinity chromatography. These results reveal that circular dichroism is an acceptable method for investigation of protein binding.

A quantitative circular dichroic investigation of the binding of the enantiomers of ibuprofen and naproxen to human serum albumin

The binding constants for racemic, R and S naproxen and ibuprofen to human serum albumin have been determined by a circular dichroic technique. The ibuprofens and naproxens show no measurable extrinsic optical activity on interaction with the protein, and so the extrinsic Cotton effect shown following the diazepam-albumin interaction is used as a probe. The presence of the drugs reduce the amount of diazepam bound as shown by the interaction is used as a probe. The presence of the drugs reduce the amount of diazepam bound as shown by the reduced size of the induced ellipticity. The calculated primary binding constants show that the S form of both drugs bind to the albumin more tightly than the R form and that the racemic forms bind less tightly than either enantiomer.

Direct quantitative determination of optically active absorbing drugs in human urine by circular dichroism. Simultaneous direct determination of beta-lactam antibiotics and proteins

Over 200 urine samples from 61 subjects were analyzed by circular dichroism spectroscopy. The results proved that this technique can be applied to the direct determination of optically active absorbing drugs in urine of subjects under multiple drug administration, independently of the presence of proteins, which can simultaneously be determined. A list of noninterfering drugs is included. The validity of the present method was confirmed by analysis of variance, the beta-lactam antibiotics ampicillin, cefoxitin, and cephalexin being chosen as model drugs and human albumin as the analytical standard for protein determination. The results demonstrated that the proposed method is accurate and precise, the correlation coefficients being higher than 0.9996. A circular dichroism and HPLC data comparison was successfully performed. The principal advantages of this method are simplicity, quickness, and economy, no derivatization or chromatographic separation steps being needed.

Protein binding investigation by difference circular dichroism: native and acetylated human serum albumins

A modified albumin was prepared by selective reaction of Lys199 with acetyl salicylic acid. Protein binding investigation was carried out on native and modified proteins by difference circular dichroism (delta CD). Acetylation of Lys199 reduces significantly the effects of the interaction between drugs in the stereoselective HSA binding at specific binding areas.

Stereoselective binding of the glucuronide of ketoprofen enantiomers to human serum albumin

Since acyl glucuronides are known to undergo deconjugation, especially in the presence of human serum albumin (HSA), only a few reports have described their reversible binding to plasma proteins. The aim of this study was to investigate the reversible binding of R and S ketoprofen glucuronides to HSA by a rapid technique, such as ultraviolet circular dichroism. Binding of R ketoprofen glucuronide only induced an extrinsic Cotton effect at 340 nm. Scatchard plot analysis revealed that R ketoprofen and its glucuronide are bound to one site of albumin with an association constant of 28.1 x 10(4) and 6.1 x 10(4) M-1, respectively. Modification of one tyrosine residue by diisopropylfluorophosphate prevented the access of ligands to sites I and II of albumin, and also fully inhibited the binding of R ketoprofen and that of its conjugate. Displacement experiments with specific probes of albumin binding sites suggested that R ketoprofen and the glucuronide are bound to site II rather than site I. However, R ketoprofen was not displaced by its conjugate. S ketoprofen glucuronide is also bound to HSA, since it decreased the binding of the antipode conjugate. However, the binding of this metabolite to albumin did not induce an extrinsic Cotton effect large enough to determine the binding constants. D-Glucuronic acid did not bind to sites I or II of albumin. This moiety is likely responsible for the lower affinity of HSA for the R ketoprofen glucuronide when compared to that for R ketoprofen, due to the hydrophilicity and/or the bulkiness of this group.

Non-labelled benzodiazepines partitioned into synaptosomal membranes: their extraction and quantification by high performance liquid chromatography

A method for the extraction and quantification by high performance liquid chromatography (HPLC) of non-labelled benzodiazepines partitioned into biological membranes has been developed. The benzodiazepine (BZD) was partitioned in a synaptosomal membrane-buffer system. The membranous pellet was separated by centrifugation and from this pellet, previously submitted or not to a proteolytic treatment and resuspended in the same buffer, the BZD was extracted by the addition of the proper volume of ethyl acetate, i.e. that capable of extracting at least 99.5% of the total drug in the aqueous phase. Optimal conditions for maximum proteolysis were: incubation of membranes at a final concentration of 1 mg protein/mL in the presence of 0.08 mg of Protease type I per mL for 90 min. at 45 degrees C. Although the efficiency of the proteolysis was demonstrated not only by kinetic but also by electrophoretic evidence, recovery of BZD in the organic solvent was not increased by the enzymatic treatment at a mass BZD/protein ratio in the range 4-4.6 10(-5) micrograms BZD/mg protein. For the HPLC quantification a reversed phase ODS column was used with methanol:water (1:1) plus 3.5% acetic acid as the mobile phase at a constant pressure and 1 mL/min flow rate. The UV detector was calibrated at 265 nm. Calibration curves were constructed by plotting peak height or peak area vs. nmoles BZD and adjusted to straight lines by a regression analysis by the least squares method. The peak height was selected as the detection method. Intra- and inter-assay precision did not exceed 10% and the total recovery was 100%.

Characterization of benzodiazepinooxazoles: crystal and molecular structure of trans and cis isomers of oxazolam

The stereochemistry of the trans and cis isomers of 2(S)- and 2(R)-oxazolam is described. A total of four molecular structures of the 11b(R) and 11b(S) diastereomers of 2(S)- and 2(R)-oxazolam have been revealed by X-ray diffraction techniques. 2(S)-Oxazolam crystallizes in space group P2(1)/c, with cell dimensions a = 16.180(2) A, b = 8.791(2) A, c = 13.574(2) A, and beta = 102.07(1) degrees. The molecule in an asymmetric unit exhibited disorder at the 2-carbon (C2) atom, and the structures for trans and cis isomers have been resolved crystallographically. 2(S)-Oxazolam forms polymeric chains with intermediation of ethanol through hydrogen bonding. 2(R)-Oxazolam crystallizes in space group P1, with cell dimensions a = 7.754(1) A, b = 8.654(1) A, c = 15.795(2) A, alpha = 104.09(1) degrees, beta = 91.19(1) degrees, and gamma = 114.00(1) degrees. A similar disorder at the C2 position has been elucidated by a mixture of trans- and cis-2(R)-oxazolams. The two 2(R)-oxazolam molecules form a dimer with a hydrogen bond net. The structural details, focusing on the conformation and molecular complex formation of both oxazolams, have been discussed in connection with the oxazolidine ring stability and the crystal polymorph.

Acute toxicity of pyrazolones

Pyrazolone intoxication accounts for most (52 percent) mild analgesic poisonings in West Germany. Severe and fatal intoxication with pyrazolones is, however, rare. In the German literature, only 50 cases have been described in the past 62 years; 80 to 90 percent of these were caused by aminopyrine, which was withdrawn from the West German market in 1978 and replaced by propyphenazone. Up to now, no fatal poisoning with propyphenazone has been reported. However, the signs and symptoms of severe intoxication are similar for both propyphenazone and aminopyrine. The acute toxicity of dipyrone is slightly lower than that of propyphenazone, whereas phenylbutazone and oxyphenbutazone clearly cause less severe reactions. Characteristic symptoms include impaired consciousness progressing to coma, and convulsions. In addition, arrhythmia and cardiogenic shock may occur. Severe aminopyrine intoxication may also be complicated by sudden apnea. Liver damage may develop after a latent period of about 24 hours, especially after phenylbutazone and oxyphenbutazone poisoning. Therapy involves supportive measures as well as gastric emptying by emesis or lavage, installation of medical charcoal, and induction of diarrhea or gut lavage. Although exact clinicotoxicologic data on hemoperfusion are not available as yet, distribution volumes, plasma half-lives, and endogenous plasma clearances as well as results of in vitro trials all suggest the efficacy of this procedure. Hemoperfusion with uncoated amberlite XAD-4 resin is, therefore, recommended for patients with severe pyrazolone intoxication.

Circular dichroic examination of the interaction of some planar acidic drugs with tryptophan-modified human serum albumin

The single tryptophan region in human serum albumin was investigated for its involvement in binding planar hydrophobic and acidic drug molecules. The lone tryptophan was alkylated with a selective and specific agent, 2-hydroxy-5 nitrobenzyl bromide. The subsequent interaction of four drug molecules with tryptophan modified, and normal albumin was examined by circular dichroism (CD). In all cases the CD signal of tryptophan modified albumin was perturbed at low drug concentrations. The predominantly alpha-helical structure of the albumin remained intact. It is suggested that the primary binding site of these four acidic planar drugs does involve the tryptophan region.

The role of albumin conformation in the binding of diazepam to human serum albumin

The effect of hydrogen, chloride and calcium ions on the binding of diazepam to human serum albumin has been studied by circular dichroism and equilibrium dialysis. In all cases the molar ellipticity of the diazepam-albumin complex increases with pH over the pH range 5 to 9. Under these conditions the free concentration of diazepam at a constant low drug to protein ratio decreases with pH. This free concentration is higher in the presence of chloride and calcium ions. With a two state conformational model for albumin it was shown, that the pH dependences of molar ellipticity of the diazepam-albumin complex and of the free concentration of diazepam are linked. It was demonstrated that the N-B transition of albumin is involved in the pH dependent binding of diazepam. The consequences of these findings for equilibrium dialysis procedures in determining free plasma levels of diazepam are discussed.

Intoxication with pyrazolones

1 About 50 severe or fatal (mostly accidental) cases of intoxication in children by pyrazolones have been reported in the German literature of the past 59 years. 2 Characteristic symptoms are impaired consciousness progressing to coma and convulsions. In addition, sudden apnoea and cardiac arrest may occur. Hepatic lesions may develop after a latent period of 12-24 hours. 3 Haemoperfusion seems to be the only therapeutic measure which is able to reduce the total body load of all pyrazolones to a toxicologically relevant extent. Actual clinico-toxicological data from poisoned patients are not available as yet; however, distribution volumes, plasma half-lives and endogenous plasma clearances as well as removal kinetics in vitro of aminophenazone (aminopyrine), propyphenazone, metamizole (dipyrone), phenylbutazone and oxyphenbutazone as point to the efficacy of haemoperfusion with amberlite XAD-4 resin.

Microcalorimetric method to determine competitive binding. Action of a psychotropic drug (dipotassium chlorazepate) on L-tryptophan . human serum albumin complex

A mathematical treatment and an original microcalorimetric method are developed to verify an eventual competitive binding between any two substances for the same macromolecule. To apply this method, a competitive binding of L-tryptophan and one benzodiazepin (dipotassium chlorazepate) for human serum albumin is perfectly demonstrated. The association constants and the enthalpy variations are equal to 14 000 +/- 2000 M-1 and --6.6 +/- 0.2 kcal/mol for human serum albumin . tryptophan complex and 13 000 +/- 1000 M-1 and --10.0 +/- 0.2 kcal/mol for human serum albumin . chlorazepate complex. In all cases the stoichiometry is equal to one. The binding of tryptophan to human serum albumin is partially stereospecific; the association constant and the enthalpy variation for D-tryptophan complex are equal, respectively, to 1000 +/- 200 M-1 and --2.6 +/- 0.3 kcal/mol.

Functional changes in cerebral 5-hydroxytryptamine metabolism in the mouse induced by anticonvulsant drugs

1 Acute administration of clonazepam, diazepam, and diphenylhydantoin to mice elevated cerebral 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA); chronic administration had less effect. 2 Acute administration of clonazepam and diazepam but not diphenylhydantoin raised cerebral trytophan levels; chronic administration of clonazepam caused a smaller elevation of cerebral tryptophan but chronic administration of diazepam still caused a large rise in cerebral tryptophan. 3 Neither clonazepam nor diazepam caused induction of drug metabolizing enzymes on chronic administration but diphenylhydantoin had a marked effect. 4 These data suggest that the altered 5-HT metabolism caused by these compounds is unrelated to a common action on tryptophan levels, and that the reduced effect of clonazepam and diazepam on chronic administration cannot be attributed to increased metabolism of these compounds. 5 Clonazepam induced abnormal head movements in mice in a dose-dependent manner. Pretreatment of animals with tranylcypromine increased the intensity of movement, although pargyline was without effect. Similar effects were observed with diazepam and diphenylhydantoin, suggesting that the increase in cerebral 5-HT caused by these compounds is of functional significance in stimulating 5-HT receptors.

Isolation and identification of a trypsin-resistant fragment of human serum albumin with bilirubin- and drug-binding properties

Extensive digestion of human serum albumin with trypsin at pH 8.8 yields essentially one main fragment which is resistant to further tryptic degradation. The fragment has been characterized by amino acid analysis, N- and C-terminal analyses, cyanogen bromide digestion, electrophoresis, ultracentrifugation and gelfiltration, and circular dichroism measurements. The results indicate that the main fragment consists of the amino acids 182-585. Repeated digestion did not degrade the isolated fragment further. The fragment mainly retains the secondary and tertiary structure of intact human serum albumin as well as its capacity to bind bilirubin and diazepam. The localization of the binding sites for these substances is discussed.

Benzodiazepines: specific competitors for the binding of L-tryptophan to human serum albumin

By means of the gel filtration technique, the effect of nine benzodiazepine derivatives on the binding of L-tryptophan to human serum albumin was investigated. Using equimolar tryptophan and benzodiazepine concentrations, all benzodiazepines with binding constants higher than 10(4) (M(-1), displace L-tryptophan from its binding site to a high degree. The mechanism of the displacement was characterized as a competition for a common binding site. Some of the benzodiazepines displace L-tryptophan to a greater extent than salicylic acid. The benzostereospecific binding to human serum albumin. This study shows that there is only one binding site on the human serum albumin molecule, which binds tryptophan and the benzodiazepines in a highly stereospecific manner. Therefore it is concluded that the benzodiazepines and L-tryptophan must have similarities in their molecular structure, so that both can bind to the common binding site in such specific manner. These considerations are discussed in regard to the known influence of benzodiazepine derivatives on the L-tryptophan metabolism in brain. A direct involvement of the reported displacement in the pharmacological actions of the drugs seems not to be relevant because of their small therapeutical plasma levels.