Improved procedure for the determination of protein binding by conventional equilibrium dialysis

Factors influencing the protein binding of a new phosphodiesterase V inhibitor, DA-8159, using an equilibrium dialysis technique

Various factors influencing the protein binding of DA-8159 to 4% human serum albumin (HSA) were evaluated using an equilibrium dialysis technique at an initial DA-8159 concentration of 5 microg/mL. It took approximately 8 h incubation to reach an equilibrium between 4% HSA and an isotonic phosphate buffer of pH 7.4 containing 3% of dextran ('the buffer') using a Spectra/Por 2 membrane (mol. wt. cut-off: 12,000--14,000) in a water bath shaker kept at 37 degrees C and at a rate of 50 oscillations per min. The extent of binding was dependent on DA-8159 concentrations, HSA concentrations, incubation temperature, buffer pH, and alpha-1-acid glycoprotein (AAG) concentrations. The binding of DA-8159 in heparinized human plasma (93.9%) was significantly higher than in rats (81.4%), rabbits (80.4%), and dogs (82.2%), and this could be due to differences in AAG concentrations in plasma.

Factors influencing the protein binding of YH-439 using an equilibrium dialysis technique. A new hepatoprotective agent

Various factors influencing the plasma protein binding of YH-439 to 4% human serum albumin (HSA) were evaluated using the equilibrium dialysis method at the initial YH-439 concentration of 2 micrograms mL-1. It took approximately 12 h of incubation to reach an equilibrium between 4% HSA and isotonic phosphate buffer of pH 7.4 containing 3% of dextran ('the buffer') using a Spectra/Por 2 membrane (molecular weight cut-off, 12,000-14,000) in a water bath shaker kept at 37 degrees C and at a rate of 50 oscillations min-1. YH-439 was fairly stable both in 4% HSA and in the 'buffer' for up to 24 h incubation. The binding of YH-439 to 4% HSA was constant (97.4 +/- 0.55%) at YH-439 concentrations ranging from 0.5 to 10 micrograms mL-1. However, the extent of binding was dependent on HSA concentrations: the values were 90.7, 94.7, 96.7, 97.0, 97.0, 97.1, and 97.5% at HSA concentrations of 0.5, 1, 2, 3, 4, 5, and 6%, respectively. The plasma protein binding decreased with increasing incubation temperature: the binding values were 98.2, 97.6, 97.2, and 96.8% when incubated at 10, 21, 26, and 37 degrees C, respectively. The binding of YH-439 was also influenced by the chloride concentration in the buffer: the binding values were 94.5, 97.0, and 96.8% for the chloride concentrations of 0, 0.249, and 0.546%, respectively. The binding of YH-439 was also dependent on the buffer pH: the percentages of free fraction were 6.0, 4.1, 3.8, 2.8, 2.7 and 2.8% for the buffer pHs of 5.0, 6.0, 6.5, 7.0, 7.4, and 8.0, respectively. The free fraction of YH-439 was slightly increased by the addition of heparin (up to 40 U mL-1), sodium azide (NaN3, up to 0.5%), and its metabolites. The protein binding of YH-439 was influenced neither by AAG, acetylsalicylic acid, or sulphisoxazole, nor by the addition of citrate or EDTA. The free fractions of YH-439 in rabbit (4.2%) and dog (4.7%) plasma seemed to be higher than in rats (2.9%) and humans (3.1%).

Factors influencing the protein binding of azosemide using an equilibrium dialysis technique

Various factors most likely to influence the plasma protein binding of azosemide to 4% human serum albumin (HSA) were evaluated using equilibrium dialysis at the initial azosemide concentration of 10 micrograms mL-1. It took approximately 8 h of incubation to reach an equilibrium between 4% HSA and isotonic phosphate buffer of pH 7.4 containing 3% dextran (the 'buffer') using a Spectra/Por 2 membrane (molecular weight cut-off, 12000-14000) in a water bath shaker kept at 37 degrees C and a rate of 50 oscillations min-1. Azosemide was fairly stable both in 4% HSA and in the 'buffer' for up to 24 h. The binding of azosemide to 4% HSA was constant (95.5 +/- 0.142%) at azosemide concentrations ranging from 5 to 100 micrograms mL-1. However, the extent of binding was dependent on HSA concentration: the values were 88.4, 91.0, 92.2, 94.2, 94.9, 94.9, and 94.9% at albumin concentrations of 0.5, 1, 2, 3, 4, 5, and 6%, respectively. The binding was also dependent on incubation temperature: the binding values were 97.0, 94.9, and 94.9% when incubated at 6, 28, and 37 degrees C, respectively. The binding of azosemide was also influenced by buffers containing various chloride ion concentrations and buffer pHs. The binding values were 95.3, 94.9 and 93.6% for the chloride ion concentrations of 0, 0.249, and 0.546%, respectively, and the unbound values were 6.8, 5.1, 3.8, 3.4, and 3.3% for buffer pHs of 5.8, 6.4, 7.0, 7.4, and 8.0, respectively. The binding of azosemide was independent of the quantity of heparin (up to 40 U mL-1), AAG (up to 0.16%), sodium azide (NaN3, up to 5%), its metabolite, M1 (up to 10 micrograms mL-1), and anticoagulants (EDTA and citrate).

Factors influencing the protein binding of vancomycin

Various factors influencing the protein binding of vancomycin were examined using equilibrium dialysis method. Four per cent human serum albumin (HSA) and/or 0.08 per cent alpha-1-acid glycoprotein (AAG), dissolved in isotonic phosphate buffer, were dialyzed against isotonic phosphate buffer of pH 7.4 using Spectrapor 2 membrane. The protein binding of vancomycin to 0.08 per cent AAG was dependent on vancomycin concentrations; the values ranged from 21.1 per cent at the vancomycin concentration of 20 micrograms ml-1 to 5.30 per cent at 2400 micrograms ml-1. However, binding to 4 per cent HSA was relatively constant, 8.79 +/- 2.43 per cent over a vancomycin concentration range of 20-2400 micrograms ml-1. The values to 4 per cent HSA alone and 0.08 per cent AAG alone did not predict the greater binding of vancomycin in the presence of both proteins, especially at higher concentrations of vancomycin; the values to 4 per cent HSA with 0.08 per cent AAG were constant, 26.3 +/- 3.74 per cent, at the vancomycin concentration range of 20-2400 micrograms ml-1. This suggested an interaction between the proteins, which resulted in enhanced binding of vancomycin. The protein binding of vancomycin to 4 per cent HSA with 0.08 per cent AAG was not influenced by the different incubation temperatures (4 degrees, 22 degrees, and 37 degrees), quantities of heparin (up to 40 units ml-1) or AAG (up to 0.16 per cent), or buffers (isotonic phosphate buffer of pH 7.4, phosphate buffer of pH 7.4 and 0.9 per cent NaCl solution) at the vancomycin concentration of 80 micrograms ml-1. Vancomycin was found to be stable in human serum albumin or in isotonic phosphate buffer of pH 7.4.

Factors influencing the protein binding of bumetanide using an equilibrium dialysis technique

Factors that influence the plasma protein binding of bumetanide were evaluated using equilibrium dialysis. It took approximately 12 h of incubation to reach an equilibrium between plasma and isotonic phosphate buffer of pH 7.4 containing 3% dextran using a Spectrapor 2 membrane (mol. wt cut-off = 12,000-14,000) in a water-bath shaker kept at 37 degrees C and at a rate of 50 oscillations per min. Bumetanide was fairly stable in both 4% human serum albumin (HSA) and in the isotonic phosphate buffer of pH 7.4 for up to 24 h. The binding of bumetanide to 4% HSA was constant (87.5 +/- 1.73%) at bumetanide concentrations ranging from 0.1 to 100 micrograms/ml. The extents of binding were 72.0, 83.3, 88.5, 90.2, 91.3 and 91.4% at albumin concentrations of 0.5, 1, 2, 3, 4 and 5 g/100 ml, respectively, and increased with a decrease in incubation temperature; the values bound were 94.6, 90.3 and 89.3% when incubated at 4, 22 and 37 degrees C, respectively. The binding of bumetanide was independent of the buffer composition used, the quantities of AAG (up to 0.32%), heparin (up to 40 units/ml), sodium azide (up to 0.5%) and anticoagulants (EDTA, heparin and citrate). The free fraction of bumetanide in rabbit plasma (2.91%) was significantly higher than in humans (1.98%) or rats (1.85%).

In vitro study of caffeic acid-bovine serum albumin interaction

Caffeic acid, a natural product with pharmacological properties, such as DOPA-decarboxylase and 5-lipooxygenase inhibition, has been tested in vitro for its binding ratio to bovine serum albumin. This study was carried out by means of four analytical methods. Equilibrium dialysis has been proved to be the most reliable in determination of total binding sites, while acid precipitation has been evaluated as a model of irreversible binding. A binding ratio between 61 and 95% was observed, also varying according the caffeic acid and albumin concentration, pH and protein conformation. Binding constants were calculated by mathematical fitting to a hyperbolic plot. Other cinnamic acids non-o-diphenolic (ferulic acid, m- and p-coumaric acids) have been proved to have a binding ratio to bovine serum albumin of less than 10%.

Protein binding studies with radiolabeled compounds containing radiochemical impurities. Equilibrium dialysis versus dialysis rate determination

The influence of radiochemical impurities in dialysis experiments with high-affinity ligands is investigated. Albumin binding of labeled decanoate (97% pure) is studied by two dialysis techniques. It is shown that equilibrium dialysis is very sensitive to the presence of impurities resulting in erroneously low estimates of the binding affinity and in inconsistent results at varying albumin concentrations. Dialysis rate determination (R. Brodersen et al. (1982) Anal. Biochem. 121, 395-408) is less sensitive to impurities.

On the binding of cinmetacin and indomethacin to human serum albumin

The binding of two non-steroidal anti-inflammatory drugs, indomethacin and cinmetacin, to human serum albumin was studied by dynamic dialysis at 37 degrees C and pH 7.4. Cinmetacin is bound more than indomethacin. The affinity constant for the primary binding site is 4.28 X 10(6) M-1 for cinmetacin and 1.4 X 10(6) M-1 for indomethacin. The protein binding of indomethacin is decreased in the presence of cinmetacin.

Binding of sulfadimethoxine to isolated human blood protein fractions

The binding of sulfadimethoxine to selected human blood protein fractions and to fresh serum has been examined by means of a new equilibrium dialysis technique which minimizes experimental error and permits the evaluation of low-level binding. Certain alpha-globulin fractions, containing mixtures of proteins, were found to bind the drug. Scatchard analysis of the binding of sulfadimethoxine to fresh serum, calculated as though all of the binding is due to albumin, gives a different result from that obtained with isolated albumin. This may be a reflection of the contribution of the alpha-globulins to the overall binding of sulfadimethoxine in fresh serum. Although sulfadimethoxine is amphoteric, it did not bind to the alpha 1-acid glycoprotein. The drug behaves as an acidic compound when binding to the blood proteins.