Effect of unbound clearance on binding parameters of valproic acid to serum proteins.

Nine healthy subjects received 400 mg sodium valproate orally in the fasting state. Binding parameters of valproic acid to serum proteins were determined by Scatchard analysis for individual series of valproic acid data. Total and unbound (intrinsic) clearances (CLt and CLu) were calculated by dividing the dose by the appropriate area under the serum drug concentration-time curve. Unbound clearance correlated positively with the product of association constant (Ka) and concentration of free protein ((P)) (P < .05). Conversely, no significant correlation was found between CLt and binding parameters. The average unbound concentration correlated negatively with both CLu and ka(P) values. The result indicates an effect of CLu on Ka(P) value of valproic acid.

MicroParticle photophysics illuminates viral bio-sensing.

The authors present an approach for specific and rapid unlabeled detection of a virus by using a microsphere-based whispering gallery mode sensor that transduces the interaction of a whole virus with an anchored antibody. They show theoretically that this sensor can detect a single virion below the mass of HIV. A micro-fluidic device is presented that enables the discrimination between viruses of similar size and shape.

Shift of whispering-gallery modes in microspheres by protein adsorption.

Biosensors based on the shift of whispering-gallery modes in microspheres accompanying protein adsorption are described by use of a perturbation theory. For random spatial adsorption, theory predicts that the shift should be inversely proportional to microsphere radius R and proportional to protein surface density and excess polarizability. Measurements are found to be consistent with the theory, and the correspondence enables the average surface area occupied by a single protein to be estimated. These results are consistent with crystallographic data for bovine serum albumin. The theoretical shift for adsorption of a single protein is found to be extremely sensitive to the target region, with adsorption in the most sensitive region varying as 1/R(5/2). Specific parameters for single protein or virus particle detection are predicted.

Binding parameters of valproic acid to serum protein in healthy adults at steady state.

Two hundred milligrams of valproic acid (VPA) was administered orally to seven healthy adults at 9:00 and 21:00 h for 5 consecutive days, including the morning dose on day 6. On the sixth day, blood samples were drawn at 0, 0.17, 0.33, 0.5, 0.75, 1, 1.5, 3, and 6 h after the morning dose. Binding of VPA to serum protein was evaluated by ultrafiltration, and total and unbound VPA concentrations were determined by fluorescence polarization immunoassay. Binding parameters of VPA to serum protein were calculated for each subject by the Scatchard analysis. The binding parameters obtained from seven subjects showed that the mean association constant (K) was 2.72 x 10(4) L/mol while the total number of binding sites (nPt) was 789 mumol/L. There were marked interindividual variations and the coefficient of variation was 42% for K and 28% for nPt. These results show that endogenous free fatty acids (FFAs) significantly reduce the binding affinity of VPA to serum albumin (p less than 0.05). In addition, they suggest the possibility that the primary binding sites for VPA can be strongly reduced by FFAs. Therefore, we consider that interindividual differences in binding parameters may be clinically important.

Can the serum protein binding of valproic acid limit the hepatic elimination?

In the previous study, we determined the in vivo binding parameters of valproic acid to serum proteins in seven healthy young adults at steady-state. In this study, we determined the effects of serum protein binding on hepatic elimination with the use of observed data obtained from our previous study of valproic acid. A regression analysis between the binding parameters and the pharmacokinetic parameters was performed. In addition, the relationship between each pharmacokinetic parameter was also analyzed. The order of association constant (K) for valproic acid-serum protein was 10(-2) l/mumol. No significant correlation was found between the binding parameters and the rate of elimination. On the other hand, the average unbound serum concentration was found to be a significantly negative correlation with the unbound (intrinsic) clearance (p = 0.0082). The product of association constant and concentration of free protein (P) correlated positively with the unbound clearance (p = 0.0233) and negatively with the average unbound and total serum concentrations (p = 0.0021 and p = 0.0029, respectively). The results indicate that the membrane permeability of valproic acid is high and that the increase of unbound clearance accompanies directly the decrease of the average unbound and total serum concentrations. Consequently, the KP values are proportional to the unbound clearance due to the rapid changes of the concentration of free protein. Therefore, the dissociation of the valproic acid-serum protein complex is not a rate-limiting factor for hepatic elimination and hence the serum protein binding cannot limit the ability of the liver to extract drug from blood.