A constant-volume ultrafiltration technique for the calculation of equilbrium binding data

A new centrifugal filtration device for free drug separation

A new centrifugal membrane filtration device (Syva Corporation) has been evaluated to determine its capabilities for separation of free phenytoin for analysis. The device is a test tube-size cylinder with two compartments separated by the membrane. In serum samples from 70 patients at the Hospital of the University of Pennsylvania, free phenytoin was prepared by the new device and by equilibrium dialysis. Levels were assayed by gas chromatography and enzyme immunoassay with good agreement at all phenytoin levels. Although pH has a significant effect on the binding of some drugs to serum proteins, phenytoin binding increased to only a small extent as pH was increased from 7.0 to 7.8 (85-88.5% bound). Centrifugal filtration with this device is a reliable, fast, and easy way to prepare free drug from serum and does not include the dilution artifact inherent in equilibrium dialysis.

Ultrafiltration in serum protein binding determinations

the ultrafiltration technique was evaluated theoretically and experimentally for use in clinical serum binding determinations. It is apparent from free energy considerations that the ultrafiltrate concentration approaches the true free concentration only as the pressure gradient causing flow reduces to zero. The theory presented accounts for the previously unexplained lower ultrafiltrate concentration observed at higher filtration pressures. Mathematical simulations of the molecular separation show that the ultrafiltrate concentration remains constant during filtration, and, thus, binding equilibria are not disturbed by this procedure, suggesting that an arbitrary restriction on the volume filtered is unnecessary. This finding greatly extends the value of the ultrafiltration technique in clinical binding determinations, especially for strongly bound, potent drugs where assays may be insufficiently sensitive to detect the extremely small free fraction reliably. These theoretical findings were verified experimentally by ultrafiltration of salicylate, ibuprofen, and carprofen in buffer, purified proteins, and whole serum.