On the temperature dependence of the unbound drug fraction in plasma: Ultrafiltration method may considerably underestimate the true value for highly bound drugs.

The value of the unbound drug fraction in plasma is a common characteristic required for drug discovery and development. The communication considers an important possibility that the unbound drug fraction at physiological temperature (37°C) could be substantially greater (more than 5 times) than the value measured by traditionally set ultrafiltration method with the incubation of drug plasma solution at 37°C followed by the centrifugation at room temperature. The consideration is based on the general thermodynamic theory of chemical equilibrium, which is applied to the particular problem of determination of protein binding by ultrafiltration method.