Determination of unbound fraction of disopyramide in plasma: a comparison of equilibrium dialysis, ultrafiltration through dialysis membranes and ultrafree anticonvulsant drug filters.

A simple, rapid ultrafiltration technique for determination of free drug concentration in plasma is described and compared with equilibrium dialysis and ultrafiltration through dialysis membranes. When used for disopyramide protein binding studies, this method requires only 1 ml of plasma and up to 20 samples may be filtered simultaneously in 20-40 min. Commercially available Ultrafree anticonvulsant drug filters are used, these are attached to 2 ml leur tip syringes, which provide the pressure gradient for filtration. Compared to equilibrium dialysis this technique is far quicker and permits protein binding to be measured at the drug concentration in the original plasma. Ultrafiltration through dialysis membranes was found to be more tedious and time-consuming than it was through Ultrafree filters. Adsorption of disopyramide from the plasma sample and protein leakage were also problems with this method. Leakage of protein did not occur with either Ultrafree filters or equilibrium dialysis. With the Ultrafree method, recovery of 14C-labeled disopyramide in buffer at 1.1 micrograms/ml and 8.4 micrograms/ml was 87% and 89% respectively. In carefully controlled experiments, a comparison of the Ultrafree method with equilibrium dialysis and ultrafiltration gave comparable values for the free fraction of the drug for total concentrations from 0.3-8 microgram/ml disopyramide.