Comparison of ultrafiltration devices for assessing theophylline protein binding

The ability of two commercially available ultrafiltration devices to assess the protein binding of theophylline was compared using human serum albumin in pH 7.4 buffer at 37 degrees C. The devices compared were the Millipore-MC filter units (Catalog number UFC3LGC00) and the Amicon Centrifree Micropartition System; both rely on centrifugal force to separate the unbound and bound molecules. The results from each device were essentially identical provided that the Millipore units were centrifuged using conditions that would not allow heat buildup to occur and thereby cause a decrease in the binding. This problem could be minimized if the Millipore units were available in a low-binding 30,000 dalton cutoff membrane instead of the 10,000 dalton membrane currently available. The use of the Millipore 30,000 dalton cutoff polysulfone membrane (UFC3TTK00) reduced the centrifugation time necessary to obtain a sufficient volume of ultrafiltrate for assay, thereby minimizing the possibility of heat generation. Furthermore, the binding of theophylline to this device was low and comparable to the binding to the Centrifree devices. Because this low degree of membrane binding may not hold for other drugs, we would suggest that a Millipore unit supplied with a low-binding, 30,000 dalton cutoff membrane could provide binding data with an efficacy comparable to that achieved with the Centrifree devices.