Mathematical and Experimental Validation of Flux Dialysis Method: an Improved Approach to Measure Unbound Fraction for Compounds with High Protein Binding and Other Challenging Properties

A flux dialysis method to measure unbound fraction (f(u)) of compounds with high protein binding and other challenging properties was tested and validated. This method is based on the principle that the initial flux rate of a compound through a size-excluding dialysis membrane is proportional to the product of the compound initial concentration, f(u), and unbound dialysis membrane permeability (P-mem). Therefore, f(u) can be determined from the initial concentration and flux rate, assuming membrane P-mem is known. Compound initial flux rates for 14 compounds were determined by dialyzing human plasma containing compound (donor side) versus compound-free plasma (receiver side) and measuring the rate of compound appearance into the receiver side. Eleven compounds had known f(u) values obtained from conventional methods (ranging from 0.000013 to 0.22); three compounds (bedaquiline, lapatinib, and pibrentasvir) had previously qualified f(u) values (e.g., <0.001). P-mem estimated from flux rates and known f(u) values did not meaningfully differ among the compounds and were consistent with previously published values, indicating that P-mem is a constant for the dialysis membrane. This P-mem constant and the individual compound flux rates were used to calculate fu values. The flux dialysis fu values for the 11 compounds were in good agreement with their reported f(u) values (all within 2.5-fold; R-2 = 0.980), confirming the validity of the method. Furthermore, the flux dialysis method allowed discrete f(u) to be estimated for the three compounds with previously qualified f(u). Theoretical and experimental advantages of the flux dialysis method over other dialysis-based protein binding methods are discussed.