Implementation of a novel method for the determination of plasma protein binding of highly bound compounds using the equilibrium dialysis method coupled with extraction to the organic phase and its comparison to known methods.

Accurate determination of plasma protein binding (PPB) is crucial in understanding the pharmacokinetics and pharmacodynamics of drugs, particularly for highly bound compounds where traditional methods may fall short. In this study, we present a pioneering approach for the precise determination of PPB that takes advantage of the lipophilicity of highly bound compounds. Twenty four highly bound compounds (with a fraction unbound (f_u) from 10^-1 to 10^-6) were tested with the most commonly used method, i.e., the rapid equilibrium dialysis (RED) method, along with its modifications adapted especially for strongly bound compounds, including dilution, presaturation, competition, and flux-dialysis methods. The results of these methods were compared to data obtained with the modification of RED coupled with extraction to organic phase, and the correlations between them were presented. Comparison studies demonstrate the accuracy of our approach across a set of highly bound compounds within a twofold range. Moreover, PPB values were determined for venetoclax, amiodarone, montelukast, and fulvestrant, for which, to the best of our knowledge and after extensive review of the literature, it has not been possible with the standard RED method until now. In conclusion, our new method can be a big step forward in finding the PPB of strongly plasma protein-bound compounds. It gives us a better understanding of how drugs and proteins interact, which helps us make safer and more effective medicines.