Plasma Protein Binding Determination for Unstable Ester Prodrugs: Remdesivir and Tenofovir Alafenamide

•The addition of an organophosphate esterase inhibitor, dichlorvos, stabilized remdesivir (RDV) during equilibrium dialysis for plasma protein binding (PPB) determination.•Dichlorvos did not impact the unbound fraction (fu) of RDV in plasma, and an equilibrium dialysis method for RDV was developed with the addition of dichlorvos.•Tenofovir alafenamide (TAF) was more stable than RDV by a factor of 3 but was not stabilized by dichlorvos. Fit-for-purpose acceptance criteria for equilibrium dialysis (ED) were implemented during PPB method development for TAF.•PPB methods for RDV and TAF were qualified and applied to PPB determinations for clinical studies. Remdesivir (RDV) and tenofovir alafenamide (TAF) are prodrugs designed to be converted to their respective active metabolites. Plasma protein binding (PPB) determination of these prodrugs is important for patients with possible alteration of free fraction of the drugs due to plasma protein changes in renal impairment, hepatic impairment, or pregnancy. However, the prodrugs’ instability in human plasma presents a challenge for accurate PPB determination. In this research work, two approaches were used in the method development and qualification for PPB assessment of RDV and TAF. For RDV, dichlorvos was used to inhibit esterase activity to stabilize the prodrug in plasma during equilibrium dialysis (ED). The impact of dichlorvos on protein binding was evaluated and determined to be insignificant by comparing the unbound fraction (fu) determined by the ED method with dichlorvos present and the fu determined by an ultrafiltration method without dichlorvos. In contrast to RDV, TAF degradation in plasma is ∼3-fold slower, and TAF stability cannot be improved by dichlorvos. Fit-for-purpose acceptance criteria for the TAF PPB method were chosen, and an ED method was developed based on these criteria. These two methods were then qualified and applied for PPB determinations in clinical studies.