Estimating Unbound Drug Concentration

Peak plasma concentrations (C_max, μM = ng/mL/MW, where MW is the molecular weight of the compound) and the unbound fraction of most drugs were obtained from the literature, or from FDA-approved package inserts. The unbound fraction of apilimod (not found in the literature) was estimated based on chemical structure, using a PK computer program, which uses a random forest machine learning method (DruMAP ver. 1.5, Mizhguchi Laboratory, Tokyo, Japan, https://drumap.nibiohn.go.jp/, Accessed 5 April 2024) [87 (link)]. Drugs that exhibited binding to plasma proteins were assumed to equilibrate rapidly between the protein-bound and unbound fractions in the plasma, and that only the unbound fraction was free to interact with cells [88 (link)]. The unbound concentration of drug in the plasma at C_max (unbound C_max) was calculated by multiplying C_max by the unbound fraction of drug.