Role of Enzyme:Inhibitor Kinetic Characterization in Predicting Pharmacodynamic Response of Irreversible Fatty Acid Amide Hydrolase (FAAH) Inhibitors
To investigate the relevance of in vitro inhibitor kinetics to in vivo pharmacodynamics (PD), a simple fluorogenic assay was used to determine the kinetic constants of PF-3845, an irreversible FAAH inhibitor. The reported in vitro potency and in vivo pharmacokinetics of a closely related compound, PF-04457845 (Ahn et al., JPET 338:114–124, 2011), were then used to predict in vivo leukocyte FAAH activity. The correspondence between predicted and reported PD results was highly dependent on two unknown factors: the leukocyte FAAH enzyme concentration and inhibitor free fraction. However, as the assumed leukocyte FAAH concentration was decreased below 1 nM, the free fraction required to give good agreement between measured and predicted PD results approached a minimum of 8%. In vitro potency and free fraction were highly dependent on each other. For example, if the potency of PF-04457845 were greater, a corresponding decrease in free fraction would be required to predict the measured data. It is concluded that PD models can provide valuable insights when viewed in light of in vitro inhibitor potency and mechanism.