Quantitative projection of human brain penetration of the H₃ antagonist PF-03654746 by integrating rat-derived brain partitioning and PET receptor occupancy

1. Unbound brain drug concentration (C_b,u), a valid surrogate of interstitial fluid drug concentration (C_ISF), cannot be directly determined in humans, which limits accurately defining the human C_b,u:C_p,u of investigational molecules. 2. For the H₃R antagonist (1R,3R)-N-ethyl-3-fluoro-3-[3-fluoro-4-(pyrrolidin-1-lmethyl)phenyl]cyclobutane-1-carboxamide (PF-03654746), we interrogated C_b,u:C_p,u in humans and nonhuman primate (NHP). 3. In rat, PF-03654746 achieved net blood-brain barrier (BBB) equilibrium (C_b,u:C_p,u of 2.11). 4. In NHP and humans, the PET receptor occupancy-based C_p,u IC₅₀ of PF-03654746 was 0.99 nM and 0.31 nM, respectively, which were 2.1- and 7.4-fold lower than its in vitro human H₃ K_i (2.3 nM). 5. In an attempt to understand this higher-than-expected potency in humans and NHP, rat-derived C_b,u:C_p,u of PF-03654746 was integrated with C_p,u IC₅₀ to identify unbound (neuro) potency of PF-03654746, nIC₅₀. 6. The nIC₅₀ of PF-03654746 was 2.1 nM in NHP and 0.66 nM in human which better correlated (1.1- and 3.49-fold lower) with in vitro human H₃ K_i (2.3 nM). 7. This correlation of the nIC₅₀ and in vitro hH₃ K_i suggested the translation of net BBB equilibrium of PF-03654746 from rat to NHP and humans, and confirmed the use of C_p,u as a reliable surrogate of C_b,u. 8. Thus, nIC₅₀ quantitatively informed the human C_b,u:C_p,u of PF-03654746.