- The brain penetration classification is shown in three colours for compounds (n = 12): restricted (Kp,uu,brain < 0.3) in orange, partially restricted (Kp,uu,brain between 0.3 and 0.7) in yellow, and unrestricted (Kp,uu,brain > 0.7) in green. In a separate column is the Efflux ratio demonstrated from a conventional transport study (also captured in Table 1). Compounds considered to be uptake substrates have been excluded (metoclopramide, doxepin, diphenhydramine, and fluoxetine)