FTY720 (2-amino-2[2-(-4-octylphenyl)ethyl]propane-1,3-diol hydrochloride) is a new sphingosine-1-phosphate receptor agonist being developed for multiple sclerosis and prevention of solid organ transplant rejection. A physiologically based pharmacokinetic model was developed to predict the concentration of FTY720 in various organs of the body. Single oral and intravenous doses of FTY720 were administered to male Wistar rats, with blood and tissue sampling over 360 h analyzed by liquid chromatography/tandem mass spectrometry. A well stirred model (perfusion rate-limited) described FTY720 kinetics in heart, lungs, spleen, muscle, kidneys, bone, and liver, with a permeability rate-limited model being required for brain, thymus, and lymph nodes. Tissue-to-blood partition coefficients (RT) ranged from 4.69 (muscle) to 41.4 (lungs). In lymph nodes and spleen, major sites for FTY720-induced changes in sequestration of lymphocytes, RT values were 22.9 and 34.7, respectively. Permeability-surface area products for brain, thymus, and lymph nodes were 39.3, 122, and 176 ml/min. Intrinsic hepatic clearance was 23,145 l/h/kg for the free drug in blood (f(ub) 0.000333); systemic clearance was 0.748 l/h/kg and terminal half-life was 23.4 h. The fraction orally absorbed was 71%. The model characterized well FTY720 disposition for this extensive dosing and tissue collection study in the rat. On scaling the model to dogs and humans, good agreement was found between the actual and predicted blood concentration-time profiles. More importantly, brain concentrations in dogs were well predicted from those of the rat. In absolute terms, the predictions were slightly lower than observed values, just under a 1.5-fold deviation, but the model accurately predicted the terminal elimination of FTY720 from the brain.