The drug transporter protein, P-glycoprotein (P-gp), is a member of the ATP-binding cassette (ABC) superfamily that is widely localized at various human tissues including the apical membranes of the gastrointestinal tract, the biliary canalicular membranes of hepatocytes, the luminal membranes of proximal tubular epithelial cells in the kidney, the testes, the placenta and the luminal membranes of endothelial cells of the blood-brain barrier (BBB). 1,2) As a drug efflux transporter, P-gp plays important role in drug absorption, distribution, and excretion. [3][4][5] Inhibition and induction of P-gp function can result in significant drug-drug interactions.The function of P-gp in BBB can significantly limit the brain uptake of its substrate drugs and affect therapeutic outcomes of central nervous system (CNS) acting drugs. By using the ABCB1a/b Ϫ/Ϫ knockout mice, P-gp has been shown to significantly limit the brain entry of a wide variety of structurally unrelated drugs. [6][7][8][9][10][11][12][13][14] Inhibition of P-gp activity can greatly increase P-gp substrate concentrations in brain and may increase their neurotoxicity. [15][16][17] Because of the importance of P-gp in CNS drug disposition, characterization of the binding affinities of CNS drugs for P-gp has clinical implications.Newer antidepressants, i.e. the selective serotonin reuptake inhibitors (SSRIs) and multi-receptor antidepressants, venlafaxine, mirtazapine, bupropion, and nefazodone have advantages over the classical tricyclic antidepressants in lower frequency to cause unwanted side effects and are extensively used worldwide due to established antidepressants efficacy. 18) However, a substantial number of patients with antidepressant therapy still exhibit treatment resistance despite increasing doses. The reason for this resistance is unknown.Recently, the transport efficacy of most of the antidepressants by P-gp has been studied by using the ABCb1a/b Ϫ/Ϫ mouse or in vitro cell culture models [10][11][12]19) except for two drugs, sertraline and bupropion. In these reports, most of the studied antidepressants (e.g. amitriptyline, nortryptyline, citalopram, and trimipramine) were shown to be substrates of P-glycoprotein. [10][11][12]19) These results suggest that the variable expression of P-gp among patients may be an important source of variability in treatment response for the antidepressants.With availability of human P-gp membranes, we have previously used an ATPase assay method to determine the drug stimulated P-gp-ATPase activity and binding affinity of several antipsychotic drugs. 20) Our results indicated that atypical antipsychotic drugs (AAPs), risperidone, and olanzapine, were effectively transported by P-gp. The findings have been verified by our subsequent in vivo Abcb1a/b gene knockout mouse experiments, 13,14) supporting the ATPase assay to provide reliable information of P-gp substrates' binding affinity. In the present report, we studied the binding affinity of sertraline, desmethylsertraline, bupropion and its three major meta...