This research project proposes studies on key components of placental drug transfer that can improve our understanding of how physiological changes during pregnancy influence the disposition and fetal exposure of drugs at different stages of gestation. Four Specific Aims will be achieved. Studies are designed to 1) define gestational age dependant changes in the gene and protein expression of three major placental drug transporters: P-glycoprotein (P-gp), the serotonin transporter (SERT) and the norepinephrine transporter (NET); 2) assess the functional activity of placental transporters across gestation using placental membrane vesicle preparations; 3) define first and early second trimester fetal exposure to P-gp, SERT and NET substrates; and 4) develop mathematical /statistical models of placental drug transfer for drugs across gestational age. These aims will address major issues of public and scientific concern regarding potential teratogenic and adverse neuro-developmental effects of drug exposure during pregnancy. There are sparse human data to understand some key components of placental drug transport, especially in the first and second trimesters. Ironically, these trimesters are the most developmentally sensitive periods of gestation. We will capitalize on the high prevalence of psychoactive drug use in women of childbearing age and utilize our unique access to healthy 1st, 2nd, and 3rd trimester placental tissue to define the role of P-gp, SERT, and NET in regulating fetal exposure to amphetamine and escitalopram. These psychoactive drugs are commonly taken by our clinical population. We will compare the results of RNA and protein expression studies from healthy women in all three trimesters (Specific Aim #1) and test the hypothesis that a high correlation will be found with functional activity of these transporters (Specific Aim #2). From our clinical population, paired maternal/fetal samples will be collected in women using amphetamines and escitalopram across gestation to assess exposure to our model substrates (Specific Aim #3). Finally, the results of our expression studies and functional activity experiments will be combined to develop mathematical models predicting fetal drug exposure to the drugs and transporters of interest (Specific Aim #4). Overall, our project will generate data to aid clinicians in choosing drugs within therapeutic categories possessing specific characteristics that predict lower fetal drug exposure during the first two trimesters of pregnancy. This translational research will promote more informed decisions regarding fetal risk from psychoactive drug use during pregnancy.