e Placental transfers of the HIV nonnucleoside reverse transcriptase inhibitor rilpivirine were investigated in 8 term human cotyledons perfused with rilpivirine (400 ng/ml) in the maternal-to-fetal direction. The mean fetal transfer rate (FTR) (fetal/maternal concentration at steady state from 15 to 90 min) was 26% ؎ 8% (mean ؎ standard deviation), and the clearance index (rilpivirine FTR/antipyrine FTR) was 61% ؎ 20%. This shows that rilpivirine crosses the placenta at a relatively high rate, suggesting that the fetus is exposed to the compound during treatment of the mother.A ntiretroviral drugs are remarkably effective for preventing vertical transmission of HIV-1, resulting in a decline in the rate of transmission, which is now less than 0.5% in France (1, 2). Current guidelines recommend antiretroviral therapy for all HIVinfected pregnant women for their own health and to prevent transmission (3, 4)., a diarylpyrimidine analog, is a nonnucleoside reverse transcriptase inhibitor (NNRTI) which is increasingly used to treat HIV infection and in recent guidelines is considered a first-line treatment. It is available in a single-tablet formulation in combination with tenofovir and emtricitabine (Complera-Eviplera). Rilpivirine is classified as a pregnancy category B drug by the Food and Drug Administration (4), which means that animal studies have failed to demonstrate a risk to the fetus, but there have been no adequate and well-controlled studies in HIV-1-infected pregnant women. To date, only one report has been published on two cases of rilpivirine use in pregnant women (5), in which the plasma concentrations of rilpivirine were lower than would be expected in nonpregnant women. Placental transfer was studied in one of these two cases. Currently, there are no data on the safety of rilpivirine use during pregnancy.Determining fetal exposure is important for all new medications in order to estimate the potential for preexposure prophylaxis and the risk for toxicities to the fetus. A number of adverse events have been reported following perinatal exposure to antiretroviral medications, including mitochondrial diseases (6). Since data from animal studies are difficult to extrapolate to humans due to differences in placental physiology, other preclinical approaches are required.The ex vivo human cotyledon is an accepted model used to study and interpret placental transfer (7,8). The purpose of this study was to investigate the placental transfer of rilpivirine in an ex vivo perfused human cotyledon.Placentas were collected after uncomplicated pregnancy with full-term delivery (Ն37 weeks gestational age) in mothers who were seronegative for HIV and hepatitis B and C viruses and had taken no medication, except for vitamin supplements and perimedullar analgesia. They were collected in a single center, the maternity ward of the Hôpitaux Universitaires Paris-Nord Val de Seine (Colombes, France), and were rapidly perfused on site. Written informed consent was obtained from each of the women who donated a placenta, acc...