Jashvant D. Unadkat

To inform fetal drug safety, it is important to determine or predict fetal drug exposure throughout pregnancy. The former is not possible in the first or second trimester. In contrast, at the time of birth, fetal drug exposure, relative to maternal exposure, can be estimated as K p,uu (unbound fetal umbilical venous (UV) plasma area under the curve (AUC)/unbound maternal plasma (MP) AUC), provided the observed UV/MP values, spanning the dosing interval, are available from multiple maternal‐fetal dyads. However, this fetal K p,uu cannot be extrapolated to other drugs. To overcome the above limitations, we have used an efflux ratio‐relative expression factor (ER‐REF) approach to successfully predict the fetal K p,uu of P‐gp substrates. Because many drugs taken by pregnant people are also BCRP substrates, here, we extend this approach to drugs that are effluxed by both placental BCRP and P‐gp or P‐gp alone. To verify our predictions, we chose drugs for which UV/MP data were available at term: glyburide and imatinib (both BCRP and P‐gp substrates) and nelfinavir (only P‐gp substrate). First, the ER of the drugs was determined using Transwells and MDCKII cells expressing either BCRP or P‐gp. Then, the ER was scaled using the proteomics‐informed REF value to predict the fetal K p,uu of the drug at term. The ER‐REF predicted fetal K p,uu of glyburide (0.43), imatinib (0.42), and nelfinavir (0.40) fell within two‐fold of the corresponding in vivo fetal K p,uu (0.44, 0.37, and 0.46, respectively). These data confirm that the ER‐REF approach can successfully predict fetal drug exposure to BCRP/P‐gp and P‐gp substrates, at term.