Limited Impact of Murine Placental MDR1 on Fetal Exposure of Certain Drugs Explained by Bypass Transfer Between Adjacent Syncytiotrophoblast Layers

Breast Cancer Resistance Protein Limits Fetal Transfer of Tadalafil in Mice

Tadalafil, a phosphodiesterase 5 (PDE5) inhibitor, is a candidate therapeutic agent for fetal growth restriction and hypertensive disorders of pregnancy. In this study, we elucidated the fetal transfer of tadalafil in comparison with that of sildenafil, the first PDE5 inhibitor to be approved. We also examined the contributions of multidrug resistance protein 1 (MDR1) and breast cancer resistance protein (BCRP) to fetal transfer. Tadalafil or sildenafil was administered to wild-type, Mdr1a/b-double-knockout or Bcrp-knockout pregnant mice by continuous infusion from gestational day (GD) 14.5 to 17.5, and the fetal-to-maternal plasma concentration ratio of unbound drug (unbound F/M ratio) was evaluated at GD 17.5. The values of unbound F/M ratio of tadalafil and sildenafil in wild-type mice were 0.80 and 1.6, respectively. The unbound F/M ratio of tadalafil was increased to 1.1 and 1.7 in Mdr1a/b-knockout and Bcrp-knockout mice, respectively, while the corresponding values for sildenafil were equal to or less than that in wild-type mice, respectively. A transcellular transport study revealed that basal-to-apical transport of both tadalafil and sildenafil was significantly higher than transport in the opposite direction in MDCKII-BCRP cells. Our research reveals that tadalafil is a newly identified substrate of human and mouse BCRP, and it appears that the fetal transfer of tadalafil is, at least in part, attributed to the involvement of BCRP within the placental processes in mice. The transfer of sildenafil to the fetus was not significantly constrained by BCRP, even though sildenafil was indeed a substantial substrate for BCRP.

Maternal and Fetal Exposure to (-)-Δ9-tetrahydrocannabinol and Its Major Metabolites in Pregnant Mice Is Differentially Impacted by P-glycoprotein and Breast Cancer Resistance Protein

(-)-Δ9-tetrahydrocannabinol (THC) is the primary pharmacological active constituent of cannabis. 11-hydroxy-THC (11-OH-THC) and 11-nor-9-carboxy-THC (THC-COOH) are respectively the active and nonactive circulating metabolites of THC in humans. While previous animal studies reported that THC could be a substrate of mouse P-glycoprotein (P-gp) and breast cancer resistance protein (Bcrp), we have shown, in vitro, that only THC-COOH is a weak substrate of human BCRP, but not of P-gp. To confirm these findings and to investigate the role of P-gp and/or Bcrp in the maternal-fetal disposition of THC and its metabolites, we administrated 3 mg/kg of THC retro-orbitally to FVB wild-type (WT), P-gp -/-, Bcrp -/-, or P-gp-/- /Bcrp-/- pregnant mice on gestation day 18 and estimated the area under the concentration-time curve (AUC) of the cannabinoids in the maternal plasma, maternal brain, placenta, and fetus, as well as the tissue/maternal plasma AUC geometric mean ratios (GMRs) using a pooled data bootstrap approach. We found that the dose-normalized maternal plasma AUCs of THC in P-gp-/- and P-gp-/- /Bcrp-/- mice, and the placenta-to-maternal plasma AUC GMR of THC in Bcrp-/- mice were 279%, 271%, and 167% of those in WT mice, respectively. Surprisingly, the tissue-to-maternal plasma AUC GMRs of THC and its major metabolites in the maternal brain, placenta, or fetus in P-gp -/-, Bcrp -/- or P-gp-/- /Bcrp-/- mice were 28-78% of those in WT mice. This study revealed that P-gp and Bcrp do not play a role in limiting maternal brain and fetal exposure to THC and its major metabolites in pregnant mice. SIGNIFICANCE STATEMENT: This study systematically investigated whether P-gp and/or Bcrp in pregnant mice can alter the disposition of THC, 11-OH-THC, and THC-COOH. Surprisingly, except for Bcrp, which limits placental (but not fetal) exposure to THC, we found that P-gp-/- , Bcrp-/- , and/or P-gp-/- /Bcrp-/- significantly decreased exposure to THC and/or its metabolites in maternal brain, placenta, or fetus. The mechanistic basis for this decrease is unclear and needs further investigation. If replicated in humans, P-gp- or BCRP-based drug-cannabinoid interactions are not of concern.

An update on placental drug transport and its relevance to fetal drug exposure

Pregnant women are often complicated with diseases that require treatment with medication. Most drugs administered to pregnant women are off-label without the necessary dose, efficacy, and safety information. Knowledge concerning drug transfer across the placental barrier is essential for understanding fetal drug exposure and hence drug safety and efficacy to the fetus. Transporters expressed in the placenta, including adenosine triphosphate (ATP)-binding cassette efflux transporters and solute carrier uptake transporters, play important roles in determining drug transfer across the placental barrier, leading to fetal exposure to the drugs. In this review, we provide an update on placental drug transport, including in vitro cell/tissue, ex vivo human placenta perfusion, and in vivo animal studies that can be used to determine the expression and function of drug transporters in the placenta as well as placental drug transfer and fetal drug exposure. We also describe how the knowledge of placental drug transfer through passive diffusion or active transport can be combined with physiologically based pharmacokinetic modeling and simulation to predict systemic fetal drug exposure. Finally, we highlight knowledge gaps in studying placental drug transport and predicting fetal drug exposure and discuss future research directions to fill these gaps.

Predicting Human Fetal Drug Exposure Through Maternal-Fetal PBPK Modeling and In Vitro or Ex Vivo Studies

Medication (drug) use in human pregnancy is prevalent. Determining fetal safety and efficacy of drugs is logistically challenging. However, predicting (not measuring) fetal drug exposure (systemic and tissue) throughout pregnancy is possible through maternal-fetal physiologically based pharmacokinetic (PBPK) modeling and simulation. Such prediction can inform fetal drug safety and efficacy. Fetal drug exposure can be quantified in 2 complementary ways. First, the ratio of the steady-state unbound plasma concentration in the fetal plasma (or area under the plasma concentration-time curve) to the corresponding maternal plasma concentration (ie, Kp,uu ). Second, the maximum unbound peak (Cu,max,ss,f ) and trough (Cu,min,ss,f ) fetal steady-state plasma concentrations. We (and others) have developed a maternal-fetal PBPK model that can successfully predict maternal drug exposure. To predict fetal drug exposure, the model needs to be populated with drug specific parameters, of which transplacental clearances (active and/or passive) and placental/fetal metabolism of the drug are critical. Herein, we describe in vitro studies in cells/tissue fractions or the perfused human placenta that can be used to determine these drug-specific parameters. In addition, we provide examples whereby this approach has successfully predicted systemic fetal exposure to drugs that passively or actively cross the placenta. Apart from maternal-fetal PBPK models, animal studies also have the potential to estimate fetal drug exposure by allometric scaling. Whether such scaling will be successful is yet to be determined. Here, we review the above approaches to predict fetal drug exposure, outline gaps in our knowledge to make such predictions and map out future research directions that could fill these gaps.