Placental Transporter Activity Research Articles

Mechanisms of Transplacental Transfer of (‐)‐∆ 9‐Tetrahydrocannabinol (THC) in the Human Dual Cotyledon, Dual Perfusion, Placenta Model

Background(‐)‐∆ 9‐Tetrahydrocannabinol (THC) is the most abundant psychoactive compound in marijuana. With legalization of marijuana, its use has increased including by pregnant women. The latter raises concerns of fetal toxicity which is likely driven by fetal THC exposure. Interestingly, in the catheterized maternal‐fetal macaque model, fetal THC exposure is lower than its corresponding maternal exposure (fetal/maternal AUC0‐inf ratio of 0.37). This difference is supported by sparse human THC umbilical vein/maternal plasma concentrations at term. The most likely explanation for these observations is that THC is effluxed by P‐glycoprotein (P‐gp) which is highly expressed in the human placenta. THC oral pharmacokinetics studies in P‐gp knock‐out mice support the hypothesis that P‐gp can transport THC. Therefore, the objective of our study was to investigate if THC is effluxed in human placentae by estimating the maternal to fetal clearance (m‐f‐CL) and fetal to maternal clearance (f‐m‐CL) using the dual cotyledon, dual perfusion, placenta model.MethodsTerm placentae were utilized from uncomplicated, unlabored, cesarean deliveries with no known history of tobacco or drug use. Two cotyledons from each placenta were perfused at physiological rates (m: 10 mL/min; f: 4 mL/min) for 2‐4 hours in a single‐pass mode with oxygenated HBSS buffer containing 0.2% BSA, 2000 U/L sodium heparin and 5 mg/mL gentamicin. To determine m‐f CL, the maternal perfusate also contained 5 μM THC, a P‐gp transporter probe (1 or 10 μM saquinavir), and a passive diffusion probe (20 μg/mL antipyrine) and effluent from the fetal vein and intervillous space were collected at multiple timepoints. To determine f‐m‐CL, the fetal perfusate of the second cotyledon contained the aforementioned drugs. All perfusions were conducted with (n = 7) and without (n = 5) a pan efflux transporter inhibitor, 4 μM valspodar. THC concentrations in the effluent were quantified using LC‐MS/MS. THC CL indices, i.e. m‐f‐CL and f‐m‐CL, normalized to the corresponding antipyrine CL, were compared in the absence and presence of valspodar.ResultsTHC m‐f‐CL index, 0.40 ± 0.15, was not significantly different from the f‐m‐CL index, 0.54 ± 0.13 (p = 0.31; two‐tailed Wilcoxon signed rank test), suggesting that THC passively diffuses across the placental barrier. Surprisingly, in the presence of valspodar, THC m‐f‐CL index, 0.24 ± 0.14, was significantly lower than the corresponding f‐m‐CL index, 0.44 ± 0.14 (p = 0.016; two‐tailed Wilcoxon signed rank test). The reason for this difference is unknown.ConclusionsBecause THC passively crosses the human placenta, changes in activity of placental transporters (e.g. drugs that inhibit/induce placental transporters or pharmacogenetics) should not alter fetal THC exposure. Further, mechanisms other than P‐gp efflux (e.g. binding to placental tissue) need to be explored to explain the fetal and maternal THC exposure of less than unity detailed in the introduction.

Gestational Age Variation in Human Placental Drug Transporters.

Patient and providers’ fear of fetal exposure to medications may lead to discontinuation of treatment, disease relapse, and maternal morbidity. Placental drug transporters play a critical role in fetal exposure through active transport but the majority of data are limited to the 3rd trimester, when the majority of organogenesis has already occurred. Our objective was to define gestational age (GA) dependent changes in protein activity, expression and modifications of five major placental drug transporters: SERT, P-gp, NET, BCRP and MRP3. Apical brush border membrane fractions were prepared from fresh 1st, 2nd and 3rd trimester human placentas collected following elective pregnancy termination or planned cesarean delivery. A structured maternal questionnaire was used to identify maternal drug use and exclude exposed subjects. Changes in placental transporter activity and expression relative to housekeeping proteins were quantified. There was evidence for strong developmental regulation of SERT, NET, P-gp, BCRP and MRP3. P-gp and BCRP decreased with gestation (r = −0.72, p < 0.001 and r = −0.77, p < 0.001, respectively). Total SERT increased with gestation but this increase was due to a decrease in SERT cleavage products across trimesters. Uncleaved SERT increased with GA (r = 0.89, p < 0.001) while cleaved SERT decreased with GA (r = −0.94, p < 0.001). Apical membrane NET overall did not appear to be developmentally regulated (r = −0.08, p = 0.53). Two forms of MRP3 were identified; the 50 kD form did not change across GA; the 160 kD form was steady in the 1st and 2nd trimester and increased in the 3rd trimester (r = 0.24, p = 0.02). The 50 kD form was expressed at higher levels. The observed patterns of SERT, NET P-gp, BCRP and MRP3 expression and activity may be associated with transporter activity or decreased placental permeability in the 1st trimester to transporter specific substrates including commonly used psychoactive medications such as anti-depressants, anti-psychotics, and amphetamines, while transport of nutrients and serotonin is important in the 1st trimester. Overall these observations are consistent with a strong protective effect during organogenesis. 3rd trimester estimates of fetal exposure obtained from cord blood likely significantly overestimate early fetal exposure to these medications at any fixed maternal dose.

Placental transport in response to altered maternal nutrition.

The mechanisms linking maternal nutrition to fetal growth and programming of adult disease remain to be fully established. We review data on changes in placental transport in response to altered maternal nutrition, including compromized utero-placental blood flow. In human intrauterine growth restriction and in most animal models involving maternal undernutrition or restricted placental blood flow, the activity of placental transporters, in particular for amino acids, is decreased in late pregnancy. The effect of maternal overnutrition on placental transport remains largely unexplored. However, some, but not all, studies in women with diabetes giving birth to large babies indicate an upregulation of placental transporters for amino acids, glucose and fatty acids. These data support the concept that the placenta responds to maternal nutritional cues by altering placental function to match fetal growth to the ability of the maternal supply line to allocate resources to the fetus. On the other hand, some findings in humans and mice suggest that placental transporters are regulated in response to fetal demand signals. These observations are consistent with the idea that fetal signals regulate placental function to compensate for changes in nutrient availability. We propose that the placenta integrates maternal and fetal nutritional cues with information from intrinsic nutrient sensors. Together, these signals regulate placental growth and nutrient transport to balance fetal demand with the ability of the mother to support pregnancy. Thus, the placenta plays a critical role in modulating maternal-fetal resource allocation, thereby affecting fetal growth and the long-term health of the offspring.

Placental transporter activity and expression in relation to fetal growth.

The question of whether there are causative or compensatory changes in placental transport physiology affecting fetal growth is considered. Reductions in uterine and umbilical blood flow in growth retardation will reduce maternofetal exchange of lipophilic solutes, such as O2 and CO2, but will not have a major effect on the transfer of hydrophilic solutes. These solutes are transferred across the placenta by paracellular diffusion, transporter protein-mediated transport and endocytosis-exocytosis. Neither paracellular diffusion nor endocytosis-exocytosis has been investigated in relation to fetal growth. The weight of evidence is that there is no change in the activity and expression of the syncytiotrophoblast GI UTI glucose transporter in fetal growth retardation. However, there is strong evidence that the activity of the system A amino acid transporter, per milligram of placental membrane protein, is altered in relation to fetal growth, but in a complex manner. There is also some weaker evidence that the activity of the Na(+)-H+ exchanger, per milligram of placental membrane protein, is directly related to birth-weight. There are no data for other solute transporters; a considerable amount of work still remains to be done in order to understand the relationship between placental function and fetal growth rate.