(-)-Δ9-tetrahydrocannabinol and pregnancy: transporter-mediated tissue distribution and drug interactions

Abstract

As cannabis use during pregnancy increases, it is important to understand the mechanisms and extent of placental transfer of (-)-trans-Δ9-tetrahydrocannabinol (THC), the primary intoxicating constitute of cannabis, along with its circulating major metabolites. In a nonhuman primate study, the fetal plasma exposure to THC is about 30% of the maternal exposure. Similarly, in a human study of daily cannabis smokers, the average THC umbilical vein-to-maternal serum concentration ratio at delivery was 0.26 ± 0.10 (n = 3). This reduced fetal exposure to THC in nonhuman primates and humans is likely attributed to the placenta’s ability to limit fetal exposure to xenobiotics. However, the exact mechanisms behind this protection are not fully understood. Since many transporters on both the maternal and fetal side of the syncytiotrophoblast are expressed in the placenta, we hypothesized that the reduced fetal exposure to THC could be attributed to apical (maternal-facing) efflux transporters [i.e., P-glycoprotein (P-gp) and/or breast cancer resistance protein (BCRP)], or basal (fetal-facing) uptake transporters [i.e., organic anion transporting polypeptide 2B1 (OATP2B1), organic cation transporter (OCT3), and/or organic anion transporter 4 (OAT4)], or both. Therefore, we evaluated, in vitro and in vivo, whether THC and its major metabolites, 11-hydroxy-THC (11-OH-THC), 11-nor-9-carboxy-THC (THC-COOH) are substrates of key placental efflux and uptake transporters at their pharmacologically relevant concentrations (Chapters 2, 3, and 4). Cannabinoid-drug interactions may occur if the cannabinoid, at its pharmacologically relevant plasma concentrations, is a significant inhibitor of the transporter involved in the drug’s tissue distribution or clearance. Therefore, we also investigated if THC and its major metabolites, 11-OH-THC and THC-COOH, are inhibitors of key placental/hepatic efflux and uptake transporters at their pharmacologically relevant concentrations (Chapters 2 and 4). To test the above hypotheses, we examined in Chapter 2, if THC and its major metabolites interact with key placental efflux transporters using cell lines that overexpress human P-gp or BCRP. At pharmacologically relevant concentrations, neither THC nor 11-OH-THC were substrates or inhibitors of P-gp or BCRP. THC-COOH, however, showed weak substrate and inhibitory activity for BCRP but not P-gp. Therefore, the placental efflux transporters P-gp and BCRP are unlikely to cause the reduced fetal-to-maternal exposure ratio of THC observed in humans or non-human primates. Moreover, THC and its metabolites are unlikely to produce cannabinoid-drug interactions at pharmacologically relevant concentrations. These findings contrast with earlier rodent studies which suggest that THC is a substrate of P-gp and Bcrp, suggesting species-specific differences in transport of THC. THC and its major metabolites are highly lipophilic with extensive nonspecific binding. Therefore, the above studies may be confounded by our inability to detect cannabinoid transport in the background of high non-specific binding. Others, using P-gp knock-out mice, have found that THC is a substrate of P-gp when it is administered orally. Therefore, in Chapter 3, we investigated maternal-fetal THC distribution and disposition in P-gp and/or Bcrp knockout pregnant mice. However, no significant changes in fetal-to-maternal area under the plasma concentration-time (AUC) ratios of THC or its metabolites were observed among all genotypes. Surprisingly, P-gp-deficient pregnant mice had significantly lower maternal brain/maternal plasma AUC ratios of THC compared to the wild type pregnant mice, suggesting an interaction of P-gp knock-out with other unknown transporters or brain fatty acid binding proteins (FABPs) to which THC binds. Since the above studies indicated that placental efflux transporters cannot explain the reduced fetal exposure to THC observed in human and non-human primates, in Chapter 4, we investigated if placental basal uptake transporters (i.e., OATP2B1, OCT3, OAT4) could be responsible for these observations. Given that THC and its metabolites are cleared by the liver, we also investigated if THC and its metabolites were substrates or inhibitors of the hepatic uptake transporters [OATP1B1, OATP1B3, OCT1, OAT4, sodium taurocholate cotransporter protein (NTCP)]. None of the cannabinoids interacted with these transporters, except for hepatic OCT1, which transported both THC and THC-COOH at their pharmacologically relevant concentrations. However, at these concentrations, they were not inhibitors of OCT1. Therefore, OATP2B1, OCT3, and OAT4 are also unlikely to be responsible for the reduced fetal exposure to THC. Also, this suggests that co-administration of OCT1 inhibitors with THC or THC-COOH could reduce the in vivo hepatic distribution of these cannabinoids provided OCT1 plays a significant role (vis-à -vis passive diffusion) in their distribution. In summary, our research indicates that the major placental efflux and uptake transporters, in humans or mice, are not responsible for limiting the observed fetal THC exposure in human and nonhuman primates, pointing to other placental transporters or alternative mechanisms. The identification of hepatic OCT1 as a transporter for THC and THC-COOH suggests possible OCT1-based drug interactions in their in vivo disposition. These findings lay the groundwork for future studies on cannabinoid pharmacokinetics, including during pregnancy, and highlight the importance of considering species differences when extrapolating from preclinical data to humans.