Absence or pharmacological blocking of placental P-glycoprotein profoundly increases fetal drug exposure

Abstract

It was recently shown that naturally occurring Mdr1a mutant fetuses of the CF-1 outbred mouse stock have no placental Mdr1a P-glycoprotein (P-gp) and that this absence is associated with increased sensitivity to avermectin, a teratogenic pesticide. To further define the role of placental drug-transporting P-gp in toxicological protection of the fetus, we used mice with a targeted disruption of the Mdr1a and Mdr1b genes. Mdr1a(+/-)/1b(+/-) females were mated with Mdr1a(+/-)/1b(+/-) males to obtain fetuses of 3 genotypes (Mdr1a(+/+)/1b(+/+), Mdr1a(+/-)/1b(+/-), and Mdr 1a(-/-)/1b(-/-)) in a single mother. Intravenous administration of the P-gp substrate drugs [(3)H]digoxin, [(14)C]saquinavir, or paclitaxel to pregnant dams revealed that 2.4-, 7-, or 16-fold more drug, respectively, entered the Mdr1a(-/-)/1b(-/-) fetuses than entered wild-type fetuses. Furthermore, placental P-gp activity could be completely inhibited by oral administration of the P-gp blockers PSC833 or GG918 to heterozygous mothers. Our findings imply that the placental drug-transporting P-gp is of great importance in limiting the fetal penetration of various potentially harmful or therapeutic compounds and demonstrate that this P-gp function can be abolished by pharmacological means. The latter principle could be applied clinically to improve pharmacotherapy of the unborn child.

Figure 1

Ratio of [³H]digoxin fetal concentration to maternal plasma concentration. Heterozygous mice were mated, and [³H]digoxin (0.05 mg/kg) was administered intravenously to pregnant dams at gestation day 15. At 4 hours or 24 hours after dosing, dams were euthanized, and fetuses from 3 litters were collected. Maternal plasma and fetal drug content and genotypes were determined as described in Methods. Open bars indicate wild-type values, hatched bars heterozygous values, and black bars Mdr1a^–/–/1b^–/– values. Values are expressed as mean [³H]digoxin concentration in fetal tissues (ng/g) normalized to maternal plasma [³H]digoxin concentration (ng/mL). Six to 16 fetuses of each genotype were analyzed. Average maternal plasma concentrations were 19.2 ± 1.6 ng/mL and 1.1 ± 0.15 ng/mL at 4 and 24 hours after digoxin administration, respectively. Error bars indicate SD. ^AP < 0.0001, ^BP < 0.005, vs. wild-type fetuses in pairwise comparison.

Figure 2

Ratio of [¹⁴C]saquinavir fetal concentration to maternal plasma concentration. Heterozygous mice were mated, and [¹⁴C]saquinavir (1 mg/kg) was administered intravenously to pregnant dams at gestation day 15. At 5 or15 minutes after dosing, dams were euthanized, and fetuses from 4 litters were collected. Maternal plasma and fetal drug content and genotypes were determined as described in Methods. Open bars indicate wild-type values, hatched bars heterozygous values, and black bars Mdr1a^–/–/1b^–/– values. Values are expressed as mean [¹⁴C]saquinavir concentration in fetal tissues normalized to maternal plasma [¹⁴C]saquinavir concentration. Four to 27 fetuses of each genotype were analyzed. Average maternal plasma concentration was 429 ± 32 ng/mL and 145.9 ± 16 ng/mL at 5 minutes and 15 minutes after [¹⁴C]saquinavir administration, respectively. All animals had also received oral administration of vehicle 2 hours before intravenous drug administration for proper comparison to PSC833-treated mice (see experiment described in Table 3). Error bars indicate SD. ^AP < 0.005 vs. wild-type fetuses in pairwise comparison.

Figure 3

Ratio of paclitaxel fetal concentration to maternal plasma concentration. Heterozygous mice were mated, and paclitaxel (10 mg/kg) was administered intravenously to pregnant dams at gestation day 15. At 1 hour after dosing, dams were euthanized, and fetuses from 4 litters were collected. Maternal plasma and fetal drug content and genotypes were determined as described in Methods. Open bars indicate wild-type values, hatched bars heterozygous values, and black bars Mdr1a^–/–/1b^–/– values. Values are expressed as mean paclitaxel concentration in fetal tissues (ng/g) normalized to maternal plasma paclitaxel concentration (ng/mL). Twelve to 14 fetuses of each genotype were analyzed. The average maternal plasma concentration was 1949 ± 219 ng/mL. Error bars indicate SD. ^AP < 0.0001 vs. wild-type fetuses in pairwise comparison.