Maternal toxicokinetics, metabolism, and embryo exposure following a teratogenic dosing regimen with 13-cis-retinoic acid (isotretinoin) in the cynomolgus monkey

The maternal pharmacokinetics, metabolism, and placental transfer of 13-cis-retinoic acid (isotretinoin) have been determined in the cynomolgus monkey using a dosing regimen which had been previously shown to result in retinoid-specific teratogenic effects [Hummler et al. (1990) Teratology 42:263-272]. The drug (2.5 mg/kg body weight) was administered by naso-gastric intubation once a day between gestational days (GD) 16-26, and twice a day between GD 27-31. Maternal plasma kinetics were determined following dosing on GD 26 and GD 31, and placental transfer was studied following the last dose on GD 31. The plasma half life of 13-cis-retinoic acid in the monkey (13.2 h) was comparable to that in the human. The main plasma metabolite in the monkey was the 13-cis-4-oxo-retinoic acid which occurred at levels lower or comparable to those of the administered drug. During multiple dosing, this metabolite accumulated to the same degree as the parent drug. All-trans-retinoic acid was present in maternal plasma in very low concentrations (2% of 13-cis-retinoic acid). The β-glucuronides of all-trans- and 13-cis-retinoic acid were further minor plasma metabolites. 13-cis-retinoic acid and its 4-oxo-metabolite reached the monkey embryo slowly but extensively during organogenesis and reached 24 h-AUC values of 956 and 590 ng · h/g embryo wet weight, resulting in embryo/maternal plasma concentration ratios of 0.41 and 0.33, respectively. The AUC value of all-trans-retinoic acid (316 ng · h/g) was only raised approximately 40% above the endogeneous AUC level (225 ng · h/g); only at two time periods examined were the embryonic all-trans-retinoic acid concentrations above endogeneous levels (at 4 h and 8 h; P < 0.01 and < 0.05, respectively; Student's t-test). The β-glucuronides of all-trans- and 13-cis-retinoic acid were not detected in the embryo. Accumulation of 13-cis-retinoic acid and the 4-oxo-metabolite during the twice-per-day dosing regimen was apparent both in maternal plasma and embryo. An interspecies comparison suggests that the half life as well as the metabolic pattern of 13-cis-retinoic acid in plasma were similar in monkey and human: 13-cis-4-oxo-retinoic acid was the main metabolite in both species and the β-glucuronides as well as all-trans-retinoic acid were minor metabolites. However, the: plasma AUC values of the administered drug and particularly the 4-oxo-metabolite were found to be lower in the monkey as compared to the human. This is one important fact why higher doses are needed in the monkey than in the human to elicit a teratogenic response. Our results indicate that the cynomolgus monkey is a good model for the human in regard to 13-cis-retinoic acid teratogenesis: we provide strong evidence that the high sensitivity of the monkey in comparison to other animal species (in rats and mice 20-50-fold higher doses are needed to elicit a teratogenic response) is due to prolonged and pronounced maternal and embryonic exposure to the parent compound and its 4-oxo-metabolite in the monkey embryo, which are implicated as proximate teratogenic agents.