Eckhoff C, Willhite C C
Hoffmann-La Roche, Inc., 340 Kingsland Street, Nutley, New Jersey 07110, USA.
Toxicol Appl Pharmacol. 1997 Sep;146(1):79-87. doi: 10.1006/taap.1997.8220.
All-trans-retinoic acid (all-trans-RA) is required in normal embryogenesis and both deficiency and excess are teratogenic. Isotretinoin (13-cis-RA) is teratogenic in all species examined; based on administered dose, humans appear most sensitive, followed by (in order or decreasing sensitivity) monkey, rabbit, hamster, mouse, and rat. Identification of the teratogenic threshold in these species is difficult because RAs are normal physiologic constituents. The rabbit no-observed-adverse-effect-level (NOAEL) and lowest-observed-adverse-effect-level (LOAEL) administered doses (3 and 15 mg/kg/day, respectively, on gestation Days 8-11) are less than the corresponding values in hamster (7.5 and 37.5 mg/kg/day, respectively, on gestation Days 7 and 8), but drawing conclusions from administered dose alone ignores differences in absorbed, metabolized, and embryonic delivered dose. Therefore, distribution and metabolism studies of 13-cis-RA at the NOAEL and LOAEL in pregnant hamsters were performed and plasma and tissue concentrations of parent compound and metabolites were compared to those found in rabbits. Metabolites of 13-cis-RA common to all species include three RAs (all-trans-RA, all-trans-4-oxoRA, 13-cis-4-oxoRA) and the glucuronide conjugate of 13-cis-RA (13-cis-RAG). As in rabbits, we found 13-cis-4-oxoRA also to be the major metabolite of 13-cis-RA in hamster plasma, peripheral tissues, and embryo. Of maternal tissues, peak 13-cis-RA concentrations were highest in liver. Total concentration of RA (13-cis-RA + 13-cis-4-oxoRA + all-trans-RA + all-trans-4-oxoRA) per gram of wet tissue was greatest in maternal liver, followed by that in lung, adipose tissue, muscle, kidney, and brain. At the NOAEL, total RA plasma Cmax in hamster was 6 times that in rabbit; at the LOAEL, hamster plasma total RA Cmax was 4 times that in rabbit. Hamster absorbed and metabolized dose (as AUC of plasma total RA) at the NOAEL and LOAEL was 2.6 and 2.4 times that in rabbit, respectively. In the embryo, hamster total RA Cmax was 2.7 times (at NOAEL) and 2.6 times (at LOAEL) that in rabbit. However, embryonic delivered dose (total RA AUC in hamster and rabbit embryo, respectively) at the NOAEL (2.08 and 2.14 microg . hr.g-1) and LOAEL (5.34 and 5.54 microg . hr . g-1) was virtually identical. Embryonic AUCs in hamster and rabbit for all-trans-RA and all-trans-4-oxoRA, metabolites which transactivate directly the nuclear RA receptors (RARs), were also very similar at the NOAEL (0.66 and 0.81 microg . hr g-1) and at the LOAEL (1.14 and 1.32 microg . hr g-1). Based on embryonic delivered dose, we suggest that 13-cis-RA is an equipotent teratogen in hamster and rabbit.
全反式维甲酸（all-trans-RA）在正常胚胎发育中是必需的，缺乏或过量都会致畸。异维甲酸（13-顺式-RA）在所有已检测的物种中都有致畸性；根据给药剂量，人类似乎最敏感，其次是（按敏感性递减顺序）猴子、兔子、仓鼠、小鼠和大鼠。由于维甲酸是正常的生理成分，因此很难确定这些物种中的致畸阈值。兔子的未观察到不良反应水平（NOAEL）和最低观察到不良反应水平（LOAEL）给药剂量（分别在妊娠第8 - 11天为3和15 mg/kg/天）低于仓鼠的相应值（分别在妊娠第7和8天为7.5和37.5 mg/kg/天），但仅根据给药剂量得出结论忽略了吸收、代谢和胚胎递送剂量的差异。因此，我们对怀孕仓鼠在NOAEL和LOAEL剂量下的13-顺式-RA进行了分布和代谢研究，并将母体化合物和代谢物的血浆及组织浓度与兔子中的进行了比较。所有物种共有的13-顺式-RA代谢物包括三种维甲酸（全反式-RA、全反式-4-氧代维甲酸、13-顺式-4-氧代维甲酸）以及13-顺式-RA的葡萄糖醛酸共轭物（13-顺式-RAG）。与兔子一样，我们发现13-顺式-4-氧代维甲酸也是仓鼠血浆、外周组织和胚胎中13-顺式-RA的主要代谢物。在母体组织中，13-顺式-RA的峰值浓度在肝脏中最高。每克湿组织中维甲酸（13-顺式-RA + 13-顺式-4-氧代维甲酸 + 全反式-RA + 全反式-4-氧代维甲酸）的总浓度在母体肝脏中最大，其次是肺、脂肪组织、肌肉、肾脏和大脑。在NOAEL时，仓鼠血浆中维甲酸的总Cmax是兔子的6倍；在LOAEL时，仓鼠血浆中维甲酸的总Cmax是兔子的4倍。仓鼠在NOAEL和LOAEL时吸收和代谢的剂量（以血浆维甲酸总量的AUC计）分别是兔子的2.6倍和2.4倍。在胚胎中，仓鼠维甲酸的总Cmax在NOAEL时是兔子的2.7倍，在LOAEL时是兔子的2.6倍。然而，在NOAEL（2.08和2.14μg·hr·g-1）和LOAEL（5.34和5.54μg·hr·g-1）时，胚胎递送剂量（分别为仓鼠和兔子胚胎中维甲酸总量的AUC）实际上是相同的。在NOAEL（0.66和0.81μg·hr·g-1）和LOAEL（1.14和1.32μg·hr·g-1）时，仓鼠和兔子胚胎中全反式-RA和全反式-4-氧代维甲酸（直接激活核维甲酸受体（RARs）的代谢物）的AUC也非常相似。基于胚胎递送剂量，我们认为13-顺式-RA在仓鼠和兔子中是等效的致畸剂。
