TERATOGENICITY OF ALL-TRANS RETINOIC ACID DURING EARLY EMBRYONIC-DEVELOPMENT IN THE CYNOMOLGUS MONKEY (MACACA-FASCICULARIS)

The embryotoxic and teratogenic potential of all-trans retinoic acid was assessed following exposure prior to and during early organogenesis in the cynomolgus monkey (Macaca fascicularis). Sixteen pregnant females were orally administered all-trans retinoic acid (Tretinoin, Hoffmann-La Roche) once daily from GD 10-20 and twice daily from GD 21-24 at three different dosages, 5 (n = 9), 10 (n = 6) and 20 mg/kg (n = 1). Adverse clinical signs resembling hypervitaminosis A were observed in one animal at 5 mg/kg, in three animals at 10 mg/kg, and in the animal treated with 20 mg/kg all-trans retinoic acid. Maternal weight loss was observed in the 10- and 20-mg/kg groups. A dose-dependent increase in embryolethality was observed, with 22% (2/9), 50% (3/6), and 100% (1/1) occurring at 5, 10, and 20 mg/kg, respectively. The majority of embryonic deaths occurred between GD 16 and 20; the incidence of these early losses was higher than in historical and concurrent controls. No malformations, but a single growth-retarded fetus, was observed in the 5-mg/kg group. Craniofacial malformations, consisting of external ear defects, mandibular hypoplasia, cleft palate, and temporal bone abnormalities, were seen in three viable fetuses in the 10-mg/kg group. Skeletal variations were common to the majority (70%, 7/10) of viable fetuses in both dose groups and were increased relative to historical controls (32%, 25/77). Unlike previous studies with 13-cis-retinoic acid during the pre- and early organogenic stages of development (Hummler et al., Teratology 42:263-272, 1990), no thymic hypo- or aplasia or heart anomalies were observed, which may be attributable to the slightly longer 13-cis retinoic acid treatment period, i.e., GD 10-27. However, external ear and temporal bone defects were common to both all-trans and 13-cis retinoic acid. The similarity observed in the malformation syndrome induced by both all-trans and 13-cis retinoic acid in the cynomolgus monkey and 13-cis retinoic acid embryopathy in humans supports this macaque species as a model for further developmental toxicity studies of vitamin A-related compounds.