Difference in Developmental Toxicity Among Structurally Similar N‐Phenylimide Herbicides in Rats and Rabbits

BACKGROUND: S-53482 is an N-phenylimide herbicide and shows a remarkable species difference in developmental toxicity between rats and rabbits. The herbicide produced embryolethality, teratogenicity (mainly ventricular septal defects and wavy ribs), and growth retardation in rats, but not in rabbits. Our objective in this study was to investigate differences in developmental toxicity among N-phenylimide compounds structurally similar to S-53482 to better characterize the developmental effects of S-53482 on rat and rabbit embryos as part of research investigation to elucidate a mechanism of rat developmental toxicity produced by S-53482. This paper is part of a series of studies to be published serially. METHODS: S-23121 or S-23031, both of which are structurally similar to S-53482, was orally administered to rats and rabbits during fetal organogenesis. Fetuses were obtained by cesarian section and examined for external, visceral, and skeletal alterations. RESULTS: S-23121 produced the similar pattern of developmental toxicity such as embryolethality, teratogenicity, and growth retardation in rats. In contrast, S-23031 showed no developmental toxicity at 1500 mg/kg. Rabbits were insensitive to all study compounds. CONCLUSIONS: The difference in developmental toxicity in rats was striking among N-phenylimide herbicides. The mechanism of action of developmental toxicity by S-53482 should account for the compound-specific difference as well as species difference between rats and rabbits.