Different effects of an N-phenylimide herbicide on heme biosynthesis between human and rat erythroid cells.

Rat developmental toxicity including embryolethality and teratogenicity (mainly ventricular septal defects and wavy ribs) were produced by S-53482, an N-phenylimide herbicide that inhibits protoporphyrinogen oxidase (PPO) common to chlorophyll and heme biosynthesis. The sequence of key biological events in the mode of action has been elucidated as follows: inhibition of PPO interferes with normal heme synthesis, which causes loss of blood cells leading to fetal anemia, embryolethality and the development of malformations. In this study we investigated whether the rat is a relevant model for the assessment of the human hazard of the herbicide.

Thomas H. Shepard, M.D., pioneer in embryology and teratology.

Dr. Thomas H. Shepard died on October 3, 2016 at the age of 93.

Mechanism of Developmental Effects in Rats Caused by an N-Phenylimide Herbicide: Transient Fetal Anemia and Sequelae during Mid-to-Late Gestation.

Background: Rat developmental toxicity including embryolethality and teratogenicity (mainly ventricular septal defects [VSDs] and wavy ribs) was produced by an N-phenylimide herbicide that inhibits protoporphyrinogen oxidase (PPO) common to chlorophyll and heme biosynthesis. Major characteristics of the developmental toxicity included species difference between rats and rabbits, compound-specific difference among structurally similar herbicides, and sensitive period. Protoporphyrin accumulation in treated fetuses closely correlated with the major characteristics.

Close link between protoporphyrin IX accumulation and developmental toxicity induced by N-phenylimide herbicides in rats.

Background: S-53482, 7-fluoro-6-[(3,4,5,6-tetrahydro)phthalimido]-4-(2-propynyl)-1,4-benzoxazin-3(2H)-one (flumioxazin), is an N-phenylimide herbicide and developmentally toxic to rats, but not to rabbits. The day of greatest sensitivity to S-53482 is gestational day (GD) 12 in rats. There is a compound-specific difference in developmental toxicity among structurally similar compounds including S-23121 (N-[4-chloro-2-fluoro-5-[(1-methyl-2-propynyl)oxy]phenyl]-3,4,5,6-tetrahydrophthalimide; teratogenic) and S-23031 (pentyl 2-chloro-4-fluoro-5-(3,4,5,6-tetrahydrophthalimido)phenoxyacetate (flumiclorac pentyl); nonteratogenic).

Dermal developmental toxicity of N-phenylimide herbicides in rats.

Background: S-53482 and S-23121 are N-phenylimide herbicides and produced embryolethality, teratogenicity (mainly ventricular septal defects and wavy ribs), and growth retardation in rats in conventional oral developmental toxicity studies. Our objective in this study was to investigate whether the compounds induce developmental toxicity via the dermal route, which is more relevant to occupational exposure, hence better addressing human health risks.

Methods: S-53482 was administered dermally to rats at 30, 100, and 300 mg/kg during organogenesis, and S-23121 was administered at 200, 400, and 800 mg/kg (the maximum applicable dose level).

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.

Further evidence for the role of free radicals in the limb teratogenicity of L-NAME.

Background: L-NAME (N(G)-nitro-(L)-arginine methyl ester), a nitric oxide synthase inhibitor, causes severe limb reduction malformations when gravid rats are treated intraperitoneally on gd-17. Hemorrhages, appearing within hours of L-NAME administration, and defects at term can be significantly reduced by co-treatment with PBN (alpha-phenyl-N-t-butylnitrone), a spin trap antioxidant. We have proposed that limb defects result from ischemia-reperfusion injury.

Involvement of mitochondria and other free radical sources in normal and abnormal fetal development.

Shepard and Mackler have documented quantitative increases in mitochondrial cristae between gestational days 10 and 14 in rats accompanied by decreased glucose utilization and increased NADH oxidase activity. Findings show a shift from glycolytic to oxidative metabolism starting at around the time of implantation. Exposure to many substances that cause transient uteroplacental hypoperfusion, including cocaine, phenytoin, calcium channel blockers, and nitric oxide synthase (NOS) inhibitors, induce limb and central nervous system (CNS) malformations while sparing the heart.