Developmental toxicity studies in rats and rabbits with 3,5,6-trichloro-2-pyridinol, the major metabolite of chlorpyrifos.

3,5,6-Trichloro-2-pyridinol (TCP), the primary metabolite of chlorpyrifos and chlorpyrifos-methyl, was evaluated for potential developmental toxicity. Groups of 32-34 bred female Fischer 344 rats were given 0, 50, 100, or 150 mg TCP/kg/day by gavage on gestation days 6-15; the fetuses were evaluated on gestation day 21. Similarly, groups of 16 inseminated female New Zealand White rabbits were given 0, 25, 100, or 250 mg TCP/kg/day by gavage on gestation days 7-19, and fetuses were evaluated on gestation day 28. No clinical signs of toxicity attributed to TCP were noted in either species. In rats, at 150 mg/kg/day, maternal effects included slight decreases in feed consumption, significantly depressed body weight gain (25% relative to controls) resulting in significantly lower maternal terminal body weights, and increased relative liver weight. At 100 mg/kg/day, maternal body weight gain in rats was depressed approximately 22%. Among rabbits, maternal effects were limited to the group given 250 mg/kg/day, which lost an average of approximately 70 g during the treatment period (vs. 140 g in the controls). There were no effects on fetal weight or viability, nor were there significant increases in any fetal alteration in either species. A slightly higher (not statistically significant) than usual incidence of central nervous system anomalies occurred in rabbits, but these anomalies were found in both treated and control groups in this study as well as contemporaneous studies of unrelated compounds. This, and the fact that these anomalies were not seen with the parent compound, chlorpyrifos, suggest that their origin was spontaneous. Thus, TCP was not considered fetotoxic or teratogenic in either rats or rabbits, even at dose levels that produced maternal toxicity.