Chlorpyrifos (O,O-diethyl-O-(3,5,6-trichloro-2-pyridyl)-phosphorothioate), an organophosphate insecticide, was evaluated for its potential to produce developmental and reproductive toxicity in rats following oral exposure. Pregnant Fischer 344 rats were given doses of 0 (corn oil vehicle), 0.1, 3.0, or 15 mg chlorpyrifos/kg/day, by gavage, on Gestation Days 6 through 15. Maternal effects noted at the two higher dose levels included decreased cholinesterase levels at 3.0 mg/kg/day and cholinergic signs (excessive salivation and tremors), decreased cholinesterase levels, and decreased body weight gain at 15 mg/kg/day. No maternal effects were apparent at 0.1 mg/kg/day. Although maternal toxicity was observed at these two higher exposure levels, no developmental effects were noted at any dose. In a two-generation reproduction study, Sprague-Dawley rats were maintained on diets supplying 0, 0.1, 1.0, or 5.0 mg chlorpyrifos/kg/day. Parental effects included decreased plasma and erythrocyte cholinesterase at 1.0 mg/kg/day, and decreased plasma, erythrocyte, and brain cholinesterase and histopathologic alterations of the adrenal zona fasciculata at 5.0 mg/kg/day. The histopathologic alterations of the adrenal were characterized as very slight to slight vacuolation (consistent with fatty change) in males, and very slight vacuolation and/or altered tinctorial properties in females. No effects on the reproductive or fertility indices or on the histopathology of reproductive tissues were observed at any dose level, and no neonatal effects were observed at 0.1 or 1.0 mg/kg/day in the F1 or F2 litters. Parental toxicity at the high dose was accompanied by decreased pup body weight and increased pup mortality in the F1 litters only. These data show that oral administration of chlorpyrifos to rats at parentally toxic dose levels was not embryolethal, embryo/fetotoxic, or teratogenic and did not adversely affect fertility or the function or structure of the reproductive organs. Although effects on neonatal growth and survival were observed at a maternally toxic dose level in one generation, this effect was not observed in the subsequent generation and, therefore, may not have been related to treatment.

This study evaluated propylene glycol monomethyl ether (PGME) in a rat 2-generation reproduction study, which included non-traditional study end points, such as sperm count and motility, developmental landmarks, estrous cyclicity, and weanling organ weights. Groups of 30 male and 30 female Sprague-Dawley rats (6-weeks-old) were exposed to 0, 300, 1000, or 3000 ppm of PGME vapors via inhalation for 6 hours/day, 5 days/week prior to mating, and 6 hours/day, 7 days/week during mating, gestation, and lactation, for 2 generations. These concentrations corresponded to estimated oral equivalent doses of 0, 396, 1325, or 3974 mg/kg/day. At 3000 ppm, toxicity in the P1 and P2 adults was marked, as evidenced by sedation during and after exposure, and mean body weights which were as much as 21% lower than controls. This marked parental toxicity was accompanied by lengthened estrous cycles, decreased fertility, decreased ovary weights, and histologic ovarian atrophy in maternal rats. In the offspring from these dams, decreased body weights, reduced survival and litter size, slight delays in puberty onset, and histologic changes in liver and thymus in the F1 and F2 offspring were observed. The nature of the reproductive/neonatal effects and their close individual animal correlation with decreased maternal body weights suggested that these effects were secondary to general toxicity and/or nutritional stress. No such reproductive/neonatal effects were observed at 1000 ppm, a concentration which caused less marked, but significant body weight effects without sedation. There were no treatment-related effects of any kind noted at 300 ppm of PGME. Therefore, the no-observable-effect level (NOEL) for reproductive/neonatal effects was 1000 ppm, and that for parental toxicity was 300 ppm.

Commercial grade propylene glycol monomethyl ether (PGME), which is composed of > 99.5% alpha-isomer and < 0.5% beta-isomer, has been shown in several studies to have a low potential for developmental toxicity. Nonetheless, questions have been raised about potential human developmental toxicity due to beta-PGME, because it can be metabolized to 2-methoxypropionic acid (MPA), a compound bearing structural similarity to the teratogen, methoxyacetic acid (MAA). Accordingly, a series of in vivo developmental toxicity, whole embryo culture, and in vivo pharmacokinetic experiments were conducted in New Zealand White rabbits (highly sensitive to these compounds) to better understand the developmental toxicity potential of MPA and the kinetics of its formation from beta-PGME. For the in vivo developmental toxicity studies, groups of 20 inseminated rabbits were gavaged with 0, 10, 26, or 78 mg/kg/day of MPA on gestation day (GD) 7-19, followed by fetal evaluation on GD 28. Results with MPA were compared with those of rabbits similarly dosed with 0, 2.5, 7.5, or 15 mg/kg/day of MAA. Developmental toxicity no-observable-effect levels (NOEL) were approximately 10-fold higher for MPA (26 mg/kg/day) than for MAA (2.5 mg/kg/day). Also, the severity of effects caused by MPA was less than that of MAA, and unlike MAA, MPA was not selectively toxic to the fetus. This differential toxicity was also seen in whole embryo cultures of GD 9 rabbit embryos, in which there were no adverse effects of MPA (1.0, 5.0 mM) or its parent compound, beta-PGME (0.5, 2.0 mM), but severe dysmorphogenesis in 100% of embryos cultured in 5.0 mM MAA. The pharmacokinetics study showed rapid and complete conversion of beta-PGME to MPA, with a relatively long elimination half-life (33-44 h) for MPA. However, peak and AUC concentrations of MPA in blood associated with the MPA LOEL dose of 78 mg/kg/day were 1.3 mM and 52.9 mM-h/l, respectively, suggesting a relatively high threshold based on internal dosimetry. Taken together, these data indicate a negligible risk of developmental toxicity due to MPA formation from the small amounts of beta-isomer present in commercial PGME.

No abstract

Human sperm count studies, historic dietary iodination, and an animal model where neonatal goitrogen administration causes unprecedented testis enlargement, together suggest an hypothesis relevant to the postulated fall in human sperm counts. We present the hypothesis with a supporting study extending the model to include iodine deficiency. In a one-generation rat reproduction study, dams were fed an iodine sufficient (control, 200 ppb I) or deficient (low iodine diet [LID], <20 ppb I) diet from prebreeding through weaning, when male offspring were divided into three groups: 1) controls from iodine sufficient dams, 2) neonatal LID (NLID) from the LID dams, fed control diet postweaning, and 3) chronic LID (CLID) from LID dams, fed a moderate LID (40 ppb I) postweaning. F1 males were euthanized on postnatal day (PND) 133+/-1. Each of the three diet groups comprised two subgroups in which testicular parameters were evaluated: 1) daily sperm production (DSP), sperm motility, morphology, and histopathology, and 2) Sertoli cell and round spermatid morphometry. In the first subgroup, NLID and CLID testes weights were 8.5% and 14.0% heavier than their unusually heavy controls (3.921 g; historical control mean approximately 3.5 g), with proportional DSP increases. Sperm motility, morphology, and testis histopathology were unaffected. In the morphometry subgroup, respective increases in NLID and CLID rats included testes weights (+28.6% and +20.3%), Sertoli cells (+24.8% and +23.9%), and round spermatids (+20.4% and +15.8%). The results indicate that neonatal iodine deficiency can significantly increase spermatogenic function in rats, and support our hypothesis concerning human sperm counts.