Two-generation reproduction study of ammonium perchlorate in drinking water in rats evaluates thyroid toxicity.

Perchlorate is an inorganic ion that has recently been detected in drinking water supplies throughout the country, but little is known about its effects on reproductive function. This two-generation reproductive study examines the effects of ammonium perchlorate on the male and female reproductive systems in rats, and on the growth and development of offspring. Adult Sprague-Dawley rats (30/sex/group) were given continuous access to ammonium perchlorate in their drinking water at doses of 0, 0.3, 3.0, and 30.0 mg/kg-day. F1 generation rats were given the same ammonium perchlorate doses as their respective P1 generation sires and dams beginning at weaning and continuing through the day of sacrifice. Standard reproductive parameters were evaluated; blood was collected for determination of serum thyroid-stimulating hormone (TSH), triiodothyronine (T3), and thyroxine (T4) levels. Histopathological examination was conducted on major tissues, including the thyroid. No significant changes in developmental parameters were observed. In the F1 generation adult rats, relative thyroid weights were significantly increased in all dose groups for female rats and in the 3.0 and 30.0 mg/kg-day dose groups for male rats. Histopathologic changes in the thyroid consisted of hypertrophy and hyperplasia that increased in incidence and severity in a dose-related manner. Dose-related, statistically significant changes in TSH and T4 or T3 occurred at doses higher than those that resulted in changes in thyroid weight and thyroid histopathology, 30 mg/kg-day. Thus, perchlorate is not a reproductive toxicant in rats when administered in the drinking water at doses up to 30 mg/kg-day, but it can affect the thyroid at doses > or =3 mg/kg-day. Based on these findings, 0.3 mg/kg-day is identified to be the no-observable-adverse-effect level (NOAEL) for this study.

Oral (drinking water) developmental toxicity study of ammonium perchlorate in New Zealand White rabbits.

This developmental toxicity study was conducted to evaluate the embryo-fetal toxicity and teratogenic potential of ammonium perchlorate in New Zealand White [Hra:(NZW)SPF] rabbits. Pregnant rabbits were given continual access to ammonium perchlorate in drinking water at target doses of 0, 0.1, 1.0, 10.0, 30.0, and 100.0 mg/kg-day on gestation days 6 through 28. The actual consumed doses in the study were 0, 0.1, 0.9, 10.4, 30.3, and 102.3 mg/kg-day. The rabbits were sacrificed on gestation day 29, and fetuses were examined for developmental alterations. In addition, blood was collected from does for determination of serum thyroid stimulating hormone (TSH), triiodothyronine (T3), and thyroxine (T4) levels and the thyroid was subjected to histopathologic examination. No maternal deaths were attributed to perchlorate exposure. Ammonium perchlorate as high as 100.0 mg/kg-day did not affect caesarean sectioning or litter parameters studied, and all values were found to be within the historical ranges of the laboratory. The litter averages for corpora lutea, implantations, litter sizes, live and dead fetuses, percent dead or resorbed conceptuses, and fetal body weights were comparable and also did not differ significantly in the six dose groups. All placentae appeared normal and no dam had a litter consisting of only resorbed conceptuses. The maternal thyroid was the target organ for ammonium perchlorate in this study. Increased incidence of thyroid follicular hypertrophy was observed in does treated with > or =10 mg/kg-day perchlorate and significantly decreased T4 was observed in does treated with > or =30 mg/kg-day. Based on these data, the maternal no-observable-adverse-effect level (NOAEL) for ammonium perchlorate was 1.0 mg/kg-day. The developmental NOAEL for ammonium perchlorate was found to be 100.0 mg/kg-day for rabbits.

Applications of mechanistic data in risk assessment: the past, present, and future.

Mechanistic data, when available, have long been considered in risk assessment, such as in the development of the nitrate RfD based on effects in a sensitive group (infants). Recent advances in biology and risk assessment methods have led to a tremendous increase in the use of mechanistic data in risk assessment. Toxicokinetic data can improve extrapolation from animals to humans and characterization of human variability. This is done by the development of improved tissue dosimetry, by the use of uncertainty factors based on chemical-specific data, and in the development of physiologically based pharmacokinetic (PBPK) models. The development of the boron RfD illustrates the use of chemical-specific data in the improved choice of uncertainty factors. The draft cancer guidelines of the U.S. Environmental Protection Agency emphasize the use of mode of action data. The first choice under the guidelines is to use a chemical-specific, biologically based dose-response (BBDR) model. In the absence of a BBDR model, mode of action data are used to determine whether low-dose extrapolation is done using a linear or nonlinear (margin of exposure) approach. Considerations involved in evaluating a hypothesized mode of action are illustrated using 1,3-dichloropropene, and use of a BBDR model is illustrated using formaldehyde. Recent developments in molecular biology, including transgenic animals, microarrays, and the characterization of genetic polymorphisms, have significant potential for improving risk assessments, although further methods development is needed. Overall, use of mechanistic data has significant potential for reducing the uncertainty in assessments, while at the same time highlighting the areas of uncertainty.

Acrylonitrile: a reevaluation of the database to support an inhalation cancer risk assessment.

Acrylonitrile (ACN) is a monomer used extensively in the production of plastics, synthetic fibers, and rubber. In previous assessments conducted by IARC and the EPA, ACN was classified as a probable human carcinogen based on limited evidence in humans and sufficient evidence in laboratory animals. Specifically, EPA had determined that there was an association between ACN exposure and lung cancer based on a study by O'Berg (1980, J. Occup. Med. 22, 245-252). However, a follow-up of this cohort (O'Berg et al., 1985, J. Occup. Med. 27, 835-840) shows no statistically significant excess of lung cancer mortality or incidence. Our evaluation of the more recent human database taken as a whole shows that there is not a clear association between ACN exposure and human cancer, yet the studies have insufficient power to be able to rule out a small increase. In laboratory rats, however, ACN has been shown to be clearly carcinogenic by the oral and inhalation routes. Applying the methodology of EPA's proposed 1996 cancer risk assessment guidelines to the rat tumor data, the estimated upper bound on the excess lifetime risk at continuous exposure to 1 microgram/m3 ACN is calculated to be in the range of 8.2 x 10(-6) to 1.1 x 10(-5).