Teratogenic potential of hexachlorocyclopentadiene in mice and rabbits

Correlation of chemical structure with reproductive and developmental toxicity as it relates to the use of the threshold of toxicological concern

In the absence of toxicological data on a chemical, the threshold of toxicological concern (TTC) approach provides a system to estimate a conservative exposure below which there is a low probability of risk for adverse health effects. The original toxicology dataset underlying the TTC was based on NOELs from repeat dose studies. Subsequently there have been several efforts to assess whether or not these limits are also protective for reproductive/developmental effects. This work expands the database of chemicals with reproductive and developmental data, presents these data in a comprehensive and transparent format and groups the chemicals according to the TTC "Cramer Class" rules. Distributions of NOAELs from each of these classes were used to assess whether the previously proposed TTC values based on repeat dose data are protective for reproductive/developmental toxicity endpoints as well. The present analysis indicates that, for each Cramer Class, the reproductive and developmental endpoints would be protected at the corresponding general TTC tiers derived by Munro et al. (1996).

Maternal toxicity: a possible etiological factor in embryo-fetal deaths and fetal malformations of rodent-rabbit species

Data from animal teratology studies were surveyed to determine whether embryo-fetal mortality and fetal malformations result from a primary action of the agent on the conceptus or if they are secondary to maternal toxicity--a consequence of administration with high dose levels of test chemicals. A fairly strong association between embryo-fetal mortality and maternal toxicity was revealed by analysis of data from hamsters, mice, rats, and rabbits in 234 studies of chemical and physical agents, of which 83 were conducted at both maternotoxic and nonmaternotoxic doses, 94 only at maternotoxic doses, and 49 at nonmaternotoxic doses. In the above studies, only nine chemicals (four each in hamsters and rabbits and one in rats) were reported to induce embryo-fetal deaths at apparently nonmaternotoxic doses. These findings tend to suggest a contributory role for maternal toxicity in the induction of embryo-fetal deaths. The previously reported hypothesis that certain fetal defects in mice may perhaps be caused by maternal toxicity was also found to be true in a review of data on hamsters, rats, and rabbits. Salient maternal toxicity-associated fetal malformations were exencephaly, encephalocele, micro- or anophalmia, and fused ribs in hamsters and defective (fused, missing, or extra) ribs, vertebrae, and sternebrae, ex-, an-, or microphthalmia, and cleft palate in rats and rabbits. These malformations occurred at low frequencies, generally with no readily apparent dose-response relationship. Presumptive evidence indicates that embryo-fetal deaths, and the above-mentioned fetal malformations in experimental animals, which in published literature are presently attributed to chemical induction for a large number of chemicals, may be a consequence of maternal toxicity per se.

Investigations on the basis for the differential toxicity of hexachlorocyclopentadiene administered to rats by various routes

The differential disposition of hexachlorocyclopentadiene (HCCP) following oral administration, as contrasted to inhalation or intravenous administration, may account for its lower toxicity by this route. Following an intravenous dose of [14C]HCCP to rats at 0.59 mg/kg, 39.0% of the radioactivity remained in the tissues at 72 h; after inhalation of vapors of [14C]HCCP (1.3-1.8 mg/kg), this amount was 11.5%. After oral doses of 4.1 or 61 mg/kg, however, the amount was only 2.4%. No detectable amount of intact HCCP was present in the lungs or kidneys of rats exposed to the chemical by inhalation, and only about 1% was converted to CO2, regardless of the route of administration. The chemical reactivity of HCCP with biological materials was evident in in vitro experiments, in which HCCP became bound to components of whole blood, plasma, liver homogenates, fecal homogenates, and intestinal contents. Thus, the lower toxicity of oral doses of HCCP may be related to its reaction with intestinal contents and its lack of absorption into tissues, in substantial amounts, as the intact, reactive form.