No abstract

The relevance of studies in dogs on regulatory testing of pesticides was examined retrospectively using data of 216 pesticides (acaricides, fungicides, growth regulators and hormones, herbicides, insecticides, molluscicides, nematicides, rodenticides, synergists for insecticides) submitted for regulatory purposes during the past 40 years to the Federal Institute of Health Protection of Consumers and Veterinary Medicine (BgVV), the competent national authority in Germany. At first the relevance of the no-observed-effect levels (NOEL) for safety assessment was evaluated for each chemical in 4-week (subacute), 13-week (subchronic) and 52/104-week (chronic) toxicity studies carried out on dogs, rats and mice. After subchronic and chronic application of fungicides the sensitivity of rats and dogs to the toxic chemicals was quite similar. However, the dog was generally a more sensitive species to toxic effects of insecticides than rat and mouse. On the other hand the NOEL was lower in the rat than the dog in chronic studies on herbicides. When the lowest-observed-effect level (LOEL) was evaluated in animal species, the dog was the most sensitive in approximately 15% of the studies. Mice were found to be the most sensitive species only in approximately 1% of the studies on 216 pesticides. Comparison of organ specific toxicity at the LOEL in subacute studies on fungicides and herbicides revealed a poor correlation of target-specific organ toxicity across species. However, in the subchronic and chronic studies (13 and 52/104 weeks) no significant differences in species-specific organ toxicity were observed in the three species rat, mouse and dog. The only exception were haematoxic effects in chronic studies on herbicides, which were more frequent in dogs (40%) than in rats and mice (20%). The results support the established concept that studies on dogs and rats are important for the safety assessment of pesticides, while studies on mice do not provide further information, except for detection of an oncogenic potential which is a further controversial issue. Further analysis of subacute, subchronic and chronic studies in dogs should reveal if all of the studies are essential for safety assessment of pesticides.

Phenotypically altered liver foci were produced in female Wistar rats by a single dose of N-nitrosomorpholine followed by promotion with phenobarbital (PB) for 20 or 28 weeks. Then treatment was changed to either hexachlorocyclohexane (HCH), or cyproterone acetate (CPA), or nafenopin (Naf) or clofibrate (Clof), two hypolipidemic drugs. Foci were identified by a positive reaction for gamma-glutamyl-transpeptidase (GGT) and other cytological markers. HCH and CPA could substitute for PB as foci promoters; in contrast, Naf and Clof decreased expression of GGT in foci resulting in a decline of number and area of detectable foci, effects particularly pronounced with Naf. Immunohistochemical investigations of serial sections revealed that Naf also reduced expression of the altered phenotype when cytochrome P450-PB and pyruvate kinase (type L) were used as foci markers, but not when glutathione-S-transferase B (GST-B) was used. Thus, the number of foci with enhanced GST-B did not decline significantly after the change from PB to Naf treatment. Furthermore, the reduction of GGT and the decrease of foci number during Naf treatment were not associated with increased evidence of cell death by apoptosis in foci, in contrast to the situation after PB withdrawal. These findings strongly suggest that the disappearance of GGT-positive foci after Naf is due to a phenotypic change resulting in a suppression of GGT expression rather than to physical elimination of foci.

Non-genotoxic hepatocarcinogens share the ability to induce liver growth in rodents. Phenobarbital (PB), as one prototype compound, promotes the development of liver tumors; altered cell foci of the clear-eosinophilic phenotype, also identified by gamma-glutamyltransferase expression, appear to be precursor lesions. These foci seem to over-respond to the growth-inducing effect of PB. In contrast, the question as to whether peroxisome inducers are also tumor promoters is still unsettled. We will present evidence which strongly suggests that the peroxisome inducer, nafenopin (Naf), promotes tumor development in rat livers by stimulating selective growth of a hitherto undescribed subtype of altered foci. This subtype is characterized by weak diffuse cytoplasmic basophilia of its hepatocytes. Initiation in rats by aflatoxin B1 followed by promotion with Naf produced numerous adenomas and carcinomas; their morphology resembled that of the weakly basophilic foci. Both clear-eosinophilic and weakly basophilic foci appear "spontaneously" in the liver of aging rats. Promotion of such lesions by PB-type compounds or peroxisome inducers may explain cancer formation by these non-genotoxic agents.