Assessment of risk of monepantel faecal residues to dung fauna.

OBJECTIVE: To evaluate the safety of monepantel and its major metabolite for dung fauna. Monepantel is a new oral anthelmintic drug for use in sheep at a dose of 2.5 mg active ingredient/kg body weight. Hazard (toxicity) is related to the expected exposure. DESIGN: The methodology was based on the Organisation for Economic Cooperation and Development (OECD) draft guideline for testing of chemicals, so dung from cattle, not sheep, and eggs of the dung fly, Scathophaga stercoraria and larvae of the dung beetle Aphodius constans were used. PROCEDURES: Monepantel or its sulfone metabolite were mixed into bovine faeces in which either dung fly eggs or dung beetle larvae were placed and their development observed. The primary endpoint was survival. Real exposure data (faecal concentrations) for comparison with the generated laboratory data were taken from a kinetics study in sheep dung. RESULTS: The no-observed effect concentration (NOEC) for monepantel was >1000 mg/kg substrate for dung flies and 250 mg/kg for dung beetles. The sulfone metabolite was slightly more toxic, with a NOEC of 500 mg/kg for dung flies and 125 mg/kg for dung beetles. CONCLUSION: A comparison of the results to the maximum concentrations of 15 mg monepantel and 4.5 mg sulfone per kg dung observed under an exaggerated dosing regime in sheep indicates that monepantel poses no risk to insect dung fauna when used as recommended. The study is considered valid because representatives of both genera were able to develop in bovine or ovine dung.

Identification of the amino-acetonitrile derivative monepantel (AAD 1566) as a new anthelmintic drug development candidate.

Anthelmintic resistance has become a global phenomenon in gastro-intestinal nematodes of farm animals, including multi-drug resistance against the three major classes of anthelmintics. There is an urgent need for an anthelmintic with a new mode of action. The recently discovered amino-acetonitrile derivatives (AADs) offer a new class of synthetic chemicals with anthelmintic activity. The evaluation of AADs was pursued applying in vitro assays and efficacy and tolerability studies in rodents, sheep, and cattle. Amongst various suitable compounds, AAD 1566 eliminated many tested pathogenic nematode species, both at larval and adult stages, at a dose of 2.5 mg/kg bodyweight in sheep and 5.0 mg/kg bodyweight in cattle. The same doses were sufficient to cure animals infected with resistant or multi-drug-resistant nematode isolates. These findings, complemented by the good tolerability and low toxicity to mammals, suggest that AAD 1566, monepantel, would be a suitable anthelmintic drug development candidate.

Trimethylphosphate: a 30-month chronic toxicity/carcinogenicity study in Wistar rats with administration in drinking water.

Trimethylphosphate (TMPO) was administered to 50 male and 50 female Wistar rats through their drinking water at doses of 0, 1, 10, or 100 mg/kg body weight up to 30 months. The dosage of 100 mg/kg was reduced to 50 mg/kg in week 54 for reasons of tolerance, and the animals were euthanized in week 100. Additional 10 animals per dose and sex were treated for 12 months and then euthanized for interim analysis. Weakness of the hind limbs, increased incidences of sunken flanks, distended abdomen, and poor general condition were observed in both sexes of the 100/50 mg/kg group beginning with week 46. Food intake was reduced in high dose males. At 10 mg/kg body weights were up to 10% (males) and at 100/50 mg/kg up to 20% (males) or 15% (females) lower than in controls. Mortality was not affected in animals receiving up to 10 mg/kg. At 100/50 mg/kg it was markedly increased, reaching about 70% at week 100. Relatively slight hematologic changes (reduced hemoglobin, hematocrit, erythrocyte counts, increased reticulocyte numbers, and thrombocyte counts as well as a shift in the differential blood count) at 100/50 mg/kg are interpreted as changes most probably secondary to the other toxic effects. Increased cholesterol concentrations in plasma, shifts in the serum protein electrophoresis (males), increased organ weights (females), and an increased incidence of necroses and lymphocytic infiltrations point to a treatment-related effect on the liver at 100/50 mg/kg. Slightly increased protein excretion, increased relative kidney weights, and an increased incidence of chronic progressive nephropathy are considered treatment-related but rather secondary effects at 100/50 mg/kg. At 100/50 mg/kg an increased incidence and severity of bilateral tubular atrophy in the testes was diagnosed. The most important toxic effect was neurotoxicity, consisting of degeneration and loss of nerve fibers in the peripheral nerves and the spinal cord, associated with myopathic changes, and occurring at 100/50 mg/kg. The no-observed-adverse-effect-level, based on the suppression of body weight gain, is 1 mg/kg in males and 10 mg/kg in females. The incidence, time of occurrence, spectrum of types, and localizations of tumors provided no indication of a tumorigenic/carcinogenic effect of the test substance. TMPO is therefore considered not to be carcinogenic in Wistar rats.

Bromopropylate: induction of hepatic cytochromes P450 and absence of covalent binding to DNA in mouse liver.

Oral administration of benzilic acid ester-based acaricide bromopropylate at daily doses of 3, 15, 100, and 300 mg/kg body wt to young adult male Tif:MAGf mice for 14 days caused slightly increased liver weights in the high-dose group. A dose-dependent increase of the microsomal cytochrome P450 content was accompanied by elevated ethoxycoumarin O-deethylase, ethoxyresorufin O-deethylase, pentoxyresorufin O-depentylase, and total testosterone hydroxylase activities. When compared with mice treated in parallel with the model compounds for hepatic xenobiotic metabolizing enzyme induction, phenobarbitone, and 3-methylcholanthrene, the enzyme activity changes observed with bromopropylate largely equalled those expressed in phenobarbitone-treated mice. Immunochemical studies with monoclonal antibodies against rat liver cytochrome P450 isoenzymes of the gene families 1A, 2B, 3A, and 4A confirmed that bromopropylate is a phenobarbitone-type inducer in the mouse liver. Titration of liver microsomal suspensions with bromopropylate yielded Type I substrate binding spectra. The specific amplitude was increased 1.5-fold when microsomes from bromopropylate-treated mice (300 mg/kg body wt) were used instead of control microsomes, indicating the induction of cytochromes P450 catalyzing the oxidative metabolism of the test compound. Single oral administration of 300 mg/kg body wt [14C]bromopropylate to male mice, without or following pretreatment for 14 days with 300 mg/kg body wt unlabeled bromopropylate, gave no indication for DNA binding of the test compound in the liver. This excludes a genotoxic potential via covalent DNA modification. The results suggest that, in analogy to phenobarbitone, bromopropylate acts as a tumor promotor rather than a tumor initiator in the mouse liver.

Toxicology of monocrotophos.

Monocrotophos is a water-soluble organophosphate insecticide with high oral and moderate dermal toxicity. The toxicologically relevant mode of action is the inhibition of ChE activities. The toxicity of organophosphate metabolites of monocrotophos is comparable with the parent compound. Glycol conjugation in plant metabolism decreased the acute toxicity significantly. Dephosphorylated metabolites showed no demonstrable acute toxicity. Repeated exposure to the compound leads to initial cumulation of the single-dose effects. At moderate dose levels, the adverse effects are counteracted by an increase of tolerance through adaptation. A study in humans demonstrated no relevant ChE depression over a 30-d period at daily dose levels of up to 0.006 mg/kg. Lifetime chronic feeding studies in rodents again indicated ChE inhibition as the only specific effect. Body weight reduction was limited to high doses. No gross or microscopic specific lesions were demonstrable; especially, there was no evidence of oncogenic effects. Genotoxicity was evident in vitro, whereas comprehensive assessment of the in vivo tests indicates no toxicologically relevant mutagenic potential in mammals. This conclusion is supported by the absence of oncogenic effects in chronic feeding trials. Findings in reproduction studies were limited to secondary fetal reactions that were triggered by maternal toxicity. Acute and repeated administration studies in hens revealed no delayed (degenerative) neurotoxic potential. Monocrotophos showed no significant potentiation with 24 other ChE inhibitors. Poisoning signs after heavy doses were controlled by therapeutic doses of atropine, preferably in combination with an oxime.

The link between dosage compensation and sex differentiation in Drosophila melanogaster.

The rate of 3H-uridine incorporation into X-chromosome and autosomal RNA was measured as an indicator of relative transcription activity in larvae carrying various Sxl mutant alleles. Hyperactivity of X chromosomes was found in heteroallelic Sxlf#1/Sxlfhv#1 and homozygous Sxlf#2 female larvae. Sxlfhv#1 homozygotes. Sxlf#1/Sxl+ heterozygotes, heteroallelic X chromosome transcription. Except for Sxlf#ba, there is a correlation between the viability of the mutants and the degree to which X-chromosomes activity is elevated. Male larvae carrying the dominant male-specific lethal mutation SxlM#1 displayed X chromosomes only half as wide as those of control larvae. However, it could not be determined whether this property is the result of a lower transcription rate of underreplication of the mutated X chromosomes. The results demonstrate that the Sxl gene plays an important role in controlling X-chromosome activity. The relationship among the various genes known to act in sex differentiation and dosage compensation is discussed.