Chlorodibromomethane in vivo Rat Liver DNA Repair Test
DWI0710a
1995

SUMMARY

Chlorodibromomethane was assessed for induction of DNA repair in hepatocytes following acute oral administration to Specific Pathogen Free outbred albino Hsd/Ola Sprague-Dawley male rats at dosages of 600 and 2000 mg/kg bodyweight. The high dose of 2000 mg/kg chosen for the DNA repair test is the maximum level recommended by the UKEMS and by OECD guidelines and is also the OECD and EEC limit dose for acute oral toxicity testing; we consider it to be an appropriate limit for use in this system.

A negative control group was treated with the vehicle, aqueous 1% methylcellulose, and a positive control group was treated with dimethylnitrosamine at 4 mg/kg (for the 2 hour expression) or 2-acetylaminofluorene at 50 mg/kg (for the 14 hour expression). Hepatocytes were isolated by enzymatic dissociation at 2 or 14 hours after exposure of the animals to the test substance. Four animals were assessed at each experimental point, with the exception that only two animals from the positive control group were assessed at each expression time.

The isolated hepatocytes were allowed to attach to glass coverslips and were cultured in vitro with (methyl-³H)thymidine at 10 µCi/ml for four hours to ‘radiolabel’ replicating DNA. The hepatocytes were ‘chased’ for 24 hours with unlabelled thymidine then they were fixed and processed for autoradiography.

DNA repair was assessed by comparing the labelling levels of hepatocyte nuclei from treated animals with control values and with the accompanying cytoplasmic labelling levels (usually a total of 150 cells per animal were examined).

Chlorodibromomethane did not cause any significant increases in either the gross nuclear grain count or the net nuclear grain count (ie the gross nuclear grain count minus the cytoplasmic grain count) at any dose level at either sampling time.

Positive control group animals showed a large and highly significant increase (P<O.OOl) in the net nuclear grain count which was accompanied by a large increase in the gross nuclear grain count.

It is concluded that Chlorodibromomethane did not elicit any evidence of DNA-damage in the rat liver in this in vivo test system.

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