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firmed that the micronucleus test in exfoliated cells is a useful tool, providing reliable informa- tion on genotoxic damage, and can be used as a tumor marker in the human population with a high risk of developing cancer. If the data are confirmed on a larger number of patients this cytogenetic tumor marker could be used in clinical routine and compared and associated with other tumor markers.

Genotoxicity of procarbazine and two deuterated analogs in mammalian cells in vitro and in vivo

Holme, J.A., E.J. Soderlund, G. Brunborg, J.K. Hongslo, B. Trygg and S.D. Nelson, Department of Toxicology, National Institute of Public Health, Oslo (Norway) and Department of Medicinal Chemistry, University of Washington, Seattle, WA (U.S.A.)

N-Isopropyl- t~-(2-methylhydrazino)-p- tolua- mide (procarbazine) is an antineoplastic drug. In spite of its strong mutagenic activity in vivo, pro- carbazine has often given negative results in short-term tests in vitro. In the present study, we found that procarbazine (50-1000 /~g/ml) in- duced DNA damage in hepatocytes measured by the automated alkaline elution method, whereas no significant increase in unscheduled DNA synthesis was seen. In hepatocytes isolated from PCB-treated rats, DNA damage was detected in both test systems at concentrations as low as 1-10 /~g/ml. DNA damage, as measured by alkaline elution and sister-chromatid exchange, was also observed in V79 cells incubated with PCB- hepatocytes. In contrast, no mutagenic activity was observed in the Salmonella typhimurium strain TA1530 co-incubated with the hepatocytes. Ex- posure of rats to low doses of procarbazine (25-50 mg/kg) caused DNA damage measured by al- kaline elution in the liver and testis, with the liver being somewhat more sensitive. The in vivo geno- toxicity of procarbazine was increased by a factor of 2-3 in both organs by PCB treatment of the rats. N-Isopropyl-et-(2-methylhydrazino)-p-( a,a- 2H2)toluamide (d2-procarbazine), was found to cause significantly less genotoxicity in control rats than either procarbazine itself, or N-isopropyl-a-

(2-( a, a, a- z H 3 )me thy lhydraz ino ) -p - to luamide (d 3-procarbazine). These results indicate that ben- zylic C-H oxidation of procarbazine is an im- portant step in the activation of procarbazine to genotoxic metabolites in uninduced rats.

Paracetamol inhibits replicative DNA synthesis and induces sister-chromatid exchanges by destruction of a tyrosyl radical or ribonucleotide reductase

Hongslo, J.K., C. Bjorge, P. Schwarze, G. Mann a, L. Thelander a and J.A. Holme, Department of Toxicology, National Institute of Public Health, Oslo (Norway) and 1 Department of Physiological Chemistry, University of Umea, Umea (Sweden)

Paracetamol is an extensively used analgesic known to cause acute liver necrosis in man and animals at higher doses. Liver cell tumors have been reported in mice after long-term feeding with doses of paracetamol that were hepatotoxic (Flaks et al. (1983) Carcinogenesis, 4, 363). Genotoxic effects of paracetamol are demonstrated in several test systems (Dybing et al. (1984) Mutation Res., 138, 21-32) and may be involved in this carcino- genic effect. In risk assessment, the mechanism leading to the observed genotoxic effect is of importance. In V79 Chinese hamster cells, we have shown that paracetamol is a potent inhibitor of replicative DNA synthesis (Hongslo et al. (1989) Toxicol. In Vitro, 3) and induces sister-chromatid exchange (SCE) (Hongslo et al. (1988) Mutation Res., 204, 333-341). Hydroxyurea, which is known to inhibit ribonucleotide reductase by destroying a tyrosyl free radical at the active site, had similar effects.

In the present study we have tested the effect of paracetamol on DNA synthesis and SCE in a mouse mammary tumor cell line TA 3 that is re- sistant to hydroxyurea due to overproduction of the subunit of ribonucleotide reductase with the active tyrosyl radical (Eriksson et al. (1984) J. Biol. Chem., 259, 11695-11700). These cells also were found to be resistant to paracetamol-induced inhibition of growth. Flow-cytometry studies showed that paracetamol caused less inhibition of DNA synthesis in the hydroxyurea-resistant cells. Furthermore, no increase in SCE was detected at