POTENT CLASTOGENICITY OF THE HUMAN CARCINOGEN ETOPOSIDE TO THE MOUSE BONE-MARROW AND MOUSE LYMPHOMA L5178Y CELLS - COMPARISON TO SALMONELLARESPONSES

The suspect human carcinogen, etoposide, is known to be genotoxic, pro ducing both gene and chromosomal mutations, probably by virtue of its ability to inhibit topoisomerase II activity. The present paper descri bes assays conducted using the Salmonella assay, the mouse lymphoma tk (+/-) assay (gene and chromosomal mutation analysis and molecular anal ysis of tk(-/-) mutants) and the mouse bone marrow micronucleus assay. Nonreproducible, weak, dose-related increases in mutation frequency i n strain TA98 (but not TA1538 or TA1537) of Salmonella typhimurium wer e observed. Etoposide was highly mutagenic at the heterozygous thymidi ne kinase (tk(+/-)) locus of L5178Y mouse lymphoma cells at concentrat ions below 0.1 mu g/ml. Mostly small colony mutants were induced, cons istent with the potent clastogenicity also observed. Molecular analysi s of mutants indicated that 83% and 92% of large and small colony muta nts, respectively, had lost the entire target gene sequence. Chromosom ally aberrant L5178Y cells were similar to 2 to 600-fold more prevalen t than small tk(-/-) mutant colonies. This suggests that the viable ta rget for etoposide-mediated clastogenesis in the selective assay is si milar to one-fifth of chromosome 11b, itself being similar to one-fort ieth of the mouse genome. An unusually potent response was observed fo r etoposide in the mouse bone marrow micronucleus assay (63.1 +/- 18 M PE/1,000 PE 24 hours after an oral dose of 1 mg/kg). The minimum detec table dose level in the assay was between 0.01 and 0.1 mg/kg. At dose levels between 1 and 15 mg/kg, an inverse dose response was observed. This reduction in assay response was not due to the small concommitant decrease in the incidence of polychromatic erythrocytes, a conclusion based on studies with N-methyl-N-nitrosourea. Animals sampled 48 hour s after dosing with etoposide (10 mg/kg) had no polychromatic erythroc ytes in the bone marrow. These observations for the micronucleus assay await explanation. The chemical structure of etoposide is displayed a nd discussed within the context of such strong mutagenic activity bein g associated with a nonelectrophilic agent. (C) 1994 Wiley-Liss, Inc.