Comparison of the mutant frequencies and mutation spectra of three non-genotoxic carcinogens, oxazepam, phenobarbital, and Wyeth 14,643, at the lambda cII locus in Big Blue (R) transgenic mice

Oxazepam (OX), a widely used benzodiazepine anxiolytic, phenobarbital (PHE) , a drug used for convulsive disorders, and Wyeth 14,643 (WY; [4-chloro-6-( 2,3-xylidino)-2-pyrimidinylthio]acetic acid), a hypolipidemic agent, are al l hepatocarcinogenic in B6C3F1 mice. They have been classified as "non-geno toxic" carcinogens since they are non-DNA reactive in in vitro assays and a re either negative or weakly positive in Salmonella typhimurium (Ames assay ). Male B6C3F1 Big Blue((R)) transgenic mice were fed 2500 ppm of OX or PHE or 500 ppm of WY in their diet, while a control group of mice received die t alone for 180 days. The mutant frequency (MF) of cII in the control mice, after correction for clonality, was 6.2 +/- 2.8 X 10(-5). The MF values fo r mice fed OX, PHE, and WY were 10.0 +/- 3.6 X 10(-5) (P < 0.05), 7.9 +/- 1 .3 X 10(-5) (P = 0.1) and 17.4 +/- 4.2 X 10(-5) (P < 0.01), respectively. T he mutation spectrum (MS) at cII from the PBE-fed mice was significantly di fferent (P < 0.05) from that of the control mice even though the MF was not , whereas the MS spectra of mice fed OX (P = 0.4) and WY (P = 0.7) were not significantly different. The PHE-derived spectrum differed from the sponta neous spectrum in the lower occurrence of G:C >C:G transversions (17 vs 1.6 %) and the higher incidence of A:T >T:A transversions (3.4 vs 9.5%). Prior to correction for clonal expansion, each treated group exhibited a high inc idence of frameshift mutations at the homopolymeric run of guanines at bp 1 79-184 (OX 21%, PHE 21%, WY 16% of the total mutations); this was not the c ase with the control group (6%). Even after clonal correction, more than 10 % of the mutations were frameshifts in the treated mice, while 5% were fram eshifts in the control mice. Despite this hypersensitive region of the gene , our findings suggest that the cII locus is less sensitive than the lac1 l ocus to mutation induction by non-DNA reactive carcinogens. (C) 2001 Elsevi er Science Inc. All rights reserved.