The genetic toxicity of 3,3 ',4,4 '-tetrachloroazobenzene and 3,3 ',4,4 '-tetrachloroazoxybenzene: discordance between acute mouse bone marrow and subchronic mouse peripheral blood micronucleus test results

3,3',4,4'-Tetrachloroazobenzene (TCAB) and 3,3',4,4'-tetrachloroazoxybenzen e (TCAOB) are dioxin-like chemicals that were investigated for toxicity in 13-week gavage studies in male and female B6C3F(1) mice and F344N rats by t he National Toxicology Program. As part of the comprehensive toxicological investigation of these chemicals, peripheral blood smears from mice treated 5 days per week for 13 weeks with 0.1-30 mg/kg/day TCAB or TCAOB were anal yzed for the frequency of micronucleated (MN) normochromatic erythrocytes ( NCE). Both chemicals produced significant increases in MN-NCE in male and f emale mice. In contrast to these positive results in subchronic exposure st udies, no significant increases were seen in acute bone marrow MN tests in male mice administered three daily injections of 50-200 mg/kg/day TCAB and TCAOB. The results with TCAB and TCAOB suggest that the routine integration of MN rests with subchronic toxicity studies may allow detection of mutage nic activity for some chemicals that fail to elicit responses in short-term , high dose tests. In addition, the integration of mutagenicity tests into general toxicity tests reduces the use of laboratory animals and the cost o f the testing. (C) 2000 Published by Elsevier Science B.V.