Low dose effects of chemicals as assessed by the flow cytometric in vivo micronucleus assay

Using flow cytometric automation of the mouse in vivo, micronucleus assay increases the sensitivity of the test. This is achieved through a very large increase in the number of cells scored, by a factor of 100×, which in turn greatly reduces the sampling error. With this method, dose–response relationships of in vivo micronucleus induction for four model agents mitomycin C (MMC), diepoxybutane (DEB), cyclophosphamide (CPA), and colchicine (COL) were studied at low dose levels. For the three clastogens MMC, DEB and CPA, linear dose–response relationships were found over the dose ranges studied, even in the very low dose region (defined as the dose region where the frequency of micronucleated erythrocytes is less than twice the baseline frequency). This is consistent with the view that no threshold should exist for genotoxic agents which target DNA. For COL a dose range was found, in which the frequency of micronucleated erythrocytes did not increase with dose, possibly indicating an in vivo threshold. The flow cytometric in vivo micronucleus assay represents one possibility for in vivo low dose–response studies.