The in vivo Mammalian Micronucleus Test (MMT) test is used for the detection of damage induced by the test substance to the chromosomes or the mitotic apparatus of erythroblasts. When a bone marrow erythroblast develops into a polychromatic erythrocyte, the main nucleus is extruded; any micronucleus that has been formed may remain behind in the otherwise anucleated cytoplasm. An increase in the frequency of micronucleated polychromatic erythrocytes in treated animals is an indication of genotoxicity. As an in vivo test, it allows consideration of factors of in vivo metabolism, pharmacokinetics and DNA-repair processes. The objective of this assay is to evaluate the test article and its metabolites for their potential to cause chromosome aberrations in vivo in rodents. The study design follows the current OECD 474  guideline “Mammalian Erythrocyte Micronucleus Test” (2014) and can be performed in accordance with Good Laboratory Practices.

This test may also be referred to as the Mouse Micronucleus Test or the Rat Micronucleus Test (depending on the species being used), or more generally as the in vivo Micronucleus test. It is typically performed in a mouse (Nucro-Technics will typically use SD-1 mice) although different species may be used depending on the test substance in question. This is one of the tests which can be used as part of the ICH S2(R1) Guideline on Genotoxicity Testing and Data Interpretation for Pharmaceuticals Intended for Human Use / FDA Battery.

A preliminary toxicity test may be performed to help design three analyzable concentrations. In the main experiment, usually five female and five male healthy animals are randomly assigned to each control and treatment groups. Positive and negative control groups will be treated with Cyclophosphamide and the vehicle, respectively. Animals are exposed to the test substance by an appropriate route. The animals are sacrificed 24 and 48 hours after treatment, the bone marrow extracted, and preparations made and stained. Slides are prepared for analysis and coded. At least 4000 polychromatic cells from each animal are examined.

A positive response shows a concentration-related response and a statistically significant increase in the incidence of micronuclei at one or more concentrations over that of the solvent controls. Positive results indicate that the test article induces chromosome breakage and/or mitotic spindle disruption. A negative response presents no statistically significant increase in the percentage of cells with micronuclei. Negative results indicate that, under the test conditions, the test article does not induce chromosome breakage or mitotic spindle disruption.

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