Mammalian Cell Mutation Test - Mouse Lymphoma Assay
The system recommended by ICH and others emplys L5178Y cells and the TK (thymidine kinase) locus. Mammalian cells generally have two copies of each gene, but this mouse lymphoma cell line has only one functional copy of the gene, and is referred to as TK+/TK-.
Thymidine kinase is not an essential enzyme. It is part of a system that recycles free thymidine that is eventually incorporated into DNA. Trifluorothymidine (TFT) is a toxic analogue of thymidine and intereferes with DNA metabolism, killing the cell. However, if the functional copy of the TK gene is lost through mutation, the TFT is not metabolized and is no longer toxic.
L5178Y cell cultures are treated with various concentrations of the test compund in both the presence and the absence of S9 mix for 3 hours. Duplicate cultures are used at each experimental point. At least four dose levels of the compound are assessed with the highest dose level being either that expected to reduced survival by 80-90% based on a scaled down preliminary test or, for non-toxic compounds, the sandard limit of 5 mg/ml or to the limit of solubility. The terated cells are washed free of compound and the cells are grown for an expression period of 48 hours hours to allow fixation of mutations.