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A mammalian cell line designed to test the mutagenic activity of anti‐herpes nucleosides

The herpes simplex virus (HSV) thymidine kinase (tk) gene was transfected into Chinese hamster ovary (CHO) 51–11 gly−cells to test its effect on the cytotoxic and mutagenic activity of anti‐herpetic nucleoside analogues. Insertion of the viral tk was verified by Southern blot analysis, by sensitivity to acyclovir, and by elevated in vitro thymidine kinase (TK) activity. TK activity was increased by superinfection with a tk− virus and inhibited by antibody to viral TK. Acyclovir (ACV) was somewhat more cytotoxic in the 51‐D3 cell line that expresses the viral TK than in the 51–11 parent line. Growth in ACV did not increase over background mutations at the hprt locus. FIAC (2′‐fluoro‐5‐iodio‐aracytosine) was slightly cytotoxic to the parent 51–11 line and the tk‐containing clone 51‐D3. FMAU (2′‐fluoro‐5‐methyl‐arauracil) had pronounced cytotoxicity in both cell lines: the 50% survival points were 1.0 μM for 51–11 cells and 0.2 μM for 51‐D3. The clone 51‐D3 was more sensitive than 51–11 to low concentrations of FIAU (2′‐fluoro‐5‐iodo‐arauracil), and when treated with FIAU 51‐D3 had a mutation frequency to glycine independence 5 times greater than that of 51–11 cells. With both cell lines the mutation frequency at the hprt locus did not increase after growth in the presence of FIAC or FIAU. A 7‐fold increase in mutation frequency at the hprt locus was detected after 51‐D3 cells were grown with iododeoxyuridine. Trifluorothymidine was more toxic to 51‐D3 than to 51–11 cells and increased the mutation frequency 2‐ fold. Cytosine‐β‐D‐arabinofuranoside showed no differential cytotoxicity on the two cell lines and did not increase the mutation frequency at the hprt locus.