Inhibition of urethan induced lung tumour growth in strain A mice by 5-methyl cytidine.

In two in vivo experiments, low doses of 5-methyl cytidine significantly suppressed the growth of urethan-induced lung tumours in strain A mice while higher doses were less effective. This inverse dose-response relationship mimics the previously reported effect of 5-methyl cytidine on mammary tumour growth in C3H mice. The finding that 5-methyl cytidine inhibits lung tumour growth as well as mammary tumour growth suggests this nucleoside may be an effective inhibitor of solid tumour growth in general. A study of the effect of 5-methyl cytidine on mouse lung tumour cell viability in vitro suggests that the direct cytotoxic effect of 5-methyl cytidine is dependent on the cell cycle. Since cell-cycle dependent agents do not inhibit lung tumour growth in vivo, 5-methyl cytidine probably inhibits mouse lung tumour growth indirectly by some effect on the host rather than by a direct cytotoxic effect on the lung tumour.