Partial Reversal of the Thymineless State in vivo

METHOTREXATE (amethopterin) blocks the activity of the enzyme dihydrofolic reductase and thereby interferes with a number of metabolic reactions involving the transfer of carbon atoms, including the production of thymidylate, de novo purine biosynthesis, and inter-conversions of several amino-acids. Partial reversal of toxicity induced by antifolics such as methotrexate by the addition of purine and thymine sources has been reported for amphibian 1 and avian 2,3 embryos, but similar experiments with rat embryos were unsuccessful 4 . The deficiencies induced by methotrexate can be completely reversed in mammalian tissue culture systems, however, by the inclusion in the nutrient fluid of a purine source, certain amino-acids, and a thymine source, usually thymidine 5–8 . If thymidine is omitted from the medium, a deficiency develops, sometimes called the thymineless state, in which the cells begin to die after approximately one generation time. It seemed desirable to attempt to reproduce this condition in vivo , in order better to define the mode of action of methotrexate, as well as to provide information which might be used to control cell growth in vivo .