Partial Reversal of the Thymineless State in vivo

Abstract

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 amphibian¹ and avian^2,3 embryos, but similar experiments with rat embryos were unsuccessful⁴. 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.