Rate of DNA synthesis in exponentially growing cell lines in the presence and absence of antimetabolites.

Experimental tumor cell lines were used to show that in the presence of 5-fluorodeoxyuridine (FdUrd), the rate of DNA synthesis remains unaltered as long as a saturating concentration of thymidine is present. This unimpeded rate of DNA synthesis in combination with FdUrd-blocked de novo thymidylate synthesis makes it possible to accurately measure the total rate of increase of DNA using tritiated thymidine of known specific activity. The observed amount of incorporated tritiated thymidine is in excellent agreement with the calculated theoretical maximal incorporation in cultures with exponentially increasing DNA and cell number.