5-Fluorouracil incorporation into human breast carcinoma RNA correlates with cytotoxicity.

We demonstrate a highly significant relationship (p less than 0.0001) between the incorporation of 5-flourouracil in total cellular RNA and loss of clonogenic survival of the human MCF-7 breast carcinoma cell line. The extent of 5-fluorouracil incorporation in RNA is concentration- and time-dependent. Identical results are obtained in experiments employing thymidine to bypass the block of thymidylate synthetase and reverse inhibition of DNA synthesis. These studies suggest that the incorporation of 5-fluorouracil in RNA is the major mechanism of cytotoxic action in this human cell line.