Enhancement of hormonal synchronization and 5-fluorouracil cytotoxicity in breast cancer cells by low concentration of thymidine.

A method is described that applies a low concentration of thymidine to achieve maximal hormonal synchronization in cultures of an established human breast cancer cell line, MCF-7. The method consists of sequential treatments with tamoxifen, estradiol, and thymidine. The increase (3.8 fold) in the S-phase fraction achieved by this method is higher than that (2.6 fold) achieved by a solely published method using tamoxifen block plus estradiol rescue. The MCF-7 cells, which have an enhanced labeling index (S-phase fraction) after being treated with the synchronizing method, have a markedly increased sensitivity to an S-phase-specific cytotoxic agent, 5-fluorouracil. This method may have potentially important implications for clinical chemotherapeutic strategy in the treatment of breast cancer.