Sequence-dependent antagonism between fluorouracil and paclitaxel in humanbreast cancer cells

The effects of 24-hr exposures to 5-fluorouracil (FUra) and paclitaxel in V arious sequences were studied in MCF-7 breast cancer cells to determine an optimal schedule for possible clinical use. In clonogenic assays, pre-expos ure to FUra followed by paclitaxel resulted in marked antagonism, while seq uential paclitaxel followed by FUra was optimal. Concurrent or pre exposure to paclitaxel did not affect [H-3]FUra metabolism, [H-3]FUra-RNA incorpora tion, or the extent of FUra-mediated thymidylate synthase inhibition. Pacli taxel led to G(2)/M phase accumulation that persisted for up to 24 hr after drug exposure, while a 24-hr FUra exposure produced S-phase accumulation. FUra pre-exposure diminished paclitaxel-associated G(2)/M phase block, wher eas subsequent exposure to FUra after paclitaxel did not. FUra exposure res ulted in transient induction of p53 and p21, which returned to basal levels 24 hr after drug removal. p53 and p21 protein content also increased marke dly during paclitaxel exposure, accompanied by phosphorylation of Bcl-2. Do uble-stranded DNA fragmentation(similar to 50 kb) was seen at 48 hr when ce lls were exposed to paclitaxel for an initial 24-hr period. Paclitaxel-asso ciated DNA fragmentation was not prevented by concurrent or subsequent expo sure to FUra. Thus, paclitaxel-mediated G(2)/M phase arrest appeared to be a crucial step in induction of DNA fragmentation. Since an initial 24-hr pa clitaxel exposure did not interfere with subsequent FUra metabolism or thym idylate synthase inhibition, and delayed exposure to FUra did not impede ei ther paclitaxel-mediated induction of mitotic blockade or DNA fragmentation , the sequence of paclitaxel followed by FUra is recommended for clinical t rials. (C) 1999 Elsevier Science Inc.