Fluoropyrimidines: a critical evaluation.

After nearly four decades of clinical experience with the fluoropyrimidines, 5-fluorouracil (5-FU) remains an integral part of chemotherapy for colorectal cancer. A range of 5-FU treatment regimens with or without biochemical modulation are currently used and toxicity appears to be schedule dependent. The use of oral 5-FU was abandoned because of erratic absorption caused by varying levels of dihydropyrimidine dehydrogenase (DPD) found in the gastrointestinal tract. This effect may be overcome by administering agents that are converted to 5-FU or by inhibiting or inactivating DPD. Xeloda((R)) (capecitabine) was designed as an orally administered, selectively tumoractivated(TM) cytotoxic agent which achieves higher levels of 5-FU in the primary tumor than in plasma or other tissues. The United States Food and Drug Administration (FDA) has approved Xeloda for use in patients with metastatic breast cancer resistant to paclitaxel and in whom further anthracycline therapy is contraindicated. Xeloda is also registered in Canada, Switzerland, Thailand and Argentina. In phase II clinical trials in colorectal cancer, Xeloda produced response rates of 21-24% and median time to disease progression of 127-230 days. Other oral agents in development for the treatment of colorectal cancer include tegafur/uracil plus oral leucovorin, S-1 and eniluracil plus oral 5-FU.