5-Fluorouracil in colorectal cancer: rationale and clinical results of frequently used schedules.

Colorectal cancer is one of the most frequent cancers in the western world. Approximately half of the patients will die of their disease because of metastases. The most active cytotoxic agent used to date is 5-fluorouracil (5-FU). However, clinical responses are achieved only in a minority of patients. Based on the current knowledge of the mechanism of action of 5-FU, many attempts have been made to improve the clinical results. These include the use of biochemical modulators and different methods of administration, and these are the subject of this review. Specifically, of five different modulators, i.e. leucovorin, methotrexate, interferon-alpha, N-(phosphonacetyl)-L-aspartate and trimetrexate glucuronate, the biochemical background and the clinical results obtained with these modulators are discussed. In order to get more insight, an overview of the 5-FU metabolism has been given. In addition, the different methods of systemic administration of 5-FU as well as possible mechanisms underlying 5-FU resistance are described.