Cancer is a term used to describe a group of malignancies that all share the common trait of developing when cells in a part of the body begin to grow out of control. Most cancers form as tumors, but can also manifest in the blood and circulate through other tissues where they grow. Cancer malignancies are most commonly treated with a combination of surgery, chemotherapy, and/or radiation therapy. The type of treatment used to treat a specific cancer depends upon several factors including the type of cancer malignancy and the stage during which it was diagnosed.
5-FU is one of the more commonly used cytotoxic agents that are used for the treatment of Breast and Colorectal cancer. This chemotherapeutic agent has the formula:

This compound has been associated with debilitating side effects such as bone marrow density loss, mucositis, nausea and vomiting. By monitoring the levels of 5-FU in the body and adjusting the dose these side effects can be better controlled and limited in patients.
At the same time, there is often a highly variable relationship between the dose of 5-FU and the resulting serum drug concentration that affects therapeutic effect. The degree of intra- and inter-individual pharmacokinetic variability of 5-FU can be as high as 10-fold (Diasio et. al. J. Clin. Invest. 81: pp 47–51, 1988, Wei et. al. J. Clin. Invest. 98: pp 610–615, 1996) and is impacted by many factors, including:                Organ function        Genetic regulation        Disease state        Age        Drug-drug interaction        Time of drug ingestion,        Mode of drug administration, and        Technique-related administration.        
As a result of this variability, equal doses of the same drug in different individuals can result in dramatically different clinical outcomes (Hon et. al. Clinical Chemistry 44, pp 388–400, 1998). The effectiveness of the same 5-FU dosage varies significantly based upon individual drug clearance and the ultimate serum drug concentration in the patient. Therapeutic drug management would provide the clinician with insight on patient variation in both oral and intravenous drug administrations. With therapeutic drug management, drug dosages could be individualized to the patient, and the chances of effectively treating the cancer without the unwanted side effects would be much higher (Nieto, Current Drug Metabolism 2: pp 53–66, 2001).
In addition, therapeutic drug management of 5-FU would serve as an excellent tool to ensure compliance in administering chemotherapy with the actual prescribed dosage and achievement of the effective serum concentration levels. It has been found that variability in serum concentration is not only due to physiological factors, but can also result from variation in administration technique and ability of the body to absorb 5-FU. As a chemotherapeutic agent, 5-FU can be administered in its pro-drug form as tegafur which has the structure:

Tegafur, when administered to a patient, is generally absorbed and metabolized into 5-FU by the patient at different rates. Therefore, in monitoring the level of 5-FU in patients by means of an immunoassay, it is important that the immunoassay be able to distinguish between tegafur, the inactive substance, and 5-FU, the active substance, into which tegafur metabolizes. The problem with antibodies to 5-FU is that they could be cross-reactive with tegafur making these immunoassays not useful.
Routine therapeutic drug management of 5-FU would require the availability of simple automated tests adaptable to general laboratory equipment. Tests that best fit these criteria are immunoassays. Currently there are no immunoassays for 5-FU available and monitoring levels of this drug is conducted by physical methods like high pressure liquid chromatography (HPLC) (Escoriaza et. al. J. of Chromatography B: Biomedical Sciences and applications, 736 (1+2): pp 97–102, 1999). In order to be most effective in monitoring drug levels the antibody should be most specific to 5-FU and display very low cross-reactivity to no cross-reactivity to related pyrimidine bases, particularly tegafur.