Capecitabine (5-Fluorouracil, 5-FU, Xeloda): Mechanism of Action and DPD Deficiency

5-Fluouracil (5-FU) and its prodrug capecitabine (Brand name: Xeloda) are two of the most commonly used drugs for anti-cancer chemotherapy in the United States. They belong to a class of drugs called fluropyrimidines which are designed to kill cancer cells when they try to divide. A prodrug is a drug that is administered in an inactive form, and then becomes activated by enzymes in the body (usually the liver). At present, 5-FU and capecitabine are indicated primarily for the treatment of colon cancer, although they can be used in certain cases of breast cancer, head and neck cancer, gastrointestinal cancer, and ovarian cancer, too. They have been shown to be effective against these cancers, although there is a significant risk of serious drug induced toxicity. Most importantly, advances in personalized medicine have discovered that this toxicity can be avoided in many cases using a simple laboratory test.

Side effects of 5-FU can be severe and include:

• Diarrhea
• Nausea
• Vomiting
• Leukopenia
• Hand-foot syndrome
• Anemia
• Stomatitis
• Hair loss

Cancer cells are rapidly dividing cells that grow at an accelerated pace and do not respond to normal control mechanisms that the body uses to restrain cell growth. Some forms of anti-cancer therapy aim to exploit this characteristic of cancer cells by targeting cells of the body that are undergoing division while sparing quiescent cells that are not rapidly dividing. Capecitabine and 5-FU are two chemotherapeutic drugs that act by this mechanism.

Capecitabine can be thought of as an oral drug that the body needs to convert to form 5-FU. The tumor selectivity of capecitabine is based on the relative abundance of enzymes between different tissues of the body. Capecitabine passes unchanged through the digestive system, where it enters the circulation and travels to the liver, which has a very high carboxylesterase activity compared to other body tissues. Carboxylesterase in the liver catalyzes the conversion of capecitabine to 5’-deoxy-5-fluorcytidine (5’-DFCR). 5’DFCR is then converted to 5’-deoxyfluoruridine via cytidine deaminase, and then to 5-Fluouracil via thymidine phosphorylase. Thymidine phosphorylase is expressed at significantly higher levels in tumor tissue than in normal tissue in multiple forms of cancer. This variability in enzyme activity allows capecitabine to be converted to 5-FU at a much higher rate in tumor cells than non-tumor cells. So, capecitabine treatment is able to deliver 5-FU to tumors while reducing the amount of 5-FU directed toward other tissues. After conversion of capecitabine to 5-FU, the antimetabolite effects are predominantly due to the inhibition of thymidylate synthase.

5-FU and capecitabine target dividing cells to stop their division and induce cell death. These drugs are taken up by dividing cells and converted into toxic catabolites by cellular enzymes. The catabolites then inhibit several pathways that cancers cells require for growth. The most important and well known pathway inhibited by 5-FU is the pathway regulating thymidine synthesis. Thymidine is one of the four nucleic acids contained in your DNA, and cancer cells need to synthesize quite a bit of it to keep growing. 5-FU catabolites compete with cellular machinery responsible for producing thymidine, and cells undergo apoptosis (programmed cell death).

A large number of patients who receive capecitabine or 5-FU treatment will experience severe side-effects. In fact, it is estimated that between 10% and 30% of patients will have serious health complications after receiving 5-FU based chemotherapy, and many of these cases are a result of dihydropyrimidine dehydrogenase (DPD) deficiency. DPD catalyzes the first and rate limiting step in the pathway of 5-FU catabolism. Approximately 80% of an administered 5-FU dose will be eliminated via the pathway of this enzyme. The other 20% of administered 5-FU will be converted to toxic metabolites within the cancer cell. These metabolites inhibit critical pathways within the cancer cell to induce cell death.

Retrospective studies have shown that 5-FU related side-effects occur more frequently in patients with low DPD activity, as measured by DPD activity in peripheral blood mononuclear cells (PBMC), compared to patients exhibiting normal DPD activity. PBMC-DPD activity has been correlated to liver DPD activity, and it is currently the main method for assessing enzyme activity, as well as the benchmark for judging the reliability of newer assays. By this method, one study found that 30-50% of patients who suffered severe 5-FU related toxicities were DPD deficient.

The issue of addressing the individual risk of 5-FU related toxicity and adverse drug events is important because of the nature of the disease being treated. Colorectal carcinoma is one of the leading causes of death in industrialized nations. Therefore, it is critical that patients with DPYD mutations and DPD deficiency are identified so that toxicity is prevented, but patients with DPYD mutations and normal DPD levels must still be permitted to get 5-FU treatment so that they receive the best available anti-cancer chemotherapy.

Investigators have examined patients suffering from 5-FU related toxicities for underlying causes of DPD deficiencies. Genetic analyses have found the gene encoding the DPD protein, DPYD, to be highly mutated, with more than 40 gene variants identified to date. Individuals with specific mutations of this gene are likely, but not guaranteed, to have DPD deficiency. Because every individual has two copies of every gene, a single mutation in DPYD does not necessarily mean that he or she will be DPD deficient.

There are several clinical tests now available for the identification of DPD deficiency in patients scheduled to receive 5-FU chemotherapy. The most common test is a type of radioenzymatic assay which measures DPD activity in peripheral blood mononuclear cells, as previously mentioned. This assay has been used for much of the DPD and 5-FU related research. There are labs available that can perform this test and identify DPD deficiency with a simple blood test.

• Molecular Diagnostic Labs
• ITT Labs
• Genelex (only genotyping, not an enzyme assay)

There is also a more rapid test being developed to test for DPD deficiency: the uracil breath test. In this assay, Carbon-13 labeled uracil is administered orally to patients and the rate of Carbon-13-CO2 exhalation is tracked over time as an indication of DPD activity. Although this is still in a testing phase, it represents a promising advance in our ability to rapidly identify individuals who are at risk for 5-FU related drug toxicities, as well as those individuals who possess a DPYD mutation but are still able to tolerate the drug.

Capecitabine and 5-FU represent the best available chemotherapeutic drugs for certain carcinomas, including forms of colon cancer. However, deficiency in the rate-limiting catabolic enzyme, DPD, can lead to serious, and sometimes even lethal, adverse drug events. Tests are now available to screen for DPD deficiency so that 5-FU and capecitabine can be administered, or not, in a personalized and safe manner.

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