First introduced in 1957, the fluoropyrimidine fluorouracil(Drug information on fluorouracil) (5-FU) remains a major chemotherapeutic agent for the treatment of gastrointestinal, breast, and head and neck cancers. Although response rates of advanced cancers to 5-FU remain in the 15% to 20% range, the complexity and variability of the drugs metabolism offer an opportunity to optimize fluoropyrimidine chemotherapy. This can be done by selecting tumors most likely to respond to the drug, altering drug scheduling, inhibiting 5-FU catabolism, modifying pharmacokinetics by administering 5-FU prodrugs, or screening individuals for genetic polymorphisms that may increase the risks of 5-FU toxicity. Capecitabine(Drug information on capecitabine) (Xeloda) is a new, orally available, tumor-selective fluoropyrimidine carbamate prodrug activated by a three-enzyme process to provide high levels of 5-FU in tumor cells.[1,2]
This article will briefly review the metabolism of 5-FU, the determinants of sensitivity to fluoropyrimidines, and strategies to optimize fluoropyrimidine therapy. It will then describe the preclinical and clinical pharmacology of capecitabine.
As shown in Figure 1, 5-FU undergoes a series of anabolic and catabolic reactions. The cytotoxicity of 5-FU appears to result from the inhibitory activity of 5-fluoro-2´-deoxyuridine-5´-monophosphate (FdUMP) on thymidylate synthase and the incorporation of fluoropyrimidines into ribonucleic acid (RNA) and deoxyribonucleic acid (DNA).[3,4] Although the incorporation of fluorouridine into RNA can produce toxicity as a result of major effects on RNA processing and function, the effects of 5-fluoro-2´-deoxyuridine-5´-triphosphate (FdUTP) incorporation into DNA are less clear. Presumably, its presence activates the excision repair process. However, since DNA repair requires the presence of thymidine triphosphate, a substrate depleted in 5-FUtreated cells, DNA strand breakage is likely to occur.
Ribonucleotide SynthesisFluorouracil follows the same pathways as uracil in pyrimidine biosynthesis. Uridine phosphorylase and uridine kinase sequentially convert 5-FU to 5-fluorouridine and then to 5-fluorouridine-5´-monophosphate (FUMP). Alternatively, FUMP can be formed by a direct reaction of 5-FU with 5-phosphoribosyl-1-pyrophosphate (PRPP) catalyzed by the enzyme orotate phosphoribosyl transferase. Cellular kinases subsequently phosphorylate FUMP to its diphosphate (FUDP) and triphosphate (FUTP) forms, with FUTP subsequently incorporated into several forms of RNA by RNA polymerase.
Deoxyribonucleotide SynthesisThe formation of FdUMP is critical to the antineoplastic activity of 5-FU. FdUMP can be formed by one of two pathways: (1) conversion of FUDP to 5-fluoro-2´-deoxyuridine-5´-diphosphate (FdUDP) by enzymatic activity of ribonucleotide reductase, or (2) direct conversion of 5-FU to 5-fluorouracil deoxyriboside (5-FUdR) by thymidine phosphorylase. Both are subsequently phosphorylated or dephosphorylated to FdUMP by thymidine kinase.
FdUMP appears to be responsible for much of the cytotoxicity of 5-FU. As a potent inhibitor of thymidylate synthase, it prevents the formation of thymidine monophosphate and thereby inhibits DNA synthesis.[3,4] Inhibition of thymidylate synthase by FdUMP depletes cellular stores of thymidine triphosphate, an important precursor of DNA. This results when FdUMP and 5,10-methylenetetrahydrofolate form a covalent ternary complex with thymidylate synthase. The stability of the fluorine-carbon bond blocks the transfer of the methylene group and the two hydrogen atoms from folate to dUMP and produces sustained inhibition of the enzymatic activity of thymidylate synthase.
The amount of 5-FU available for antineoplastic activity is directly regulated by its catabolism.[5,6] The initial and rate-limiting step in 5-FU catabolism is the reduction of the pyrimidine ring to 5-fluoroureidopropionic acid (FUPA) by hepatic dihydropyrimidine dehydrogenase (DPD). This enzyme is also found in the intestinal mucosa and other tissues. FUPA is subsequently converted to fluoro-beta-alanine by beta-alanine synthase. Following administration of an intravenous dose of 5-FU, the drug is rapidly catabolized by the liver, with the majority (60% to 90%) excreted in the urine as inactive metabolites. Biliary excretion accounts for 2% to 3% of the total. Deficiency of DPD has been estimated to occur in 2% to 5% of the population and has the potential to dramatically increase the risk of 5-FU toxicity due to impaired clearance of the drug.[7,8]
A number of clinical and metabolic factors determine the efficacy of the fluoropyrimidines. These include the sensitivity of the tumor, drug scheduling, cellular factors (eg, activity of the anabolic and catabolic enzymes, the presence of a pool of nucleotide precursors, folate and other cofactors, intact apoptotic pathways), and the particular formulation of 5-FU.
Pivotal Role of Thymidine Phosphorylase
Thymidine phosphorylase is a key enzyme in DNA synthesis and is responsible for the reversible conversion of thymidine to thymine. Thymidine phosphorylase is also a potent angiogenic growth factor and is identical to platelet-derived endothelial cell growth factor. Since thymidine phosphorylase can directly activate 5-FU by converting it to 5-FUdR, levels of thymidine phosphorylase are critical in the efficacy of fluoropyrimidines. Levels of thymidine phosphorylase are higher in a wide variety of solid tumors than in the adjacent nonneoplastic tissue. For these reasons, thymidine phosphorylase represents an attractive target for fluoropyrimidine therapy. Since the last step of capecitabine activation is by intratumoral thymidine phosphorylase, this selectivity can increase tumor cytotoxicity and decrease systemic exposure to the effects of 5-FU.
Intrinsic Mechanisms of Resistance
Some tumors are inherently more sensitive than others to the metabolic actions of 5-FU. For example, although gastrointestinal, breast, and head and neck cancers are relatively sensitive to 5-FU, the drug has little or no activity against malignant melanomas. In addition, resistance may develop during the course of disease. Mechanisms of resistance include insufficient delivery of drug to the target tissue, decreased anabolism of 5-FU to the nucleotide form, increased clearance of 5-FU nucleotides, alterations in the kinetics of thymidylate synthase, depletion of folate cofactors, and/or the level of the competing substrate, deoxyuridine monophosphate (dUMP).
Strategies to Optimize 5-FU Therapy
Knowledge of 5-FU pharmacokinetics, cellular factors that contribute to resistance, and 5-FU metabolism provides an opportunity to modify drug scheduling, as well as to rationally design drugs that can increase the intrinsic efficacy of 5-FU.
PharmacokineticsFluorouracil shows significant and unpredictable differences in absorption following administration of an oral dose. Consequently, the drug is not routinely administered by the oral route. However, it also shows significant variations in plasma concentrationtime profiles after both intravenous bolus and infusion administration. At higher doses, the nonlinear pharmacokinetics make it difficult to predict plasma levels and toxicity.
At least some of this pharmacokinetic variability may be due to individual differences in DPD enzymatic activity. Coadministration of 5-FU with DPD inhibitors such as eniluracil has the potential to minimize the variability of 5-FU pharmacokinetics between patients, decrease the circadian variation in 5-FU levels during continuous infusion therapy, and improve the therapeutic index of the drug.[3,12]
SchedulingThe schedule of administration can have a significant effect on the efficacy and toxicity of 5-FU. Fluorouracil has been administered by oral, intravenous bolus, and continuous infusion techniques. Continuous infusion schedules are associated with a lower incidence of bone marrow and gastrointestinal toxicity. Although the therapeutic index of continuous infusion 5-FU vs bolus administration appears to be improved in patients with colorectal cancer, no improvement has been observed in survival rates.
Scheduling may also influence the method of cell death following 5-FU therapy. Preclinical studies suggest that bolus administration may result in cell death that is more likely mediated by the disruption of RNA metabolism than by a thymidylate synthaserelated mechanism.
Cellular FactorsA variety of cellular factors influence the efficacy of 5-FU therapy (Table 1). The level and activity of thymidylate synthase, as well as intracellular folate pools, are critical factors in 5-FU activity. For example, in a series of patients with gastric cancer treated with 5-FU, the level of thymidylate synthase in the primary tumor was related to tumor response and survival. In a group of patients with advanced colorectal cancer, intratumoral thymidylate synthase expression detected by immunohistochemistry was inversely correlated with tumor response to 5-FU chemotherapy (P = .0001).
Intratumoral folate levels are also an important variable in determining response to 5-FU therapy. In a group of patients with head and neck cancers, all patients who responded to 5-FU therapy had intratumoral reduced folate levels above the median for the entire population. Overexpression of DPD in tumor cells also appears to be an important factor in resistance to 5-FU in some patients. Indeed, the combination of high thymidylate synthase and high DPD levels in tumor cells appears to account for most cases of clinical resistance to 5-FU.
As discussed below, the effectiveness of capecitabine is critically dependent upon the intratumoral level of thymidine phosphorylase, the enzyme necessary for its intracellular activation.
5-FU Prodrugs5-FU prodrugs offer the opportunity to alter the scheduling of the drug, improve pharmacokinetics, increase tumor specificity, and potentially improve the therapeutic index. Available and investigational 5-FU prodrugs include capecitabine, tegafur(Drug information on tegafur), UFT (a fixed 1:4 ratio of tegafur and uracil), S-1, and BOF-A2 (see Table 2).[3,19,20]