The Pharmacogenomics Journal (2005) 5, 226–243 & 2005 Nature Publishing Group All rights reserved 1470-269X/05 $30.00 www.nature.com/tpj REVIEW Genetic factors influencing Pyrimidine- antagonist chemotherapy JG Maring1 ABSTRACT 2 Pyrimidine antagonists, for example, 5-fluorouracil (5-FU), cytarabine (ara-C) HJM Groen and gemcitabine (dFdC), are widely used in chemotherapy regimes for 2 FM Wachters colorectal, breast, head and neck, non-small-cell lung cancer, pancreatic DRA Uges3 cancer and leukaemias. Extensive metabolism is a prerequisite for conversion EGE de Vries4 of these pyrimidine prodrugs into active compounds. Interindividual variation in the activity of metabolising enzymes can affect the extent of 1Department of Pharmacy, Diaconessen Hospital prodrug activation and, as a result, act on the efficacy of chemotherapy Meppel & Bethesda Hospital Hoogeveen, Meppel, treatment. Genetic factors at least partly explain interindividual variation in 2 The Netherlands; Department of Pulmonary antitumour efficacy and toxicity of pyrimidine antagonists. In this review, Diseases, University of Groningen & University Medical Center Groningen, Groningen, The proteins relevant for the efficacy and toxicity of pyrimidine antagonists will Netherlands; 3Department of Pharmacy, be summarised. In addition, the role of germline polymorphisms, tumour- University of Groningen & University Medical specific somatic mutations and protein expression levels in the metabolic Center Groningen, Groningen, The Netherlands; pathways and clinical pharmacology of these drugs are described. Germline 4Department of Medical Oncology, University of Groningen & University Medical Center polymorphisms of uridine monophosphate kinase (UMPK), orotate phos- Groningen, Groningen, The Netherlands phoribosyl transferase (OPRT), thymidylate synthase (TS), dihydropyrimidine dehydrogenase (DPD) and methylene tetrahydrofolate reductase (MTHFR) Correspondence: and gene expression levels of OPRT, UMPK, TS, DPD, uridine phosphorylase, Dr JG Maring, Department of Pharmacy, uridine kinase, thymidine phosphorylase, thymidine kinase, deoxyuridine Diaconessen Hospital Meppel & Bethesda Hospital Hoogeveen, Hoogeveenseweg 38, triphosphate nucleotide hydrolase are discussed in relation to 5-FU efficacy. 7943KA Meppel, The Netherlands. Cytidine deaminase (CDD) and 50-nucleotidase (5NT) gene polymorphisms Tel: þ 31 522 234900 and CDD, 5NT, deoxycytidine kinase and MRP5 gene expression levels and Fax: þ 31 522 243900 their potential relation to dFdC and ara-C cytotoxicity are reviewed. E-mail: [email protected] The Pharmacogenomics Journal (2005) 5, 226–243. doi:10.1038/sj.tpj.6500320 Keywords: pyrimidine antagonists; 5-fluorouracil; gemcitabine; cytarabine; poly- morphisms; mutations; gene expression INTRODUCTION Pyrimidine antagonists belong to the group of antimetabolite anticancer drugs and show structural resemblance with naturally occurring nucleotides (see Figure 1). Their action is accomplished through incorporation as false precursor in DNA or RNA or through inhibition of proteins involved in nucleotide metabolism. The most commonly used pyrimidine antagonists are 5-fluorouracil (5-FU), gemcitabine (dFdC) and cytarabine (ara-C). Newer oral variants of 5-FU are capecitabine and tegafur. 5-FU and its analogues are used, for example, in the treatment of colorectal, breast and head and neck cancer,1–3 whereas dFdC is especially prescribed for non-small-cell lung cancer and pancreatic cancer.4,5 Ara- C is used in the treatment of leukaemia.6 All pyrimidine antagonists are prodrugs Received: 17 December 2004 Revised: 3 May 2005 and intracellular conversion into cytotoxic nucleosides and nucleotides is Accepted: 5 May 2005 needed to produce cytotoxic metabolites. Proteins, involved in pyrimidine Pyrimidine antagonist pharmacogenetics JG Maring et al 227 Figure 1 Overview of the chemical structures of the naturally occurring pyrimidines cytosine and uracil and the synthetic pyrimidine analogues cytarabine, dFdC, capecitabine, 5-Fu and tegafur. metabolism, handle these synthetic drugs, as if they were activity results from inhibition of thymidylate synthase (TS) naturally occurring substrates. The extensive metabolism of by FdUMP, as well as from incorporation of 5-FU metabolites pyrimidine antagonists implies that the intracellular con- into RNA and DNA. Only a small part of the 5-FU dose is centrations of cytotoxic metabolites, thus indirectly the activated via these routes, as in humans 80–90% of the potential antitumour effects, largely depend on intracellular administered dose is degraded to 5,6 dihydrofluorouracil metabolic enzyme activity. (DHFU) by dihydropyrimidine dehydrogenase (DPD). DHFU The aim of this review is to summarise pharmacogenomic can be further degraded to fluoro-b-ureidopropionate data regarding proteins related to the efficacy and toxicity of (FUPA) by dihydropyrimidinase (DPYS) and subsequently pyrimidine antagonists and to identify potential predictive to fluoro-b-alanine (FBAL) by b-ureidopropionase (BUP1). 5- and/or prognostic genetic factors for toxicity and treatment FU degradation occurs in all tissues, including tumour outcome. The impact of germline polymorphisms as well as tissue, but is highest in the liver.9 tumour-specific somatic mutations and protein expression levels on the clinical pharmacology and metabolic pathways of these drugs will be discussed. Capecitabine and tegafur Capecitabine is an oral prodrug of 5-FU. After absorption Metabolic Pathways of Pyrimidine Antagonists from the gut, capecitabine is converted into 50-deoxy-5- 5-Fluorouracil fluorocytidine (50-dFCR) by carboxyl-esterase in the liver, The anabolic conversion of 5-FU into nucleotides is essential and subsequently further converted into 50-deoxy-5-fluoro- for its action. Several enzymes of the pyrimidine metabolic uridine (50-dFUR) by cytidine deaminase (CDD). Finally, 5- pathway are required for the conversion of 5-FU to FU results from bioconversion of 50-dFUR by TP10 (see Figure nucleotides.7 Cytotoxic nucleotides can be formed by three 2). Tegafur is another oral 5-FU prodrug, that is converted routes, as illustrated in Figure 2: (1) conversion of 5-FU to 5- into 5-FU by cytochrome P450 (CYP450) enzymes in the fluoro-uridine-monophosphate (FUMP) by orotate phos- liver. CYP2A6 is the main CYP450 enzyme involved in phoribosyl transferase (OPRT); (2) sequential conversion of tegafur activation, but CYP1A2 and CYP2C8 also play a role 5-FU to FUMP by uridine phosphorylase (UP) and uridine (see Figure 2). Tegafur is combined with uracil in a molar kinase; (3) sequential conversion of 5-FU to 5-fluoro-deoxy- proportion of 1 : 4 available in the commercial preparation uridine-monophosphate (FdUMP) by thymidine phospho- UFTs. Uracil is a competitive substrate for DPD and its role rylase (TP) and thymidine kinase (TK).8 The antitumour in UFTs is to diminish 5-FU catabolism by DPD. www.nature.com/tpj Pyrimidine antagonist pharmacogenetics JG Maring et al 228 Figure 2 Metabolism of 5-FU and 5-FU analogues. For explanation of symbols and metabolic routes, see text. dFdC and ara-C ara-C, which halts polymerase progression at the analogue insertion site. The masked termination of dFdC makes the The metabolic pathways of dFdC and ara-C are almost inserted analogue more resistant to removal from DNA.12 alike11,12 (see Figure 3). Membrane transport of both dFdC Other differences with ara-C are the faster membrane and ara-C is mediated by equilibrative nucleoside transpor- transport velocity of dFdC, the greater effectiveness of dFdC ters. Subsequently, dFdC is phosphorylated into dFdC phosphorylation by dCK and the longer intracellular monophosphate (dFdCMP) by deoxycytidine kinase (dCK). retention of dFdCTP. These factors may, at least in part, The same enzyme is responsible for the intracellular explain the different spectra of antitumour activity of both phosphorylation of ara-C into cytarabine monophosphate drugs. Only a small part of the dFdC and ara-C dose is (ara-CMP). dCK is the rate-limiting enzyme in the biotrans- responsible for the cytotoxic effects, since more than 90% of formation of both dFdC and ara-C. Inactivation of dFdCMP the dose is inactivated by the enzyme CDD into dFdU and and ara-CMP can occur through dephosphorylation by 50- uracil-arabinoside (ara-U), respectively. nucleotidase (5NT). Monophosphates, escaping from de- phosphorylation are available for further phosphorylation Germline Polymorphisms and Fluoropyrimidine Efficacy into di- and triphospates by dCMP kinase and nucleoside 5-Fluorouracil diphosphate kinase, respectively. dFdC diphosphate (dFdCDP) is a potent inhibitor of the enzyme ribonucleotide Genetic polymorphisms of enzymes involved in the meta- reductase (RNR), which will lead to depletion of deoxycy- bolic activation pathway of 5-FU have been described for the tidine diphosphate (dCDP) and deoxycytidine triphosphate enzymes uridine monophosphate kinase (UMPK) and (dCTP) in the cell. This may favour the incorporation of orotate phosphorylase transferase (OPRT). Three allelic dFdCTP into DNA. Moreover, dFdC has the unique property variants of UMPK have been recognised in the human that, after incorporation of dFdC monophosphate in DNA, population: UMPK1, UMPK2 and UMPK3.14 The UMPK1 one more deoxynucleotide molecule can be inserted. This allele is associated with about three times the catalytic stops DNA polymerase.13 This pattern is distinct from that of activity of the UMPK2 allele. Therefore, UMPK2 homo- The Pharmacogenomics Journal