Patients were treated for colorectal cancer and received chemotherapy consisting of leucovorin 20mgm?2 plus 5-fluorouracil kinase inhibitor Sunitinib 425mgm?2. Blood sampling was carried out on day 1 of the first cycle. The 5-fluorouracil area under the curve0��3h in the index patient was 24.1mghl?1 compared to 9.8��3.6 (range 5.4�C15.3) mghl?1 in control patients. The 5-fluorouracil clearance was 520mlmin?1 vs 1293��302 (range 980�C1780) mlmin?1 in controls. The activity of dihydropyrimidine dehydrogenase in mononuclear cells was lower in the index patient (5.5nmolmgh?1) compared to the six controls (10.3��1.6, range 8.0�C11.7nmolmgh?1). Sequence analysis of the dihydropyrimidine dehydrogenase gene revealed that the index patient was heterozygous for a IVS14+1G>A point mutation.

Our results indicate that the inactivation of one dihydropyrimidine dehydrogenase allele can result in a strong reduction in 5-fluorouracil clearance, causing severe 5-fluorouracil induced toxicity. British Journal of Cancer (2002) 86, 1028�C1033. DOI: 10.1038/sj/bjc/6600199 www.bjcancer.com ? 2002 Cancer Research UK Keywords: DPD, 5-fluorouracil, pharmacokinetics, DPYD gene, mutation, pharmacogenetics Fluorouracil (5-FU) is widely used in chemotherapeutic regimens for the treatment of breast-, colorectal- and head- and neck cancer. The cytotoxic mechanism of 5-FU is complex, requiring intracellular bioconversion of 5-FU into cytotoxic nucleotides (see Figure 1). Inhibition of thymidylate synthase by the metabolite 5-fluoro-2��-deoxyuridine-5��-monophosphate is thought to be the main mechanism of cytotoxicity (Pinedo and Peters, 1988) .

The cytotoxicity is caused by only a small part of the administered 5-FU dose, as the majority of 5-FU is rapidly metabolised into inactive metabolites. The initial and rate-limiting enzyme in the catabolism of 5-FU is dihydropyrimidine dehydrogenase (DPD), catalysing the reduction of 5-FU into 5,6-dihydrofluorouracil (DHFU). Several groups have suggested a major role of DPD in the regulation of 5-FU metabolism and thus in the amount of 5-FU available for cytotoxicity (Harris et al, 1990; Fleming et al, 1992a; Lu et al, 1993; Etienne et al, 1994). Indeed, in patients with DPD enzyme deficiency, 5-FU chemotherapy is associated with severe, life-threatening toxicity (Van Kuilenburg et al, 2000a). Moreover, a markedly prolonged elimination half-life of 5-FU has been observed in a patient with complete deficiency of DPD enzyme activity (Diasio et al, 1988). Several mutations in the dihydropyrimidine dehydrogenase gene (DPYD), which encodes Brefeldin_A for the DPD enzyme have recently been identified (Van Kuilenburg et al, 2000a; Collie-Duguid et al, 2000).