The primary purpose of this study is to prospectively determine whether capecitabine and 5-FU-induced toxicity is preventable by dose reduction prior to start of the first administration in patients heterozygous or homozygous mutant for DPYD*2A, and to determine whether this strategy is cost-effective. Secondly, an individualized treatment algorithm for capecitabine and 5-FU therapy in DPYD*2A mutant patients will be developed and the pharmacokinetic profile of capecitabine and 5-FU will be assessed.

Patients exhibiting a genetically determined disorder (DPYD*2A) in the metabolic degradation of the frequently used anticancer agents capecitabine and 5-FU (fluoropyrimidines) are at high risk of development of severe and life-threatening toxicity during standard treatment with these compounds. Treatment and recovery of this fluoropyrimidine-induced severe toxicity often requires prolonged periods of hospitalization.

Screening for DPYD*2A in patients to treat with fluoropyrimidine drugs with subsequent dose adjustments in mutant individuals prior to start of therapy will possibly reduce the number of severe toxicity events. Furthermore, by reducing the frequency and/or duration of hospitalization, substantial medical costs can be saved, making this a cost-effective strategy.

Inclusion Criteria:

• Histological proof of cancer
• patient is considered for treatment with capecitabine or 5-FU
• hetero- or homozygous mutant for DPYD*2A
• able and willing to give written informed consent
• able and willing to undergo blood sampling for pharmacokinetic analysis
• life expectancy 3 months or longer
• acceptable safety laboratory values (ANC, platelet count, ASAT, ALAT, creatinine,
• WHO performance status 0-2
• no radio- or chemotherapy within the last 3 weeks prior to study entry

Exclusion Criteria:

• patients with known alcoholism, drug addiction and/or psychotic disorders that are not suitable for adequate follow-up
• women who are pregnant or breast-feeding