An observational study to assess the ability of the thymine loading test to prospectively categorise patients with gastrointestinal or breast cancer who cannot tolerate fluoropyrimidine treatment.

This study is an observational study. Thymine test results will not affect any treatment decisions. At a scheduled assessment clinic visit, >24 hours prior to the commencement of treatment with 5-FU/capecitabine containing schedules: A baseline blood sample will be obtained (one 10 ml sample) as well as a baseline spot urine sample. A cheek (buccal) cell sample will also be obtained. This will involve using a cytobrush to take a brushing of the inside of the cheek (like using a toothbrush on the inside of the mouth). This will be done twice (one on each side of the mouth) and will require a mouth wash with water prior to the first brushing. The participants will then be administered thymine (250 mg, oral gelatine capsule) with a drink of water. A cumulative (total) urine sample will be collected for the next 4 hours. All patients who have been given the thymine test will then continue on their planned chemotherapy treatment. All patients will be assessed for adverse events related to 5-FU/capecitabine treatment. Information will be collected at each scheduled clinic visit for 8 – 9 weeks while on normal chemotherapy treatment. Analysis will involve urine sample for thymine and its metabolite (dihydrothymine or DHT). Cheek cells will be used to assess 5-FU uptake in vitro and genomic DNA (from blood) will be used to analysing variation in the DPYD gene and further analysis of genes associated with 5-FU side effects.