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.

Interventions: 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.
Primary outcome(s): The primary outcome is the presence of severe DM toxicity. All diarrhoea (D) and mucositis (M) toxicities will be identified and coded according to CTCAE criteria. Patients with grade 2 or less GI events or haematotoxicity, or grade 1 or less hand-foot syndrome will be coded as minimal toxicity. For patients with a Grade 3 or higher D or M toxicity a review will be carried out by a clinical PI (blinded to thymine test results and genomic information).[Information will be collected at each scheduled clinic visit for 8 – 9 weeks while on normal chemotherapy treatment.
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Study Design: Purpose: Natural history;Duration: Longitudinal;Selection: Defined population;Timing: Prospective