Feasibility and population exposure of 5‐fluorouracil using therapeutic drug monitoring (PREDICT‐5FU): A multicentre clinical trial

Aim PREDICT‐5FU aimed to document 5‐fluorouracil (5FU) exposure in a cancer population and to evaluate the feasibility of 5FU and capecitabine therapeutic drug monitoring (TDM) in patients receiving standard doses and schedules. Methods Multicentre, prospective, observational single‐arm study. Eligible adult patients received 5FU (infusional ≥24 h) or capecitabine. Patients were treated for gastrointestinal, breast and head‐and‐neck cancers at four Australian hospitals. TDM was performed in consecutive cycles until target area under the curve (AUC) was reached. Pharmacogenetic testing was performed for all patients. Results Fifty patients (24 males, 26 females) were recruited. Median age was 63 years; common diagnoses were lower gastrointestinal cancers 40% (20/50) and metastatic disease 80% (40/50). The majority received 5FU (38/50, 76%) over 46 h. Only 36% of 5FU patients achieved target AUC when dosed based on body surface area; 61% were below and 3% above target range. Post TDM‐adjusted dosing, target AUC was achieved in 58% of patients (22% absolute increase vs . BSA dosing, p = 0.03), within median three cycles (range 1–5). DPYD variant allele carriers (3/4) had upfront reduced dosing due to heterozygosity; all were below the target AUC and one experienced Grade 3 toxicity. There was no correlation between dihydrouracil: uracil ratio [UH2/U] or uracilemia [U] and DPYD genotype. TDM results were reported with an average of 4 days from sampling. Conclusion TDM dosing is feasible and increases the proportion of patients reaching target AUC. Findings are relevant across all cancers treated with 5FU, and particularly for DPYD variant allele carriers receiving upfront dose reductions.