Plasma 5-Fluorouracil Exposure, Clinical Outcomes, and Therapeutic Drug Monitoring in Advanced Colorectal Cancer

Background/Objectives: Body-surface-area-based dosing of continuous-infusion 5-fluorouracil does not account for inter-individual pharmacokinetic variability. This pilot study explored whether patient-level representative plasma 5-fluorouracil exposure within the target area under the concentration–time curve (AUC) range was associated with clinical outcomes. It evaluated a prototype immunochromatographic assay as a preliminary monitoring tool. Methods: Fifteen patients with unresectable advanced or recurrent colorectal cancer who received continuous-infusion 5-fluorouracil-based chemotherapy were prospectively evaluated between 1 January 2017 and 30 April 2018. Plasma 5-fluorouracil levels were measured during eight treatment cycles at three time points in each cycle. Representative AUC values were calculated using median concentrations across cycles and interpreted as exploratory patient-level exposure indices. Tumor response, grade ≥2 adverse events, progression-free survival, and overall survival were assessed descriptively. Results: The median representative AUC was 24.3 mg·h/L. Eight patients (53.3%) were within the target range of 20–30 mg·h/L, whereas seven (46.7%) were outside it. Disease control was observed in 7 of 8 patients (87.5%) within the target range and in 3 of 7 patients (42.9%) outside it. Grade ≥2 adverse events were less frequent in the target-range group (2/8, 25.0%) than in the outside-range group (6/7, 85.7%; p = 0.041). Progression-free survival was numerically longer in the target-range group (17.2 vs. 9.2 months, p = 0.36), while overall survival did not differ clearly (p = 0.76); these survival analyses were exploratory. The prototype immunochromatographic assay showed a favorable correlation with the My-5FU assay (R2 = 0.762), but Bland–Altman analysis showed relatively wide limits of agreement. Conclusions: Target plasma 5-fluorouracil exposure was associated with lower clinically relevant toxicity and may support favorable tumor control in this pilot cohort. The prototype immunochromatographic method demonstrated preliminary feasibility for rapid plasma 5-FU monitoring but requires further validation before routine dose adjustment.