Background: Concurrent chemoradiation with 5-fluorouracil (5-FU) is widely accepted for treatment of abdominal malignancy. Nonetheless, the interactions between radiation and 5-FU remain unclear. We evaluated the influence of abdominal irradiation on the pharmacokinetics of 5-FU in rats.
Methods: The radiation dose distributions of cholangiocarcinoma patients were determined for the low dose areas, which are generously deposited around the intrahepatic target volume. Then, corresponding single-fraction radiation was delivered to the whole abdomen of Sprague-Dawley rats from a linear accelerator after computerized tomography-based planning. 5-FU at 100 mg/kg was intravenously infused 24 hours after radiation. A high-performance liquid chromatography system equipped with a UV detector was used to measure 5-FU in the blood. Ultrafiltration was used to measure protein-unbound 5-FU.
Results: Radiation at 2 Gy, simulating the daily human treatment dose, reduced the area under the plasma concentration vs. time curve (AUC) of 5-FU by 31.7% compared to non-irradiated controls. This was accompanied by a reduction in mean residence time and incremental total plasma clearance values, and volume of distribution at steady state. Intriguingly, low dose radiation at 0.5 Gy, representing a dose deposited in the generous, off-target area in clinical practice, resulted in a similar pharmacokinetic profile, with a 21.4% reduction in the AUC. This effect was independent of protein binding capacity.
Conclusions: Abdominal irradiation appears to significantly modulate the systemic pharmacokinetics of 5-FU at both the dose level for target treatment and off-target areas. This unexpected and unwanted influence is worthy of further investigation and might need to be considered in clinical practice.