Relationships between body composition parameters and fluorouracil pharmacokinetics

Aims To verify whether fluorouracil (FU) clearance (CL) and volume of distribution (Vss) are better correlated with specific body compartments, such as body cell mass (BCM), total body water (TBW) or fat free mass (FFM), rather than with body surface area (BSA) or total body weight (BW). Methods Thirty‐four patients (13 females and 21 males) affected by colorectal cancer and receiving FU as adjuvant therapy entered the study. CL and Vss were determined after a 2 min i.v. injection of FU (425 mg m−2) and leucovorin (20 mg m−2). Body composition, in terms of BCM, TBW and FFM, was evaluated non‐invasively by bioelectrical impedance analysis (BIA). Results Significant but poor correlations were found between CL or Vss and most anthropometric parameters, including BIA‐derived measures (r2 range=0.10–0.21). However, when multiple regression analysis was performed with sex, TBW and FFM as independent variables, the correlations improved greatly. The best correlation was obtained between CL and sex (r2=0.44) and between Vss and sex (r2=0.36). FFM‐normalized CL was significantly higher in women than in men (0.030±0.008 vs 0.022±0.005 l min−1 kg−1; 95% CI of difference 0.012, 0.003; P=0.003), suggesting that FU metabolism is more rapid in females. Surprisingly, Vss was highly correlated with CL (r2=0.67; CL=0.52+Vss×0.040). This finding may either be explained by extensive drug metabolism in extra‐hepatic organs or by variable inactivation on first‐pass through the lung. Both these hypotheses need experimental validation. Conclusions The pharmacokinetics of FU are better predicted by FFM and TBW than by standard anthropometric parameters and predictions are sex‐dependent. The use of BIA may lead to improved dosing with FU.