Evaluating variability in fluorouracil (FU) exposure in obese and non-obese patients with colorectal cancer using full weight-based dosing.

Abstract

409 Background: Approximately 30% of cancer patients are obese. Controversy exists on how to best use body weight to safely and effectively dose chemotherapy. Additionally, body surface area (BSA)-based dosing is associated with high pharmacokinetic (PK) variability and is a poor indicator of optimal drug exposure. Methods: We estimated FU exposure post cycle 1, as indicated by area under the curve (AUC), in 58 colorectal cancer patients receiving mFOLFOX6 (FU 2400 mg/m2 [BSA calculated using actual body weight (ABW)] over 46 hours) +/- bevacizumab. AUCs were determined using an immunoassay at Myriad Laboratories. The primary objective was to identify variability in FU exposure in obese and non-obese patients using body mass index (BMI), BSA, and ABW. Groups were compared using two group t-tests. Results: No significant difference in AUC was observed between obese (BMI ≥ 30 kg/m2) and non-obese (< 30 kg/m2) patients. A significantly lower AUC was observed in patients with a BSA ≥ 2.0 m2 compared to those < 2.0 m2 (p=0.02). The AUC for obese and non-obese patients ranged from 7-34 and 12-38 mg*hr/L, respectively. Only 32% and 28% of obese and non-obese patients, respectively, were within the previously reported therapeutic target AUC 20-25 mg*hr/L. Males had significantly lower AUCs compared to females (mean 18.7 v 22.4, p=0.03). FU AUC was not strongly correlated with BMI, BSA, or ABW (Pearson correlation of -0.03, -0.26, and -0.15, respectively). Conclusions: BSA-based dosing is associated with variable FU exposure. Lower AUCs in patients with a BSA ≥ 2.0 m2 supports both ASCO recommendations for full-weight-based dosing in obese patients and the value of dosing assessment across patient populations. Further study of AUC-based dosing is warranted as our data do not support a reduction in variable exposure based on BMI, BSA, or ABW dosing. [Table: see text]