ABCC4 polymorphism indirectly reduces systemic exposure to a capecitabine metabolite 5'-deoxy-5-fluorouridine in Japanese subjects.

Aims: Capecitabine converts to 5-fluorouracil (5-FU) in 3 steps. We previous demonstrated a significant association between area under the plasma concentration-time curve (AUC) of a metabolite 5'-deoxy-5-fluorouridine (5'-DFUR) and capecitabine-induced toxicity. However, critical factors affecting the 5'-DFUR AUC remain unclear. This study investigated the effects of the ABCC4 rs3742106 (G > T) which down regulates ABCC4 protein expression, on 5'-DFUR AUC, and analysed the underlying mechanisms.

Methods: ABCC4 rs3742106 and 5'-DFUR AUC were prospectively analysed in 37 Japanese patients with colorectal cancer who received capecitabine plus oxaliplatin. 5'-DFUR transport was analysed using ABCC4-expressing membrane vesicles. A physiologically based pharmacokinetic (PBPK) model was constructed to identify potential drug-metabolizing enzymes responsible for 5'-DFUR AUC and affected by rs3742106. ABCC4 expression in human hepatoma HepaRG cells was suppressed by small interfering RNA against ABCC4, followed by gene expression measurement of the enzymes.

Results: AUC/dose of 5'-DFUR in patients with ABCC4 rs3742106 G/T or T/T genotype was significantly lower than other patients (P = .0258). However, 5'-DFUR was not transported by ABCC4 expressed in membrane vesicles. PBPK model analyses revealed that activity of thymidine phosphorylase (TP), which converts 5'-DFUR to 5-FU, may be increased by the ABCC4 rs3742106, most strongly contributing to the decrease in 5'-DFUR AUC. Transfection of siABCC4 in HepaRG cells increased intracellular cyclic adenosine monophosphate (cAMP) and enhanced TP mRNA expression. Addition of a cAMP analogue, 8-bromo-cAMP, also induced TP mRNA.

Conclusion: The ABCC4 polymorphism reduces 5'-DFUR AUC by an indirect mechanism. An increase in hepatic cAMP, which upregulates TP expression, was proposed as a hypothetical mechanism for this polymorphic change.