PII-068 - REDUCED SYSTEMIC EXPOSURE TO A CAPECITABINE METABOLITE 5'-DFUR IN ABCC4 POLYMORPHISM MAY BE ASSOCIATED WITH INCREASES IN CAMP LEVEL AND SUBSEQUENT UPREGULATION OF THYMIDINE PHOSPHORYLASE

N. Matsumoto¹, A. Oishi², Y. Kubota¹, H. Ishida³, K. Shimada⁴, Y. Masuo², Y. Kato², K. Fujita¹; ¹Showa University, ²Kanazawa University, ³Showa University Northern Yokohama Hospital, ⁴Showa University Koto Toyosu Hospital.

Background: Capecitabine is a prodrug that converts to 5-fluorouracil (5-FU) in three steps. A prospective study by us demonstrated that area under the plasma concentration-time curve (AUC) of a metabolite, 5'-deoxy-5-fluorouridine (5'-DFUR), was significantly associated with capecitabine-induced toxicity. To date, roles of ABCC4 on the systemic exposure to 5'-DFUR remains unclear. ABCC4 rs3742106 (G>T) is a functional polymorphism that may upregulate the expression of ABCC4 protein by altering the microRNA binding. Here, we examined the effects of ABCC4 rs3742106 on 5'-DFUR pharmacokinetics and analyzed the underlined mechanisms.

Methods: ABCC4 rs3742106 and 5'-DFUR AUC were prospectively analyzed in colorectal cancer patients who were administered capecitabine plus oxaliplatin. 5'-DFUR transport by ABCC4 was analyzed using ABCC4 expressing inside-out membrane vesicles. PBPK model was established to identify drug-metabolizing enzyme which was responsible for 5'-DFUR AUC and affected by ABCC4 rs3742106. ABCC4 expression in HepaRG cells was suppressed by the addition of siRNA against ABCC4, followed by measurement of intracellular cAMP level cAMP-Glo assay kit TM.

Results: Significantly lower AUC/dose of 5'-DFUR was observed in patients having ABCC4 rs3742106 G/T or T/T genotype than other patients (P=0.0245). However, the vesicle experiments showed no transport of 5′-DFUR by ABCC4. PBPK model analyses revealed that expression of thymidine phosphorylase (TP), which converted 5'-DFUR to 5-FU, may be increased by the polymorphic genotype of ABCC4, most strongly contributing to the decrease in the AUC of 5'-DFUR. The transfection of the siABCC4 to human hepatoma cell line HepaRG cells resulted in the increase of intracellular concentration of cAMP and enhanced the expression of TP mRNA. The addition of a cAMP analogue 8-Br-cAMP to the HepaRG cells led to the increase of the expression of TP mRNA.

Conclusion: We found that ABCC4 polymorphism reduced systemic exposure to 5'-DFUR which is associated with capecitabine toxicity. An increase in hepatic cAMP which upregulates the expression of TP may be involved in such polymorphic change.