Numerical analysis of apparent decitabine uptake in HCT116 cells: Incorporation of a bidirectional first-order kinetic parameter for ENT1 transport and Michaelis-Menten parameters for subsequent phosphorylation.

Decitabine (DAC), a DNA methylation inhibitor, is transported into cancer cells mainly via equilibrative nucleoside transporter 1 (ENT1) and subsequently phosphorylated by deoxycytidine kinase (dCK). We previously reported that apparent DAC uptake into cells may be described using a simple compartment model with clearance for facilitated diffusion (PS) and subsequent phosphorylation (CLmet). In the present study, time course of apparent intracellular [3H]-DAC uptake was analyzed numerically, and PS and CLmet values were calculated using the compartment model in human colon cancer HCT116 cells. PS at 0.1 μM [3H]-DAC was markedly decreased in the presence of 100 μM irinotecan or etoposide, while CLmet was markedly decreased in the presence of 100 μM cytarabine or gemcitabine. CLmet at 0.1-10 μM [3H]-DAC varied in a concentration-dependent manner and was described by Michaelis-Menten parameters Km,met and Vmax,met. In conclusion, DAC uptake mainly via ENT1 may be described by a bidirectional first-order kinetic parameter, while phosphorylation by dCK may be described by Michaelis-Menten parameters.