| Abstract: | Cytochrome P‐450s (CYPs) are important biopolymers for the maintenance of cellular function. If metabolic activity of the CYP in the cells can be estimated, so can the function of metabolism, which is closer to the organism. In this research, the method of measuring the drug metabolic activity inside the cell by making use of an electrochemical technique was examined. Human hepatoma GS‐3A4‐HepG2 cells of which the cytochrome P‐4503A4 (CYP3A4) drug metabolic activity is found to be the same as that of primary hepatocytes were used in the experiment. The GS‐3A4‐HepG2 cells were cultured on an indium‐tin oxide (ITO) electrode until they became confluent. Substrate testosterone and inhibitor ketoconazole of CYP3A4 were exposed to cells cultured on an ITO electrode, and the reaction was observed by noting the electrochemical impedance measurement. Impedance was decomposed into the resistance component and the reactance component, and each was examined in detail. As a result, according to testosterone concentration change, there was a remarkable time change in the reactance component. A similar impedance measurement was done by using human hepatoma HepG2 cells in which the drug metabolic activity had extremely decreased. Nevertheless, no time change in the reactance component that was noticed in GS‐3A4‐HepG2 cells was observed. Next, the amount of metabolite in the solution after impedance measurement was measured by means of liquid chromatography‐tandem mass spectroscopy (LC‐MS/MS). In the experiment with GS‐3A4‐HepG2 cells, a testosterone concentration‐dependent correlation was observed between the reactance component change and the amount of metabolite. But, in the impedance measurement by ketoconazole, the change in reactance components was not observed in either the GS‐3A4‐HepG2 cells or the HepG2 cells. Ketoconazole and the heme iron in CYP3A4 effect the coordination bond, but ketoconazole was not metabolized by CYP3A4. It was confirmed that the time change in the reactance component which was caused by the testosterone was detected neither in the cells that take up the substrate, nor in the coordination bond between the CYP enzyme and the drug. Therefore, the time change in the remarkable reactance component observed by this electrochemical impedance measurement is dependent on drug metabolic activity. An electrochemical drug metabolic activity measuring method with the human hepatoma GS‐3A4‐HepG2 cells was able to be established. Copyright © 2004 John Wiley & Sons, Ltd. |
