REVERSAL OF DOXORUBICIN RESISTANCE BY THE AMILORIDE ANALOG EIPA IN MULTIDRUG-RESISTANT HUMAN COLON-CARCINOMA CELLS

Although multidrug resistance (mdr) may arise through a variety of mec hanisms, the most widely studied and accepted form is associated with an increased concentration of P-glycoprotein (P-gp), a 170 kd protein found in the membrane fraction of a number of mammalian cells. Since m di seems to be related to the ability of resistant cells to extrude dr ugs and the circumvention of mdr is supposed to be due to the restored ability to accumulate drugs, membrane has been regarded as the crucia l site for such a regulation and an important role for membrane ion ex changers has been suggested. The aim of this work was to elucidate whe ther the Na+/H+ antiporter is involved in the mechanism of regulation and circumvention of mdr and if 5-(N-ethyl-N-isopropyl) amiloride (EIP A), a selective inhibitor of the Na+/H+ exchanger, can modulate the fu nctional expression of the mdr phenotype. The effect of EIPA on doxoru bicin (DX) resistant cells (LoVo/DX) obtained from a human colon adeno carcinoma cell line (LoVo) was studied. EIPA at concentrations ranging from 10 to 50 mu M was able to increase the antibiotic cytotoxicity i n the resistant Lovo/DX cells. The reversal of DX resistance parallele d an increase of the ability of the cells to accumulate the drug. Both drug loading and sensitivity to the inhibitory effect of DX on cell p roliferation were restored by EIPA in a dose-dependent way. These resu lts suggest a new mechanism of mdr reversal and indicate that amilorid e and its derivatives may be useful in reversing DX resistance and in enhancing the clinical effectiveness of chemotherapeutics.