INHIBITION OF MDR1 GENE-EXPRESSION BY ANTIMESSENGER OLIGONUCLEOTIDES LOWERS MULTIPLE-DRUG RESISTANCE

The multiple drug resistance of neoplastic cells is mediated by overex pression of the human MDR1 gene, which encodes the transmembrane efflu x pump P-glycoprotein. In both cell lines and human tumors the MDR phe notype closely correlates with MDR1 mRNA and P-glycoprotein levels. Re version of the MDR phenotype was attempted in human colorectal adenoca rcinoma doxorubicin (Dx)-resistant cells (LoVo/Dx) by long-term admini stration of an equimolecular mixture of three unmodified ODNs (18mer) targeted to adjacent binding sites of the MDR1 mRNA and carried by a s ynthetic cationic lipid (DOTAP). Three different experimental paramete rs were used to evaluate the antimessenger agent's effectiveness in co mparison with a random sequence ODN: the level of cell resistance to D x; the level of P-glycoprotein (determined by flow cytometry); the lev el of MDR1 mRNA (determined by quantitative RT-PCR). Experimental data indicate that the level of both the MDR1 mRNA and the P-glycoprotein is reduced by approximately 50% by treatment of LoVo/Dx cells with a 1 0 mu M total concentration of the aODN mixture every 24 h for 15 days. In agreement with these findings, sensitivity to Dx of the antimessen ger agent-treated LoVo/Dx cells was almost doubled in comparison with random sequence ODN-treated controls.