Novel chemically modified oligonucleotides provide potent inhibition of P-glycoprotein expression

One major form of multiple drug resistance (MDR) to cancer therapeutic agents is mediated by overexpression of P-glycoprotein, a membrane ATPase that serves as a drug efflux pump. In humans, this protein is the product of the MDR1 gene. We have used chemically modified antisense oligonucleotides to reduce expression of P-glycoprotein in multidrug-resistant fibroblasts and colon carcinoma cells. Although several types of oligonucleotides were tested, compounds having a phosphorothioate backbone and a methoxyethoxy (ME) group at the 2' position of the ribose ring proved to have the greatest potency. Thus, phosphorothioate 2'-ME oligonucleotides directed against either the AUG codon region or the stop codon region of the MDR1 message produced substantial (50-70%) inhibition of P-glycoprotein expression at concentrations of < or = 50 nM. In addition, such treatment resulted in augmented drug uptake as measured by flow cytometry. Unmodified phosphorothioate compounds of the same sequence were active only in the micromolar range. We also tested the ability of several potential delivery agents to enhance the pharmacological effectiveness of anti-MDR1 oligonucleotides. Both commercial Lipofectin, a well known liposomal transfection agent, and a liposomal preparation based on a novel "facial amphiphile" were effective in enhancing their pharmacological effects of antisense oligonucleotides. A Starburst dendrimer, a type of cationic polymer, was also effective in oligonucleotide delivery. This report emphasizes that significant improvements in antisense pharmacology can be made through judicious use of both chemical modifications of oligonucleotides and appropriate delivery systems.