An oligomer complementary to the 5' end region of MDR1 gene decreases resistance to doxorubicin of human adenocarcinoma-resistant cells.

Acquired resistance to doxorubicin and other anti-cancer drugs is generally dependent on gene amplification of a specific nucleotide sequence, the MDR1 gene. Verapamil, cyclosporin and other drugs have been used to circumvent the resistance in experimental models in vitro and/or in vivo. We have attempted to reverse the MDR phenotype by treating human adenocarcinoma resistant cells with 20 mers of synthetic unmodified oligodeoxynucleotide MDR1 antisenses. Five ODNs towards different mRNA regions and three different schedules of ODN antisense administration were tested. We found that FCS concentration greatly influenced the stability of ODN, whereas heat-inactivated FCS had no effect. The kinetics of ODN cellular uptake suggest the presence of a saturable receptor. Among the five antisense ODNs used, the most efficient was the oligomer (ODN-1) complementary to 20 bases upstream of the AUG initiation codon. No effect was observed with antisense complementary to the nucleotide binding sites. Administration of ODN-1 every 12 hr for 72 hr partially reversed the MDR phenotype. Approximately 60% of the cells lost their resistance to doxorubicin and did not form colonies in the presence of the drug. The MDR1 mRNA was transiently down-regulated so that the level of gp170 was slightly reduced. The incomplete switch off of MDR1 gene expression may be ascribed to the large abundance and great stability of MDR1 messenger RNA. Moreover, the inactivity of the two ODNs complementary to the NBS protein domains suggests that translation inhibition is ineffective. It is likely that ODN-4 and ODN-5 complement a large number of mRNAs competing for duplex formation, because these sequences are highly conserved among many proteins.