1. Multidrug resistance is the major obstacle to successful cancer chemotherapy. Circumventing multidrug resistance therefore represents a high priority for clinical anti-cancer treatment. Among many reversal strategies, antisense oligodeoxynucleotides may offer a molecular targeting tool for overcoming cellular multidrug resistance.

2. Two 17-mer phosphorothioate antisense oligomers, complementary to the 5′ end of the ATG initiator codon-containing region and loop-forming site (located at nucleotides 991–1007 from the first ATG codon) in mdr-1 cDNA sequence, were synthesized. The purpose was to study their effects on the function and expression of P-glycoprotein and mdr-1 gene.

3. The results showed that 10 μmol/l antisense oligomers could significantly inhibit the growth of multidrug resistant K562/Adm cells cultured in adriamycin-containing medium. No such effect was observed for parental (sensitive) K562/S cells. Intracellular daunorubicin accumulation increased greatly in the K562/Adm cells after they were treated with oligomers for 48 h and P-glycoprotein synthesis was strikingly reduced.

4. Further investigation with [α-32P]dCTP incorporation by the reverse transcriptase—polymerase chain reaction method revealed that antisense oligomers could result in a reduction in the level of mdr-1 mRNA, probably through hindering mdr-1 gene transcription.

5. The high reversal efficiency and specificity of antisense oligomers in regulating mdr-1 gene expression suggest a potential clinical application in gene therapy for drug resistant malignancies.

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