Potent growth inhibition of leukemic cells by novel ribbon-type antisense oligonucleotides to c-myb1

We studied the effects of antisense oligonucleotides (AS oligos) with a nov el structure. The AS oligos were covalently closed to avoid exonuclease act ivities by enzymatic ligation of two identical molecules. The AS oligos of a ribbon type (RiAS oligos) consist of two loops containing multiple antise nse sequences and a stem connecting the two loops. Three antisense sequence s targeting different binding sites were placed in a loop that was designed to form a minimal secondary structure by itself. RiAS oligos were found to be stable because they largely preserved their structural integrity after 24 h incubation in the presence of either exonuclease III or serums. When a human promyelocytic cell line, HL-60, was treated with RiAS oligos to c-my b, c-myb expression was effectively ablated, Cell growth was inhibited by > 90% determined by both the 3-[4,5-dimethythiazol-2-yl]-2,5-diphenyltetrazol ium bromide assay and [H-3]thymidine incorporation. Further, when the leuke mic cell line K562 was treated with c-myb RiAS oligos, colony formation on soft agarose was reduced by 92 +/- 2%. These results suggest that RiAS olig os may be employed for developing molecular antisense drugs as well as for the functional study of a gene.