Nonspecific cellular accumulation of 99mTc-labeled oligonucleotides in culture is influenced by their guanine content

Specific accumulation of radiolabeled antisense oligonucleotides as a result of binding by an antisense mechanism to target mRNAs in tumor has been repeatedly observed. However, the mechanisms responsible for nonspecific cellular accumulation remain almost completely unexplored. We have occasionally observed in cell culture nonspecific accumulations of 99mTc-labeled sense, scrambled or random control oligonucleotides in tumor cells comparable to or even higher than those of the corresponding antisense oligonucleotides. We have also observed that these nonspecific accumulations of control oligonucleotides are sequence dependent. To explore the influence of base composition on nonspecific accumulation, we used MCF-7 breast cancer cells, along with 10 uniform phosphorothioates and 5 uniform phosphodiesters oligonuc leotides. Three of the phosphorothioates were antisense against different sites within the survivin mRNA, and two were the corresponding sense and scrambled controls. In addition, the accumulations in MCF-7 cells of radiolabeled poly A, poly C, poly T and poly GGGA phosphorothioate oligonucleotides were also studied to explore the influence of each nitrogenous base on the nonspecific cell accumulations of phosphorothioate oligonucleotides. Our results show that guanine content is an important determinant of nonspecific cellular accumulations under the conditions of this investigation. If this observation can be shown to be universally applicable to other cell types, then the selection of control sequences in studies of antisense tumor targeting should avoid those that are guanine rich, if possible.