New cellular RNA elimination method for cell-based SELEX of modified RNA aptamers

Aptamers are synthetic singlestranded RNA or DNA molecules that have high specificity to bind tar� get molecules by means of unique spatial structure for� mation. The method of the systematic evolution of ligands by exponential enrichment (SELEX) is used to obtain aptamers (1). Different objects can be the tar� gets for selection ranging from metal ions and small organic molecules to proteins and unicellular organ� isms. Aptamers that recognize the cells of certain type due to affinity to specific surface proteins are of great interest. As a rule, live cells are used as targets for the selection of these aptamers. The fields of application of these aptamers include the targeted delivery of drugs into cells and the specific detection of cells (2-4). Both DNA and RNA libraries are widely used to select aptamers. The advantages of RNAbased aptamers are their ability to form more diverse and stable tertiary structures than DNAbased aptamers; nonetheless, additional chemical modifications are needed to increase their biological stability. Thus, pyrimidine nucleosides within escort RNA aptamers are usually substituted by 2'�modified analogues, as a rule, by 2'�fluoroor 2'�aminonucleosides (5). A general scheme for the selection of 2'�modified RNA aptamers by cell� based SELEX method includes the following stages: (1) the incubation of the RNA library with target cells;