In vitro selection using a dual RNA library that allows primerless selection.

High affinity target-binding aptamers are identified from random oligonucleotide libraries by an in vitro selection process called Systematic Evolution of Ligands by EXponential enrichment (SELEX). Since the SELEX process includes a PCR amplification step the randomized region of the oligonucleotide libraries need to be flanked by two fixed primer binding sequences. These primer binding sites are often difficult to truncate because they may be necessary to maintain the structure of the aptamer or may even be part of the target binding motif. We designed a novel type of RNA library that carries fixed sequences which constrain the oligonucleotides into a partly double-stranded structure, thereby minimizing the risk that the primer binding sequences become part of the target-binding motif. Moreover, the specific design of the library including the use of tandem RNA Polymerase promoters allows the selection of oligonucleotides without any primer binding sequences. The library was used to select aptamers to the mirror-image peptide of ghrelin. Ghrelin is a potent stimulator of growth-hormone release and food intake. After selection, the identified aptamer sequences were directly synthesized in their mirror-image configuration. The final 44 nt-Spiegelmer, named NOX-B11-3, blocks ghrelin action in a cell culture assay displaying an IC50 of 4.5 nM at 37 degrees C.