A major goal of translational cancer research is to develop targeted therapies that can specifically inhibit the expression or function of proteins that play an essential role in oncogenesis. There is considerable interest in using synthetic DNA or RNA oligonucleotides to achieve this goal because of their ability to recognize specified nucleic acid sequences or protein structures with high affinity. Several oligonucleotide-based strategies, including antisense, small interfering RNAs (siRNAs), protein-binding aptamers and immunostimulatory oligonucleotides, have produced potent anti-cancer effects in pre-clinical studies. However, clinical trials of therapeutic (antisense) oligonucleotides have been generally disappointing and this has been attributed, in part, to their inefficient uptake by cancer cells. The Principal Investigator and her collaborators have developed a novel antiproliferative oligonucleotide named AGRO100. This molecule has recently been tested in a clinical trial involving patients with advanced cancer and has demonstrated a remarkable lack of toxicity combined with promising clinical activity. Unlike most other oligonucleotides, AGRO100 is taken up efficiently and selectively by cancer cells in culture and in vivo. We hypothesize that these extraordinary properties are related to the unusual G-quadruplex structure of AGR0100 and its ability to bind specifically to a protein that is expressed at high levels on the surface of cancer cells. The long-term goal of this project is to develop oligonucleotides that are avidly and selectively taken up by cancers in vivo. Such tumor-targeting sequences could be incorporated into oligonucleotide-based therapeutics or conjugated to chemotherapy drugs in order to enhance their efficacy and reduce unpleasant side effects. In this application, we propose to elucidate the mechanism involved in the preferential uptake of AGRO100 by tumors and to identify sequence or structural motifs that lead to efficient oligonucleotide internalization by cancer cells. The first specific aim is to characterize the cellular internalization of AGRO100 and confirm the role of nucleolin in this process. The second aim is to use a SELEX approach to identify oligonucleotides (from combinatorial libraries) that have efficient and selective uptake by cancer cells. The third aim is to incorporate the optimal tumor-targeting sequences into antisense, siRNA and immunomodulatory oligonucleotides in order to determine if this leads to superior uptake and activity.