An Aptamer That Rapidly Internalizes into Cancer Cells Utilizes the Transferrin Receptor Pathway

SIMPLE SUMMARY: It has been a continuous effort to develop innovative targeted cancer treatments to overcome the non-specific toxicity associated with chemotherapy. Antibody-drug conjugates (ADCs) highlight the clinical benefits of specifically delivering drugs to cancer cells via targeting agents. Aptamers are high-affinity ligands that can be made synthetically and employed as targeting molecules in place of antibodies. Our group previously reported an aptamer, E3, that can target a wide range of cancer cell lines and deliver highly toxic drugs into cancer cells in vitro and in vivo. We report that E3 enters cancer cells by specifically targeting transferrin receptor 1. ABSTRACT: Strategies to direct drugs specifically to cancer cells have been increasingly explored, and significant progress has been made toward such targeted therapy. For example, drugs have been conjugated into tumor-targeting antibodies to enable delivery directly to tumor cells. Aptamers are an attractive class of molecules for this type of drug targeting as they are high-affinity/high-specificity ligands, relatively small in size, GMP manufacturable at a large-scale, amenable to chemical conjugation, and not immunogenic. Previous work from our group revealed that an aptamer selected to internalize into human prostate cancer cells, called E3, can also target a broad range of human cancers but not normal control cells. Moreover, this E3 aptamer can deliver highly cytotoxic drugs to cancer cells as Aptamer-highly Toxic Drug Conjugates (ApTDCs) and inhibit tumor growth in vivo. Here, we evaluate its targeting mechanism and report that E3 selectively internalizes into cancer cells utilizing a pathway that involves transferrin receptor 1 (TfR 1). E3 binds to recombinant human TfR 1 with high affinity and competes with transferrin (Tf) for binding to TfR1. In addition, knockdown or knockin of human TfR1 results in a decrease or increase in E3 cell binding. Here, we reported a molecular model of E3 binding to the transferrin receptor that summarizes our findings.