Search

Prostate-specific membrane antigen (PSMA) is a well-characterized tumor marker associated with prostate cancer and neovasculature of most solid tumors. PSMA-specific ligands are thus being developed to deliver imaging or therapeutic agents to cancer cells. Here, we report on a crystal structure of human PSMA in complex with A9g, a 43-bp PSMA-specific RNA aptamer, that was determined to the 2.2 Å resolution limit. The analysis of the PSMA/aptamer interface allows for identification of key interactions critical for nanomolar binding affinity and high selectivity of A9g for human PSMA. Combined with in silico modeling, site-directed mutagenesis, inhibition experiments and cell-based assays, the structure also provides an insight into structural changes of the aptamer and PSMA upon complex formation, mechanistic explanation for inhibition of the PSMA enzymatic activity by A9g as well as its ligand-selective competition with small molecules targeting the internal pocket of the enzyme. Additionally, comparison with published protein–RNA aptamer structures pointed toward more general features governing protein-aptamer interactions. Finally, our findings can be exploited for the structure-assisted design of future A9g-based derivatives with improved binding and stability characteristics.

Current therapies for advanced-stage prostate cancer have shown limited efficacy due to the molecular complexity of this aggressive disease and the unwanted side effects that result from the treatments themselves. Chemotherapeutic drug cocktails are currently the preferred treatment option to inhibit multiple targets simultaneously, thereby reducing drug-resistance in advanced-stage disease. However, owing to the non-selective nature of these drugs, targeted approaches that eliminate toxicity to non-target tissues and reduce the amount of drug that needs to be administered to the patient are warranted. Prostate-specific membrane antigen (PSMA), a transmembrane receptor expressed on malignant prostate cancer cells, has been identified as a promising therapeutic target for targeted therapy of prostate cancer. PSMA-targeted agents have included small molecules, antibodies, and nucleic acid aptamers. This review focuses on oligonucleotide-based ligands (DNA and RNA aptamers) that target PSMA and their use in imaging and therapeutic applications for prostate cancer. This review covers important concepts pertaining to the clinical translation of PSMA aptamers (safety, stability, and pharmacokinetics) and highlight existing hurdles and future prospects.

The relative ease of isolating aptamers with high specificity for target molecules suggests that molecular recognition may be common in the folds of natural RNAs. We show here that, when expressed in cells, aptamers can increase the intracellular concentrations of their small molecule ligands. We have named these aptamers as DRAGINs (Drug Binding Aptamers for Growing Intracellular Numbers). The DRAGIN property, assessed here by the ability to enhance the toxicity of their ligands, was found for some, but not all, aminoglycoside aptamers. One aptamer protected cells against killing by its ligand. Another aptamer promoted killing as a singlemer and protected against killing as a tandemer. Based on a mathematical model, cell protection vs. killing is proposed as governed by aptamer affinity and access to the inner surface of the cell membrane, with the latter being a critical determinant. With RNA molecules proposed as the earliest functional polymers to drive the evolution of life, we suggest that RNA aptamer-like structures present in primitive cells might have selectively concentrated precursors for polymer synthesis. Riboswitches may be the evolved forms of these ancient aptamer-like “nutrient procurers”. Aptamers with DRAGIN capability in the modern world could be applied for imaging cells, in synthetic cell constructs, or to draw drugs into cells to make “undruggable” targets accessible to small molecule inhibitors.