Targeting prostate cancer with synthetic RNA ligands

Prostate cancer represents a serious health concern as the most diagnosed form of cancer in men and the second leading cause of cancer death in the Western world. Current treatments for prostate cancer are non-targeted and result in a number of undesirable, non-specific effects, highlighting the need for novel, targeted therapeutics in the treatment of prostate cancer.

Prostate Specific Membrane Antigen (PSMA) offers great promise in the targeting of prostate cancer for imaging and therapy. PSMA is a transmembrane carboxypeptidase with cell surface expression several orders of magnitude higher in cancerous prostatic epithelia than found in other tissue and PSMA is constitutively internalized into cells. The unique expression profile of PSMA and its constitutive internalization offer great value in the targeted delivery of therapeutics to prostate cancer cell.

In 2002, two synthetic RNA ligands, aptamers, were selected for their ability to inhibit the enzymatic activity of PSMA. In 2006, the utility of these aptamers in the delivery of cytotoxic siRNA across the cell membrane was demonstrated in vivo using aptamer-siRNA chimeras. However, those experiments were performed by intratumoral injection, and systemic administration will be necessary for use in the clinic.

In this thesis, we improve PSMA targeted chimeras to serve as more powerful therapeutics in the treatment of prostate cancer. We optimize existing aptamer-siRNA chimeras for increased potency and stability and improved pharmacokinetics to enable systemic administration. We truncate the PSMA binding aptamers for amenability to large-scale chemical synthesis. With emerging roles for PSMA enzymatic activity in the prostate cancer disease we identify aptamers that are suitable for chemical synthesis and retain inhibitory properties against PSMA. Finally, we assess the use of aptamers as synthetic ligands in the functional inhibition of PSMA mediated motility in prostate cancer.

Our results demonstrate the ability of aptamer-siRNA chimeras to specifically kill PSMA-expressing cells with cytotoxic siRNA upon systemic injection. We confirm a newly reported role for PSMA in the promotion of cell motility and demonstrate the ability of aptamers to effectively neutralize PSMA-mediated motility. The results presented within argue strongly for the functional utility of aptamers in the treatment of prostate cancer.