A Theranostic “SMART” Aptamer for Targeted Therapy of Prostate Cancer
Authors: Vittorio de Franciscis
Journal: Molecular Therapy (2014)

The development of therapeutic molecules that specifically recognize cancer cells has reinforced the hope of developing patient-tailored treatments. By combining greater efficacy with selective targeting of cancer cells, treatments aim to inhibit tumor cell growth and invasion while sparing the surrounding normal cells, thus reducing the adverse cytotoxic side effects caused by conventional chemotherapy.1Toniatti C Jones P Graham H Pagliara B Draetta G Oncology drug discovery: planning a turnaround.Cancer Discov. 2014; 4: 397-404Crossref PubMed Scopus (45) Google Scholar To date, monoclonal antibodies (mAbs) represent the largest class of therapeutic agent for the specific recognition of cell surface antigens. Several mAbs have been approved and are widely used in oncology as diagnostic tools, selective therapeutics, or delivery moieties, including targeted delivery of therapeutic agents and nanoparticles. mAbs are less toxic than conventional chemotherapeutics, but they are large, complex molecules that are expensive to produce, which has hampered a broader translation to the clinic.2Scott AM Allison JP Wolchok JD Monoclonal antibodies in cancer therapy.Cancer Immun. 2012; 12: 14-22PubMed Google Scholar A report by Dassie et al. in this issue of Molecular Therapy now provides preclinical characterization of a different kind of targeted molecule, a nucleic acid aptamer, a sort of "smart" molecule that has been shown to be a safe and effective alternative for the therapy of prostate cancer, one of the most aggressive cancers.3Dassie JP Hernandez LI Thomas GS Long ME Rockey WM Howell CA et al.Targeted inhibition of prostate cancer metastases with an RNA aptamer to prostate-specific membrane antigen.Mol Ther. 2014; 22: 1910-1922Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar Aptamers are single-chain oligonucleotides that are selected from high-complexity RNA (or DNA) pools. By assuming specific folding, aptamers tightly bind to and inhibit protein targets. Chemically modified aptamers exhibit low immunogenicity and toxicity, and an increased half-life in the circulation, making them very attractive and effective therapeutics.4Keefe AD Pai S Ellington A Aptamers as therapeutics.Nat Rev Drug Discov. 2010; 9: 537-550Crossref PubMed Scopus (1261) Google Scholar They thus represent a promising alternative to antibodies, owing to their high specificity of target recognition and the fact that animal cells are not required for their production, which is instead performed in vitro relatively rapidly and with high batch-to-batch fidelity.5Cerchia L Franciscis V Targeting cancer cells with nucleic acid aptamers.Trends Biotechnol. 2010; 28: 517-525Abstract Full Text Full Text PDF PubMed Scopus (187) Google Scholar,6Sun H Zhu X Lu PY Rosato RR Tan W Zu Y Oligonucleotide aptamers: new tools for targeted cancer therapy.Mol Ther Nucleic Acids. 2014; 3: e182Abstract Full Text Full Text PDF PubMed Scopus (377) Google Scholar Prostate cancer is the most common cancer in the male population and a leading cause of death in Western countries. Current standard therapies include surgery, radiation therapy, and adjuvant hormone therapy. These approaches are somewhat effective in the early stages of disease, when cancer is still confined to the prostate gland. However, a significant proportion of patients relapse and rapidly progress to advanced metastatic castration-resistant prostate cancer (mCRPC). Upon diagnosis, the patient with mCRPC has a mean survival time of 12–18 months, and no curative treatment exists. The therapeutic compounds currently used in the clinic are taxanes, which act by inhibiting mitotic cell division. They are often used in combination with steroids, such as prednisone. The high toxicity of these compounds prevents their prolonged use, however, and there is a clear need for more effective and safer therapeutic options for mCRPC. In this respect, the selective targeting of prostate cancer cells has recently become a major challenge. To address the need, several new compounds targeting neoangiogenesis and restoring the response of immune cells have been recently developed, and protocols for their use in combination have been established.7Conway RE Petrovic N Li Z Heston W Wu D Shapiro LH Prostate-specific membrane antigen regulates angiogenesis by modulating integrin signal transduction.Mol Cell Biol. 2006; 26: 5310-5324Crossref PubMed Scopus (150) Google Scholar,8Cereda V Formica V Massimiani G Tosetto L Roselli M Targeting metastatic castration-resistant prostate cancer: mechanisms of progression and novel early therapeutic approaches.Expert Opin Investig Drugs. 2014; 23: 469-487Crossref PubMed Scopus (17) Google Scholar On the other hand, the prostate-specific membrane antigen (PSMA) is the most prevalent prostate cancer cell biomarker. It is a 100-kDa transmembrane glycosylated protein endowed with NAALADase/glutamate carboxypeptidase II activity that is expressed on the surface of prostate epithelial cells and in the neovasculature of many solid tumors.9Ghosh A Heston WD Tumor target prostate specific membrane antigen (PSMA) and its regulation in prostate cancer.J Cell Biochem. 2004; 91: 528-539Crossref PubMed Scopus (591) Google Scholar In normal cells, PSMA is poorly expressed, mainly as a soluble splice variant in the cell cytoplasm. The levels of expression of PSMA are high in almost all prostate cancer cells and further increased in the later stages of the disease, and are therefore associated with more aggressive tumors and circulating tumor cells of the prostate. Aptamers, mAbs, and peptides have been developed for targeted delivery of drugs or imaging agents to PSMA-expressing cells.8Cereda V Formica V Massimiani G Tosetto L Roselli M Targeting metastatic castration-resistant prostate cancer: mechanisms of progression and novel early therapeutic approaches.Expert Opin Investig Drugs. 2014; 23: 469-487Crossref PubMed Scopus (17) Google Scholar,10Dassie JP Liu XY Thomas GS Whitaker RM Thiel KW Stockdale KR et al.Systemic administration of optimized aptamer–siRNA chimeras promotes regression of PSMA-expressing tumors.Nat Biotechnol. 2009; 27: 839-849Crossref PubMed Scopus (476) Google Scholar,11Olson WC Olson WC Antibody–drug conjugates targeting prostate-specific membrane antigen.Front Biosci (Landmark Ed). 2014; 19: 12-33Crossref PubMed Scopus (17) Google Scholar A radiolabeled antibody specific to PSMA (mAb 7E11) is routinely used in clinical practice to target PSMA-positive prostate cancer cells (ProstaScint scan) and has thus become an invaluable tool to monitor the extent of disease. However, despite recent evidence implicating PSMA in matrix degradation and angiogenesis, the function of PSMA activity in tumor development, invasion, and spread is poorly defined.12Zhao LY Mao XP Chao KY Guo SJ Qiu SP Prostate-specific membrane antigen can promote in vivo osseous metastasis of prostate cancer cells in mice.Braz J Med Biol Res. 2012; 45: 737-745Crossref PubMed Scopus (6) Google Scholar,13Baum V Bühler P Gierschner D Herchenbach D Fiala GJ Schamel WW et al.Antitumor activities of PSMA×CD3 diabodies by redirected T-cell lysis of prostate cancer cells Immunotherapy.. 2013; 5: 27-38Google Scholar In the new study, Dassie et al. explored the possibility of exploiting the high binding specificity of an anti-PSMA aptamer as a "smart drug" to target prostate cells and mCRPC. Two anti-PSMA aptamers dubbed A9 and A10, developed approximately a decade ago by Coffey and colleagues,14Lupold SE Hicke BJ Lin Y Coffey DS Identification and characterization of nuclease-stabilized RNA molecules that bind human prostate cancer cells via the prostate-specific membrane antigen.Cancer Res. 2002; 62: 4029-4033PubMed Google Scholar have been widely used as targeting agents for prostate-specific delivery of imaging or therapeutic compounds. A9g is a truncated version of A9 that shows enhanced binding affinity to PSMA and, importantly, can inhibit its NAALADase/glutamate carboxypeptidase II activity with an IC50 of 10 nM (ref. 15Rockey WM Hernandez FJ Huang SY Cao S Howell CA Thomas GS et al.Rational truncation of an RNA aptamer to prostate-specific membrane antigen using computational structural modeling.Nucleic Acid Ther. 2011; 21: 299-314Crossref PubMed Scopus (87) Google Scholar). Dassie and colleagues used the A9g aptamer, a 2′-F modified RNA, to inhibit PSMA enzymatic activity in prostate cancer cells and showed that it strongly inhibits cell migration and invasion.3Dassie JP Hernandez LI Thomas GS Long ME Rockey WM Howell CA et al.Targeted inhibition of prostate cancer metastases with an RNA aptamer to prostate-specific membrane antigen.Mol Ther. 2014; 22: 1910-1922Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar Inhibition of the enzymatic activity is required for inhibition of cell migration; an inert aptamer (A10-3.2) had no effect. The authors then developed a mouse model of metastatic prostate cancer characterized by high osteotropism. PSMA-positive, luciferase-expressing prostate cancer cells (22Rv1(1.7)) were injected into the left ventricle of the mouse heart, and, as in human disease, 80–90% of the metastases localized to bone (Figure 1). Treating animals with A9g by systemic administration significantly reduced the spread of prostate cancer cells in this model, and only 10% of mice developed bone metastases, thus showing the efficacy of aptamer treatment. The RNA aptamer was also safe; there were no changes in the appearance, weight, or behavior of treated animals over the 4 weeks of treatment with the A9g aptamer. Short nucleic acid–based compounds such as short interfering RNA or aptamers can activate the innate immune system by a mechanism that is dependent on the nucleic acid composition, as well as the structure and type of cell exposed to the agent. Typically, 2′-F modified RNA-based aptamers have been shown to be poor activators of innate immunity.4 Indeed, Dassie et al. report stable levels of inflammatory cytokines (interleukin-6), types I (IFN-β) and II (IFN-γ) interferons and viral RNA recognition genes (OAS-1, IFIT1) in spleen and liver (through 0.5 to 24 hours) upon systemic administration of A9g aptamer in immunocompetent mice. An important advantage of aptamers is the great ease of chemical modification that facilitates conjugation to various molecules, including fluorophores and radiopharmaceuticals. To determine the biodistribution and pharmacokinetic properties of A9g, Dassie et al. coupled the aptamer to the near-infrared (NIR) fluorophore IRDye 800CW. NIR-labeled aptamers were then injected into the tail vein of mice bearing PC-3 (PSMA-positive) tumors and monitored over 72 hours using bioluminescence imaging. RNA aptamers (A9g and the nonbinding A9g.6) were found uniformly distributed throughout the body by 5 minutes but were rapidly cleared from circulation. Importantly, A9g was retained in the PSMA-positive tumor mass up to 72 hours after treatment, whereas the control A9g.6 remained undetectable. Furthermore, by using xenograft mice bearing PSMA-positive and PSMA-negative tumors implanted subcutaneously on both flanks of the same animal, the authors found that NIR-A9g localized to the PSMA-positive tumor in a selective manner, because no signal was detected in PSMA-negative tumors grafted in the same animal. Therefore, by ruling out any mechanism of nonspecific accumulation of macromolecules in tumor tissues, the results demonstrate the therapeutic efficacy of an anti-PSMA aptamer for advanced prostate cancer. These findings obtained in a prostate model indicate the use of aptamers as an alternative path for the generation of more effective and safer therapeutics in oncology. Given the high versatility of aptamers, the results offer a blueprint for the design of theranostic molecules able to selectively target cancer cells and their metastases in advanced-stage tumors or even as an intraoperative treatment. Further preclinical studies should verify in vivo the advantages of aptamers as theranostic "smart" drugs.