The medical community is in the midst of a war on cancer. As we gain more intelligence about cancer cells and the tactics they use to survive, we are developing new strategies. Aptamers are one of the newest weapons in our arsenal. Aptamers can be used to defend the immune system from tactics of evasion, to actively trigger immune response, and to target delivery of therapeutics to cancer cells.
Evasion of the Immune System: Aptamers Fight Back
The immune system is a sophisticated network of cells and signaling molecules designed to detect and eliminate threats. In many cancers, tumor cells have developed ways to manipulate the host’s own immune system to suppress immune activity or disguise themselves to avoid detection. In some cases, tumor cells send immunosuppressive signals, limiting the hosts ability to attack cancer cells. Researchers in France used HSP70-specific aptamers to block the interaction of HSP70-expressing, tumor-derived exosomes with toll-like receptors (TLR2) on myeloid-derived suppressive cells (MDSC), blocking the activation of MDSCs and decreasing imunosuppression. (3) Some tumor cells have been shown to assume control of T regulatory cells (Treg), immunosuppressive cells, to evade an immune response. Researchers in Spain combined a CD28-targeting aptamer with a P60 Foxp3 inhibitor peptide for targeted reduction of Treg immunosuppressive activity. (5)
Triggering Immune Response
One tactic in the fight against cancer involves helping the immune system to identify cancer cells by increasing tumor immunogenicity. Nonsense-mediated mRNA decay (NMD) is a cellular quality control mechanism designed to degrade incomplete mRNAs. (1) Researchers in Spain combined an anti-CD40 aptamer with a shRNA that inhibits SMG1, an important kinase in MSD. The result is the production and expression of nonsense proteins on the cell surface of CD40+ cells and enhanced immunogenicity. (8) In a case of direct immune activation, researchers in Korea developed RIG-1 aptamers that increase RIG1-mediated IFN beta production. Activation of this immune pathway was shown to inhibit viral replication when tested with Newcastle disease virus (NDV) and Vesicular stomatitis virus (VSV). (4)
Cell Targeting in Immunotherapy
Aptamers selective for specific cancer cells can facilitate targeted delivery of a wide range of therapeutic agents, including antibodies, toxins, siRNAs, and aptamers. Researchers in Jiangsu, China utilized aptamers for targeted delivery of the chemotherapeutic drug doxorubicin in mice. Using whole cell SELEX, the researchers identified an aptamer with high affinity and selectivity for the 4T1 mouse mammary breast cancer cell line. By conjugating 4T1 cell-specific aptamers to doxorubicin-loaded liposomes, they increased internalization of doxorubicin by 4T1 cancer cells, increased tumor suppression, and reduced toxicity to normal cells. (7) An aptamer conjugate consisting of a VEGF-specific targeting aptamer and an agonistic 4-1BB aptamer showed an improved therapeutic index and reduced toxicity compared to a 4-1BB antibody therapeutic. (6,2) In several studies, aptamer-based, targeted drug delivery has been shown to increase drug efficacy, lower required dose, and decrease side effects.
Many antibody-based immunotherapies are associated with toxicity that limits dosage and effectiveness. While aptamers themselves can be powerful cancer-fighting agents offering reduced immunogenicity and enhanced cellular access, the use of aptamers as combinational therapies designed to block immune suppression, boost the natural immune response, and deliver targeted treatment appear to be winning strategies in the war on cancer.
Image of cytotoxic T cells attacking an oral squamous cancer cell courtesy of Rita Elena Sarda, National Cancer Institute \ Duncan Comprehensive Cancer Center at Baylor
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2. Gilboa, E. et al. (2015). Reducing toxicity of immune therapy using aptamer-targeted drug delivery. Cancer Immunol Res; 3(11).
3. Gobbo, J. et al. (2016). Restoring anti-cancer immune response by targeting tumor-derived exosomes with a HSP70 peptide aptamer. JNCI Natl. Cancer Inst. 108(3). Doi: 10.1093/jnci/djv330.
4. Hwang, S. et al. (2012). 5′-Triphosphate-RNA-independent activation of RIG-I via RNA aptamer with enhanced antiviral activity. Nucleic Acids Research. 40(6) :2724-2733.
5. Lozano, T. et al. (2016). Targeting inhibition of Foxp3 by a CD28 20-Fluro oligonucleotide aptamer conjugated to P60-peptide enhances active cancer Immunotherapy. Biomaterials. 91:73-80. Dx.doi.org/10.1016/j.biomaterials.2016.03.007
6. Schrand B, et al. (2014). Targeting 4–1BB costimulation to the tumor stroma with bispecific aptamer conjugates enhances the therapeutic index of tumor immunotherapy. Cancer Immunol Res. 2:867–77.
7. Song, X. et al. (2015). Targeted delivery of doxorubicin to breast cancer cells by aptamer functionalized DOTAP/DOPE liposomes. Oncology Reports. Pages 1953-60. Doi.org/10.3892/or.2015.4136.
8. Soldevilla, M. M. et al. (2015). 2-fluoro-RNA oligonucleotide CD40 targeted aptamers for the control of B lymphoma and bone-marrow aplasia. Biomaterials. 67:274-285.