Abstract A25: Targeting costimulation to the tumor stroma with bispecific oligonucleotide aptamers.
Authors: Schrand Brett , Brenneman Randall , Berezhnoy Alexey , Gilboa Eli 
Published: August 2014

Abstract
The paucity of costimulation at the tumor site compromises the ability of tumor-specific T cells to eliminate the tumor. The recent FDA approval of Ipilumimab, an antibody that blocks the inhibitory action of CTLA-4, has underscored the therapeutic potential of using monoclonal antibodies as ligands to stimulate protective immunity in human patients. Nonetheless, systemic administration of immunostimulatory ligands carries the risk of activating low avidity autoreactive T cells that could result in immune related pathologies. Indeed, treatment with Ipilimumab is associated with significant dose-limiting autoimmune toxicities. Similarly, treatment with an agonistic antibody that binds to 4-1BB, an immune stimulatory receptor expressed on activated CD8+ T cells, while demonstrating antitumor efficacy in cancer patients was associated with severe hepatic toxicity that led to the suspension of the clinical trial.
Arguably, targeting costimulatory ligands to the disseminated tumor lesions of the patient would reduce drug associated toxicities. We have previously shown that systemic delivery of a bispecific aptamer composed of an agonistic 4-1BB binding aptamer, the therapeutic aptamer, conjugated to a an aptamer that bound to a product expressed on the surface of a tumor cell, the targeting aptamer, led to inhibition of tumor growth, was more effective than, and synergized with, vaccination, and exhibited a superior therapeutic index compared to nontargeted costimulation with 4-1BB antibodies or 4-1BB aptamers. The chemically synthesized oligonucleotide aptamers offer significant advantages over antibodies in terms of synthesis, cost, and conjugation chemistry to generate bispecific dimeric ligands.
Nonetheless, the main current limitation of tumor targeted costimulation stems from the fact that the therapeutic cargo, the costimulatory (aptamer) ligand, has to be displayed on the surface of the targeted tumor cells. Consequently one is limited to target receptors that do not internalize upon ligand binding. While such receptor-ligand interactions exist, case in point is Rituximab, an anti-CD19 antibody that kills their targets via antibody-dependent cell cytotoxicity, most receptor-ligand complexes are internalized, and consequently the targeting choices are severely limited. Here we describe an approach that addresses this limitation and broadens the scope of tumor-targeted costimulation whereby the therapeutic aptamer ligand is targeted not to a product expressed on the surface of the tumor cells, but rather to a product that is secreted into the tumor stroma. Thus, the tumor infiltrating T cells will encounter the ligand and receive immune stimulation in advance to engaging their cognate MHC-restricted antigen on the tumor cells. In support of this approach we have shown that agonistic 4-1BB aptamers conjugated to aptamers which bind to either of two secreted products, VEGF or osteopontin, potentiated vaccine-induced protective antitumor immunity in mice that was superior to that of the non-targeted 4-1BB aptamers. Targeting secreted products will also significantly broaden the scope of tumor-targeted costimulation because secreted products like VEGF or osteopontin are broadly expressed in many tumors of disparate origin, unlike cell surface expressed products like PSMA or Her2 that exhibit a much more restricted expression pattern.
Citation Format: Brett Schrand, Randall Brenneman, Alexey Berezhnoy, Eli Gilboa. Targeting costimulation to the tumor stroma with bispecific oligonucleotide aptamers. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology: Multidisciplinary Science Driving Basic and Clinical Advances; Dec 2-5, 2012; Miami, FL. Philadelphia (PA): AACR; Cancer Res 2013;73(1 Suppl):Abstract nr A25.