Abstract 3217: Preclinical evaluation of the recombinant dendritic cell growth factor CDX-301 (Flt3L), and AST-008, a TLR9 agonist SNA
Authors: Thomas Lawrence J. , He Li-Zhen , Gergel Lauren E. , Forsberg Eric M. , Do Elizabeth Q. , Boyer James M. , Baronas April R. , Rocheleau Mallary , Borrelli Kathleen M. , Wasiuk Anna , Weidlick Jeffrey , Marsh Henry C. , Anderson Bart R. , Nallagatla SubbaRao , Kang Richard , Kandimalla Ekambar R. , Keler Tibor 
Published: August 2019

Abstract
Recent studies have highlighted the critical role of CD103+/CD141+ dendritic cells (DCs) in antitumor immunity. CDX-301 is a soluble, recombinant human FLT3 ligand (Flt3L). Flt3L is a hematopoietic cytokine which stimulates the proliferation and differentiation of various blood cell progenitors, including CD103+/CD141+ DCs. CDX-301 is in clinical development for multiple cancers and may hold significant opportunity for synergistic development in combination with other immunotherapies, in particular toll-like receptor 9 (TLR9) agonists that are known to promote maturation and activation of DCs. AST-008 is a TLR9 agonist oligonucleotide in a spherical nucleic acid (SNA) format and is in clinical development for multiple cancers in combination with pembrolizumab. SNAs are densely packed, radial arrangements of oligonucleotides around a nanoparticle core. SNAs have increased cellular uptake, nuclease stability, and affinity to targets compared with linear oligonucleotides. AST-008 induces potent TH1-type immune responses in vitro, in mice and non-human primates, and has shown potent antitumor activity as a monotherapy and enhanced checkpoint inhibitor activity in several murine tumor models. AST-008 increases tumor-infiltrating lymphocytes, interferon-inducible gene expression, and activation and expansion of CD8+ T cells with reduced T-regulatory cells in the tumor microenvironment. We examined the effects of the combination of CDX-301 and a murine version of AST-008, muAST-008, in mice on DCs and on antitumor efficacy in a tumor model. Mice were implanted with MC38 murine colon adenocarcinoma tumor cells (day 0) and animals were treated with either CDX-301 (5 μg, days 2-8 i.p.) only, muAST-008 only (1 mg/kg or 3 mg/kg, days 9, 16, 23 and 30 p.t.), or the combination of CDX-301 (5 μg, days 2-8) with muAST-008 (1 mg/kg or 3 mg/kg, days 9, 16, 23 and 30). Treatment with CDX-301 and muAST-008 showed an additive effect in retarding tumor growth and prolonging survival. To help understand the mechanisms involved in the antitumor effects, animals were treated with a similar course of CDX-301 and muAST-008 and spleens were harvested for flow cytometry evaluation 2 days after the last CDX-301 dose. For these studies, we also included a second DC activating agent, an agonist anti-CD40 mAb FGK45.5 (50 μg i.p. together with the last dose of CDX-301) among the combinations. We observed a significant increase in the percentage of CD103+CD8+ cDCs by the addition of muAST-008 to CDX-301 treatment. In addition, muAST-008 led to the up-regulation of activation markers on dendritic cells, which was markedly enhanced when combined with CD40 activation. These data demonstrate that muAST-008 leads to systemic activation of CDX-301 expanded DCs, leading to more potent anti-tumor immunity and support the potential of combining CDX-301 and AST-008 in augmenting the immunotherapy of cancers.
Citation Format: Lawrence J. Thomas, Li-Zhen He, Lauren E. Gergel, Eric M. Forsberg, Elizabeth Q. Do, James M. Boyer, April R. Baronas, Mallary Rocheleau, Kathleen M. Borrelli, Anna Wasiuk, Jeffrey Weidlick, Henry C. Marsh, Bart R. Anderson, SubbaRao Nallagatla, Richard Kang, Ekambar R. Kandimalla, Tibor Keler. Preclinical evaluation of the recombinant dendritic cell growth factor CDX-301 (Flt3L), and AST-008, a TLR9 agonist SNA [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3217.