Background CEA (CEACAM5) is a well-validated cell-surface antigen that is highly expressed in multiple solid tumors. Bolt's immune-stimulating antibody conjugates (ISACs) direct a TLR7/8 agonist into tumors to activate tumor-infiltrating myeloid cells and initiate a broad innate and adaptive anti-tumor immune response.1 The favorable properties of CEA, including robust cell surface expression, low internalization rate, and limited normal tissue expression, suggest that the antigen may be a suitable ISAC target. We are evaluating an anti-CEA ISAC, BDC-2034, as a multi-functional approach to treat CEA-expressing cancers.
Methods Anti-CEA antibodies were tested for binding affinity and specificity, CEA-targeted antibody-dependent cellular phagocytosis (ADCP), and myeloid-mediated tumor cell killing. Selected antibodies were conjugated to proprietary TLR7/8 agonists, and the resulting CEA ISACs were evaluated for in vitro myeloid activation and in vivo efficacy against xenograft tumors.
Results Antibody CEA1 binds to the CEA protein with high affinity (EC50 = 0.25 nM), binds selectively to CEA-positive tumor cell lines, and mediates ADCP more efficiently than a reference anti-CEA antibody, labetuzumab (figure 1). We generated BDC-2034 by conjugating a potent TLR7/8 agonist to CEA1. BDC-2034 tumor cell binding drives myeloid effector cell ADCP, agonist delivery to TLR7 and TLR8 in endosomes, and secretion of cytokines critical for innate and adaptive immunity (including IL-12p70, CXCL10, and TNFa). In the HPAF II + cDC co-culture model, IL-12p70 is induced with EC50 = 1.2 nM, and the level of induction is at least ten-fold higher than with ISACs using labetuzumab (figure 2). Potent cellular activity is strictly dependent on tumor cell CEA expression; in whole blood, in the absence of CEA-expressing tumor cells, cytokine induction was only observed at approximately 100-fold higher concentrations. BDC-2034 inhibits the growth of HPAF II xenograft tumors in SCID/beige mice with a minimal efficacious dose (MED) of 1 mg/kg, demonstrating anti-tumor activity solely through innate immune activation (figure 3). The TLR7/8 agonist in BDC-2034 has relatively poor activity in mice; a surrogate CEA1 ISAC with a mouse TLR7-activating agonist achieved MED = 0.5 mg/kg in the HPAF II model, with eradication of all tumors at the 5 mg/kg dose.
Conclusions These pre-clinical data demonstrate the potential of BDC-2034 to treat CEA-expressing human cancers. Most importantly, the antigen-dependent induction of immune-stimulating cytokines promises a robust immune response that combines the activation of innate and adaptive arms.
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