Abstract 1542: Generation of an antibody-drug conjugate-optimized TLR 7/8 agonist payload

Ligation of toll-like receptors 7 and 8 (TLR7/8) can potently activate innate immune cells, including tumor-associated macrophages, to prime downstream T cell activation and drive potent, lasting anti-tumor immunity. TLR7/8 agonists have long been pursued as an anti-cancer therapeutic because of their potential in re-programing the immune system but the clinical utility has been limited by systemic toxicity. Several means of decreasing this systemic toxicity have been investigated, including intra-tumoral administration, nanoparticle encapsulation and conjugation to a tumor-targeting antibody (antibody-drug conjugates, ADCs). ADCs are a clinically validated technology designed to target drugs to disease tissues to reduce the systemic toxicity of highly potent payloads and improve anti-tumor activity. We have developed an imidazoquinoline-based dual TLR7 and TLR8 small molecule agonist that has been specifically designed as an ADC payload. The initial compound was chosen from a set of imidazoquinoline-based small molecules using HEK293 reporter cells expressing either TLR7 or TLR8. This newly identified TLR7/8 agonist potently reactivated immunosuppressive macrophages to produce inflammatory cytokines, increased phagocytosis of tumor cells and enhanced T cell activation and proliferation. A lead payload candidate based on a modified version of the initial TLR7/8 agonist was designed to decrease drug permeability to minimize non-targeted systemic immune activation. The lead TLR7/8 agonist was shown to be significantly less potent than the first-generation compound as a small molecule but demonstrated enhanced immune-stimulating capability when conjugated to a direct immune-targeting antibody. The increased potency as an ADC is hypothesized to be driven by enhanced intracellular retention of the less cell permeable payload and/or a higher binding affinity to the TLR7 or 8 receptors, which was predicted via modeling. The greater in vitro potency of the lead payload was also observed in vivo in an MC38 syngeneic tumor model where 33% complete tumor cures were observed with the lead versus only tumor delay with the permeable version. Drug linkers employing the lead payload were evaluated by varying payload linkage chemistry, drug linker hydrophobicity, and drug release mechanisms using in vivo anti-tumor activity in different syngeneic mouse tumor models as a readout. The optimized drug linker design increased the in vitro and in vivo potency up to 4-fold when c