Targeted Delivery of TLR7 Agonists to the Tumor Microenvironment Enhances Tumor Immunity via Activation of Tumor-Resident Myeloid Cells

Toll-like receptors (TLR) are phylogenetically conserved mediators of innate immunity that are essential for establishing adaptive immune responses against invading pathogens. TLR7 is an endosomal receptor expressed predominantly in myeloid and B cells. Activation of TLR7 induces Type I interferon and proinflammatory responses; therefore, targeting TLR7 is a promising strategy for antitumor therapy. Although the use of bacterial components to trigger innate immune responses in cancer patients started a century ago, the effectiveness of systemic TLR agonists has been rather underwhelming in clinical trials, partly due to nonspecific immune activation leading to safety and tolerability issues. Antibody–drug conjugates (ADCs) constitute a proven therapeutic modality amenable to systemic administration with limited toxicity concerns via a targeted delivery platform. We generated TLR7 agonist–antibody conjugates that recognize tumor antigens expressed on the surface of tumor cells. Generated ADCs demonstrated robust activity in in vitro tumor antigen-presenting cell (APC) coculture systems as indicated by dose-dependent upregulation of PD-L1 and CD86 on macrophages. TLR7 agonist-ADC provided superior tumor growth control compared to intravenously (IV) administrated free TLR7 agonist. Treatment with TLR7 agonist-ADC led to prolonged activation of myeloid cells in the tumor microenvironment (TME) with minimum immune activation in the periphery. Systemic and tissue exposure studies demonstrated tumor-specific free drug release by targeted ADC treatment. In summary, the TLR7 agonist-ADC can potentially activate immune cells in the TME to generate tumor antigen-specific T-cell responses, making it an attractive approach for precision cancer therapy.