Background Induction of immunogenic cell death (ICD) and activation of the cyclic GMP-AMP synthase stimulator of interferon gene (cGAS-STING) pathway are two potent anticancer immunotherapeutic. Activation of the cGAS-STING signaling pathway induces the expression of type I interferons and proinflammatory cytokines, culminating in promotion of a robust adaptive antitumor immunity. As part of this innate immune signaling pathway, STING is ubiquitously expressed in immune and nonimmune cells. Immune-stimulating antibody conjugates (ISAC) combining tumor-targeting monoclonal antibodies with STING agonist allow localized delivery of immune activators into tumors, downstream priming T cells activation and resulting as well in a higher Therapeutic Index (TI) in the clinic. Here we describe HER2-ISAC, designed to selectively deliver our currently phase I clinical drug DN015089 (CTR20212462) to HER2+ cells through ADC format. This approach enabled systemic delivery and enhanced prolonged drug exposure to tumor microenvironment (TME). The activation of tumor-specific cellular immunity by STING agonists can also amplify ICD-triggered immune responses and enhance the infiltration of immune cells into the tumor, led to enhanced inhibition of antigen-negative tumor and long-term immunological memory.

Methods DN028073 (STING agonist conjugated to an anti-HER2 mAb by TME-sensitive cleavage linker) was evaluated in preclinical in vitro and in vivo systems to characterize potency, stability, Fc-related mechanism, and antitumor activity alone and in combination with a-PD-1 treatment.

Results HER2-mediated delivery of DN028073 triggered dose dependent activation of the STING signaling pathway and STING induced gene expression, as well as robust activation of innate and adaptive immune activity both in vitro and in vivo. Following systemic administration of DN028073 in preclinical syngeneic mouse studies, divers cytokines exposure was monitored in plasma and in tumor. Compared with intratumorally administration of DN015089, the expression of chemokines was prolonged, leading to better downstream T-cell activation. In CT-26 tumor bearing immunocompetent mice, treatment with DN028073 (1mg/kg) treatment alone achieved strong antitumor activity better than treatment with Enhertu ® (10mg/kg). When combined with a-PD-1 treatment, better anti-tumor response has been achieved with a 60% CR rate.

Conclusions We aim to leverage robust immune responses to investigate the potential induction of epitope spreading against cold tumor. The activation of cGAS-STING signaling will probably contribute to tumor ICD and downstream lead to the recruitment of cytotoxic T cells (CTLs) to the tumor site. In conclusion, our study highlights the localized delivery of STING agonist to tumor may induce ICD and the immune-enhancing defense, which holds significant implications for future research and clinical applications.