DA-3501 (AT-211): Evaluation of immune-mediated antitumor activity and combination immunotherapy potential of a site-specific CLDN18.2-targeting antibody-drug conjugate (ADC)

Background: Claudin 18.2 (CLDN18.2) is a clinically validated therapeutic target in gastric and pancreatic cancers. However, acquired resistance to CLDN18.2-directed antibodies such as zolbetuximab and trastuzumab highlights the need for next-generation therapeutics with distinct mechanisms of action. DA-3501 (AT-211) is a CLDN18.2-targeting antibody-drug conjugate (ADC) developed using the AbClick® site-selective conjugation platform. The antibody component exhibits enhanced binding affinity, internalization efficiency, and molecular stability. This study aimed to evaluate the immune-mediated antitumor activity of DA-3501 and explore its potential for combination immunotherapy. Methods: To assess efficacy in resistant disease contexts, immune organoids derived from gastric cancer patients with acquired resistance to trastuzumab and zolbetuximab were used to evaluate DA-3501 activity. Antitumor effects were analyzed in immune cell co-culture systems to assess cytotoxicity, CLDN18.2-dependent responses, and immune activation markers. Furthermore, in vivo efficacy studies incorporating immune checkpoint inhibitor (ICI) combination groups were conducted under immunocompetent conditions to investigate immune-mediated mechanisms and the combinatorial potential of DA-3501. Results: DA-3501 demonstrated potent and selective cytotoxicity toward CLDN18.2-positive tumor cells and induced immune cell activation consistent with IgG1-mediated antibody-dependent cellular cytotoxicity (ADCC). In vivo, DA-3501 exhibited robust immune-mediated antitumor activity both as a monotherapy and in combination with ICIs, indicating complementary interactions between the ADC's direct cytotoxic payload and immune-modulatory mechanisms. Conclusions: DA-3501 (AT-211) is a site-selectively conjugated CLDN18.2-targeting ADC that exerts strong immune-mediated antitumor effects, even in models derived from patients resistant to existing antibody therapies. These results highlight the dual mechanism of action of DA-3501-combining direct cytotoxicity and Fc-driven immune activation-and support the development of rational ADC-immunotherapy combinations as a promising strategy in the evolving landscape of ADC-based cancer treatment.