Abstract 921: Combination of antibody-targeted amanitin conjugates (ATAC) with immune checkpoint inhibitors shows synergistic therapeutic effect in vitro and in vivo
Authors: Decker Kristin , Dranova Irina , Orlik Christian , Palfi Aniko , Mueller Christoph , Singh Ram Kumar , Orlowski Robert Z. , Hechler Torsten , Pahl Andreas , Kulke Michael 
Published: July 2021

Abstract
Background:
Antibody-targeted amanitin conjugates (ATACs) are a new class of antibody-drug conjugates (ADCs) using amanitin as a toxic payload. Amanitin binds to the RNA polymerase II and thereby efficiently inhibits the cellular transcription process. The present study highlights the benefit of combining ATACs with immune checkpoint inhibitors. The combination of these two modalities leads to an induction of immunogenic cell death (ICD) in vitro and a synergistic anti-tumor effect in vivo. Hence, the combination of ATACs with immune checkpoint inhibitors (ICI) provides a promising approach for potential further cancer treatment.
Material and methods:
Cell lines: BT-474 (breast ductal carcinoma); BJAB, Raji, (Burkitt lymphoma)
Antibody: engineered monoclonal antibody produced at Heidelberg Pharma
Toxic warhead: Cysteine reactive amanitin-linker constructs were synthesized at Heidelberg Pharma and conjugated site-specifically to the antibody.
Animal models: Raji cells with/without human peripheral blood mononuclear cells (PBMCs) were implanted subcutaneously in mice. Treatment: ADC (ATAC): single dose i.v.; immune checkpoint inhibitor: Q3D x6.
Results:
The treatment of target-positive cell lines (e.g., BT-474 and BJAB) with corresponding ATACs led to the induction of three ICD hallmarks in vitro. In addition to increased surface expression of calreticulin (CRT), ATAC-treated tumor cells secreted adenosine triphosphate (ATP) and released high-mobility group box 1 protein (HMGB1). In contrast, this was not observed when the same cells were treated with a non-targeting ATAC.
In a subcutaneous CDX model, mixed with human PBMCs, the combined administration of target specific ATACs with an immune checkpoint inhibitor led to an increased anti-tumor effect indicated by significant tumor growth inhibition as compared to single treatment with ATAC or ICI. This synergistic effect was not observed in mice bearing tumors without human PBMCs.
Conclusions:
Antibody-targeted amanitin conjugates (ATACs) induced immunogenic cell death in vitro and resulted in an increased anti-tumor effect in combination with an immune checkpoint inhibitor in a subcutaneous CDX model if human PBMCs were present.
Consequently, the data presented provide a rationale to the use of ATACs in combination therapy with immune checkpoint inhibitors.
Citation Format: Kristin Decker, Irina Dranova, Christian Orlik, Aniko Palfi, Christoph Mueller, Ram Kumar Singh, Robert Z. Orlowski, Torsten Hechler, Andreas Pahl, Michael Kulke. Combination of antibody-targeted amanitin conjugates (ATAC) with immune checkpoint inhibitors shows synergistic therapeutic effect in vitro and in vivo [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 921.