Abstract 915: Preclinical evaluation of anti-CD37 antibody-targeted amanitin conjugates (ATAC) in B-cell malignancies
Authors: Voss Stephanie , Palfi Aniko , Mueller Christoph , Pahl Andreas , Hechler Torsten , Kulke Michael 
Published: July 2021

Abstract
Background:
ATACs (antibody-targeted amanitin conjugates) comprise a new class of antibody-drug conjugates (ADCs) using amanitin as a toxic payload. Amanitin binds to the RNA pol II and thereby efficiently inhibits the cellular transcription process. In the current study, in vitro and in vivo data of ATACs targeting CD37 (member of the tetraspanin family) are presented. CD37 is a transmembrane protein expressed exclusively on cells of the immune system and mainly on mature B-cells, as well as in many B-cell malignancies, including Richter's Syndrome (RS) or Richter´s transformation. RS is a transformation of B cell chronic lymphocytic leukemia, refractory to treatment and carries a poor prognosis, and hence is considered an ideal target for amanitin-based ADCs.
Material and methods:
Cell lines: CD37-positive: Mec-2 (B-chronic lymphocytic leukaemia); Raji-Luc, Ramos (Burkitt´s lymphoma); CD37-negative: HL-60 (acute lymphocytic leukemia)
Antibody: anti-CD37 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.
Cell proliferation assay: Quantitative determination of cell viability was performed by CellTiter Glo 2.0 assay (Promega).
Animal models: Disseminating Mouse xenograft tumor models (Mec-2 and Raji-Luc) and CD37-positive patient derived xenograft (PDX; Richter's Syndrome) models were performed in single-dose experiments. Tolerability was assessed in non-human primates (NHP).
Results:
All tested anti-CD37 ATACs showed in vitro cytotoxicity in the picomolar range on CD37-positive cell lines. No cytotoxic activity was observed on CD37-negative cells.
In mouse xenograft models, 80 - 100% overall survival was achieved with two anti-CD37 ATACs at doses of 1/16 MTD (Mec-2 model) and 1/64 MTD (Raji Luc Model). Single-dose treatment caused rapid and complete tumor remission already 7 days after treatment.
The tested ATACs revealed good tolerability in mice.
In NHP, the anti CD37 ATACs revealed good tolerability. Hematology and clinical chemistry parameters were unaffected except liver transaminases and LDH: A mild to moderate and transient increase was observed.
Conclusions:
Targeted cytotoxic drug delivery to CD37 positive cell lines was achieved by using anti-CD37 ATACs. The mode of action of the payload amanitin led to an efficient anti-tumor potential in vitro and in vivo with good tolerability in non-human primates. Using ADCs in the therapy of B-cell lymphomas, including malignancies that underwent Richter's transformation, is a promising approach, especially by using a payload whose mode of action differs from other commonly used toxins, like ATACs.
Citation Format: Stephanie Voss, Aniko Palfi, Christoph Mueller, Andreas Pahl, Torsten Hechler, Michael Kulke. Preclinical evaluation of anti-CD37 antibody-targeted amanitin conjugates (ATAC) in B-cell malignancies [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 915.