Abstract 237: Preclinical evaluation of anti-HER2 Antibody Targeted Amanitin Conjugates (ATACs) on HER2low breast cancer with chromosome 17p deletion (2019)

Background: Triple negative breast cancer (TNBC) is the most difficult to treat subtype of breast cancer with limited therapeutic options. At least 50% of TNBC patients have low epidermal growth factor receptor 2 (HER2; ERBB2) expression with the majority harboring hemizygous loss of POLR2A/chromosome 17p. For these patients the treatment with antibody-targeted amanitin conjugates (ATACs) targeting HER2 is a new promising approach. ATACs comprise a new class of antibody-drug conjugates (ADCs) using amanitin as toxic payload and are able to kill antigen low expressing cells. Amanitin binds to the eukaryotic RNA polymerase II and thereby efficiently inhibits the cellular transcription process. In the current study, in vitro and in vivo data of ATACs targeting human HER2low as well as tolerability studies are presented. HER2low TNBC is considered an interesting target for amanitin-based ADCs. Material and methods: Different HER2 expressing cell lines were treated with anti-HER2 ATACs. Cysteine reactive amanitin-linkers were conjugated site-specifically to engineered cysteine residues of an anti-HER2 antibody yielding ATACs with a DAR of 2.0. Quantitative determination of cell viability was analyzed by BrdU ELISA assay. Subcutaneous mouse xenograft models with HER2-positive cell lines were performed with single-dose treatments. In addition, ATAC efficacy was tested in HER2low heterogeneous TNBC patient derived xenograft (PDX) models with and without POLR2A deletion. Tolerability of ATACs was assessed in mice and non-human primates (NHP). Results: Anti-HER2 ATACs showed in vitro cytotoxicity on HER2 + high and low cell lines in low nanomolar to picomolar range. In mouse xenograft models, the anti-HER2 ATACs caused dose-dependent tumor regression independent of Her2 expression level. In HER2low heterogeneous TNBC PDX models anti-HER2 ATACs caused dose-dependent tumor regression. The efficacy of anti-Her2 ATACs was more pronounced in PDX models with hemizygous loss of TP53 and POLR2A reflecting a 17p deletion. Safety profiling of an optimized anti-Her2 ATAC in cynomolgus monkeys revealed a good tolerability indicating a good therapeutic window for 17p deleted TNBC. Conclusions: Targeted cytotoxic drug delivery to HER2 positive cell lines was achieved by using anti-HER2 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 NHP studies. TNBC PDX models with HER2low expression were sensitive to ATAC treatment. Loss of POLR2A/chromosome 17p increased susceptibility to anti-HER2 ATAC making 17p del TNBC a suitable indication for optimized anti Her2 ATACs. Citation Format: Christian Breunig, Aniko Palfi, Michael Kulke, Christian Lutz, Christoph Muller, Torsten Hechler, Andreas Pahl. Preclinical evaluation of anti-HER2 Antibody Targeted Amanitin Conjugates (ATACs) on HER2low breast cancer with chromosome 17p deletion [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 237.