Abstract 1761: Treatment with antibody-targeted amanitin conjugates induces tolerability in preclinical models without triggering tolerance

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Abstract Background: Amatoxin-based antibody-drug conjugates (ATACs) are a new class of ADCs using amanitin, a specific inhibitor of RNA polymerase II, as toxic payload. A first ATAC targeting B-cell maturation antigen (BCMA) is currently being tested in phase I/II clinical trials in multiple myeloma patients. During the clinical trials, the effect of repeated dosing on safety as well as anti-tumor efficacy of the ATAC will be assessed. In preclinical studies a reduction of toxicity was observed after repeated dosing with ATACs regardless of the target or antibody. In the present study, we demonstrate the effect of ATAC pre-treatment on tolerability in pre-clinical models. Material and Methods: Antibody: Engineered monoclonal antibodies 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: Tolerability in CB17 Scid mice and Cynomolgus monkeys; Efficacy in a subcutaneous LNCaP prostate cancer model; Treatment: ADC (ATAC): escalating dosing i.v. q21d (tolerability) or repeated dosing i.v. q7dx4 (efficacy). Results: The impact of repeated dosing on the tolerability of a non-targeting ATAC was tested in CB17 Scid mice. The mice were initially treated with the maximal tolerated dose (MTD) of an ATAC, or PBS followed by a second higher dose 21 days after the first dose. Pre-treatment improved the tolerability of a second, higher dose of the same non-targeting ATAC from 12.5% to 80% survival. As ATAC toxicity is mainly caused by liver toxicity, the liver damage markers AST, ALT and LDH are used as toxicity markers in non-human primate studies. The induction of liver damage markers observed after repeated or escalated dosing was significantly lower than the induction after single or first dose treatment. Taken together, these data indicate that ATAC treatment can induce tolerability in animal models. Furthermore, in a subcutaneous LNCaP prostate cancer model, multiple ATAC dosing did improve the anti-tumor efficacy as compared to single dose treatment, indicating that the induced tolerability is not associated with reduced anti-tumor efficacy. Conclusions: ATAC pre-treatment induces tolerability in preclinical animal models without a loss in efficacy. Since this effect of ATAC pre-treatment was observed across different animal species, it likely translates also into humans. Consequently, if reduced liver toxicity and less adverse effects after repeated dosing with ATACs are also observed in the upcoming clinical trials, it might have a significant impact on treatment regimen and clinical success of ATACs. Citation Format: Kristin Decker, Marisa Schmitt, Can Araman, Franziska Ebeling, Irina Dranova, Torsten Hechler, Anikó Pálfí, Andreas Pahl, Michael Kulke. Treatment with antibody-targeted amanitin conjugates induces tolerability in preclinical models without triggering tolerance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1761.

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