Abstract 633: Amanitin-based ADCs with an improved therapeutic index

Abstract Antitumoral activity of monoclonal antibodies can be dramatically enhanced by conjugation to toxic small molecules. Beside the recent approval of Kadcyla (T-DM1) and Adcetris (SGN-35) more than 30 antibody-drug conjugates (ADC) have entered clinical trials, promising to strengthen the therapeutic capabilities for cancer treatment in the next decade. Surprisingly most ADCs are based on one of few toxic compounds only and on an even smaller number of toxicity mechanisms: Most antibodies are coupled to the microtubuli-targeting auristatins and maytansins. Toxins that operate through such a mechanism could suffer from limited activity in different cancer indications and in cells expressing resistance mechanisms. Accordingly the use of new drugs that function via alternative toxicity mechanisms could enhance the therapeutic potential of ADCs. In the present study we evaluated the antitumoral potency of monoclonal antibodies conjugated to small molecules from the amatoxin family. Amanitin, the most well-known toxin of the amatoxin family, binds to the eukaryotic RNA pol II and thereby inhibits the cellular transcription at very low concentrations. In our experiments, we tested random- and site-specific strategies to covalently conjugate amanitin to antibodies and generated conjugates with low aggregation and high affinity for the target antigen. We compared the cytotoxic activity of stable and cleavable linker ADCs and the stability of such constructs in plasma. Overall we observed high stability in plasma after incubation for four days. Moreover we demonstrated superior efficacy of amanitin-based ADCs in cancer xenograft models compared to tubulin-inhibiting toxins. For site-specific conjugation antibodies were engineered at different locations to incorporate cysteines (Thiomabs). Thiomabs with different positions of the engineered cysteine were compared in vitro and in vivo to identify the best position for amanitin-thiomab-conjugates. Safety profiling of the two best Thiomabs in cynomolgous monkey revealed a substantial in improvement in tolerability and of the therapeutic index by using site-specific engineered amanitin-ADCs. The data support the development of amanitin-based ADCs by using a toxin with a new mode of action and with a favorable therapeutic index. Citation Format: Torsten Hechler, Christoph Müller, Andreas Pahl, Jan Anderl. Amanitin-based ADCs with an improved therapeutic index. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 633. doi:10.1158/1538-7445.AM2015-633