Abstract 664: Amanitin-based antibody-drug conjugates targeting the prostate-specific membrane antigen

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 maytansines. 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 a monoclonal antibody targeting the prostate-specific membrane antigen (PSMA) conjugated to small molecules from the amatoxin family. PSMA is a membrane antigen overexpressed in prostate cancer and an attractive target for an ADC approach, as it shows low expression by most normal tissues and sufficient internalization after antibody binding. 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 several random- and site-specific strategies to covalently conjugate amanitin to the antibody and generated conjugates with low aggregation and high affinity for the target antigen. Using a series of PSMA-expressing cells we compared the cytotoxic activity of stable and cleavable linker ADCs and the stability of such constructs in plasma. Overall we observed picomolar activity of ADCs after incubation for three to five days with PSMA-positive prostate cancer cells independent of the hormone-sensitivity status. Moreover we demonstrated high activity of amanitin-based anti-PSMA ADCs in prostate cancer xenograft models. The data encourage the evaluation of these agents in a clinical advanced prostate cancer study. Note: This abstract was not presented at the meeting. Citation Format: Torsten Hechler, Michael Kulke, Christoph Mueller, Andreas Pahl, Jan Anderl. Amanitin-based antibody-drug conjugates targeting the prostate-specific membrane antigen. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association