Abstract 2984: Design and characterization of bispecific engineered toxin bodies for targeted cancer therapy

Engineered Toxin Bodies (ETBs) are a distinct class of targeted immunotoxins in development as anti-cancer therapeutics by Molecular Templates. ETBs drive a potent and targeted response mediated by antibody-like binding, induced internalization, and enzymatic ribosomal inhibition via the delivery of a Shiga-like toxin subunit A (SLTA) that has been proprietarily modified to avoid innate and adaptive immune recognition. MTEM has a pipeline of scFv targeted ETBs, including its clinical lead MT-3724. MT-3724 targets CD20 for treatment of B-cell lymphoma and has shown promising signs of activity in heavily pretreated patients. The novel MOA inherent to ETB therapeutics allows for activity in relapsed/refractory settings, as previously acquired resistance mechanisms should not hinder efficacy. To further expand the therapeutic benefit of the platform, MTEM is characterizing ETBs that are targeted through multiple binding domains. Bispecific ETBs that target two epitopes on the same receptor, or two distinct cell surface molecules both expressed on cancer cells, may allow for enhanced activity profiles. These possibilities include: (i) activity in the presence of a competitive binding protein (ii) sustained activity when one target molecule is shed or downregulated, (iii) synergistic binding events to increase overall potency, and (iv) increased specificity towards cancer over normal tissue. MTEM has performed in vitro experiments to explore the pairing of binding domains into a single fusion protein, resulting in two targeting arms and a single SLTA subunit. Included in these efforts are the design and characterization of ETBs harboring a single domain antibody derived from camelids, or VHH domains, paired in tandem with other VHH domains or with scFv binding domains to produce both bispecific and biparatopic ETBs. Bispecific ETBs have been generated to engage a variety of target combinations, relevant to both solid and hematologic cancer treatment. In vitro characterization studies conducted include cellular cytotoxicity assays and on-cell binding assays. Bispecific ETBs highlighted in this presentation demonstrate functionality of each targeting arm within a single ETB molecule. In addition, we have shown that one arm can bind target and elicit cytotoxicity even when the other target is blocked. Furthermore, we are learning that some bispecific combinations are cooperative and achieve activities in vitro not observed with the parent single-target ETBs from which they were derived. MTEM is exploring therapeutically relevant target combinations towards the development of a bispecific clinical lead. By pairing the biology and potency of ETB-mediated killing with the expanded targeting possibilities afforded by bispecific molecules, we aim to address unmet medical needs in cancer therapy. Citation Format: Aimee Iberg, Garrett L. Cornelison, Caleigh Howard, Paul Amador, Steven Rivera, Michael Jamaleddine, Garrett L. Robinson, Erin K. Willert. Design and characterization of bispecific engineered toxin bodies for targeted cancer therapy [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 2984.