Abstract 335: c-KIT targeted ETBs for cancer therapy and HSC transplant conditioning

Abstract Engineered toxin bodies (ETBs) are next-generation immunotoxins that harbor an antibody-derived targeting domain and a cytotoxic payload derived from Shiga-like toxin-1 catalytic subunit A (SLTA). SLTA has been engineered to reduce innate immunogenicity and therefore increase the safety of ETBs whilst retaining potent cytotoxic properties. When targeted via a binding domain, SLTA is internalized and routes to the cytosol leading to irreversible ribosome inactivation and ultimately cell death. ETBs have been developed to treat a wide variety of cancers, including breast, lymphoma, multiple myeloma and PD-L1 positive solid tumors. This technology holds promise for non-oncology indications as well, particularly as a targeted and non-genotoxic conditioning regimen for hematopoietic stem cell (HSC) ablation to prepare patients for autologous stem cell transplant. c-KIT (CD117) is a well-known marker of hematopoietic stem and progenitor cells and is overexpressed in a high percentage of certain cancers including GIST, SCLC and AML. While tyrosine kinase inhibitors such as imatinib are effective therapies for c-KIT mutant GIST, resistance often occurs by the development of secondary mutations in the intracellular signaling domains. Thus, c-KIT represents a potential ETB target for both oncological and HSC transplant conditioning indications. Here we present data highlighting the in vitro potency and efficacy of CD117-targeting ETBs on cancer cell lines as well as on primary human CD34+ HSCs. CD117-targeted ETBs demonstrate exquisite specificity in vitro by killing only target positive CD34+ cells at picomolar potency while sparing the progenitors that lack CD117 expression. However, relatively low CD117 receptor levels on CD34+ HSCs prevent complete killing in vitro, limiting the observed efficacy. To overcome this challenge, an additional cytotoxic payload was conjugated to the ETB molecule to capitalize on ETBs’ unique internalization and routing properties to deliver a secondary mechanism of action. ETB-drug conjugates (ETB-DCs) exhibited improved cytotoxicity in vitro, especially in CD117-low target cells. Moving forward, we plan to explore both the ETB and the ETB-DC therapeutic index in vivo with a series of CD117+ tumor efficacy and HSC depletion models. Citation Format: Caleigh Howard, Shu Wiley, Wenzhao Dong, Andrea Mendiola, Veronica Partridge, Sara LeMar, Paul Amador, Amit K. Chaudhary, Joseph D. Dekker, Jay Zhao, Ross Durland, Aimee Iberg. c-KIT targeted ETBs for cancer therapy and HSC transplant conditioning [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 335.