Abstract 3435: GTB-5550 (cam16-IL15-camB7H3) trispecific killer engager (TriKE®) drives natural killer cell activation and antibody dependent cellular cytotoxicity against head and neck squamous cell carcinomas
Authors: Khaw Melissa , Davis Zachary , Zorko Nicholas , Berk Greg , Choy Gavin , MacMillan Margaret , Felices Martin , Miller Jeffrey S. 
Published: June 2022

Abstract
Worldwide, Head and Neck Squamous Cell Carcinomas (HNSCC) account for about 900,000 cases and 400,000 deaths. In some settings, like Fanconi anemia (FA), patients receive curative treatments (allogeneic stem cell transplantation), only to develop HNSCC in early adulthood at a high rate of incidence. Current treatment strategies for non-FA HNSCC patients include surgery, chemotherapy and radiotherapy. However, these are not viable treatment options for FA HNSCC patients due to their low tolerance for the high toxicity levels of chemotherapy and radiation. Therefore, there is a critical need for novel and targeted therapeutic interventions for the treatment of FA HNSCC patients.
B7H3, a checkpoint member of the B7 and CD28 families, is overexpressed on several solid tumors but is absent or not expressed on healthy tissues. It is a promising target for immunotherapy, and recent basket trials, particularly in the prostate cancer, have demonstrated strong clinical signals. Here we developed and tested the ability of GTB-5550, a tri-specific killer engager (TriKE) that includes a B7H3 targeting component, to direct NK cell killing to B7H3-expressing Head and Neck cancer targets. This TriKE molecule includes an NK cell engaging domain containing a humanized camelid nanobody against CD16, a camelid nanobody against B7H3 and a wild type IL-15 sequence between the two engagers. We assessed B7H3 expression by flow cytometry of wild-type HNSCC cells and a paired version with a CRISPER KO of the FANCA gene and determined that the KO had no effect on B7H3 expression. Thus, GTB-5550 activity against HNSCC should be present on both normal HNSCC and FA-HNSCC settings.
NK cell responses against HNSCC lines in the presence of GTB-5550 were assessed through either flow cytometry based functional assays, to evaluate NK cell degranulation and cytokine secretion, or IncuCyte imaging assays, to directly assess target killing. NK cell degranulation and IFN-gamma production of GTB-3550-treated samples were higher compared to that of control samples treated with B7H3 single domain or IL-15 alone. GTB-5550 also induced more HNSCC target cell killing by NK cells compared to treatment with the B7H3 single domain or IL-15 alone irrespective of the FANCA gene. Ongoing experiments will evaluate functionality of GTB-5550 on FA patient samples as well as in spheroid assays. Taken together, this data shows that a GTB-5550 is able to drive NK cell activity against B7H3-expressing HNSCC cells, which presents potential for a B7H3-targeted TriKE to be used to be implemented clinically to treat HNSCC or FA-HNSCC patients.
Citation Format: Melissa Khaw, Zachary Davis, Nicholas Zorko, Greg Berk, Gavin Choy, Margaret MacMillan, Martin Felices, Jeffrey S. Miller. GTB-5550 (cam16-IL15-camB7H3) trispecific killer engager (TriKE®) drives natural killer cell activation and antibody dependent cellular cytotoxicity against head and neck squamous cell carcinomas [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 3435.