GT Biopharma, Inc. announced preclinical data to be presented at the hybrid 48thEuropean Society for Blood and Marrow Transplantation Annual Meeting (EBMT). The poster presentation titled, "Tri-specific Killer Engager (TriKE®) against B7-H3 enhances NK cell mediated killing of multiple myeloma," is presented by Aimee Merino, MD, PhD, Assistant Professor of Medicine, Division of Hematology, Oncology and Transplantation at the University of Minnesota. GTB-5550 (B7-H3 TriKE) is the Company's tri-specific killer engager (TriKE) with camelid single-chain Fv fragments against B7-H3 (CD276) and CD16 linked by IL-15 to enhance NK cell killing of myeloma.

GTB-5550 is part of GT Biopharma's portfolio of lead TriKE product candidates being investigated as a mono- and combination therapy against multiple myeloma. Background - Natural Killer (NK) cell-based therapies hold great promise in treating multiple myeloma. One method to enhance NK cell specificity against myeloma is antibody dependent cellular cytotoxicity through a CD16 receptor.

B7-H3 (CD276) was targeted as its expression in myeloma is associated with decreased progression free survival, it exhibits low expression on healthy tissue, and it is expressed on myeloid derived suppressor cells (MDSC), which promote myeloma growth. Study design and analysis – The study compared the ability of peripheral blood NK cells with or without GTB-5550 to kill myeloma cells in live imaging IncuCyte Zoom assays with escalating doses of TriKE. Maximal killing occurred with 3 nM concentration.

Testing was performed across four different myeloma cell lines (H929, MM1S, RPMI-8226, U266). In the study, the efficacy of GTB-5550 was also tested in combination with the proteasome inhibitor bortezomib (10 nM) and the immunomodulatory drug lenalidomide (5 mM). Cytotoxicity curves were compared by repeated measures ANOVA and performed in triplicate.

Statistically significant increase in NK cell mediated killing across all lines when 3nM B7-H3-TriKE was added. Against U266 and MM1S, B7-H3-TriKE significantly enhanced killing at effector:target (E:T) ratios of 2:1 and 4:1. RPMI-8226 showed relatively high resistance to NK cell cytotoxicity but B7-H3-TriKE enhanced killing at E:T of 4:1. H929 cells were more potently killed in the presence of B7-H3-TriKE at E:T of 2:1 but there was no difference in killing at E:T 4:1 likely due to high natural cytotoxicity in both groups. Combination therapy with GTB-5550, NK cells, and lenalidomide showed synergistic killing of H929 cells after 48 hours of live cell imaging (p=0.047) but combination with bortezomib did not further enhance killing as compared to NK cells and TriKE alone.

Both lenalidomide and bortezomib showed a trend toward improved killing against MM1S when given with NK cells and B7-H3 TriKE but it did not reach statistical significance. Combination therapy with B7-H3-TriKE, NK cells, and lenalidomide or bortezomib showed synergistic killing of RPMI-8226 cells after 48 hours of live cell imaging (p<0.001 and 0.015 respectively). Bortezomib combined with GTB-5550 and NK cells enhanced killing in U266 cells (p=0.037).