Preclinical Efficacy of CD276 (B7-H3) CAR T in Acute Myeloid Leukemia

Immuno-therapies selectively directed against leukemia-restricted antigens (with limited to no expression in normal tissue) are desirable for optimizing efficacy without on-target off-tumor toxicity, although this goal has been elusive in acute myeloid leukemia (AML). One such tumor-restricted target is CD276 (B7-H3), an immune checkpoint inhibitor upregulated in numerous malignancies including AML. Through transcriptomic evaluation of over 2000 AML patients, we have previously characterized CD276 expression to be highly expressed in both pediatric and adult AML and absent in normal hematopoietic cells (Kirkey, Blood, 2023). We have also demonstrated that CD276 is highly enriched in patients with high-risk AML, including KMT2A-rearranged, CBFA2T3::GLIS2, and KAT6A::CREBBP fusion genes (Kirkey, Blood, 2023). Flow-cytometry analysis of 1,500 pediatric AML patients demonstrated homogenous cell surface expression of CD276 in 22% (Kirkey, Blood, 2023). Using a CD276- targeting second generation CAR with TE9 binder and CD28 costimulatory domain developed at the University College, London, we conducted both in vitro and in vivo preclinical studies to evaluate efficacy and target-specificity in AML models.

In vitro cytotoxicity assays following co-incubation of CD276-positive NOMO-1 cells and CD276 negative Kasumi-1 cells with either CD276 CAR T or unmodified T cells showed robust target specific cell killing across a variety of effector: target cell ratios in the NOMO-1 cells with no significant cytotoxicity in the CD276-negative Kasumi-1 cells (p<0.0001). Cytokine production assessed via Human Luminex Assay with supernatant collection following 24hr co-incubation at a 1:1 effector to target ratio in a CD276-positive cell line revealed proinflammatory cytokine production in the CD276 CAR T treated co-cultures to be 100-300 fold higher than in the unmodified T cell treated co-cultures, demonstrating significant reactivity of the CD276 CAR T cells (p<0.001) with negligible cytokine production in the antigen negative cell line.

In vivo evaluations of the CD276 CAR T cells were conducted in an aggressive KMT2A-r AML cell-derived xenograft model. NSG mice were transplanted with 1e6 ffLuciferase transduced NOMO-1 cells and allowed 1 week for leukemia engraftment. Mice were then treated with 3.5e6 unmodified T cells or CD276 CAR T cells and leukemic burden was monitored by weekly bioluminescent imaging and flow cytometry analysis of peripheral blood. All CAR T-cell treated mice showed dramatic disease reduction as indicated by a significant reduction in radiance (>1 log fold decrease) within 6 days of CAR T cell infusion and clearance of disease maintained through the duration of the experiment. In contrast, all control animals who received unmodified T cells had progressive disease evidenced by >2 log fold increase in radiance.

We demonstrated potent and target specific efficacy of CD276 CAR T in an aggressive model of AML. As a tumor-specific antigen, CD276 represents an ideal therapeutic target in a very high-risk cohort of AML. This CAR T is advancing into clinical development pending completion of ongoing IND enabling studies.