Nonclinical Safety Assessment of AMG 553, an Investigational Chimeric Antigen Receptor T-Cell Therapy for the Treatment of Acute Myeloid Leukemia

Feline McDonough Sarcoma-like tyrosine kinase 3 (FLT3), a tyrosine-protein kinase involved in hematopoiesis, is detectable on the cell surface of approximately 80% of leukemia isolates from adult patients with acute myeloid leukemia (AML). AMG 553 is an investigational chimeric antigen receptor (CAR) T-cell immunotherapy for the treatment of AML. FLT3 expression analysis and in vitro and in vivo studies were leveraged to evaluate the nonclinical safety of AMG 553. Cynomolgus monkeys administered autologous anti-FLT3 CAR T cells demonstrated no evidence of CAR T-cell-mediated toxicity, expansion, or persistence, likely due to restricted cell surface FLT3 protein expression in healthy animals. This highlights the limited value of such in vivo studies for safety assessment of the CAR T-cell modality when directed against a target with restricted expression. To complement these studies and directly evaluate the potential toxicities of eliciting T-cell-mediated cytotoxicity against cells with surface expression of FLT3 protein in vivo, data from cynomolgus monkey toxicology studies with 2 bispecific T-cell engager molecules targeting FLT3 were leveraged; findings were consistent with the targeted killing of bone marrow cells expressing cell surface FLT3. Potential AMG 553-induced cytotoxicity was assessed against a wide range of normal human primary cells and cell lines; cytotoxicity was observed against FLT3-positive AML cell lines and a percentage of primary bone marrow CD34(+) cells. In conclusion, the nonclinical safety data suggest that AMG 553 can target FLT3 protein on AML cells, whereas only affecting a percentage of normal hematopoietic stem and progenitor cells, supporting clinical development.