Engineering a CD123xCD3 bispecific scFv immunofusion for the treatment of leukemia and elimination of leukemia stem cells.

Engineered bispecific antibodies that recruit cytotoxic lymphocytes to kill specific tumor cells have been showing promising clinical results. Here, we describe a bispecific single-chain Fv (scFv) immunofusion or BIf to target CD123(+) leukemia, that contains an anti-CD123 scFv fused at the N-terminus of human IgG1 hinge-C(H)2-C(H)3, and an anti-CD3 scFv fused at C-terminus. When expressed from transfected CHO-S cells, CD123xCD3 BIf forms a homodimer that provides a structure of N-terminal tumor-targeting domain that closely resembles natural antibody. The CD123xCD3 dimeric structure also provides binding affinity to CD123(+) tumor cells with a Kd of 10(-10) M, one to two orders of magnitude stronger than traditional bispecific antibody constructs. The location of the anti-CD3 scFv at C-terminus of BIf reduces the binding affinity to CD3(+) T cells by two orders, which could help to prevent non-specific T-cell activation. CD123xCD3 BIf is able to achieve T-cell-mediated target cell killing activities at low pM levels with E/T ratios as low as 2. Overall, the inclusion of human IgG1 constant region in BIf construct increases target cell-binding affinity; potentially increases serum half-life by its larger size and FcRn-mediated salvage system; and includes the abilities to activate the additional antibody-mediated cellular cytotoxicities.