Efficient production of bispecific antibodies (BsAbs) in single mammalian cells is essential for basic research and industrial manufacturing. However, preventing unwanted pairing of heavy chains (HCs) and light chains (LCs) is a challenging task. To address this, we created an engineering technology for preferential cognate HC/LC and HC/HC paring called FAST-Ig (Four-chain Assembly by electrostatic Steering Technology – Immunoglobulin), and applied it to NXT007, a BsAb for the treatment of hemophilia A. We introduced charged amino-acid substitutions at the HC/LC interface to facilitate the proper assembly for manufacturing a standard IgG-type BsAb. We generated CH1/CL interface-engineered antibody variants that achieved > 95% correct HC/LC pairing efficiency with favorable pharmacological properties and developability. Among these, we selected a design (C3) that allowed us to separate the mis-paired species with an unintended pharmacological profile using ion-exchange chromatography. Crystal structure analysis demonstrated that the C3 design did not affect the overall structure of both Fabs. To determine the final design for HCs-heterodimerization, we compared the stability of charge-based and knobs into hole-based Fc formats in acidic conditions and selected the more stable charge-based format. FAST-Ig was also applicable to stable CHO cell lines for industrial production and demonstrated robust chain pairing with different subclasses of parent BsAbs. Thus, it can be applied to a wide variety of BsAbs both preclinically and clinically.

Emicizumab, a factor (F)VIIIa-function mimetic therapeutic bispecific antibody (BsAb) to FIXa and FX, has become an indispensable treatment for people with hemophilia A (PwHA). However, non-clinical studies suggest that the maximum cofactor activity of emicizumab is lower than international standard activity (100 IU/dL of FVIII), leaving room for further improvement. Since not all PwHA experienced zero treated bleeds, increased cofactor activity would be beneficial for such patients. Here, we report NXT007, a BsAb against FIXa and FX developed through further engineering of emicizumab. While emicizumab has a common light chain, advances in antibody engineering enabled us to identify a more potent BsAb with two distinct new light chains, and following extensive mutational optimization of the two emicizumab-derived heavy chains and two light chains, the resulting NXT007 exerted in vitro thrombin generation (TG) activity in hemophilia A plasma corresponding to that at 100 IU/dL of FVIII when coagulation is triggered by tissue factor. NXT007 demonstrated potent hemostatic activity in an acquired hemophilia A model in non-human primates at much lower dosage than emicizumab, consistent with an around 30-fold dose shift in the in vitro TG activity between NXT007 and emicizumab. Moreover, together with Fc engineering that enhanced FcRn binding and reduced in vivo clearance, we demonstrate that NXT007 could be effective at much lower dosage with a longer dosing interval compared to emicizumab. These non-clinical results suggest that NXT007 is expected to maintain a non-hemophilic range of coagulation potential in PwHA and provides a rationale for its clinical testing.