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Background COBRA™ (COnditional Bispecific Redirected Activation) T-cell engagers are designed to target solid tumors as a single polypeptide chain prodrug that becomes activated by proteolysis in the tumor microenvironment. One COBRA molecule comprises seven Ig domains: three single-domain antibodies (sdAbs) recognizing a tumor target or human serum albumin (HSA), and CD3ε-binding variable fragment heavy chain (VH) and variable fragment light chain (VL) and their inactivated counterparts, VHi and VLi. Pairing of VH and VL, and VLi and VHi into single-chain variable fragments (Fv) is prevented by shortened inter-domain linkers. Instead, VH and VL are expected to interact with VLi and VHi, respectively, thus making a diabody whose binding to CD3ε on the T-cells is impaired. Methods We analyzed the structure of an epidermal growth factor receptor (EGFR) COBRA in solution using negative stain electron microscopy (EM) and small-angle X-ray scattering (SAXS). Results We found that this EGFR COBRA forms stable monomers with a very dynamic interdomain arrangement. At most, only five domains at a time appeared ordered, and only one VH-VL pair was found in the Fv orientation. Nonenzymatic posttranslational modifications suggest that the CDR3 loops in the VL-VHi pair are exposed but are buried in the VH-VLi pair. The MMP9 cleavage rate of the prodrug when bound to recombinant EGFR or HSA is not affected, indicating positioning of the MMP9-cleavable linker away from the EGFR and HSA binding sites. Conclusion Here, we propose a model for EGFR COBRA where VH and VLi form an Fv, and VL and VHi do not, possibly interacting with other Ig domains. SAXS and MMP9 cleavage analyses suggest that all COBRA molecules tested have a similar structural architecture.

Structural arrangement of the VH and VL domains in the COBRA™ T-cell engaging single-chain diabody