a, The pyridazinedione method for the generation of functionalized Fabs. The Fab is first reduced with TCEP to liberate the cysteines of the single interchain disulfide bond. The reduced Fab is then reacted with the Br2PD of choice, via an addition-elimination mechanism whereby the thiols sequentially displace each Br atom to generate a stable covalent linkage between the heavy and light chains of the protein. BBS, Borate buffered saline. b, The previously developed method for the generation of bsAbs with pyridazinediones by means of SPIEDAC click chemistry. c, Proposed mechanism of action of a sialidase-containing CiTE. The CiTE binds to a target cancer cell through HER2-engagement and to a T cell through the CD3 co-receptor, crosslinking the two cells. The sialidase enzyme removes sialic acid from both target and effector (T) cell to enhance immune activation, leading to more potent T cell-mediated cytotoxicity. The CiTE is functionalized