Spatial hindrance‐based pro‐antibodies (pro‐Abs) are engineered antibodies to reduce monoclonal antibodies' (mAbs) on‐target toxicity using universal designed blocking segments that mask mAb antigen‐binding sites through spatial hindrance. By linking through protease substrates and linkers, these blocking segments can be removed site‐specifically. Although many types of blocking segments have been developed, such as coiled‐coil and hinge‐based Ab locks, the molecular structure of the pro‐Ab, particularly the region showing how the blocking fragment blocks the mAb, has not been elucidated by X‐ray crystallography or cryo‐EM. To achieve maximal effect, a pro‐Ab must have high antigen‐blocking and protease‐restoring efficiencies, but the unclear structure limits its further optimization. Here, we utilized molecular dynamics (MD) simulations to study the dynamic structures of a hinge‐based Ab lock pro‐Ab, pro‐Nivolumab, and validated the simulated structures with small‐ and wide‐angle X‐ray scattering (SWAXS). The MD results were closely consistent with SWAXS data (χ2best‐fit = 1.845, χ2allMD = 3.080). The further analysis shows a pronounced flexibility of the Ab lock (root‐mean‐square deviation = 10.90 Å), yet it still masks the important antigen‐binding residues by 57.3%–88.4%, explaining its 250‐folded antigen‐blocking efficiency. The introduced protease accessible surface area method affirmed better protease efficiency for light chain (33.03 Å2) over heavy chain (5.06 Å2), which aligns with the experiments. Overall, we developed MD‐SWAXS validation method to study the dynamics of flexible blocking segments and introduced methodologies to estimate their antigen‐blocking and protease‐restoring efficiencies, which would potentially be advancing the clinical applications of any spatial hindrance‐based pro‐Ab.
