A novel insulin-like growth factor II-based masking domain for conditional activation of therapeutic antibodies

infection challenge model at the dose tested. IGF-II-s showed no detectable IGF receptor binding and provided stronger masking than native IGF-II. This generalized approach was demonstrated across multiple antibodies, enabling efficient protease-dependent conditional activation of trastuzumab (anti-HER2), an anti-IL-1β antibody, and bevacizumab (anti-VEGF). Together, these results established an engineered IGF-II-based masking domain as a versatile, IGF-II receptor-silent platform enabling protease-dependent conditional activation of therapeutic antibodies, with potential to improve safety and expand therapeutic windows.