Bioinorganic Site‐Specific Conjugation of Multi‐Arm Linkers to Create Pharmaceuticals With Targeting and Effector Functions

ABSTRACT The development of bioconjugates with high payload loading and defined stoichiometry remains a persistent challenge in medicinal chemistry. In this Account, we present a modular multi‐arm linker platform that decouples payload loading from site‐specific antibody conjugation. This platform utilizes a central peptide core with multiple linking arms for attaching effector/targeting molecules and a single coupling arm (e.g., maleimide) for antibody conjugation. To achieve site‐specificity without the need for enzymatic processing, we employ a bioinorganic strategy: a computationally designed zinc‐binding motif (ACPGHA) fused to the antibody C‐terminus. Density functional theory calculations suggest Zn(II) binds and deprotonates the engineered cysteine to a reactive thiolate for rapid, chemoselective Michael addition to the linker's maleimide. We demonstrate how the modular multi‐arm linker platform, paired with Zn 2+ ‐mediated site‐specific conjugation, enables the modular construction of (i) a theranostic antibody‐radionuclide conjugate for pancreatic cancer and (ii) a potent antibody‐drug conjugate for multiple myeloma. This integration of coordination chemistry, computational design/modeling, and protein engineering provides a robust framework for constructing homogeneous bioconjugates with precise stoichiometry. Beyond cytotoxic payload delivery, the same multi‐arm linker architecture enables pharmacokinetic modulation of a peptide agonist via conjugation of two albumin‐targeting fatty acids.