Chelator-Free Radiometal Labeling Inside Engineered Affibodies

Affibodies are remarkably stable three-helix bundle proteins that can be engineered to selectively bind target proteins. When combined with radioactive metals, they serve as imaging agents or cancer therapeutics, depending on the metal used. Traditionally, this involves bifunctional linkers that attach large chelators to the affibody via reactive groups. Here, we present an alternative approach that eliminates the need for such linkers by burying the metal within the core of the affibody, surrounded by its three helices. A simple engineered triple cysteine motif, with one cysteine in each helix, stably binds Bi(III), Pb(II), In(III), and Ga(III), which are commonly used in imaging and radiotherapy. Quantitative metal uptake is instantaneous at room temperature and physiological pH, and all metal-affibody complexes remain fully intact for one week at 4 °C. All retain their metal cargo when challenged with cellular concentrations of glutathione, while only the bismuth-affibody complex withstands a challenge with 100 equivalents of strong chelators, even over two weeks. We demonstrate that, a bismuth‑loaded affibody retains binding affinity to the HER2 receptor comparable to the wildtype affibody, while selectively binding and retaining ²¹³Bi, a promising radioisotope for targeted alpha therapy, thereby enabling targeting of HER2‑overexpressing SKBR3 cancer cells.