Radionuclide therapy using ABD-fused ADAPT scaffold protein: Proof of Principle

Molecular recognition in targeted therapeutics is typically based on immunoglobulins. Development of engi-neered scaffold proteins (ESPs) has provided additional opportunities for the development of targeted therapies. ESPs offer inexpensive production in prokaryotic hosts, high stability and convenient approaches to modify their biodistribution. In this study, we demonstrated successful modification of the biodistribution of an ESP known as ADAPT (Albumin-binding domain Derived Affinity ProTein). ADAPTs are selected from a library based on the scaffold of ABD (Albumin Binding Domain) of protein G. A particular ADAPT, the ADAPT6, binds to human epidermal growth factor receptor type 2 (HER2) with high affinity. Preclinical and early clinical studies have demonstrated that radiolabeled ADAPT6 can image HER2-expression in tumors with high contrast. However, its rapid glomerular filtration and high renal reabsorption have prevented its use in radionuclide therapy. To modify the biodistribution, ADAPT6 was genetically fused to an ABD. The non-covalent binding to the host's albumin resulted in a 14-fold reduction of renal uptake and appreciable increase of tumor uptake for the best variant, Lu-177-DOTA-ADAPT6-ABD035. Experimental therapy in mice bearing HER2-expressing xenografts demonstrated more than two-fold increase of median survival even after a single injection of 18 MBq Lu-177-DOTA-ADAPT6ABD035. Thus, a fusion with ABD and optimization of the molecular design provides ADAPT derivatives with attractive targeting properties for radionuclide therapy.

Automated summary

Molecular recognition in targeted therapeutics is usually based on immunoglobulins. Engineered scaffold proteins have provided new opportunities for targeted therapies due to their inexpensive production, high stability, and convenient modification of biodistribution. The modification of the biodistribution through fusion with albumin-binding domain has proven effective in reducing renal uptake and increasing tumor uptake, improving the targeting properties of ADAPT derivatives for radionuclide therapy.