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Abstract

Small engineered scaffold proteins have attracted attention as probes for radionuclide-based molecular imaging. One class of these imaging probes, termed ABD-Derived Affinity Proteins (ADAPT), has been created using the albumin-binding domain (ABD) of streptococcal protein G as a stable protein scaffold. In this study, we report the development of a clinical lead probe termed ADAPT6 that binds HER2, an oncoprotein overexpressed in many breast cancers that serves as a theranostic biomarker for several approved targeting therapies. Surface-exposed amino acids of ABD were randomized to create a combinatorial library enabling selection of high-affinity binders to various proteins. Furthermore, ABD was engineered to enable rapid purification, to eradicate its binding to albumin, and to enable rapid blood clearance. Incorporation of a unique cysteine allowed site-specific conjugation to a maleimido derivative of a DOTA chelator, enabling radionuclide labeling, (111)In for SPECT imaging and (68)Ga for PET imaging. Pharmacologic studies in mice demonstrated that the fully engineered molecule (111)In/(68)Ga-DOTA-(HE)3-ADAPT6 was specifically bound and taken up by HER2-expressing tumors, with a high tumor-to-normal tissue ratio in xenograft models of human cancer. Unbound tracer underwent rapid renal clearance followed by high renal reabsorption. HER2-expressing xenografts were visualized by gamma-camera or PET at 1 hour after infusion. PET experiments demonstrated feasibility for discrimination of xenografts with high or low HER2 expression. Our results offer a preclinical proof of concept for the use of ADAPT probes for noninvasive in vivo imaging.  

Author information

Garousi J1, Lindbo S2, Nilvebrant J2, Åstrand M2, Buijs J1, Sandström M1, Honarvar H1, Orlova A3, Tolmachev V4, Hober S2.

1Department of Immunology, Genetics and Pathology, Uppsala University, Uppsala, Sweden.

2Department of Protein Technology, KTH Royal Institute of Technology, Stockholm, Sweden.

3Preclinical PET Platform, Department of Medicinal Chemistry, Uppsala University, Uppsala, Sweden.

4Department of Immunology, Genetics and Pathology, Uppsala University, Uppsala, Sweden. vadimir.tolmachev@igp.uu.se.