Site-Specific Radiometal Labeling and Improved Biodistribution Using ABY-027, A Novel HER2-Targeting Affibody Molecule-Albumin-Binding Domain Fusion Protein

Because of their better penetration, smaller targeting proteins may be superior to antibodies for radioimmunotherapy of solid tumors. Therefore, Affibody molecules (6.5 kDa) have a potential for being suitable as targeted moiety for radiolabeled therapeutic proteins. Previous studies have demonstrated that a fusion of an Affibody molecule with an albumin-binding domain (ABD) provides a strong noncovalent binding to albumin in vivo. This strong noncovalent binding can be used for reduction of the renal uptake of the Affibody molecule while maintaining a size smaller than that of an antibody, which is important when using residualizing radionuclide labels conjugated to Affibody molecules. The goal of this study was to design and evaluate a new targeting Affibody-ABD fusion protein with improved bio-distribution properties for radionuclide therapy. Methods: A novel Affibody-based construct, ZHER2: 2891-ABD035-DOTA (ABY-027), was created by fusion of the reengineered HER2-binding Affibody molecule ZHER2: 2891 to the N terminus of the high-affinity ABD(035), and a maleimido-derivative of DOTA was conjugated at the C terminus of the construct. Binding and processing of Lu-177-ABY-027 by HER2-expressing cells were evaluated in vitro. Targeting of HER2-expressing SKOV-3 xenografts was evaluated in BALB/C nu/nu mice and compared with targeting of previously reported ABD-(Z(HER2: 342))(2). Results: The binding affinity (dissociation constant) of ABY-027 to HER2 (74 pM) was the same as for the parental ZHER2: 2891 (76 pM). ABY-027 was stably labeled with Lu-177 and In-111 with preserved specific binding to HER2-expressing cells in vitro. In vivo receptor saturation experiments demonstrated that targeting of SKOV-3 xenografts in BALB/C nu/nu mice was HER2-specific. 177Lu-ABY-027 demonstrated substantially (2-to 3-fold) lower renal and hepatic uptake than previously assessed HER2-specific Affibody-based albumin-binding agents. Tumor uptake of radiolabeled ABY-027 at 48 h after injection was 2-fold higher than that for previously reported ABD-(ZHER2: 342) 2. Conclusion: An optimized molecular design of an ABD fusion protein resulted in an Affibody molecule construct with better properties for therapy. Fully preserved in vivo targeting of the fusion protein was shown in xenografted mice. Site-specific coupling of DOTA provides a uniform conjugate and creates the potential for labeling with a broad range of therapeutic radionuclides. The biodistribution of Lu-177-ABY-027 in a murine model suggests it is more suitable for therapy than alternative approaches.