Evaluation of a novel ¹⁷⁷Lu-labelled therapeutic Affibody molecule with a deimmunized ABD domain and improved biodistribution profile

Purpose: Fusion of Affibody molecules with an albumin-binding domain (ABD) provides targeting agents, which are suitable for radionuclide therapy. To facilitate clinical translation, the low immunogenic potential of such constructs with targeting properties conserved is required. Methods: The HER2-targeting Affibody molecule ZHER2:2891 was fused with a deimmunized ABD variant and DOTA was conjugated to a unique C-terminal cysteine. The novel construct, PEP49989, was labelled with ¹⁷⁷Lu. Affinity, specificity, and in vivo targeting properties of [¹⁷⁷Lu]Lu-PEP49989 were characterised. Experimental therapy in mice with human HER2-expressing xenografts was evaluated. Results: The maximum molar activity of 52 GBq/µmol [¹⁷⁷Lu]Lu-PEP49989 was obtained. [¹⁷⁷Lu]Lu-PEP49989 bound specifically to HER2-expressing cells in vitro and in vivo. The HER2 binding affinity of [¹⁷⁷Lu]Lu-PEP49989 was similar to the affinity of [¹⁷⁷Lu]Lu-ABY-027 containing the parental ABD035 variant. The renal uptake of [¹⁷⁷Lu]Lu-PEP49989 was 1.4-fold higher, but hepatic and splenic uptake was 1.7-2-fold lower than the uptake of [¹⁷⁷Lu]Lu-ABY-027. The median survival of xenograft-bearing mice treated with 21 MBq [¹⁷⁷Lu]Lu-PEP49989 (> 90 days) was significantly longer than the survival of mice treated with vehicle (38 days) or trastuzumab (45 days). Treatment using a combination of [¹⁷⁷Lu]Lu-PEP49989 and trastuzumab increased the number of complete tumour remissions. The renal and hepatic toxicity was minimal to mild. Conclusion: In preclinical studies, [¹⁷⁷Lu]Lu-PEP49989 demonstrated favourable biodistribution and a strong antitumour effect, which was further enhanced by co-treatment with trastuzumab.