Evaluation of an Anti-HER2 Nanobody Labeled with 225Ac for Targeted α-Particle Therapy of Cancer

Human epidermal growth factor receptor type 2 (HER2) is overexpressed in numerous carcinomas. Nanobodies (Nbs) are the smallest antibody-derived fragments with beneficial characteristics for molecular imaging and radionuclide therapy. Therefore, HER2-targeting nanobodies could offer a valuable platform for radioimmunotherapy, especially when labeled with α-particle emitters, which provide highly lethal and localized radiation to targeted cells with minimal exposure to surrounding healthy tissues. In this study, the anti-HER2 2Rs15d-nanobody was conjugated with 2-(4-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (p-SCN-Bn-DOTA) and radiolabeled with an α-emitter 225Ac with a high yield (>90%) and a radiochemical purity above 95%. The 225Ac-DOTA-Nb binding affinity was 4.12 ± 0.47 nM with an immunoreactive fraction above 80%. Binding to low HER2-expressing MDA-MB-231 cells was negligible, whereas HER2-overexpressing SKOV-3 cells could be blocked with an excess of unlabeled nanobody, confirming the specificity of binding. Noncompeting binding to HER2 was observed in the presence of an excess of trastuzumab. The cell-associated fraction of 225Ac-DOTA-Nb was 34.72 ± 16.66% over 24 h. In vitro, the radioconjugate was toxic in an HER2-mediated and dose-dependent manner, resulting in IC50 values of 10.2 and 322.1 kBq/mL for 225Ac-DOTA-Nb and the 225Ac-DOTA control, respectively, on SKOV-3 cells, and 282.2 kBq/mL for 225Ac-DOTA-Nb on MDA-MB-231 cells. Ex vivo biodistribution studies, performed in mice bearing subcutaneous HER2-overexpressing and low HER2-expressing tumors, showed a fast uptake in SKOV-3 tumors compared to MDA-MB-231 (4.01 ± 1.58% ID/g vs 0.49 ± 0.20% ID/g after 2 h), resulting also in high tumor-to-normal tissue ratios. In addition, coinjection of 225Ac-DOTA-Nb with Gelofusine reduced kidney retention by 70%. This study shows that 225Ac-DOTA-Nb is a promising new radioconjugate for targeted α-particle therapy and supports its further development.