Preclinical Evaluation of (225)Ac-Labeled Single-Domain Antibody for the Treatment of HER2(pos) Cancer

Human epidermal growth factor receptor type 2 (HER2) is overexpressed in various cancers; thus, HER2-targeting single-domain antibodies (sdAb) could offer a useful platform for radioimmunotherapy. In this study, we optimized the labeling of an anti-HER2-sdAb with the α-particle-emitter (225)Ac through a DOTA-derivative. The formed radioconjugate was tested for binding affinity, specificity and internalization properties, whereas cytotoxicity was evaluated by clonogenic and DNA double-strand-breaks assays. Biodistribution studies were performed in mice bearing subcutaneous HER2(pos) tumors to estimate absorbed doses delivered to organs and tissues. Therapeutic efficacy and potential toxicity were assessed in HER2(pos) intraperitoneal ovarian cancer model and in healthy C57Bl/6 mice. [(225)Ac]Ac-DOTA-2Rs15d exhibited specific cell uptake and cell-killing capacity in HER2(pos) cells (EC(50) = 3.9 ± 1.1 kBq/mL). Uptake in HER2(pos) lesions peaked at 3 hours (9.64 ± 1.69% IA/g), with very low accumulation in other organs (<1% IA/g) except for kidneys (11.69 ± 1.10% IA/g). α-camera imaging presented homogeneous uptake of radioactivity in tumors, although heterogeneous in kidneys, with a higher signal density in cortex versus medulla. In mice with HER2(pos) disseminated tumors, repeated administration of [(225)Ac]Ac-DOTA-2Rs15d significantly prolonged survival (143 days) compared to control groups (56 and 61 days) and to the group treated with HER2-targeting mAb trastuzumab (100 days). Histopathologic evaluation revealed signs of kidney toxicity after repeated administration of [(225)Ac]Ac-DOTA-2Rs15d. [(225)Ac]Ac-DOTA-2Rs15d efficiently targeted HER2(pos) cells and was effective in treatment of intraperitoneal disseminated tumors, both alone and as an add-on combination with trastuzumab, albeit with substantial signs of inflammation in kidneys. This study warrants further development of [(225)Ac]Ac-DOTA-2Rs15d.