Utility of At-Trastuzumab for the Treatment of Metastatic Gastric Cancer in the Liver: Evaluation of a Preclinical α-Radioimmunotherapy Approach in a Clinically Relevant Mouse Model.

A liver metastasis from a primary gastric cancer (LMGC) is relatively common and results in an extremely poor prognosis due to a lack of effective therapeutics. We here demonstrate in a clinically relevant mouse model that an α-particle radioimmunotherapy approach with At-labeled trastuzumab has efficacy against LMGCs that are positive for human epidermal growth factor receptor 2 (HER2). At was produced in a cyclotron via a Bi (α,2n) At reaction. At-trastuzumab was subsequently generated using a single-step labeling method. NCI-N87 cells (HER2-positive human gastric cancer cells) carrying a luciferase gene were intrasplenically transplanted into severe combined immunodeficiency mice to generate an HER2-positive LMGC model. A biodistribution study was then conducted through the intravenous injection of At-trastuzumab (1 MBq) into these LMGC xenograft mice. In parallel with this experimental therapy, phosphate-buffered saline, intact trastuzumab, or At-nonspecific human IgG (1 MBq) was injected into control groups. The therapeutic efficacy was evaluated by monitoring tumor changes by chemiluminescence imaging. Body weights, white blood cell counts, and serum markers of tissue damage were monitored at regular intervals. Microdosimetry using a CR-39 plastic detector was also performed. The biodistribution analysis revealed an increased uptake of At-trastuzumab in the metastasized tumors that reached approximately 12% of the injected dose per gram of tissue (%ID/g) at 24 h. In contrast, its uptake to the surrounding liver was about 4 %ID/g. The LMGCs in the mouse model reduced dramatically at 1 wk after the single systemic injection of At-trastuzumab. No recurrences were observed in 6 of 8 mice treated with this single injection, and their survival time was significantly prolonged compared with the control groups, including the animals treated with At-nonspecific antibodies. No severe toxicities or abnormalities in terms of body weight, white blood cell number, liver function, or kidney parameters were observed in the At-trastuzumab group. Microdosimetric studies further revealed that At-trastuzumab had been delivered at an 11.5-fold higher dose to the LMGC lesions than to the normal liver. α-radioimmunotherapy with At-trastuzumab has considerable potential as an effective and safe therapeutic option for LMGC.