Locoregional therapy with α‐emitting trastuzumab against peritoneal metastasis of human epidermal growth factor receptor 2‐positive gastric cancer in mice

Peritoneal metastasis of gastric cancer (PMGC) is incurable and thus has an extremely poor prognosis. We have found, however, that locoregionally administered trastuzumab armed with astatine‐211 ((211)At‐trastuzumab) is effective against human epidermal growth factor receptor 2 (HER2)‐positive PMGC in a xenograft mouse model. We first observed that (211)At‐trastuzumab can specifically bind and effectively kill NCI‐N87 (N87) cells, which are HER2‐positive human metastatic GC cells, both in vitro and in s.c. tumors. We established a PMGC mouse model using N87 xenografts stably expressing luciferase to test α‐particle radioimmunotherapy with (211)At‐trastuzumab against PMGC. Biodistribution analysis in this PMGC mouse model revealed that the i.p. administration of (211)At‐trastuzumab (1 MBq) was a more efficient means of delivery of (211)At into metastatic tumors than i.v. injection; the maximum tumor uptake with i.p. administration was over 60% injected dose per gram of tissue (%ID/g) compared to approximately 18%ID/g with i.v. injection. Surprisingly, a single i.p. injection of (211)At‐trastuzumab (1 MBq) was sufficient to completely eradicate intraperitoneally disseminated HER2‐positive GC xenografts in two of six treated mice by inducing DNA double‐strand breaks, and to drastically reduce the tumor burden in another three mice. No bodyweight loss, leukocytopenia, or significant biochemical changes in liver or kidney function were observed in the treatment group. Accordingly, locoregionally administered (211)At‐trastuzumab significantly prolonged the survival time of HER2‐positive PMGC mice compared with control treatments. Our results provide a proof‐of‐concept demonstration that locoregional therapy with (211)At‐trastuzumab may offer a new treatment option for HER2‐positive PMGC.