LAUSR

Objectives The 111In-labeled anti-HER2 antibody trastuzumab modified with a nuclear-localizing sequence (NLS) peptide (111In-trastuzumab-NLS) is a radiopharmaceutical candidate for Auger electron radioimmunotherapy (AE-RIT). However, in-vivo action of 111In-trastuzumab-NLS is poorly understood in intraperitoneal tumors. We aimed to elucidate the nuclear targeting activity of 111In-trastuzumab-NLS in a mouse model of intraperitoneal tumors. Methods Trastuzumab, trastuzumab-NLS-S with shorter NLS peptides, and trastuzumab-NLS-L with longer NLS peptides were tested in an intraperitoneal tumor xenograft. The AE-emitting radionuclide 111In was labeled with these antibodies. The cell-binding activity, nuclear importation, and cytotoxicity of those radiolabeled antibodies were examined in human cancer cell lines. Analyses of the biodistribution and in-vivo nuclear importation of 111In were conducted in a mouse model. Results The two111In-trastuzumab-NLS variants delivered the radionuclide into the nucleus more efficiently and had a comparable cytotoxicity to 111In-trastuzumab against human gastric cancer cells, although had a lower cell binding affinity. 111In-trastuzumab-NLS-L exhibited both a superior tumor uptake and in vivo nuclear transportation of the radionuclide than 111In-trastuzumab. Conclusion Nuclear targeting using 111In-trastuzumab-NLS promotes a more efficient tumor cell uptake and subsequent nuclear translocation of the 111In AE-emitting radionuclide in vivo. This radio-immunoconjugate will likely be an effective agent for HER2-targeting by AE-RIT.