"When we systemically administered our nanoagent in mice bearing human breast cancer cells, it efficiently accumulated in tumors, robustly generated reactive oxygen species and completely eradicated the cancer without adverse effects ..."
So it kills human cancer and doesn't harm the mouse in the process.
Mice models of cancer are useful, but you should never be too surprised when something that works in mice doesn't work in the clinic, xenografting or no. Cancer is complicated.