  Radiotherapy is often the most straightforward first line cancer treatment
for solid tumors. While it is highly effective against tumors, there is also
collateral damage to healthy proximal tissues especially with high doses. The
use of radiosensitizers is an effective way to boost the killing efficacy of
radiotherapy against the tumor while drastically limiting the received dose and
reducing the possible damage to normal tissues. Here, we report the design and
application of a good radiosensitizer by using ultrasmall gold nanoclusters
with a naturally occurring peptide (e.g., glutathione or GSH) as the protecting
shell. The GSH coated gold nanoclusters can escape the RES absorption, leading
to a good tumor uptake (8.1% ID/g at 24 h post injection). As a result, the
as-designed Au nanoclusters led to a strong enhancement for radiotherapy, as
well as a negligible damage to normal tissues. After the treatment, the
ultrasmall gold nanoclusters can be efficiently cleared by the kidney, thereby
avoiding potential long term side effects caused by the accumulation of gold
atoms in the body. Our data suggest that the ultrasmall peptide protected Au
nanoclusters are a promising radiosensitizer for cancer radiotherapy.
