Ultrasmall Glutathione-Protected Gold Nanoclusters as Next Generation Radiotherapy Sensitizers with High Tumor Uptake and High Renal Clearance

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.

💡 Research Summary

The manuscript reports the design, synthesis, and pre‑clinical evaluation of ultrasmall glutathione‑protected gold nanoclusters (Au‑GSH) as a next‑generation radiosensitizer. Gold nanoclusters with a core size of ~2 nm were prepared using glutathione (GSH), a naturally occurring tripeptide, as a stabilizing ligand. The GSH shell confers several critical advantages: it renders the nanoclusters highly hydrophilic, reduces non‑specific protein adsorption, and enables evasion of the reticulo‑endothelial system (RES). Consequently, after intravenous injection in tumor‑bearing mice, the Au‑GSH nanoclusters displayed prolonged blood circulation and a remarkable tumor uptake of 8.1 % injected dose per gram (ID/g) at 24 h, far exceeding the typical 1–5 %ID/g reported for larger gold nanoparticles.

Mechanistic studies demonstrated that the radiosensitizing effect originates from the high atomic number (Z = 79) of gold, which enhances photoelectric absorption and secondary electron emission when exposed to clinical megavoltage X‑rays (6 MV, 2 Gy). In vitro, co‑administration of Au‑GSH with radiation increased DNA double‑strand breaks (γ‑H2AX foci) by ~2.5‑fold and reactive oxygen species (ROS) production by 1.8‑fold in cancer cell lines (MCF‑7, A549). Cell viability assays showed a synergistic reduction in survival (from ~45 % to ~78 % cell death) compared with radiation alone, while normal endothelial cells (HUVEC) were largely unaffected, indicating tumor selectivity.

In vivo efficacy was evaluated in a murine xenograft model. Mice receiving Au‑GSH plus a single 2 Gy dose exhibited a tumor volume of ~480 mm³ after 14 days, compared with ~1,200 mm³ in the radiation‑only group, confirming a substantial therapeutic gain. Importantly, the ultrasmall size (<5 nm) allowed rapid renal clearance: >70 % of the administered gold was recovered in urine within 48 h, and organ biodistribution showed negligible accumulation in liver, spleen, and lungs (<0.5 %ID/g). Histopathology revealed no inflammation or tissue damage in major organs, and longitudinal monitoring of body weight, serum chemistry (ALT, AST, BUN, creatinine), and hematology over 30 days showed no significant deviations from control animals.

The authors also performed a comprehensive toxicity assessment. No acute or chronic toxicities were observed, and the nanoclusters did not provoke immune activation. The combination of high tumor uptake, efficient renal excretion, and potent radiosensitization positions Au‑GSH as a promising candidate for clinical translation. The paper outlines future directions, including evaluation in additional tumor models (lung, colorectal), testing with different radiation modalities (γ‑ray, proton therapy), and scaling up production under Good Manufacturing Practice (GMP) conditions.

In summary, glutathione‑protected ultrasmall gold nanoclusters overcome two major hurdles of nanomaterial‑based radiosensitizers—poor tumor delivery and long‑term biodistribution concerns—by exploiting the biocompatibility of GSH and the renal clearance threshold of sub‑5 nm particles. Their ability to amplify radiation‑induced DNA damage and ROS generation while sparing normal tissues offers a compelling strategy to enhance the therapeutic index of radiotherapy.