| Summary: | Peptide receptor radionuclide therapy with <sup>177</sup>Lu-DOTATATE improves the outcome of patients with somatostatin receptor (SSTR)-expressing neuroendocrine tumours. Nevertheless, stable disease has been the main response pattern observed, with some rare complete responses. Lu-177 exerts about two-thirds of its biological effects via the indirect effects of ionizing radiation that generate reactive oxygen species, eventually leading to oxidative damage and cell death. This provides a rationale for targeting the antioxidant defence system in combination with <sup>177</sup>Lu-DOTATATE. In the present study, the radiosensitizing potential and the safety of depleting glutathione (GSH) levels using buthionine sulfoximine (BSO) during <sup>177</sup>Lu-DOTATATE therapy were assessed in vitro and in vivo using a xenograft mouse model. In vitro, the combination resulted in a synergistic effect in cell lines exhibiting a BSO-mediated GSH decrease. In vivo, BSO neither influenced <sup>177</sup>Lu-DOTATATE biodistribution nor induced liver, kidney or bone marrow toxicity. In terms of efficacy, the combination resulted in reduced tumour growth and metabolic activity. Our results showed that disturbing the cell redox balance using a GSH synthesis inhibitor increased <sup>177</sup>Lu-DOTATATE efficacy without additional toxicity. Targeting the antioxidant defence system opens new safe treatment combination opportunities with <sup>177</sup>Lu-DOTATATE.
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