[Investigator-Initiated Clinical Trial of 211At-NaAt against Refractory Thyroid Cancer].

Radioactive iodine has long been used clinically for patients with differentiated thyroid cancer. Radioiodine(131I) is used for the ablation of thyroid remnants or treatment of metastatic thyroid cancer. However, some patients with multiple metastases are refractory to repetitive 131I treatment, despite the targeted regions showing sufficient iodine uptake. In such patients, β- particle therapy using 131I is inadequate and another strategy is needed using more effective radionuclide targeting the sodium/iodide symporter(NIS). Astatine(211At)is receiving increasing attention as an α-emitter for targeted radionuclide therapy. 211At is a halogen element with similar chemical properties to iodine. α particles emitted from 211At has higher linear energy transfer as compared to β particles from 131I and exert a better therapeutic effect by inducing DNA double strand breaks and free radical formation. We showed that increase of the radiochemical purity of astatide of 211At solution by addition of ascorbic acid was associated with significantly enhanced uptake of 211At by both normal thyroid tissue and differentiated thyroid cancer cells. The treatment effect of 211At solution in the K1-NIS xenograft model was dose-dependent and was associated with prolonged survival, suggesting the potential applicability of targeted α therapy for the treatment of advanced differentiated thyroid cancer. Thus, targeted α therapy using 211At is highly promising for the treatment of advanced differentiated thyroid cancer. We have already started the clinical trial of 211At-NaAt in Osaka University Hospital since November 2021 after getting the approval by IRB and PMDA investigation. We would like to get the proof of concept that astatine can be used safely and effectively in patients, aiming at the drug approval as a targeted α therapeutic from Japan.