The present study showed that [211At]NaAt effectively induced more DSBs, with significantly fewer colonies in the in vitro assay, compared with [131I]NaI. In tumour xenograft mice, the tumour-growth suppression effects were higher in the 211At group than in the 131I group despite the lower administered doses in the 211At group and shorter physical half-life of 211At.
RAI therapy is now widely used for patients with differentiated thyroid cancer as post-surgical ablation for high-risk patients and for the treatment of recurrence or metastasis. However, a more effective treatment is necessary for patients for whom RAI therapy is insufficient [3, 13]. 211At is considered to be a potential therapeutic agent for the treatment of differentiated thyroid cancer, and we previously reported the dose-dependent tumour-suppressive effects of [211At]NaAt solution . Because 211At is transported into cells via NIS, similar to 131I, its alpha particle emission can achieve a better anti-tumour effect than 131I [7, 8]. In addition, outpatient [211At]NaAt treatment is possible with minimum radiation exposure to the public and caregivers, although hospitalisation is essential in many countries when administering high dose therapy of [131I]NaI . Thus, [211At]NaAt therapy may be considered as a better choice for differentiated thyroid cancer with promising therapeutic effects without the need for hospitalisation.
In the present study, we found that 211At caused more DSBs in a dose-dependent manner. Previously, we reported the dose-related induction of DSBs by 211At-AAMT in PANC1 cells . Alpha particles emitted from 211At have higher LET than beta particles emitted from 131I, and the severity and complexity of DNA damage significantly increase with alpha irradiation . Low LET induces more single strand breaks or isolated DSBs, which can be rejoined 5 h after irradiation . However, high LET irradiation induces more non-rejoining DSBs and clustered lesions, and clustered lesion number and size increase after high LET irradiation, making the repair of DSBs difficult [18–20]. The number and rejoining ability of DSBs in cells decreased after alpha-irradiation, and thus the lethality of DSBs increases with alpha-irradiation .
In addition, we discovered the lower clonogenicity of [211At]NaAt solution in vitro, suggesting a lower survival rate of K1-NIS cells treated with [211At]NaAt solution. Previous studies have reported that the survival rate of cells treated with 177Lu, a beta particle emitter, was also higher than cells treated with 225Ac, an alpha particle emitter [22, 23]. The results support that high-LET alpha-emitters have more powerful cell-killing properties, with higher DSB formation. In addition, in previous studies, we found that [211At]NaAt dose-dependently lowers the viability of K1-NIS cells than [131I]NaI in vitro , which also proved the higher cytotoxicity of 211At. Thus, the results of this study, which showed higher induction efficiency of DBSs of an alpha particle emitter (211At) than a beta particle emitter (131I), are consistent with the findings of the previous studies and supported the results of the colony formation assay in the present study.
Because of the abundant NIS expression in the thyroid gland, salivary gland, and stomach, the relative uptake of 211At and 131I was similar at 3 h. However, the 131I uptake in tissues except the thyroid gland rapidly decreased at 24 h, whereas 131I uptake in the thyroid gland was still observed. In previous studies, short retention time of iodide was also observed in NIS-expressing xenografts. 125I accumulation in Tc-rNIS xenograft peaked at 90 min, but decreased to half at 6 h . Meanwhile, the uptake of 125I by organs except the thyroid gland ceased at 19 h after administration . In contrast, the clearance of 211At was slow, and a similar trend was observed in normal rats in a previous study . In addition, Cobb et al. reported that the uptake of 211At was higher than 125I in the human grafts implanted to mouse (moderately differentiated follicular carcinomas), but lower in the normal mouse thyroid gland . The clearance of 211At is slower than that of 125I in patient-derived xenografts between 4 and 24 h after administration, which suggested the possibility that 211At was retained longer in human thyroid cancers, supporting the findings of the present study.
The higher number of DSBs caused by alpha-particles and higher uptake of 211At in the tumour suggest the possible advantage of 211At in the treatment of tumours with NIS expression. Thus, as shown in Fig. 4, we observed more effective therapeutic effects in mice administrated 0.4 MBq 211At than 4 MBq 131I, confirming the stronger tumour-suppressive ability of 211At. Meanwhile, the higher absorbed dose of 211At in the tumour was also confirmed. The strong treatment response of [225Ac]PSMA-617, a targeted alpha therapy, in patients with metastatic prostate cancer who are resistant to [177Lu]PSMA-617, also suggest a more beneficial effect of alpha particles than beta particles in clinical application .
Regarding the possible toxicity of [211At]NaAt solution in normal organs, we did not find severe adverse effects with a high administered dose of [211At]NaAt solution (up to 50 MBq/kg) in our previous studies in normal mice (9,10). In these studies, thyroid gland ablation, transient bone marrow suppression in the high-dose group (decline in the number of white blood cells and platelet count), and pathological changes in the testis were observed. No pathological abnormalities were observed in the other major organs. However, caution should be exercised regarding species difference in biodistribution between mice and humans. Thus, it is necessary to start with a low-dose of [211At]NaAt solution in the investigator-initiated clinical trial, which is scheduled to start in 2021. However, low administered doses may result in insufficient tumour suppression or recurrence due to the dose-dependent therapeutic effects of 211At. Repeated administrations of [211At]NaAt can be considered for future applications in clinical practice.
Severe xerostomia has been reported in patients who received alpha-targeted therapy using [225Ac]PSMA-617 [27, 28]. Therefore, it is necessary to decrease the potential adverse effects in the clinical application of [211At]NaAt. The tandem therapy of [225Ac]PSMA-617 and [177Lu]PSMA-617 has been shown to enhance efficacy while reducing adverse effects , suggesting that the combination of [211At]NaAt and [131I]NaI may also enhance therapeutic effects with lower toxicity.
This study had some limitations. First, the present study involved a small number of mice due to the limited allowance in our institution for use of radioactivity by legal regulations. Second, we did not compare the adverse effects of the histopathological evaluation between [211At]NaAt and [131I]NaI administration. In our previous extended single-dose toxicity study of [211At]NaAt, we found no pathological abnormalities in high-uptake organs, such as the salivary gland and stomach, up to 50 MBq/kg . Toxicity in humans will be elucidated during dose escalation in the clinical trial, which will start from a minimal dose. Finally, we did not evaluate the therapeutic effects of the combination of [211At]NaAt and [131I]NaI, which may have better clinical applications.