Difference in thyroid uptake between Astatine-211 and Iodine-123 in normal rats: a comparative study between oral and intravenous administration.

Objectives Astatine-211 (At-211), an α-particle emitting radionuclides, is a member of the halogens and can be used as a heavier analog of iodine. α-Particle therapy using At-211 might replace radioiodine therapy for refractory thyroid cancer, because α-particle emitting radionuclides would be more effective than β-emitters. However, normal thyroid uptake of At-211 has not been thoroughly investigated yet. In the present study, we evaluated sequential imaging of At-211 in normal rats after oral or intravenous bolus administration and compared to the iodine-123 (I-123) study with the same protocol.

Methods The rats were assigned either of 4 groups (n=2, each): group A rats received At-211 NaAt intravenously, group B rats received At-211 NaAt orally, group C rats received I-123 NaI intravenously, and group D rats received I-123 NaI orally. All the rats were fed low iodine diet for 2 weeks prior to the radionuclide admission. The planar imaging of rats was conducted using a SIEMENS E.Cam gamma camera. For the study of group A and B, energy photo peak was adjusted at 79 keV ± 10% to acquire the polonium X-rays inherent to the At-211 decay. The gamma ray energy photo peak for the study of group C and D was adjusted at 159 keV ± 10%. In group A and B, the imaging was performed at 30 min, 3, 18, 24, and 48 hr after administration. In group C and D, 6 and 12 hr imaging after administration was added. We evaluated the difference of sequential images, and estimated thyroid uptake ratios in all groups.

Results In group A, the peak thyroid uptakes were observed at 3 hr after administration and the uptake decreased slowly. In group B, thyroid uptakes gradually increased during 30 min to 18 hr or more hour time points. In group C and D, all rats showed rapid thyroid accumulations within 3 hr after administration. Mean thyroid uptake ratio of At-211 at 3 and 24 hr of administration was 3.30 ± 0.65 % and 1.32 ± 0.78 % in group A and 0.89 ± 0.36 % and 0.35 ± 0.51 % in group B, respectively. Mean thyroid uptake ratio of I-123 at 3 and 24 hr of administration was 22.4 ± 4.05 % and 23.6 ± 3.47 % in group C and 12.4 ± 1.82 % and 15.3 ± 0.39 % in group D, respectively. Mean thyroid uptake ratios of group A and B were less than 15% of group C and D.

Conclusions The present study firstly reported sequential changes of At-211 NaAt distribution in normal thyroid glands of living rats. Although the thyroid uptake ratio of At-211 is lower than that of I-123, manner of slow At-211 accumulation by oral administration suggests a continuous supply of an α-emitter in the radionuclide therapy of thyroid cancer. Acknowledgements: This study was supported by the KAKENHI Grant-in-Aid for Grant (No. 15K09955) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.