T. Hongyo

What We Learned from Mouse Studies on Stable Iodine Tablets and Their Alternatives During Exposure to Radioactive Iodine

T. Hongyo (Osaka University, Japan)

Introduction

If exposure to a large amount of radioactive iodine due to a nuclear power plant accident or the like is expected, intake of stable iodine in advance would reduce the uptake of radioactive iodine in the thyroid gland. However, in the Fukushima nuclear power plant accident, few stable iodine tablets were distributed to ordinary people. We cannot rule out the possibility of another nuclear-related accident occurring in the future. If such an accident occurs, cases where it is difficult to obtain stable iodine tablets or some people cannot take iodine due to iodine allergy or the like may also be assumed. Therefore, it is considered as very important to examine in advance the presence of any substances that can be alternatives for stable iodine tablets and the conditions for the intake of such substances.

Material and Method

Stable iodine tablets or their alternative substances were first orally administered to 4-week-old male and female ICR mice, and I-131 (74 kBq/mouse) was orally administered six hours later. Gamma ray measurement in the thyroid gland was performed 24 hours later to examine the inhibitory effect on the uptake of I-131 in the thyroid gland. The ratio of I-131 taken in the thyroid gland per weight to the total dosage of I-131 (I-131 thyroid uptake rate) was studied by comparison. Substances containing iodine, such as dried kelp, kelp tea, oral rinse containing povidone-iodine, and eggs of chickens fed with seaweed-blended feedstuff were administered, and substances containing no iodine, such as potassium perchlorate and mefenamic acid were further administered. In addition, 12-week-old mother mice were administered with the same substances as those for the 4-week-old mice as well as I-131, and the uptake of I-131 in the thyroid gland of their 0- to 15-day-old infant mice was also observed.

Findings

(1) With the substances containing iodine, the uptake of I-131 in the thyroid gland was inhibited according to the iodine content, regardless of their type.

(2) As for the substances containing no iodine, potassium perchlorate showed an effect equivalent to stable iodine tablets, suggesting that it can serve as an alternative substance for stable iodine tablets for those who are prohibited from ingesting iodine. In addition, mefenamic acid (Pontal), which is used as an anti-inflammatory analgesic, also showed an inhibitory effect on the uptake of I-131 in the thyroid gland.

(3) Even though an amount of iodine equivalent to the iodine intake of an average Japanese person was administered to the mice for 10 days, no inhibitory effect on the uptake of I-131 in the thyroid gland was observed. This suggests that, in the event of radioactive iodine exposure, it is necessary to administer stable iodine tablets (or alternative substances) regardless of the daily iodine intake.

(4) In terms of the I-131 uptake in the thyroid gland via milk per weight of infant mice, the younger the mouse (especially 0- and 1-day-old mice), the more uptake of I-131 was observed.

(5) An inhibitory effect on I-131 uptake via milk from substances containing iodine in infant mice was observed. However, the younger the mouse, the weaker of the effect became. This suggests that, even if stable iodine tablets are administered to a mother mouse, its effect may not be fully conveyed to her newborn infant mice.

放射性ヨウ素被ばく時の安定ヨウ素剤とその代替物質に関するマウスの研究からわかったこと

本行忠志 (Osaka University, Japan)

はじめに

原発事故等で大量の放射性ヨウ素による被ばくが予想される時、あらかじめ安定ヨウ素を摂取していれば、放射性ヨウ素の甲状腺への取込みが抑制されるが、福島原発事故では、安定ヨウ素剤は一般の人にはほとんど配布されなかった。今後も核に関連する事故が起こる可能性は否定できず、その際、安定ヨウ素剤の入手が困難な場合や、ヨウ素アレルギー等でヨウ素摂取ができない場合も想定される。そこで、安定ヨウ素剤の代替となり得る物質の存在やその摂取条件を調べておくことは非常に重要と考えられる。

材料と方法

4週齢雌雄 ICR マウスにあらかじめ安定ヨウ素剤やその代替物質を経口投与し、その 6 時間後に I 131(74kBq/ 匹を経口投与した。そして、 24 時間後に甲状腺のガンマ線計測を行い、 I-131 の甲状腺取込みの抑制効果を調べた。 I-131の総投与量に対する体重あたりの甲状腺への取込みの割合（ I-131 甲状腺摂取率）を比較検討した。ヨウ素含有物質として、干しコンブ、コンブ茶、ポビドンヨード含嗽液、ヨード卵等を投与し、さらにヨウ素非含有物として過塩素酸カリウムやメフェナム酸等を投与した。また、 12 週齢雌母マウスに 4 週齢マウスと同様の物質および I-131 を投与し、その乳仔マウス 0 15 日齢の I-131の甲状腺取込みも観察した。

わかったこと

(1) ヨウ素含有物質においてはその種類にかかわらず、ヨウ素の含有量に応じて I 131 の甲状腺取込みは抑制された。

(2) ヨウ素非含有物質においては過塩素酸カリウムが安定ヨウ素剤と同程度の効果を示し、ヨウ素摂取禁の人への安定ヨウ素剤代替物質となり得ることが示唆された。また、消炎鎮痛薬のメフェナム酸（ポンタール）にも I-131 の甲状腺取り込み抑制効果が認められた。

(3) 日本人のヨウ素摂取量の相当量をマウスに 10 日間投与しても I-131 の甲状腺取込みの抑制効果は認められなかった。このことは、放射性ヨウ素被ばく時には、日頃のヨウ素摂取量にかかわらず安定ヨウ素剤（あるいは代替物質）を投与する必要があることを示唆している。

(4) 経母乳による乳仔マウスの I 131 の体重あたりの甲状腺取込みは若齢ほど（特に0 および 1 日齢）多く見られた。

(5) 経母乳による乳仔マウスのヨウ素含有物による I 131 の取込みの抑制効果

が認められたが、若齢ほどその効果が弱く、母親へ安定ヨウ素剤を投与し

ても新生児にはその効果が十分伝わらない可能性が示唆された。