Biodistribution and Dosimetry of Free ²¹¹At, ¹²⁵I⁻ and ¹³¹I⁻ in Rats

Abstract

¹³¹I is widely used for therapy in the clinic and ¹²⁵I and ¹³¹I, and increasingly ²¹¹At, are often used in experimental studies. It is important to know the biodistribution and dosimetry for these radionuclides to determine potential risk organs when using radiopharmaceuticals containing these radionuclides. The purpose of this study was to investigate the biodistribution of ¹²⁵I⁻, ¹³¹I⁻, and free ²¹¹At in rats and to determine absorbed doses to various organs and tissues. Male Sprague Dawley rats were injected simultaneously with 0.1–0.3 MBq ¹²⁵I⁻ and 0.1–0.3 MBq ¹³¹I⁻, or 0.05–0.2 MBq ²¹¹At and sacrificed 1 hour to 7 days after injection. The activities and activity concentrations in organs and tissues were determined and mean absorbed doses were calculated. The biodistribution of ¹²⁵I⁻ was similar to that of ¹³¹I⁻ but the biodistribution of free ²¹¹At was different compared to ¹²⁵I⁻ and ¹³¹I⁻. The activity concentration of radioiodine was higher compared with ²¹¹At in the thyroid and lower in all extrathyroidal tissues. The mean absorbed dose per unit injected activity was highest to the thyroid. ¹³¹I gave the highest absorbed dose to the thyroid, and ²¹¹At gave the highest absorbed dose to all other tissues studied.

Introduction

Radioactive iodine has long been used for medical purposes. ¹³¹I is the radionuclide most widely used for treatment of various thyroid disorders, such as hyperthyroidism and thyroid cancer. This is due to the fact that iodide mainly accumulates in the thyroid gland and in differentiated thyroid carcinoma cells, but also since ¹³¹I has decay properties favorable for treatment. Iodine is transported into the thyroid follicular cells by the sodium iodide symporter (NIS).¹

Astatine is the heaviest of the halogens, the chemical group also containing iodine. Astatine is thought to have properties similar to iodine in regard to thyroid uptake and excretion, for example, ²¹¹At is probably transported partly by NIS,^2,3 although ²¹¹At seems to have metallic properties resembling those of polonium.^4,5 Due to an assumed near optimal LET for therapy ²¹¹At is very suitable for therapeutic applications.^{6 211}At emits, for example, α-particles with a very short range in tissue compared to most of the electrons emitted by ¹³¹I, together with a lower photon emission.

Animal models are often used to test new radiopharmaceuticals for treatment of tumor diseases, including both mice and rats. This is an important area of research to improve already existing and new treatment methods, especially for disseminated tumor disease, when using radioactive substances labeled with ¹³¹I or ²¹¹At. It is common that a small amount of ¹³¹I⁻ or free ²¹¹At is released from the radiopharmaceutical in vivo. It is then important to know the biodistribution and the absorbed dose to normal tissues for ¹³¹I and ²¹¹At to determine potential risk organs when using such radiopharmaceuticals for treatment. To our knowledge, no biodistribution data of free ²¹¹At can be found in humans. For pharmaceuticals directly labeled with radioiodine the released iodine will most probably be in the iodide chemical form. For indirectly iodinated pharmaceuticals, the chemical form of the released radioiodine might vary. For astatine At⁻ is the best defined valence state and it is expected to be stable in acidic and basic solutions containing reducing agents.⁷ However, to our knowledge no information can be found on the behavior of At⁻ in solutions without reducing agents. In this article, we therefore, use the term “free astatine” to include all possible oxidation states of astatine occurring in vivo.

The biodistribution of ¹²⁵I⁻, ¹³¹I⁻ and free ²¹¹At has been studied in mice.^8–12 The results demonstrate clear differences between the radionuclides and show a higher accumulation of ²¹¹At compared to radioiodine in all organs and tissues except the thyroid. The difference in size between rats and mice makes organ sampling easier in rats, particularly for the smaller organs (such as thyroid which is one of the risk organs), which can be difficult to entirely excise alone in mice. This may result in a more extensive use of rat animal models. To our knowledge, biodistribution data of iodide and free ²¹¹At in rats has been presented in only one article, published in 1953.¹³ There is thus, a need to repeat such a study. Furthermore, at that time the astatine chemistry was only known to some extent.

The purpose of this study was to investigate and compare the biokinetics of ¹²⁵I⁻, ¹³¹I⁻, and free ²¹¹At in rats and to determine absorbed doses from these radionuclides to various organs and tissues.

Materials and Methods

Animal model

Male Sprague Dawley rats (Scanbur AB) weighing 180–210 g were used in all studies. The rats were kept in groups of 5 individuals per cage. Drinking water and autoclaved food were given ad libitum. For 5 days, before injection, the animals were given autoclaved food with reduced iodine content (0.05 ppm), at all other times it was regular autoclaved laboratory food (iodine content 2 ppm). The studies were approved by the Ethics committee for Animal Research at University of Gothenburg.

Radionuclides

¹²⁵I was delivered (as NaI) from PerkinElmer and ¹³¹I was delivered (as NaI) from GE Healthcare. ²¹¹At was produced through the ²⁰⁹Bi(α,2n)²¹¹At reaction and delivered from the Cyclotron and PET Unit at Rigshospitalet in Copenhagen, Denmark. The extraction procedure of the ²¹¹At has previously been presented.¹⁴ Throughout this article, ²¹¹At refers to “free astatine” which probably consists not only of ²¹¹At⁻ but also, to some extent, of other oxidation states.

The decay data for ¹²⁵I, ¹³¹I and ²¹¹At is shown in Table 1.

Table 1.

Decay Data for ¹²⁵I, ¹³¹I, and ²¹¹At¹⁷

			Energy per decay (keV/Bq·s)
Radionuclide	Decay mode	T1/2	Electrons	α-Particles	Photons
125I	EC	59 days	17	—	42
131I	β−	8.0 days	190	—	380
211At	α, EC	7.2 hours	0.048	6900	27

α-particle data for ²¹¹At was calculated from the complex disintegration of ²¹¹At and ²¹¹Po.

Administration and organ sampling

Radioactive solutions were prepared using sodium radioiodide dissolved in phosphate-buffered saline (PBS, pH 7.2). The syringes and needles were weighed individually before and after adding the solution, and also after injection, to determine the amount of radioactive solution administered. The rats were divided into two study groups. The first study comprised of 30 animals, which were further divided into six groups with 5 animals in each group. These were injected intravenously with 0.1–0.3 MBq ¹²⁵I⁻ and 0.1–0.3 MBq ¹³¹I⁻ in 0.15 mL of PBS. The rats were sacrificed by cardiac puncture under anaesthesia with Mebumal (sodium pentobarbital 60 mg/mL; Apoteksbolaget AB) at 1, 6, 18, 24, 72 hours, or 7 days after injection. The animals were immediately dissected and radioactivity measurements were made ex vivo on trachea (including thyroid), salivary glands, blood (0.5–1 mL, from cardiac puncture), lungs, heart, liver (right lobe), kidney (right), stomach, muscle from neck, brain, spleen, and sections of large and small intestine.

The second study involved 20 rats divided into four groups with five animals in each group and injected with 0.05–0.2 MBq ²¹¹At in 0.15 mL of PBS. Also here, the syringes and needles were weighed individually before and after adding the solution, and also after injection. The rats were sacrificed as described above at 1, 5, 18, or 24 hours after injection. The animals were immediately dissected and all tissue samples were weighed. Radioactivity measurements were made ex vivo on thyroid, salivary glands, blood (1 mL, from cardiac puncture), lungs, heart, liver (right lobe), kidney (right), stomach, muscle from neck, spleen, and sections of large and small intestine.

Radioactivity measurements

Injected activity was individually determined by measurements on the syringes before injection using an ion chamber (CRC-15 dose calibrator; Capintec), and also by measuring syringe remnants and batch solutions in a gamma-counter (Wallac 1480 WIZARD^® 3′′; Wallac Oy). Radioactivity measurements made ex vivo on the rat organs were performed using the gamma-counter. Corrections were made for dead time loss, self-attenuation, spill-over (when performing measurements on ¹²⁵I and ¹³¹I simultaneously), radioactive decay, and background. The measurement time was adjusted to allow the counts in the sample to exceed 1000 counts above the background level; this resulted in a detection error, which was less than 3%.

The activities in the organs and tissues at time t, a_tissue(t), were calculated as percent of the injected activity (%IA):

Eq. 1

and the activity concentrations at time t, c_tissue(t), as percent of the injected activity per gram tissue (%IA/g):

Eq. 2

where A_tissue(t) is the activity in the sample at time t corrected for radioactive decay to t=0, A_inj is the activity injected at time t=0, and m_tissue is the mass of the tissue sample.

The total blood volume (in mL) of the animal, V_blood, was determined using

Eq. 3

where m_body is the body weight in grams.¹⁵

For those tissues where the entire organ was not measured (blood, liver, small intestine and large intestine), calculations of a_tissue(t) were performed by correcting A_tissue(t) to correspond to the whole of the organ (assuming homologous activity distribution inside the organ). The total organ mass was calculated using previous measurements.^13,15 When calculating the c_thyroid(t) from the trachea sample (including thyroid) it was assumed that the trachea had no uptake of iodine and the weight used in this calculation was the mean weight of the thyroids collected in the second study.

Absorbed dose calculations

The mean absorbed dose to the various organs and tissues from the different radionuclides was calculated according to the Medical Internal Radiation Dose (MIRD) formalism:

Eq. 4

where Ã_tissue is the cumulated activity in the organ, E is the mean emitted energy per disintegration and ø is the absorbed fraction. For absorbed dose calculations on thyroids containing radioiodine, the mean weight of the thyroids collected in the biodistribution study of ²¹¹At was used.

Ã_tissue was calculated from the data using

Eq. 5

where t_D is the time to which the absorbed dose contribution is to be calculated, and λ is the radioactivity decay constant. Due to the short range of α-particles and electrons compared to the size of most of the rat organs, the self-absorbed fraction was set to 1.0 for ¹²⁵I and ²¹¹At. For ¹³¹I the self-absorbed fraction was calculated from previously presented results.¹⁶ The cross-absorbed fraction was set to 0 for all organs for all three radionuclides. The activity distribution in the source-organs was assumed to be homogeneous.

E was assumed to include only the electrons emitted for the radioiodine isotopes, and for ²¹¹At E was assumed to concern only the α-disintegrations and was calculated from the complex disintegration of ²¹¹At and ²¹¹Po (Table 1).¹⁷ Table 1 also shows the contribution calculated from the EC disintegration chain of ²¹¹At to ²⁰⁷Bi.

Statistical analysis

The statistical uncertainties in the measurements are represented by standard error of the mean (SEM).

Results

Biodistribution of ¹²⁵I⁻, ¹³¹I⁻ and ²¹¹At

The biodistribution of ¹²⁵I⁻ and ¹³¹I⁻ was determined 1 hour to 7 days after injection [Table 2 shows c_tissue(t), and Fig. 1 shows a_tissue(t)], and the biodistribution of ²¹¹At was determined 1–24 hours after injection [c_tissue(t) is shown in Table 3, and a_tissue(t) in Fig. 1]. The biodistribution of ¹²⁵I⁻ was similar to that of ¹³¹I⁻ and only the data from ¹³¹I⁻ is shown in Figure 1. The thyroid gland showed the highest accumulation of all three radionuclides with a_thyroid=16%IA (SEM=1.5%IA) or c_thyroid=400%IA/g (SEM=40%IA/g) for ¹²⁵I, while for ¹³¹I the uptake was 17%IA (SEM=2.6%IA) or 430%IA/g (SEM=60%IA/g), and for ²¹¹At 2.8%IA (SEM=0.5%IA) or 75%IA/g (SEM=15%IA/g) after 18 hours. For thyroid, the activity concentration was highest at 18 hours for all three radionuclides. In all organs and tissues except the thyroid, the activity concentration of ²¹¹At was higher compared with ¹²⁵I⁻ and ¹³¹I⁻. This was particularly evident for lungs and spleen. ²¹¹At was also retained longer in extrathyroidal tissues compared to ¹²⁵I⁻ and ¹³¹I⁻. The stomach, which is known to express NIS,¹⁸ had a high concentration of all three radionuclides. The activity concentration in the stomach was highest after 6 hours for ¹²⁵I⁻ and ¹³¹I⁻ and after 18 hours for ²¹¹At. The brain had the lowest activity concentration of radioiodine of all organs and tissues studied.

Table 2.

The Activity Concentration [c_tissue(t)], %IA/g, of ¹²⁵I⁻ and ¹³¹I⁻ in Rats (n=5) at 1 Hour to 7 Days After Simultaneous Injection of 0.1–0.3 MBq of ¹²⁵I⁻ and 0.1–0.3 MBq of ¹³¹I⁻

	ctissue(t) (%IA/g)
Tissue	1 hour	6 hours	18 hours	24 hours	72 hours	7 days
125I−
Blood	0.44 (0.02)	0.26 (0.02)	0.09 (0.01)	0.10 (0.01)	0.023 (0.002)	0.006 (0.001)
Brain	0.023 (0.003)	0.018 (0.003)	0.007 (0.001)	0.009 (0.002)	0.0042 (0.0004)	0.0007 (0.0001)
Heart	0.150 (0.003)	0.13 (0.02)	0.035 (0.005)	0.039 (0.005)	0.011 (0.001)	0.0028 (0.0005)
Kidneys	0.30 (0.01)	0.19 (0.02)	0.07 (0.01)	0.07 (0.01)	0.023 (0.002)	0.007 (0.001)
Large intestine	0.22 (0.02)	0.16 (0.01)	0.05 (0.01)	0.05 (0.01)	0.012 (0.003)	0.0035 (0.0005)
Liver	0.18 (0.01)	0.12 (0.01)	0.056 (0.003)	0.06 (0.01)	0.025 (0.003)	0.007 (0.001)
Lungs	0.29 (0.01)	0.27 (0.07)	0.11 (0.01)	0.07 (0.01)	0.019 (0.002)	0.0057 (0.0004)
Muscle	0.081 (0.004)	0.15 (0.09)	0.04 (0.01)	0.14 (0.05)	0.011 (0.003)	0.005 (0.001)
Salivary glands	0.19 (0.01)	0.15 (0.02)	0.05 (0.01)	0.07 (0.01)	0.017 (0.002)	0.005 (0.001)
Small intestine	0.28 (0.04)	0.37 (0.12)	0.10 (0.02)	0.13 (0.04)	0.024 (0.003)	0.004 (0.001)
Spleen	0.20 (0.02)	0.25 (0.11)	0.044 (0.003)	0.04 (0.01)	0.011 (0.002)	0.0029 (0.0003)
Stomach	1.2 (0.1)	4.1 (0.6)	0.61 (0.12)	0.64 (0.13)	0.10 (0.01)	0.006 (0.001)
Thyroid	50 (6)	270 (60)	400 (40)	340 (40)	110 (40)	40 (14)
131I−
Blood	0.46 (0.03)	0.22 (0.02)	0.09 (0.01)	0.09 (0.01)	0.022 (0.002)	0.006 (0.001)
Brain	0.025 (0.003)	0.017 (0.003)	0.007 (0.001)	0.008 (0.001)	0.0045 (0.0004)	0.0011 (0.0001)
Heart	0.16 (0.01)	0.11 (0.02)	0.039 (0.005)	0.036 (0.004)	0.010 (0.002)	0.0030 (0.0005)
Kidneys	0.27 (0.02)	0.17 (0.02)	0.07 (0.01)	0.06 (0.01)	0.023 (0.002)	0.007 (0.001)
Large intestine	0.23 (0.02)	0.13 (0.01)	0.06 (0.01)	0.046 (0.004)	0.011 (0.003)	0.0035 (0.0004)
Liver	0.18 (0.02)	0.11 (0.01)	0.059 (0.005)	0.05 (0.01)	0.024 (0.003)	0.007 (0.001)
Lungs	0.32 (0.02)	0.18 (0.03)	0.07 (0.01)	0.06 (0.01)	0.019 (0.002)	0.0061 (0.0005)
Muscle	0.09 (0.01)	0.14 (0.08)	0.05 (0.01)	0.13 (0.05)	0.012 (0.003)	0.005 (0.001)
Salivary glands	0.20 (0.02)	0.13 (0.02)	0.047 (0.005)	0.07 (0.02)	0.017 (0.002)	0.005 (0.001)
Small intestine	0.30 (0.05)	0.30 (0.10)	0.11 (0.02)	0.11 (0.03)	0.025 (0.003)	0.004 (0.001)
Spleen	0.22 (0.03)	0.19 (0.08)	0.045 (0.005)	0.036 (0.004)	0.011 (0.002)	0.0031 (0.0004)
Stomach	1.2 (0.1)	3.9 (0.6)	0.62 (0.12)	0.54 (0.09)	0.10 (0.01)	0.006 (0.001)
Thyroid	50 (9)	270 (70)	430 (60)	360 (80)	110 (40)	40 (14)

Data are given as the mean (SEM). The data are corrected for physical decay.

SEM, standard error of the mean.

FIG. 1.

The mean activity [a_tissue(t)], %IA, of ¹³¹I⁻ and ²¹¹At in rats (n=5) at 1 hour to 7 days after simultaneous injection of 0.1–0.3 MBq ¹²⁵I⁻ and 0.1–0.3 MBq ¹³¹I or 0.05–0.2 MBq of ²¹¹At. Data are given as the mean±standard error of the mean. The data from ¹²⁵I⁻ was similar to that of ¹³¹I⁻.

Table 3.

The Activity Concentration [c_tissue(t)], %IA/g, of ²¹¹At in Rats (n=5) at 1–24 Hours After Injection of 0.05–0.2 MBq of ²¹¹At

	ctissue, At-211(t) (%IA/g)
Tissue	1 hour	5 hours	18 hours	24 hours
Blood	0.29 (0.02)	0.26 (0.01)	0.22 (0.01)	0.160 (0.005)
Heart	0.53 (0.04)	0.49 (0.03)	0.34 (0.01)	0.29 (0.01)
Kidneys	1.2 (0.1)	1.1 (0.1)	0.72 (0.04)	0.61 (0.02)
Large intestine	0.46 (0.04)	0.63 (0.19)	0.30 (0.02)	0.28 (0.03)
Liver	0.41 (0.03)	0.36 (0.02)	0.25 (0.01)	0.22 (0.01)
Lungs	3.0 (0.2)	2.50 (0.05)	1.8 (0.1)	1.60 (0.03)
Muscle	0.32 (0.04)	0.34 (0.05)	0.24 (0.02)	0.23 (0.04)
Salivary glands	0.95 (0.11)	0.76 (0.03)	0.52 (0.01)	0.47 (0.02)
Small intestine	0.82 (0.06)	0.95 (0.21)	0.50 (0.03)	0.43 (0.01)
Spleen	3.0 (0.2)	2.4 (0.1)	1.7 (0.1)	1.5 (0.1)
Stomach	2.3 (0.4)	3.1 (0.3)	3.6 (0.5)	2.4 (0.2)
Thyroid	35 (9)	41 (11)	75 (15)	37 (3)

Data are given as the mean (SEM). The data are corrected for physical decay.

Dosimetry

The dosimetric calculations for ¹²⁵I, ¹³¹I, and ²¹¹At show that the thyroid received the highest absorbed dose per injected activity, with D_{thyroid, I-125}=2600 mGy/MBq and D_{thyroid, I-131}=21000 mGy/MBq (Table 4) and D_{thyroid, At-211}=18000 mGy/MBq (Table 5) at t=∞. The highest mean absorbed doses to extrathyroidal tissues and organs were found in stomach, heart, and small intestine for ¹²⁵I and ¹³¹I, and in stomach, lungs and spleen for ²¹¹At. ²¹¹At delivered a much higher absorbed dose per injected activity to all extrathyroidal organs compared to ¹²⁵I and ¹³¹I (at t=∞), and ¹²⁵I delivered the lowest absorbed dose per unit injected activity.

Table 4.

The Mean Absorbed Dose Per Unit Injected Activity, mGy/MBq, of ¹²⁵I and ¹³¹I in Rats Obtained at Different Points in Time After Injection (n=5) and the Corresponding m_tissue Used for Calculations (n=30)

		D(t) (mGy/MBq)
Tissue	mtissue(g)	18 hours	24 hours	72 hours	7 days	∞
125I
Blood	0.67 (0.01)	0.40 (0.01)	0.46 (0.01)	0.72 (0.04)	0.85 (0.05)	0.87 (0.05)
Brain	0.76 (0.04)	0.027 (0.002)	0.033 (0.002)	0.05 (0.01)	0.08 (0.01)	0.08 (0.01)
Heart	0.59 (0.02)	1.3 (0.1)	1.3 (0.1)	1.4 (0.1)	1.5 (0.1)	1.5 (0.1)
Kidney	0.71 (0.02)	0.30 (0.02)	0.34 (0.02)	0.56 (0.03)	0.70 (0.04)	0.74 (0.04)
Large intestine	0.33 (0.01)	0.20 (0.02)	0.23 (0.02)	0.38 (0.03)	0.44 (0.04)	0.46 (0.03)
Liver	1.95 (0.05)	0.18 (0.01)	0.22 (0.01)	0.45 (0.03)	0.58 (0.04)	0.61 (0.04)
Lung	0.66 (0.03)	0.34 (0.08)	0.40 (0.09)	0.61 (0.08)	0.70 (0.08)	0.73 (0.08)
Muscle	0.23 (0.01)	0.15 (0.05)	0.21 (0.05)	0.63 (0.15)	0.74 (0.15)	0.76 (0.15)
Salivary glands	0.36 (0.01)	0.22 (0.02)	0.26 (0.02)	0.44 (0.05)	0.55 (0.05)	0.57 (0.05)
Small intestine	0.74 (0.02)	0.39 (0.08)	0.46 (0.08)	0.89 (0.20)	1.0 (0.2)	1.0 (0.2)
Spleen	0.71 (0.02)	0.25 (0.06)	0.28 (0.06)	0.40 (0.08)	0.46 (0.08)	0.48 (0.08)
Stomach	1.41 (0.03)	3.9 (0.4)	4.3 (0.4)	5.9 (0.5)	6.3 (0.5)	6.3 (0.5)
Thyroid	0.040 (0.002)	470 (60)	680 (80)	1700 (100)	2400 (200)	2600 (300)
131I
Blood	0.67 (0.01)	4.4 (0.1)	5.0 (0.1)	7.4 (0.3)	8.5 (0.4)	8.6 (0.4)
Brain	0.76 (0.04)	0.30 (0.02)	0.36 (0.02)	0.58 (0.04)	0.78 (0.07)	0.82 (0.07)
Heart	0.59 (0.02)	13 (1)	14 (1)	15 (1)	16 (1)	16 (1)
Kidney	0.71 (0.02)	3.0 (0.2)	3.4 (0.2)	5.6 (0.2)	6.8 (0.3)	7.0 (0.4)
Large intestine	0.33 (0.01)	2.1 (0.1)	2.4 (0.1)	3.7 (0.2)	4.1 (0.2)	4.2 (0.2)
Liver	1.95 (0.05)	2.1 (0.1)	2.4 (0.1)	4.5 (0.3)	5.7 (0.4)	5.9 (0.4)
Lung	0.66 (0.03)	3.2 (0.3)	3.6 (0.3)	5.5 (0.3)	6.3 (0.3)	6.5 (0.3)
Muscle	0.23 (0.01)	1.5 (0.5)	2.0 (0.4)	5.8 (1.2)	7.0 (1.2)	7.1 (1.2)
Salivary glands	0.36 (0.01)	2.2 (0.2)	2.5 (0.3)	4.1 (0.5)	4.9 (0.5)	5.0 (0.5)
Small intestine	0.74 (0.02)	4.2 (0.8)	4.9 (0.9)	8.8 (2.0)	10 (2)	10 (2)
Spleen	0.71 (0.02)	2.5 (0.5)	2.8 (0.5)	3.9 (0.7)	4.5 (0.6)	4.6 (0.6)
Stomach	1.41 (0.03)	41 (4)	45 (4)	60 (5)	64 (5)	64 (5)
Thyroid	0.040 (0.002)	4200 (700)	6100 (800)	15000 (2000)	19000 (2000)	21000 (2000)

Data are given as the mean (SEM).

Table 5.

The Mean Absorbed Dose Per Unit Injected Activity, mGy/MBq, of ²¹¹At in Rats Obtained at Different Points in Time After Injection (n=5) and the Corresponding m_tissue Used for Calculations (n=20)

		D(t) (mGy/MBq)
Tissue	mtissue(g)	18 hours	24 hours	∞
Blood	0.96 (0.04)	87 (3)	93 (3)	98 (3)
Heart	0.53 (0.02)	170 (10)	180 (10)	190 (20)
Kidney	0.65 (0.02)	370 (20)	390 (20)	390 (20)
Large intestine	0.35 (0.01)	200 (40)	210 (40)	210 (40)
Liver	1.97 (0.08)	160 (10)	160 (10)	230 (20)
Lung	0.60 (0.01)	1000 (100)	1100 (100)	1100 (100)
Muscle	0.23 (0.01)	120 (10)	120 (10)	130 (10)
Salivary glands	0.43 (0.02)	250 (10)	270 (10)	280 (10)
Small intestine	0.78 (0.02)	310 (50)	330 (50)	340 (50)
Spleen	0.69 (0.02)	950 (50)	1000 (50)	1000 (50)
Stomach	1.33 (0.02)	1000 (100)	1200 (100)	1200 (100)
Thyroid	0.040 (0.002)	16000 (4000)	17000 (4000)	18000 (4000)

Data are given as the mean (SEM).

Discussion

No significant difference was found between the biodistributions of ¹²⁵I⁻ and ¹³¹I⁻ (Table 2). This was expected since the very small difference in atomic mass should not have an impact on the biokinetics of a radionuclide. Results showed that the trachea (including thyroid) and stomach selectively accumulated iodide over time (Table 2 and Fig. 1). This was also true for ²¹¹At (Table 3 and Fig. 1) and was expected since both organs are known to express NIS.¹⁸ The higher concentration of ¹²⁵I⁻ and ¹³¹I⁻ than ²¹¹At in thyroid could only to a low extent be explained by the difference in thyroid tissue sampling technique between the studies (a possible overestimation of ¹²⁵I⁻ and ¹³¹I⁻ concentration). No other organs showed significant increase in activity concentration from 1 to 5–6 hours after injection. The salivary glands, which are also known to express NIS,¹⁸ showed low activity concentrations of all three radionuclides. The biodistribution of ²¹¹At also showed high activity concentrations in lungs and spleen and to a lesser extent in the kidneys. This supports our previous findings that the uptake/transport of ²¹¹At is also dependent on mechanisms other than NIS.³ Results also showed that the retention time of ²¹¹At was longer compared with¹²⁵I⁻ and ¹³¹I⁻. A possible explanation for this is that the biochemical properties of ²¹¹At in vivo, which are largely unknown, might change after injection. Another possibility is the existence of an uptake mechanism different from NIS.

Two animals showed unexpectedly low activity concentrations of both ¹²⁵I⁻ and ¹³¹I⁻ in the thyroid at 6 and 72 hours and 7 days and resulted in high SEM values for these time points (Table 2). No obvious abnormalities could be found in these individuals, and the activity concentrations in other organs and tissues did not differ. A possible reason is that the animals in question had a slower metabolic rate. The individual food intake of the animals was not monitored and differences might change the amount of stable iodine accumulated in the thyroid before injection with radioiodide. Results from our pilot studies without the reduced iodine diet showed similar unexpectedly low activity concentrations of radioiodide, but in a larger number of animals (data not shown). The findings might be explained by the relatively high iodine concentration (2 ppm) in regular laboratory food compared to normal human diets (about 0.1 ppm). Therefore, the food with reduced iodine content (0.05 ppm), which better resembles a normal human diet was used, which seems to have reduced the effect.

In the previous study, from 1953 on female Long-Evans rats kept on standard laboratory chow, the maximum uptake in thyroid for ¹³¹I and ²¹¹At occurred 24 hours after injection.¹³ In the present study, the maximum activity of ¹³¹I found in the thyroid was about half of the maximum activity found by Hamilton et al. (17%IA compared to 28%IA), although our rats had probably received food with lower iodide content (the iodide content was not specified in the older study). Similar relations (lower concentrations in the present study) were found for most of the other organs and tissues (except spleen which had a similar maximum uptake). The reason for this discrepancy is not known; however, the most possible explanation is differences in stable iodine intake via the food and rat strain. The differences in sex and amount of administered ¹³¹I (0.1–0.3 vs. 1.4 MBq) would most probably not affect the biodistribution. In the older study no organs or tissues except the thyroid gland showed a selective accumulation of radioiodide over time. This is in contrast to the results found in the present study, which also showed a selective accumulation in the stomach.

The maximum activity of ²¹¹At found in the thyroid in the older study was about one-tenth of the maximum activity of radioiodide, while it was about one-fifth in the present study. The maximum activity of ²¹¹At in the thyroid in our study (at 18 hours) was similar to that found previously (at 24 hours), (2.8%IA compared to 2.7%IA). For the other organs and tissues, the older study gave somewhat higher maximum activities in general. In both studies, only the thyroid gland and the stomach showed selective accumulation of ²¹¹At over time.

In a third limited study from 1954, the thyroidal uptake of ²¹¹At was 1.2%IA 1.5 hours after injection.¹⁹ This is comparable to the results found in the present study (1.1%IA 1 hour after injection).

Studies on the biodistribution of ²¹¹At in nude mice^11,12 have shown that the uptake of ²¹¹At was highest in the thyroid gland, lungs, spleen, and stomach. These studies also demonstrated higher activity concentrations of ²¹¹At compared to radioiodine in extrathyroidal organs and tissues and Lundh et al.¹² also showed a longer retention time of ²¹¹At compared to ¹²⁵I⁻. These results are all in accordance with the results found in this study. In contrast to the results found in this study, previous studies on nude mice present a maximum thyroidal uptake of ²¹¹At at approximately 4 hours after injection.^10,12 The reason for this difference is probably due to lower metabolic rates in rats compared to mice.²⁰

In the dosimetric calculations, only the electrons and α-particles were considered (see Table 1 for decay data). This assumes no photon contribution to the absorbed dose and resulted only in a small underestimation of the absorbed dose. Previous studies have shown that the photon absorbed dose contribution is, for example, about 7% for the mouse thyroid gland for ¹²⁵I.²¹ Concerning ²¹¹At the energy released from electrons and photons per decay is negligible compared to that from the alpha particles. However, these results are from animal models and a translation of this data to the human system must be performed with caution, for all three radionuclides, as the photon contribution may be higher in clinical situations.^22,23

Results from the absorbed dose calculations showed that the thyroid received the maximum mean absorbed dose per unit injected activity, for all three radionuclides (Tables 4 and 5). This was expected due to the high activity concentration (400%IA/g for ¹²⁵I⁻, 430%IA/g for ¹³¹I⁻ and 75%IA/g for ²¹¹At). The highest mean absorbed dose per unit injected activity to the thyroid was found for ¹³¹I followed by ²¹¹At (2.1·10⁴ mGy/MBq compared to 1.8·10⁴ mGy/MBq, values compared at t=∞), which is explained by the much higher activity concentrations found for ¹³¹I⁻ and the much shorter half-life of ²¹¹At. For the other organs and tissues ²¹¹At delivered a higher absorbed dose per unit injected activity in general, due to the larger amount of energy deposited per disintegration and longer retention time of ²¹¹At compared to ¹²⁵I⁻ and ¹³¹I⁻. In a clinical application, the tissue accumulation of ²¹¹At may be blocked, which would potentially reduce the mean absorbed dose to extrathyroidal tissue.¹¹

Conclusions

The biodistribution of free ²¹¹At is different compared to ¹²⁵I⁻ and ¹³¹I⁻. The thyroid gland was found to accumulate ¹²⁵I⁻, ¹³¹I⁻, and ²¹¹At selectively, although the activity concentration of ²¹¹At was only about one-fifth of that of radioiodide. Absorbed dose calculations showed that ²¹¹At gave the highest mean absorbed dose per unit injected activity to all extrathyroidal tissues.

Acknowledgments

The authors thank Lilian Karlsson for her skillful handling of the animals and Dr. Sture Lindegren for the generous supply of ²¹¹At. This study was supported by grants from the European Commission FP7 Collaborative Project TARCC HEALTH-F2-2007-201962, the Swedish Research Council, the Swedish Cancer Society, BioCARE—a National Strategic Research Program at University of Gothenburg, the Swedish Radiation Safety Authority and the King Gustav V Jubilee Clinic Cancer Research Foundation. The work was performed within the EC COST Action BM0607.

Disclosure Statement

The authors declare that they have no conflict of interest.