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6-Iodo-L-m-Tyr for tumor imaging
茨城県立医療大学紀要
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S

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第 24 巻

Original Article

Volume 24

Implications for the Use of 6-Radioiodinated L-meta-tyrosine as a
Tumor Imaging Agent: Mouse Biodistribution, Renal
Autoradiography, and Colon Cancer DLD-1 Cell Studies
Naoto Shikano1), Nobuyuki Yagi1), Ryuichi Nishii2), Syuichi Nakajima1), Masato Ogura1),
Ayako Ikeda1), Masato Kobayashi3), Naoto Yamaguchi4), Keiichi Kawai3)
1)

Department of Radiological Sciences, Ibaraki Prefectural University of Health Sciences
Diagnostic and Therapeutic Nuclear Medicine Team Department of Molecular Imaging and Theranostics, National Institute ofRadiological Science
3)
Division of Health Science, Graduate School of Health Sciences, Kanazawa University
4)
Center for Medical Science, Ibaraki Prefectural University of Health Sciences
2)

Abstract
Introduction: 125I-6-iodo-L-meta-tyrosine (6-125I-L-mTyr) has been used to measure cerebral aromatic L-amino aciddecarboxylase activity but there is no report of its use as a tumor imaging agent. We therefore conducted mouse biodistribution,
renal autoradiography and DLD-1 colon cancer cell studies to investigate the utility of 6-125I-L-mTyr as a tumor imaging agent.
Methods: 6-125I-L-mTyr (11.1 kBq/0.1 mL for biodistribution, 670 kBq/0.1 mL for autoradiography) was administered to
6-week-old male ddY mice by injection into the tail vein and the time-course of 6-125I-L-mTyr biodistribution was determined.
After five minutes, the kidney was dissected and renal autoradiography was performed. Separately, DLD-1 cells (5.0×105)
suspended in 5 mL of Dulbecco’s Modified Eagles Medium containing fetal bovine serum were cultured in 6 cm diameter
culture dishes under typical conditions and time course measurements of the uptake of 6-125I-L-mTyr (18.5 kBq) into the cells
were obtained. Furthermore, the transport systems were analyzed by uptake-inhibition experiments using 2-amino-bicyclo[2,2,1]
heptane-2-carboxylic acid (a system L–specific inhibitor) and 2-(methylamino)isobutyric acid (a system A–specific inhibitor).
Results: The biodistribution and renal autoradiogram studies demonstrated low accumulation of 6-125I-L-mTyr in normal mouse
tissues and rapid clearance from the blood. Membrane transport inhibition studies indicated that approximately 90% of 6-125I-LmTyr was transported by system L into DLD-1 cells.
Conclusions: The DLD-1 cell experiments showed that metabolically stable 6-125I-L-mTyr accumulates in tumor cells via
system L transport and the mouse experiments showed that 6-125I-L-mTyr is excreted rapidly in the urine. 6-125I-L-mTyr can beused as a tumor imaging agent.
Key words: 6-iodo-L-meta-tyrosine, system L, neutral amino acid, DLD-1 cells, renal distribution, biodistribution

Address: Naoto Shikano. Department of Radiological Sciences, Ibaraki Prefectural University of Health Sciences, 4669-2 Ami, Ami-machi,
Inashiki-gun, Ibaraki 300-0394, Japan
Phone: +81-29-840-2217, Fax: +81-29-840-2317, E-mail address: sikano@ipu.ac.jp

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茨城県立医療大学紀要

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1. Introduction

第 24 巻frequency of colon cancer has recently increased.5) In
addition, the cellular accumulation of L-[14C(U)]tyrosine,

We previously conducted studies on the utility ofthe parent compound used to design the drug 6-125I-L-

radioactive iodine–labeled meta-tyrosine (m-Tyr) as amTyr, was compared to that of 6-125I-L-mTyr. 125I has adiagnostic imaging agent for use in single-photon

long half-life and is suitable for basic studies whereas

emission computed tomography (SPECT)1-4) and reported

123

the high metabolic stability and amino acid (AA) transport

used clinically.

affinity of L-meta-tyrosine (L-mTyr).

1)

I-labeled compounds have a shorter half-life and are

mTyr was first

separated into D- and L-isomers using high-performance
2. Materials and Methods

liquid chromatography (HPLC), then both isomers werelabelled with non-carrier-added

I. The geometric125

isomers of radioiodinated L-meta-tyrosine, 125I-6-iodo-Lmeta-tyrosine (6-125I-L-mTyr) and125

I-4-iodo-L-meta-

The animal experiments were approved by the ethics
committees of the universities affiliated with this study.

tyrosine (4-125I-L-mTyr), were separated by HPLC fromthe reaction mixture of labelled and non-carrier-added
I-L-meta-tyrosine ( I-L-mTyr).

125

125

2.1. Materials
The radiolabel precursor (D,L-mTyr) was purchased

2)

6- I-L-mTyr (Fig. 1) was reported to bind preferentiallyfrom Sigma-Aldrich Japan Co. 125I-NaI was purchasedto cerebral aromatic L-AA decarboxylase.2) We proposedfrom Amersham Pharmacia Biotech. L-[14C(U)] tyrosinethat 6-125I-L-mTyr, a new radiopharmaceutical, enters thewas purchased from American Radiolabeled Chemicals

brain via a stereospecific AA transport system and used

Inc. DLD-1 (human colon adenocarcinoma, JCRB9094)

SPECT to assess both the AA transport system and tocells were purchased from JCRB. Plastic tissue culture

quantify

L-3,4-

dishes (6 cm diameter, Cat. no. 150288) and plastic culture

dihydroxyphenylalanine (DOPA) decarboxylase. SPECT

flasks (surface area 25 cm2, Cat. no. 163371) were

provides information regarding the functional status ofpurchased from Nalge Nunc International. 2-Amino-

dopaminergic presynaptic neurons.

bicyclo[2,2,1]heptane-2-carboxylic acid (BCH, a system

125

metabolically

activeTyrosine is an amino acid and is required for protein

L–specific inhibitor), 2-(methylamino)isobutyric acidsynthesis during tumor cell growth and proliferation.

(MeAIB, a system A–specific inhibitor), natural L/D-

Consequently, it is likely that radioiodinated tyrosine

AAs, and reagent grade chemicals were purchased fromderivatives, including 6-123/125I-L-mTyr, would accumulate

Kanto Chemical Co., Inc.

in tumors.
In the present study, we focused on 6-125I-L-mTyr and2.2. Labeled AAs

conducted biodistribution and kidney autoradiography

L-mTyr was separated from D,L-mTyr by high-studies in mice to assess the basic properties of 6- I-L-

performance liquid chromatography (HPLC) using a

mTyr as a radiopharmaceutical for tumor imaging. Next,

Crownpak CR(-) chiral column (Daicel Chemical

we investigated the transport mechanism and accumulation

Industries, Ltd.). The separated L-mTyr was labeled withof 6-125I-L-mTyr in DLD-1 colon cancer cells. The

125

125

I-NaI (8.1×1019 [Bq/mol]) and chloramine-T (Kanto

Chemical Co., Inc.). Radioiodination of L-mTyr produces
two major geometric isomers, 6-iodo- and 4-iodo-LmTyr,2) which were separated by HPLC using a Nova-Pak
C18 column (Waters Co.). The radiochemical yield wasdetermined using a silica gel thin-layer chromatographykit (Merck). The labeled compounds were used in theirFig. 1 Structure of 6-iodo-L-meta-tyrosine (6-I-L-mTyr).

noncarrier-added condition.

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6-Iodo-L-m-Tyr for tumor imaging

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was removed, then the cells were washed twice with 5.0

2.3. Biodistribution Study in Mice
In vivo experiments were performed to investigate themL of cold PBS and solubilized in 2.0 mL of NaOH (2 N).

biodistribution of 6- I-L-mTyr administered to mice.

The radioactivity of each aliquot (500 µL) was counted

Male ddY mice (6 weeks old, 3-5 per group) were injectedusing an auto-well counter (Aloka, Japan; ARC-380). The

with 0.1 mL of saline containing 11.1 kBq of 6-125I-L-

results for each set of experimental conditions are reported

mTyr via the tail vein. The animals were anesthetized withas the mean of 4 dishes.

125

ether and sacrificed painlessly by heart puncture at 2, 5,
10, 15 and 30 min after injection, then the brain, heart,

2.6. Labeled AA Uptake Inhibition Studylung, liver, pancreas, spleen, kidney, stomach andTransport system L in DLD-1 cells was analyzed byintestines were removed and weighed, and the blood wasremoving the culture fluid from the culture dishes, adding

sampled. The radioactivity in each organ was determined

5 mL of 37°C PBS, and incubating the dishes for 10 min

using a well-type scintillation counter (ARC-300; Aloka).

at 37°C, 5% CO2, pH 7.4. The PBS was removed and

The biodistribution of the radioactivity was determined

uptake-inhibition experiments were performed using

for each organ as percent dose/organ and on a per-organ-

6-125I-L-mTyr and the system L-specific and systemweight basis as percent dose/g.

A-specific inhibitors BCH and MeAIB.6) The concentration
dependence of uptake by transport system L wasinvestigated by adding 0.1, 0.5, 1.0, 5.0 or 10.0 mM BCH

2.4. In vivo autoradiography
A ddY male mouse weighing 30 g was injected

to 5 mL of 37°C Na+-free PBS containing 18.5 kBq ofintravenously with 0.1 mL of 6-125I-L-mTyr (670 kBq),

carrier-free 6-125I-L-mTyr dissolved in 137 mM choline-

then 5 minutes later was anesthetized, sacrificed

Cl, 3.7 mM KCl, 8.0 mM K2HPO4, 1.5 mM KH2PO4, 1.8

painlessly, and the kidneys were immediately dissected,

mM CaCl2·2H2O and 1 mM MgCl·6H2O. The dishes were

washed, placed in an embedding medium and frozen at

incubated for 10 min under the conditions described-15°C for 24 hours. A cryostat microtome (Handex,

above, then the uptake solution was removed and the

Shiraimatsu) was used to generate 20 mm axial slices

surfaces of the adhering cells were rinsed twice with 5 mLfrom top to bottom, then the slices were air-dried at

of ice-cold PBS. Next, 2 mL of NaOH (2 N) was added-15°C for 24 hours. The slices were placed in contact

and the dishes were left at room temperature for 24 h towith an imaging plate (Fuji Photo Film; BASTR2040,

solubilize the cells. The radioactivity in an aliquot (500

20×40 cm) for at least 24 hours and the resulting imagesµL) of each sample was measured.

were processed using Bio-Imaging Analyzer (Fuji Photo
Film; BAS-2000).

Transport system A was analyzed in DLD-1 cells using
an uptake solution comprising PBS containing Na+ andthe specific transport system inhibitor MeAIB at 0.1, 0.5,
1.0, 5.0 or 10.0 mM to examine the concentration

2.5. Time Course of 6- I-L-mTyr Uptake
125

DLD-1 cells were cultured to about 80% confluence

dependence of uptake. The dependence of transport on the(3-4 days after inoculation), then the medium was replaced

specific transport system inhibitor concentration waswith 5 mL of phosphate-buffered saline (PBS) containing

determined from the radioactivity levels and the transport

Na+ (37°C, pH 7.4). The cells were incubated at 37°C forsystem was analyzed at the concentration showing the

10 min in a 5% CO2 atmosphere and then the PBS was

greatest inhibitory effect.

replaced with 2.0 mL of uptake solution containing 18.5

We examined the system L specificity of transport ofkBq of 6- I-L-mTyr. Incubation was conducted using

the labeled AAs in DLD-1 cells. The cells were incubated

PBS containing Na uptake solution (NaCl, 137 mM;

in 2 mL of incubation medium with/without Na+ andKCl, 3.7 mM; Na2HPO4, 8.0 mM; KH2PO4, 1.5 mM;

containing 18.5 kBq of 6-125I-L-mTyr or L-[14C(U)]CaCl2·2H2O, 1.8 mM; MgCl·6H2O, 1 mM). The cells

tyrosine for 10 min at 37°C. Next, the cells were treated

were incubated for 5, 10, 15, 30 and 60 min, the medium

with final concentrations of either 5 mM BCH without

125+

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茨城県立医療大学紀要

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Na+ or MeAIB with Na+ in the incubation medium (for6- I-L-mTyr), or 1 mM BCH without Na or MeAIB with
125

+

A

c

第 24 巻mp

B

C

Na+ in incubation medium (for L-[14C(U)] tyrosine). In
medium without Na+, NaCl was replaced with the same
concentration of choline-Cl. We measured the radioactivity
associated with the solubilized cells in 2N NaOH using
the ARC-380 well-type scintillation counter for 6-125I-LmTyr and an LS6500 liquid scintillation counter (Beckman
Instruments) for L-[14C(U)] tyrosine.
2.7. Statistical Analysis
Data were collated as mean ± standard deviation of 3-5

Fig. 2(A) Kidney tissue, (B) fusion image, (C) renal
autoradiography at 5 min after i.v. administration. S1 cells are
distributed in the periphery of the renal cortex (c) whereas S3cells are distributed medially, with some S3 cells found in the
medulla (m). S2 cells are distributed in the middle of the cortex.
A high level of accumulation was observed in renal papilla (p).

measurements, and each experiment was performed in
duplicate. Results were analyzed using the Student’s t test.
Probability levels of P<0.001 were considered to indicate
statistical significance.

3.3. Time Course of 6-125I-L-mTyr Uptake
When the dishes containing DLD-1 cells prepared as
described in Section 2.5 were incubated for 10 min with
6-125I-L-mTyr, 9.90±1.00% of the amino acid was taken
up into the cells and thereafter the amount of 6-125I-L-

3. Results

mTyr in the cells gradually decreased (Fig. 3). Therefore,transport system analyses using a specific transport system
inhibitor were conducted 15 min after the administration

3.1. Biodistribution Study in Mice
6-125I-L-mTyr rapidly moved out of the blood and intoof 6-125I-L-mTyr.

the organs following i.v. administration to normal mice
(Table 1 A, B) and its accumulation in the pancreas

3.4. Labeled AA Uptake Inhibition Study

showed in vivo kinetics characteristic of AAs. Essentially

The inhibition of transport system L was dependent on

no radioactivity accumulation was seen in the stomach.

the concentration of the inhibitor, and maximum inhibition

There was some accumulation in the intestine and liver on

was observed at 5 mM (Fig. 4-A). Consequently, wea per-organ basis but it was minimal on a per-organ-

analyzed transport system L using a BCH concentration

weight basis. The physiological accumulation of 6-125I-L-of 5 mM.

mTyr in normal mouse tissues was low and similar to thatof 4- I-L-mTyr.
125

The inhibition of transport system A was not dependent
on the concentration of the inhibitor. There were some2)

concentrations at which increased uptake was observed,
but there were no statistically significant differences

3.2. In vivo autoradiography
Figure 2 show the results of renal autoradiography 5

between these concentrations (Fig. 4-B). Therefore, we

min after i.v. administration of 6- I-L-mTyr to normal

analyzed transport system A at the MeAIB concentration

mice. Low radioactivity was observed in segment 1 (S1)that provided the greatest inhibitory effect, 5 mM.

125

and segment 2 (S2) of proximal tubules located in lesions,

The contribution of transport system L to the uptake ofsimilar to that of 4-125I-L-mTyr.3) Radioactivity levels in

6-125I-L-mTyr was 91.0±3.0%, which was statisticallythe medulla were very high 5 min after administration.

significant (P<0.001), whereas the contribution ofRadioactivity in segment 3 (S3) located in lesions could

transport system A to 6-125I-L-mTyr uptake was 9.0±4.0%,

not be observed due to the high level of radioactivity inwhich was not statistically significant (P<0.05) (Fig. 4-C).

vicinal tissue such as renal papilla.

Similarly, the contribution of transport system L to theuptake of L-[14C(U)] tyrosine was 97.0±1.0%, which was

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17

6-Iodo-L-m-Tyr for tumor imaging

Table 1 Biodistribution of 6-125I-L-mTyr in normal mice

A. Biodistribution of 6-125I-L-mTyr in mice.

%Dose/g, *%Dose/mL

2 min

5 min

10 min15 min30 minBlood*

8.919±0.831

5.460±0.766

4.395±0.683-

2.828±0.362

Brain

0.721±0.082

0.725±0.139

0.637±0.187

0.623±0.096

0.576±0.064

Pancreas 14.949±3.128

21.599±3.606

31.168±11.725

33.819±10.340

6.643±0.947

Spleen4.640±0.790

3.579±0.537

1.754±0.721

1.275±0.232

1.461±0.041

Stomach4.529±0.759

3.358±0.521

3.032±0.861

2.998±0.456

2.588±0.374

lntestine

4.034±0.542

3.037±0.722

2.611±0.354

2.351±0.158

1.808±0.293

Liver

9.322±0.943

8.686±1.491

9.327±0.911

9.321±0.724

7.940±2.134

Kidney19.029±3.119

17.563±1.813

18.885±1.810

15.782±2.583

13.290±2.327

Heart

6.939±1.029

4.122±1.492

3.498±0.452

2.904±0.288

1.784±0.150

Lung

6.951±0.714

5.105±0.504

4.406±0.464

4.019±0.591

3.148±0.342

15 min30 minAll values are mean ±SD. n=3-5.
B. Biodistribution of 6-125I-L-mTyr in mice.

%Dose/Organ

2 min

5 min

10 minBlood3.568±0.333

2.184±0.306

1.758±0.273-

1.131±0.145

Brain

0.309±0.034

0.311±0.065

0.269±0.083

0.253±0.049

0.246±0.030

Pancreas2.304±0.652

3.310±1.539

2.245±0.698

2.122±0.465

1.025±0.111

Spleen0.386±0.050

0.338±0.065

0.176±0.070

0.153±0.028

0.095±0.023

Stomach1.614±0.282

1.108±0.154

1.046±0.228

0.917±0.209

0.714±0.059

lntestine

8.927±1.876

6.374±1.641

5.438±0.576

4.784±0.664

3.750±0.610

Liver

9.897±1.396

9.401±2.515

10.950±0.742

10.000±0.559 8.766±2.423

Kidney6.641±0.922

5.995±0.304

7.091±0.915

5.756±1.371

4.727±0.945

Heart

0.735±0.245

0.521±0.184

0.329±0.065

0.274±0.030

0.160±0.007

Lung

1.209±0.191

0.949±0.298

0.686±0.110

0.566±0.275

0.490±0.084

All values are mean ± SD. n=3-5.
statistically significant (P<0.001) and the contribution of

12transport system A to L-[ C(U)] tyrosine uptake was10

8.0±3.0%, which was not statistically significant (Fig. 5).
Therefore, the uptake of both L-[14C(U)] tyrosine and6-125I-L-mTyr occurred via similar transport systems.

Uptake [%uptake of administered dose]

14

8
6
4
2
0

0

20

40

60

80

Time [min]Fig. 3 Time course of 6-125I-L-mTyr uptake by DLD-1 cells(n = 4).

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茨城県立医療大学紀要18

第 24 巻A

PBS (Na+(+))

120

PBS (Na+(-))

0

120

60
40
20
0

1

2

3

4

5

6

7

8

9

10

14

Concentration of BCH [mM]B

80
System L

60
40
20

Control100% uptake [relative value]

100

0

120

MeAIB
(1 mM)

Na+-freeBCH(1 mM)

Fig. 5 The effects of inhibitors (MeAIB, BCH) and incubationin Na+-free medium on L-[14C(U)] tyrosine uptake by DLD1 cells. Values represent mean ± SD as the percentage of theadministered dose (n = 4).

80
60
40
20
0
0

1

2

3

4

5

6

7

8

9

10

Concentration of MeAIB [mM]

C
PBS (Na+(+))
PBS (Na+(-))

cerebral dopaminergic presynaptic function.2) Based on
the results of rat brain experiments using inhibitors, wehypothesized that 6-125I-L-mTyr would be useful as adiagnostic imaging agent that reflects both the activity ofbrain AA transporters and the level of DOPA decarboxylase
activity. Therefore, in the present study, we investigated120
6-125I-L-mTyr % uptake [relative value]

L-[ C(U)] tyrosine % uptake [relative value]

uptake [relative value]

80

%

100

the possibility of using 6-125I-L-mTyr as a tumor imaging

100

agent.
Kloss et al. proposed that a suitable radiotracer

80

synthesized using the radioiodination method requires the60

attachment of radioiodine in such a way that the

System L

40

physiological properties of the compound are not20

affected.7) Although this is straightforward for a large
molecule such as a protein, it is challenging for small

0
Control

MeAIB(5 mM)

Na+-freecompounds such as tyrosine. The molecular weight of

BCH(5 mM)

mTyr is about 181 Da and the atomic weight of the stable

Fig. 4
Concentration-dependent inhibition curves of(A) BCH and (B) MeAIB for 6- 125I-L-mTyr. (C): Effects ofinhibitors (MeAIB, BCH) and incubation in Na+-free medium
on 6-125I-L-mTyr uptake by DLD-1 cells. Values represent mean
± SD as the percentage of the administered dose (n = 4)).

isotope of iodine is 127 Da. Therefore, the atomic weightof iodine accounts for approximately 41% of the molecularweight of 6-125I-L-mTyr. We anticipated that the additionof iodine at the 6-position of the bulky, hydrophobic
benzene ring of mTyr would decrease the affinity of the
AA transport system for 6-125I-L-mTyr AA and that iodine

4. Discussion

in the 4-position of the benzene ring (i.e., 4-125I-L-mTyr)might exhibit different properties for AA transport.

In designing this study we took into consideration

Kloss et al. further proposed that the radioiodinatedprevious reports of the measurement of DOPA

compound should not be broken down quickly by general

decarboxylase activity and the development 6- I-L-mTyrmetabolic pathways.7) 6-125I-L-mTyr has been reported toas a cranial nerve transmission function imaging agent forbe metabolically stable.2) The accumulation of radioactivity

125

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6-Iodo-L-m-Tyr for tumor imaging

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due to 3-123I-iodo-alpha-methyl-L-tyrosine (123I-IMT) andthe S2 region of the kidney proximal tubule, suggesting4- I-L-mTyr in the stomach were similar, indicatingthat there are different mechanisms for the renal handlinghigh resistance of 6- I-L-mTyr towards de-iodinationof 6-125I-L-mTyr or 4-125I-L-mTyr and 125I-IMT, possibly

and suggesting that the use of 4-125I-L-mTyr would preventdue to the alpha-methylation of Tyr.3)

125

3)

125

undesirable radiation exposure due to radioactive iodine
released by de-iodination reactions.

6-125I-L-mTyr is primarily transported across themembrane via the AA transport system and thus we

The results of the biodistribution study of 6- I-L-mTyrconducted uptake inhibition experiments targeting thissummarized in Table 1 indicate rapid blood clearance,system. Various AA membrane transport systems havemaximum tissue uptake, and prolonged retention in the

been described, including transport system L, transportorgans, comparable to the biodistribution of other AA

system A, and transport system ASC. Transport systems A

analogs

and ASC depend on the presence of Na+, and they can be

125(18F-FET),

such

as

O-(2- F-fluoroethyl)-L-tyrosine
18

3-18F-fluoro-alpha-methyl-L-tyrosine,anddistinguished on the basis of specific inhibition by

123

I-IMT.3) 6-125I-L-mTyr exhibited higher uptake (%artificial AAs. For example, transport system A is Na+

Dose/g) in the pancreas than in the liver and tyrosine isdependent, and uptake is inhibited by the artificial AA

known to accumulate in pancreatic cells and is used for

MeAIB. Similarly, transport system ASC is also Na+

the synthesis of enzymes. The higher accumulation in thedependent, but it is not inhibited by MeAIB. Transport

pancreas is consistent with the characteristics of AA

system L is independent of Na+, and it is specificallybiodistribution.

inhibited by a different artificial AA, BCH.6,9)

There was a higher accumulation of 125I-IMT in all

Here we examined 6-125I-L-mTyr uptake into DLD-1

organs studied compared to 4- or 6-radioiodinated L-meta-

cells to determine the dependence of uptake on thetyrosine. In particular, the accumulation of 125I-IMT inconcentration of MeAIB, a specific inhibitor of transport

kidney was about 5-fold higher than that of 4- or

system A, or the concentration of BCH, a specific inhibitor

6-radioiodinated L-meta-tyrosine.

of transport system L. Our aim was to determine what3)

After glomerular filtration, natural AAs are generally

concentration of each of these compounds should be usedlargely reabsorbed and most are resorbed by proximal

in inhibition experiments. We found that 6-125I-L-mTyrtubules in segment 1 (S1) to S3 cells; consequently, there

uptake into DLD-1 cells showed no clear dependence on

is little excretion of AAs. In contrast, a large amount of

the concentration of MeAIB and thus we used 5 mM

radioactivity due to 6-125I-L-mTyr had moved to the renal

MeAIB in the inhibition experiments (Fig. 4-B).

papilla 5 min after i.v. administration (Fig. 2). These two

Conversely, the uptake of 6-125I-L-mTyr into DLD-1 cells

observations, of very early excretion of radioactivity

decreased as the BCH concentration increased and thus

through the urine and low radioactivity in the S1 to S3

we chose a concentration of 5 mM BCH for the uptake

region, suggest that 6- I-L-mTyr may not be efficiently

inhibition experiments (Fig. 4-A).

8)

125

reabsorbed, although it was difficult to observe
radioactivity in the S3 region, as mentioned above.

The uptake of 6-125I-L-mTyr did not differ significantly
whether or not Na+ was present in the PBS, indicating that

We previously reported the accumulation of secreted

neither of the Na+-dependent transport systems (i.e.,

125

I-IMT, an alpha-methylated Tyr analog, in the cortex ofsystems A and ASC) is involved in 6-125I-L-mTyr uptake

proximal tubule S2 areas, similar to that of 99mTc-

by DLD-1 cells. Transport system L is primarily

mercaptoacetyltriglycine (MAG3) and p-aminohippuric

responsible for 6-125I-L-mTyr uptake (Fig. 4-C), asacid (PAH), which are typical proximal tubule secretionsupported by real-time PCR results demonstrating

compounds. Probenecid is an organic anion secretion

expression of the LAT1, LAT2, LAT3, and LAT4

inhibitor that markedly inhibits the accumulation oftransporter genes of transport system L and of the 4F2hc

secreted MAG3, PAH andIn

coupling factor in DLD-1 cells (data not shown).10-14)

contrast, 6-125I-L-mTyr did not accumulate significantly in

Similarly, the cellular accumulation of L-[14C(U)]tyrosine,3)

I-IMT in S2 areas.

125

3)

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茨城県立医療大学紀要20

第 24 巻

Conflict of interest

the parent compound used to design the drug 6-125I-LmTyr, is primarily due to transport system L (Fig. 5).
Based on the results and discussion above, we suggestthat 6-radioiodinated L-meta-tyrosine (e.g.; 6-123I-L-mTyr,

The authors declare that they have no conflict ofinterest.

6-124I-L-mTyr) may be useful as a tumor imaging agent for
the following reasons. After i.v. administration to normal
Acknowledgement

mice, 6-125I-L-mTyr was rapidly transported out of theblood and into the tissues (Table 1 A, B), supporting its
usefulness as a molecular imaging agent for tumors. Also,

We wish to thank Yuko Hamai, Sanae Matsutani, Naomi

the accumulation of 6- I-L-mTyr in the pancreas is

Tojyo, Yumi Suzuki, Yuko Hirotsu, Sayaka Imai and Kei

characteristic of the in vivo kinetics of AAs. Almost no

Satoh (Ibaraki Prefectural University) for their excellent

radioactivity accumulation was observed in the stomach,technical assistance. This work was supported by Grants-

indicating the high resistance of 6-125I-L-mTyr to de-in-Aid for Scientific Research (#26461801) from theiodination. This suggests that it will be possible to avoid

Ministry of Education, Science, Sports and Culture ofundesirable radiation exposure of the thyroid, stomach,

Japan and the Japan Society for the Promotion of Science.

and other organs due to radioactive iodine released by de-

Financial support was also provided by a Japaniodination reactions. There was some accumulation of

Atherosclerosis Research Foundation Grant in 2016-

radioactive iodine in the intestine and liver on a per-organ

2018.

125

basis, but it was minor on the per-organ-weight basis.
Accumulation in normal mouse tissues was low overall,
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和文抄録
123

I-6-iodo-L-meta-tyrosine (6-123I-L-mTyr) は，脳の芳香族 L- アミノ酸脱炭酸酵素活性測定に使用できると報告されたが，腫瘍診断薬としての使用可能性の報告はない。我々は 125I- 標識体を用い，腫瘍診断薬としての 6-123I-L-mTyr の有用性を基礎的に調べるために，6 週齢雄性 ddY 系マウスの体内分布，腎臓オートラジオ
グラフィー，DLD-1 大腸癌細胞により集積に関わるアミノ酸輸送系に関する検討を行った。体内分布から，正常マウス組織における 6-125I-L-mTyr の低集積性および血液から尿への迅速なクリアランスが示された。
DLD-1 膜輸送阻害実験では，約 90％が L 系輸送を介して腫瘍細胞に蓄積することが示された。 6-125I-L-mTyr
は，腫瘍診断薬として使用することができると考えられる。

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