ORIGINAL ARTICLE
Annals of Nuclear Medicine Vol. 18, No. 3,263-270, 2004

Renal accumulation and excretion of radioiodinated
3-iodo- t -methyl-L-tyrosine
Naoto SHIKANO,*Keiichi KAWAI,**Syuichi NAKAJIMA,**Ryuichi NIsHII,*** Leo Garcia FLORESII,***
Akiko KUBODERA,**** Nobuo KUBOTA,*Nobuyoshi ISHIKAWA*and Hideo SAJI******Department of Radiological Sciences, Ibaraki Prefectural University of Health Sciences**School of Health Sciences, Faculty of Medicine, Kanazawa University***Department of Radiology, Miyazaki Medical College****Faculty of Pharmaceutical Sciences, Science University of Tokyo*****Graduate School of Pharmaceutical Sciences, Kyoto University

Objective: We investigated mechanisms of renal accumulation of radioiodinated 3-iodo-a-methylL-tyrosine (IMT), which has been used clinically for tumor imaging and as an amino acid transport
marker in studies of brain and pancreas function. Methods: In this study, we used 125I-or 123I-labeled
IMT ([125I]IMT or [123I]IMT) as the transport marker. Partition coefficients of [125I]IMT were determined for hypothetic urine at pH ranging from 5 to 8. The examination of uptake and inhibition of[125I]IMT was performed using normal human renal proximal tubule epithelial cells (RPTEC),
which are characteristic of the proximal convoluted tubule. The plasma protein binding ratio of[125I]IMT was determined using rats. In the in vivo experiments using mice, we examinedbiodistribution and excretion inhibition, and performed whole body autoradiography. Also, renal
SPECT using [123I]IMT was performed using a normal canine. Results: Very low iipophilicity of[125I]IMT in hypothetic urine suggests that a carrier-mediated pathway contributes to its marked
kidney accumulation. [125I]IMT uptake into RPTEC was significantly inhibited by 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid (BCH) in a sodium-dependent manner, suggesting reabsorption mainly via system B~ in apical membrane of proximal tubule. Plasma protein binding ratioof IMT was 45.4 _+5.6%. At 6 hr after administration of IMT to mice, excretion via urinary tract was
77.51% of injected dose, and excretion into feces was 0.25%. Furosemide, ethacrynic acid andprobenecid inhibited tubular secretion of [125I]IMT in mice. We obtained very clear autoradiographs of mouse renal cortex and a canine renal SPECT image (S2-1ike region). Conclusions: We
believe that [123I]IMT is useful for kidney imaging. In future studies, we plan to examine the useof [123I]IMT in diagnosis of disease.
Key words: amino acid transport, artificial amino acid, renal cortex, 3-iodo-ct-methyl-L-tyrosine

INTRODUCTION
THE ARTIFICIALamino acid 3-iodo-tx-methyi-L-tyrosine
Received January 1, 2004, revision accepted February 19
2004.
For reprint contact: Naoto Shikano, M.S., Department ofRadiological Sciences, Ibaraki Prefectural University of Health
Sciences, 4669-2 Ami, Ami-machi, Inashiki-gun, Ibaraki 3000394, JAPAN.
E-mail: sikano@ipu.ac.jp

Vol. 18, No. 3, 2004

(IMT) has been developed as a functional imaging agent
for amino acid transport in the brain and pancreas.l-3
It has also been successfully used clinically for SPECT
imaging of brain tumors and whole-body scintigraphy fortumor imaging.4-7 Renal physiological accumulation ofradiolabeled IMT is higher than that of radiolabeled Tyr.
We have also observed marked renal accumulation andrapid urinary excretion of IMT in mice. 8 Clarification ofthese accumulation mechanisms could aid in development of methods for reducing physiological accumulation, which obstructs abdominal tumor imaging. Also, we

Original Article 263

believe that it would facilitate evaluation of radiolabeled
IMT as a renal imaging agent. To investigate the pathways
responsible for IMT accumulation in the kidney, we
performed IMT uptake and inhibition experiments using
125I-labeled IMT ([125I]IMT) and normal human renal
proximal tubule epithelial cells (RPTEC), which are characteristic of the proximal convoluted tubule. Partition
coefficients were determined for hypothetic urine at pH
ranging from 5 to 8 (hypothetic urine pH). In the in vivo
experiments using mice, we performed whole body autoradiography and examined biodisttibution and excretion
inhibition. Also, renal SPECT in a normal canine was
performed using 123I-labeled IMT ([123I]IMT).
MATERIALS AND METHODS

Animal experiments were approved by the ethics committees of the affiliated universities.

Materials and preparation of labeled compounds
Reagent-grade 2-amino-bicyclo[2,2,1 ]heptane-2-carboxylic acid (BCH), 2-(methylamino)isobutyric acid
(MeAIB), 2-aminoisobutyric acid (AIB), probenecid, furosemide, ethacrynic acid and a-methyl-L-tyrosine wereacquired from Sigma-Aldrich Japan K.K. (Tokyo, Japan).
Chloramine-T and other chemicals of reagent grade werepurchased from Kanto Chemical Co. (Tokyo, Japan).
The radioisotope [125I]NaI (8.1 • 1019 Bq/mol) was
obtained from Amersham Pharmacia Biotech UK(Buckinghamshire, UK). [123I]NaI (8.79 • 1018 Bq/mol)
solution was provided by Nihon Medi-Physics Co., Hyogo,
Japan. For preparation of labeled compounds, non-cartier-added [ 1251]IMT, [1231]IMT and 4-[ 1251]iodo-L-metatyrosine (4-[125I]mTyr) were prepared by the conventional chloramine-T method, using a-methyl-L-tyrosine
as a precursor, as previously reported. 1-3,8 A Nova-Pak
C18 (3.9 • 300 mm; Waters, Milford, MA, USA) wasused for separation and purification. Silica gel thin-layer
chromatography kit (TLC, catalogue number Art. 5553)
was obtained from Merck (Darmstadt, Germany). [123I]Nisopropyl-p-iodoamphetamine ([123I]IMP), technetium99m-diethylenetriaminepentaacetic acid ([99mTc]DTPA)
and technetium-99m-mercaptoacetylglycylglycylglycine([99mTc]MAG3) were acquired from Nihon MediPhysics Co., Hyogo, Japan.
For cell experiments, normal human renal proximal
tubule epithelial cells (RPTEC), which are characteristicof the proximal convoluted tubule, were obtained from
Cambrex Bio Science Walkersville, Inc. (Walkersville,
MD, USA). Plastic tissue culture flasks (surface area, 25cm 2) were purchased from Nalge Nunc International(Roskilde, Denmark).
For 125I-labeled compounds, radioactivity was measured using an ARC-380 well-type scintillation counter(Aloka). For 14C-labeled compounds, radioactivity wasmeasured using an LS6500 liquid scintillation counter

264 NaotoShikano, Keiichi Kawai, Syuichi Nakajima, et al(Beckman Instruments, Fullerton, CA, USA).

Measurements of partition coefficients
The partition coefficients of [125I]IMT and [123I]IMP were
measured using a pre-mixed combination of 2.0 ml of noctanol (organic phase) and 2.0 ml of 0.1 M phosphatebuffer (aqueous phase). The radioactive sample (10 ~l;
0.68 kBq) was added to this combination, followed bymixing twice for 1 min each time at room temperature,using a mechanical mixer. After centtifugation, the radioactivity of 0.2 ml aliquots was measured.

Cell line study
RPTEC cells were maintained by serial passages in 25cm 2 cell culture flasks in 8 ml of complete medium using
REGMTM Bullet Kit (Cambrex Bio Science Walkersville,
Inc., Walkersville, MD, USA), in 500 ml of REBM TM
culture medium and a REGM TM Single Qouts mixture
containing hEGF (10 I~g/l), insulin (5 llg/l), hydrocortisone (0.5 I~g/l), fetal calf serum (0.5%), epinephrine
triiodothyronine (0.5 lig/l), transferrin (6.5 ~tg/l) and GA100 (1 ml/l) in an atmosphere of 5% CO2: 95% air at 37~
Subculturing was performed every 4 days using 0.02%
EDTA and 0.05% trypsin. Cells were used within the 3rd
passage.
Uptake was measured using RPTEC cell semi-confluent
monolayers grown on 60 mm plastic culture dishes(Falcon; Becton Dickinson, Lincoln Park, NJ, USA).
The incubation medium comprised 145 mM NaCi, 3 mM
KCI, 1 mM CaCI2, 0.5 mM MgCI2, and 5 mM 2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES), witha final pH of 7.4. In the Na+-dependence experiment,
NaCI was replaced with the same concentration ofcholine-Cl. After removal of culture medium, each dishwas washed once with 5 ml of incubation medium for 10
min at 37~ Cells were then incubated with 2 ml ofincubation medium containing 18.5 kBq of non-carrieradded [J25I]IMT for several different time periods at
37~
For the inhibition study, L-Tyr, D-Tyr, BCH (an inhibitor of system L in sodium-independent transport, and an
inhibitor of system B~ in sodium-dependent transport) or
MeAIB (an inhibitor of system A) was added at a final
concentration of 1.0 mM. Cells were incubated for 5 rain
at 37~ with 18.5 kBq of non-carrier-added [125I]IMT.
After incubation, media were aspirated, and monolayers were rinsed twice rapidly using 5 ml of ice-cold
incubation medium. Cells were solubilized in 1.5 ml of 1
N NaOH. Then, radioactivity associated with solubilized
cells was measured.

Measurements of protein binding
Serum protein binding of [125I]IMT, 4-[125I]mTyr,[99mTc]MAG3 and [99mTc]DTPA was evaluated byultrafiltration (Ultracent-10; Tosoh, Tokyo, Japan). A
mixture (0.9 ml) of [125I]IMT (220 kBq) in saline (20/.tl)

Annals of Nuclear Medicine

and rat serum from a male SD rat (age, 6 weeks) was
centrifuged at 3,000 rpm for 10 min at room temperature(Himac CF 7D; Hitachi, Tokyo, Japan). Radioactivity(counts/20 pl) of the initials and filtrates was measured
using a well-type scintillation counter (ARC-380; Aloka,
Tokyo, Japan); these data were used to determine protein
binding and free fraction rates.

Renal accumulation and excretion experiments in mice
Biodistribution and urinary excretion experiments in mice
were based on a method described previously. 1'9 For thebiodistribution experiment, groups of 5 male ddY mice
(weight, 30 g; age, 6 weeks) received 0.1 ml of [125I]IMT(18.5 kBq) by injection into the tail vein. Mice wereanesthetized with ether inspiration and sacrificed by heart
puncture at 1, 3, 5, 10, 15, 30, 60 and 120 min after[125I]IMT administration. Organs were dissected.
For the urinary excretion experiment, groups of 5 ddY
male mice (weight, 30 g; age, 6 weeks) were injected with
0.1 ml saline containing [125I]IMT (55.5 kBq) via the tailvein, and then placed in metabolic cages under feeding
conditions. Urine and feces were collected at 5, 10, 15, 30,
60, 180 and 360 min after [125I]IMT administration.
To examine inhibition of urinary excretion of [125I]IMT,
the mice received an injection of probenecid, furosemide
or ethacrynic acid (50, 10 and 10 mg/kg bodyweightrespectively) with [125I]IMT(55.5 kBq). 1~For probenecidloading, the probenecid was pre-administered via the tailvein 10 min prior to injection of [125I]IMT.11 Urine wascollected 10 min after administration. Radioactivity ofsamples was quantified using the above-mentioned welltype scintillation counter.
In vivo whole body autoradiography in mice
For autoradiography, 0.1 ml of [t25I]IMT (670 kBq) was
injected into the tail vein of male ddY mice (n = 4; weight,
30 g; age, 6 weeks). After 5 min, the mice were sacrificed
by inspiration of excess ether, placed in carboxymethyl
cellulose embedding medium (Nacalai, Kyoto, Japan),and frozen at -15~ for at least 12 hr. An Autocryotome
NA-200F (Nakagawa, Tokyo, Japan) was used to cut 20/.tm coronal sections from posterior to anterior. Sections
were then dried at -15~ for another 24 hr. Tissue slices
were kept in contact with BAS-TR2040 imaging plates(Fuji Photo Film, Kanagawa, Japan) for 24 hr. Images
were processed using a BAS 2000 Bio-Imaging Analyzer(Fuji Photo Film).

dominal imaging was performed using a ring-type SPECTcamera (SET-030W; Shimadzu, Kyoto, Japan; FWHM,
12 mm; slice thickness, 24 mm; slice interval, 30 mm).
RESULTS

Materials and preparation of labeled compounds
Labeling of a-methyl-L-tyrosine produced [125I]IMT and[123I]IMT with labeling efficiency of more than 80%.
After purification with HPLC, we obtained [12sI]IMT and[123I]IMT with greater than 95% radiochemical purity.
Measurements of partition coefficients
Partition coefficients of [125I]IMT and [123I]IMP are
shown in Figure 1. At pH 7.0 and 7.4, the partition coefficient of [125I]IMT (-0.714 + 0.007 and -0.741 + 0.010,

2.0
.~
e.)

1.0

o

0

~

_

_

~

[123I]IMP[125I]IMT
4-[125I]mTyr

o
.,~
q.o
-2.0

i

i

I

i

i

i

4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0
pH

Fig. 1 Partition coefficients of [125I]IMT and [123I]IMP atvarious pH: log(n-octanol/0.1 M phosphate buffer). Values
represent the mean + S.D. (n = 5).

125
o
O

o

100
75

O

~

50

"~

25
0

SPECT imaging of canine abdomen
Canine SPECT was performed as described by Kawai etal. 3 A male beagle dog (body weight, 10 kg) was anesthetized with pentobarbitai (Nembutal; Abbott Laboratories,
Abbott Park, IL) and positioned on the cradle in supine
position. [123I]IMT solution (48 MBq in saline) was
injected through the femoral vein. Imaging was performed at 2.5-minute intervals for 60 minutes. The ab-Vol. 18, No. 3, 2004

Fig. 2 Effects of inhibitors (l mM) on accumulation of[125I]IMT in Na+-containingmedium (black column) and Na§
free medium (white column). Each column represents mean +S.D. of 7 to 8 normal human renal proximaltubule epithelialcell
monolayers. * p < 0.001, ** p < 0.01.

Original Article 265

respectively) was less than that of [123I]IMP (0.832 +0.007 and 1.043 + 0.009, respectively), which is a wellknown lipophilic amineJ 2 From pH 5 to 8, partition coefficients of [1251]IMTand 4-[125I]mTyr were similarly low.

Cell line study
Time course analysis of [125I]IMT uptake into RPTEC
shows that steady-state levels were reached 25 min after
starting incubation (data not shown). An incubation timeof 10 min was chosen for the inhibition study. The resultsof the [125I]IMT uptake inhibition experiments are shownin Figure 2. L-Tyr significantly inhibited [125I]IMT uptake (p < 0.01). o-Tyr had a smaller inhibitory effect on[125I]IMT uptake than L-Tyr. Replacing sodium withcholine significantly reduced [125I]IMT transport into
RPTEC cell monolayers.
Transport of [125I]IMT into semi-confluent RPTEC
monolayers was predominantly (72.16%) mediated by a
Na+-independent system, with a minor contribution(19.56%) by a Na§
system (Fig. 2). At 1 mM,
the amino acids BCH (with and without sodium) and LTyr inhibited accumulation of [125I]IMT at similar mag-Table 1 Protein binding rate in rat serum
TracerProtein binding rate (%)

n[125I]IMT
4-[125I]mTyr[99mTc]MAG3

45.4 + 5.6
43.0 + 8.5
80.8 + 6.3
2.3 + 11.4

6
5
4
5

[99mTc]DTPA

Values represent mean + S.D.

A

nitudes (p < 0.001). This suggests that system B~ is a main
contributor in Na+-dependent [1251]IMTtransport. MeAIB
also exhibited a significant inhibitory effect (p < 0.01). LArg, a basic amino acid, did not inhibit [125I]IMT transport or accumulation (Fig. 2).

Measurements of protein binding
In the in vitro binding assay with the SD rat, plasma
protein binding of [125I]IMT was approximately 45.4 +
5.6% (Table 1). The values of other compounds tested areshown in Table 1.
Renal accumulation and excretion experiments in mice
Figure 3 (A) shows kidney accumulation of [125I]IMT (%injected dose/organ) in mice. Peak radioactivity (33.48 +6.95%) was observed at 5 min after injection. Kidney-toother-tissue ratios of [125I]IMT accumulation in mice areshown in Table 2. At 5 min after injection, [125I]IMT
accumulation in the kidney was.followed by accumulation in the pancreas and adrenal gland, and radioactivityof the kidney was at least 15-fold greater than that of othertissues, except the pancreas and adrenal gland. During thefirst 30 min, we observed rapid blood clearance (Fig. 3(A)) and urinary excretion (Fig. 3 (B)) of [125I]IMT. Threehours after administration, more than 70% of the [I25I]IMT
was found in the urine. At 6 hr after injection, [125I]IMT
excretion into feces was only 0.25 _+ 0.14%. Figure 4
shows inhibition of urinary excretion of [I25I]IMT bydrugs in mice. Probenecid, furosemide and ethacrynicacid inhibited urinary excretion of [I25I]IMT by 11.10%(p < 0.01), 47.21% (p < 0.01) and 62.24% (p < 0.1),

B
100
9

Kidney]Bl~176

10
[._,~

100
~D

~

75

~

50

0
..o

.~

25

o

0.1
0

i

i

50

100

. ,=-

150

0

0.0

Time (min)

i

i

i

2.0

4.0

6.0

8.0

Time (hr)Fig. 3 Kidney accumulation,blood clearance (A) and urinary excretion (B) of [1251]lMTin mice. Values
represent the mean _+S.D. (n = 5). *, % injected dose/m/.

Table 2 Kidney-to-other-tissue ratios of [125 1]IMTaccumulation in mice at 5 min after i.v.
Kidney/Blood

Kidney/Pancreas

Kidney/Liver

Kidney/Adrenal

Kidney/Stomach

Kidney/Intestine

15.30 + 0.53

2.74 + 0.04

22.17 + 0.69

4.36 + 0.01

23.82 + 0.39

20.20 + 0.81

Ratio of accumulation: % injected dose/g tissue. Values represent the mean _+S.D. (n = 5).

266 Naoto Shikano, Keiichi Kawai, Syuichi Nakajima, et al

Annals of Nuclear Medicine

Fig. 4 Inhibition of urinary excretion of [125I]IMT by drugs inmice at 10 min after i.v. Values represent the mean + S.D. (n =5). Statistical comparisons between groups were performed
using Student's t test. *, p < 0.01, **, p < 0.05, ***, p < 0.1.

Fig. 5 Whole body autoradiography of [125I]IMT-injected miceat 5 min after i.v., in gray scale (A) and color scale (B).

Fig. 6 Preliminary SPECT image of canine abdomen (A) and time-activity curve (B) after intravenous
injection of IMT. High accumulation of [123I]IMTin kidney cortex is shown in (A). In (B), red line shows
radioactivity of non-target tissue (in the square between the kidneys in (A)), and blue line shows kidney
radioactivity (in the square of the left-kidney in (A)). The retention of [123I]IMT in kidney is clearly
indicated by the very high ratios of radioactivity in the kidney to background radioactivity in canine
abdomen.

respectively, compared to control (100%), at 10 min afteradministration of [z25I]IMT.
In vivo whole body autoradiography in mice
Figure 5 shows a typical whole body autoradiograph,demonstrating localization of [125I]IMT in mice at 5 minafter administration. The highest radioactivity was foundVol. 18, No. 3, 2004

in the kidney. In the kidney, 96.33+ 1.38% of the [125I]IMT
accumulated in the renal cortex, and 3.67 _+ 1.38% accumulated in the medulla.

SPECT imaging of canine abdomen
As shown in Figure 6 (A), a clear canine kidney cortex
image was obtained. Figure 5 (B) shows that kidney

Original Article267

accumulation peaked at approximately 15 min after injection, followed by gradual decline. At 60 min after injection, retention in kidney was about 65% of peak accumulation. The maximum ratio of kidney count to backgroundcount in canine abdomen was approximately 7.0.

Metabolically stable IMT has been derived from 3-iodoL-tyrosine, based on reports that a-methylation confers
metabolic stability on L-Tyr, 13'14and based on a prediction that a-methylation would also confer metabolicstability on 3-iodo-L-tyrosine. We recently examined thefate of injected [125I]IMT and 3-[125I]iodo-L-tyrosine([125I]IT) in mouse kidney and urine, l At 10 min after
intravenous injection of [125I]IMT into mice, 94.93 +
1.43% of radioactivity in the kidney was due to intact[125I]IMT, and 2.21 + 1.45% was due to deiodinated freeI-; in urine, these respective values were 93.09 + 2.45%
and 3.81 + 2.00%. Protein incorporation of [125I]IMT was
not detected in kidney or any other organs examined. At
10 min after injection of [125I]IT, 14.07 + 4.55% ofradioactivity in the kidney was due to intact [125I]IT, and75.07 + 6.13% was due to deiodinated free I-; in urine,
these respective values were 4.79 + 1.52% and 94.21 +
1.19%. These findings indicate that IMT is metabolicallyvery stable in the urinary tract, compared to [125I]IT. They
also show that neutral amino acid movement up to urinary
excretion can be traced by measuring radioactivity of
intact IMT; radioactivity of metabolites has little effect onthese measurements.
In our comparison of partition coefficients between[J25I]IMT and [123I]IMP, from pH 5 to 8 (hypothetic urine
pH range), the lower coefficient for [125I]IMT suggeststhat [125I]IMT is more hydrophilic than [123I]IMP, which
is a typical lipophilic compound. 13Very high pH (8 to 10)
causes [125I]IMT to become an anion, increasing its hydrophilicity. Within the pH range of urine, tissue andblood, the [ 125i]IMT n-octanol/phosphate buffer partition
coefficient was too low for simple diffusion of [125I]IMT
into the cells to occur as a result of its lipophilicity.13This
indicates that cartier-mediated transport is the main gateway for IMT into the proximal tubule cells under normal
blood and urine pH conditions.
We obtained very clear autoradiographs of mouse
renal cortex and a canine renal SPECT image (S2-1ike region). Although we cannot simply compare accumulationlevels of IMT and [99mTc]MAG3 (a clinical renal function imaging radiopharmaceutical), because they aretransported by different mechanisms, there are similarities between excretion of IMT and excretion of[99mTc]MAG3.11 In both cases, most of the injected compound is excreted into urine with markedly high accumulation in kidney cortex (S2-1ike region), and excretion into
feces is negligible. A surprising finding is that the peakkidney accumulation of [125I]IMT (102.62 + 0.93% in-jected dose/g tissue; 5 min after administration) 8 far
exceeded that of [99mTc]MAG3 (64.32 + 12.83%; 2 minafter administration) in an experiment in which mice were
injected with [99mTc]MAG3 using the same method used
for [125I]IMT.
The plasma protein binding ratio of [125I]IMT was
markedly less than that of [99mTc]MAG3. This suggeststhat about half of the [125I]IMTdoes not undergo glomerular filtration, resulting in secretion in the proximal tubule.
A probenecid-sensitive secretion pathway is thought tobe an important contributor to excretion of [125I]IMT.
Probenecid blocks organic anion transport, l~ There are 2
structural differences between natural L-Tyr and [125I]IMT:
tx-methylation and iodination of 3 position of phenolgroup of L-Tyr. These differences may be responsible forthe greater affinity of IMT for probenecid-sensitive secretion. L-Tyr accumulation in kidney is moderate, in contrast to the high accumulation of IMT. 15 Affinity foraccumulative organic anion transport system may contribute to the high accumulation of IMT in kidney cortex(proximal tubule of S2-1ike region). Like probenecid, thediuretics furosemide and ethacrynic acid inhibited secretion of [125I]IMT. It is interesting that these drugs inhibittransport ofp-aminohippurate (PAH) and [99mTc]MAG3
via organic anion transporter 1.11,16.17
The present results suggest that reabsorption of[125I]IMT in apical membrane of proximal tubule is mediated by system B ~ because [125I]IMTuptake into RPTEC
was significantly inhibited by BCH in a sodium-dependent manner. Up to 63.73% of total sodium-dependent[125I]IMT uptake into RPTEC was apparently mediatedby system B ~ and the remaining sodium-dependent uptake was apparently mediated by system A.
Several studies on IMT transport have been performed
with tumor cell lines. 18-21 These studies have revealed
that the majority of IMT transport takes place via the Na+independent system L (>70%), and that relatively minor
uptake takes place via the Na§
system B ~(<20%). Proximal tubule cells have system L in thebasolateral membrane and system B~247
in the apical membrane. 22 Further research is needed to determine which
isoforms of the transporters in these systems mediate IMTtransport in the kidney.
System L is important because some essential aminoacids are transported via this system in plasma membrane. 22 Of the system L transporters, human L-typeamino acid transporter 1 (hLAT1) and its isoform hLAT2
are the main neutral amino acid transporters in plasma
membrane. 23,24Formation of a hetero-dimer with heavy
chain of 4F2 antigen (4F2hc) is required for functional
expression of hLAT1 and hLAT2. 23,24Recently, we examined the isoform selectivity of [125I]IMT transport via
hLATi and hLAT2 in Xenopus laevis oocytes coexpressing 4F2hc. 25 The results indicate that [J25I]IMTtransport selectively occurs via hLAT1-4F2hc heterodimer, whereas transport of L-Tyr (structural parent of

268 NaotoShikano, Keiichi Kawai, Syuichi Nakajima, et al

Annals of Nuclear Medicine

DISCUSSION

IMT) was not isoform-selective. 25 Expression of hLAT1
is ubiquitous, and IMT is a substrate of hLAT1.24-26 In theproximal tubule, hLAT2-4F2hc expression is observed inbasolateral membrane of epithelial cells. 23 This isoform is
involved in secretion and reabsorption of neutral aminoacids, as an amino acid exchanger. 23 Natural amino acids,
including mother L-Tyr, are reabsorbed so extensively
that only small amounts of natural amino acids are foundin urine. 22 Our finding of rapid urinary excretion of[125I]IMT may be due to its lack of affinity for hLAT24F2hc.
Radioiodinated amino acids have been used as imaging
agents in the majority of tumor imaging studies. Based on
the clear image obtained in the present study, we believethat [123I]IMT is a useful imaging agent for kidney studies. Renal amino acid transport is disturbed in a number of
diseases. In future studies, we plan to investigate use of
IMT in the diagnosis of kidney diseases.
CONCLUSIONSWith high kidney-to-other-tissue ratios, we observed
markedly high accumulation of metabolically stable IMT
in an S2-1ike region of kidney cortex in mice. A clear
kidney cortex image was obtained in the canine SPECT
experiment. A c c u m u l a t i o n of I M T is mediated byprobenecid-sensitive membrane transport and BCH-sensitive membrane transport in a Na§
manner.
We believe that [123I]IMT is useful for kidney imaging,
and we plan to investigate use of IMT in the diagnosis of
disease in future studies.

ACKNOWLEDGMENTS
We wish to thank Sae Fujimoto, Yuki Nobayashi, Junko Yagi,
and Takashi Kotani (Ibaraki Prefectural University) for their
excellent technical assistance. This work was supported by
Grants-in-Aid for Scientific Research (#10770451, #14770498,#13557075 and #15659283) from the Ministry of Education,
Science, Sports and Culture of Japan and the Japan Society for
the Promotion of Science. Financial support was also provided
by the lbaraki Prefectural University Research Project (9808-3,
0118 - 1 and 0220-1) and an Ibaraki Prefectural University Grantsin-Aid for the Encouragement for Young Scientists 2001, 2002
and 2004.

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Vol. 18, No. 3, 2004

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Konishi J, et al. Canine SPECT studies for cerebral aminoacid transport by means of J23I-3-iodo-a-methyl-L-tyrosine
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