SHORT COMMUNICATION 
Annals of Nuclear Medicine Vol.9, No.l, 47-50, 1995 

Canine SPECT studies for cerebral amino acid transport by means of 123I-3-iodo-a-methyl-L-tyrosine and preliminary kinetic analysis 

Keiichi KAWAI,* Yasuhisa FUJIBAYASHl,** Yoshiharu YONEKURA,*** Kenichi TANAKA,* Hideo SAJI,** Junji KONISHI,*** Akiko KUBODERA* and Akira YOKOYAMA** 
*Faculty of Pharmaceutical Sciences, Science University of Tokyo 
**Faculty of Pharmaceutical Sciences and ***School of Medicine, Kyoto University 

We have already reported that 123I-3-iodo- a-methyl-L-tyrosine (123I-L-AMT) is superior as a singlephoton emitter labeled radiopharmaceutical reflecting cerebral amino acid transport. In this study, we investigated the distribution of 123I-L-AMT in the canine head by means of SPECT and kinetically analyzed the data in the brain. As a result, clear SPECT images of the canine brain were obtained. Kinetic analysis with a 2-compartment model, including or expressing membrane transport of the amino acid, was performed with time-activity curves in the arterial blood and in the cerebral region. The results of the analysis coincided closely with the experimental data and the relevance of the model was strongly suggested. Therefore 123I-L-AMT is considered to be useful as a single photon radiopharmaceutical which enables us to measure the cerebral amino acid transport rate. 

Key words : cerebral amino acid transport, 3-iodo-a-methyl-L-tyrosine, kinetic analysis, SPECT 

INTRODUCTION 

QUANTITATIVE DIAGNOSIS with not only PET but also SPECT has recently come to be desired in nuclear medicine.1 In our research on single photon emitter labeled radiopharmaceuticals, which offered wider applicability than positron emitter labeled ones, 123I-3-iodo-a-methyl-Ltyrosine (123I-L-AMT) was selected for the measurement of cerebral amino acid transport. As we have already reported, 123I-L-AMT has shown high brain accumulation, rapid blood clearance, the affinity for the carriermediated and stereoselective active transport system of the brain, and stability against catabolism including enzymatic deiodination.2 Therefore 123I-L-AMT has appeared to be a useful radiopharmaceutical for the selective measurement of cerebral amino acid transport. In the present study, imaging of the canine head with 123I-L-AMT by SPECT and preliminary quantitative analysis were carried out. For this study, the time-activity curves in the cerebral region and in the arterial blood were obtained by SPECT and arterial blood collection, respectively. Kinetic analysis with these data was performed with a 2-compartment model considering only cerebral amino acid transport. MATERIALS AND METHODS 
Preparation of 123I-L-AMT 
Non-carrier added 123I-L-AMT (8.79 x 10^12 Bq/umol, theoretical) was prepared by the conventional chloramine-T method with Nal23I solution provided by Nihon MediPhysics, Japan, previously reported.2 All other chemicals used were of reagent grade. 

Comparative canine SPECT studies with 123I-L-AMTand 123I -IMP 
A male beagle dog (10 kg body weight, no fasting condition) was anesthetized with pentobarbital sodium (NembutalR, Abbott Laboratories), and 123I-L-AMT solution (48 MBq in saline) was injected through the femoral vein. Imaging was performed at 2.5 minutes intervals for 60 minutes with a ring-type SPECT machine (SET-030W, Shimadzu, FWHM: 12 mm, slice thickness: 24 mm, slice interval: 30 mm). Seven days after the experiment, 123I-dl-N-isopropyl-p-iodoamphetamine hydrochloride (123I-IMP 67 MBq, Nihon Medi-Physics) imaging was carried out in the same dog. 

Canine SPECT studies of l23I-L-AMT and kinetic analysis of cerebral amino acid transport 
A male mongrel dog (12 kg body weight, no fasting condition) was injected with 123I-L-AMT solution (26 MBq in saline) through a foreleg vein and head SPECT images parallel to the orbitomeatal (OM) line were obtained at 2.5 minutes intervals for 60 minutes with a ring-type SPECT machine (SET-030W, Shimadzu). Arterial blood samples were collected through the catheter set in the femoral aorta, weighed and radioactivity was measured with a well-type scintillation counter (ARC-300, Aloka). The time-activity curve in the cerebral region was calculated. The activity per I ml in the arterial blood was substituted in equation ( l) and the constants P1 to P6 were calculated. With the obtained constants and the activity data per boxel in the cerebral region, the rate constants for cerebral amino acid transport, kl and k2 in equation (2), were obtained. Both kinetic analyses were fitted by the Simplex method.
 Cb = Pl x Exp(-P2t) + P3 x Exp(-P4t) + P5 X Exp(-P6t) (1) 
dCf/dt = k1Cb - k2Cf (2) 
where 
Cb: radioactivity in the arterial blood (cpm/ml blood) 
P1-P6: blood clearance parameters 
Cf: radioactivity in the cerebral region (cpmn/boxel) 
t: time after intravenous injection (minutes) 

RESULTS 

Comparative canine SPECT studies with l23I-L-AMTand 123I -IMP
A canine head SPECT image added imaging data for 10 to 30 minutes after intravenous injection of 123I-L-AMT solution is shown in Figure la. The high brain accumulation of 123I-L-AMT was clearly visualized as a circle in the lower part of the photograph. The crescent-shaped accumulation may represent the salivary glands or the nasal mucosa. Figure lb shows the head SPECT image of the same dog for 10 to 30 minutes after intravenous injection of 123I-IMP solution at the same position. Only the brain was visualized by 123I-IMP, which is well known as a brain-seeking amine with high lipophilicity. 

Canine SPECT studies of 123I-L-AMT and kinetic analysis of cerebral amino acid transport A head SPECT image of another dog parallel to the OM line 10 to 30 minutes after intravenous injection of 123I-LAMT and a region of interest set in the SPECT image are shown in Figure 2. The high accumulation of 123 I-L-AMT in the brain was also visualized as a circle, and the time-radioactivity curve of the cerebral region was obtained (Fig. 3). The time-activity curve of the canine brain resembled that of the human cerebral cortex during 124I-LAMT PET studies.3 The results of kinetic analysis by the Simplex method are shown in Figures 3 and 4. The 3 exponential equation (I) showed the best fit for the arterial blood clearance curve. The blood clearance parameters, P1 to P6, fitted to equation (1) were 1.441 x 10^5 cpm/ml, l.318 minute^-1, 4.311 x 10^4 cpm/ml, 2.735 x l0^-1 minute^-1, 3.367 x 104 cpm/ml and 7.292 x l0^-3 minute^-1, respectively (r = 0.9999). These constants were introduced into equation (2). From the radioactivity data for the cerebral region, the rate constants k1 and k2 were converted to 7.28 x l0^-6 ml blood/boxel/minute and 6.27 x 10^-2 minute^-1, respectively. The obtained curves closely fitted the original time-radioactivity curve (r = 0.9573). 

DISCUSSION 

Various mental disorders have been associated with changes in cerebral amino acid metabolism; for example, a defect in tyrosine transport has been suggested in schizophrenic patients.4 Some studies on the use of radiolabeled amino acids with cyclotron-produced short-lived radionuclides have been attempted,5 but they are limited to PET centers; a radioiodinated amino acid, providing similar data, would offer wider applicability. 

Radioiodinated AMT has already been reported as an imaging agent for pancreas,6,7 melanoma,8,9 and brain tumors.3,10 We reported that 123I-L-AMT showed high brain accumulation in mice and in rats.2 The brain uptake index and brain slice studies indicated the affinity of 123I-LAMT for carrier-mediated and stereoselective active transport systems similar to14C-L-tyrosine, respectively. Langen et al. also reported saturation of 123I-L-AMT uptake of human brain by loading L-amino acids.ll The tissue homogenate analysis revealed that most of the accumulated radioactivity belonged to intact 123I-L-AMT, an indication of its metabolic stability. The high brain accumulation of 123I-L-AMT therefore indicated only cerebral L-tyrosine transport. 

In these studies, the high accumulation of 123I-L-AMT in canine brain was clearly visualized similar to that of 123I-IMP. The salivary glands or the nasal mucosa might also be imaged in contrast with 123I-IMP, it might be an example that 123I-L-AMT showed the characteristics as an amino acid. The higher regional accumulation of 123I-LAMT in rat striatum and midbrain reported by Yoshizumi et al. 12 was not observed in these canine cerebral SPECT images. Even if there were greater accumulation of 123I-LAMT in these regions, the canine brain is too small to image the specific accumulation in these regions. Based on previous results, 2-compartment kinetic model shown in Figure 5 was applied to the kinetic analysis of canine SPECT data for cerebral amino acid transport. The time activity curves in the cerebral region and in the arterial blood were obtained by SPECT and arterial blood collection, respectively. The kinetic analysis with these data was performed by means of a 2-compartment model considering only cerebral amino acid transport. The result of the analysis closely fitted the experimental data and the relevance of this model was strongly suggested. The kinetic analysis of cerebral amino acid transport might provide useful information on brain tumors, schizophrenia and other diseases. Therefore 123I-L-AMT is considered to be useful as a single photon radiopharmaceutical for measuring the cerebral amino acid transport rate. 

ACKNOWLEDGMENTS 

The authors wish to thank Nihon Medi-Physics Co. Ltd. for the generous supply of Na123I solution. 

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