SHORT COMMUNICATION 
Annals of Nuclear Medicine Vol. 15, No. 3, 293-296, 2001 
Copper-62 ATSM as a hypoxic tissue tracer in myocardial ischemia 
Norio TAKAHASHI,* Yasuhisa FUJIBAYASHI,** Yoshiharu YONEKURA,** Michael J. WELCH,**** Atsuo WAKI,** Tatsuro TSUCHIDA,* Norihiro SADATO,** Katsuya SUGIMOTO,* Akira NAKANO,*** Jong-Dae LEE*** and Harumi ITOH* 
*Department of Radiology, **Biomedical Imaging Research Center, and ***First Department of lnternal Medicine, Fukui Medical University, Fukui, Japan ****Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, MO, USA 
Copper-62 labeled diacetyl-bis(N4-methylthiosemicarbazone) (62Cu-ATSM) has been proposed as a generator produced positron-emitting tracer for hypoxic tissue imaging. To clarify the usefulness of 62Cu-ATSM for myocardial ischemia 62Cu-ATSM PET was performed in 7 patients with coronary artery disease. Increased myocardial uptake of 62Cu-ATSM was observed (myocardium/ blood ratio: 3.09) in one patient with unstable angina, who had increased 18F-fluorodeoxyglucose (18F-FDG) uptake under the fasting condition. The other 6 patients, who were clinically stable, did not have increased 62Cu-ATSM uptake, although abnormal 18F-FDG uptake was seen in 4 patients. This preliminary study suggests that 62Cu-ATSM is a promising PET tracer for hypoxic imaging in acute ischemia. 
Key words: copper-62 ATSM, hypoxia, coronary artery disease, fluorine-18 FDG, PET 
INTRODUCTION 
VISUALIZATION OF HYPOXIC TISSUE is important for the evaluation of ischemic change in the brain and heart, and for the characterization of tumors. Nitroimidazole com-pounds are of great interest because of their selective accumulation in hypoxic tumorsl as well as ischemic tissues.2 Various groups have attempted to design nitroimidazole-based drugs labeled with 18F,3 123I,4 131I,5 or 99mTc6 for imaging hypoxia but these tracers had low target accumulation due to slow blood clearance and low membrane permeability .7 
62Cu labeled diacetyl-bis(N4-methylthiosemicarbazone) (62Cu-ATSM) has been proposed as a generator-based positron-emitting tracer for imaging hypoxia 8 62Cu-PTSM, developed as a perfusion tracer, is easily reduced by the electron transport system in mitochondria, which can explain its retention.9 On the other hand, 62Cu-ATSM, 
an analogue of 62Cu-PTSM, cannot be reduced by normal mitochondria due to its low redox potential. Therefore, 62Cu-ATSM is not retained in the brain and heart, al-though it has high membrane permeability. The hypoxiaselective retention of 62Cu-ATSM requires an abnormally high NADH concentration caused by oxygen depletion, and also intact mitochondria. It was reported that 62Cu-ATSM has shown sign of high myocardial accumulation in a perfused rat heart model under hypoxic conditions 8 as well as in an in vivo rat model immediately after LAD occlusion.lo It was also reported that as the blood flow decreased the 62Cu-ATSM accumulation increased; but at flow rates that were approximately 40(~o of normal, the uptake began to decrease.10 
To clarify the usefulness of 62Cu-ATSM in myocardial ischemia 62Cu-ATSM PET was performed in 7 patients with coronary artery disease. 
MATERIALS AND METHODS 
Copper-62 was obtained with a 62Zn/62Cu generator sys-tem from [62Zn]ZnC12 solution.11 Cu-ATSM was synthesized according to the method of Gingas et al.,12 and confirmed by elemental analysis and mass spectrometry. 62Cu-ATSM was prepared as follows8: Briefly, 4 ml of 62Cu-glycine (non-carrier added 62Cu) solution obtained from the generator was mixed with 0.2 ml of ATSM solution (0.4 mM in dimethyl sulfoxide). The radiochemical purity of 62Cu-ATSM was confirmed by HPLC in combination with authentic Cu-ATSM. 
The study involved 7 patients with coronary artery disease (4 male, 3 female, age range 63-80 yr). Six patients had prior myocardial infarction and I patient was diagnosed with unstable angina because she had a few angina pectoris attacks per day, which were refractory to medical treatment (Table 1). The study was approved by the Ethical Committee of Fukui Medical University and written informed consent was obtained from all the subjects before the PET study. 
PET was performed with a high-resolution, whole-body PET scanner with an 18-ring detector arrangement (Advance, GE Medical Systems, Milwaukee, WI, USA). The physical characteristics of this scanner have been described in detail by DeGrado et al. 13 Briefly, the system permits the simultaneous acquisition of 35 transaxial images with an interslice spacing of 4.25 mm. Both axial and transaxial resolution are 4.2 mm, allowing multidirectional reconstruction of the images without loss of resolution. The FOV and the pixel size of the reconstructed images were 256 and 2 mm, respectively. A 10-min transmission scan was acquired with a 68Ge/68Ga source for attenuation correction, followed by intrave-
nous injection of 370 to 740 MBq of 62Cu-ATSM over 30 sec. Static scan was performed for 10 min (10-20 min post injection). In order to compare 62Cu-ATSM images with blood flow and glucose metabolism of the myocardium, nitrogen-13 ammonia (13NH3) and 18F-fluorodeoxyglucose (18F-FDG) PET was performed after overnight fast within a week. Static PET images were acquired over 10 min beginning 10 min after an intravenous injection of 13NH3 (740 MBq). 18F-FDG (370 MBqy was then injected intravenously, and static images acquired over 10 min beginning 60 min after the injection. 
Eleven circular regions of interest (ROI) were placed on the 62Cu-ATSM PET images of the left myocardium (Fig. 1), and left atrium (0.6 cm2 and 1.8 cm2 respectively, in area). The myocardial activity of 62Cu-ATSM was normalized by the arterial blood activity, which was derived from the ROI placed over the left atrium of the PET image (uptake ratio). The upper limit of 62Cu-ATSM was defined as 2.6, which was the mean + 2SD in the 4 normal subjects in our previous report.14 
The standardized uptake value (SUV) images of 18F-FDG were calculated with the following formula: 
The same ROIs as used in the 62Cu-ATSM PET images of the left ventricle were placed on both the 13NH3 and 18F-FDG images. In the study of myocardial perfusion, the myocardial uptake percent was calculated after normalization to each peak value in the study. The uptake ratio of 62Cu-ATSM was compared with myocardial blood flow (%) and glucose metabolism (SUV) under the fasting condition. The normal range of 18F-FDG uptake was defined as < 3.5 mg/ml (SUV), which was previously re ported.15 
RESULTS 
Increased 62Cu-ATSM uptake was observed (3.09) in one segment of a patient with unstable angina, who had increased 18F-FDG uptake under the fasting condition (Fig. 2). The other 6 patients, who were clinically stable, did not have increased 62Cu-ATSM uptake, although abnormal 18F-FDG uptake was seen in 18 segments, in 4 of the patients (Fig. 3). Enhanced uptake of 62Cu-ATSM was not seen in the moderately low flow area (Fig. 4). 
DISCUSSION 
Although increased glucose metabolism was seen in 5 of 7 patients, only I patient with unstable angina had enhanced myocardial uptake of 62Cu-ATSM. In addition, no increase in 62Cu-ATSM uptake was observed in the moderately low flow area, which was apparent in the rat acute ischemia model.8,lo As the impairment of contractile function reduces the oxygen demand of hypoperfused myocardium in hibernating myocardium, 16lowered oxygen demand may reduce electron transport in the mitochondria. Accordingly, retention of 62Cu-ATSM was not increased in the chronically ischemic myocardium. After long duration of ischemia, the myocardium is irreversibly injured, and the leakage of intramitochondrial enzymes occurs, which is necessary for 62Cu-ATSM retention. Accordingly, 62Cu-ATSM is considered to be a PET tracer for hypoxic imaging in acute ischemia, highly sensitive to the intactness of mitochondria. 18F-FDG uptake might indicate abnormality of myocardial metabolism, but not intactness of the energy production system. 
Although we have not compared 62Cu-ATSM and 18F-fluoromisonidazole (18F-FMISO) in this study, 62Cu-ATSM has two advantages. First, 62Cu can be obtained by a generator system from 62Zn, which has a 9 hr half-life and could be delivered for long distances. The second advantage is that the faster myocardial uptake of 62Cu-ATSM than 18F-FMISO allows more rapid imaging of ischemic but viable myocardium. In 18F-FMISO PET, the difference between normal and hypoxic tissues does not become clear until 2 hours post injection due to slow blood clearance.17 62Cu-ATSM PET imaging can be done within 20 min after injection due to its high membrane permeability.10 Therefore, the more efficient washout kinetics of 62Cu-ATSM in acute ischemia in comparison with 18F-FMISO offers the possibility of a faster and more efficient means of evaluating of myocardial hypoxia by PET imaging. 
There are some limitations to this study. The number of patients was small and the results are preliminary but enhanced uptake of 62Cu-ATSM was observed in a patient with unstable angina, and imaging was completed only 20 minutes after the injection. Although experimental studies have already supported the possibility of identifying myocardial hypoxia with the other agents for imaging hypoxia 18F-FMISO and 99mTc-2-nitroimidazole BMS 181321,17,18 neither of them has been used success-fully in visualizing hypoxic myocardium in human subjects. 
In conclusion, this preliminary study suggests that 62Cu-ATSM is a promising PET tracer for hypoxic imaging in acute ischemia not in chronic ischemia. Further clinical trials will needed to determine the usefulness of 62Cu-ATSM for myocardial ischemia. 
ACKNOWLEDGMENT 
We thank the Nihon Medi-Physics Co. Ltd., Japan for supplying 62Zn. 
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