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Basic Science Investigation |
1 Department of Nuclear Medicine, Klinikum Rechts der Isar, Technische Universität München, Munich, Germany; and 2 Department of Discovery Chemistry and Department of Discovery Biology, Bristol Myers Squibb Medical Imaging, North Billerica, Massachusetts
Correspondence: For correspondence contact: Markus Schwaiger, MD, Nuklearmedizinische Klinik der Technischen Universität München, Ismaninger Strasse 22, 81675 Munich, Germany. E-mail: markus.schwaiger{at}tum.de
| ABSTRACT |
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Key Words: PET myocardial perfusion imaging
| INTRODUCTION |
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At present, several blood flow tracers are available for use with PET. The availability of these blood flow tracers depends on local production either by a cyclotron facility in close proximity or by a generator, as in the case of 82Rb. This limitation has restricted the application of PET flow measurements. Therefore, the need for an 18F-labeled flow tracer is well appreciated. Such a tracer can be distributed as a single-dose unit in a fashion similar to that of 18F-FDG or 99mTc-labeled flow tracers, widening the application of PET perfusion imaging.
Recently, the first 18F-labeled radiopharmaceutical for the assessment of myocardial perfusion was proposed (8), underscoring the interest in such a tracer. The purpose of the present study was to characterize a new 18F-radiolabeled PET perfusion tracer (18F-BMS-747158-02) in a rat model. 18F-BMS-747158-02 is a pyridazinone analog designed to target mitochondrial complex I of the electron transport chain with high affinity. Uptake into the heart is selective because of the high density of mitochondria in myocardial tissue. In addition to biodistribution, the in vivo pharmacokinetics in the rat myocardium as well as surrounding tissues were measured by dynamic small-animal PET imaging. Studies with perfused isolated rat hearts were used to determine the first-pass myocardial extraction fraction of this tracer at different flow rates.
| MATERIALS AND METHODS |
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Tracer Preparation
Dry cryptate ([K
2.2.2.]+18F–) was resolubilized with a solution of BMS-747155-01 in dry acetonitrile. The mixture was stirred at 90°C for 10 min. 18F-BMS-747158-02 was purified by preparative high-performance liquid chromatography. After dilution, the product fraction 18F-BMS-747158-02 was immobilized on a reversed-phase cartridge, washed, and eluted with ethanol. The solvent was removed under reduced pressure. Quality control was performed with an analytic high-performance liquid chromatography system. The entire synthesis was completed within 60 min. The overall radiochemical yield was 50%–60%, and the radiochemical purity was 99%. The value of log POct/PBS was 0.76 ± 0.01 (mean ± SD), indicating moderate lipophilicity.
Biodistribution Study
For the biodistribution study, 6.3–7.0 MBq of 18F-BMS-747158-02 in 150 µL of phosphate-buffered saline (pH 7.4) was injected into a lateral tail vein. The animals were anesthetized with isoflurane (Abbott GmbH & Co.; 1.5% by volume, 2.0 L of O2/min) during tracer injection. The animals were awake during the uptake period of 10 min (n = 3) or 60 min (n = 3), after which they were euthanized by carbon dioxide inhalation. The organs of interest were rapidly dissected and weighed. Organ activity levels were determined by well counting with the appropriate energy window and decay correction. Data were expressed as percentage injected dose per gram (%ID/g).
Perfused Isolated Rat Heart Study
Hearts were quickly excised from anesthetized rats (intraperitoneal sodium pentobarbital [0.25 mL]; Merial GmbH), placed in ice-chilled Krebs–Henseleit bicarbonate buffer (glucose at 10 mmol/L), and cannulated via the aorta. Krebs–Henseleit buffer, oxygenated with a mixture of 95% oxygen and 5% carbon dioxide, was used to perfuse the heart (without recirculation; n = 3, n = 4, and n = 4 for flow velocities of 5, 8.3, and 16.6 mL/min, respectively). The heart was stabilized for a period of 15 min, after which a bolus injection of
0.925 MBq of 18F-BMS-747158-02 (0.037 MBq/µl) was given. Whole-heart radioactivity was measured over 10 min with a pair of bismuth germanate detectors interfaced to coincidence detection circuitry. Care was taken to keep the counting rate within the linear response range of the system, to prevent dead-time–induced counting rate losses during tracer injection. The total coincidence counting rate was measured as a function of time and corrected for decay. The curve was analyzed by fitting an exponentially decaying function to the data representing the tissue washout component (from 100 s to 600 s). The fitted curve was extrapolated to the time of the maximum counting rate, and the value obtained was divided by the measured maximum counting rate. This ratio is a measure of the extraction fraction (Fig. 1; please note that for visibility, only data up to 100 s are shown, together with the extrapolated fitted curve). Because the curve was essentially linear in the fit region, possible differences between an exponential fit and a linear fit were studied. No differences were found for the determination of the extraction fraction (data not shown).
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37 MBq of 18F-BMS-747158-02 in 500 µL of phosphate-buffered saline (pH 7.4). To increase myocardial blood flow, we performed stress studies during the infusion of adenosine via a separate catheter placed in a different lateral tail vein (11). Adenosine (140 µg/kg/min; Sanofi-Synthelabo, GmbH) was infused over a period extending from 5 min before to 2 min after the start of the imaging procedure with an infusion pump. Measurements were obtained for 6 animals in the rest group and 6 animals in the stress group. Data were acquired in list mode format for 20 min, and histograms were used to produce a 29-frame sinogram (18 x 5 s, 7 x 30 s, 2 x 150 s, and 2 x 5 min). The sinogram was reconstructed into an image of 128 x 128 x 95 voxels by use of filtered backprojection with a ramp filter with a cutoff at the Nyquist frequency. The voxel size equaled 0.433 x 0.433 x 0.796 mm3, and the effective spatial resolution was less than 2.0 mm. Data were normalized and corrected for randoms, dead time, and decay.
Image Analysis
Image analysis was performed with the ASIPro software package (Siemens Medical Solutions) as well as with the Munich Heart software package (12). The latter software allows for image reorientation, enabling viewing of the heart along the short, vertical long, and horizontal long axes.
Regions of interest (ROIs) were manually defined in transaxial slices of the frame reconstructed from data measured between 15 and 20 min after injection. Blood-pool (left ventricular cavity) and myocardial ROIs were drawn on 3 contiguous slices near the basal plane of the heart. Lung and liver ROIs were drawn close to the heart. The mean activities in the ROIs (in arbitrary units, proportional to cps/mL) were plotted as a function of time (time–activity curves). These regional time–activity curves were used to determine uptake ratios for myocardium to liver, myocardium to blood, and myocardium to lung.
Tracer retention (1/s) was defined as the myocardial activity concentration divided by the integral under the input function. For calculation of tracer retention at 1 and 2 min after injection, the ROIs described earlier were used to define myocardial uptake. The time integral of the input function for 1 and 2 min was defined by fitting a
-variate function to the measured input function. The homogeneity of myocardial tracer uptake was determined with the Munich Heart software. Polar map representations of uptake after 2 min were derived. The values in the polar map representation of the tracer indicated the percentage of mean tracer uptake in the ROI, normalized to the maximum myocardial uptake (expressed as a percentage of the maximum).
Statistical Methods
Values are reported as mean (SD). Statistical significance was tested with a standard t test.
| RESULTS |
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0.05).
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Figure 3 shows an imaging example of the uptake of 18F-BMS-747158-02 in the myocardium at 15–20 min after injection. High contrast between the myocardium and blood, lungs, and liver was seen. Right ventricular activity was delineated, indicating high extraction and retention, as well as low nonspecific binding of the tracer.
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| DISCUSSION |
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13N-Ammonia is widely used as a flow tracer because of its high myocardial extraction fraction and its relatively long physical half-life (10 min), which together facilitate good image quality. Several studies have validated 13N-ammonia as a marker for the qualitative as well as quantitative assessment of regional myocardial blood flow (16–18). Tracer kinetic models are used to compensate for decreasing myocardial extraction fractions at higher flows, yielding accurate measurements of flow over a wide flow range (4,5). A pattern of heterogeneous retention of 13N-ammonia has been observed in healthy volunteers, limiting the specificity of perfusion defect detection, especially in the lateral wall of the left ventricle (1,4).
82Rb offers the advantage of being a generator-produced radiopharmaceutical with a generator shelf life of about 4 wk. The very short half-life of 82Rb, 76 s, and the relatively low myocardial extraction fraction result in image quality very similar to that provided by 99mTc-based SPECT tracers. This tracer has been validated extensively and is being used clinically as a routine generator-produced radiopharmaceutical in the application of PET and PET/CT for the work-up of patients with CAD (19–22).
Recently, 18F-labeled fluorobenzyl triphenyl phosphonium, a member of the class of potentiometric lipophilic phosphonium cations originally developed for measurement of the mitochondrial membrane potential, was introduced for myocardial perfusion imaging (8). The accumulation of the tracer depends primarily on the mitochondrial membrane potential. Furthermore, the myocardium-to-liver uptake ratio is approximately 1, hampering image contrast for myocardial structures.
In the present study, 18F-BMS-747158-02 was characterized with respect to its usefulness as a PET tracer for myocardial perfusion imaging. The chemical structure of 18F-BMS-747158-02 represents a modification of the pyridazinone insecticide pyridaben. These compounds act through tight binding to mitochondrial complex I, the first component of the electron transport chain, which is highly conserved. 18F-BMS-747158-02 is selectively taken up into the heart because of the high density of mitochondria in cardiac muscle (23,24). The lipophilicity of the compound and its high binding affinity (50% inhibitory concentration of
13.8 nM (25)) result in a very high binding capacity.
Our study of the first-pass extraction fraction of the tracer in an isolated rat heart showed that the extraction fraction was higher than 90% and essentially flow independent (5–15 mL/min). Therefore, we conclude that delivery of the tracer by myocardial flow remains the limiting factor for retention, fulfilling the requirement for an almost ideal flow tracer.
This high extraction fraction is surprising, because the flow rates in the non–blood-perfused rat heart are considerably higher than those under physiologic conditions. Other common SPECT and PET perfusion tracers that are retained in the myocardium have considerably lower values. The SPECT tracer 99mTc-sestamibi has an average myocardial extraction fraction of 0.38 (SD = 0.09) for a flow range of 0.52–3.19 mL/min/g (26). For 13N-ammonia, an extraction fraction of 0.82 (SD = 0.06) has been found at a control flow after injection into the left circumflex coronary artery (27); lower extraction fractions have been found at higher flows (in mL/min/100 g; calculated as extraction fraction = 1 – 0.607–125/flow). For 82Rb, a value of 0.42 (SD = 0.06) has been found after injection into the femoral vein at normal resting flow rates (0.75–1.5 mL/min/g) (28). The observation of an essentially flow-independent extraction fraction for 18F-BMS-747158-02 implies a linear relationship between uptake and myocardial blood flow, which is an important attribute for stress flow measurements. To our knowledge, the observed increase in tracer retention in the rat heart during pharmacologic stress testing with adenosine infusion is the first documentation of the expected relationship between uptake and perfusion. The stress-to-rest retention ratio of approximately 1.9 in the present study corresponds well to the reported value of 1.8 (SD = 0.6) for adenosine stress testing with validated tracer kinetic models of 13N-ammonia in the rat heart (11). However, further studies with larger animals and labeled microspheres as a gold standard are being performed in our laboratory to define the relationship between 18F-BMS-747158-02 PET flow measurements and microsphere-derived blood flow in the heart (29).
Because this tracer is bound to mitochondrial structures, it needs to be clarified whether alterations in the metabolic state of the myocardium (hypoxia or ischemia) affect the regional retention of the tracer. Such alterations have been observed with 99mTc-sestamibi, which is also retained in mitochondrial structures (30). However, small metabolically induced changes in tissue retention may not affect the overall diagnostic performance of an imaging approach, as shown by blood flow data obtained with 99mTc.
The metabolic fate of the tracer was not evaluated in the present study. An observed slight increase in blood activity over time (Table 1) may suggest the presence of metabolites with different tissue uptake characteristics. Therefore, tracer retention estimates were restricted to the first 2 min after tracer injection, when metabolic effects are expected to be negligible.
The relatively slow washout of the tracer allows for imaging protocols with high count statistics. Gated data acquisition would provide measurements of ventricular volumes and wall motion. Patients can be injected during peak exercise on a treadmill and transferred for imaging after the completion of the exercise protocol.
| CONCLUSION |
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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