The feasibility of imaging myocardial ischemic/reperfusion injury using (99m)Tc-labeled duramycin in a porcine model

Abstract

When pathologically externalized, phosphatidylethanolamine (PE) is a potential surrogate marker for detecting tissue injuries. (99m)Tc-labeled duramycin is a peptide-based imaging agent that binds PE with high affinity and specificity. The goal of the current study was to investigate the clearance kinetics of (99m)Tc-labeled duramycin in a large animal model (normal pigs) and to assess its uptake in the heart using a pig model of myocardial ischemia-reperfusion injury.

Methods: The clearance and distribution of intravenously injected (99m)Tc-duramycin were characterized in sham-operated animals (n=5). In a closed chest model of myocardial ischemia, coronary occlusion was induced by balloon angioplasty (n=9). (99m)Tc-duramycin (10-15mCi) was injected intravenously at 1hour after reperfusion. SPECT/CT was acquired at 1 and 3hours after injection. Cardiac tissues were analyzed for changes associated with acute cellular injuries. Autoradiography and gamma counting were used to determine radioactivity uptake. For the remaining animals, (99m)Tc-tetrafosamin scan was performed on the second day to identify the infarct site.

Results: Intravenously injected (99m)Tc-duramycin cleared from circulation predominantly via the renal/urinary tract with an α-phase half-life of 3.6±0.3minutes and β-phase half-life of 179.9±64.7minutes. In control animals, the ratios between normal heart and lung were 1.76±0.21, 1.66±0.22, 1.50±0.20 and 1.75±0.31 at 0.5, 1, 2 and 3hours post-injection, respectively. The ratios between normal heart and liver were 0.88±0.13, 0.80±0.13, 0.82±0.19 and 0.88±0.14. In vivo visualization of focal radioactivity uptake in the ischemic heart was attainable as early as 30min post-injection. The in vivo ischemic-to-normal uptake ratios were 3.57±0.74 and 3.69±0.91 at 1 and 3hours post-injection, respectively. Ischemic-to-lung ratios were 4.89±0.85 and 4.93±0.57; and ischemic-to-liver ratios were 2.05±0.30 to 3.23±0.78. The size of (99m)Tc-duramycin positive myocardium was qualitatively larger than the infarct size delineated by the perfusion defect in (99m)Tc-tetrafosmin uptake. This was consistent with findings from tissue analysis and autoradiography.

Conclusion: (99m)Tc-duramycin was demonstrated, in a large animal model, to have suitable clearance and biodistribution profiles for imaging. The agent has an avid target uptake and a fast background clearance. It is appropriate for imaging myocardial injury induced by ischemia/reperfusion.

Keywords: Apoptosis; Duramycin; Ischemia–reperfusion injury; Phosphatidylethanolamine.

Figure 1

^99mTc-duramycin clearance and distribution in control animals. A) Blood clearance profile as a function of time. Solid line: Two-exponential fitting for estimation of blood clearance half-lives. B) Organ clearance profiles derived from serial SPECT data, for the heart (○), lung (■) and liver (▲) as a function of time. C) Serial anterior whole-body planar images acquired at 30, 60, 120 and 180 minutes after intravenous ^99mTc-duramycin injection.

Figure 2

Imaging ischemic myocardial injury using ^99mTc-duramycin in a pig model of myocardial ischemia and reperfusion. A) Whole-body anterior planar image acquired at 30 minutes post injection. Radioactivity uptake in the ischemic heart region can be differentiated from the background, as marked by an arrow. Injection site via the ear vein is marked with an asterisk. Left and right kidneys are labeled as K_L and K_R, respectively. The bladder is marked as BL. B) and C) Short- and long-axial SPECT/CT fusion images acquired at 1 hour post injection, with a hot spot ^99mTc-duramycin uptake at the anterior left ventricular free wall (arrow). D) and E) Short- and long-axial images acquired at 3 hour from the same animal. Consecutive slices are shown in Supplementary Figures 2 and 3, for 1 and 3 hour data, respectively.

Figure 3

A representative example of a relatively small, apical region of ischemic injury detected in SPECT/CT using ^99mTc-duramycin. A) Short-axial CT of the thorax. B) and C) A small area of ischemic and reperfusion injury was identified (arrows) at the apical region of the heart as a result of distal circumflex occlusion.

Figure 4

Imaging small region of acute ischemic myocardial injury using ^99mTc-Duramycin. A) Short-axial CT of the thorax. B and C) A small area of ischemic and reperfusion damage is identified (arrows) at the apical region of the heart as a result of distal circumflex occlusion.

Figure 5

Histological analyses. A) TTC stained (upper) and autoradiography (lower) of the same short-axial heart sections. B) TUNEL staining of tissue harvested from the periphery of the ischemic core. C) Ultrastructural changes associated with ischemia/reperfusion injury, as identified with transmission electron microscopy. D) H&E staining of damaged myocardium at the ischemic zone.