In vivo dynamic imaging of myocardial cell death using 99mTc-labeled C2A domain of synaptotagmin I in a rat model of ischemia and reperfusion

Abstract

Objectives: This study was designed to investigate the capability of a small-animal SPECT imager, FastSPECT II, for dynamic rat heart imaging and to characterize the in vivo kinetic properties of 99mTc-C2A-glutathione-s-transferase (GST), a molecular probe targeting apoptosis and necrosis, in detecting cell death in ischemic-reperfused rat hearts.

Methods: C2A-GST was radiolabeled with 99mTc via 2-iminothiolane thiolation. Myocardial ischemia-reperfusion was induced by 30-min ligation of the left coronary artery followed by 120-min reperfusion in seven rats. FastSPECT II cardiac images of 99mTc-C2A-GST in list-mode acquisition were recorded for 2 h using FastSPECT II.

Results: Tomographic images showed a focal radioactive accumulation (hot spot) in the lateral and anterior walls of the left ventricle. The hot spot was initially visualized 10 min after injection and persisted on the 2-h images. Quantitative analysis demonstrated that the hot-spot radioactivity increased significantly within 30 min postinjection and experienced no washout up to the end of the 2-h study. The ratio of the hot spot/viable myocardium was 4.52+/-0.24, and infarct-to-lung ratio was 8.22+/-0.63 at 2 h postinjection. The uptake of 99mTc-C2A-GST in the infarcted myocardium was confirmed by triphenyl tetrazolium chloride staining and autoradiography analysis.

Conclusions: FastSPECT II allows quantitative dynamic imaging and functional determination of radiotracer kinetics in rat hearts. An in vivo kinetic profile of 99mTc-C2A-GST in the ischemic-reperfused rat heart model was characterized successfully. The pattern of accelerated 99mTc-C2A-GST uptake in the ischemic area at risk after reperfusion may be useful in detecting and quantifying ongoing myocardial cell loss induced by ischemia-reperfusion.

Figure 1

Representative dynamic tomographic images of ^99mTc-C2A-GST using a series of the same transaxial slice at each time point in a rat heart with ischemia-reperfusion (injected dose = 3.4 mCi). The rat was anesthetized with 1.2% isoflurane. The number in the upper right corner represents the post-injection time. The integrated images were parsed and split into dynamic 1-minute images. The cardiac blood pool (dashed mark) is evident on the 1-minute image. A good infarct definition with a regional hot spot in the left ventricular wall was achieved 10–30 minutes after radiotracer administration. The hot spot becomes increasingly prominent in size and radioactivity from 10 to 120 minutes post-injection. The arrow on 120-minute indicates the site of the infarct.

Figure 2

Representative FastSPECT II transversal (left), coronal (middle), and sagittal (right) tomographic slices 2 hours post-injection of ^99mTc-C2A-GST in a rat heart with myocardial infarction (injected dose = 3.4 mCi, 5-minute acquisition). The rat was anesthetized with 1.2% isoflurane. ^99mTc-C2A-GST “hot spot” accumulations localized in the lateral wall, anterior wall, and apex of the left ventricle. The radioactive distribution of the remote normal myocardium and the lungs was low. The prominent liver uptake of ^99mTc-C2A-GST was observed on the coronal and sagittal slices.

Figure 3

^99mTc-C2A-GST time-activity curves from infarcted myocardium, remote viable zone, and lungs in the rat hearts with ischemia-reperfusion injury. The curves were corrected by decay and acquisition time. * = P < 0.05 compared to remote viable zone, # = P < 0.05 compared to lung.

Figure 4

Ratios of the infarct/normal myocardium and the infarct/lung activity increased with time following ^99mTc-C2A-GST administration in the ischemic-reperfused rat hearts.

Figure 5

Photographs of TTC staining (first and third row) and autoradiograph images (second and fourth row) from a representative ischemic-reperfused rat heart. The location and size of myocardial infarction (negative TTC staining) were consistent with the positive accumulation of ^99mTc-C2A-GST determined by autoradiograph imaging.