Development and application of non-invasive imaging methods for the evaluation of acute myocardial infarction

Acute myocardial infarction (AMI) is a common presentation of ischemic heart disease. Early diagnostic and prognostic assessments will be beneficial in patient care and treatments. Current diagnosis of AMI is made by integrating the following three main criteria: (1) clinical history of ischemic type chest pain (2) regional wall motion abnormalities on an electrocardiogram (ECG) (3) changes of serum cardiac biomarkers. Nevertheless, to date, there still lacks a decisive diagnostic method of AMI in clinical settings.My graduate study is focused on the non-invasive imaging of AMI as well as the characterizations of imaging tracers, using an experimental rat model of myocardial ischemia/reperfusion. The outline of the thesis can be divided into three main steps: (1) using a non-specific imaging tracer of gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA) to determine the infarct size with delayed enhancement magnetic resonance imaging (deMRI) (2) characterization of 99mTc-C2A-GST (fusion protein of C2A domain of synaptotagmin I and glutathione S-transferase) with target-specific binding activities to the membrane anionic phospholipids as a radiotracer using nuclear imaging techniques (3) the characterization of the specific phosphatidylethanolamine (PtdE)-binding peptide, 99mTc-duramycin, as a novel imaging tracer.We investigate the earliest time point at which the infarct size can be accurately measured by deMRI, with the validation of tetrazolium staining as the histological gold standard. T1-weighted images of deMRI were acquired with a spin-echo sequence after intravenous Gd-DTPA injection. DeMRI measurement of infarct size was performed and quantitatively compared with tetrazolium staining of the corresponding tissue section. We found that deMRI can provide accurate infarct size measurement as early as 2 hours after AMI.The exposure of anionic phospholipids is a near-universal molecular signature for both types of cell death, apoptosis and necrosis, and enables the simultaneous detection of these distinct types of cell death. Based on the phosphatidylserine (PtdS)-binding of activity C2A-GST toward apoptosis and necrosis in vitro, we quantitatively characterized the temporal and spatial distribution of 99mTc-C2A-GST. In vivo planar imaging of AMI in rats was performed on a gamma-camera. We found that radiolabeled C2A-GST bound both apoptotic and necrotic cells. Temporally, the radioactivity uptake in the area-at-risk maximized when the radiotracer was injected before 3 hours of reperfusion. In planar imaging, the infarct was clearly identifiable as prominent hot-spot uptake. In autoradiography, the distribution of radioactivity predominantly coregistered with the infarcted regions determined by histology. The data indicate that 99mTc-C2A-GST has an uptake profile in the area-at-risk that is appropriate for imaging myocardial cell death in the acute phase.Then we investigated whether the uptake of 99mTc-C2A-GST in the acute phase of myocardial infarction is associated with cardiac dysfunction in follow-ups. The in vivo signal detected was correlated with wall motion score index (WMSI) at 1 and 3 weeks follow-ups measured by echocardiography. A significant correlation was found between 99mTc-C2A-GST uptake in the acute phase and functional abnormality at 1 and 3 weeks. This demonstrates the potential diagnostic and prognostic value of 99mTc-C2A-GST.Better tracer kinetics will help facilitate tracer diffusion and interactions with its binding targets in tissues. Decreasing the molecular size of the imaging tracer will in turn accelerate the diffusion rate of the tracer crossing the capillary wall, thus the maximum target-to-background ratio (TBR) in the target tissue can be achieved in a shorter period of time after administration. Duramycin binds phosphatidylethanolamine (PtdE) with high affinity and exclusive specificity. An outstanding feature of the peptide is its low molecular weight, as well as a fast blood clearance. Duramycin holds the potential to be a new generation imaging tracer of AMI due to its unique binding activities and physiological properties and the availability of exposed PtdE during acute myocardial cell death. It is anticipated that such improvements of duramycin will lead to a greater uptake in the target tissue with higher TBRs. We developed a fast labeling technique of 99mTc-duramycin and evaluated it as a novel molecular probe for acute myocardial cell death. The results suggested that 99mTc-duramycin can be synthesized quickly with an optimized radiolabeling technique, resulting high radiochemical purity (RCP). There was a 30-fold higher in the uptake of 99mTc-duramycin in apoptotic cells than that in viable cells. This specific binding can be competitively inhibited when PtdE-containing liposomes were present. In nuclear planar images, the 99mTc-duramycin uptake in the infarct region quickly becomes obvious shortly after injection because of the specific binding of 99mTc-duramycin to the infarcted myocardium and its fast blood clearance with low uptake in viable myocardium, lungs and liver as background. As a conclusion, 99mTc-duramycin has a strong potential to be a new generation molecular probe for imaging of acute myocardial cell death.