Abstract 2954: In Vivo Molecular Imaging of Myocardial Angiogenesis Using ^99m Tc-RAFT-RGD, a Novel α _v β ₃ Integrin-Specific Tracer

Abstract

Introduction - Myocardial angiogenesis following reperfusion of an infarcted area affects postinfarct remodelling and may therefore impact on patient prognosis. The non-invasive molecular imaging of angiogenesis is thus of potential clinical relevance in this setting. Angiogenic vessels overexpress the α v β 3 integrin. Tc-RAFT-RGD is a novel 99m Tc-labelled tracer composed of four α v β 3 integrin-specific cyclo(RGDfK) sequences tethered on a cyclodecapeptide platform. Hypothesis - We hypothesized that Tc-RAFT-RGD binding to the α v β 3 integrin would allow the molecular imaging of myocardial angiogenesis in vivo. Methods - Myocardial infarction was induced by left anterior descending coronary artery occlusion for 45 min prior to reperfusion in 24 anesthetized male Wistar rats. Fourteen days later, the animals were injected intravenously with Tc-RAFT-RGD (n = 16) or Tc-RAFT-RAD (negative control, n = 8). Tracer uptake was assessed 60 min following injection in 12 and 4 animals injected with Tc-RAFT-RGD and Tc-RAFT-RAD, respectively. In those animals, the hearts were quickly excised for autoradiographic imaging, nitroblue tetrazolium (NBT) infarct staining, CD31 immunostaining of neovessels, and for gamma-well counting of myocardial tracer activity in the infarcted and normal areas. The remaining animals were dedicated to in vivo dual-isotope pinhole SPECT imaging following the simultaneous injection of Tl-201 and Tc-RAFT-RGD (n = 4) or Tc-RAFT-RAD (n = 4). Results - Neovessels were observed in the infarcted area using CD31 immunostaining. The Tc-RAFT-RGD infarcted-to-normal zone activity ratios by gamma-well counting and ex vivo imaging (2.6 ± 0.6 and 3.9 ± 1.0, respectively) were significantly higher than those of Tc-RAFT-RAD (1.7 ± 0.5 and 2.1 ± 0.4, respectively, P< 0.05). Quantification of short-axis images from SPECT acquisitions indicated similar Tl-201 defect magnitudes in animals injected with Tc-RAFT-RGD and Tc-RAFT-RAD (0.3 ± 0.1 and 0.4 ± 0.1, P =NS). The infarcted area was readily visible in vivo with Tc-RAFT-RGD but not with Tc-RAFT-RAD (infarct-to-normal zone activity ratio, 2.8 ± 0.6 and 1.6 ± 0.4, respectively, P< 0.05). Conclusions - Tc-RAFT-RGD allowed the experimental in vivo molecular imaging of myocardial angiogenesis.