[⁶⁸Ga]Ga-NODAGA-E[(cRGDyK)]₂ Angiogenesis PET/MR in a Porcine Model of Chronic Myocardial Infarction

Abstract

Angiogenesis is crucial in tissue repair and prevents scar tissue formation following an ischemic event such as myocardial infarction. The ischemia induces formation of new capillaries, which have high expression of integrin α_vβ₃. [⁶⁸Ga]Ga-NODAGA-E[(cRGDyK)]₂ ([⁶⁸Ga]Ga-RGD) is a promising PET-radiotracer reflecting angiogenesis by binding to integrin α_vβ₃. A Göttingen mini-pig underwent transient catheter-induced left anterior descending artery (LAD) occlusion for 120 min, and after 8 weeks was imaged on a Siemens mMR 3T PET/MR system. A large antero-septal infarction was evident by late gadolinium enhancement (LGE) on the short axis and 2–4 chamber views. The infarcted area corresponded to the area with high [⁶⁸Ga]Ga-RGD uptake on the fused PET/MR images, with no uptake in the healthy myocardium. To support the hypothesis that [⁶⁸Ga]Ga-RGD uptake reflects angiogenesis, biopsies were sampled from the infarct border and healthy myocardium. Expression of α_vβ₃ was evaluated using immunohistochemistry. The staining showed higher α_vβ₃ expression in the capillaries of the infarct border compared to those in the healthy myocardium. These initial data confirm in vivo detection of angiogenesis using [⁶⁸Ga]Ga-RGD PET in a translational model, which overall support the method applicability when evaluating novel cardio-protective therapies.

To evaluate angiogenesis following transient occlusion of the LAD artery, a Göttingen minipig underwent PET/MR examination [1,2]. A bolus injection of approximately 100 MBq [⁶⁸Ga]Ga-RGD was administered through a central venous catheter. A 15 min PET acquisition was performed 30 min after the injection. After the PET acquisition, gadolinium was injected in order to examine the final infarct size.

Figure 1. Anatomical cardiac MR images of a Göttingen minipig following gadolinium injection in short axis (A), two-chamber (B) and four-chamber view (C). The animal had undergone 120 min invasive LAD occlusion 8 weeks prior to the scan [3]. The green line in (B) illustrates the placement of the short axis slice shown in (A). A large antero-septal infarction is evident by late gadolinium enhancement (LGE) on all views (orange arrows). On the midventricular short axis image (A), the LGE appears subendocardial, while towards the apex of the heart, the infarct is transmural, indicating a large infarction. All images were obtained using a Siemens mMR 3T PET/MR system and flex body coils. The infarcted area corresponded to the area with high [⁶⁸Ga]Ga-RGD uptake on the fused PET/MR images ((D–F), red arrows), with no [⁶⁸Ga]Ga-RGD uptake observed in the healthy parts of the myocardium. RV: Right ventricle, LV: Left ventricle, LA: Left atrium, RA: Right atrium, SUV: Standardized uptake value.

Figure 1. Anatomical cardiac MR images of a Göttingen minipig following gadolinium injection in short axis (A), two-chamber (B) and four-chamber view (C). The animal had undergone 120 min invasive LAD occlusion 8 weeks prior to the scan [3]. The green line in (B) illustrates the placement of the short axis slice shown in (A). A large antero-septal infarction is evident by late gadolinium enhancement (LGE) on all views (orange arrows). On the midventricular short axis image (A), the LGE appears subendocardial, while towards the apex of the heart, the infarct is transmural, indicating a large infarction. All images were obtained using a Siemens mMR 3T PET/MR system and flex body coils. The infarcted area corresponded to the area with high [⁶⁸Ga]Ga-RGD uptake on the fused PET/MR images ((D–F), red arrows), with no [⁶⁸Ga]Ga-RGD uptake observed in the healthy parts of the myocardium. RV: Right ventricle, LV: Left ventricle, LA: Left atrium, RA: Right atrium, SUV: Standardized uptake value.

Figure 2. To confirm infarct severity ex-vivo, the excised heart was divided into 4 axial slices and stained using 1% triphenyl-tetrazolium-chloride (TTC) solution (A–D). The TTC staining in (B) shows a subendocardial infarction, while in (C,D), the TTC staining is transmural. The TTC staining corresponds to patterns seen by LGE on the corresponding MRI images (E–H) and fused PET/MR images (I–L). The red, yellow and green arrows in (C) mark the infarct zone, infarct border zone and healthy myocardium, respectively. These areas correspond to the immunohistochemical images in Figure 3. Scale bar (20 mm) is shown in (D) for size reference, SUV: Standardized uptake value.

Figure 2. To confirm infarct severity ex-vivo, the excised heart was divided into 4 axial slices and stained using 1% triphenyl-tetrazolium-chloride (TTC) solution (A–D). The TTC staining in (B) shows a subendocardial infarction, while in (C,D), the TTC staining is transmural. The TTC staining corresponds to patterns seen by LGE on the corresponding MRI images (E–H) and fused PET/MR images (I–L). The red, yellow and green arrows in (C) mark the infarct zone, infarct border zone and healthy myocardium, respectively. These areas correspond to the immunohistochemical images in Figure 3. Scale bar (20 mm) is shown in (D) for size reference, SUV: Standardized uptake value.

This study was performed as a hypothesis-generating exploration of the evaluation of integrin α_vβ₃ expression following transient occlusion of the LAD artery. The use of [⁶⁸Ga]Ga-RGD as a prognostic marker for functional outcome following MI has been explored in both animal and human studies [4]. The [⁶⁸Ga]Ga-RGD tracer in blocking studies shows a high sensitivity towards α_vβ₃ [5], indicating the potential in future studies with a radiotracer. However, the minipig was euthanized following the PET/MR scan in order to examine histology and immunohistochemistry. This study shows promising PET images of [⁶⁸Ga]Ga-RGD, but further studies need to be performed in order to evaluate the prognostic value of the tracer.