Molecular imaging-guided repair after acute myocardial infarction by targeting the chemokine receptor CXCR4

AIMS

Balance between inflammatory and reparative leucocytes allows optimal healing after myocardial infarction (MI). Interindividual heterogeneity evokes variable functional outcome complicating targeted therapy. We aimed to characterize infarct chemokine CXC-motif receptor 4 (CXCR4) expression using positron emission tomography (PET) and establish its relationship to cardiac outcome. We tested whether image-guided early CXCR4 directed therapy attenuates chronic dysfunction.

METHODS AND RESULTS

Mice (n = 180) underwent coronary ligation or sham surgery and serial PET imaging over 7 days. Infarct CXCR4 content was elevated over 3 days after MI compared with sham (%ID/g, Day 1:1.1 ± 0.2; Day 3:0.9 ± 0.2 vs. 0.6 ± 0.1, P < 0.001), confirmed by flow cytometry and histopathology. Mice that died of left ventricle (LV) rupture exhibited persistent inflammation at 3 days compared with survivors (1.2 ± 0.3 vs. 0.9 ± 0.2% ID/g, P < 0.001). Cardiac magnetic resonance measured cardiac function. Higher CXCR4 signal at 1 and 3 days independently predicted worse functional outcome at 6 weeks (rpartial = -0.4, P = 0.04). Mice were treated with CXCR4 blocker AMD3100 following the imaging timecourse. On-peak CXCR4 blockade at 3 days lowered LV rupture incidence vs. untreated MI (8% vs. 25%), and improved contractile function at 6 weeks (+24%, P = 0.01). Off-peak CXCR4 blockade at 7 days did not improve outcome. Flow cytometry analysis revealed lower LV neutrophil and Ly6Chigh monocyte content after on-peak treatment. Patients (n = 50) early after MI underwent CXCR4 PET imaging and functional assessment. Infarct CXCR4 expression in acute MI patients correlated with contractile function at time of PET and on follow-up.

CONCLUSION

Positron emission tomography imaging identifies early CXCR4 up-regulation which predicts acute rupture and chronic contractile dysfunction. Imaging-guided CXCR4 inhibition accelerates inflammatory resolution and improves outcome. This supports a molecular imaging-based theranostic approach to guide therapy after MI.

Multimodal and Multiscale Analysis Reveals Distinct Vascular, Metabolic and Inflammatory Components of the Tissue Response to Limb Ischemia

Ischemia triggers a complex tissue response involving vascular, metabolic and inflammatory changes.

Methods: We combined hybrid SPECT/CT or PET/CT nuclear imaging studies of perfusion, metabolism and inflammation with multicolor flow cytometry-based cell population analysis to comprehensively analyze the ischemic tissue response and to elucidate the cellular substrate of noninvasive molecular imaging techniques in a mouse model of hind limb ischemia.

Results: Comparative analysis of tissue perfusion with [^99mTc]-Sestamibi and arterial influx with [^99mTc]-labeled albumin microspheres by SPECT/CT revealed a distinct pattern of response to vascular occlusion: an early ischemic period of matched suppression of tissue perfusion and arterial influx, a subacute ischemic period of normalized arterial influx but impaired tissue perfusion, and a protracted post-ischemic period of hyperdynamic arterial and normalized tissue perfusion, indicating coordination of macrovascular and microvascular responses. In addition, the subacute period showed increased glucose uptake by [¹⁸F]-FDG PET/CT scanning as the metabolic response of viable tissue to hypoperfusion. This was associated with robust macrophage infiltration by flow cytometry, and glucose uptake studies identified macrophages as major contributors to glucose utilization in ischemic tissue. Furthermore, imaging with the TSPO ligand [¹⁸F]-GE180 showed a peaked response during the subacute phase due to preferential labeling of monocytes and macrophages, while imaging with [⁶⁸Ga]-RGD, an integrin ligand, showed prolonged post-ischemic upregulation, which was attributed to labeling of macrophages and endothelial cells by flow cytometry.

Conclusion: Combined nuclear imaging and cell population analysis reveals distinct components of the ischemic tissue response and associated cell subsets, which could be targeted for therapeutic interventions.

Multimodality imaging of blood-brain barrier impairment during epileptogenesis

Insult-associated blood-brain barrier leakage is strongly suggested to be a key step during epileptogenesis. In this study, we used three non-invasive translational imaging modalities, i.e. positron emission tomography, single photon emission computed tomography, and magnetic resonance imaging, to evaluate BBB leakage after an epileptogenic brain insult. Sprague-Dawley rats were scanned during early epileptogenesis initiated by status epilepticus. Positron emission tomography and single photon emission computed tomography scans were performed using the novel tracer [⁶⁸Ga]DTPA or [^99mTc]DTPA, respectively. Magnetic resonance imaging included T2 and post-contrast T1 sequence after infusion of Gd-DTPA, gadobutrol, or Gd-albumin. All modalities revealed increased blood-brain barrier permeability 48 h post status epilepticus, mainly in epileptogenesis-associated brain regions like hippocampus, piriform cortex, thalamus, or amygdala. In hippocampus, Gd-DTPA-enhanced T1 magnetic resonance imaging signal was increased by 199%, [⁶⁸Ga]DTPA positron emission tomography by 37%, and [^99mTc]DTPA single photon emission computed tomography by 56%. Imaging results were substantiated by histological detection of albumin extravasation. Comparison with quantitative positron emission tomography and single photon emission computed tomography shows that magnetic resonance imaging sequences successfully amplify the signal from a moderate amount of extravasated DTPA molecules, enabling sensitive detection of blood-brain barrier disturbance in epileptogenesis. Imaging of the disturbed blood-brain barrier will give further pathophysiologic insights, will help to stratify anti-epileptogenic treatment targeting blood-brain barrier integrity, and may serve as a prognostic biomarker.

Preparation and 68Ga-radiolabeling of porous zirconia nanoparticle platform for PET/CT-imaging guided drug delivery

This paper describes the preparation of gallium-68 (⁶⁸Ga) isotope labeled porous zirconia (ZrO₂) nanoparticle (NP) platform of nearly 100nm diameter and its first pharmacokinetic and biodistribution evaluation accomplished with a microPET/CT (μPet/CT) imaging system. Objectives of the investigations were to provide a nanoparticle platform which can be suitable for specific delivery of various therapeutic drugs using surface attached specific molecules as triggering agents, and at the same time, suitable for positron emission tomography (PET) tracing of the prospective drug delivery process. Radiolabeling was accomplished using DOTA bifunctional chelator. DOTA was successfully adsorbed onto the surface of nanoparticles, while the ⁶⁸Ga-radiolabeling method proved to be simple and effective. In the course of biodistribution studies, the ⁶⁸Ga-labeled DOTA-ZrNPs showed proper radiolabeling stability in their original suspension and in blood serum. μPet/CT imaging studies confirmed a RES-biodistribution profile indicating stable nano-sized labeled particles in vivo. Results proved that the new method offers the opportunity to examine further specifically targeted and drug payload carrier variants of zirconia NP systems using PET/CT imaging.