Defining the success of cardiac gene therapy: how can nuclear imaging contribute?

Image-guided therapies for myocardial repair: concepts and practical implementation

Cell- and molecule-based therapeutic strategies to support wound healing and regeneration after myocardial infarction (MI) are under development. These emerging therapies aim at sustained preservation of ventricular function by enhancing tissue repair after myocardial ischaemia and reperfusion. Such therapies will benefit from guidance with regard to timing, regional targeting, suitable candidate selection, and effectiveness monitoring. Such guidance is effectively obtained by non-invasive tomographic imaging. Infarct size, tissue characteristics, muscle mass, and chamber geometry can be determined by magnetic resonance imaging and computed tomography. Radionuclide imaging can be used for the tracking of therapeutic agents and for the interrogation of molecular mechanisms such as inflammation, angiogenesis, and extracellular matrix activation. This review article portrays the hypothesis that an integrated approach with an early implementation of structural and molecular tomographic imaging in the development of novel therapies will provide a framework for achieving the goal of improved tissue repair after MI.

Cardiac hybrid imaging

Computed tomography coronary angiography (CTCA) and myocardial perfusion imaging techniques (single photon emission computed tomography, SPECT, or positron emission tomography, PET) are established non-invasive modalities for the diagnosis of coronary artery disease (CAD). Cardiac hybrid imaging consists of the combination (or 'fusion') of both modalities and allows obtaining complementary morphological (coronary anatomy, stenoses) and functional (myocardial perfusion) information in a single setting. However, hybrid cardiac imaging has also generated controversy with regard to which patients should undergo such integrated examinations for clinical effectiveness and minimization of costs and radiation dose. The feasibility and clinical value of hybrid imaging has been documented in small cohort studies and selected series of patients. Hybrid imaging appears to offer superior diagnostic and prognostic information compared with stand-alone or side-by-side interpretation of data sets. Particularly in patients with multivessel disease, the hybrid approach allows identification of flow-limiting coronary lesions and thereby provides useful information for the planning of revascularization procedures. Furthermore, integration of the detailed anatomical information from CTCA with the high molecular sensitivity of SPECT and PET may be useful to evaluate targeted molecular and cellular abnormalities in the future. While currently still restricted to specialized cardiac centres, the ongoing efforts to reduce radiation exposure and the increasing clinical interest will further pave the way for an increasing use of cardiac hybrid imaging in clinical practice.

Clinical cardiovascular molecular imaging

Molecular imaging holds the promise of becoming a key diagnostic modality in cardiovascular medicine by allowing visualization of specific targets and pathways that precede or underlie changes in morphology, physiology, and function. As such, molecular imaging aims at detecting precursors or early stages of cardiovascular disease and at monitoring and guiding novel, increasingly specific and versatile cardiovascular therapies. Imaging of myocardial metabolism and autonomic innervation are already used in current practice, and a wide variety of other targets and probes is on the horizon. This focused review provides an overview of the opportunities and challenges that molecular imaging faces to fulfill its promises in clinical cardiovascular medicine.

Quantum mechanics/molecular mechanics investigation of the mechanism of phosphate transfer in human uridine-cytidine kinase 2

The mechanisms of enzyme-catalyzed phosphate transfer and hydrolysis reactions are of great interest due to their importance and abundance in biochemistry. The reaction may proceed in a stepwise fashion, with either a pentavalent phosphorane or a metaphosphate anion intermediate, or by a concerted SN2 mechanism. Despite much theoretical work focused on a few key enzymes, a consensus for the mechanism has not been reached, and examples of all three possibilities have been demonstrated. We have investigated the mechanism of human uridine-cytidine kinase 2 (UCK2, EC 2.7.1.48), which catalyzes the transfer of a phosphate group from ATP to the ribose 5'-hydroxyl of cytidine and uridine. UCK2 is normally expressed in human placenta, but is overexpressed in certain cancer cells, where it is responsible for activating a class of antitumor prodrugs. The UCK2 mechanism was investigated by generating a 2D potential energy surface as a function of the P-O bonds forming and breaking, with energies calculated using a quantum mechanics/molecular mechanics potential (B3LYP/6-31G(d):AMBER). The mechanism of phosphate transfer is shown to be concerted, and is accompanied by concerted proton transfer from the 5'-hydroxyl to a conserved active site aspartic acid that serves as a catalytic base. The calculated barrier for this reaction is 15.1 kcal/mol, in relatively good agreement with the experimental barrier of 17.5 kcal/mol. The interactions of the enzyme active site with the reactant, transition state, and product are examined for their implications on the design of anticancer prodrugs or positron emission tomography (PET) reporter probes for this enzyme.

Radionuclide reporter gene imaging for cardiac gene therapy

INTRODUCTION

In the field of cardiac gene therapy, angiogenic gene therapy has been most extensively investigated. The first clinical trial of cardiac angiogenic gene therapy was reported in 1998, and at the peak, more than 20 clinical trial protocols were under evaluation. However, most trials have ceased owing to the lack of decisive proof of therapeutic effects and the potential risks of viral vectors. In order to further advance cardiac angiogenic gene therapy, remaining open issues need to be resolved: there needs to be improvement of gene transfer methods, regulation of gene expression, development of much safer vectors and optimisation of therapeutic genes. For these purposes, imaging of gene expression in living organisms is of great importance. In radionuclide reporter gene imaging, "reporter genes" transferred into cell nuclei encode for a protein that retains a complementary "reporter probe" of a positron or single-photon emitter; thus expression of the reporter genes can be imaged with positron emission tomography or single-photon emission computed tomography. Accordingly, in the setting of gene therapy, the location, magnitude and duration of the therapeutic gene co-expression with the reporter genes can be monitored non-invasively. In the near future, gene therapy may evolve into combination therapy with stem/progenitor cell transplantation, so-called cell-based gene therapy or gene-modified cell therapy.

CONCLUSION

Radionuclide reporter gene imaging is now expected to contribute in providing evidence on the usefulness of this novel therapeutic approach, as well as in investigating the molecular mechanisms underlying neovascularisation and safety issues relevant to further progress in conventional gene therapy.

Noninvasive imaging of ex vivo intracoronarily delivered nonviral therapeutic transgene expression in heart

We developed a clinically applicable approach for noninvasive monitoring of reporter-therapeutic linked gene expression in the whole heart of large animals using PET imaging and further validated the efficacy and cardiac adverse effects of reporter-therapeutic linked gene transfer in a rabbit cervical heterotopic functional heart transplant model. Cationic liposome complexed with a vector containing a herpes simplex virus type 1 mutant thymidine kinase (HSV1-sr39tk) as the reporter gene and a recombinant human immunosuppressive cytokine, interleukin-10 (hIL-10), as the therapeutic gene was ex vivo intracoronarily delivered into cardiac allografts before implantation. Long-term HSV1-sr39tk and hIL-10 transgene and protein overexpression associated with myocardial PET reporter probe 9-(4-[18F]fluoro-3-hydroxymethylbutyl)guanine ([18F]FHBG) accumulation was observed in the allografts. The expression of the HSV1-sr39tk gene was significantly correlated with the hIL-10 gene expression and the total myocardial [18F]FHBG accumulation quantified as a percentage of intravenously injected [18F]FHBG dose. A homogeneous distribution of [18F]FHBG accumulation was seen in the whole heart similar to the distribution of [18F]fluorodeoxyglucose, a PET glucose metabolism probe. The immunosuppressive therapeutic efficacy remained the same in allografts treated with reporter-therapeutic linked gene and therapeutic gene only. No cardiac adverse effect was found. Our results demonstrate for the first time that PET reporter-therapeutic linked gene imaging is applicable for noninvasively monitoring ex vivo intracoronarily delivered therapeutic transgene expression in the whole heart.

NOGA-guided analysis of regional myocardial perfusion abnormalities treated with intramyocardial injections of plasmid encoding vascular endothelial growth factor A-165 in patients with chronic myocardial ischemia: subanalysis of the EUROINJECT-ONE multicenter double-blind randomized study

BACKGROUND

The aim of this substudy of the EUROINJECT-ONE double-blind randomized trial was to analyze changes in myocardial perfusion in NOGA-defined regions with intramyocardial injections of plasmid encoding plasmid human (ph)VEGF-A(165) using an elaborated transformation algorithm.

METHODS AND RESULTS

After randomization, 80 no-option patients received either active, phVEGF-A165 (n=40), or placebo plasmid (n=40) percutaneously via NOGA-Myostar injections. The injected area (region of interest, ROI) was delineated as a best polygon by connecting of the injection points marked on NOGA polar maps. The ROI was projected onto the baseline and follow-up rest and stress polar maps of the 99m-Tc-sestamibi/tetrofosmin single-photon emission computed tomography scintigraphy calculating the extent and severity (expressed as the mean normalized tracer uptake) of the ROI automatically. The extents of the ROI were similar in the VEGF and placebo groups (19.4+/-4.2% versus 21.5+/-5.4% of entire myocardium). No differences were found between VEGF and placebo groups at baseline with regard to the perfusion defect severity (rest: 69+/-11.7% versus 68.7+/-13.3%; stress: 63+/-13.3% versus 62.6+/-13.6%; and reversibility: 6.0+/-7.7% versus 6.7+/-9.0%). At follow-up, a trend toward improvement in perfusion defect severity at stress was observed in VEGF group as compared with placebo (68.5+/-11.9% versus 62.5+/-13.5%, P=0.072) without reaching normal values. The reversibility of the ROI decreased significantly at follow-up in VEGF group as compared with the placebo group (1.2+/-9.0% versus 7.1+/-9.0%, P=0.016). Twenty-one patients in VEGF and 8 patients in placebo group (P<0.01) exhibited an improvement in tracer uptake during stress, defined as a >or =5% increase in the normalized tracer uptake of the ROI.

CONCLUSIONS

Projection of the NOGA-guided injection area onto the single-photon emission computed tomography polar maps permits quantitative evaluation of myocardial perfusion in regions treated with angiogenic substances. Injections of phVEGF A165 plasmid improve, but do not normalize, the stress-induced perfusion abnormalities.

Imaging after vascular gene therapy

Targets for cardiovascular gene therapy currently include limiting restenosis after balloon angioplasty and stent placement, inhibiting vein bypass graft intimal hyperplasia/stenosis, therapeutic angiogenesis for cardiac and lower-limb ischemia, and prevention of thrombus formation. While catheter angiography is still standard method to follow-up vascular gene transfer, other modern imaging techniques, especially intravascular ultrasound (IVUS), magnetic resonance (MR), and positron emission tomography (PET) imaging provide complementary information about the therapeutic effect of vascular gene transfer in humans. Although molecular imaging of therapeutic gene expression in the vasculatures is still in its technical development phase, it has already offered basic medical science an extremely useful in vivo evaluation tool for non- or minimally invasive imaging of vascular gene therapy.

Noninvasive Imaging of Cardiac Gene Expression and Its Future Implications for Molecular Therapy

Innovative approaches for cardiovascular molecular therapy are rapidly evolving, and translational efforts from experimental to clinical application are increasing. Gene and cell therapy hold promise for treatment of heart disease, but despite progress, some basic principles are still under development. Open issues are, e.g., related to the optimal method for delivery, to therapeutic efficacy, to time course and magnitude of gene expression, and to the fate of transplanted cells in target and remote areas. The use of reporter genes and labeled reporter probes for noninvasive imaging provides the methodology to address these questions by assessment of location, magnitude, and persistence of transgene expression in the heart and the whole body. Coexpression of a reporter gene allows for indirect imaging of the expression of a therapeutic gene of choice. Furthermore, reporter genes can be transferred to stem cells prior to transplantation for serial monitoring of cell viability using gene product imaging. Additionally, functional effects of therapy on the tissue level can be identified using established imaging approaches to determine blood flow, metabolism, innervation, or cell death. Measures of transgene expression can then be linked to physiologic effects and will refine the understanding of basic therapeutic mechanisms. Noninvasive gene-targeted imaging will thus enhance the determination of therapeutic effects in cardiovascular molecular therapy in the future.

Noninvasive Imaging of Transgene Expression by Use of Positron Emission Tomography in a Pig Model of Myocardial Gene Transfer

BACKGROUND

Radionuclide imaging of reporter gene expression may be useful for noninvasive monitoring of clinical cardiac gene therapy. Experience until now, however, has been limited to small animals.

METHODS AND RESULTS

To evaluate feasibility in a clinically applicable setting, pigs were studied by conventional positron emission tomography (PET) 2 days after regional intramyocardial injection of control adenovirus or adenovirus carrying herpesviral thymidine kinase reporter gene (HSV1-tk). Myocardial blood flow was quantified by use of [13N]ammonia. Subsequently, kinetics of the reporter substrate [124I]-2'-fluoro-2'-deoxy-5-iodo-1-beta-d-arabino-furanosyluracil (FIAU) were assessed over a period of 2 hours. Areas infected with adenovirus expressing HSV1-tk showed significantly elevated FIAU retention during the first 30 minutes after injection. At later times, washout was observed, and retention was not different from that in areas infected with control virus or remote myocardium. Early in vivo FIAU uptake correlated with ex vivo images, autoradiography, and immunohistochemistry for reporter gene product after euthanasia. After intramyocardial injection of both adenoviruses, myocardial blood flow was mildly elevated compared with that in remote areas, consistent with histological signs of regional inflammation.

CONCLUSIONS

In vivo quantification of regional myocardial transgene expression is feasible with clinical PET methodology, the radioiodinated reporter probe FIAU, and the HSV1-tk reporter gene. Radioactivity efflux after specific initial uptake was not observed previously in tumor studies, suggesting that tissue-specific differences in nucleoside metabolism influence reporter probe kinetics. By coregistering reporter gene expression with additional biological parameters such as myocardial blood flow, PET allows for noninvasive characterization of the success of cardiac gene transfer along with its functional correlates.