Myocardial perfusion imaging agents: SPECT and PET

Construction and Preliminary Evaluation of Two 18F-Labeled Radiopharmaceuticals for Myocardial Perfusion Imaging

AIMS

PET myocardial perfusion imaging (MPI) is the gold standard for the noninvasive diagnosis of ischemic myocardial. Construction of 18F-labeled PET MPI probe showed benefits to reduce the imaging cost, and enhance the image quality and patient-friendliness.

METHODS

Two 18F-labeled MPI probes (18F-BoMPI) were developed. Detailed in vitro/vivo evaluation including photophysical properties, in vitro stability, myocardial cell uptake kinetics and mechanisms, cytotoxicity and IC50, biodistribution and plasma clearance curve were investigated. Resting and stressing myocardial perfusion PET imaging were performed in healthy and myocardial ischemic mice.

RESULTS

18F-BoMPI could be quickly labeled and easily postprocessed, and demonstrated excellent in vitro stability. Cell assays indicated that 18F-BoMPI exhibited mitochondria-targeting but potential-independent myocardial uptake. In vivo evaluation revealed the effective myocardial uptake and rapid background clearance. PET MPI confirmed effective probe accumulation in the healthy heart, but rapidly clearance in the background, making heart clearly delineated in the images. Ischemic myocardial could be clearly distinguished as the region of radioactivity sparsity in PET MPI.

CONCLUSION

The 18F-labeled probes showed great potentials to reduce the practicability threshold of PET MPI.

SPECT/CT and PET/CT, related radiopharmaceuticals, and areas of application and comparison

The paper begins by identifying the key historical elements in the development of nuclear medicine imaging, focusing on the Anger camera and single photon emission computed tomography (SPECT) technologies. In this context, key reference is made to the physics of detection in Anger camera systems, especially key components such as the sodium iodide crystal, the function and performance of photomultiplier tubes, and the collimator design. It is discovered that within each component of technology, there are fundamental physical relationships that govern the performance of each component, and that overall image quality is the result of the complex interaction of all such elements. The increasing use of SPECT/CT imaging is described and illustrated with a range of typical clinical applications, which include brain, spinal, cardiac, and cancer studies. The use of CT imaging functionality allows for SPECT image correction based on compensation for absorption within tissue. Reference is also made to the basics of positron emission tomography (PET) imaging and, in particular, to the integration of PET/CT systems where the anatomy profile of the CT image is used to provide correction for photon absorption. A summary is provided of the radionuclides and radiopharmaceuticals commonly used in PET/CT imaging and a range of image studies referenced includes those of nasopharyngeal carcinoma, lung cancer investigation, brain investigation (cancer detection and dementia) and cardiac function. Reference is made to the development of "time of flight" (TOF) technology for improving of image resolution in PET/CT systems. Furthermore, SPECT/CT and PET/CT imaging systems are compared, where a key factor identified is the significantly higher number of photons detected with PET/CT technology and improved image resolution.

Technetium-99m Radiopharmaceuticals for Ideal Myocardial Perfusion Imaging: Lost and Found Opportunities

The favorable nuclear properties in combination with the rich coordination chemistry make technetium-99m the radioisotope of choice for the development of myocardial perfusion tracers. In the early 1980s, [^99mTc]Tc-Sestamibi, [^99mTc]Tc-Tetrofosmin, and [^99mTc]Tc-Teboroxime were approved as commercial radiopharmaceuticals for myocardial perfusion imaging in nuclear cardiology. Despite its peculiar properties, the clinical use of [^99mTc]Tc-Teboroxime was quickly abandoned due to its rapid myocardial washout. Despite their widespread clinical applications, both [^99mTc]Tc-Sestamibi and [^99mTc]Tc-Tetrofosmin do not meet the requirements of an ideal perfusion imaging agent due to their relatively low first-pass extraction fraction and high liver absorption. An ideal radiotracer for myocardial perfusion imaging should have a high myocardial uptake; a high and stable target-to-background ratio with low uptake in the lungs, liver, stomach during the image acquisition period; a high first-pass myocardial extraction fraction and very rapid blood clearance; and a linear relationship between radiotracer myocardial uptake and coronary blood flow. Although it is difficult to reconcile all these properties in a single tracer, scientific research in the field has always channeled its efforts in the development of molecules that are able to meet the characteristics of ideality as much as possible. This short review summarizes the developments in ^99mTc myocardial perfusion tracers, which are able to fulfill hitherto unmet medical needs and serve a large population of patients with heart disease, and underlines their strengths and weaknesses, the lost and found opportunities thanks to the developments of the new ultrafast SPECT technologies.

The indications and utility of adjunctive imaging modalities for chronic total occlusion (CTO) intervention

Coronary chronic total occlusions (CTO) are common in patients undergoing coronary angiography, yet the optimal management strategy remains uncertain, with conflicting results from randomized trials. Appropriate patient selection and careful periprocedural planning are imperative for successful patient management. We review the role of adjunctive imaging modalities including myocardial perfusion imaging (MPI), cardiac magnetic resonance imaging (CMR), echocardiography and computed tomography coronary angiography (CTCA) in myocardial ischemic quantification, myocardial viability assessment, as well as procedural planning for CTO revascularization. An appreciation of the value, indications and limitations of these modalities prior to planned intervention are essential for optimal management.

A combined static-dynamic single-dose imaging protocol to compare quantitative dynamic SPECT with static conventional SPECT

BACKGROUND

SPECT myocardial perfusion imaging (MPI) is a clinical mainstay that is typically performed with static imaging protocols and visually or semi-quantitatively assessed for perfusion defects based upon the relative intensity of myocardial regions. Dynamic cardiac SPECT presents a new imaging technique based on time-varying information of radiotracer distribution, which permits the evaluation of regional myocardial blood flow (MBF) and coronary flow reserve (CFR). In this work, a preliminary feasibility study was conducted in a small patient sample designed to implement a unique combined static-dynamic single-dose one-day visit imaging protocol to compare quantitative dynamic SPECT with static conventional SPECT for improving the diagnosis of coronary artery disease (CAD).

METHODS

Fifteen patients (11 males, four females, mean age 71 ± 9 years) were enrolled for a combined dynamic and static SPECT (Infinia Hawkeye 4, GE Healthcare) imaging protocol with a single dose of 99mTc-tetrofosmin administered at rest and a single dose administered at stress in a one-day visit. Out of 15 patients, eleven had selective coronary angiography (SCA), 8 within 6 months and the rest within 24 months of SPECT imaging, without intervening symptoms or interventions. The extent and severity of perfusion defects in each myocardial region was graded visually. Dynamically acquired data were also used to estimate the MBF and CFR. Both visually graded images and estimated CFR were tested against SCA as a reference to evaluate the validity of the methods.

RESULTS

Overall, conventional static SPECT was normal in ten patients and abnormal in five patients, dynamic SPECT was normal in 12 patients and abnormal in three patients, and CFR from dynamic SPECT was normal in nine patients and abnormal in six patients. Among those 11 patients with SCA, conventional SPECT was normal in 5, 3 with documented CAD on SCA with an overall accuracy of 64%, sensitivity of 40% and specificity of 83%. Dynamic SPECT image analysis also produced a similar accuracy, sensitivity, and specificity. CFR was normal in 6, each with CAD on SCA with an overall accuracy of 91%, sensitivity of 80%, and specificity of 100%. The mean CFR was significantly lower for SCA detected abnormal than for normal patients (3.86±1.06 vs 1.94±0. 0.67, P < 0.001).

CONCLUSIONS

The visually assessed image findings in static and dynamic SPECT are subjective, and may not reflect direct physiologic measures of coronary lesion based on SCA. The CFR measured with dynamic SPECT is fully objective, with better sensitivity and specificity, available only with the data generated from the dynamic SPECT method.

68Ga-Galmydar: A PET imaging tracer for noninvasive detection of Doxorubicin-induced cardiotoxicity

Background

Cancer patients undergoing Doxorubicin (DOX) treatment are susceptible to acute and chronic cardiac anomalies, including aberrant arrhythmias, ventricular dysfunction, and heart failure. To stratify patients at high risk for DOX -related heart failure (CHF), diagnostic techniques have been sought. While echocardiography is used for monitoring LVEF and LV volumes due to its wide-availability and cost-efficiency, it may not identify early stages of the initiation of DOX-induced systolic heart failure. To address these limitations, PET tracers could also provide noninvasive assessment of early and reversible metabolic changes of the myocardium.

Objective

Herein, we report a preliminary investigation of ⁶⁸Ga-Galmydar potential to monitor Dox-induced cardiomyopathy in vivo, ex vivo, and in cellulo employing both nuclear- and optical imaging.

Methods and results

To assess ⁶⁸Ga-Galmydar ability for monitoring DOX-induced cardiomyopathy, microPET imaging was performed 5 d post treatment of rats either with a single dose of DOX (15 mg/kg) or vehicle as a control (saline) and images were co-registered for anatomical reference using CT. Following tail-vein injection of the radiotracer in rats at 60 min, micro-PET/CT static scan (10 min acquisition), ⁶⁸Ga-Galmydar demonstrated 1.91-fold lower uptake in hearts of DOX-treated (standard uptake value; SUV: 0.92, n = 3) rats compared with their vehicle treated (SUV: 1.76, n = 3) control counterparts. For correlation of PET imaging data, post-imaging quantitative biodistribution studies were also performed, wherein excised organs were counted for γ activity, and normalized to injected dose. The post imaging pharmacokinetic data also demonstrated heart uptake values of 2.0 fold lower for DOX treated rats(%ID/g; DOX: 0.44 ± 0.1, n = 3) compared to their vehicle-treated controls (%ID/g; Control: 0.89 ± 0.03, n = 3, p = 0.04). Employing the fluorescent traits of Galmydar, live cell fluorescence imaging indicated a gradual decrease in uptake and retention of Galmydar within mitochondria of H9c2 cells following DOX-treatment, while indicating dose-dependent and time-dependent uptake profiles. Following depolarization of electronegative transmembrane gradients at the mitochondrial membrane, the uptake of the probe was decreased in H9c2 cells, and the uptake profiles were found to be identical, using both fluorescence and radiotracer bioassays. Finally, the decreased uptake of the metalloprobe in H9c2 cells also correlated with caspase-3 expression resulting from DOX-induced cardiotoxicity and cell death.

Conclusions

⁶⁸Ga-Galmydar could provide a noninvasive assessment of DOX-related and likely reversible metabolic changes at earliest stages. Further studies with other chemotherapeutics (potentially capable of inducing cardiomyopathy) are underway.

Gallium-68: methodology and novel radiotracers for positron emission tomography (2012–2017)

Commercial 68Ge/68Ga generators provide a means to produce positron emission tomography agents on site without use of a cyclotron. This development has led to a rapid growth of academic literature and patents ongallium-68 (68Ga). As 68Ga positron emission tomography agents usually involve a targeting moiety attached to a metal chelator, the development lends itself to the investigation of theragnostic applications; the 68Ga-based diagnostic is utilized to determine if the biological target is present and, if so, a therapeutic isotope (e.g., 177Lu, 225Ac) can be complexed with the same scaffold to generate a corresponding radiotherapeutic. This review considers patents issued between 2012 and 2017 that contain a 68Ga-labeled molecule indexed by Chemical Abstract Services (a division of the American Chemical Society).

Chemical Speciation and Metallomics

Chemical speciation approaches is an inherent part of metallomics, once metals/metalloids and organic structures need to be currently evaluated for attaining metallomics studies. Then, this chapter focuses on the applications of the chemical speciation applied to the human health risk, food and human diet, drugs, forensic, nanoscience, and geological metallomics, also pointing out the advances in such area. Some aspects regarding sample preparation is commented along this chapter, and some strategies for maintaining the integrity of the metallomics information are also emphasized.

Role of Cardiac PET in Clinical Practice

OPINION STATEMENT

Early identification of atherosclerosis and at-risk lesions plays a critical role in reducing the burden of cardiovascular disease. While invasive coronary angiography serves as the gold standard for diagnosing coronary artery disease, non-invasive imaging techniques provide visualization of both anatomical and functional atherosclerotic processes prior to clinical presentation. The development of cardiac positron emission tomography (PET) has greatly enhanced our capability to diagnose and treat patients with early stages of atherosclerosis. Cardiac PET is a powerful, versatile non-invasive diagnostic tool with utility in the identification of high-risk plaques, myocardial perfusion defects, and viable myocardial tissue. Cardiac PET allows for comparisons of myocardial function both at time of rest and stress, providing accurate assessments of both myocardial perfusion and viability. Furthermore, novel PET techniques with unique radiotracers yield clinically relevant data on high-risk plaques in active progressive atherosclerosis. While PET exercise stress tests were previously difficult to perform given short radiotracer half-life, the development of the novel radiotracer Flurpiridaz F-18 provides a promising future for PET exercise stress imaging. In addition, hybrid imaging with computed tomography angiography (CTA) and cardiac magnetic resonance (CMR) provides integration of cardiac function and structure. In this review article, we discuss the principles of cardiac PET, the clinical applications of PET in diagnosing and prognosticating patients at risk for future cardiovascular events, compare PET with other non-invasive cardiac imaging modalities, and discuss future applications of PET in CVD evaluation and management.

The elements of life and medicines

Which elements are essential for human life? Here we make an element-by-element journey through the periodic table and attempt to assess whether elements are essential or not, and if they are, whether there is a relevant code for them in the human genome. There are many difficulties such as the human biochemistry of several so-called essential elements is not well understood, and it is not clear how we should classify elements that are involved in the destruction of invading microorganisms, or elements which are essential for microorganisms with which we live in symbiosis. In general, genes do not code for the elements themselves, but for specific chemical species, i.e. for the element, its oxidation state, type and number of coordinated ligands, and the coordination geometry. Today, the biological periodic table is in a position somewhat similar to Mendeleev's chemical periodic table of 1869: there are gaps and we need to do more research to fill them. The periodic table also offers potential for novel therapeutic and diagnostic agents, based on not only essential elements, but also non-essential elements, and on radionuclides. Although the potential for inorganic chemistry in medicine was realized more than 2000 years ago, this area of research is still in its infancy. Future advances in the design of inorganic drugs require more knowledge of their mechanism of action, including target sites and metabolism. Temporal speciation of elements in their biological environments at the atomic level is a major challenge, for which new methods are urgently needed.

Comparison of occupational radiation exposure from myocardial perfusion imaging with Rb-82 PET and Tc-99m SPECT

OBJECTIVE

Rubidium-82 (Rb-82) PET myocardial perfusion imaging (MPI) has superior diagnostic accuracy, at least similar prognostic value, and lower patient radiation exposure when compared with technetium-99m single-photon emission computed tomography (Tc-99m SPECT) MPI. The aim of this study was to compare occupational radiation exposure from the two modalities and show that improvements for the patient do not come at a cost to staff.

MATERIALS AND METHODS

Electronic personal dosimeters were worn by staff involved in the administration and imaging of routine clinical Tc-99m SPECT and Rb-82 PET MPI, and during tracer production and QC. To estimate dose to the staff in the event of a medical emergency, a survey meter was placed in close contact with the patient during Rb-82 infusion and imaging, and immediately after administration for Tc-99m SPECT.

RESULTS

Mean (SD) whole-body effective dose to staff during a single MPI procedure was 0.4 (0.4) μSv for Rb-82 PET (1110 MBq) and 3.3 (1.7) μSv for Tc-99m SPECT (350 MBq). Staff effective dose during tracer production and QC was low (<0.2 μSv/patient) and comparable between tracers. An additional effective dose was measured at close contact to the patient during, and immediately after, tracer administration, although this will not pose a significant radiation risk to staff with either technique as long as this is not routine practice.

CONCLUSION

There is a significant reduction in effective dose during Rb-82 PET when compared with Tc-99m SPECT MPI because of the short half-life of Rb-82 and reduced patient contact.

Characterization of the four isomers of (123)I-CMICE-013: a potential SPECT myocardial perfusion imaging agent

Myocardial perfusion imaging (MPI) with single photon emission computed tomography (SPECT) is widely used in the assessment of coronary artery disease (CAD). We have developed (123)I-CMICE-013 based on rotenone, a mitochondrial complex I (MC-1) inhibitor, as a promising new MPI agent. Our synthesis results in a mixture of four species of (123)I-CMICE-013 A, B, C, D. In this study, we separated the four species and evaluated their biodistribution and imaging properties. The cold analogs (127)I-CMICE-013 A, B, C, D were isolated and characterized and their chemical structures proposed.

METHODS

(123)I-CMICE-013 was synthesized by radiolabeling rotenone with Na(123)I in trifluoroacetic acid (TFA) with iodogen as the oxidizing agent at 60°C for 45min, and the four species were separated by RP-HPLC. The cold analogs (127)I-CMICE-013 A, B, C and D were isolated with a similar procedure and characterized by NMR and mass spectrometry. Biodistribution and microSPECT imaging studies were carried out on normal rats.

RESULTS

We propose the mechanism of the rotenone iodination and the structures of the four species. First, I(+) forms an intermediate three-membered ring with 6' and 7' carbons. Second, the lone electron pair of the water molecule attacks the 6' or 7'-carbon, following by the formation of 6'-OH, and 7'-I bonds as in major products C and D, or 6'-I and 7'-OH bonds as in minor products A and B. The weaker 6'-I bond in the intermediate prompts the nucleophilic attachment of water at the favorable 6'-carbon to generate C and D. MicroSPECT images of (123)I-CMICE-013 A, B, C, D in rats showed clear visualization of myocardium and little interference from lung and liver. The imaging time activity curves and biodistribution data showed complex profiles for the four isomers, which is not expected from the structure activity relationship theory.

CONCLUSION

(123/127)I-CMICE-013 A and B are constitutional isomers with C and D, while A and C are diastereomers of B and D, respectively. Overall, the biological characteristics of the four species are not correlated perfectly with their molecular structures.

Evaluation of ¹⁸F-labeled BODIPY dye as potential PET agents for myocardial perfusion imaging

INTRODUCTION

Despite the great potential of positron emission tomography/computed tomography (PET/CT) in cardiovascular disease imaging, one of the major limitations is the availability of PET probes with desirable half-lives and reasonable cost. In this report, we hypothesized that lipophilic cationic BODIPY dye could be selectively accumulated in cardiac muscle, possibly for the development of novel PET myocardial perfusion imaging (MPI) probes.

METHODS

A (18)F-labeled BODIPY dye ([(18)F]1) was synthesized efficiently through a fluoride exchange reaction catalyzed by the Lewis acid tin chloride (SnCl₄). The compound was first evaluated by a cellular uptake assay in vitro, followed by biodistribution and microPET imaging studies in vivo.

RESULTS

[(18)F]1 was obtained in more than 90% labeling yield, with >98% radiochemical purity. The HEK-293 cellular uptake assay showed that the preferential uptake of [(18)F]1 could be related to the cell membrane potential. The biodistribution data demonstrated high levels of [(18)F]1 accumulation in the heart. In the biodistribution study in mice, the radioactivity uptake in the heart, blood, liver and lung was 3.01 ± 0.44, 0.39 ± 0.09, 0.69 ± 0.07, 1.71 ± 0.27%ID/g, respectively, at 3h post-injection (p.i.). The heart-to-lung and heart-to-liver ratios are 1.76 ± 0.14 and 4.37 ± 0.51 at 3h p.i., respectively. Volume-of-interest analysis of the microPET images correlated well with the biodistribution studies in mice. The heart was clearly visualized in normal rats, with 0.72 ± 0.18, 0.69 ± 0.18, 0.67 ± 0.20 and 0.59 ± 0.17%ID/g uptake at 0.5, 1, 2 and 4h p.i., respectively.

CONCLUSIONS

(18)F-labeled BODIPY dye showed good heart uptake and heart-to-blood and heart-to-lung contrast. A (18)F-labeled BODIPY dyes may represent a new category of cationic PET agents for myocardial perfusion imaging.

⁸²Rb PET myocardial perfusion imaging is superior to ⁹⁹mTc-labelled agent SPECT in patients with known or suspected coronary artery disease

PURPOSE

We compared the quality, interpretive confidence and interreader agreement between SPECT and PET myocardial perfusion imaging (MPI) in the same group of patients.

METHODS

The study group comprised 27 patients (age 55 ± 8.5 years, 12 men) with known or suspected coronary artery disease (CAD) who had undergone gated rest/stress MPI with (99m)Tc-labelled agent SPECT (with and without attenuation correction, AC), and subsequent clinical confirmation with (82)Rb PET. Three experienced readers blinded to the clinical information interpreted all MPI studies.

RESULTS

Interreader agreement was significantly superior for PET studies than for SPECT studies. Following consensus interpretation, the quality of 22 % of the non-AC SPECT studies, 33 % of the AC SPECT studies and 63 % of the PET studies was assessed as excellent or good (p = 0.016). Interpretations were definitely normal or abnormal in 7 % of non-AC SPECT studies, 30 % of AC SPECT studies and 85 % of PET studies (p = 0.046). In 13 patients who had received either invasive coronary angiography or CT angiography with no significant CAD, the true-positive rate for significant CAD was higher for PET, and the true-negative rate was equal for PET and AC SPECT, and lower for non-AC SPECT.

CONCLUSION

(82)Rb PET MPI, used as a confirmatory test after SPECT, offers improved image quality, interpretive confidence and interreader agreement.

Advances in PET myocardial perfusion imaging: F-18 labeled tracers

Coronary artery disease and its related cardiac disorders represent the most common cause of death in the USA and Western world. Despite advancements in treatment and accompanying improvements in outcome with current diagnostic and therapeutic modalities, it is the correct assignment of these diagnostic techniques and treatment options which are crucial. From a diagnostic standpoint, SPECT myocardial perfusion imaging (MPI) using traditional radiotracers like thallium-201 chloride, Tc-99m sestamibi or Tc-99m tetrofosmin is the most utilized imaging technique. However, PET MPI using N-13 ammonia, rubidium-82 chloride or O-15 water is increasing in availability and usage as a result of the growing number of medical centers with new-generation PET/CT systems taking advantage of the superior imaging properties of PET over SPECT. The routine clinical use of PET MPI is still limited, in part because of the short half-life of conventional PET MPI tracers. The disadvantages of these conventional PET tracers include expensive onsite production and inconvenient on-scanner tracer administration making them unsuitable for physical exercise stress imaging. Recently, two F-18 labeled radiotracers with longer radioactive half-lives than conventional PET imaging agents have been introduced. These are flurpiridaz F 18 (formerly known as F-18 BMS747158-02) and F-18 fluorobenzyltriphenylphosphonium. These longer half-life F-18 labeled perfusion tracers can overcome the production and protocol limitations of currently used radiotracers for PET MPI.

Myocardial uptake of 7'-(Z)-[(123)I]iodorotenone during vasodilator stress in dogs with critical coronary stenoses

BACKGROUND

There is a well-recognized need for a new generation of single photon emission computed tomography (SPECT) perfusion tracers with improved myocardial extraction over a wide flow range. Radiotracers that target complex I of the mitochondrial electron transport chain have been proposed as a new class of myocardial perfusion imaging agents. 7-(Z)-[(125)I]iodorotenone ((125)I-ZIROT) has demonstrated superior myocardial extraction and retention characteristics in rats and in isolated perfused rabbit hearts. We sought to fully characterize the biodistribution and myocardial extraction versus flow relationship of (123)I-ZIROT in an intact large-animal model.

METHODS AND RESULTS

The (123)I-ZIROT was administered during adenosine A(2A) agonist-induced hyperemia in 5 anesthetized dogs with critical left anterior descending (LAD) stenoses. When left circumflex (LCx) flow was maximal, (123)I-ZIROT and microspheres were coinjected and the dogs were euthanized 5 minutes later. (123)I-ZIROT biodistribution was evaluated in 2 additional dogs by in vivo planar imaging. At (123)I-ZIROT injection, transmural LAD flow was unchanged from baseline (mean±SEM, 0.90±0.22 versus 0.87±0.11 mL/[min · g]; P=0.92), whereas LCx zone flow increased significantly (mean±SEM, 3.25±0.51 versus 1.00±0.17 mL/[min · g]; P<0.05). Myocardial (123)I-ZIROT extraction tracked regional myocardial flow better than either thallium-201 or (99m)Tc-sestamibi from previous studies using a similar model. Furthermore, the (123)I-ZIROT LAD/LCx activity ratios by ex vivo imaging or well counting (mean±SEM, 0.42±0.08 and 0.45±0.1, respectively) only slightly underestimated the LAD/LCx microsphere flow ratio (0.32±0.09).

CONCLUSIONS

The ability of (123)I-ZIROT to more linearly track blood flow over a wide range makes it a promising new SPECT myocardial perfusion imaging agent with potential for improved coronary artery disease detection and better quantitative estimation of the severity of flow impairment.

Cardiac imaging and safety evaluation of BMS747158, a novel PET myocardial perfusion imaging agent, in chronic myocardial compromised rabbits

BACKGROUND

BMS747158 labeled with (18)F is being developed for PET myocardial perfusion imaging. Imaging studies showed clear detection of necrotic tissue in acute myocardial infarcted (MI) animals and a good safety profile in normal animals. This study evaluated BMS747158 imaging and cardiovascular safety in a rabbit model of chronic MI with cardiac compromise.

METHODS AND RESULTS

Chronic MI rabbits were developed by the left coronary artery ligation followed by 4 weeks recovery. Cardiac PET imaging with BMS747158 (~1.5 mCi, iv) in control rabbits showed clear and uniform myocardial uptake. However, imaging in chronic MI rabbits demonstrated obvious defect area in the left ventricular wall. Before BMS747158 injection, baseline electrocardiogram (ECG) waveforms in lead II configuration were normal with positive QRS complexes in control rabbits. In contrast, MI rabbits exhibited negative QRS complexes with enlarged Q waves and inverted T waves. Baseline values of mean intra-arterial pressure (AP, 61 +/- 6 vs 89 +/- 11 mmHg), systolic AP (79 +/- 11 vs 114 +/- 11 mmHg) and diastolic AP (53 +/- 4 vs 76 +/- 10 mmHg) were lower in MI than in control rabbits. Heart rate (162 +/- 36 vs 159 +/- 8 beat/minute) and QTc interval (corrected by Fridericia method, 288 +/- 17 vs 319 +/- 17 ms) were comparable. BMS747158 administration induced no changes from baseline in any of the measured cardiovascular parameters and ECG waveforms in either control or MI rabbits.

CONCLUSIONS

Cardiac imaging with BMS747158 allows clear detection of chronic MI without producing any cardiovascular alterations in cardiac compromised rabbits.

Recent advances and future trends in multimodality cardiac imaging

The cardiovascular imaging field has experienced marked growth and technical advancement in the past several decades. In the future, multimodality imaging will provide enhanced characterisation of disease states. Myocardial perfusion imaging will become more quantitative, permitting measurement of absolute blood flow and coronary flow reserves during stress states. A greater use of positron emission tomography (PET) can be expected for both assessing blood flow quantitatively and molecular imaging of atherosclerotic plaques and myocardial disease states. SPECT and PET imaging of myocardial metabolism and cardiac neuronal imaging have already shown great promise for identifying high-risk patients with coronary heart disease and nonischaemic cardiomyopathy. Further progress will occur in computed tomography imaging of the heart and coronary arteries and cardiac magnetic resonance imaging including quantitative estimates of coronary blood flow, coronary and peripheral vessel plaque characterisation, and detection of myocardial cellular dysfunction. Fusion imaging, in which two disparate image data sets are merged into one functional image, will become commonplace. Major breakthroughs in CV imaging will depend on discoveries in basic research, further refinement of instrumentation and software for image processing and analysis, and outcomes research demonstrating the worth of imaging technologies in reducing cardiovascular death and morbidity.

Minimizing liver uptake of cationic Tc radiotracers with ether and crown ether functional groups

Ischemia-related diseases, particularly coronary artery disease (CAD), account for the majority of deaths worldwide. Myocardial ischemia is a serious condition and the delay in reperfusion of ischemic tissues can be life-threatening. This is particular true in the aged population. Rapid and accurate early detection of myocardial ischemia is highly desirable so that various therapeutic regiments can be given before irreversible myocardial damage occurs. Myocardial perfusion imaging with radiotracers is an integral component in evaluations of patients with known or suspected CAD. (99m)Tc-Sestamibi and (99m)Tc-Tetrofosmin are commercial radiopharmaceuticals currently available for myocardial perfusion imaging. Despite their widespread clinical applications, both (99m)Tc-Sestamibi and (99m)Tc-Tetrofosmin do not meet the requirements of an ideal perfusion imaging agent, largely due to their high liver uptake. The intense liver uptake makes it difficult to interpret the heart activity in the inferior and left ventricular wall. Photon scattering from the high liver radioactivity accumulation remains a significant challenge for diagnosis of heart diseases. This review will summarize the most recent research efforts to minimize the liver uptake of cationic (99m)Tc radiotracers by using ether and crown ether-containing chelators. Fast liver clearance will shorten the duration of imaging protocols (< 30 min post-injection), and allow for early acquisition of heart images with high quality. Improvement of heart/liver ratio may permit better detection of the presence and extent of coronary artery disease. Identification of such a new radiotracer that allows for the improved noninvasive assessment of myocardial perfusion would be of considerable benefit in treatment of patients with suspected CAD.

Effects of food intake and anesthetic on cardiac imaging and uptake of BMS747158-02 in comparison with FDG

BACKGROUND

BMS747158-02 is an (18)F-labeled agent being developed for PET myocardial perfusion imaging. This study examined impacts of feeding state and anesthetic on cardiac imaging and uptake of this agent in rats in comparison with (18)F-fluorodeoxyglucose (FDG).

METHODS AND RESULTS

Studies were performed in rats either nonfasted or food deprived for 20 hours and anesthetized with either sodium pentobarbital (Pentob) or ketamine and xylazine (Ket/Xyl). Influences of the feeding state and anesthesia were examined by measurement of blood glucose levels, and tissue biodistribution and cardiac imaging of BMS747158-02 and FDG. The blood glucose levels were lower in fasted than nonfasted rats before anesthesia (91 +/- 11 vs 122 +/- 10 mg/dL) and the levels did not significantly change when anesthetized with Pentob. However, the levels increased markedly by 262 +/- 64 mg/dL in nonfasted rats anesthetized with Ket/Xyl. At 60 minutes post-injection, the heart uptake of FDG was significantly lower in fasted than nonfasted rats (0.2 +/- 0.1 vs 2.8 +/- 1.5%ID/g). However, the heart uptake of BMS747158-02 did not differ under these conditions (3.3 +/- 0.9 vs 3.6 +/- 0.9%ID/g, respectively). In nonfasted rats, the heart uptake of FDG was markedly lower when anesthetized with Ket/Xyl than with Pentobl (0.2 +/- 0.1 vs 2.8 +/- 1.5%ID/g). In contrast, the heart uptake of BMS747158-02 was similar with both anesthetics (3.6 +/- 0.5 vs 3.6 +/- 0.9%ID/g). Consistent with the biodistribution studies, the myocardium was not visible following FDG imaging in fasted rats, but clearly seen with BMS747158-02 in both fasted and nonfasted rats anesthetized with either anesthetic.

CONCLUSIONS

Unlike FDG, BMS747158-02 cardiac images are clear and not affected by the feeding state and anesthetics.

Noninvasive assessment of myocardial ischemia in the future: fusion of anatomical and functional images?

Evaluation of: Gaemperli O, Schepis T, Valenta I et al.: Functionally relevant coronary artery disease: comparison of 64-section CT angiography with myocardial perfusion SPECT. Radiology 248(2), 414–423 (2008). Computed tomography coronary angiography (CTCA) is increasingly applied for noninvasive detection of coronary stenoses, but a major limitation of this method is the overestimation of coronary stenosis and the lack of functional information, leading to disappointing low positive-predictive values, in contrast to the high negative-predictive values. Myocardial perfusion imaging with single photon emission computed tomography (SPECT) provides functional and prognostic information, but does not give an insight into the coronary anatomy. In the recent paper in Radiology, Gaemperli et al. compared CTCA with both SPECT and conventional coronary angiography. As others, they demonstrated high specificity, but lower sensitivity of CTCA alone. However, future investigations should focus on the integration of anatomical and functional information as assessed by CTCA and SPECT. An integrated SPECT–CT may further improve diagnostic and prognostic information, and tailor patient selection for invasive treatment.

Assessment of 18F-labeled mitochondrial complex I inhibitors as PET myocardial perfusion imaging agents in rats, rabbits, and primates

PURPOSE

Myocardial extractions of mitochondria complex I (MC-I) inhibitors were high and well correlated with flow. This study assessed the potential of MC-I inhibitors to be developed as myocardial perfusion imaging (MPI) agents.

METHODS

RP1003, RP1004, and RP1005 representing three classes of MC-I inhibitor were synthesized and radio-labeled with (18)F. These agents were evaluated for IC(50) values, tissue biodistribution, and cardiac PET imaging. (18)F-RP1004 was further examined for first-pass extraction and by imaging in non-human primates (NHP) and rats following coronary ligation.

RESULTS

RP1003, RP1004, and RP1005 exhibited high MC-I inhibitory activity with IC(50) of 3.7, 16.7, and 14.4 nM. Heart uptakes in rats (percent injected dose per gram tissue) at 15 and 60 min after injection were 3.52 +/- 0.36 and 2.68 +/- 0.20 for (18)F-RP1003, 2.40 +/- 0.21 and 2.67 +/- 0.27 for (18)F-RP1004, and 2.28 +/- 0.12 and 1.81 +/- 0.17 for (18)F-RP1005. The heart to lung and liver uptake ratios were favorable for cardiac imaging with these agents. In isolated perfused rabbit hearts, the uptake of (18)F-RP1004 increased from 0.74 +/- 0.19 to 1.68 +/- 0.39 mL/min/g at flow rates of 1.66 to 5.06 mL/min/g. These values were higher than or similar to that of (99m)Tc-sestamibi. Cardiac imaging with these agents in rats and rabbits allowed visualization of the heart with minimal lung interference and rapid liver activity clearance. Imaging with (18)F-RP1004 also showed clear myocardium and marked liver activity washout in the NHP and clear detection of the perfusion-deficit area associated with left coronary artery ligation in the rat.

CONCLUSION

MC-I inhibitors have the potential to be a new class of MPI agent.

Whole-body biodistribution and radiation dosimetry of the new cardiac tracer 99mTc-N-DBODC

Our purpose was to evaluate the safety profile and biodistribution behavior in healthy human volunteers of the new myocardial perfusion tracer bis[(dimethoxypropylphosphanyl)ethyl]ethoxyethylamine N,N'-bis(ethoxyethyl)dithiocarbamato nitrido technetium(V) (99mTc-N-DBODC).

METHODS

Ten healthy male volunteers were injected with 99mTc-N-DBODC under both stress and rest conditions. Anterior and posterior planar gamma-camera images were collected at 5, 30, 60, 240, and 1,440 min after injection, with organ uptake quantified by region-of-interest analysis. Tracer kinetics in body fluids were determined by collecting blood and urine samples at different time points.

RESULTS

After injection, 99mTc-N-DBODC showed significant accumulation in the myocardium and prolonged retention. Under rest conditions, uptake in the heart, lungs, and liver at 5 min after injection was 1.67% +/- 0.13%, 1.16% +/- 0.07%, and 10.85% +/- 1.72%, respectively, of administered activity. Under stress conditions, heart uptake was significantly higher (2.07% +/- 0.22%). Radioactivity in the liver decreased to 3.64% +/- 0.98% and 2.37% +/- 0.48% at 60 and 240 min, respectively, after injection. This rapid liver clearance led to favorable heart-to-liver ratios, reaching values of 0.74 +/- 0.13 at rest and 1.26 +/- 0.28 during exercise 60 min after tracer administration. Radiation dose estimates were comparable to those obtained with other myocardial perfusion cationic compounds.

CONCLUSION

The high uptake in the myocardium and the fast liver washout of 99mTc-N-DBODC will allow SPECT images of the left ventricle to be acquired early and with excellent quality.

Imaging of angiogenesis in cardiology

In the past decade, there have been major improvements in our understanding of angiogenesis at the genetic, molecular and cellular levels. Concentrated efforts in this area have led to new therapeutic approaches to ischaemic heart disease using angiogenic factors, gene therapy and progenitor cells. Despite very promising experimental results in animal studies, large clinical trials have failed to confirm the results in patients with coronary artery disease. Important questions such as selection of growth factors and donor cells, as well as the timing, dose and route of administration, have been raised and need to be answered. Molecular imaging approaches which may provide specific markers of the angiogenic process (e.g. integrin expression in endothelial cells) have been introduced and are expected to address some of these questions. Although few clinical imaging results are currently available, animal studies suggest the potential role of molecular imaging for characterisation of the angiogenetic process in vivo and for the monitoring of therapeutic effects.

Quinazoline derivatives as MC-I inhibitors: evaluation of myocardial uptake using Positron Emission Tomography in rat and non-human primate

Several quinazoline derivatives were made as mitochondrial complex 1 inhibitors. Compound 4 showed an IC(50) of 11.3 nM and was the most potent compound of this series. The (18)F analog of 4, [(18)F] 4, was injected in the rat and showed high and rapid heart uptake, fast liver clearance, and low blood uptake. Images obtained using a microPET showed clear delineation of the myocardium in normal rats and perfusion deficit in ischemic rats. In the non-human primate, [(18)F] 4 showed rapid uptake and clearance from the myocardium and high liver uptake.

Molecular cardiovascular imaging using scintigraphic methods

Molecular cardiovascular imaging plays an increasingly important role both in basic research and in clinical diagnosis. Scintigraphic methods have long been used to study pathophysiological changes on a cellular and molecular level, and they are likely to remain important molecular imaging modalities in the foreseeable future. This article provides an overview over current developments in cardiovascular molecular imaging using scintigraphic methods. The focus lies on imaging of cardiac innervation, plaque instability, hypoxia and angiogenesis, gene expression and stem and progenitor cell migration and proliferation.