C-11 acetate has excellent reproducibility for quantification of myocardial oxidative metabolism

Short-term effects of sacubitril/valsartan therapy on myocardial oxygen consumption and energetic efficiency of cardiac work in heart failure with reduced ejection fraction: A randomized controlled study

AIMS

We sought to evaluate the mechanism of angiotensin receptor-neprilysin inhibitor (ARNI) sacubitril/valsartan therapy and compare it with a valsartan-only control group in patients with heart failure with reduced ejection fraction (HFrEF).

METHODS AND RESULTS

The study was a phase IV, prospective, randomized, double-blind, parallel-group study in patients with New York Heart Association class II-III heart failure and left ventricular ejection fraction (LVEF) ≤35%. During a 6-week run-in period, all patients received valsartan therapy, which was up-titrated to the highest tolerated dose level (80 mg bid or 160 mg bid) and then randomized to either valsartan or sacubitril/valsartan. Myocardial oxygen consumption, energetic efficiency of cardiac work, cardiac and systemic haemodynamics were quantified using echocardiography and 11 C-acetate positron emission tomography before and after 6 weeks of therapy (on stable dose) in 55 patients (ARNI group: n = 27, mean age 63 ± 10 years, LVEF 29.2 ± 10.4%; and valsartan-only control group: n = 28, mean age 64 ± 8 years, LVEF 29.0 ± 7.3%; all p = NS). The energetic efficiency of cardiac work remained unchanged in both treatment arms. However, both diastolic (-4.5 mmHg; p = 0.026) and systolic blood pressure (-9.8 mmHg; p = 0.0007), myocardial perfusion (-0.054 ml/g/min; p = 0.045), and left ventricular mechanical work (-296; p = 0.038) decreased significantly in the ARNI group compared to the control group. Although myocardial oxygen consumption decreased in the ARNI group (-5.4%) compared with the run-in period and remained unchanged in the control group (+0.5%), the between-treatment group difference was not significant (p = 0.088).

CONCLUSIONS

We found no differences in the energetic efficiency of cardiac work between ARNI and valsartan-only groups in HFrEF patients. However, ARNI appears to have haemodynamic and cardiac mechanical effects over valsartan in heart failure patients.

Radiosynthesis, Preclinical, and Clinical Positron Emission Tomography Studies of Carbon-11 Labeled Endogenous and Natural Exogenous Compounds

The presence of positron emission tomography (PET) centers at most major hospitals worldwide, along with the improvement of PET scanner sensitivity and the introduction of total body PET systems, has increased the interest in the PET tracer development using the short-lived radionuclides carbon-11. In the last few decades, methodological improvements and fully automated modules have allowed the development of carbon-11 tracers for clinical use. Radiolabeling natural compounds with carbon-11 by substituting one of the backbone carbons with the radionuclide has provided important information on the biochemistry of the authentic compounds and increased the understanding of their in vivo behavior in healthy and diseased states. The number of endogenous and natural compounds essential for human life is staggering, ranging from simple alcohols to vitamins and peptides. This review collates all the carbon-11 radiolabeled endogenous and natural exogenous compounds synthesised to date, including essential information on their radiochemistry methodologies and preclinical and clinical studies in healthy subjects.

Obesity-associated Blunted Subcutaneous Adipose Tissue Blood Flow After Meal Improves After Bariatric Surgery

CONTEXT

Glucose-dependent insulinotropic peptide (GIP) and meal ingestion increase subcutaneous adipose tissue (SAT) perfusion in healthy individuals. The effects of GIP and a meal on visceral adipose tissue (VAT) perfusion are unclear.

OBJECTIVE

Our aim was to investigate the effects of meal and GIP on VAT and SAT perfusion in obese individuals with type 2 diabetes mellitus (T2DM) before and after bariatric surgery.

METHODS

We recruited 10 obese individuals with T2DM scheduled for bariatric surgery and 10 control individuals. Participants were studied under 2 stimulations: meal ingestion and GIP infusion. SAT and VAT perfusion was measured using 15O-H2O positron emission tomography-magnetic resonance imaging at 3 time points: baseline, 20 minutes, and 50 minutes after the start of stimulation. Obese individuals were studied before and after bariatric surgery.

RESULTS

Before bariatric surgery the responses of SAT perfusion to meal (P = .04) and GIP-infusion (P = .002) were blunted in the obese participants compared to controls. VAT perfusion response did not differ between obese and control individuals after a meal or GIP infusion. After bariatric surgery SAT perfusion response to a meal was similar to that of controls. SAT perfusion response to GIP administration remained lower in the operated-on than control participants. There was no change in VAT perfusion response after bariatric surgery.

CONCLUSION

The vasodilating effects of GIP and meal are blunted in SAT but not in VAT in obese individuals with T2DM. Bariatric surgery improves the effects of a meal on SAT perfusion, but not the effects of GIP. Postprandial increase in SAT perfusion after bariatric surgery seems to be regulated in a GIP-independent manner.

Divergence of acetate uptake in proinflammatory and inflammation-resolving macrophages: implications for imaging atherosclerosis

BACKGROUND

Metabolic divergence of macrophages polarized into different phenotypes represents a mechanistically relevant target for non-invasive characterization of atherosclerotic plaques using positron emission tomography (PET). Carbon-11 (11C)-labeled acetate is a clinically available tracer which accumulates in atherosclerotic plaques, but its biological and clinical correlates in atherosclerosis are undefined.

METHODS AND RESULTS

Histological correlates of 14C-acetate uptake were determined in brachiocephalic arteries of western diet-fed apoE-/- mice. The effect of polarizing stimuli on 14C-acetate uptake was determined by proinflammatory (interferon-γ + lipopolysaccharide) vs inflammation-resolving (interleukin-4) stimulation of murine macrophages and human carotid endarterectomy specimens over 2 days. 14C-acetate accumulated in atherosclerotic regions of arteries. CD68-positive monocytes/macrophages vs smooth muscle actin-positive smooth muscle cells were the dominant cells in regions with high vs low 14C-acetate uptake. 14C-acetate uptake progressively decreased in proinflammatory macrophages to 25.9 ± 4.5% of baseline (P < .001). A delayed increase in 14C-acetate uptake was induced in inflammation-resolving macrophages, reaching to 164.1 ± 21.4% (P < .01) of baseline. Consistently, stimulation of endarterectomy specimens with interferon-γ + lipopolysaccharide decreased 14C-acetate uptake to 66.5 ± 14.5%, while interleukin-4 increased 14C-acetate uptake to 151.5 ± 25.8% compared to non-stimulated plaques (P < .05).

CONCLUSIONS

Acetate uptake by macrophages diverges upon proinflammatory and inflammation-resolving stimulation, which may be exploited for immunometabolic characterization of atherosclerosis.

A new concept for the production of 11C-labelled radiotracers

Background

The GMP-compliant production of radiopharmaceuticals has been performed using disposable units (cassettes) with a dedicated synthesis module. To expand this “plug ‘n’ synthesize” principle to a broader scope of modules we developed a pressure controlled setup that offers an alternative to the usual stepper motor controlled rotary valves. The new concept was successfully applied to the synthesis of N-methyl-[¹¹C]choline, L-S-methyl-[¹¹C]methionine and [¹¹C]acetate.

Results

The target gas purification of cyclotron produced [¹¹C]CO₂ and subsequent conversion to [¹¹C]MeI was carried out on a TRACERlab Fx C Pro module. The labelling reactions were controlled with a TRACERlab Fx FE module. With the presented modular principle we were able to produce N-methyl-[¹¹C]choline and L-S-methyl-[¹¹C]methionine by loading a reaction loop with neat N,N'-dimethylaminoethanol (DMAE) or an ethanol/water mixture of NaOH and L-homocysteine (L-HC), respectively and a subsequent reaction with [¹¹C]MeI. After 18 min N-methyl-[¹¹C]choline was isolated with 52% decay corrected yield and a radiochemical purity of > 99%. For L-S-methyl-[¹¹C]methionine the total reaction time was 19 min reaction, yielding 25% of pure product (> 97%). The reactor design was used as an exemplary model for the technically challenging [¹¹C]acetate synthesis. The disposable unit was filled with 1 mL MeMgCl (0.75 M) in tetrahydrofuran (THF) bevore [¹¹C]CO₂ was passed through. After complete release of [¹¹C]CO₂ the reaction mixture was quenched with water and guided through a series of ion exchangers (H⁺, Ag⁺ and OH⁻). The product was retained on a strong anion exchanger, washed with water and finally extracted with saline. The product mixture was acidified and degassed to separate excess [¹¹C]CO₂ before dispensing. Under these conditions the total reaction time was 18 ± 2 min and pure [¹¹C]acetate (n = 10) was isolated with a decay corrected yield of 51 ± 5%.

Conclusion

Herein, we described a novel single use unit for the synthesis of carbon-11 labelled tracers for preclinical and clinical applications of N-methyl-[¹¹C]choline, L-S-methyl-[¹¹C]methionine and [¹¹C]acetate.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41181-022-00159-y.

Myocardial perfusion quantification with Rb-82 PET: good interobserver agreement of Carimas software on global, regional, and segmental levels

Purpose

To estimate the interobserver agreement of the Carimas software package (SP) on global, regional, and segmental levels for the most widely used myocardial perfusion PET tracer—Rb-82.

Materials and methods

Rest and stress Rb-82 PET scans of 48 patients with suspected or known coronary artery disease (CAD) were analyzed in four centers using the Carimas SP.

We considered values to agree if they simultaneously had an intraclass correlation coefficient (ICC) > 0.75 and a difference < 20% of the median across all observers.

Results

The median values on the segmental level were 1.08 mL/min/g for rest myocardial blood flow (MBF), 2.24 mL/min/g for stress MBF, and 2.17 for myocardial flow reserve (MFR). For the rest MBF and MFR, all the values at all the levels fulfilled were in excellent agreement. For stress MBF, at the global and regional levels, all the 24 comparisons showed excellent agreement. Only 1 out of 102 segmental comparisons (seg. 14) was over the adequate agreement limit—23.5% of the median value (ICC = 0.95).

Conclusion

Interobserver agreement for Rb-82 PET myocardial perfusion quantification analyzed with Carimas is good at any LV segmentation level—global, regional, and segmental. It is good for all the estimates—rest MBF, stress MBF, and MFR.

Reproducibility and Repeatability of Assessment of Myocardial Light Chain Amyloidosis Burden Using 18F-Florbetapir PET/CT

BACKGROUND

18F-florbetapir PET is emerging as an excellent quantitative tool to quantify cardiac light chain (AL) amyloidosis burden. The primary aim of this study was to determine interobserver reproducibility and intraobserver repeatability, defined per the recommendations of the Quantitative Imaging Biomarker Alliance technical performance group, of PET 18F-florbetapir retention index (RI) in patients with cardiac AL amyloidosis.

METHODS

The study cohort comprised 37 subjects with systemic AL amyloidosis enrolled in the prospective study: Molecular Imaging of Primary Amyloid Cardiomyopathy (clinical trials.gov NCT: 02641145). Using 10 mCi of 18F-florbetapir, a 60-minute dynamic cardiac scan was acquired. Global and segmental left ventricular estimates of retention index (RI) of 18F-florbetapir were calculated (Carimas 2.9 software, Turku, Finland). RI was analyzed twice, at least 24 hours apart, by two independent observers. Intraobserver repeatability and interobserver reproducibility were evaluated using Bland-Altman plots and scatter plots with fitted linear regression curves.

RESULTS

All reproducibility (interobserver, r = 0.98) and repeatability (intraobserver, R=0.99 for each observer) measures of 18F-florbetapir RI are excellent. On the Bland-Altman plots, the agreement limits for global 18F-florbetapir RI were high and ranged for reproducibility (interobserver) from - 9.3 to + 9.4% (Fig. 1), and for repeatability (observer 1 from - 10.8 to + 10.7% and from - 9.2 to + 11.4%, for observer 2).

CONCLUSIONS

The present study showed excellent interobserver reproducibility and intraobserver repeatability of 18F-florbetapir PET retention index in patients with cardiac AL amyloidosis.

Basal and cold-induced fatty acid uptake of human brown adipose tissue is impaired in obesity

Fatty acids (FA) are important substrates for brown adipose tissue (BAT) metabolism, however, it remains unclear whether there exists a difference in FA metabolism of BAT between lean and obese healthy humans. In this study we evaluated supraclavicular BAT fatty acid uptake (FAU) along with blood perfusion in lean and obese subjects during cold exposure and at room temperature using positron emission tomography (PET)/computed tomography (CT). Additionally, tissue samples were taken from supraclavicular region (typical BAT region) from a subset of subjects to evaluate histological presence of BAT. Non-shivering cold stress elevated FAU and perfusion of BAT in lean, but not in obese subjects. Lean subjects had greater FAU in BAT compared to obese subjects during cold exposure and interestingly also at room temperature. The higher BAT FAU was related to younger age and several indicators of superior systemic metabolic health. The subjects who manifested BAT histologically had several folds higher BAT FAU compared to subjects with no such histological manifestation. Together, obese subjects have less active tissue in supraclavicular region both in basal and cold-activated state and the FA metabolism of BAT is blunted in obesity.

Positron emission tomography/MRI for cardiac diseases assessment

Functional imaging tools have emerged in the last few decades and are increasingly used to assess the function of the human heart in vivo. Positron emission tomography (PET) is used to evaluate myocardial metabolism and blood flow. Magnetic resonance imaging (MRI) is an essential tool for morphological and functional evaluation of the heart. In cardiology, PET is successfully combined with CT for hybrid cardiac imaging. The effective integration of two imaging modalities allows simultaneous data acquisition combining functional, structural and molecular imaging. After PET/CT has been successfully accepted for clinical practices, hybrid PET/MRI is launched. This review elaborates the current evidence of PET/MRI in cardiovascular imaging and its expected clinical applications for a comprehensive assessment of cardiovascular diseases while highlighting the advantages and limitations of this hybrid imaging approach.

Carbon Flux as a Measure of Prostate Cancer Aggressiveness: [11C]-Acetate PET/CT

Purpose: Dynamic [¹¹C]-acetate positron emission tomography (PET) can be used to study tissue perfusion and carbon flux simultaneously. In this study, the feasibility of the quantification of prostate cancer aggressiveness using parametric methods assessing [¹¹C]-acetate kinetics was investigated in prostate cancer subjects. The underlying uptake mechanism correlated with [¹¹C]-acetate influx and efflux measured in real-time in vitro in cell culture. Methods: Twenty-one patients with newly diagnosed low-to-moderate risk prostate cancer underwent magnetic resonance imaging (MRI) and dynamic [¹¹C]-acetate PET/CT examinations of the pelvis. Parametric images of K₁ (extraction × perfusion), k₂ (oxidative metabolism) and V_T (=K₁/k₂, anabolic metabolism defined as carbon retention) were constructed using a one-tissue compartment model with an arterial input function derived from pelvic arteries. Regions of interest (ROIs) of the largest cancer lesion in each patient and normal prostate tissue were drawn using information from MRI (T2 and DWI images), biopsy results, and post-surgical histopathology of whole prostate sections (n=7). In vitro kinetics of [¹¹C]-acetate were studied on DU145 and PC3 cell lines using LigandTracer^® White equipment for the measurement of the radioactivity uptake in real-time at 37°C. Results: Mean prostate specific antigen (PSA) was 8.33±3.92 ng/mL and median Gleason Sum 6 (range 5-7). K₁, V_T and standardized uptake values (SUVs) were significantly higher in cancerous prostate tissues compared to normal ones for all patients (p<0.001), while k₂ was not (p=0.26). PSA values correlated to early SUVs (r=0.50, p=0.02) and K₁ (r=0.48, p=0.03). Early and late SUVs correlated to V_T (r>0.76, p<0.001) and K₁ (r>0.64, p<0.005). In vitro studies demonstrated higher extraction and retention (p<0.01) of [¹¹C]-acetate in the more aggressive PC3 cells. Conclusion: Parametric images could be used to visualize the [¹¹C]-acetate kinetics of the prostate cancer exhibiting elevated extraction associated with the cancer aggressiveness. The influx rate of [¹¹C]-acetate studied in cell culture also showed dependence on the cancer aggressiveness associated with elevated lipogenesis. Dynamic [¹¹C]-acetate/PET demonstrated potential for prostate cancer aggressiveness estimation using parametric-based K₁ and V_T values.

Review of cardiovascular imaging in the Journal of Nuclear Cardiology 2018. Part 1 of 2: Positron emission tomography, computed tomography, and magnetic resonance

In this review, we summarize key articles that have been published in the Journal of Nuclear Cardiology in 2018 pertaining to nuclear cardiology with advanced multi-modality and hybrid imaging including positron emission tomography, cardiac-computed tomography, and magnetic resonance. In an upcoming review, we will summarize key articles that relate to the progress made in the field of single-photon emission computed tomography. We hope that these sister reviews will be useful to the reader to navigate the literature in our field.

Repeatable and reproducible measurements of myocardial oxidative metabolism, blood flow and external efficiency using 11C-acetate PET

BACKGROUND

Non-invasive approaches to investigate myocardial efficiency can help track the progression of heart failure (HF). This study evaluates the repeatability and reproducibility of 11C-acetate positron emission tomography (PET) imaging of oxidative metabolism.

METHODS AND RESULTS

Dynamic 11C-acetate PET scans were performed at baseline and followup (47 ± 22 days apart) in 20 patients with stable HF with reduced ejection fraction. Two observers blinded to patients' clinical data used FlowQuant® to evaluate test-retest repeatability, as well as intra- and inter-observer reproducibility of 11C-acetate tracer uptake and clearance rates, for the measurement of myocardial oxygen consumption (MVO2), myocardial external efficiency (MEE), work metabolic index (WMI), and myocardial blood flow. Reproducibility and repeatability were evaluated using intra-class-correlation (ICC) and Bland-Altman coefficient-of-repeatability (CR). Test-retest correlations and repeatability were better for MEE and WMI compared to MVO2. All intra- and inter-observer correlations were excellent (ICC = 0.95-0.99) and the reproducibility values (CR = 3%-6%) were significantly lower than the test-retest repeatability values (22%-54%, P < 0.001). Repeatability was improved for all parameters using a newer PET-computed tomography (CT) scanner compared to older PET-only instrumentation.

CONCLUSION

11C-acetate PET measurements of WMI and MEE exhibited excellent test-retest repeatability and operator reproducibility. Newer PET-CT scanners may be preferred for longitudinal tracking of cardiac efficiency.

Radiopharmaceutical tracers for cardiac imaging

Cardiovascular disease (CVD) is the leading cause of death and disease burden worldwide. Nuclear myocardial perfusion imaging with either single-photon emission computed tomography or positron emission tomography has been used extensively to perform diagnosis, monitor therapies, and predict cardiovascular events. Several radiopharmaceutical tracers have recently been developed to evaluate CVD by targeting myocardial perfusion, metabolism, innervation, and inflammation. This article reviews old and newer used in nuclear cardiac imaging.