11C-meta-hydroxyephedrine: a promising PET radiopharmaceutical for imaging the sympathetic nervous system

Automated Radiolabeling and Evaluation of [18F]FPMBBG: A Novel Cardiac Neuronal PET Imaging Agent

This study reports the automated radiosynthesis and evaluation of [18F]FPMBBG, a radiopharmaceutical designed to target the norepinephrine transporter (NET). A newly developed fully protected benzylguanidine precursor, which prevents interference from non-protected benzylguanidine part during the nucleophilic process, has enabled a one-pot two-step fully automated cassette-based synthesis of [18F]FPMBBG. This advancement enhances the feasibility of the synthesis, ensures reproducibility, and allows for the production of substantial quantities of the radiotracer, paving the way for future clinical applications. [18F]FPMBBG was prepared in radiochemical yield of ~ 23% (n = 6, decay-corrected) within 70 min, with a radiochemical purity exceeding 98%, and molar activity of > 2 GBq/μmol. In studies using miniature Bama pigs, [18F]FPMBBG showed favorable distribution, providing high-contrast cardiac images at an early stage. Moreover, desipramine inhibition studies confirmed the high NET specificity of [18F]FPMBBG. The efficient automated synthesis, robust heart uptake, and minimal background signal highlight [18F]FPMBBG as a promising PET tracer for assessing cardiac sympathetic neuronal function.

Radiopharmaceuticals: navigating the frontier of precision medicine and therapeutic innovation

This review article explores the dynamic field of radiopharmaceuticals, where innovative developments arise from combining radioisotopes and pharmaceuticals, opening up exciting therapeutic possibilities. The in-depth exploration covers targeted drug delivery, delving into passive targeting through enhanced permeability and retention, as well as active targeting using ligand-receptor strategies. The article also discusses stimulus-responsive release systems, which orchestrate controlled release, enhancing precision and therapeutic effectiveness. A significant focus is placed on the crucial role of radiopharmaceuticals in medical imaging and theranostics, highlighting their contribution to diagnostic accuracy and image-guided curative interventions. The review emphasizes safety considerations and strategies for mitigating side effects, providing valuable insights into addressing challenges and achieving precise drug delivery. Looking ahead, the article discusses nanoparticle formulations as cutting-edge innovations in next-generation radiopharmaceuticals, showcasing their potential applications. Real-world examples are presented through case studies, including the use of radiolabelled antibodies for solid tumors, peptide receptor radionuclide therapy for neuroendocrine tumors, and the intricate management of bone metastases. The concluding perspective envisions the future trajectory of radiopharmaceuticals, anticipating a harmonious integration of precision medicine and artificial intelligence. This vision foresees an era where therapeutic precision aligns seamlessly with scientific advancements, ushering in a new epoch marked by the fusion of therapeutic resonance and visionary progress.

Does quantification of [11C]meta-hydroxyephedrine and [13N]ammonia kinetics improve risk stratification in ischemic cardiomyopathy

BACKGROUND

In ischemic cardiomyopathy patients, cardiac sympathetic nervous system dysfunction is a predictor of sudden cardiac arrest (SCA). This study compared abnormal innervation and perfusion measured by [11C]meta-hydroxyephedrine (HED) vs [13N]ammonia (NH3), conventional uptake vs parametric tracer analysis, and their SCA risk discrimination.

METHODS

This is a sub-study analysis of the prospective PAREPET trial, which followed ischemic cardiomyopathy patients with reduced left ventricular ejection fraction (LVEF ≤ 35%) for events of SCA. Using n = 174 paired dynamic HED and NH3 positron emission tomography (PET) scans, regional defect scores (%LV extent × severity) were calculated using HED and NH3 uptake, as well as HED distribution volume and NH3 myocardial blood flow by kinetic modeling.

RESULTS

During 4.1 years follow-up, there were 27 SCA events. HED defects were larger than NH3, especially in the lowest tertile of perfusion abnormality (P < .001). Parametric defects were larger than their respective tracer uptake defects (P < .001). SCA risk discrimination was not significantly improved with parametric or uptake mismatch (AUC = 0.73 or 0.70) compared to HED uptake defect scores (AUC = 0.67).

CONCLUSION

Quantification of HED distribution volume and NH3 myocardial blood flow produced larger defects than their respective measures of tracer uptake, but did not lead to improved SCA risk stratification vs HED uptake alone.

PET Tracers for Imaging Cardiac Function in Cardio-oncology

PURPOSE OF REVIEW

Successful treatment of cancer can be hampered by the attendant risk of cardiotoxicity, manifesting as cardiomyopathy, left ventricle systolic dysfunction and, in some cases, heart failure. This risk can be mitigated if the injury to the heart is detected before the onset to irreversible cardiac impairment. The gold standard for cardiac imaging in cardio-oncology is echocardiography. Despite improvements in the application of this modality, it is not typically sensitive to sub-clinical or early-stage dysfunction. We identify in this review some emerging tracers for detecting incipient cardiotoxicity by positron emission tomography (PET).

RECENT FINDINGS

Vectors labeled with positron-emitting radionuclides (e.g., carbon-11, fluorine-18, gallium-68) are now available to study cardiac function, metabolism, and tissue repair in preclinical models. Many of these probes are highly sensitive to early damage, thereby potentially addressing the limitations of current imaging approaches, and show promise in preliminary clinical evaluations. The overlapping pathophysiology between cardiotoxicity and heart failure significantly expands the number of imaging tools available to cardio-oncology. This is highlighted by the emergence of radiolabeled probes targeting fibroblast activation protein (FAP) for sensitive detection of dysregulated healing process that underpins adverse cardiac remodeling. The growth of PET scanner technology also creates an opportunity for a renaissance in metabolic imaging in cardio-oncology research.

Reproducible Quantification of Regional Sympathetic Denervation with [11C]meta-Hydroxyephedrine PET Imaging

BACKGROUND

Regional cardiac sympathetic denervation is predictive of sudden cardiac arrest in patients with ischemic cardiomyopathy. The reproducibility of denervation scores between automated software programs has not been evaluated. This study seeks to (1) compare the inter-rater reliability of regional denervation measurements using two analysis programs: FlowQuant® and Corridor4DM®; (2) evaluate test-retest repeatability of regional denervation scores.

METHODS

N = 190 dynamic [11C]meta-hydroxyephedrine (HED) PET scans were reviewed from the PAREPET trial in ischemic cardiomyopathy patients with reduced left ventricular ejection fraction(LVEF ≤ 35%). N = 12 scans were excluded due to non-diagnostic quality. N = 178 scans were analyzed using FlowQuant and Corridor4DM software, each by two observers. Test-retest scans from N = 20 patients with stable heart failure were utilized for test-retest analysis. Denervation scores were defined as extent × severity of relative uptake defects in LV regions with < 75% of maximal uptake. Results were evaluated using intraclass correlation coefficient (ICC) and Bland-Altman coefficient of repeatability (RPC).

RESULTS

Inter-observer, inter-software, and test-retest ICC values were excellent (ICC = 94% to 99%) and measurement variability was small (RPC < 11%). Mean differences between observers ranged .2% to 1.1% for Corridor4DM (P = .28), FlowQuant (P < .001), and between software programs (P < .001). Kaplan-Meier analysis demonstrated HED scores from both programs were predictive of SCA.

CONCLUSION

Inter-rater reliability for both analysis programs was excellent and test-retest repeatability was consistent. The minimal difference in scores between FlowQuant and Corridor4DM supports their use in future trials.

Positron Emission Tomography Imaging of Regional Versus Global Myocardial Sympathetic Activity to Improve Risk Stratification in Patients With Ischemic Cardiomyopathy

BACKGROUND

Current risk assessment approaches fail to identify the majority of patients at risk of sudden cardiac arrest (SCA). Noninvasive imaging of the cardiac sympathetic nervous system using single-photon emission computed tomography and positron emission tomography offers the potential for refining SCA risk assessment. While various [11C]meta-hydroxyephedrine quantification parameters have been proposed, it is currently unknown whether regional denervation or global innervation yields greater SCA risk discrimination. The aim of the study was to determine whether the global innervation parameters yield any independent and additive prognostic value over the regional denervation alone.

METHODS

In a post hoc competing-risks analysis of the PAREPET trial (Prediction of Arrhythmic Events With Positron Emission Tomography), we compared global innervation and regional denervation parameters using the norepinephrine analog [11C]meta-hydroxyephedrine for SCA risk discrimination. Patients with ischemic cardiomyopathy (n=174) eligible for an implantable cardioverter-defibrillator for the primary prevention of SCA were recruited into the trial. [11C]meta-hydroxyephedrine uptake and clearance rates were measured to assess global (left ventricle mean) retention index and volume of distribution. Regional defects were quantified as the percentage of the left ventricle having values <75% of the maximum.

RESULTS

During a median follow-up of 4.2 years, there were 56 cardiac-related deaths, of which 26 were SCAs. For any given regional denervation volume, there was substantial heterogeneity in global innervation scores. Global retention index and distribution volume did not decrease until regional defects exceeded 40% left ventricle. Global scale parameters, retention index, and distribution volume (area under the curve=0.61, P=0.034, P=0.046, respectively), yielded inferior SCA risk discrimination compared to regional heterogeneity (area under the curve=0.74).

CONCLUSIONS

Regional denervation volume has superior cause-specific mortality prediction for SCA versus global parameters of sympathetic innervation. These results have widespread implications for future cardiac sympathetic imaging, which will greatly simplify innervation analysis. Registration: URL: https://www.clinicaltrials.gov; Unique identifier: NCT01400334.

Molecular imaging of norepinephrine transporter-expressing tumors: current status and future prospects

The human norepinephrine transporter (hNET) is a transmembrane protein responsible for reuptake of norepinephrine in presynaptic sympathetic nerve terminals and adrenal chromaffin cells. Neural crest tumors, such as neuroblastoma, paraganglioma and pheochromocytoma often show high hNET expression. Molecular imaging of these tumors can be done using radiolabeled norepinephrine analogs that target hNET. Currently, the most commonly used radiopharmaceutical for hNET imaging is meta-[123I]iodobenzylguanidine ([123I]MIBG) and this has been the case since its development several decades ago. The γ-emitter, iodine-123 only allows for planar scintigraphy and single photon emission computed tomography imaging. These modalities typically have a poorer spatial resolution and lower sensitivity than positron emission tomography (PET). Additional practical disadvantages include the fact that a two-day imaging protocol is required and the need for thyroid blockade. Therefore, several PET alternatives for hNET imaging are actively being explored. This review gives an in-depth overview of the current status and recent developments in clinical trials leading to the next generation of clinical PET ligands for imaging of hNET-expressing tumors.

Reproducible quantification of cardiac sympathetic innervation using graphical modeling of carbon-11-meta-hydroxyephedrine kinetics with dynamic PET-CT imaging

BACKGROUND

Graphical methods of radiotracer kinetic modeling in PET are ideal for parametric imaging and data quality assurance but can suffer from noise bias. This study compared the Logan and Multilinear Analysis-1 (MA1) graphical models to the standard one-tissue-compartment (1TC) model, including correction for partial-volume effects, in dynamic PET-CT studies of myocardial sympathetic innervation in the left ventricle (LV) using [11C]HED.

METHODS

Test and retest [11C]HED PET imaging (47 ± 22 days apart) was performed in 18 subjects with heart failure symptoms. Myocardial tissue volume of distribution (VT) was estimated using Logan and MA1 graphical methods and compared to the 1TC standard model values using intraclass correlation (ICC) and Bland-Altman analysis of the non-parametric reproducibility coefficient (NPC).

RESULTS

A modeling start-time of t* = 5 min gave the best fit for both Logan and MA1 (R2 = 0.95) methods. Logan slightly underestimated VT relative to 1TC (p = 0.002), whereas MA1 did not (p = 0.96). Both the MA1 and Logan models exhibited good-to-excellent agreement with the 1TC (MA1-1TC ICC = 0.96; Logan-1TC ICC = 0.93) with no significant differences in NPC between the two comparisons (p = 0.92). All methods exhibited good-to-excellent test-retest repeatability with no significant differences in NPC (p = 0.57).

CONCLUSIONS

Logan and MA1 models exhibited similar agreement and variability compared to the 1TC for modeling of [11C]HED kinetics. Using t* = 5 min and partial-volume correction produced accurate estimates of VT as an index of myocardial sympathetic innervation.