Clinical evaluation of no-carrier-added meta-[123I]iodobenzylguanidine for myocardial scintigraphy

Novel [18F]-Labeled Meta-Bromobenzylguanidine Derivatives: Potential Positron Emission Tomography Imaging Probes for the Norepinephrine Transporter

To develop novel norepinephrine transporter (NET)-targeting positron emission tomography (PET) probes with optimal pharmacokinetic properties, a series of meta-bromobenzylguanidine derivatives was synthesized. 4-Fluorodiethoxyethane-3-bromobenzylguanidine (compound 12) showed relatively good affinity for the NET (IC50 = 1.00 ± 0.04 μM). The corresponding radiotracer 18F-12 was prepared in high radiochemical purity (>98%) via a three-step method. The in vitro cellular uptake results demonstrated that 18F-12 was specifically taken up by NET-expressing SK-N-SH cells by the uptake-1 mechanism. Biodistribution studies in mice showed that 18F-12 exhibited high cardiac uptake (10.45 ± 0.66 %ID/g at 5 min p.i. and 6.44 ± 0.40 %ID/g at 120 min p.i.), faster liver clearance, and a lower dose of absorbed radiation than [123I]-labeled meta-iodobenzylguanidine ([123I]MIBG). Small animal PET imaging confirmed the high heart-to-background ratio of 18F-12 and the uptake-1 mechanism specific for the NET in rats, indicating its potential as a promising PET radiotracer for cardiac sympathetic nerve imaging.

[Functional neuroimaging in the diagnosis of patients with Parkinsonism: Update and recommendations for clinical use]

Functional Neuroimaging has been traditionally used in research for patients with different Parkinsonian syndromes. However, the emergence of commercial radiotracers together with the availability of single photon emission computed tomography (SPECT) and, more recently, positron emission tomography (PET) have made them available for clinical practice. Particularly, the development of clinical evidence achieved by functional neuroimaging techniques over the past two decades have motivated a progressive inclusion of several biomarkers in the clinical diagnostic criteria for neurodegenerative diseases that occur with Parkinsonism. However, the wide range of radiotracers designed to assess the involvement of different pathways in the neurodegenerative process underlying Parkinsonian syndromes (dopaminergic nigrostriatal pathway integrity, basal ganglia and cortical neuronal activity, myocardial sympathetic innervation), and the different neuroimaging techniques currently available (scintigraphy, SPECT and PET), have generated some controversy concerning the best neuroimaging test that should be indicated for the differential diagnosis of Parkinsonism. In this article, a panel of nuclear medicine and neurology experts has evaluated the functional neuroimaging techniques emphazising practical considerations related to the diagnosis of patients with uncertain origin parkinsonism and the assessment Parkinson's disease progression.

Preclinical assessment of strategies for enhancement of metaiodobenzylguanidine therapy of neuroendocrine tumors

By virtue of its high affinity for the norepinephrine transporter (NET), [(131)I]metaiodobenzylguanidine ([(131)I]MIBG) has been used for the therapy of tumors of neuroectodermal origin for more than 25 years. Although not yet universally adopted, [(131)I]MIBG targeted radiotherapy remains a highly promising means of management of neuroblastoma, pheochromocytoma, and carcinoids. Appreciation of the mode of conveyance of [(131)I]MIBG into malignant cells and of factors that influence the activity of the uptake mechanism has indicated a variety of means of increasing the effectiveness of this type of treatment. Studies in model systems revealed that radiolabeling of MIBG to high specific activity reduced the amount of cold competitor, thereby increasing tumor dose and minimizing pressor effects. Increased radiotoxicity to targeted tumors might also be achieved by the use of the α-particle emitter [(211)At]astatine rather than (131)I as radiolabel. Recently it has been demonstrated that potent cytotoxic bystander effects were induced by [(131)I]MIBG, [(123)I]MIBG, and [(211)At]meta-astatobenzylguanidine. Discovery of the structure of bystander factors could increase the therapeutic ratio achievable by MIBG targeted radiotherapy. [(131)I]MIBG combined with topotecan produced supra-additive cytotoxicity in vitro and tumor growth delay in vivo. The enhanced antitumor effect was consistent with a failure to repair DNA damage. Initial findings suggest that further enhancement of efficacy might be achieved by triple combination therapy with drugs that disrupt alternative tumor-specific pathways and synergize not only with [(131)I]MIBG abut also with topotecan. With these ploys, it is expected that advances will be made toward the optimization of [(131)I]MIBG therapy of neuroectodermal tumors.

Determination of the survival rate in patients with congestive heart failure stratified by ¹²³I-MIBG imaging: a meta-analysis from the studies performed in Japan

BACKGROUND

The goals of this meta-analysis were to determine survival rates in patients with heart failure (HF) assessed by ¹²³I-MIBG imaging results using recently published studies and to determine the prognostic value of ¹²³I-MIBG imaging.

METHODS

We reviewed published cohort studies carried out in Japan that compared the prognosis of patients with their ¹²³I-MIBG activity quantified as late heart-to-mediastinum ratio (H/M) or washout rate by performing a PubMed search for articles in English up to December 2006. Studies were selected if they analyzed a clearly defined lethal outcome (cardiovascular death) using life tables to estimate the odds ratio at 24 months after enrollment.

RESULTS

Of 158 articles related to cardiac ¹²³I-MIBG, seven referred to studies that met the inclusion criteria: 5 evaluated H/M via ¹²³I-MIBG in a total of 866 patients and 4 calculated washout rate in a total of 491 patients. A low H/M indicated a high risk of cardiac death: pooled odds ratio, 5.2; 95% confidence interval (CI) of 3.1-5.7. A high washout was also associated with lethal events with a pooled odds ratio of 2.8 (CI: 1.6-5.0). The association between washout and cardiac death was heterogeneous (Chi-square = 11.0, P < 0.02), whereas that between late H/M and fatal events was homogeneous (Chi-square = 2.3, P = no significance).

CONCLUSION

This meta-analysis of published studies of ¹²³I-MIBG studies conducted in Japan indicated that both a decreased cardiac ¹²³I-MIBG activity (H/M) and an increased washout rate are indicative of a poor prognosis in patients with chronic heart failure.

Clinical performance and radiation dosimetry of no-carrier-added vs carrier-added 123I-metaiodobenzylguanidine (MIBG) for the assessment of cardiac sympathetic nerve activity

PURPOSE

We hypothesized that assessment of myocardial sympathetic activity with no-carrier-added (nca) (123)I-meta-iodobenzylguanidine (MIBG) compared to carrier-added (ca) (123)I-MIBG would lead to an improvement of clinical performance without major differences in radiation dosimetry.

METHODS

In nine healthy volunteers, 15 min and 4 h planar thoracic scintigrams and conjugate whole-body scans were performed up to 48 h following intravenous injection of 185 MBq (123)I-MIBG. The subjects were given both nca and ca (123)I-MIBG. Early heart/mediastinal ratios (H/M), late H/M ratios and myocardial washout were calculated. The fraction of administered activity in ten source organs was quantified from the attenuation-corrected geometric mean counts in conjugate views. Radiation-absorbed doses were estimated with OLINDA/EXM software.

RESULTS

Both early and late H/M were higher for nca (123)I-MIBG (ca (123)I-MIBG early H/M 2.46 +/- 0.15 vs nca (123)I-MIBG 2.84 +/- 0.15, p = 0.001 and ca (123)I-MIBG late H/M 2.69 +/- 0.14 vs nca (123)I-MIBG 3.34 +/- 0.18, p = 0.002). Myocardial washout showed a longer retention time for nca (123)I-MIBG (p < 0.001). The effective dose equivalent (adult male model) for nca (123)I-MIBG was similar to that for ca (123)I-MIBG (0.025 +/- 0.002 mSv/MBq vs 0.026 +/- 0.002 mSv/MBq, p = 0.055, respectively).

CONCLUSION

No-carrier-added (123)I-MIBG yields a higher relative myocardial uptake and is associated with a higher myocardial retention. This difference between nca (123)I-MIBG and ca (123)I-MIBG in myocardial uptake did not result in major differences in estimated absorbed doses. Therefore, nca (123)I-MIBG is to be preferred over ca (123)I-MIBG for the assessment of cardiac sympathetic activity.

No-carrier-added versus carrier-added123I-metaiodobenzylguanidine for the assessment of cardiac sympathetic nerve activity

PURPOSE

No-carrier-added (nca) MIBG is primarily associated with specific uptake (i.e. uptake-1 mechanism). We evaluated the hypothesis that nca MIBG will be less influenced by changes in extra-neuronal uptake (i.e. uptake-2 mechanism) compared with carrier-added (ca) MIBG.

METHODS

No-carrier-added MIBG was compared with ca MIBG of two different manufacturers (ca MIBG-1 and ca MIBG-2, with a specific activity of 200 Mq/mumol and 40 MBq/mumol MIBG respectively) in rats (n=6 per group): controls, blocking uptake-1 (desipramine) and blocking uptake-2 (phenoxybenzamine hydrochloride). Dedicated pinhole SPECT was performed 2 h after injection of the radiotracer. After SPECT, biodistribution was assessed [% injected dose per gram tissue (%ID)].

RESULTS

No-carrier-added MIBG had the highest absolute cardiac uptake. Although a clear trend was observed, nca MIBG was not statistically significantly different from ca MIBG-1 (0.31+/-0.05 %ID vs 0.25+/-0.01 %ID,p=0.05). Blocking uptake-1 resulted in a significant decrease in absolute cardiac uptake only for nca MIBG (0.22+/-0.03 %ID,p=0.004). Blocking uptake-2 resulted in a significant reduction in ca MIBG-1 cardiac uptake (0.14+/-0.02 %ID,p=0.0001), but not in the cardiac uptake of nca MIBG or MIBG-2. SPECT showed the highest relative cardiac uptake for nca MIBG. Poor contrast between myocardium and surrounding tissue hampered assessment of relative cardiac uptake on SPECT of both ca MIBG-1 and ca MIBG-2.

CONCLUSION

No-carrier-added MIBG yields a higher myocardial uptake than ca MIBG and is associated with a higher specific as well as a lower non-neuronal uptake. We therefore conclude that for the scintigraphic assessment of the myocardial sympathetic nervous system, nca MIBG is to be preferred over ca MIBG.

Cardiac neurotransmission SPECT imaging

The sympathetic nervous system has great influence on cardiovascular physiology. Cardiac neurotransmission single photon emission computed tomography (SPECT) imaging allows in vivo noninvasive assessment of presynaptic reuptake and storage of neurotransmitters, which offers characterization of the cardiac neuronal function in different diseases of the heart and other altered metabolic or functional conditions. Therefore assessment of the integrity of cardiac sympathetic innervation may help in the diagnosis of these disorders, as well as in prognostication, and will result in better therapy and outcome. At present, the most widely available SPECT tracer by which to assess cardiac neurotransmission is metaiodobenzylguanidine labeled with iodine 123. This article focuses on reviewing the characteristics of cardiac SPECT imaging with I-123 metaiodobenzylguanidine and its role in the assessment of pathophysiologic changes during relevant clinical conditions.

Challenging the neuronal MIBG uptake by pharmacological intervention: effect of a single dose of oral amitriptyline on regional cardiac MIBG uptake

PURPOSE

Imaging with metaiodobenzylguanidine (MIBG) is used for the assessment of neuronal dysfunction in various cardiovascular disorders. Although valuable information is obtained by resting MIBG imaging, it is conceivable that competitive interference with the re-uptake mechanism would exaggerate MIBG defects and might unmask subclinical neuronal dysfunction. Tricyclic antidepressants, such as amitriptyline, have been reported to significantly increase cardiac MIBG washout and inhibit uptake into presynaptic neurons. This study was undertaken to assess whether a single oral dose of amitriptyline could influence cardiac MIBG distribution.

METHODS

Six patients (aged 62-81 years; four males, two females) who had demonstrated a normal cardiac MIBG scan during work-up for movement disorders were studied. The patients underwent a second 123I-MIBG study after oral administration of 25 mg amitriptyline within 1 week. Single-photon emission computed tomography images were acquired at 4 h to assess the regional distribution of MIBG, after generation of polar maps and employing a 20-segment model. Mean percentage of peak activity was calculated for each segment at rest and after amitriptyline administration.

RESULTS

After amitriptyline administration, there was a decrease in regional MIBG uptake in 10+/-4 segments per patient [62/120 segments (52%): 37 segments with a 5-10% decrease, 25 segments with a >10% decrease]. This change was statistically significant in lateral (P=0.003), apical (P<0.0001) and inferior (P=0.03) regions.

CONCLUSION

A single oral dose of amitriptyline can induce changes in the uptake and retention of cardiac MIBG, indicating the feasibility of use of pharmacological intervention in cardiac neurotransmission imaging.

Brugada syndrome with atypical ECG: downsloping ST-segment elevation in inferior leads

We present an unusual case of a young Thai immigrant, symptomatic, who had suffered prior episodes of syncope with strong family background: male, first-degree relatives, younger than 45 years old who had died suddenly. The rest ECG, with the patient asymptomatic at the time, showed persistent ST-segment elevation, in inferior leads and "mirror" image in the anterior wall, which were not modified with sublingual nitrates, in absence of demonstrable structural heart disease by chest X-rays and echocardiogram, hypothermia, ischemia, or electrolytic disorders. Holter monitoring revealed at dawn, a short episode of polymorphic ventricular tachycardia of short onset extrasystole coupling, which evolved into asystole and sudden cardiac death. We believe this is a sudden unexplained death syndrome, although we did not have a chance to conduct a genetic study.