Cardiac uptake-1 inhibition by high circulating norepinephrine levels in patients with pheochromocytoma

Assessment of myocardial sympathetic innervation with 18F-FDOPA-PET/CT in patients with autonomic dysfunction: feasibility study in IPD patients

BACKGROUND

Dysfunction and denervation of myocardial nor-adrenergic sympathetic neurons has been documented in IPD patients with dysautonomia. The aim of this study was to evaluate the feasibility of single tracer imaging of myocardial sympathetic and cerebral striatal involvement in these patients.

METHODS

Twenty-two controls (mean-age 59.09 ± 12.39 years, 15 men) with no clinical autonomic-dysfunction and normal striatal-uptake in 18F-FDOPA-PET/CT; and 28 patients (mean-age 58.18 ± 8.25 years, 18 men) with autonomic-dysfunction (in Autonomic Function Tests) and striatal dopaminergic-dysfunction were enrolled. Both cardiac-PET/CT (40 minutes post IV-injection of 185-259MBq 18F-FDOPA) and Brain-PET/CT (60 minutes post-IV) were acquired in same session. ROIs were drawn over the entire left ventricular myocardium, individual walls and mediastinum for quantification. Patients and controls were followed-up for 26.93 ± 5.43 months and 37.91 ± 8.63 months, respectively.

RESULTS

Striatal and myocardial-parameters were significantly lower in patients compared to controls; with Myocardium/mediastinal ratio (MwMR) yielding the area-under-the-curve of .941 (P < .001). MwMR correlated negatively with the drop in systolic blood pressure (SBP) during AFTs {Pearson-coefficient (-).565, P = .002}. Mean MwMR in patients with abnormal-AFTs was significantly lower than patients with borderline-AFTs (1.39 ± .12 vs 1.55 ± .10; P = .002). 9/20 patients with abnormal-AFTs showed functional worsening during follow-up, compared to 2/8 with borderline-AFTs.

CONCLUSION

Single tracer, single session imaging of striatal and cardiac sympathetic dysfunction in patients with advanced IPD is feasible with use of 18F-FDOPA. Significantly reduced 18F-FDOPA uptake is seen in the myocardium of the IPD patients with sympathetic dysfunction.

Roles of cardiac sympathetic neuroimaging in autonomic medicine

Sympathetic neuroimaging is based on the injection of compounds that either radiolabel sites of the cell membrane norepinephrine transporter (NET) or that are taken up into sympathetic nerves via the NET and radiolabel intra-neuronal catecholamine storage sites. Detection of the radioactivity is by planar or tomographic radionuclide imaging. The heart stands out among body organs in terms of the intensity of radiolabeling of sympathetic nerves, and virtually all of sympathetic neuroimaging focuses on the left ventricular myocardium. The most common cardiac sympathetic neuroimaging method worldwide is ¹²³I-metaiodobenzylguanidine (¹²³I-MIBG) scanning. ¹²³I-MIBG scanning is used routinely in Europe and East Asia in the diagnostic evaluation of neurogenic orthostatic hypotension (nOH), to distinguish Lewy body diseases (e.g., Parkinson disease with orthostatic hypotension (OH), pure autonomic failure) from non-Lewy body diseases (e.g., multiple system atrophy) and to distinguish dementia with Lewy bodies from Alzheimer's disease. In the USA, ¹²³I-MIBG scanning has been approved by the Food and Drug Administration for the evaluation of pheochromocytoma and some forms of heart failure-but not for the above-mentioned differential diagnoses. Positron emission tomographic methods based on imaging agents such as ¹⁸F-dopamine are categorized as research tools, despite more than a quarter century of clinical experience with these modalities. Considering that ¹²³I-MIBG scanning is available at most academic medical centers in the USA, cardiac sympathetic neuroimaging by this methodology merits consideration as an autonomic test, especially in patients with nOH.

Catecholamines influence myocardial 123I MIBG uptake in neuroblastoma patients

Aim: Cardiac 123I metaiodobenzylguanidine (MIBG) imaging can be influenced by several factors. We evaluated the relationship between catecholamine measurements and cardiac 123I MIBG uptake in neuroblastoma patients. Patients, methods: 30 neuroblastoma patients were retrospectively assessed on cardiac 123I MIBG uptake and urinary catecholamine dopamine and metabolites, homovanillic acid (HVA) and vanillylmandelic acid (VMA). Cardiac 123I MIBG uptake was quantified by heart-to-mediastinum (H/M) ratios, which were calculated into standard deviation scores (SDS) using age-specific reference values. Results: In 17 (57%) and 12 patients (40%) H/M ratio measurements were below –1.0 and –2.0 SDS at diagnosis. A significant inverse correlation between the average of urine metabolites HVA and VMA, and H/M ratio SDS was observed (r -.39, p = 0.04). Furthermore, there was a significant correlation between the urinary catecholamine metabolite HVA and H/M ratio SDS (r -.40, p = 0.04). Conclusion: Routine calculation of H/M ratios in 123I MIBG scintigrams of neuroblastoma patients is not helpful because it will not identify cardiac ventricular dysfunction in this patient category. A low H/M ratio on 123I MIBG scintigraphy is explained by increased cathecholamine levels secreted by neuroblastoma tumours.

Sympathoneural and adrenomedullary responses to mental stress

This concept-based review provides historical perspectives and updates about sympathetic noradrenergic and sympathetic adrenergic responses to mental stress. The topic of this review has incited perennial debate, because of disagreements over definitions, controversial inferences, and limited availability of relevant measurement tools. The discussion begins appropriately with Cannon's "homeostasis" and his pioneering work in the area. This is followed by mental stress as a scientific idea and the relatively new notions of allostasis and allostatic load. Experimental models of mental stress in rodents and humans are discussed, with particular attention to ethical constraints in humans. Sections follow on sympathoneural responses to mental stress, reactivity of catecholamine systems, clinical pathophysiologic states, and the cardiovascular reactivity hypothesis. Future advancement of the field will require integrative approaches and coordinated efforts between physiologists and psychologists on this interdisciplinary topic.

Concepts of scientific integrative medicine applied to the physiology and pathophysiology of catecholamine systems

This review presents concepts of scientific integrative medicine and relates them to the physiology of catecholamine systems and to the pathophysiology of catecholamine-related disorders. The applications to catecholamine systems exemplify how scientific integrative medicine links systems biology with integrative physiology. Concepts of scientific integrative medicine include (i) negative feedback regulation, maintaining stability of the body's monitored variables; (ii) homeostats, which compare information about monitored variables with algorithms for responding; (iii) multiple effectors, enabling compensatory activation of alternative effectors and primitive specificity of stress response patterns; (iv) effector sharing, accounting for interactions among homeostats and phenomena such as hyperglycemia attending gastrointestinal bleeding and hyponatremia attending congestive heart failure; (v) stress, applying a definition as a state rather than as an environmental stimulus or stereotyped response; (vi) distress, using a noncircular definition that does not presume pathology; (vii) allostasis, corresponding to adaptive plasticity of feedback-regulated systems; and (viii) allostatic load, explaining chronic degenerative diseases in terms of effects of cumulative wear and tear. From computer models one can predict mathematically the effects of stress and allostatic load on the transition from wellness to symptomatic disease. The review describes acute and chronic clinical disorders involving catecholamine systems-especially Parkinson disease-and how these concepts relate to pathophysiology, early detection, and treatment and prevention strategies in the post-genome era.

Assessment of cardiac autonomic neuronal function using PET imaging

The autonomic nervous system is the primary extrinsic control of cardiac performance, and altered autonomic activity has been recognized as an important factor in the progression of various cardiac pathologies. Molecular imaging techniques have been developed for global and regional interrogation of pre- and postsynaptic targets of the cardiac autonomic nervous system. Building on established work with the guanethidine analogue ¹²³I-metaiodobenzylguanidine (MIBG) for single-photon emission tomography (SPECT), development of radiotracers and protocols for positron emission tomography (PET) investigation of autonomic signaling has expanded. PET is limited in availability and requires specialized centers for radiosynthesis and interpretation, but the higher resolution allows for improved regional analysis and kinetic modeling provides more true quantification than is possible with SPECT. A wider array of radiolabeled catecholamines, analogues of catecholamines, and receptor ligands have been characterized and evaluated. Sympathetic neuronal PET tracers have shown promise in the identification of several cardiac pathologies. In particular, recent studies have elucidated a mechanistic role for heterogeneous sympathetic innervation in the development of lethal ventricular arrhythmias. Evaluation of cardiomyocyte adrenergic receptor expression and the parasympathetic nervous system has been slower to develop, with clinical studies beginning to emerge. This review summarizes the clinical and the experimental PET tracers currently available for autonomic imaging and discusses their application in health and cardiovascular disease, with particular emphasis on the major findings of the last decade.

Malignant Pheochromocytomas and Paragangliomas: A Phase II Study of Therapy with High-Dose 131I-Metaiodobenzylguanidine (131I-MIBG)

Thirty patients with malignant pheochromocytoma (PHEO) or paraganglioma (PGL) were treated with high-dose 131I-MIBG. Patients were 11-62 (mean 39) years old: 19 patients males and 11 females. Nineteen patients had PGL, three of which were multifocal. Six PGLs were nonsecretory. Eleven patients had PHEO. All 30 patients had prior surgery. Fourteen patients were refractory to prior radiation or chemotherapy before 131I-MIBG. Peripheral blood stem cells (PBSCs) were collected and cryopreserved. 131I-MIBG was synthesized on-site, by exchange-labeling 131I with 127I-MIBG in a solid-phase Cu2+-catalyzed exchange reaction. 131I-MIBG was infused over 2 h via a peripheral IV. Doses ranged from 557 mCi to 1185 mCi (7.4 mCi/kg to 18.75 mCi/kg). Median dose was 833 mCi (12.55 mCi/kg). Marrow hypoplasia commenced 3 weeks after 131I-MIBG therapy. After the first 131I-MIBG therapy, 19 patients required platelet transfusions; 19 received GCSF; 12 received epoeitin or RBCs. Four patients received a PBSC infusion. High-dose 131I-MIBG resulted in the following overall tumor responses in 30 patients: 4 sustained complete remissions (CRs); 15 sustained partial remissions (PRs); 1 sustained stable disease (SD); 5 progressive disease (PD); 5 initial PRs or SD but relapsed to PD. Twenty-three of the 30 patients remain alive; deaths were from PD (5), myelodysplasia (1), and unrelated cause (1). Overall predicted survival at 5 years is 75% (Kaplan Meier estimate). For patients with metastatic PHEO or PGL, who have good *I-MIBG uptake on diagnostic scanning, high-dose 131I-MIBG therapy was effective in producing a sustained CR, PR, or SD in 67% of patients, with tolerable toxicity.