Biodistribution and metabolism of [N-methyl-11C]m-hydroxyephedrine in the rat

[11C]mHED PET follows a two-tissue compartment model in mouse myocardium with norepinephrine transporter (NET)-dependent uptake, while [18F]LMI1195 uptake is NET-independent

PURPOSE

Clinical positron emission tomography (PET) imaging of the presynaptic norepinephrine transporter (NET) function provides valuable diagnostic information on sympathetic outflow and neuronal status. As data on the NET-targeting PET tracers [¹¹C]meta-hydroxyephedrine ([¹¹C]mHED) and [¹⁸F]LMI1195 ([¹⁸F]flubrobenguane) in murine experimental models are scarce or lacking, we performed a detailed characterization of their myocardial uptake pattern and investigated [¹¹C]mHED uptake by kinetic modelling.

METHODS

[¹¹C]mHED and [¹⁸F]LMI1195 accumulation in the heart was studied by PET/CT in FVB/N mice. To test for specific uptake by NET, desipramine, a selective NET inhibitor, was administered by intraperitoneal injection. [¹¹C]mHED kinetic modelling with input function from an arteriovenous shunt was performed in three mice.

RESULTS

Both tracers accumulated in the mouse myocardium; however, only [¹¹C]mHED uptake was significantly reduced by excess amount of desipramine. Myocardial [¹¹C]mHED uptake was half-saturated at 88.3 nmol/kg of combined mHED and metaraminol residual. After [¹¹C]mHED injection, a radiometabolite was detected in plasma and urine, but not in the myocardium. [¹¹C]mHED kinetics followed serial two-tissue compartment models with desipramine-sensitive K₁.

CONCLUSION

PET with [¹¹C]mHED but not [¹⁸F]LMI1195 provides information on NET function in the mouse heart. [¹¹C]mHED PET is dose-independent in the mouse myocardium at < 10 nmol/kg of combined mHED and metaraminol. [¹¹C]mHED kinetics followed serial two-tissue compartment models with K₁ representing NET transport. Myocardial [¹¹C]mHED uptake obtained from PET images may be used to assess cardiac sympathetic integrity in mouse models of cardiovascular disease.

Recent Advances and Clinical Applications of PET Cardiac Autonomic Nervous System Imaging

PURPOSE OF REVIEW

The purpose of this review was to summarize current advances in positron emission tomography (PET) cardiac autonomic nervous system (ANS) imaging, with a specific focus on clinical applications of novel and established tracers.

RECENT FINDINGS

[11C]-Meta-hydroxyephedrine (HED) has provided useful information in evaluation of normal and pathological cardiovascular function. Recently, [11C]-HED PET imaging was able to predict lethal arrhythmias, sudden cardiac death (SCD), and all-cause mortality in heart failure patients with reduced ejection fraction (HFrEF). In addition, initial [11C]-HED PET imaging studies have shown the potential of this agent in elucidating the relationship between impaired cardiac sympathetic nervous system (SNS) innervation and the severity of diastolic dysfunction in HF patients with preserved ejection fraction (HFpEF) and in predicting the response to cardiac resynchronization therapy (CRT) in HFrEF patients. Longer half-life 18F-labeled presynaptic SNS tracers (e.g., [18F]-LMI1195) have been developed to facilitate clinical imaging, although no PET radiotracers that target the ANS have gained wide clinical use in the cardiovascular system. Although the use of parasympathetic nervous system radiotracers in cardiac imaging is limited, the novel tracer, [11C]-donepezil, has shown potential utility in initial studies. Many ANS radioligands have been synthesized for PET cardiac imaging, but to date, the most clinically relevant PET tracer has been [11C]-HED. Recent studies have shown the utility of [11C]-HED in relevant clinical issues, such as in the elusive clinical syndrome of HFpEF. Conversely, tracers that target cardiac PNS innervation have been used less clinically, but novel tracers show potential utility for future work. The future application of [11C]-HED and newly designed 18F-labeled tracers for targeting the ANS hold promise for the evaluation and management of a wide range of cardiovascular diseases, including the prognostication of patients with HFpEF.

Molecular and cellular pathophysiology of preclinical Alzheimer's disease

Although the two pathological hallmarks of Alzheimer's disease (AD), senile plaques composed of amyloid-β (Aβ) peptides and neurofibrillary tangles (NFTs) consisting of hyperphosphorylated tau, have been studied extensively in postmortem AD and relevant animal and cellular models, the pathogenesis of AD remains unknown, particularly in the early stages of the disease where therapies presumably would be most effective. We and others have demonstrated that Aβ plaques and NFTs are present in varying degrees before the onset and throughout the progression of dementia. In this regard, aged people with no cognitive impairment (NCI), mild cognitive impairment (MCI, a presumed prodromal AD transitional state, and AD all present at autopsy with varying levels of pathological hallmarks. Cognitive decline, a requisite for the clinical diagnosis of dementia associated with AD, generally correlates better with NFTs than Aβ plaques. However, correlations are even higher between cognitive decline and synaptic loss. In this review, we illustrate relevant clinical pathological research in preclinical AD and throughout the progression of dementia in several areas including Aβ and tau pathobiology, single population expression profiling of vulnerable hippocampal and basal forebrain neurons, neuroplasticity, neuroimaging, cerebrospinal fluid (CSF) biomarker studies and their correlation with antemortem cognitive endpoints. In each of these areas, we provide evidence for the importance of studying the pathological hallmarks of AD not in isolation, but rather in conjunction with other molecular, cellular, and imaging markers to provide a more systematic and comprehensive assessment of the multiple changes that occur during the transition from NCI to MCI to frank AD.

Quantification of cardiac sympathetic nerve density with N-11C-guanyl-meta-octopamine and tracer kinetic analysis

Most cardiac sympathetic nerve radiotracers are substrates of the norepinephrine transporter (NET). Existing tracers such as (123)I-metaiodobenzylguanidine ((123)I-MIBG) and (11)C-(-)-meta-hydroxyephedrine ((11)C-HED) are flow-limited tracers because of their rapid NET transport rates. This prevents successful application of kinetic analysis techniques and causes semiquantitative measures of tracer retention to be insensitive to mild-to-moderate nerve losses. N-(11)C-guanyl-(-)-meta-octopamine ((11)C-GMO) has a much slower NET transport rate and is trapped in storage vesicles. The goal of this study was to determine whether analyses of (11)C-GMO kinetics could provide robust and sensitive measures of regional cardiac sympathetic nerve densities.

METHODS

PET studies were performed in a rhesus macaque monkey under control conditions or after intravenous infusion of the NET inhibitor desipramine (DMI). Five desipramine dose levels were used to establish a range of available cardiac NET levels. Compartmental modeling of (11)C-GMO kinetics yielded estimates of the rate constants K1 (mL/min/g), k2 (min(-1)), and k3 (min(-1)). These values were used to calculate a net uptake rate constant K(i) (mL/min/g) = (K1k3)/(k2 + k3). In addition, Patlak graphical analyses of (11)C-GMO kinetics yielded Patlak slopes K(p) (mL/min/g), which represent alternative measurements of the net uptake rate constant K(i). (11)C-GMO kinetics in isolated rat hearts were also measured for comparison with other tracers.

RESULTS

In isolated rat hearts, the neuronal uptake rate of (11)C-GMO was 8 times slower than (11)C-HED and 12 times slower than (11)C-MIBG. (11)C-GMO also had a long neuronal retention time (>200 h). Compartmental modeling of (11)C-GMO kinetics in the monkey heart proved stable under all conditions. Calculated net uptake rate constants K(i) tracked desipramine-induced reductions of available NET in a dose-dependent manner, with a half maximal inhibitory concentration (IC50) of 0.087 ± 0.012 mg of desipramine per kilogram. Patlak analysis provided highly linear Patlak plots, and the Patlak slopes Kp also declined in a dose-dependent manner (IC50 = 0.068 ± 0.010 mg of desipramine per kilogram).

CONCLUSION

Compartmental modeling and Patlak analysis of (11)C-GMO kinetics each provided quantitative parameters that accurately tracked changes in cardiac NET levels. These results strongly suggest that PET studies with (11)C-GMO can provide robust and sensitive quantitative measures of regional cardiac sympathetic nerve densities in human hearts.

Evaluating presynaptic and postsynaptic innervation in heart failure

Congestive heart failure increasingly contributes to the overall morbidity and mortality associated with cardiovascular disease. Although significant advances in therapies allow patients to feel better and have improved functional status and survival, not all patients respond equally to these therapies. Moreover, for any given level of left ventricular systolic dysfunction, it is difficult to predict who will have progressive heart failure leading to death or transplantation or who will die suddenly. It has long been recognized that the sympathetic nervous system plays a major role in the morbidity and mortality associated with congestive heart failure from systolic left ventricular dysfunction. Although some of the sympathetic effect occurs at the systemic level, malfunction at the ventricular myocyte-sympathetic nerve terminal interface is likely a major contributor to sudden death and progressive heart failure. Imaging the cardiac sympathetic nervous system can be used to evaluate this myoneural interface and to predict outcome.

Pharmacokinetic study of three cardiovascular drugs by high-performance liquid chromatography using pre-column derivatization with 9,10-anthraquinone-2-sulfonyl chloride

A new method was developed to analyze three cardiovascular drugs in rat plasma, Mexiletine hydrochloride (MXL), Methoxamine hydrochloride (MTX), and Metaraminol bitartrate (MTR), by high-performance liquid chromatography (HPLC) using 9,10-anthraquinone-2-sulfonyl chloride (ASC) as the derivatization reagent. The derivatization modes and conditions for this method were optimized. The quantitative analysis was achieved using a C18 column at room temperature (25 degrees C), with various volume ratios of methanol-water as the mobile phase and a detection wavelength at 256 nm. Analytical linearity was obtained for the method over the concentration range of 0.04-8.0 microg mL(-1) for all the three drugs. The lower limit of quantification (LLOQ) was 0.04 microg mL(-1). This method was successfully applied to the analysis of the three drugs in rat plasma and their pharmacokinetic studies. The t1/2 values of the three drugs in rats were found to be 5.38+/-0.61, 4.49+/-0.53, and 3.70+/-0.19 h for MXL, MTX, and MTR, respectively.

Radiolabeled phenethylguanidines: novel imaging agents for cardiac sympathetic neurons and adrenergic tumors

The norepinephrine transporter (NET) substrates [123I]-m-iodobenzylguanidine (MIBG) and [11C]-m-hydroxyephedrine (HED) are used as markers of cardiac sympathetic neurons and adrenergic tumors (pheochromocytoma, neuroblastoma). However, their rapid NET transport rates limit their ability to provide accurate measurements of cardiac nerve density. [11C]Phenethylguanidine ([11C]1a) and 12 analogues ([11C]1b-m) were synthesized and evaluated as radiotracers with improved kinetics for quantifying cardiac nerve density. In isolated rat hearts, neuronal uptake rates of [11C]1a-m ranged from 0.24 to 1.96 mL min-1 (g wet wt)-1, and six compounds had extremely long neuronal retention times (clearance T1/2 > 20 h) due to efficient vesicular storage. Positron emission tomography (PET) studies in nonhuman primates with [11C]1e, N-[11C]guanyl-m-octopamine, which has a slow NET transport rate, showed improved myocardial kinetics compared to HED. Compound [11C]1c, [11C]-p-hydroxyphenethylguanidine, which has a rapid NET transport rate, avidly accumulated into rat pheochromocytoma xenograft tumors in mice. These encouraging findings demonstrate that radiolabeled phenethylguanidines deserve further investigation as radiotracers of cardiac sympathetic innervation and adrenergic tumors.

Desipramine binding to noradrenaline reuptake sites in cardiac sympathetic neurons in man in vivo

Noradrenergic reuptake blockade is a recognised mechanism of antidepressant action, but the extent of the blockade necessary for therapeutic effect is not known and plasma levels do not provide a guide to therapy. We report a method to assess noradrenaline reuptake blockade in vivo in man using [11C]meta-hydroxyephedrine and the multiple organs' coincidences counter. Eight healthy volunteers had two scans, one with tracer alone and one after preloading with desipramine 50-75 mg p.o. In all subjects, there was an increased washout rate of the radioligand from the heart following preloading (t=4.38; P<0.003) as well as a decrease of the area under the [11C]meta-hydroxyephedrine time activity curve (t=7. 4; P=0.001). In one subject who had three doses of desipramine, the increase in washout rate was dose-dependent. In conclusion, [11C]meta-hydroxyephedrine in the multiple organs' coincidences counter gives a valid, low radiation method to assess noradrenergic reuptake blockade in the clinic.