Michael S. Haka

Noninvasive evaluation of sympathetic nervous system in human heart by positron emission tomography.

The noninvasive functional characterization of the cardiac sympathetic nervous system by imaging techniques may provide important pathophysiological information in various cardiac disease states. Hydroxyephedrine labeled with carbon 11 has been developed as a new catecholamine analogue to be used in the in vivo evaluation of presynaptic adrenergic nerve terminals by positron emission tomography (PET). To determine the feasibility of this imaging approach in the human heart, six normal volunteers and five patients with recent cardiac transplants underwent dynamic PET imaging after intravenous injection of 20 mCi [11C]hydroxyephedrine. Blood and myocardial tracer kinetics were assessed using a regions-of-interest approach. In normal volunteers, blood 11C activity cleared rapidly, whereas myocardium retained 11C activity with a long tissue half-life. Relative tracer retention in the myocardium averaged 79 +/- 31% of peak activity at 60 minutes after tracer injection. The heart-to-blood 11C activity ratio exceeded 6:1 as soon as 30 minutes after tracer injection, yielding excellent image quality. Little regional variation of tracer retention was observed, indicating homogeneous sympathetic innervation throughout the left ventricle. In the transplant recipients, myocardial [11C]hydroxyephedrine retention at 60 minutes was significantly less (-82%) than that of normal volunteers, indicating only little non-neuronal binding of the tracer in the denervated human heart. Thus, [11C]hydroxyephedrine, in combination with dynamic PET imaging, allows the noninvasive delineation of myocardial adrenergic nerve terminals. Tracer kinetic modeling may permit quantitative assessment of myocardial catecholamine uptake, which will in turn provide insights into the effects of various disease processes on the neuronal integrity of the heart.

Evidence for regional catecholamine uptake and storage sites in the transplanted human heart by positron emission tomography.

Positron emission tomography in combination with the newly introduced catecholamine analogue [11C]hydroxyephedrine ([11C]HED) enables the noninvasive delineation of sympathetic nerve terminals of the heart. To address the ongoing controversy over possible reinnervation of the human transplant, 5 healthy control subjects and 11 patients were studied after cardiac transplant by this imaging approach. Regional [11C]HED retention was compared to regional blood flow as assessed by rubidium-82. Transplant patients were divided into two groups. Group I had recent (less than 1 yr, 4.4 +/- 2.3 mo) surgery, while group II patients underwent cardiac transplantation more than 2 yr before imaging (3.5 +/- 1.3 yr). [11C]HED retention paralleled blood flow in normals, but was homogeneously reduced in group I. In contrast, group II patients revealed heterogeneous [11C]HED retention, with increased uptake in the proximal anterior and septal wall. Quantitative evaluation of [11C]HED retention revealed a 70% reduction in group I and 59% reduction in group II patients (P less than 0.001). In group II patients, [11C]HED retention reached 60% of normal in the proximal anterior wall. These data suggest the presence of neuronal tissue in the transplanted human heart, which may reflect regional sympathetic reinnervation.

Mouse brain distribution of a carbon-11 labeled vesamicol derivative: Presynaptic marker of cholinergic neurons

The regional mouse brain distribution of a new carbon-11 labeled derivative of vesamicol, [11C]-5-(N-methylamino)benzovesamicol [( 11C]MABV) is reported. Radiotracer concentrations in vivo are in the rank order of striatum greater than cortex greater than hippocampus greater than hypothalamus greater than cerebellum, consistent with reported distributions of other presynaptic cholinergic neuronal markers. In time course studies, striatum/cerebellum and cortex/cerebellum ratios for (-)-[11C]MABV continue to increase to values of 13 and 5, respectively, 75 min after i.v. injection of [11C]MABV. The specific binding in striatum and cortex is lowered by pretreatment with (+/-)-vesamicol, and shows stereoselectivity with lower uptake and lower ratios for the (+)-enantiomer. (-)-enantiomer. (-)-[11C]MABV is proposed as a positron-emitting radioligand for the in vivo study of presynaptic cholinergic neurons.

Biodistribution, dosimetry, metabolism and monkey PET studies of [18F]GBR 13119. Imaging the dopamine uptake system in vivo

The in vivo characteristics of a new radiotracer, [18F]GBR 13119, have been examined. Full body biodistribution in rats has been determined and the expected human dosimetry calculated. Pharmacological specificity of in vivo regional brain distribution in rats was examined. Blockage of specific binding was accomplished by dopamine reuptake inhibitors but no effect was observed for pretreatment with serotonin or norepinephrine reuptake inhibitors. Preliminary examination of rat blood shows the presence of radiolabeled metabolites, which can be rapidly identified using bonded-phase (Sep-Pak) chromatography. Finally, the striatum of living primates has been imaged using PET and i.v. administration of [18F]GBR 13119. These results represent the intermediate steps in the development of [18F]GBR 13119 as a radiotracer for the study of the dopamine uptake system in man.

Synthesis and regional mouse brain distribution of [11C]Nisoxetine, a norepinephrine uptake inhibitor

Nisoxetine, a selective and high affinity (IC50 = 1 nM) inhibitor of NE reuptake, has been radiolabeled in high specific activity (greater than 600 Ci/mmol) by the alkylation of the nor-methyl precursor with [11C]CH3I. Synthetic yields are good (40-60% from [11C]methyl iodide, corrected for decay, 20 min synthesis), with the product purified by HPLC. In vivo studies of regional brain distribution in CD-1 mice show uptake and retention of tracer in the cortex, striatum, hypothalamus and thalamus, with the highest levels in the hypothalamus and cortex. Specific binding in the cortex and hypothalamus can be reduced by preadministration of 7 mg/kg i.v. unlabeled nisoxetine. The possible value of [11C]nisoxetine as a PET imaging agent is discussed.

Synthesis of [18F]GBR13119, a presynaptic dopamine uptake antagonist

[18F]GBR13119 (1-[(4-[18F]fluorophenyl)-(phenyl)methoxy]ethyl-4-(3-phenylpropyl) piperazine) has been prepared in no carrier added form by a four-step synthesis from [18F]fluoride. Isolated yields are 7-10% (uncorrected) in a synthesis time of 120 min. The product is obtained in high specific activity (greater than 1000 Ci/mmol) and high radiochemical purity (greater than 99%) without chromatographic purification. Small amounts of chemical impurity, identified as the nitro-substituted analog by independent synthesis, can be removed by HPLC. [18F]GBR13119 is proposed as a new radiotracer for the presynaptic dopamine uptake system.

Imaging the Dopamine Uptake Site with Ex Vivo [18F]GBR 13119 Binding Autoradiography in Rat Brain

We studied the binding of [18F]GBR 13119 {1‐[[(4‐[18F]fluorophenyl) (phenyl)methoxy]ethyl]‐4‐(3‐phenylpropyl)piperazine} to rat brain with autoradiography after intravenous injection. The rank order of binding was dorsal striatum > nucleus accumbens = olfactory tubercle > sub‐stantia nigra = ventral tegmental area > other areas. Binding was blocked by prior injection of dopamine uptake blockers but not by injection of dopamine receptor antagonists or drugs that bind to the dialkylpiperazine site. Unilateral 6‐hydroxy dopamine lesions of dopamine neurons caused a marked decrease in striatal and nigral binding on the side of the lesion. We conclude that intravenous injection of [18F]GBR 13119 provides a useful marker of presynaptic dopamine uptake sites.

Regional brain distribution of [18F]GBR 13119, a dopamine uptake inhibitor, in CD-1 and C57BL/6 mice.

We have examined the regional brain distribution of [18F]GBR 13119 (18F: beta +, T1/2 = 110 min), a dopamine uptake inhibitor, in CD-1 and C57BL/6 mice. High levels of binding are observed in the striatum of both species, with striatum/cerebellum ratios of 3-4 at 60 min after injection of the radiotracer. Striatum radioactivity and striatum/cerebellum ratios are more than 50% reduced in C57BL/6 mice treated chronically with the neurotoxin MPTP. We conclude mice are an appropriate model for the in vivo study of the dopamine uptake system, and that [18F]GBR 13119 may be a suitable in vivo marker for degeneration of striatal dopaminergic neurons.