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Cheng KT.


Molecular Imaging and Contrast Agent Database (MICAD) [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2004-2013.
2006 Jan 25 [updated 2008 Mar 17].


1R-[11C]Phenylephrine ([11C]PHEN) is a radioligand developed for positron emission tomography (PET) imaging of the sympathetic nervous system (SNS). It is a catecholamine analog labeled with 11C, a positron emitter with a physical t½ of 20.4 min (1, 2). Many diseases affect the SNS, and imaging of the pathologic changes of adrenergic transmission has been an important area of PET research (3, 4). Most postganglionic sympathetic neurons in the autonomic nervous system release the neurotransmitter, norepinephrine (NE), which stimulates adrenergic receptors in various effector organs (5). There are different types of adrenergic receptors, and NE stimulates α1, β1 and certain β2 receptors. The NE transporter (NET) is a transmembrane protein located in the adrenergic nerve terminals that is responsible for active reuptake (uptake-1) of NE released by neurons (6). NE is stored in the neuronal vesicles and is released on stimulation. Significant expression of NET is found in major organs of the SNS such as the heart and brain. There is substantial evidence that aberrations in cardiac SNS function contribute to the morbidity and mortality associated with cardiac diseases (7). Molecular probes with structures closely related to NE can be used to assess the integrity of presynaptic sympathetic nerve terminals in various diseases. meta-Iodobenzylguanidine ([123]MIBG), a single-photon emission tomography (SPECT) agent, has been developed and used for neuronal imaging (2). Efforts have been made to develop a positron-emitting tracer because of the inadequate quantitative information and lower spatial resolution obtained by SPECT imaging with MIBG. [11C]meta-Hydroxyephedrine ([11C]mHED) was first developed based on metaraminol, a synthetic false transmitter analog of NE, that accumulates in nerve terminals in the same way as NE (8). [11C]PHEN is structurally similar to [11C]mHED and lacks only the α-methyl group. This difference appears to render [11C]PHEN susceptible to oxidative deamination by axoplasmic monoamine oxidase (MAO). Thus, [11C]PHEN is transported into sympathetic nerve varicosities by NET and taken up into storage vesicles (9, 10). It is most likely cleaved by MAO at the α-carbon into two by-products: an unlabeled aldehyde and [11C]methylamine. [11C]Methylamine is a freely diffusible molecule that rapidly leaves the neuron and enters the systemic circulation. In principle, heart PET imaging with [11C]PHEN may allow the assessment of heart MAO activity.

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