A novel PET marker for in vivo quantification of myelination
Graphical abstract
Introduction
In the central nervous system (CNS), myelin membranes play a critical role in protecting neurons for speedy and accurate signal transduction.1 Destruction of or changes in myelination are considered some of the primary causes of neurological disorders such as multiple sclerosis (MS).2, 3 Current efforts have focused on the delineation of molecular mechanisms of myelination and the development of novel therapies aimed at the prevention of demyelination and the promotion of remyelination.4 These studies require a molecular imaging tool that permits direct detection and quantification of myelin changes in vivo.
Thus far, magnetic resonance imaging (MRI) has been the primary modality in brain imaging to detect lesions associated with the destruction and/or repair of myelin.5, 6, 7, 8 Because any change in MRI signal intensity reflects only a change in tissue water content, many other macroscopic tissue injuries that affect water content, such as inflammation and edema, can also be visualized by MRI. As a result, MRI signal intensities are not specific for myelin changes. Indeed, use of MRI has been found to be dissociated from clinical outcomes of current therapies for MS.9 It is thus necessary to develop a direct imaging marker that is specific for changes in myelin content. For this reason, we set out to develop myelin-imaging agents for positron emission tomography (PET). PET is a functional imaging technique that is suitable for in vivo studies of biochemical and metabolic processes at the molecular level.10 For directly monitoring myelin changes in the brain, appropriate radiotracers must be developed that readily penetrate the blood–brain barrier (BBB) and localize in brain regions in proportion to the extent of myelination. Once developed, these radiotracers can be used in conjunction with PET as a novel imaging marker to directly and quantitatively assess the extent of demyelination or remyelination. This will provide a direct clinical efficacy endpoint measure of myelin change and become a potentially powerful tool in efficacious evaluation of myelin repair therapies.
Currently, a very limited number of small-molecule probes (SMP) for PET imaging of myelination in vivo studies have been developed in MS as represented by [11C]PK11195, which is a radiotracer developed to characterize peripheral benzodiazepine receptors (PBR) expressed by microglial cells.11 [11C]PK11195-PET is often used to study the correlation of microglia activation with tissue destruction and disease progression in MS patients.12, 13 However, [11C]PK11195-PET is not a specific marker of demyelination, a hallmark that is characteristic of MS. [11C]PK11195-PET imaging is capable of imaging only inflammation and does not provide any correlation of disease progression with the degree of myelination in the brain.
Recently, we have studied a novel series of stilbene derivatives as myelin imaging agents, which show promising binding properties with high affinity and specificity for myelin.14 Based on structure–activity relationship studies, we have identified a lead agent, termed MeDAS, that is suitable for in vivo imaging studies. In continuation of our previous work,14 we radiolabeled MeDAS with C-11 and conducted a series of in vitro, ex vivo, and in vivo studies including tissue staining, biodistribution, autoradiography, and microPET in a transgenic Plp-Akt-DD mouse model. In this model, myelination in the brain is enhanced as a result of the over-expression of a constitutively active serine/threonine kinase Akt in oligodendrocytes, leading to hypermyelination in the white matter regions, especially in the corpus callosum.15 These studies are designed to validate [11C]MeDAS as a promising SMP for PET imaging of myelination in vivo. Compared to immuno-PET, which requires use of radiolabeled antibodies, SMP-PET is advantageous in that radiosynthesis of the imaging agents is more practical and their structures and pharmacokinetics can be clearly characterized. In vivo [11C]MeDAS-PET studies in Plp-Akt-DD mice in comparison with normal controls showed that [11C]MeDAS is a sensitive and specific radiotracer for PET imaging of myelination.
Section snippets
Radiochemistry
The synthesis of the target [11C]MeDAS was accomplished using (E)-4,4′-diaminostilbene (DAS) in acetone with [11C]methyl triflate ([11C]CH3OTf) as outlined in Scheme 1. The cyclotron-derived [11C]carbon dioxide was converted to [11C]methyl iodide ([11C]CH3I) by an automatic synthesis modular FXc-box (General Electric Medical Systems) using the direct iodination of [11C]methane produced by an on-site cyclotron. [11C]CH3OTf was generated by reaction of the produced [11C]CH3I with silver triflate
Conclusion
In this work, we demonstrated that a stilbene derivative, N-methyl-4,4′-diamino stilbene ([11C]MeDAS) binds to myelin fibers with high specificity and sensitivity. An in vivo biodistribution study in mice showed high permeability across the BBB. Distinct labeling of myelin fibers by [11C]MeDAS was observed in the corpus callosum through in situ fluorescent staining and ex vivo autoradiography. Quantitative [11C]MeDAS microPET studies permitted direct differentiation in myelination between Plp
General remarks
Black-Gold® (AG390) was purchased from Millipore, Bedford, MA. Anti-myelin basic protein (MBP) antibodies (AB980) were obtained from Chemicon-Millipore, Bedford, MA, and IRDye 800CW Goat Anti-Rabbit (926-32211) was purchased from Li-COR Biosciences, Lincoln, NE. SWR/J mice were obtained from the Jackson Laboratory, Bar Harbor, MN, and the Plp-Akt-DD mice were obtained as previously described.15
Radiosynthesis
Carbon-11 was produced in the form of [11C]carbon dioxide ([11C]CO2) at Scanditronix MC17 cyclotron by
Acknowledgements
We gratefully acknowledge financial support through Grants from the Department of Defense, National Multiple Sclerosis Society, and NIH/NINDS (R01 NS061837). We also thank George Bakale for assistance in preparation of the manuscript.
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