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Am J Nucl Med Mol Imaging 2014;4(1):29-38

Original Article
Design, synthesis, and evaluation of hydroxamic acid-based molecular probes for
in vivo imaging of histone deacetylase (HDAC) in brain

Changning Wang, Thomas E Eessalu, Vanessa N Barth, Charles H Mitch, Florence F Wagner, Yijia Hong, Ramesh Neelamegam,
Frederick A Schroeder, Edward B Holson, Stephen J Haggarty, Jacob M Hooker

Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Harvard Medical
School, Charlestown, MA 02129, USA; Eli Lilly & Co., Indianapolis, IN 46285, USA; Stanley Center for Psychiatric Research, Broad
Institute of Harvard and MIT, 7 Cambridge Center, Cambridge, MA 02142, USA; Department of Molecular and Cell Biology, University
of California, Berkeley, CA 94720, USA; Chemical Neurobiology Laboratory, Departments of Neurology and Psychiatry, Center for
Human Genetic Research, Massachusetts General Hospital, 185 Cambridge Street, Boston, MA 02114, USA

Received August 29, 2013; Accepted October 1, 2013; Epub December 15, 2013; Published January 1, 2014

Abstract: Hydroxamic acid-based histone deacetylase inhibitors (HDACis) are a class of molecules with therapeutic potential
currently reflected in the use of suberoylanilide hydroxamic acid (SAHA; Vorinostat) to treat cutaneous T-cell lymphomas (CTCL).
HDACis may have utility beyond cancer therapy, as preclinical studies have ascribed HDAC inhibition as beneficial in areas such as
heart disease, diabetes, depression, neurodegeneration, and other disorders of the central nervous system (CNS). However, little is
known about the pharmacokinetics (PK) of hydroxamates, particularly with respect to CNS-penetration, distribution, and retention. To
explore the rodent and non-human primate (NHP) brain permeability of hydroxamic acid-based HDAC inhibitors using positron
emission tomography (PET), we modified the structures of belinostat (PXD101) and panobinostat (LBH-589) to incorporate
carbon-11. We also labeled PCI 34051 through carbon isotope substitution. After characterizing the in vitro affinity and efficacy of
these compounds across nine recombinant HDAC isoforms spanning Class I and Class II family members, we determined the
brain uptake of each inhibitor. Each labeled compound has low uptake in brain tissue when administered intravenously to rodents
and NHPs. In rodent studies, we observed that brain accumulation of the radiotracers were unaffected by the pre-administration of
unlabeled inhibitors. Knowing that CNS-penetration may be desirable for both imaging applications and therapy, we explored
whether a liquid chromatography, tandem mass spectrometry (LC-MS-MS) method to predict brain penetrance would be an
appropriate method to pre-screen compounds (hydroxamic acid-based HDACi) prior to PET radiolabeling. LC-MS-MS data were
indeed useful in identifying additional lead molecules to explore as PET imaging agents to visualize HDAC enzymes in vivo.
However, HDACi brain penetrance predicted by LC-MS-MS did not strongly correlate with PET imaging results. This underscores the
importance of in vivo PET imaging tools in characterizing putative CNS drug lead compounds and the continued need to discover
effect PET tracers for neuroepigenetic imaging. (ajnmmi1308006).

Keywords: Hydroxamic acid, HDAC inhibitors, epigenetic, PET, brain, imaging

Address correspondence to: Dr. Jacob M Hooker, Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology,
Massachusetts General Hospital, Harvard Medical School, 149 13th StreetSuite 2301, Charlestown, MA 02129, USA. Tel:
617-726-6596; Fax: 617-726-7422; E-mail: hooker@nmr.mgh.harvard.edu