Novel Histone Deacetylase Class IIa Selective Substrate Radiotracers for PET Imaging of Epigenetic Regulation in the Brain

Abstract

Histone deacetylases (HDAC’s) became increasingly important targets for therapy of various diseases, resulting in a pressing need to develop HDAC class- and isoform-selective inhibitors. Class IIa deacetylases possess only minimal deacetylase activity against acetylated histones, but have several other client proteins as substrates through which they participate in epigenetic regulation. Herein, we report the radiosyntheses of the second generation of HDAC class IIa–specific radiotracers: 6-(di-fluoroacetamido)-1-hexanoicanilide (DFAHA) and 6-(tri-fluoroacetamido)-1-hexanoicanilide ([18F]-TFAHA). The selectivity of these radiotracer substrates to HDAC class IIa enzymes was assessed in vitro, in a panel of recombinant HDACs, and in vivo using PET/CT imaging in rats. [18F]TFAHA showed significantly higher selectivity for HDAC class IIa enzymes, as compared to [18F]DFAHA and previously reported [18F]FAHA. PET imaging with [18F]TFAHA can be used to visualize and quantify spatial distribution and magnitude of HDAC class IIa expression-activity in different organs and tissues in vivo. Furthermore, PET imaging with [18F]TFAHA may advance the understanding of HDACs class IIa mediated epigenetic regulation of normal and pathophysiological processes, and facilitate the development of novel HDAC class IIa-specific inhibitors for therapy of different diseases.

Unpacking the Pathogen Box—An Open Source Tool for Fighting Neglected Tropical Disease

“…are the hydroxamic acid groups, which bind to the active site zinc cations, and are absent in compound 508 . The IIa class of human HDAC differs from other zinc‐dependent HDACs in that they are inefficient deacaetylases, but rather act as epigenetic regulators by associating with client protein substrates . Despite this, class IIa HDACS also feature the zinc‐binding site, which interacts with the hydroxamic acid group of many HDAC inhibitors.…”

Molecular Imaging of Sirtuin1 Expression–Activity in Rat Brain Using Positron-Emission Tomography–Magnetic-Resonance Imaging with [18F]-2-Fluorobenzoylaminohexanoicanilide

“…Moreover, 11 C-Martinostat has been already translated into the clinic for mapping HDAC1 distribution in the human brain in vivo. 24 PET−CT−MR imaging with these novel imaging agents enabled the monitoring of the pharmacokinetics (PK) and BBB penetrance of nonradiolabeled drug analogues, dose−occupancy studies of HDAC class I enzymes, 25 and the dose-dependent inhibition of HDAC class IIa 18,19 enzyme activity in the brain. However, despite the biological importance of HDAC class III enzymes, especially SIRT1, PET imaging of SIRT1 expression−activity has not yet been reported.…”

“…17 Previously, we successfully used this approach for the development of the HDAC class IIa specific substrate-type radiotracers [ 18 F]FAHA 18 and [ 18 F]TFAHA. 19 The current approach was based on previous reports demonstrating that SIRT1 is able to cleave large aromatic chemical groups, such as phenylacetyl, 20 from the ε -amine of lysine. Although other SIRT enzymes can cleave long acyl chains from lysine, they are not capable of cleaving aromatic-ring-containing moieties.…”

Neuroepigenetics of Post-Traumatic Stress Disorder

“…Therefore, postmortem human brain studies of dysregulated HDAC isoform expression in PTSD will be needed to benchmark HDAC isoform-specific density and distribution in different regions of the brain. Further developments stemming from this tracer 125 , as well as synthesis and preclinical testing of other promising epigenetic radiotracers, such as [18F]-TFAHA (selective for HDAC IIa) 126 , have been reported and may improve and expand our capabilities for in vivo epigenetic measurements.…”