Radiosynthesis of ¹¹C‑Levetiracetam: A Potential Marker for PET Imaging of SV2A Expression

The multistep preparation of ¹¹C-levetiracetam (¹¹C-LEV) was carried out by a one-pot radiosynthesis with 8.3 ± 1.6% (<i>n</i> = 8) radiochemical yield in 50 ± 5.0 min. Briefly, the propionaldehyde was converted to propan-1-imine <i>in situ</i> as labeling precursor by incubation with ammonia. Without further separation, the imine was reacted with ¹¹C-HCN to form ¹¹C-aminonitrile. This crude was then reacted with 4-chlorobutyryl chloride and followed by hydrolysis to yield ¹¹C-LEV after purification by chiral high-performance liquid chromatography (HPLC). Both the radiochemical and enantiomeric purities of ¹¹C-LEV were >98%

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

<div><p>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 <i>in vitro</i>, in a panel of recombinant HDACs, and <i>in vivo</i> 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 <i>in vivo</i>. 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.</p></div

Synthesis of [¹⁸F]DFAHA and [¹⁸F]TFAHA.

<p>Reaction conditions are as follows: f) [¹⁸F]KF, K_2,2,2; 0.4mL ACN, 105°C 25 min.; g)[¹⁸F]KF, K_2,2,2; 0.4mL ACN, 105°C 25 min.</p

Synthesis of DFAHA and TFAHA precursors.

<p>Reaction conditions are as follows: c) pyridine, acetyl chloride added drop-wise at 0°C, stirred overnight at RT d) 0.9 eq. SOCl₂, 12 hr. stirred under argon at 40°C, catalytic DMF; e) DCM, triethylamine added drop-wise at 0°C, stirred 24 hr. under argon at RT.</p

The mechanism of action of HDAC4.

<p>As compared to HDAC class I, HDAC class IIa enzymes exhibit significantly reduced ability to deacetylate. In class IIa enzymes the His976 located in the same position in the catalytic site as Tyr306 in class I HDACs does not serve as a hydrogen bond donor to bind to the carbonyl oxygen of the leaving acetyl group and thus reduces the susceptibility of carbonyl carbon to nucleophilic attack by the water, as in HDAC class I (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0133512#pone.0133512.g004" target="_blank">Fig 4</a>).</p

The structures of compounds and docking scores with HDAC8, a HDAC Class I enzyme.

<p>A lower docking score indicates higher affinity.</p

Synthesis of DFAHA and TFAHA.

<p>Reaction conditions are as follows: a) RT overnight; b) 2mL DCM, stirred overnight.</p

The three parts of a HDAC substrate, the cap (red), the linker (green), and the leaving group (blue).

<p>The three parts of a HDAC substrate, the cap (red), the linker (green), and the leaving group (blue).</p