[18F]Bavarostat

Histone deacetylase 6 (HDAC6) is involved in microtubule stabilisation and protein degradation, and is attracting attention as a therapeutic target molecule for neurodegenerative diseases. [¹⁸F]EKZ-001 is a positron emission tomography (PET) radioligand that can visualise HDAC6 in the brain and allows exploration of the relationship between diseases and HDAC6 expression levels and distribution. The present study reports the radiosynthesis of a carbon-11-labelled version of EKZ-001 that is chemically identical to [¹⁸F]EKZ-001. This radiosynthesis adopted a conventional ¹¹C-labelling method with [¹¹C]methyl triflate instead of the originally reported ruthenium-mediated ¹⁸F-deoxyfluorination. Preliminary biological evaluations using [¹¹C]EKZ-001 were also performed.

Carbon-11-labelling was conducted with a desmethyl precursor and [¹¹C]methyl triflate in acetone, which was selected after solvent optimisation. After the ¹¹C-labelling reaction at room temperature for 5 min, [¹¹C]EKZ-001 was purified using semipreparative high-performance liquid chromatography. The solvent of the fraction containing the product was removed by evaporation, and the resulting residue was formulated in physiological saline containing ethanol and ascorbic acid. The radioligand kinetics in the brain were assessed in normal mice by small-animal PET with and without coadministration of unlabelled EKZ-001. Metabolite analysis was also performed in the brain and plasma at 15 and 30 min after radioligand administration in normal mice. Radioligand binding in the mouse brain was evaluated by in vitro autoradiography and compared with HDAC6 immunostaining.

[¹¹C]EKZ-001 was obtained in a radiochemical yield of 35.5 ± 4.2 % (decay corrected to [¹¹C]CO₂; n = 3), with a radiochemical purity and molar activity at the end of the synthesis of 96.0 ± 1.1 % and 111.2 ± 32.3 MBq/nmol, respectively. PET imaging in normal mice demonstrated an excellent brain uptake of [¹¹C]EKZ-001 that was reduced by coadministration of unlabelled EKZ-001. [¹¹C]EKZ-001 was stable in the mouse brain, and the proportion of [¹¹C]EKZ-001 radioactivity remained intact at 85 % even 30 min after administration. In autoradiography using mouse brain sections, [¹¹C]EKZ-001 showed specific binding in regions generally consistent with HDAC6 immunostaining.

[¹¹C]EKZ-001 can be synthesised in a conventional way with [¹¹C]methyl triflate and used as a radioligand for HDAC6 imaging in the brain.

Fluorine is increasingly being incorporated into pharmaceuticals to modulate their physical properties. Furthermore, 18F can be utilized in positron emission tomography (PET) probes for in vivo imaging. Bavarostat is a selective inhibitor of histone deacetylase 6 (HDAC6), and when derivatized with 18F is used to map out HDAC6 localisation within the brain. As HDAC6 is implicated in Alzheimer’s disease due to its function as a Tau deacetylase, it may serve as an effective biomarker for this disease. The active site cavity of HDAC6 contains two phenylalanine residues that form an aromatic crevice, F583 and F643, that engage in offset π‐π interactions with phenylhydroxymate binding groups. Moreover, the fluoroaromatic ring of Bavarostat orientates such that the C–F group binds in the aromatic crevice instead of being exposed to solvent. Structure activity relationships were determined for a series of fluorophenylhydroxymates to inform the design of fluorinated inhibitors of HDAC6 as well PET probes. Our studies show the aromatic crevice of HDAC6 preferentially binds C–F groups in this series of inhibitors. The inhibitors show differences in zinc binding orientations, likely due to the electronic influence of fluorine on the hydroxymate zinc‐binding group.