Development of a Fluorinated Class-I HDAC Radiotracer Reveals Key Chemical Determinants of Brain Penetrance

Abstract

Despite major efforts, our knowledge about many brain diseases remains remarkably limited. Epigenetic dysregulation has been one of the few leads toward identifying the causes and potential treatments of psychiatric disease over the past decade. A new positron emission tomography radiotracer, [11C]Martinostat, has enabled the study of histone deacetylase in living human subjects. A unique property of [11C]Martinostat is its profound brain penetrance, a feature that is challenging to engineer intentionally. In order to understand determining factors for the high brain-uptake of Martinostat, a series of compounds was evaluated in rodents and non-human primates. The study revealed the major structural contributors to brain uptake, as well as a more clinically relevant fluorinated HDAC radiotracer with comparable behavior to Martinostat, yet longer half-life.

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.…”

Radiosynthesis and preliminary evaluation of an 18F‐labeled tubastatin A analog for PET imaging of histone deacetylase 6

“…27,28 The majority of HDAC radioligands, including the first SAHA-based HDAC radioligand [ 18 F] FAHA, are hydroxamic acid based with others being benzamide based or carboxylic acid based. 32,33 In particular, the carbon-11-labeled ligand [ 11 C]Martinostat, which has high binding affinity for class I HDACs, underwent a first-in-human study to evaluate neuroepigenetic regulation. 24 However, Hooker et al reported that hydroxamic acid-based radioligands containing an adamantane group, which is often conjugated with drugs to improve their brain penetrance, 30,31 readily crossed the BBB in rodents and nonhuman primates.…”

“…36,42 The chemical structure of 1 consists of three motifs: a hydroxamic acid zinc-binding group that localizes a zinc ion within the HDAC active site, a carboline-based cap group that interacts with the protein surface of the binding pocket, and a benzyl linker group that bridges the F I G U R E 1 Chemical structures of tubastatin A and its radiolabeled analogs two groups. We intended to perform this radiosynthesis without intermediary solid-phase extraction or high-performance liquid chromatography (HPLC) purification steps, which have been incorporated in other hydroxamic acid-based radioligands, 32,40,43 because these processes often complicate automated radiosynthesis. Kalin et al demonstrated that a substituent could be introduced at the nitrogen atom of tetrahydropyridine of the β-carboline regioisomer of 1 without seriously disrupting HDAC6 selectivity.…”

“…24 However, Hooker et al reported that hydroxamic acid-based radioligands containing an adamantane group, which is often conjugated with drugs to improve their brain penetrance, 30,31 readily crossed the BBB in rodents and nonhuman primates. 32,33 In particular, the carbon-11-labeled ligand [ 11 C]Martinostat, which has high binding affinity for class I HDACs, underwent a first-in-human study to evaluate neuroepigenetic regulation. 34 As expected, [ 11 C]Martinostat demonstrated excellent uptake in the human brain, and higher uptake was observed in the cortical gray matter compared with the white matter.…”

“…Radiosynthesis of [ 18 F]3 was achieved using a two-step reaction composed of 18 F-fluorination by general nucleophilic substitution and conversion of a methylester group into a hydroxamic acid group. We intended to perform this radiosynthesis without intermediary solid-phase extraction or high-performance liquid chromatography (HPLC) purification steps, which have been incorporated in other hydroxamic acid-based radioligands, 32,40,43 because these processes often complicate automated radiosynthesis. In addition, preclinical evaluations such as binding selectivity assays and biodistribution assays were performed to assess the potential utility of [ 18 F]3 for visualization of HDAC6.…”