PET Imaging Demonstrates Histone Deacetylase Target Engagement and Clarifies Brain Penetrance of Known and Novel Small Molecule Inhibitors in Rat

Abstract

Histone deacetylase (HDAC) enzymes have been demonstrated as critical components in maintaining chromatin homeostasis, CNS development, and normal brain function. Evidence in mouse models links HDAC expression to learning, memory, and mood-related behaviors; small molecule HDAC inhibitor tool compounds have been used to demonstrate the importance of specific HDAC subtypes in modulating CNS-disease-related behaviors in rodents. So far, no direct evidence exists to understand the quantitative changes in HDAC target engagement that are necessary to alter biochemistry and behavior in a living animal. Understanding the relationship between target engagement and in vivo effect is essential in refining new ways to alleviate disease. We describe here, using positron emission tomography (PET) imaging of rat brain, the in vivo target engagement of a subset of class I/IIb HDAC enzymes implicated in CNS-disease (HDAC subtypes 1, 2, 3, and 6). We found marked differences in the brain penetrance of tool compounds from the hydroxamate and benzamide HDAC inhibitor classes and resolved a novel, highly brain penetrant benzamide, CN147, chronic treatment with which resulted in an antidepressant-like effect in a rat behavioral test. Our work highlights a new translational path for understanding the molecular and behavioral consequences of HDAC target engagement.

Kinetic Analysis and Quantification of [11C]Martinostat for in Vivo HDAC Imaging of the Brain

“…[ 11 C]Martinostat, Figure 1, is a hydroxamic acid with uniquely high brain penetration (with a measured in vitro partition coefficient Log D = 2.03) relative to other hydroxamic acids in this class 19, 20 . Our initial evaluation in rodents and nonhuman primates demonstrated high binding and appropriate retention of [ 11 C]Martinostat in the brain 11, 19 . In addition, [ 11 C]Martinostat binds selectively and reversibly to its target enzymes, implying that [ 11 C]Martinostat could be a useful imaging tool to quantify the drug-occupancy relationship 11 .…”

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

“…Histone deacetylases (HDACs) are epigenetic enzymes which have received considerable attention as potential drug targets, based on animal models and post mortem studies 6–12 . Even though their potential application includes a number of psychiatric and neurodegenerative diseases, only cancer treatment is currently an approved application of HDAC inhibitors in humans 13 .…”

“…[ 11 C]Martinostat is an HDAC radiotracer with exceptionally high brain uptake 12, 15, 16 . The currently available version of Martinostat is labelled with carbon-11, which limits its practicality, since logistics are constrained by its 20 min half-life 17, 18 .…”

Epigenetic Mechanisms in Psychiatric Diseases and Epigenetic Therapy

“…However, chronic treatment with Cpd‐60 was not well tolerated [Schroeder et al, ]. CN147, an ortho‐amino‐anilide HDAC inhibitor was effective as an antidepressant [Schroeder et al, ].…”