Certain enzymes called histone deacetylases, or HDACs, are part of the epigenetic machinery that regulates gene transcription. In neurological disorders, HDACs change expression in regions throughout the brain, but their dynamic contribution to human disease development over time is unknown. Wey et al. therefore developed and applied an HDAC imaging probe, called Martinostat, to visualize HDAC expression in the living brain. Martinostat was previously tested in rodents and nonhuman primates, and here, it is used for the first time in humans. The authors saw surprisingly conserved regions of HDAC expression in the healthy brain, suggesting tightly regulated epigenetic processes. In human stem cell–derived neural progenitor cells, Martinostat engaged the subset HDACs that regulate downstream genes important for neuroplasticity, memory, and neurodegeneration, supporting its use in monitoring and understanding brain pathologies like Alzheimer’s disease.

Epigenetic dysfunction is implicated in many neurological and psychiatric diseases, including Alzheimer’s disease and schizophrenia. Consequently, histone deacetylases (HDACs) are being aggressively pursued as therapeutic targets. However, a fundamental knowledge gap exists regarding the expression and distribution of HDACs in healthy individuals for comparison to disease states. Here, we report the first-in-human evaluation of neuroepigenetic regulation in vivo. Using positron emission tomography with [ 11 C]Martinostat, an imaging probe selective for class I HDACs (isoforms 1, 2, and 3), we found that HDAC expression is higher in cortical gray matter than in white matter, with conserved regional distribution patterns within and between healthy individuals. Among gray matter regions, HDAC expression was lowest in the hippocampus and amygdala. Through biochemical profiling of postmortem human brain tissue, we confirmed that [ 11 C]Martinostat selectively binds HDAC isoforms 1, 2, and 3, the HDAC subtypes most implicated in regulating neuroplasticity and cognitive function. In human stem cell–derived neural progenitor cells, pharmacologic-level doses of Martinostat induced changes in genes closely associated with synaptic plasticity, including BDNF (brain-derived neurotrophic factor) and SYP (synaptophysin), as well as genes implicated in neurodegeneration, including GRN (progranulin), at the transcript level, in concert with increased acetylation at both histone H3 lysine 9 and histone H4 lysine 12. This study quantifies HDAC expression in the living human brain and provides the foundation for gaining unprecedented in vivo epigenetic information in health and disease.

Although there is strong evidence for localized HDAC dysfunction in CNS disease, epigenetic models cannot recapitulate dynamic human-environment interactions and therefore may not accurately reflect in vivo human biology. Moreover, until now, there has been no method to visualize in vivo epigenetic mechanisms in humans. We developed the positron emission tomography (PET) epigenetic imaging agent, [ 11 C]Martinostat, previously described in rodents and nonhuman primates (NHPs) ( 12 , 18 , 19 ). Our previous work in rodents demonstrated the specific and reversible binding properties of [ 11 C]Martinostat and that the agent engaged recombinant class I HDACs (isoforms 1, 2, and 3) and class IIb HDAC (isoform 6) with low nanomolar affinities ( 18 ). Because [ 11 C]Martinostat demonstrated excellent brain penetrance, it was used to determine whether clinically relevant HDAC inhibitors, such as suberoylanilide hydroxamic acid (SAHA) and CI-994, crossed the blood-brain barrier and exhibited target occupancy in rodents ( 12 ). Most recently, we performed studies in NHPs to characterize the kinetic properties of [ 11 C]Martinostat and to estimate nondisplaceable binding of [ 11 C]Martinostat with pharmacologic blockades in preparation for human studies ( 19 ). In addition to the brain, [ 11 C]Martinostat showed high specific binding and fast binding kinetics appropriate for PET imaging in heart, pancreas, spleen, and kidneys ( 18 , 19 ). Here, we translate [ 11 C]Martinostat for clinical research use and quantify human epigenetic regulation.

In addition to the overall level of HDAC expression within the brain, spatially localized variation of HDACs is also highly impactful in neuronal plasticity, memory, and behavior. For example, intrahippocampal injection of short hairpin RNA against Hdac2 selectively normalized HDAC2 levels and restored neuroplasticity-associated gene transcription, synaptic density, and cognitive behavior in a mouse model of AD ( 6 ). In contrast to the high level of hippocampal HDAC2 in animal models and postmortem human tissue from AD patients, deficient HDAC2 expression was observed in the frontal cortex of postmortem AD tissue, highlighting the importance of tightly regulated localized HDAC expression ( 15 ). Analogously, focal genetic deletion of Hdac3 in the hippocampus and the nucleus accumbens enhanced long-term memory and acquisition of cocaine-associated place preference in mice, respectively ( 5 , 16 ). Although understanding of the full compendium of genes under HDAC-dependent regulation in defined regions of the brain is incomplete, HDAC2 chromatin immunoprecipitation studies in hippocampal tissue have identified several immediate-early genes (for example, BDNF and CDK5) involved in learning and memory, as well as multiple genes involved in synaptic plasticity (for example, SYP and SYT1) as downstream targets ( 3 , 6 , 17 ). Collectively, these studies provide support that localized HDAC expression levels drive pivotal epigenetic mechanisms that modulate neuronal function.

Disorders of the central nervous system (CNS), including Alzheimer’s disease (AD), schizophrenia, depression, and addiction, are increasingly recognized to involve dysregulation of epigenetic machinery. Among all, histone deacetylases (HDACs)—a family of chromatin-modifying enzymes that dynamically regulates gene transcription—are the most frequently implicated ( 1 , 2 ). A subset of HDACs has already been linked to neuronal development, synaptic plasticity, and cognition ( 3 , 4 ). For example, postmortem human brain tissue analyses and in vivo rodent studies exposed HDAC1, HDAC2, and HDAC3 as antagonists of learning and memory and contributors to AD and mood disorders ( 3 , 5 – 9 ). Genetic manipulations or pharmacologic inhibition of aberrant HDAC2 and HDAC3 activity rescued behavioral defects in rodent models of both AD and mood disorders ( 6 , 7 , 10 – 14 ). HDAC inhibitors were also proposed as a targeted treatment of frontotemporal lobar degeneration, owing to mutations that cause haploinsufficiency of the progranulin-encoding gene GRN ( 14 ). Collectively, these studies implicate a direct relationship between the levels of class I HDACs (isoforms 1, 2, and 3) and neuronal function.

Human neural progenitor cells were treated with DMSO (Veh), Martinostat (MSTAT; 0.5, 2.5, or 5.0 μM), and SAHA (10 μM) for 24 hours. ( A ) Whole-cell lysates were prepared (n = 3). # Because treatment with 5.0 μM Martinostat was toxic to cells, whole-cell lysates from three replicates were combined into one pool to obtain sufficient protein for this dose. Equivalent amounts of total protein were compared through Western blotting. Histone acetylation immunoreactive band intensity values were normalized to GAPDH intensity values. Data are means ± SD (n = 3). P values compare drug treatments to Veh, determined by repeated-measures two-way ANOVA (α = 0.05 with Dunnett’s multiple comparisons correction). ( B ) RNA was extracted (n = 3) and converted into complementary DNA (cDNA). mRNA transcript levels of memory/neuronal plasticity–related (BDNF, EGR1, CDK5, SYT1, and SYP) and monogenic neurological disorder–related (GRN and FXN) genes were compared through qPCR and normalized to GAPDH mRNA levels. Data are means ± SEM (n = 3 cDNA per condition with three technical qPCR replicates per cDNA). P values compare drug treatments to Veh, determined by repeated-measures two-way ANOVA (α = 0.05 with Dunnett’s multiple comparisons correction).

To link [ 11 C]Martinostat uptake with downstream HDAC substrate signaling and gene expression, we treated human stem cell–derived neural progenitor cells with increasing concentrations of Martinostat. Acetylation levels of established class I HDAC substrates, histone H3 lysine 9 (H3K9) and histone H4 lysine 12 (H4K12), were determined using Western blotting ( 3 , 24 ). Treatment with 2.5 and 5.0 μM Martinostat increased H3K9 and H4K12 acetylation levels as compared to vehicle control ( Fig. 5A ). Treatment with 5.0 μM Martinostat elevated acetylation to a level equivalent to or greater than 10 μM SAHA ( Fig. 5A ). Messenger RNA (mRNA) transcript levels of memory-related ( 3 , 6 , 24 ), neuroplasticity-related ( 3 ), and neurological disease–related genes ( 17 ) were measured through quantitative polymerase chain reaction (qPCR). Treatment with 2.5 and/or 5.0 μM Martinostat increased brain-derived neurotrophic factor (BDNF), early growth response protein 1 (EGR1), cyclin-dependent kinase 5 (CDK5), synaptotagmin (SYT1), synaptophysin (SYP), and progranulin (GRN) expression compared to vehicle control, but not frataxin (FXN) ( Fig. 5B ). Treatment with 2.5 μM Martinostat elevated BDNF and SYP (about 20- and 10-fold, respectively) to a level equivalent to or greater than 10 μM SAHA ( Fig. 5B ). Together, these results indicate that Martinostat engages the subset HDACs that deacetylate targets including H3K9 and H4K12, to regulate downstream genes important for neuroplasticity (BDNF, EGR1, CDK5, SYT1, SYP, and GRN).

Competition autoradiography was performed in postmortem baboon brain tissue to compare the specific binding of [ 11 C]Martinostat in gray and white matter. [ 11 C]Martinostat binding in white matter was more biased by nonspecific uptake than in gray matter ( Fig. 4C ). Together, our in vivo imaging and ex vivo biochemistry data indicate that [ 11 C]Martinostat binds to a subset of class I HDACs (isoforms 1, 2, and 3) across the human and baboon brains.

( A ) Whole-cell lysates were prepared from postmortem human SFG and CC (n = 3 replicate donor pools with two donors per pool). Thermal shift assays were performed with increasing concentrations of Martinostat (0, 0.0032, 0.016, 0.080, 0.40, 2.0, and 10 μM). Thermal stabilization of HDACs 1, 2, 3, 6, and 8 was compared through Western blotting with scaled immunoreactive band intensity values represented as an averaged heat map (n = 3). The imaging-derived dissociation constant (K d ) for [ 11 C]Martinostat in NHP brain is indicated by the black arrow ( 19 ). See fig. S6 for original Western blotting data. ( B ) Whole-cell lysates were prepared from postmortem human SFG (n = 3 replicate donor pools with two donors per pool), as well as dorsolateral prefrontal cortex, hippocampus, and anterior cingulate (n = 3 replicate donor pools with three donors per pool). Thermal shift assays were performed with increasing concentrations of Martinostat (0, 0.16, 0.80, 4.0, 20, and 100 μM). Thermal stabilization of HDACs 1, 2, 3, 6, and 8 was compared through Western blotting with scaled immunoreactive band intensity values represented as an averaged heat map (n = 3). See figs. S7 to S9 for original Western blotting data. ( C ) Baboon brain (n = 1) was sectioned to include gray matter and white matter regions in the same slice. Tissue was coincubated with ~100 μCi of [ 11 C]Martinostat and either 0 or 2 μM nonradiolabeled Martinostat. Grayscale autoradiographic images were colored using a standard lookup table (royal scale in Image J) to reflect [ 11 C]Martinostat intensity (left). Region-specific baseline and blocking intensity values were quantitated from each slice (right). Data are means ± SD (n = 22 0-μM slices, n = 10 2-μM slices; one image per slice; one region of interest per brain region). P values were determined by ordinary two-way ANOVA (α = 0.05 with Sidak’s multiple comparisons correction).

Thermal shift assays evaluate target engagement, such that inhibitor binding increases the thermal stability of a target protein, as compared to a vehicle control ( 22 , 23 ). To determine the HDAC isoform selectivity of Martinostat, thermal shift assays were performed with clarified human brain homogenate and increasing concentrations of Martinostat. Martinostat stabilized HDAC1, HDAC2, and HDAC3 in both the SFG and the CC at nanomolar concentrations ( Fig. 4A , with individual biological replicates in fig. S6). No significant stabilization of either HDAC6 or HDAC8 (negative control) was observed. The former suggests differences between the accessibility of endogenous HDAC6 complex and Martinostat binding, relative to recombinant protein ( 18 ). To assess heterogeneity in HDAC isoform selectivity across gray matter regions, we compared SFG binding to the dorsolateral prefrontal cortex, hippocampus, and anterior cingulate. On the basis of thermal stabilization data, Martinostat exhibited a relatively uniform binding profile in gray matter with target engagement observed at concentrations around and above 0.160 μM ( Fig. 4B , with individual biological replicates in figs. S7 to S9).

HDAC2 and HDAC3 expression level differences between the SFG and the CC could not be attributed to nuclear density, according to quantification of the number of nuclei per field of view in postmortem baboon brain tissue (fig. S5). We observed that the CC had an increased number of nuclei compared to the SFG, which suggested that lower HDAC expression in the CC was not due to a depletion of cells in this brain region (fig. S5). As nuclear size (area per nucleus) was smaller in the CC than in the SFG, the total nuclear area per field of view was equivalent between these regions, further refuting that HDAC expression levels are driven by nuclear density.

Whole-cell lysates were prepared from postmortem human SFG and CC (n = 3 replicate donor pools with two donors per pool), as well as dorsolateral prefrontal cortex (DLPFC), hippocampus (Hipp), and anterior cingulate (Ant Cing) (n = 3 replicate pools with three donors per pool). ( A ) Equivalent amounts of total protein were compared to human recombinant HDAC standards through Western blotting. # The HDAC2 recombinant standard was tagged with glutathione S-transferase (GST), resulting in increased molecular weight. ( B and C ) Comparison of HDAC expression between white matter (CC) and gray matter (SFG) regions (B) and among different gray matter regions (C). HDAC immunoreactive band intensity values were normalized to glyceraldehyde-3-phosphate dehydrogenase (GAPDH) intensity values. HDAC expression levels were calculated per milligram of total extracted protein. Solid lines represent mean expression values. Donor pools are denoted by black, gray, and open circles. P values were determined by unpaired t test (B) and ordinary one-way analysis of variance (ANOVA) (α = 0.05 with Tukey’s multiple comparisons correction) (C).

To assess regional differences in [ 11 C]Martinostat binding in the human brain, we biochemically profiled postmortem brain tissue from gray matter regions [superior frontal gyrus (SFG), dorsolateral prefrontal cortex, hippocampus, and anterior cingulate] and a white matter region [corpus callosum (CC)] (table S5). Quantitative protein levels of HDAC1, HDAC2, HDAC3, and HDAC6 were determined by Western blotting ( Fig. 3A ). Significantly lower amounts of HDAC2 and HDAC3 were found in the CC relative to the SFG ( Fig. 3B ). No significant differences in HDAC expression were noted among the dorsolateral prefrontal cortex, hippocampus, or anterior cingulate—all gray matter regions ( Fig. 3C ). The average expression levels of HDAC2, HDAC3, and HDAC6 were similar in the SFG (0.12 to 0.16 pmol/mg total protein), with the notable exception of HDAC1 (1.7 pmol/mg total protein). Although high HDAC1 expression was observed across all brain regions tested ( Fig. 3 ), we cannot exclude the possibility that these values are driven by postmortem neuronal death ( 20 , 21 ).

Group-level analyses showed that the average gray matter SUV 60-90 min was nearly double that of white matter ( Fig. 2C , fig. S3, and table S4), and heterogeneous binding was observed among gray matter regions examined. Besides the white matter, the lowest [ 11 C]Martinostat uptake was observed in the hippocampus and amygdala, and the highest was observed in the putamen and cerebellum ( Fig. 2C , fig. S3, and table S4). To facilitate intersubject comparison of regional HDAC distribution, we normalized regional SUV 60-90 min to individual subjects’ white matter SUV 60-90 min as SUV 60-90 min ratios (SUVR 60-90 min ). SUVR 60-90 min showed that the regional distribution patterns of [ 11 C]Martinostat binding were consistent in all subjects ( Fig. 2C ) and on consecutive scans in single subjects. In preliminary test/retest scans (3 hours apart) in the same individual, SUVR 60-90 min showed less than 3% variability (fig. S4).

( A ) Mean images (left) and standard deviation (inset, to the lower right of each composite image) of SUV 60-90 min from healthy volunteers (n = 8). The images are overlaid onto the MNI152 standard brain, where x, y, and z indicate the coordinate of each image plane shown. ( B ) Correlation of regional V T values, derived from a two-tissue compartmental model using metabolite-corrected arterial plasma as an input function and SUV 60-90 min . Data are means ± SD (n = 6 subjects), and each circle symbol represents a separate brain region (n = 14 brain regions). P value determined with Pearson correlation analysis. ( C ) Regional SUV 60-90 min and SUV ratios (SUVR 60-90 min ) of cortical, subcortical, cerebellar, and white matter volumes of interest (VOIs). Individual pairs of brain regions that are significantly different from each other are listed in table S4. Each dashed line represents SUVR 60-90 min from a single subject (n = 8).

Regional standardized uptake values from 60 to 90 min after radiotracer administration (SUV 60-90 min ), an image-based indicator of binding to HDACs ( Fig. 2A ), correlated positively with V T values ( Fig. 2B ). The image-based SUV 60-90 min had less intersubject variability [coefficient of variation (CV) is 11.2 to 19.2% across brain regions] than the blood data–derived V T values (CV is 22.0 to 39.2% across brain regions) ( Fig. 2B ). SUV 60-90 min may therefore be an appropriate surrogate outcome measurement for V T and can be used in future studies to eliminate arterial blood sampling and reduce sample size because of its smaller variation. As with all surrogate measures, validation relative to a full treatment of the data using arterial blood in each patient population will be required.

( A ) [ 11 C]Martinostat (injected dose, 4.7 mCi; specific activity, 1.1 mCi/nmol) images averaged from 60 to 90 min after radiotracer injection (SUV 60-90 min ; SUV = radioactivity per injected dose per body weight) from a representative subject overlaid on anatomical magnetic resonance (MR) image. ( B ) [ 11 C]Martinostat SUVR 60-90 min images of individual subjects. To facilitate intersubject comparison of regional HDAC distribution, we normalized regional SUV 60-90 min to an individual subject’s white matter SUV 60-90 min as SUV 60-90 min ratios (SUVR 60-90 min ). The SUVR 60-90 min images were also coregistered with an MNI152 standard human atlas brain.

To visualize HDAC expression in the living human brain, we performed [ 11 C]Martinostat PET imaging on eight healthy volunteers (four males and four females; mean age ± SD, 28.6 ± 7.6 years) (table S1). The uptake of [ 11 C]Martinostat reached a maximum at ~30 min after injection and showed minimal decrease during the 90-min scan (fig. S1). The retention of radioactivity is a unique feature of [ 11 C]Martinostat, which allows for a stable quantification of HDAC expression levels. Regional heterogeneity, such as different levels of [ 11 C]Martinostat uptake between gray and white matter tissues, was observed at the individual subject level ( Fig. 1 ). Quantitative analysis using compartmental modeling on individual subjects’ dynamic PET data allowed us to determine the distribution volume (V T ), a measure of radiotracer binding that is normalized to the activity present in circulating blood, and rate constants describing the pharmacokinetics of [ 11 C]Martinostat (fig. S1 and tables S2 and S3). V T values were stable beyond 50 min, with less than 10% variability when compared to the 90-min data (fig. S2).

DISCUSSION

This first-in-human epigenetic imaging study with [11C]Martinostat establishes that HDACs are highly expressed throughout the healthy brain with region-specific distribution, including distinct differences between gray and white matter and differences between cortical and subcortical gray matter regions. On the basis of our previous in vitro profiling with recombinant HDACs (18) and our ex vivo profiling with postmortem human and baboon brain tissues, the [11C]Martinostat signal in the brain originated from binding class I HDACs (isoforms 1, 2, and 3), which are relevant to cognition, memory, and mood regulation (3, 13, 16, 25). Notably, Martinostat stabilized these isoforms at a concentration of ~0.1 μM, which is consistent with the imaging-derived dissociation constant (K d ) for [11C]Martinostat in the NHP brain (18). In contrast with previous in vitro recombinant inhibition data (18), Martinostat did not appear to stabilize HDAC6 in the brain regions that we assessed, although it is worth noting that the recombinant assay provided a more than fivefold lower median inhibitory concentration for HDAC6 when compared to isoforms 1, 2, and 3. At high concentrations, α-tubulin acetylation may be increased by Martinostat, thus implicating potential HDAC6 binding at therapeutic-relevant concentrations.

Our imaging data revealed that in vivo HDAC expression is higher in cortical gray matter than in white matter, which was confirmed for HDAC2 and HDAC3 by postmortem human tissue analyses. We postulate that HDAC complexes in each brain tissue type may affect the selectivity of Martinostat and other HDAC inhibitors, including those currently used as U.S. Food and Drug Administration–approved drugs. HDAC complex–directed selectivity of HDAC inhibitors has been shown previously through chemoproteomic approaches (26, 27), and additional work will be required to elucidate the HDAC complexes most represented by the [11C]Martinostat signal.

Beyond regional differences in HDAC distribution, the most striking observation was the consistency of [11C]Martinostat binding patterns between individual subjects. Because epigenetic machinery, and thus HDAC expression, is a highly dynamic process, we did not fully expect a spatially conserved pattern of HDAC expression between individuals. This result not only suggests that HDAC expression is tightly regulated and may represent a state function, but also reiterates the importance of localized levels of HDACs as they directly relate to gene transcription (1). We anticipate that regional [11C]Martinostat uptake differences between healthy and diseased individuals will be detectable given the conversed baseline expression that we have measured. The use of [11C]Martinostat imaging may eventually enable precision medicine approaches for disease stratification and treatment based on epigenetic aberrations in the human brain. As hippocampal HDAC2 overexpression has been found in postmortem brain tissue from AD patients (6), [11C]Martinostat PET imaging holds great potential for detecting aberrant hippocampal HDAC expression and assessing novel HDAC therapeutics in AD patients.

Because we envision and will apply [11C]Martinostat to measure HDAC expression in patient populations, it is critical that the outcome measurements are reliable, reproducible, and noninvasive. By comparing the standard deviation of the mean of V T and SUV 60-90 min across brain regions, we found that intersubject variability was smaller using SUV 60-90 min analysis than V T . These results support the use of SUV 60-90 min in future studies to eliminate arterial blood sampling when patient enrollment would be limited by the invasiveness and risk of this procedure. Perhaps as important, PET studies with [11C]Martinostat may be sufficiently powered with a smaller sample size when SUV 60-90 min is chosen as the outcome measurement instead of V T . However, validation studies will be required to evaluate whether SUVs are appropriate surrogates for V T values in different patient populations.

To begin to connect HDAC imaging with [11C]Martinostat to gene regulation in the human brain, we compared mRNA transcript level changes elicited by pharmacologically relevant doses of Martinostat in human stem cell–derived neural progenitor cells. The concentrations of Martinostat used to treat neural progenitor cells were ~1000-fold higher than tracer-level doses used for in vivo [11C]Martinostat imaging. Tracer-level doses are intended to achieve low occupancy and thus should not perturb HDAC enzyme activity and downstream gene expression. However, by using pharmacologically relevant doses for neural progenitor cell studies, the downstream targets of Martinostat-bound HDACs were revealed and provide insight into imaging signal interpretations. For example, these data suggest that in regions where [11C]Martinostat binding in the human brain is lowest, such as the hippocampus, the levels of HDAC-regulated genes, such as BDNF, are elevated. The hippocampus was previously shown to be consistently enriched in BDNF (17, 28–31).

Besides genes implicated in memory and neuroplasticity, Martinostat enhanced the mRNA expression of GRN encoding the glycoprotein progranulin. GRN mutations are a major cause of autosomal dominant frontotemporal lobar degeneration (14). The demonstration here that Martinostat treatment increases GRN mRNA levels supports the value of HDAC-targeted therapies as a disease-modifying treatment for this type of dementia. Moreover, because HDAC inhibitors are the subject of current clinical investigation for frontotemporal lobar degeneration, measuring HDAC expression in the human brain with [11C]Martinostat imaging may provide a critically needed tool for determining optimal doses of therapeutics and for patient stratification should levels of HDACs change in the disease state.

We recognize several limitations in our current study. First, the imaging data presented here are from a cohort of eight healthy subjects and thus we cannot characterize changes in “normal” HDAC expression (for example, as a function of age). Future studies will expand our imaging cohort to include more healthy subjects as well as multiple HDAC dysfunction-associated patient populations, including AD and schizophrenia, to investigate the in vivo relevance of HDAC expression in neurological and psychiatric diseases. Another limitation is that quantitative HDAC levels in postmortem brain tissue are relative to recombinant HDAC standards and do not reflect the absolute values of HDAC expression in the living brain, as postmortem HDAC levels may be affected by artifacts such as postmortem interval. Additionally, owing to the low throughput of thermal shift assays with Western blot–based detection and limited tissue availability, we found it necessary to pool multiple postmortem brain samples into three biological replicates, rather than analyze individual thermal shift assays for each donor, which may have revealed a higher variability of Martinostat selectivity. Last, neural progenitor cell studies uncovered only a subset of downstream Martinostat-bound HDAC substrates and gene targets. Future studies using acetyl proteomic profiling, RNA sequencing, and chemoproteomics are needed to fully understand the biological pathways detected by [11C]Martinostat.

In conclusion, this first-in-human epigenetic imaging study reveals that HDACs are highly expressed throughout the healthy brain with a conserved regional distribution between individuals. Our study uncovers region-specific variations in HDAC inhibitor binding, which we postulate is due to differences between the HDAC complex identities in those regions. Together, our neuroimaging and biochemical experiments provide a critical foundation for how to quantify epigenetic activity in the living brain and in turn accomplish HDAC inhibition in the CNS as a therapy for human brain disorders.