Chemoproteomics Sheds Light on Epigenetic Targets of [¹¹C]Martinostat in the Human Brain

Initiation of research programs to investigate binding specificity based on in vivo positron emission tomography (PET) imaging results can provide rich opportunities to improve data interpretation, gain biological insight, and inform hypothesis development. Here, we profile the binding specificity of the neuroepigenetic imaging probe, [¹¹C]Martinostat. In vivo neuroimaging studies using [¹¹C]Martinostat have uncovered differential regional uptake in relation to age and biological sex and in patients with schizophrenia, bipolar disorder, Alzheimer's disease, and low-back pain compared to healthy controls. Previous studies using recombinant proteins and thermal shift assays in postmortem tissue indicate that [¹¹C]Martinostat engages class I and putatively class IIb histone deacetylases (HDACs). While HDACs serve multiple functions, including regulation of chromatin remodeling and gene transcription, it is not known how differences in HDAC expression may arise across brain regions. HDACs functionally interact with a diverse array of multisubunit complexes, and engagement with associated binding partners may contribute to these differences. To further assess target engagement of [¹¹C]Martinostat, we designed a synthetic probe based on the inhibitor structural scaffold for use in competition experiments followed by proteomic analysis in postmortem tissue. The synthetic probe, called Compound 4, appears to interact with the class I HDAC paralog HDAC2 and the class IIb paralog HDAC6 in a robust manner. We also uncovered unique interacting partners, including synaptic proteins from the synaptotagmin (SYT) family of proteins and neuronal pentraxin 2 (NPTX2). Further work to investigate HDAC associations with interacting proteins across regions of the human brain is needed to better understand neuroepigenetic dysregulation in psychiatric and neurological conditions.