Histone deacetylases (HDACs) are epigenetic writers that – as their name suggests – remove acetyl groups from lysine residues in histones. They have been pursued as anticancer targets for decades; vorinostat was approved back in 2006 (and arguably has fragment origins). The catalytic site of HDACs contains a zinc ion, and many inhibitors include zinc-binding moieties, often hydroxamic acids. However, the high affinity of these metallophilic fragments leads to inhibition of other zinc-containing enzymes, causing toxicity. In a new ACS Med. Chem. Lett. paper, Emiliano Tamanini, Shin Miyamura, and colleagues at Astex and Otsuka provide alternative chemistries. (Emiliano presented part of this story at Pacifichem last year.)
The researchers were specifically interested in HDAC2, which has been implicated in neurodegenerative diseases such as Alzheimer’s. Because of the chronic nature of these conditions, selectivity was all the more important, as was brain penetration. Screens of the Astex fragment library, along with a set of known zinc-binding fragments, yielded 35 crystallographically validated hits. These included compound 3, which forms bidentate interactions with the catalytic zinc through the amine and carbonyl moieties.
An additional fragment (compound 4, not shown) bound in a pocket at the “foot” of the catalytic site, which is normally partially blocked by a side chain residue. Yet another fragment bound near the entrance tunnel. Growing compound 3 in both directions led to compound 7, the first molecule of the series with measurable activity in a fluorescence assay. Rigidification and further growing led to compound 9, with low micromolar activity.
Interestingly, merging compound 9 directly with foot-pocket binder compound 4 did not improve activity, but adding a chlorine atom to fill a small subpocket increased affinity two-fold (compound 13). Finally, optimization of the moiety at the entrance tunnel (the isoindoline of compound 13) yielded compound 17, with high nanomolar potency.
Compound 17 caused increased acetylation of H4K12 in cells. More importantly, it showed good brain exposure when orally dosed in mice, and H4K12 acetylation was observed in mouse brain tissue. The compound hits HDAC1 and HDAC8 with similar potency as HDAC2, and these could be contributing to the biological effect. Nonetheless, while potency and selectivity still need to be improved, compound 17 is an attractive lead for further optimization.
In a comment to the Notum work we highlighted back in May, Daniel Beck wondered whether “fragments are especially good starting points for CNS target campaigns.” This paper suggests the answer is yes.