This past July Practical Fragments was devoted almost entirely to bromodomains, an important type of epigenetic protein. Protein lysine methyltransferases (PKMTs) are another significant class: 51 human enzymes that transfer a methyl group from the cofactor S-adenosylmethionine (SAM) to the side chain amine of lysines, typically in histones. In two recent papers in ACS Med. Chem. Lett., researchers from Novartis describe how they discovered inhibitors of DOT1L, a target for certain leukemias.
The first paper, by Frédéric Stauffer and colleagues, started with a fragment screen of the DOT1L catalytic domain using surface plasmon resonance (SPR). This led to the discovery of compound 1, which is teetering on the edge of molecular obesity (at least for a fragment) but did show activity in a functional assay as well as binding by NMR. Moreover, a co-crystal structure revealed that it binds in a new pocket near the SAM binding site, primarily through hydrophobic and stacking interactions.
Replacing the potentially unstable pyrrole with a quinoline led to compound 3, and subsequent structure-based design led to compound 5. Interestingly, while the methoxy substituent on compound 5 was installed to form a hydrogen bond with the protein, this instead caused a shift in binding mode – a reminder that fragments don’t always retain their original orientations during optimization. This new binding mode provided a vector to grow through a narrow channel into another pocket, ultimately resulting in compound 8, with low nanomolar activity.
The second paper, by Christoph Gaul and colleagues, started with a high-throughput screen (HTS). One low micromolar hit turned out to be a (felicitous) regioisomeric impurity from a commercial supplier. Crystallography revealed that this binds in the same pocket as the fragment in the previous paper, and subsequent medicinal chemistry led to low nanomolar inhibitors such as compound 3’. Unfortunately these turned out to have low permeability, probably due to the high number of hydrogen bond donors and acceptors. Fragmenting compound 3’ led to compound 4’, with a dramatic loss in potency, but structure-based design ultimately led to potent molecules such as compound 12’. This compound is also selective against other PKMTs, cell active, and orally bioavailable in rats.
These two papers provide a nice window into the complexity of lead discovery. In contrast to other examples, the fragment made largely hydrophobic interactions, while the HTS hit made numerous hydrogen bonds. Both hits bound in a new pocket, a reminder that secondary ligand binding sites are common. And in both cases, extensive medicinal chemistry was necessary and led to molecules that scarcely resemble their starting points. Interestingly, a previously described clinical candidate against this target, EPZ-5676, was identified by yet another approach: structure-based design starting from the cofactor SAM. All of which is to say that there are lots of ways to find inhibitors, and they don’t always fall into neat categories.