When the market is more than 20 Billion dollars, you will find everyone working there. And so, with this recent publication, we have another entrant into the BACE inhibitor from Fragments competition, discussed previously here. This is the fifth by my count, the first being from Astra Zeneca.
In this paper, Eli Lilly describes their efforts using fragments to generate "the first orally available non-peptidic BACE1 inhibitor that produces profound Abeta-lowering effects in animals." They screened ~8000 compounds at 4.76mM that generated a number of low-affinity, but highly "LEAN" fragments (discussed below). Of most interest were the amino-benzothiazine (1) and amino-thiadiazine (2) compounds.
The authors note that co-crystallilzation was a key advance for their understanding of this system. The co-crystal showed two copies of (1) with high active site occupancy and in the "open-flap" conformation. One copy engaged the catalytic dyad and the other spanned the S1-S3 cavity. This data let them recognize that the planarity of the molecules were not optimal for fragment growth, so they "de-planarized" them, leading to (3). Only one enantiomer of (3) was recognized by BACE. The co-crystal of this compound showed binding identical to the original fragment, one copy engaging the catalytic dyad and one in the S1-S3 region. Addition of the S3 moiety pyrimidine led to (4). Fluorination of the central ring reduced in vivo clearance and and realized a significant increase in potency, while maintaining atom efficiency (5).
The crystal structure of (5) shows that this molecule retains an optimal H-bonding network, efficiently traverses S1, and projects the pyrimidine into S3.
Compound (5) was tested in animal models and pre-clinically in healthy human volunteers given orally. It showed significant reduction in Abeta levels in brain and CSF. Retinal pathology became a concern in longer term animal studies and the compound was not taken any further.
This paper shows the power of Fragments in discovering novel scaffolds for important targets. It is also important to note that the modified fragment hit retained the same binding as the original fragment hit.
The other contribution that the Lilly group brings out in this paper is the concept of LEAN (Ligand Efficiency by Atom Number): -log (IC50)/Number of heavy atoms. This is one of many ways people have developed to gauge the efficiency of their ligand hits, I think this is the simplest to use. As can be seen from the Lilly data, a LEAN of >=0.30 is an efficient molecule. For those of us who don't do logs in our head well, this lends it itself to a simple cheat sheet:
I can send a copy of this spreadsheet to anyone who wants.