Fragments vs PDE10A: Astellas’ turn

The 11 members of the phosphodiesterase (PDE) family cleave cyclic nucleotides such as cAMP and cGMP to regulate cell signaling. These enzymes are established drug targets – sildenefil inhibits PDE5, for example. PDE10A inhibitors have been heavily investigated for a variety of neurological disorders, and fragments have played a role in several efforts: we’ve highlighted work from Merck, AstraZeneca, and Zenobia/PARC on this target. A new paper in Chem. Pharm. Bull. by Ayaka Chino and colleagues describes work from Astellas.

A previous HTS screen at the company had led to a series of low nanomolar inhibitors, but these had metabolic liabilities and also inhibited CYP3A4. Thus, the researchers turned to fragments. No details are given as to library size, screening method, or hit rate, though it is worth noting that Astellas has previously reported fragment screening by crystallography. Compound 2 turned out to be a hit, and examination of the crystallographically determined binding mode proved quite useful. (Astute readers will also note the similarity of compound 2 to one of the Merck fragments.)

Because the chlorophenyl moiety was pointing towards solvent, the researchers decided to lop this off  to lower both lipophilicity and molecular weight. Previous publications had also revealed the presence of a “selectivity pocket”, and the researchers therefore grew towards this pocket, yielding molecules such as compound 7. Further tweaking led to compound 13, with low nanomolar potency. In contrast to the HTS-derived lead, this molecule was metabolically stable in vitro and showed negligible inhibition against a panel of 13 CYP enzymes.

This is a nice – albeit brief – example of how fragments can generate new chemical matter even against an extensively explored class of enzymes. Plenty of questions remain around pharmacokinetics, selectivity, and brain penetration, but the paper does end by promising that more will be revealed.