One of the cornerstones underpinning fragment-based lead discovery is molecular complexity: fragments are less complex than larger molecules, and are thus likely to bind to more sites on more proteins. In theory, then, you want relatively simple fragments, and in fact Astex has actually formalized this with the concept of the “minimal pharmacophore”, in which each fragment contains a single pharmacophore (such as a hydrogen bond donor next to a hydrogen bond acceptor). But this is not the only way to build a fragment library; in 2016 we noted a paper out of the University of Dundee describing fragment libraries built with “caps” for easy derivatization. In a new paper in ChemMedChem, Paul Wyatt, Peter Ray, and collaborators at the University of Dundee and GlaxoSmithKline describe a screen with this “functional group complexity” (FGC) library.
The researchers were interested in the protein InhA, a drug target for Mycobacterium tuberculosis, the organism causing the eponymous disease. A relatively small library of 1360 fragments was assembled from six different sources, loosely defined by the authors:
- 573 commercial fragments
- 170 “3D” fragments from the 3DFrag consortium
- 326 of the designed FGC fragments
- 46 commercial fragments chosen based on known InhA inhibitors
- 124 “inventory” fragments
- 121 “project” fragments
Previous work had suggested that more potent molecules seemed to reduce the STD signals for the NADH cofactor, so these molecules (32 fragments) were prioritized. The 13 FGC fragments represented a hit rate of 4%, nearly double the 2.4% for the library as a whole.
All 149 of the initial fragments were tested in a biochemical assay at 0.5 mM, but only 4 gave measurable inhibition – too few to draw conclusions. Five compounds were characterized crystallographically bound to InhA, including two of the FGC fragments. This information was used to merge two fragments, compound 24 (an FGC fragment) and compound 12 (a commercial fragment), yielding a mid-micromolar inhibitor. Adding a “magic methyl” gave a satisfactory ten-fold boost in potency. Fragment 24 was also merged with a previously reported molecule, compound 3a, to produce compound 42.
These results suggest that more heavily functionalized fragments don’t necessarily have a lower hit rate, albeit for a small library and a single target. And as we noted last year, molecular complexity is difficult to define; it is not immediately obvious that FGC fragment 24 is actually more complex than commercial fragment 12. The old cliché still holds: more data are needed.