The researchers were interested in the M. tuberculosis enzyme pantothenate synthetase (PS) as a potential therapy for TB. Using a number of biophysical techniques including thermal shifts, NMR, and isothermal titration calorimetry, Abell and colleagues identified indole fragment 1 as a low-affinity binder from a library of about 1300 fragments (see figure below). X-ray crystallography revealed that the fragment binds in the ATP-binding site. An attempt to partially mimic the triphosphate by introducing negatively charged moieties led to modest improvements in potency (compounds 1a, 1b, and 2). Compound 2 bound in a similar position as compound 1, with the advantage that the methyl group off the sulfonamide is nicely positioned for further growing the molecule. Replacing this methyl group with a methylpyridine produced compound 4, increasing the affinity by about two orders of magnitude while maintaining ligand efficiency, and crystallography revealed that this moiety binds in the P2 pocket. Thus, the fragment growing approach began with an indole of low millimolar affinity and produced a molecule with low micromolar affinity after several iterations.
At the same time, the researchers also identified benzofuran fragment 5 (see figure below) and discovered that it binds in the P1 pocket some distance from the indole fragment 1, suggesting the two could be linked. In fact, a crystal structure revealed that the two fragments are able to bind to PS simultaneously. Linking these together through the acylsulfonamide linker employed above led to compound 8, with a potency similar to that obtained from fragment growing. Compounds 4 and 8 structurally resemble each other, but although the indole fragment of each binds in the same location, the terminal fragments (the methylpyridine in compound 4 and the benzofuran fragment in compound 8) bind in different locations, the former in the P2 pocket with the later in the P1 pocket. However, the benzofuran is somewhat twisted relative to the binding mode it adopts as a free fragment.
As the researchers observe, the ligand efficiency of compound 8 derived from fragment linking is lower than those derived from fragment growing, though even the molecules developed from growing have lower ligand efficiencies than the initial fragments.
The researchers conclude:
The two strategies resulted in similar compounds with similar potencies. This outcome obscures the fact that although the linking strategy appears more elegant, the limited repertoire of linkers is likely to compromise the binding of the original fragments. In comparison, the fragment-growing strategy provides more freedom for development at each stage and allows more room for further optimization.
True. But, the fragment linking strategy does provide a clear starting point for further optimization. The researchers did not describe how they selected the methylpyridyl fragment in compound 4 or how many other moieties they tested; 5-methylpyridine-2-sulfonamide does not seem like the first reagent one would grab from the shelf. However, the methylpyridine fragment is not dissimilar to the benzofuran fragment: swap the (hydrogen-bond accepting) oxygen for the (hydrogen-bond accepting) nitrogen, and the methyl would sit in a similar position as the phenyl ring (see figure above). In other words, medicinal chemistry on compound 8 could lead quite naturally to compound 4.