Teddy recently summarized how fragment-based approaches have been used to develop bromodomain inhibitors at GlaxoSmithKline, and they’re certainly not alone: Mark Bunnage of Pfizer described how they used fragment-based methods to discover nanomolar inhibitors of BRD4 that are active in cells. This work was done in collaboration with the Structural Genomics Consortium (SGC), and one well-characterized compound, PFI-1, is being released as a chemical probe to the worldwide research community with no intellectual property entanglements.
There was a heavy emphasis on metrics, and as we saw in the poll last year ligand efficiency and LLE (sometimes also called LipE) seem to be dominant. However, the latter metric is used more for advanced leads than for fragments. Siegfried Reich from the Lilly Biotechnology Center (neé SGX) said of LLE that it is less important where you start than where you end up, though starting from a very polar fragment gives you the luxury of adding lipophilicity during optimization.
Slight changes to fragments can often cause them to bind in different orientations, and the same fragment may bind differently in closely related proteins, but Siegfried argued these multiple orientations could be advantageous by providing multiple opportunities for optimization. Siegfried also mentioned that deconstruction of HTS hits to fragments has been successful at identifying fragments with high ligand-efficiency that could subsequently be optimized to new series.
Richard Law of Evotec gave as clear account as possible of fragment molecular orbital (FMO) calculations, a high-level quantum mechanical method for understanding protein-ligand interactions. Although quantum mechanical calculations are notorious for taking hours, days, or even weeks to run, the calculations can be done much more efficiently by breaking larger molecules into fragments.
Richard also kept thorough notes at a break-out discussion I moderated, and was kind enough to share them; I’ve also made a few additions. There were nine people from both large and small organizations, all but one from industry.
- As we’ve seen here, SPR seems to be the most common fragment-finding technique; in his presentation, Walter Huber said that it is the primary screening method at Roche.
- "Reverse SPR" - the Graffinity/Novalix technology we’ve discussed previously, has been applied to over 100 targets. It is reversed because the small molecules are immobilised on the chip, in multiple orientations to present different moieties to the target protein. This is also a larger library (~25,000).
- NMR was not being used as much as SPR, and no one at the table was using 19F NMR, though its use does appear to be growing, and compound suppliers are coming out with 19F fragment libraries.
- FCS++ is being used at Evotec; it has the advantage of being high-throughout but still highly sensitive and therefore accommodates a larger than average fragment library (~20,000).
- There is increasing desire for sp3 (3-dimensional) fragments and a move away from planar fragments, though one participant had seen chemists shy away from too many chiral centers.
- Advantage of more vectors for SBDD, and additional solubility versus otherwise equivalent flat compounds.
- Despite these advantages, often planar fragments yield more hits - likely because planar compounds are more likely to form dispersive/non-specific interactions, whereas sp3-fragments must form very specific H-bond interactions in order to bind. Does sp3 therefore also enrich for enthalpic binders?
Use of indices
LE was used by everyone at the table, whereas LipE/LLE were not used until lead compound stage. BEI/SEI and other metrics were not really used.
FBLD vs HTS
Fragment screening is being used on most programs at many companies, in parallel with HTS and virtual screens. A subset of targets is addressed only with fragment screening either because of target-specific information or specific requirements to lower costs of screening.
Fragment screening of GPCRs
There was very limited experience using FBLD on G-protein coupled receptors, though one company is trying to use nanodiscs to stabilize GPCRs for fragment screening. As more crystal structures are solved people may become more comfortable tackling this target class.
Finally, there was widespread agreement that fragments are ideal for designing in the desired physico-chemical properties of molecules as fragments are developed. The parent fragment is not biasing, and could often be med-chemed away. Richard Law offered the analogy of a rock-climber:
An HTS gets you halfway up the cliff, but the route to the top may not be from where you are, or may just be too difficult to find from that position. Whereas a fragment hit is at the base of cliff but enables you to see and select the exact route to the top that you need.
If you attended the break-out discussion or the meeting please comment on your impressions, and if you missed this conference don’t worry – there are many more great events coming up throughout the year.