Objective

Fragment-based lead discovery is now a well-established as a powerful approach to early drug or lead discovery: since small (<250Da) compounds (“fragments”) tend to bind relatively promiscuously, hits can be readily identified by screening against comparatively small compound libraries (100s-1000s). What remains challenging is that hits typically bind weakly: not only must the screening technique be sufficiently sensitive, but potency can only be achieved through considerable synthetic elaboration. Historically, the most sensitive primary screening technique of all, direct observation in crystal structures, has been too challenging to be achievable by but a few labs world-wide. Equally, no consensus has yet emerged on systematic strategies for synthetic follow-up. Now, the XChem facility at beamline I04-1 at Diamond Light Source had established X-ray screening as a routine medium-throughput experiment with a capacity of up to 1000 compound soaks per week, implementing the full experiment from soaking to dataset. The facility has been taking regular users since October 2015, with dedicated weekly beamtime supporting over 30 per year. The highly streamlined process includes image recognition for crystal targeting, soaking by acoustic dispensing, robot-assisted harvesting, unattended X-ray data collection, automatic data integration, and pan-dataset electron density analysis for detecting hits. The technology was developed as a joint research project with the Protein Crystallography group of the SGC at Oxford University, and has been validated on a series of diverse targets, all of which have yielded hits. A “poised” fragment library has been developed that provides clear and robust routes to first-shell follow-up: combined with new algorithms for prioritizing compounds, the ultimate ambition is to establish how potency can be achieved cheaply from very limited initial experiments.