29 April 2014

Ninth Annual Fragment-based Drug Discovery Meeting, Part 2

The first major fragment event of 2014 drew around 500 people to San Diego last week. This is part of CHI’s three-day Drug Discovery Chemistry conference, and although the official FBDD track was only one of six, it is a testimony to the vitality of the field that fragments made appearances in most of the other sessions. With 17 talks in the FBDD track alone this post will not attempt to be comprehensive; Teddy has already shared some impressions here.

Jim Wells (UCSF) gave a magisterial keynote address that emphasized how useful fragments can be for tackling difficult targets such as protein-protein interactions (PPIs). In fact, many of the talks in the protein-protein interaction track relied on fragments. That’s not to say it’s easy. Rod Hubbard (University of York and Vernalis) emphasized that advancing fragments to leads against such targets can take a long time and often requires patience that strains the management of many organizations. Fragment hits against PPIs usually have lower ligand efficiencies (0.23-0.25 kcal/mol/HA if you’re lucky), and improving potency can be a bear. Rhian Holvey (University of Cambridge) presented a nice example of how she was able to find millimolar fragments that bind to the anti-mitotic target TPX2, potentially blocking its interaction with importin-alpha, but even structural information was not enough to get to potent inhibitors.

G-protein coupled receptors (GPCRs) were thought to be unsuitable for fragments until recently, but both Iwan de Esch (whose work has been profiled several times, including here and here) and Jan Steyaert (Vrije University) presented success stories. In fact, Jan has only been working with the Maybridge fragment library for a few months, but has found agonists, antagonists, and inverse agonists for several GPCRs.

Another example of a difficult target is lactate dehydrogenase A (LDHA). We’ve previously highlighted cases where fragment linking was used to get to nanomolar binders (here and here); Mark Elban (GlaxoSmithKline) presented an example of fragment growing and using information from a high-throughput screen (HTS) to get to nanomolar binders. Mark also discussed a particularly disturbing false positive: HTS had generated dozens of confirmed hits spanning 7 chemotypes, but upon closer inspection it turned out that all of them came from a single vendor, and that – unreported by the vendor – they were all oxalate salts. Oxalate is a low micromolar inhibitor of LDHA, and is invisible in proton NMR, so I’m sure this was not fun to track down.

Ben Davis (Vernalis) also presented great examples of false positives and false negatives, and how to avoid them. In particular, the WaterLOGSY NMR technique is great for weeding out aggregators when run in the absence of protein.

A common theme throughout the conference was the integration of fragments with other methods, such as HTS. Nick Skelton (Genentech) actually titled his presentation “Fragment vs. HTS hits: does it have to be a competition?” Kate Ashton (Amgen) discussed how using information from a fragment screen helped solve pharmacokinetic issues with an HTS-derived hit. And Steven Taylor (Boehringer Ingelheim) presented a similar example (also covered here) of using fragments to fix a more advanced lead. Steven noted that fragment-based methods are now fully integrated into the organization, which marks a significant change from Sandy Farmer’s presentation at this meeting four years ago.

The roundtables are great opportunities to swap ideas and get feedback; Teddy already mentioned the excellent roundtable he chaired, but I wanted to also give a shout-out to one organized by Derek Cole (Takeda) focused on "practical aspects of fragment screening." We recently discussed discussed fragments that destabilize proteins in thermal shift assays, and it turns out that folks from both the Broad Institute and Takeda have also crystallographically characterized such fragments. There was the sense that either stabilizers or destabilizers should be considered hits, though the latter were less likely to lead to crystal structures than the former.

Finally, on the subject of library design, Damian Young (Baylor College of Medicine) described using diversity-oriented synthesis (DOS) to generate more “three-dimensional” fragments. He is planning to build a library of roughly 3000 fragments which he hopes to make widely available to the community; these should help answer the question of whether the third dimension is really an advantage.

The importance of library design was also emphasized by Valerio Berdini (Astex); they are currently on their seventh generation library, about 40% of which is non-commercial, and half of whose members have been solved in one or more of 6000+ crystal structures. Relevant to the rule of three, Astex is moving to ever smaller fragments, with an average of 12.6 non-hydrogen atoms, ClogP = 0.6, and MW = 179. Indeed, despite assertions that PPIs may require larger fragments, Rod noted that at Vernalis the average fragments hits against PPIs are only slightly larger (MW = 202 vs 189 against all targets) and more lipophilic (ClogP 1.2 vs 0.8).

CHI has already announced that next year’s meeting will be held in San Diego from April 21-23. As it will be the ten year anniversary, they’re planning something big, so put it on your calendar now!

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