FBLD 2014, the fifth in an illustrious series of conferences, took place in Basel, Switzerland last week. Organizers Wolfgang Jahnke (Novartis), Michael Hennig (Roche), and Rod Hubbard (University of York & Vernalis) put together a fantastic event. With 35 talks, 45 posters, and more than 200 delegates, I won’t attempt to give more than a few impressions here. In addition to Teddy’s (and others’) Tweets, Derek Lowe put up several posts (see here, here, and here); please share your thoughts below.
Harren Jhoti delivered a lively and wide-ranging opening keynote summarizing the past 15 years of FBLD as viewed from Astex. Among many other innovations, researchers there are responsible for the Rule of 3, which has been the subject of some debate. Harren emphasized that the “Voldemort Rule” should not be a strait-jacket. Like the Kobayashi Maru, rules are there to be broken, though you need to be something of a James T. Kirk to do so effectively.
Astex has produced what is likely the largest collection of protein-fragment crystal structures, and Harren noted that many proteins appear to have fragment binding sites outside the active site: across 25 different proteins, the average number of total sites is slightly greater than 2. Astex is increasingly targeting these sites for allosteric lead discovery.
The theme of crystallography carried through the conference. As Armin Ruf (Roche) exhorted, “more crystals, more structures.” One challenge is that not all crystal forms are suitable for fragments, and it is not always clear from the outset which forms will work. Armin described their chymase project in which an initial crystal form gave 8 fragment structures, but additional crystal forms allowed them to obtain 6 more. Without the different crystal forms these later fragments would have been crystallographic false negatives, yet the potential of different crystal forms to reveal more hits is under-appreciated: Armin noted that the majority of recent fragment papers reported using only a single crystal form.
The importance of crystallography was emphasized again by Nick Skelton (Genentech), who discussed their NAMPT program (which we covered here). In this project, which utilized dozens of crystal structures, a single atom change to a fragment could completely and unpredictably alter the binding mode.
Obtaining a good crystal is not necessarily easy, though. Andreas Lingel (Novartis) described their efforts to produce a form of B-RAF that would diffract to higher resolution and allow fragment soaks (as opposed to co-crystallization). They tried reducing the “surface entropy” by mutating glutamate and lysine residues to alanine, but only one of a dozen or so mutants expressed well and gave superior crystals. Although this turned out to be useful, the team is still at a loss to explain why the specific mutants are effective.
Continuing the theme of crystallography, Matt Clifton (Beryllium) described what looks to be a significant advance for the protein MCL-1. (This is a collaboration with the Broad Institute, and we previously noted some of their progress here.) The researchers have developed a maltose-binding protein (MBP) fusion of this oncology target that diffracts to 1.9 Å in the absence of any ligand. (MBP fusions are used to help crystallize challenging proteins.) Since they developed this construct in May of this year, the researchers have already solved more crystal structures than had been reported publicly to date, and uncovered some interesting findings. For example, the initial fragment that Steve Fesik’s group found binds in a similar manner to one of his more potent later leads, as does one of the AbbVie fragments; however another AbbVie fragment binds in a somewhat different fashion than the elaborated lead.
The subject of how to effectively sample chemical space was another theme, and to this end Alison Woolford (Astex) proposed the concept of a “minimal pharmacophore”: the minimal interactions necessary to drive fragment binding. Researchers at Astex have systematically cataloged several dozen of these, which include such simple entities as amines, acids, aromatic chlorides, and more abstract concepts such as a 1-bond donor-acceptor (think pyrazole). Alison showed an interesting graph with targets on the y-axis and minimal pharmacophores on the x-axis which revealed some obvious patterns such as the preference of donor-acceptor minimal pharmacophores by kinases, but there were unexpected features as well. In a sense, this is an empirical realization of early docking studies, but it also has interesting implications for library design. For example, Alison suggested avoiding fragments with more than one minimal pharmacophore, as these will not be able to effectively sample as many different sites on a protein: with two pharmacophores, a fragment would be limited to binding sites having matching recognition elements to both rather than just one. This idea ties in with the concept of molecular complexity, but from a more chemocentric point of view.
On the subject of chemistry, Dalia Hammoudeh (St Jude’s Hospital) gave a lovely talk on her experiences developing allosteric inhibitors of DHPS, an antibiotic target. Among other fragment hits from the Maybridge library, one was ostensibly 4-trifluoromethylbenzylamine, but turned out to actually be the Schiff base of this with the corresponding aldehyde. Yet another reminder to always carefully check what you think you have.
Practical Fragments has previously discussed the Genentech MAP4K4 program (here and here), and Terry Crawford gave a nice summary. One of the challenges they faced was that their initial leads had excellent brain penetration, leading to animal toxicity. This forced them to increase size and polar surface area. Although it was problematic in this case, this emphasizes how small and drug-like fragment-derived leads can be. Indeed Vicki Nienaber, who was a prime mover behind the original FBLD 2008 meeting, has devoted much of her efforts at Zenobia to CNS targets.
Finally, Derek Lowe (Vertex) gave a rollicking history of the drug industry, ending with his view of where fragments fit in. He noted that chemists – Valinor not withstanding – play a central role, and in that sense the field is a departure from the general trend of the past decade or so. It still remains to be seen how many of the 30+molecules FBLD has delivered to the clinic will come out the other side, but at least for now the field is thriving. As Chris Lipinski stated last year, “if I had to single out one technology that really took me by surprise and has been very successful, it has been fragment screening.”