So here's the GSK paper on applying the DNA-encoded library technology to a protein-protein target. I'm particularly interested in seeing the more exotic techniques applied to hard targets like these, because it looks like there are plenty of them where we're going to need all the help we can get. In this case, they're going after integrin LFA-1. That's a key signaling molecule in leukocyte migration during inflammation, and there was an antibody (Raptiva, efalizumab) on the market, until it was withdrawn for too many side effects. (It dialed down the immune system rather too well). But can you replace an antibody with a small molecule?
A lot of people have tried. This is a pretty well-precedented protein-protein interaction for drug discovery, although (as this paper mentions), most of the screens have been direct PPI ones, and most of the compounds found have been allosteric - they fit into another spot on LFA-1 and disrupt the equilibrium between a low-affinity form and the high-affinity one. In this case, though, the GSK folks used their encoded libraries to screen directly against the LFA-1 protein. As usual, the theoretical number of compounds in the collection was bizarre, about 4 billion compounds (it's the substituted triazine library that they've described before).
An indanyl amino acid in one position on the triazine seemed to be a key SAR point in the resulting screen, and there were at least four other substituents at the next triazine point that kept up its activity. Synthesizing these off the DNA tags gave double-digit nanomolar affinities (if they hadn't, we wouldn't be hearing about this work, I'm pretty sure). Developing the SAR from these seems to have gone in classic med-chem fashion, although a lot of classic med-chem programs would very much like to be able to start off with some 50 nM compounds. The compounds were also potent in cell adhesion assays, with an interesting twist - the team also used a mutated form of LFA-1 where a disulfide holds it fixed in the high-affinity state. The known small-molecule allosteric inhibitors work against wild-type in this cell assay, but wipe out against the locked mutant, as they should. These triazines showed the same behavior; they also target the allosteric site.
That probably shouldn't have come as a surprise. Most protein-protein interactions have limited opportunities for small molecules to affect them, and if there's a known friendly spot like the allosteric site here, you'd have to expect that most of your hits are going to be landing on it. You wonder what might happen if you ran the ELT screen against the high-affinity-locked mutant protein - if it's good enough to work in cells, it should be good enough to serve in a screen for non-allosteric compounds. The answer (most likely) is that you sure wouldn't find any 50 nM leads - I wonder what you'd find at all? Running four billion compounds across a protein surface and finding no real hits would be a sobering experience.
The paper finishes up by showing the synthesis of some fluorescently tagged derivatives, and showing that these also work in cell assay. The last sentence is : "The latter phenomena provided an opportunity for ELT selections against a desired target in its natural state on cell surface. We are currently exploring this technology development opportunity." I wonder if they are? For the same reasons given above, you'd expect to find mostly allosteric binders, and those already seem to be findable. And it's my impression that this is the early-stage ELT stuff (the triazine library), plus, when you look at the list of authors, there are several "Present address" footnotes. So this work was presumably done a while back and is just now coming into the light.
So the question of using this technique against PPI targets remains open, as far as I can tell. This one had already been shown to yield small-molecule hits, and it did so again, in the same binding pocket. What happens when you set out into the unknown? Presumably, GlaxoSmithKline (and the other groups pursuing encoded libraries) know a lot more about than the rest of us do. Surely some screens like this have been run. Either they came up empty - in which case we'll never hear about them - or they actually yielded something interesting, in which case we'll hear about them over the next few years. If you want to know the answer before then, you're going to have to run some yourself. Isn't that always the way?