The first era of medicinal chemistry was finding small molecules to target proteins. It's still going strong and will continue to do so. The second era that promises a treasure trove at least in principle is finding small molecules for disrupting protein-protein (PP) interactions. So many important processes in our body are regulated by these crucial interactions that finding small molecules to modulate them could promise a bonanza of new therapies.
But disrupting PP interactions is fundamentally different from designing a small molecule to bind to a (mostly) well-defined active site on a protein. PP interaction surfaces are rather flat and expansive and depend on subtle interactions between amino acids that add up to provide substantial binding affinity. Designing a small molecule to disrupt such interactions is somewhat like disrupting the sliding of one door hinge against another by lodging a grain of sand between the two. However, the picture has been simplified somewhat in recent years by the identification of "hotspots"- key amino acid epitopes that provide the bulk of binding interaction. If one could design a small molecule that could provide this major contribution to the free energy of binding, one could have an effective drug that could target PP interactions. After all, a grain of sand can indeed inhibit hinge sliding if it's placed in the right position.
One of the big players in investigating small molecule PP interaction agents has been Jim Wells, formerly at Genentech and Sunesis and now at UCSF. He has a nice Naturereview that traces recent successful examples of small molecule PP interaction antagonists. Wells considers six or seven successful stories involving important proteins playing key roles in health and disease. For example, disrupting the inhibition of the pro-apoptotic BAD and BAK by the anti-apoptotic Bcl-2 and Bcl-xl, disrupting the binding of interleukin IL-2 to its receptor, and disrupting the binding of HDM2 to the tumour suppressor p53.
But more importantly, Wells then address some widely held myths about PP interactions that seem to drive pessimism in the field. They are worth taking a look at:
Myth 1: It's very difficult to find a small molecule that would lodge between the rather disordered flat surfaces of two proteins.
While this is true, in practice as is exemplified by almost all the examples, the protein surface does not remain flat when a molecule binds to it. Loops and side chain adapt and form shallow pockets and dents to which the molecule can bind. The fallacy in believing the myth is to assume that small-molecule protein surface binding is a rigid body interaction. It's not, and there's a strong element of induced fit in the process. This is probably the single most important thing to keep in mind, that small molecules will form their own small pockets and bind well to initially flat protein surfaces.
Myth 2: Small molecules that disrupt PP interactions are too large to be drugs
This is part of the partly substantiated myth that large molecules usually don't become drugs, because of many factors including ROF problems. But many molecules disrupting PP interactions cited in the review are about 500-700 Da, perhaps a little large but not intractable as drugs. The authors also calculated the ligand efficiency which is the free energy of binding per non-hydrogen (heavy) atom for the ligands, and found that it was comparable to that of kinase or protease inhibitors. Clearly with sound med chem efforts, it won't be too difficult to have such drugs. Interestingly, since the molecules occupy about half the binding site that the parts of the native protein partner do, their ligand efficiency is almost twice.
Myth 3: Small molecules disrupting PP interactions won't be potent
Just not true. Almost all the molecules found in the cited examples had mid to low nanomolar Ki values, almost as good as the binding constants for the partner proteins.
Myth 4: Screening would not help find novel small molecule PP modulators
Again, not true. Most of the cited molecules were found by HTS. Interestingly, there may be even more wealth in HTS than we have now. As the authors explain, HTS hits are fundamentally going to be limited by chemotypes present in the libarries. After all we can do only as well as chemical space in existing libraries. Existing libraries contain many molecules targeted against kinases, GPCRs and other well-known targets for which common privileged structures have been deduced. But because of the diversity of PP interactions, it is improbable that common scaffolds will exist for disrupting them, and libraries will have to contain novel scaffolds to get better hits. Given this fact, it's impressive and encouraging that the existing libraries could come up with such potent structures for disrupting a few PP interactions. Remarkably, even with such different scaffolds, the ligand efficiency remains more or less constant for the cases studied.
Clearly the field of small-molecule-PP interactions is alive and kicking. In the next few years, hopefully computational, screening and NMR approaches will converge to discover novel agents for these important processes.
Wells, J.A., McClendon, C.L. (2007). Reaching for high-hanging fruit in drug discovery at protein-protein interfaces. Nature, 450(7172), 1001-1009. DOI: 10.1038/nature06526