Membrane proteins such as G-protein coupled receptors (GPCRs) represent a large fraction of drug targets. These are mostly overlooked by the fragment community, for two reasons. First, assays for low affinity binders are difficult to develop. Second, the proteins usually lack structural information useful for advancing fragment hits. Earlier this year Heptares provided a lovely solution to both problems by generating stabilized mutant GPCRs, which could be screened using surface plasmon resonance (SPR) and characterized crystallographically. In a new paper in ACS Med. Chem. Lett., Iva Navratilova, Andrew Hopkins, Robert Lefkowitz, and a multinational team at the University of Dundee, Duke University, and the University of North Carolina Chapel Hill report using SPR to screen fragments against a wild-type GPCR.
The researchers chose the human β2 adrenergic receptor, which has served as a model GPCR for a variety of biological and biophysical studies. They expressed this with a His10 tag on the C-terminus and used a conventional nickel chip to immobilize the protein in the presence of detergent. The immobilized protein was able to bind to a known agonist and antagonist with dissociation constants similar to those reported in the literature, suggesting that it was folded correctly.
Next, 656 fragments were screened against the protein at 50 micromolar each. Using a surface containing β2 adrenergic receptor blocked with a known high-affinity, slowly dissociating agonist as a reference, the researchers looked for fragments that bound selectively to the surface containing the unblocked protein. A total of 81 fragments were then examined more closely in dose-response curves, yielding five confirmed hits, with dissociation constants ranging from 17 nM to 22 micromolar.
All five of these hits were tested in a conventional radioligand competition assay, confirming their binding. Interestingly, four of the five ligands were N-arylpiperazines, a class of molecules that the Heptares team also found as ligands for the β1 adrenergic receptor. When tested against this GPCR most were not selective, but one did show some selectivity for β2 adrenergic receptor against a panel of 27 GPCRs.
The fragment hits were then tested for activity in a cell-based assay, and all of them inhibited signaling. This illustrates a general complication with binding (versus functional) assays: with simple enzymes, once you’ve found a binder, it is probably either an inhibitor or has no effect. With GPCRs, a binder could be an agonist, an antagonist, a partial agonist, an inverse agonist, a neutral antagonist, or something else entirely; you need to go into cells quickly to figure out what you’ve got.
I do wonder whether it would be possible to screen at higher concentrations to look for weaker ligands, particularly for more challenging GPCRs for which no small molecule ligands are known. Still, the fact that SPR works as well as it does for a native GPCR is quite impressive. I suspect that we’ll see more and more fragment screening by SPR on membrane proteins. Whether folks will be comfortable optimizing fragment hits in the absence of high-resolution structures, though, remains to be seen.