Discovery and SAR of novel pyrazole-based thioethers as cathepsin S inhibitors: part 1.

A series of pyrazole-based thioethers were prepared and found to be potent cathepsin S inhibitors. A crystal structure of 13 suggests that the thioether moiety may bind to the S3 pocket of the enzyme. Additional optimization led to the discovery of aminoethylthioethers with improved enzymatic activity and submicromolar cellular potency.

Fragment-based discovery of nonpeptidic BACE-1 inhibitors using tethering.

BACE-1 (beta-site amyloid precursor protein cleaving enzyme), a prominent target in Alzheimer's disease drug discovery efforts, was surveyed using Tethering technology to discover small molecule fragment ligands that bind to the enzyme active site. Screens of a library of >15000 thiol-containing fragments versus a panel of BACE-1 active site cysteine mutants under redox-controlled conditions revealed several novel amine-containing fragments that could be selectively captured by subsets of the tethering sites. For one such hit class, defined by a central aminobenzylpiperidine (ABP) moiety, X-ray crystal structures of BACE mutant-disulfide conjugates revealed that the fragment bound by engaging both catalytic aspartates with hydrogen bonds. The affinities of ABP fragments were improved by structure-guided chemistry, first for conjugation as thiol-containing fragments and then for stand-alone, noncovalent inhibition of wild-type (WT) BACE-1 activity. Crystallography confirmed that the inhibitors bound in exactly the same mode as the disulfide-conjugated fragments that were originally selected from the screen. The ABP ligands represent a new type of nonpeptidic BACE-1 inhibitor motif that has not been described in the aspartyl protease literature and may serve as a starting point for the development of BACE-1-directed Alzheimer's disease therapeutics.

Pyrazole-based cathepsin S inhibitors with arylalkynes as P1 binding elements.

A crystal structure of 1 bound to a Cys25Ser mutant of cathepsin S helped to elucidate the binding mode of a previously disclosed series of pyrazole-based CatS inhibitors and facilitated the design of a new class of arylalkyne analogs. Optimization of the alkyne and tetrahydropyridine portions of the pharmacophore provided potent CatS inhibitors (IC50=40-300 nM), and an X-ray structure of 32 revealed that the arylalkyne moiety binds in the S1 pocket of the enzyme.

Allosteric inhibition of protein tyrosine phosphatase 1B.

Obesity and type II diabetes are closely linked metabolic syndromes that afflict >100 million people worldwide. Although protein tyrosine phosphatase 1B (PTP1B) has emerged as a promising target for the treatment of both syndromes, the discovery of pharmaceutically acceptable inhibitors that bind at the active site remains a substantial challenge. Here we describe the discovery of an allosteric site in PTP1B. Crystal structures of PTP1B in complex with allosteric inhibitors reveal a novel site located approximately 20 A from the catalytic site. We show that allosteric inhibitors prevent formation of the active form of the enzyme by blocking mobility of the catalytic loop, thereby exploiting a general mechanism used by tyrosine phosphatases. Notably, these inhibitors exhibit selectivity for PTP1B and enhance insulin signaling in cells. Allosteric inhibition is a promising strategy for targeting PTP1B and constitutes a mechanism that may be applicable to other tyrosine phosphatases.

Aminoethylenes: a tetrahedral intermediate isostere yielding potent inhibitors of the aspartyl protease BACE-1.

A series of novel beta-site amyloid precursor protein cleaving enzyme (BACE-1) inhibitors containing an aminoethylene (AE) tetrahedral intermediate isostere were synthesized and evaluated in comparison to corresponding hydroxyethylene (HE) compounds. Enzymatic inhibitory values were similar for both isosteres, as were structure-activity relationships with respect to stereochemical preference and substituent variation (P2/P3, P1, and P2'); however, the AE compounds were markedly more potent in a cell-based assay for reduction of beta-secretase activity. The incorporation of preferred P2/P3, P1, and P2' substituents into the AE pharmacophore yielded compound 7, which possessed enzymatic and cell assay IC(50)s of 26 nM and 180 nM, respectively. A three-dimensional crystal structure of 7 in complex with BACE-1 revealed that the amino group of the inhibitor core engages the catalytic aspartates in a manner analogous to hydroxyl groups in HE inhibitors. The AE isostere class represents a promising advance in the development of BACE-1 inhibitors.

Integrating fragment assembly and biophysical methods in the chemical advancement of small-molecule antagonists of IL-2: an approach for inhibiting protein-protein interactions.

Fragment assembly has shown promise for discovering small-molecule antagonists for difficult targets, including protein-protein interactions. Here, we describe a process for identifying a 60 nM inhibitor of the interleukin-2 (IL-2)/IL-2 receptor (IL-2Ralpha) interaction. By use of fragment-based approaches, a compound with millimolar affinity was evolved to a hit series with low micromolar activity, and these compounds were optimized into a lead series with nanomolar affinity. Fragment assembly was useful not only for hit identification, but also for lead optimization. Throughout the discovery process, biophysical methods and structural biology demonstrated that compounds bound reversibly to IL-2 at the IL-2 receptor binding site.

Potent small-molecule binding to a dynamic hot spot on IL-2.

The complexes between IL-2 and two similar small molecules, one a lead compound and the other a potent, affinity-optimized compound, were determined by X-ray crystallography. The lead compound (IC50 = 6 muM) bound to a hot spot on IL-2 in a groove that is not apparent in either the unliganded protein or a complex between IL-2 and a weakly bound drug fragment. The affinity-optimized compound (IC50 = 0.06 muM), which has an added aromatic acid fragment, bound in the same groove as the lead compound. In addition, a novel binding site was formed for the aromatic acid which is unseen in the complex with the lead compound. Thus, the hot spot on IL-2 is highly dynamic, with the protein changing form at multiple sites to maximize packing for each compound. Binding-site rigidity is often thought to play a role in high-affinity interactions. However, in this case, specific contacts between the small molecule and the protein are made despite the adaptivity of the hot spot. Given the change in morphology that was observed in IL-2, it is unlikely that a potent inhibitor could have been found by rational design. Therefore, fragment assembly methods offer the stochastic advantage of finding fragments in flexible protein regions where structural changes are unpredictable.

Discovery and characterization of cooperative ligand binding in the adaptive region of interleukin-2.

The cytokine hormone interleukin-2 (IL-2) contains a highly adaptive region that binds small, druglike molecules. The binding properties of this adaptive region have been explored using a "tethering" method that relies on the formation of a disulfide bond between the protein and small-molecule ligands. Using tethering, surface plasmon resonance (SPR), and X-ray crystallography, we have discovered that the IL-2 adaptive region contains at least two cooperative binding sites where the binding of a first ligand to one site promotes or antagonizes the binding of a second ligand to the second site. Cooperative energies of interaction of -2 kcal/mol are observed. The observation that the adaptive region contains two adjacent sites may lead to the development of tight-binding antagonists of a protein-protein interaction. Cooperative ligand binding in the adaptive region of IL-2 underscores the importance of protein dynamics in molecular recognition. The tethering approach provides a novel and general strategy for discovering such cooperative binding interactions in specific, flexible regions of protein structure.

Discovery of a new phosphotyrosine mimetic for PTP1B using breakaway tethering.

Protein tyrosine phosphatases play important roles in many signaling cascades involved in human disease. The identification of druglike inhibitors for these targets is a major challenge, and the discovery of suitable phosphotyrosine (pY) mimetics remains one of the key difficulties. Here we describe an extension of tethering technology, "breakaway tethering", which is ideally suited for discovering such new chemical entities. The approach involves first irreversibly modifying a protein with an extender that contains both a masked thiol and a known pY mimetic. The extender is then cleaved to release the pY mimetic, unmasking the thiol. The resulting protein is screened against a library of disulfide-containing small molecule fragments; any molecules with inherent affinity for the pY binding site will preferentially form disulfides with the extender, allowing for their identification by mass spectrometry. The ability to start from a known substrate mimimizes perturbation of protein structure and increases the opportunity to probe the active site using tethering. We applied this approach to the anti-diabetic protein PTP1B to discover a pY mimetic which belongs to a new molecular class and which binds in a novel fashion.

Binding of small molecules to an adaptive protein-protein interface.

Understanding binding properties at protein-protein interfaces has been limited to structural and mutational analyses of natural binding partners or small peptides identified by phage display. Here, we present a high-resolution analysis of a nonpeptidyl small molecule, previously discovered by medicinal chemistry [Tilley, J. W., et al. (1997) J. Am. Chem. Soc. 119, 7589-7590], which binds to the cytokine IL-2. The small molecule binds to the same site that binds the IL-2 alpha receptor and buries into a groove not seen in the free structure of IL-2. Comparison of the bound and several free structures shows this site to be composed of two subsites: one is rigid, and the other is highly adaptive. Thermodynamic data suggest the energy barriers between these conformations are low. The subsites were dissected by using a site-directed screening method called tethering, in which small fragments were captured by disulfide interchange with cysteines introduced into IL-2 around these subsites. X-ray structures with the tethered fragments show that the subsite-binding interactions are similar to those observed with the original small molecule. Moreover, the adaptive subsite tethered many more compounds than did the rigid one. Thus, the adaptive nature of a protein-protein interface provides sites for small molecules to bind and underscores the challenge of applying structure-based design strategies that cannot accurately predict a dynamic protein surface.