An inhibitor of Bcl-2 family proteins induces regression of solid tumours.

Proteins in the Bcl-2 family are central regulators of programmed cell death, and members that inhibit apoptosis, such as Bcl-X(L) and Bcl-2, are overexpressed in many cancers and contribute to tumour initiation, progression and resistance to therapy. Bcl-X(L) expression correlates with chemo-resistance of tumour cell lines, and reductions in Bcl-2 increase sensitivity to anticancer drugs and enhance in vivo survival. The development of inhibitors of these proteins as potential anti-cancer therapeutics has been previously explored, but obtaining potent small-molecule inhibitors has proved difficult owing to the necessity of targeting a protein-protein interaction. Here, using nuclear magnetic resonance (NMR)-based screening, parallel synthesis and structure-based design, we have discovered ABT-737, a small-molecule inhibitor of the anti-apoptotic proteins Bcl-2, Bcl-X(L) and Bcl-w, with an affinity two to three orders of magnitude more potent than previously reported compounds. Mechanistic studies reveal that ABT-737 does not directly initiate the apoptotic process, but enhances the effects of death signals, displaying synergistic cytotoxicity with chemotherapeutics and radiation. ABT-737 exhibits single-agent-mechanism-based killing of cells from lymphoma and small-cell lung carcinoma lines, as well as primary patient-derived cells, and in animal models, ABT-737 improves survival, causes regression of established tumours, and produces cures in a high percentage of the mice.

Discovery of a potent inhibitor of the antiapoptotic protein Bcl-xL from NMR and parallel synthesis.

The antiapoptotic proteins Bcl-x(L) and Bcl-2 play key roles in the maintenance of normal cellular homeostasis. However, their overexpression can lead to oncogenic transformation and is responsible for drug resistance in certain types of cancer. This makes Bcl-x(L) and Bcl-2 attractive targets for the development of potential anticancer agents. Here we describe the structure-based discovery of a potent Bcl-x(L) inhibitor directed at a hydrophobic groove on the surface of the protein. This groove represents the binding site for BH3 peptides from proapoptotic Bcl-2 family members such as Bak and Bad. Application of NMR-based screening yielded an initial biaryl acid with an affinity (K(d)) of approximately 300 microM for the protein. Following the classical "SAR by NMR" approach, a second-site ligand was identified that bound proximal to the first-site ligand in the hydrophobic groove. From NMR-based structural studies and parallel synthesis, a potent ligand was obtained, which binds to Bcl-x(L) with an inhibition constant (K(i)) of 36 +/- 2 nM.

NMR-based modification of matrix metalloproteinase inhibitors with improved bioavailability.

The NMR-based discovery of biaryl hydroxamate inhibitors of the matrix metalloproteinase stromelysin (MMP-3) has been previously described (Hajduk et al. J. Am. Chem. Soc. 1997, 119, 5818-5827). While potent in vitro, these inhibitors exhibited no in vivo activity due, at least in part, to the poor pharmacokinetic properties of the alkylhydroxamate moiety. To circumvent this liability, NMR-based screening was implemented to identify alternative zinc-chelating groups. Using this technique, 1-naphthyl hydroxamate was found to bind tightly to the protein (K(D) = 50 microM) and was identified as a candidate for incorporation into the lead series. On the basis of NMR-derived structural information, the naphthyl hydroxamate and biaryl fragments were linked together to yield inhibitors of this enzyme that exhibited improved bioavailability. These studies demonstrate that the NMR-based screening of fragments can be effectively applied to improve the physicochemical or pharmacokinetic profile of lead compounds.

Discovery of a novel small molecule binding site of human survivin.

Survivin is one of the most tumor-specific genes in the human genome and is an attractive target for cancer therapy. However, small-molecule ligands for survivin have not yet been described. Thus, an interrogation of survivin which could potentially both validate a small-molecule therapy approach, and determine the biochemical nature of any of survivin's functions has not been possible. Here we describe the discovery and characterization of a small molecule binding site on the survivin surface distinct from the Smac peptide-binding site. The new site is located at the dimer interface and exhibits many of the features of highly druggable, biologically relevant protein binding sites. A variety of small hydrophobic compounds were found that bind with moderate affinity to this binding site, from which one lead was developed into a group of compounds with nanomolar affinity. Additionally, a subset of these compounds are adequately water-soluble and cell-permeable. Thus, the structural studies and small molecules described here provide tools that can be used to probe the biochemical role(s) of survivin, and may ultimately serve as a basis for the development of small molecule therapeutics acting via direct or allosteric disruption of binding events related to this poorly understood target.