MultiMCS: a fast algorithm for the maximum common substructure problem on multiple molecules.

Several efficient correspondence graph-based algorithms for determining the maximum common substructure (MCS) of a pair of molecules have been published in the literature. The extension of the problem to three or more molecules is however nontrivial; heuristics used to increase the efficiency in the two-molecule case are either inapplicable to the many-molecule case or do not provide significant speedups. Our specific algorithmic contribution is two-fold. First, we show how the correspondence graph approach for the two-molecule case can be generalized to obtain an algorithm that is guaranteed to find the optimum connected MCS of multiple molecules, and that runs fast on most families of molecules using a new divide-and-conquer strategy that has hitherto not been reported in this context. Second, we provide a characterization of those compound families for which the algorithm might run slowly, along with a heuristic for speeding up computations on these families. We also extend the above algorithm to a heuristic algorithm to find the disconnected MCS of multiple molecules and to an algorithm for clustering molecules into groups, with each group sharing a substantial MCS. Our methods are flexible in that they provide exquisite control on various matching criteria used to define a common substructure.

Discovery of benzisoxazoles as potent inhibitors of chaperone heat shock protein 90.

Heat shock protein 90 (Hsp90) is a molecular chaperone that is responsible for activating many signaling proteins and is a promising target in tumor biology. We have identified small-molecule benzisoxazole derivatives as Hsp90 inhibitors. Crystallographic studies show that these compounds bind in the ATP binding pocket interacting with the Asp93. Structure based optimization led to the identification of potent analogues, such as 13, with good biochemical profiles.

4-(Phenylaminomethylene)isoquinoline-1,3(2H,4H)-diones as potent and selective inhibitors of the cyclin-dependent kinase 4 (CDK4).

The cyclin-dependent kinases (CDKs), as complexes with their respective partners, the cyclins, are critical regulators of cell cycle progression. Because aberrant regulations of CDK4/cyclin D1 lead to uncontrolled cell proliferation, a hallmark of cancer, small-molecule inhibitors of CDK4/cyclin D1 are attractive as prospective antitumor agents. The series of 4-(phenylaminomethylene)isoquinoline-1,3(2H,4H)-dione derivatives reported here represents a novel class of potent inhibitors that selectively inhibit CDK4 over CDK2 and CDK1 activities. In the headpiece of the 4-(phenylaminomethylene)isoquinoline-1,3(2H,4H)-dione, a basic amine substituent is required on the aniline ring for the CDK4 inhibitory activity. The inhibitory activity is further enhanced when an aryl or heteroaryl substituent is introduced at the C-6 position of the isoquinoline-1,3(2H,4H)-dione core. We present here SAR data and a CDK4 mimic model that explains the binding, potency, and selectivity of our CDK4 selective inhibitors.

Antitumor activity of HKI-272, an orally active, irreversible inhibitor of the HER-2 tyrosine kinase.

HER-2 belongs to the ErbB family of receptor tyrosine kinases, which has been implicated in a variety of cancers. Overexpression of HER-2 is seen in 25-30% of breast cancer patients and predicts a poor outcome in patients with primary disease. Trastuzumab (Herceptin), a monoclonal antibody to HER-2, is specifically approved for HER-2-positive breast cancer but is active only in a subset of these tumors. Blocking HER-2 function by a small molecule kinase inhibitor, therefore, represents an attractive alternate strategy to inhibit the growth of HER-2-positive tumors. HKI-272 is a potent inhibitor of HER-2 and is highly active against HER-2-overexpressing human breast cancer cell lines in vitro. It also inhibits the epidermal growth factor receptor (EGFR) kinase and the proliferation of EGFR-dependent cells. HKI-272 reduces HER-2 receptor autophosphorylation in cells at doses consistent with inhibition of cell proliferation and functions as an irreversible binding inhibitor, most likely by targeting a cysteine residue in the ATP-binding pocket of the receptor. In agreement with the predicted effects of HER-2 inactivation, HKI-272 treatment of cells results in inhibition of downstream signal transduction events and cell cycle regulatory pathways. This leads to arrest at the G(1)-S (Gap 1/DNA synthesis)-phase transition of the cell division cycle, ultimately resulting in decreased cell proliferation. In vivo, HKI-272 is active in HER-2- and EGFR-dependent tumor xenograft models when dosed orally on a once daily schedule. On the basis of its favorable preclinical pharmacological profile, HKI-272 has been selected as a candidate for additional development as an antitumor agent in breast and other HER-2-dependent cancers.

2-(Quinazolin-4-ylamino)-[1,4]benzoquinones as covalent-binding, irreversible inhibitors of the kinase domain of vascular endothelial growth factor receptor-2.

A series of 2-(quinazolin-4-ylamino)-[1,4] benzoquinone derivatives that function as potent covalent-binding, irreversible inhibitors of the kinase domain of vascular endothelial growth factor receptor-2 (VEGFR-2) has been prepared by ceric ammonium nitrate oxidation of substituted (2,5-dimethoxyphenyl)(6,7-disubstituted-quinazolin-4-yl)amines and by displacement of the chlorine atom of substituted 2-chloro-5-(6,7-disubstituted-quinazolin-4-ylamino)-[1,4]benzoquinones with various amines, anilines, phenols, and alcohols. Enzyme studies were conducted in the absence and presence of glutathione and plasma. Several of the compounds inhibit VEGF-stimulated autophosphorylation in intact cells. Kinetic experiments were performed to study the reactivity of selected inhibitors toward glutathione. Reactivities correlated with LUMO energies calculated as averages of those of individual conformers weighted by the Boltzmann distribution. These results and molecular modeling were used to rationalize the biological observations. The compounds behave as non-ATP-competitive inhibitors. Unequivocal evidence, from mass spectral studies, indicates that these inhibitors form a covalent interaction with Cys-1045. One member of this series displays antitumor activity in an in vivo model.

Optimization of 6,7-disubstituted-4-(arylamino)quinoline-3-carbonitriles as orally active, irreversible inhibitors of human epidermal growth factor receptor-2 kinase activity.

A series of new 6,7-disubstituted-4-(arylamino)quinoline-3-carbonitrile derivatives that function as irreversible inhibitors of human epidermal growth factor receptor-2 (HER-2) and epidermal growth factor receptor (EGFR) kinases have been prepared. These compounds demonstrated enhanced activities for inhibiting HER-2 kinase and the growth of HER-2 positive cells compared to our EGFR kinase inhibitor 86 (EKB-569). Three synthetic routes were used to prepare these compounds. They were prepared mostly by acylation of 6-amino-4-(arylamino)quinoline-3-carbonitriles with unsaturated acid chlorides or by amination of 4-chloro-6-(crotonamido)quinoline-3-carbonitriles with monocyclic or bicyclic anilines. The third route was developed to prepare a key intermediate, 6-acetamido-4-chloroquinoline-3-carbonitrile, that involved a safer cyclization step. We show that attaching a large lipophilic group at the para position of the 4-(arylamino) ring results in improved potency for inhibiting HER-2 kinase. We also show the importance of a basic dialkylamino group at the end of the Michael acceptor for activity, due to intramolecular catalysis of the Michael addition. This, along with improved water solubility, resulted in compounds with enhanced biological properties. We present molecular modeling results consistent with the proposed mechanism of inhibition. Binding studies of one compound, 25o (C-14 radiolabeled), showed that it binds irreversibly to HER-2 protein in BT474 cells. Furthermore, it demonstrated excellent oral activity, especially in HER-2 overexpressing xenografts. Compound 25o (HKI-272) was selected for further studies and is currently in phase I clinical trials for the treatment of cancer.

A fresh look at pharmaceutical screening library design.

Most current methods for the design of pharmaceutical screening libraries centre around compound diversity. We present arguments for a different approach, involving a fixed number of analogs around a set of medicinally relevant scaffolds. Most current approaches to screening library design emphasize wide coverage of chemical space at the expense of poor local representation. We propose constructing uniform libraries around fixed ring scaffolds with "adequate" representation so as not to miss potentially active series.

Synthesis and structure-activity relationships of 6,7-disubstituted 4-anilinoquinoline-3-carbonitriles. The design of an orally active, irreversible inhibitor of the tyrosine kinase activity of the epidermal growth factor receptor (EGFR) and the human epidermal growth factor receptor-2 (HER-2).

A series of of 6,7-disubstituted-4-anilinoquinoline-3-carbonitrile derivatives that function as irreversible inhibitors of EGFR and HER-2 kinases have been prepared. These inhibitors have, at the 6-position, butynamide, crotonamide, and methacrylamide Michael acceptors bearing water-solublilizing substituents. These compounds were prepared by acylation of 6-amino-4-(arylamino)quinoline-3-carbonitriles with unsaturated acid chlorides or mixed anhydrides. We performed competitive reactivity studies showing that attaching a dialkylamino group onto the end of the Michael acceptor results in compounds with greater reactivity due to intramolecular catalysis of the Michael addition. This, along with improved water-solubility results in compounds with enhanced biological properties. We present molecular modeling results consistent with the proposed mechanism of inhibition. One compound, 5 (EKB-569), which shows excellent oral in vivo activity, was selected for further studies and is currently in phase I clinical trials for the treatment of cancer.

A novel approach to combinatorial library design.

We address the problem of designing a general-purpose combinatorial library to screen for pharmaceutical leads. Conventional approaches focus on diversity as the primary factor in designing such libraries. We suggest making screening libraries out of a set of pharmaceutically relevant scaffolds, with multiple analogs per scaffold. The rationale for this rests on the fact that even though the hit-rate in active series is much higher than in the database as a whole, often a large fraction of the compounds in active series are inactive. This is especially true when the series has not been optimized for the target under study. We introduce the concept of hit-rate within a series and use historic screening data to arrive at a crude estimate for it. We then use simple probability arguments to show that 50-100 compounds are required in each series in order to be nearly certain of finding at least one active compound in each true active series for any given target.

A structural informatics approach to mine kinase knowledge bases.

In this paper, we describe a combination of structural informatics approaches developed to mine data extracted from existing structure knowledge bases (Protein Data Bank and the GVK database) with a focus on kinase ATP-binding site data. In contrast to existing systems that retrieve and analyze protein structures, our techniques are centered on a database of ligand-bound geometries in relation to residues lining the binding site and transparent access to ligand-based SAR data. We illustrate the systems in the context of the Abelson kinase and related inhibitor structures.

A protein relational database and protein family knowledge bases to facilitate structure-based design analyses.

The Protein Data Bank is the most comprehensive source of experimental macromolecular structures. It can, however, be difficult at times to locate relevant structures with the Protein Data Bank search interface. This is particularly true when searching for complexes containing specific interactions between protein and ligand atoms. Moreover, searching within a family of proteins can be tedious. For example, one cannot search for some conserved residue as residue numbers vary across structures. We describe herein three databases, Protein Relational Database, Kinase Knowledge Base, and Matrix Metalloproteinase Knowledge Base, containing protein structures from the Protein Data Bank. In Protein Relational Database, atom-atom distances between protein and ligand have been precalculated allowing for millisecond retrieval based on atom identity and distance constraints. Ring centroids, centroid-centroid and centroid-atom distances and angles have also been included permitting queries for pi-stacking interactions and other structural motifs involving rings. Other geometric features can be searched through the inclusion of residue pair and triplet distances. In Kinase Knowledge Base and Matrix Metalloproteinase Knowledge Base, the catalytic domains have been aligned into common residue numbering schemes. Thus, by searching across Protein Relational Database and Kinase Knowledge Base, one can easily retrieve structures wherein, for example, a ligand of interest is making contact with the gatekeeper residue.

CONFIRM: connecting fragments found in receptor molecules.

A novel algorithm for the connecting of fragment molecules is presented and validated for a number of test systems. Within the CONFIRM (Connecting Fragments Found in Receptor Molecules) approach a pre-prepared library of bridges is searched to extract those which match a search criterion derived from known experimental or computational binding information about fragment molecules within a target binding site. The resulting bridge 'hits' are then connected, in an automated fashion, to the fragments and docked into the target receptor. Docking poses are assessed in terms of root-mean-squared deviation from the known positions of the fragment molecules, as well as docking score should known inhibitors be available. The creation of the bridge library, the full details and novelty of the CONFIRM algorithm, and the general applicability of this approach within the field of fragment-based de novo drug design are discussed.

A family of ring system-based structural fragments for use in structure-activity studies: database mining and recursive partitioning.

In earlier work from our laboratory, we have described the use of the ring system and ring scaffold as descriptors. We showed that these descriptors could be used for fast compound clustering, novelty determination, compound acquisition, and combinatorial library design. Here we extend the concept to a whole family of structural descriptors with the ring system as the centerpiece. We show how this simple idea can be used to build powerful search tools for mining chemical databases in useful ways. We have also built recursive partition trees using these fragments as descriptors. We will discuss how these trees can help in analyzing complex structure-activity data.

Discovery of novel inhibitors of the ZipA/FtsZ complex by NMR fragment screening coupled with structure-based design.

ZipA is a membrane anchored protein in Escherichia coli that interacts with FtsZ, a homolog of eukaryotic tubulins, forming a septal ring structure that mediates bacterial cell division. Thus, the ZipA/FtsZ protein-protein interaction is a potential target for an antibacterial agent. We report here an NMR-based fragment screening approach which identified several hits that bind to the C-terminal region of ZipA. The screen was performed by 1H-15N HSQC experiments on a library of 825 fragments that are small, lead-like, and highly soluble. Seven hits were identified, and the binding mode of the best one was revealed in the X-ray crystal structure. Similar to the ZipA/FtsZ contacts, the driving force in the binding of the small molecule ligands to ZipA is achieved through hydrophobic interactions. Analogs of this hit were also evaluated by NMR and X-ray crystal structures of these analogs with ZipA were obtained, providing structural information to help guide the medicinal chemistry efforts.

Syntheses and EGFR kinase inhibitory activity of 6-substituted-4-anilino [1,7] and [1,8] naphthyridine-3-carbonitriles.

The syntheses and EGFR kinase inhibitory activity of a series of 6-substituted-4-anilino [1,7] and [1,8] naphthyridine-3-carbonitriles are described. Both reversible and irreversible binding inhibitors were prepared. These series were compared with each other and with the corresponding 4-anilinoquinoline-3-carbonitriles. Compounds having a 1,7-naphthyridine core structure can retain high potency while those with a 1,8-naphthyridine core are significantly less active. These results are consistent with molecular modeling observations.

Modulation of selective serotonin reuptake inhibitor and 5-HT1A antagonist activity in 8-aza-bicyclo[3.2.1]octane derivatives of 2,3-dihydro-1,4-benzodioxane.

2,3-Dihydro-1,4-benzodioxanes with aryl 8-aza-bicyclo[3.2.1]oct-3-ene attachments 2 produce compounds with potent 5-HT-T affinity, and weak 5-HT(1A) affinity and alpha(1) affinity. This compares with 2,3-dihydro-1,4-benzodioxanes containing 8-aza-bicyclo[3.2.1] octan-3-ol attachments 4 which possess potent 5-HT(1A) affinity, moderate to good selectivity over alpha(1) and little 5-HT-T affinity. A 3-benzothiophene analogue of 4 (30) was synthesized which possesses potent 5-HT(1A) affinity and especially good selectivity over both alpha(1) and 5-HT-T.

Syntheses and EGFR and HER-2 kinase inhibitory activities of 4-anilinoquinoline-3-carbonitriles: analogues of three important 4-anilinoquinazolines currently undergoing clinical evaluation as therapeutic antitumor agents.

The syntheses and biological evaluations of 4-anilinoquinoline-3-carbonitrile analogues of the three clinical lead 4-anilinoquinazolines Iressa, Tarceva, and CI-1033 are described. The EGFR and HER-2 kinase inhibitory activities and the cell growth inhibition of the two series are compared with each other and with the clinical lead EKB-569. Similar activities are observed between these two series.