Receptor-based Pharmacophore Model Research Articles

Small Molecule Inhibitors of the MDM2-p53 Interaction Discovered by Ensemble-Based Receptor Models

Five nonpeptide, small-molecule inhibitors of the human MDM2-p53 interaction are presented, and each inhibitor represents a new scaffold. The most potent compound exhibited a Ki of 110 +/- 30 nM. These compounds were identified using our multiple protein structure (MPS) method which incorporates protein flexibility into a receptor-based pharmacophore model that identifies appropriate hotspots of binding. Docking the inhibitors with an induced-fit docking protocol suggested that the inhibitors mimicked the three critical binding residues of p53 (Phe19, Trp23, and Leu26). Docking also predicted a new orientation of the scaffolds that more fully fills the binding cleft, enabling the inhibitors to take advantage of additional hydrogen-bonding possibilities not explored by other small molecule inhibitors. One inhibitor in particular was proposed to probe the hydrophobic core of the protein by taking advantage of the flexibility of the binding cleft floor. These results show that the MPS technique is a promising advance for structure-based drug discovery and that the method can truly explore broad chemical space efficiently in the quest to discover potent, small-molecule inhibitors of protein-protein interactions. Our MPS technique is one of very few ensemble-based techniques to be proven through experimental verification of the discovery of new inhibitors.

Protein Flexibility and Species Specificity in Structure-Based Drug Discovery: Dihydrofolate Reductase as a Test System

In structure-based drug discovery, researchers would like to identify all possible scaffolds for a given target. However, techniques that push the boundaries of chemical space could lead to many false positives or inhibitors that lack specificity for the target. Is it possible to broadly identify the appropriate chemical space for the inhibitors and yet maintain target specificity? To address this question, we have turned to dihydrofolate reductase (DHFR), a well-studied metabolic enzyme of pharmacological relevance. We have extended our multiple protein structure (MPS) method for receptor-based pharmacophore models to use multiple X-ray crystallographic structures. Models were created for DHFR from human and Pneumocystis carinii. These models incorporate a fair degree of protein flexibility and are highly selective for known DHFR inhibitors over drug-like non-inhibitors. Despite sharing a highly conserved active site, the pharmacophore models reflect subtle differences between the human and P. carinii forms, which identify species-specific, high-affinity inhibitors. We also use structures of DHFR from Candida albicans as a counter example. The available crystal structures show little flexibility, and the resulting models give poorer performance in identifying species-specific inhibitors. Therapeutic success for this system may depend on achieving species specificity between the related human host and these key fungal targets. The MPS technique is a promising advance for structure-based drug discovery for DHFR and other proteins of biomedical interest.

Refining the Multiple Protein Structure Pharmacophore Method: Consistency across Three Independent HIV-1 Protease Models

Developing methods to incorporate protein flexibility into structure-based drug design is an important challenge. Our approach uses multiple protein structures (MPS) to create a receptor-based pharmacophore model of the desired target. We have previously demonstrated the success of the method by applying it to human immunodeficiency virus-1 protease (HIV-1p). Our models, based on an apo structure, discriminated known HIV-1p inhibitors from druglike inactive compounds and also accurately identified bound conformations of known inhibitors. Here, we test the method by applying it to all three unbound crystal structures of HIV-1p. We have also improved our method with denser probe mapping of the binding site and refined our selection criteria for pharmacophore elements. Our improved protocol has led to the development of a consistent 8-site pharmacophore model for HIV-1p, which is independent of starting structure, and a robust MPS pharmacophore method that is more amenable to automation.

Computational screening of inhibitors for HIV-1 integrase using a receptor based pharmacophore model

The HIV (human immuno-deficiency virus) integrase has a crucial role in viral replication. Moreover, it has no cellular homologue in humans. Hence, it is considered as an attractive drug target. Many inhibitors against the integrase protein has been designed and discussed. The Y-3 inhibitor (4-acetyl amino-5-hydroxy naphthalene - 2, 7- disulfonic acid) is already known to inhibit HIV-1 integrase. However, it is not suitable as a drug like candidate due to its high cyto-toxicity. In this report, a pharmacophore model for HIV integrase is described using the already known Y-3 inhibitor binding site. Fourteen compounds chemically related to the Y-3 inhibitor were generated using the described pharmacophore model and reported. Subsequent computational analysis showed that these compounds have interactions with the Y3 binding site and their possible utility as an integrase inhibitor is discussed.

Structure-Based Optimization of Azole Antifungal Agents by CoMFA, CoMSIA, and Molecular Docking

In a continuing effort to develop highly potent azole antifungal agents, the three-dimensional quantitative structure-activity relationship methods, CoMFA and CoMSIA, were applied using a set of novel azole antifungal compounds. The binding mode of the compounds at the active site of lanosterol 14alpha-demethylase was further explored using the flexible docking method. Various hydrophobic, van der Waals, pi-pi stacking, and hydrogen bonding interactions were observed between the azoles and the enzyme. Based on results from the molecular modeling, a receptor-based pharmacophore model was established to guide the rational optimization of the azole antifungal agents. Thus, a total of 57 novel azoles were designed and synthesized by a three-step optimization process. In vitro antifungal assay revealed that the antifungal activities of these novel azoles were greatly improved, which confirmed the reliability of the model from molecular modeling.

Novel Strategies for the Design of New Potent and Selective Human A3 Receptor Antagonists: An Update

A computer-aided approach has been developed in order to understand the molecular pharmacology of human A3R, and specifically, to lead to the discovery and structural refinement of new, potent and selective human A3R antagonists. This review focuses on our combined target-based and ligand-based drug design strategy, recently applied to provide more accurate information about the recognition mode on human A3R of some pyrazolotriazolopyrimidine and triazoloquinoxalinone analogs. The 3D rhodopsin-based homology model of human A3R has represented the starting point of our approach. A high throughput molecular docking method on the considered antagonists has allowed us to generate a receptor-based pharmacophore model. A novel "Y-shaped" pharmacophore binding motif has been proposed for both pyrazolotriazolopyrimidine and triazoloquinoxalinone derivatives. Moreover, related receptor-based 3D-QSAR analysis has been carried out to provide a suitable tool for prediction of the antagonists binding affinity on human A3R.

A Searchable Database for Comparing Protein−Ligand Binding Sites for the Analysis of Structure−Function Relationships

The rapid expansion of structural information for protein-ligand binding sites is potentially an important source of information in structure-based drug design and in understanding ligand cross reactivity and toxicity. We have developed a large database of ligand binding sites extracted automatically from the Protein Data Bank. This has been combined with a method for calculating binding site similarity based on geometric hashing to create a relational database for the retrieval of site similarity and binding site superposition. It contains an all-against-all comparison of binding sites and holds known protein-ligand binding sites, which are made accessible to data mining. Here we demonstrate its utility in two structure-based applications: in determining site similarity and in aiding the derivation of a receptor-based pharmacophore model. The database is available from http://www.bioinformatics.leeds.ac.uk/sb/.

Dynamic Receptor-Based Pharmacophore Model Development and Its Application in Designing Novel HIV-1 Integrase Inhibitors

We present here a dynamic receptor-based pharmacophore model representing the complementary features of the active site region of HIV-1 integrase (IN), which was developed from a series of representative conformations of IN. Conformations of IN were sampled through a molecular dynamics study of the catalytic domain of an IN monomer, and an ensemble of representative IN structures were collected via a probability-based representative conformer sampling method that considers both the potential energy and the structural similarity of the protein conformations. The dynamic pharmacophore model was validated by a set of 128 known inhibitors, and the results showed that over 72% of the active inhibitors (IC(50) lower than 20 microM) could be successfully identified by the dynamic model. Therefore, we screened our in-house database of commercially available compounds against this model and successfully identified a set of structurally novel IN inhibitors. Compounds 7 and 18 with IC(50)s of 8 microM and 15 microM, respectively, against the strand transfer reaction were the most potent. Moreover, 7, 8 and 20 showed a 5-fold selectivity for the strand transfer reaction over 3'-processing.

A structure-based design approach for the identification of novel inhibitors: application to an alanine racemase.

We report a new structure-based strategy for the identification of novel inhibitors. This approach has been applied to Bacillus stearothermophilus alanine racemase (AlaR), an enzyme implicated in the biosynthesis of the bacterial cell wall. The enzyme catalyzes the racemization of L- and D-alanine using pyridoxal 5'-phosphate (PLP) as a cofactor. The restriction of AlaR to bacteria and some fungi and the absolute requirement for D-alanine in peptidoglycan biosynthesis make alanine racemase a suitable target for drug design. Unfortunately, known inhibitors of alanine racemase are not specific and inhibit the activity of other PLP-dependent enzymes, leading to neurological and other side effects. This article describes the development of a receptor-based pharmacophore model for AllaR, taking into account receptor flexibility (i.e. a 'dynamic' pharmacophore model). In order to accomplish this, molecular dynamics (MD) simulations were performed on the full AlaR dimer from Bacillus stearothermophilus (PDB entry, 1 sft) with a D-alanine molecule in one active site and the non-covalent inhibitor, propionate, in the second active site of this homodimer. The basic strategy followed in this study was to utilize conformations of the protein obtained during MD simulations to generate a dynamic pharmacophore model using the property mapping capability of the LigBuilder program. Compounds from the Available Chemicals Directory that fit the pharmacophore model were identified and have been submitted for experimental testing. The approach described here can be used as a valuable tool for the design of novel inhibitors of other biomolecular targets.

A study of the binding requirements of calyculin A and dephosphonocalyculin A with PP1, development of a molecular recognition model for the binding interactions of the okadaic acid class of compounds with PP1

The interactions of the okadaic acid class of compounds, with special emphasis on the solution structures of calyculin A and dephosphonocalyculin A with PP1 are reported. After examination of the interactions of all docked structures, a receptor based pharmacophore model for the interactions of the protein phosphatase inhibitors has been developed. Calyculin A or dephosphonocalyculin A can interact with the enzyme in either a manner similar to the reported crystal structure, or in an extended form. The inhibitors require two essential regions interacting with the hydrophobic region and the central metal binding regions of the enzyme. This simplified model is consistent with previously published models of the okadaic acid class of compounds with PP1.

Developing a dynamic pharmacophore model for HIV-1 integrase.

We present the first receptor-based pharmacophore model for HIV-1 integrase. The development of "dynamic" pharmacophore models is a new method that accounts for the inherent flexibility of the active site and aims to reduce the entropic penalties associated with binding a ligand. Furthermore, this new drug discovery method overcomes the limitation of an incomplete crystal structure of the target protein. A molecular dynamics (MD) simulation describes the flexibility of the uncomplexed protein. Many conformational models of the protein are saved from the MD simulations and used in a series of multi-unit search for interacting conformers (MUSIC) simulations. MUSIC is a multiple-copy minimization method, available in the BOSS program; it is used to determine binding regions for probe molecules containing functional groups that complement the active site. All protein conformations from the MD are overlaid, and conserved binding regions for the probe molecules are identified. Those conserved binding regions define the dynamic pharmacophore model. Here, the dynamic model is compared to known inhibitors of the integrase as well as a three-point, ligand-based pharmacophore model from the literature. Also, a "static" pharmacophore model was determined in the standard fashion, using a single crystal structure. Inhibitors thought to bind in the active site of HIV-1 integrase fit the dynamic model but not the static model. Finally, we have identified a set of compounds from the Available Chemicals Directory that fit the dynamic pharmacophore model, and experimental testing of the compounds has confirmed several new inhibitors.