Current Pharmaceutical Design - Volume 18, Issue 30, 2012

Pharmacophore searches have become a popular tool for virtual screening of libraries to identify novel active substances that can be potentially developed into drugs. While they have been applied for years on common drug targets, their application in the discovery of protein-protein interaction inhibitors remains limited. This review describes current pharmacophore modelling methods applied in the discovery of novel inhibitors targeting protein-protein interactions. We first address the mimicry of protein-protein interactions with their respective inhibitors as observed in crystal structure complexes. This mimicry can be exploited to derive a pharmacophore query from protein-protein complex structures. We then discuss several cases where pharmacophore queries were utilized for the discovery of first-in-class inhibitors of their respective protein-protein interaction targets. These examples have demonstrated the usefulness of pharmacophore modelling in the quest for protein-protein interaction inhibitors.

The conformational flexibility of protein targets is being increasingly recognized in the drug discovery and design processes. When working on a particular disease-related biochemical pathway, it is of crucial importance to carefully select druggable protein binding pockets among all those cavities that may appear transiently or permanently on the respective protein surface. In this review, we will focus on the conformational dynamics of proteins that governs the formation and disappearance of such transient pockets on protein surfaces. We will also touch on the issue of druggability of transiently formed pockets. For example, protein cavities suitable to bind small drug-like molecules show an increased pocket size and buriedness when compared to empty sites. Interestingly, we observed in molecular dynamics simulations of five different protein systems that the conformational transitions on the protein surface occur almost barrierless and large pockets are found at similar frequencies as small pockets, see below. Thus, the dynamical processes at protein surfaces are better visualized as fluid-like motion than as energetically activated events. We conclude by comparing two computational tools, EPOS and MDpocket, for identifying transient pockets in PDK1 kinase. We illustrate how the obtained results depend on the way in which corresponding pockets in different molecular dynamics snapshots are connected to each other.

Most processes in living organisms occur through an intricate network of protein-protein interactions, in which any malfunctioning can lead to pathological situations. Therefore, current research in biomedicine is starting to focus on protein interaction networks. A detailed structural knowledge of these interactions at molecular level will be necessary for drug discovery targeting protein-protein interactions. The challenge from a structural biology point of view is determining the structure of the specific complex formed upon interaction of two or several proteins, and/or locating the surface residues involved in the interaction and identify which of them are the most important ones for binding (hot-spots). In this line, an increasing number of computer tools are available to complement experimental efforts. Docking algorithms can achieve successful predictive rates in many complexes, as shown in the community assessment experiment CAPRI, and have already been applied to a variety of cases of biomedical interest. On the other side, many methods for interface and hotspot prediction have been reported, based on a variety of evolutionary, geometrical and physico-chemical parameters. Computer predictions are reaching a significant level of maturity, and can be very useful to guide experiments and suggest mutations, or to provide a mechanistic framework to the experimental results on a given interaction. We will review here existing computer approaches for proteinprotein docking, interface prediction and hot-spot identification, with focus to drug discovery targeting protein-protein interactions.

Physical interactions among proteins constitute the backbone of cellular function, making them an attractive source of therapeutic targets. Although the challenges associated with targeting protein-protein interactions (PPIs) -in particular with small molecules - are considerable, a growing number of functional PPI modulators is being reported and clinically evaluated. An essential starting point for PPI inhibitor screening or design projects is the generation of a detailed map of the human interactome and the interactions between human and pathogen proteins. Different routes to produce these biological networks are being combined, including literature curation and computational methods. Experimental approaches to map PPIs mainly rely on the yeast two-hybrid (Y2H) technology, which have recently shown to produce reliable protein networks. However, other genetic and biochemical methods will be essential to increase both coverage and resolution of current protein networks in order to increase their utility towards the identification of novel disease-related proteins and PPIs, and their potential use as therapeutic targets.

During the last decades, a large amount of evidence has been gathered on the importance of protein-protein interactions in tuning and regulating most important biological processes. Since many of these pathways are deeply involved in diseases, extensive research efforts have been undertaken towards the modulation of protein-protein interactions. At the early stage of this challenge most of the attention was drawn to the drawbacks of such a therapeutic approach. Encouragingly, however, several recent studies provided a proof of concept that protein-protein interactions are actually valuable targets and that they do have a promising therapeutic potential. This review is divided into three sections. In the first section we summarize the general features of protein-protein interfaces, focusing on the characteristics that make them different from classical protein-ligand binding sites, as well as on problematic aspects that hamper the application of classical drug discovery approaches. In the second section, we present how some of the characteristics of protein-protein interactions can be exploited fruitfully in drug design, hence focusing on the druggability of protein-protein interfaces. Methods successfully applied to protein-protein interactions will be introduced, giving special attention to the computational ones. In the third section, three case studies are presented. First, we describe protein-protein interaction modulators targeting HDM2 and the computational methods applied to identify them. Next, we present the retrospective application of the discussed approaches on the well-examined target IL-2. We conclude with a prospective application to the NHR2 protein, a target just recently validated experimentally with the aid of computational methods.

Protein-protein interactions (PPI) are involved in vital cellular processes and are therefore associated to a growing number of diseases. But working with them as therapeutic targets comes with some major hurdles that require substantial mutations from our way to design drugs on historical targets such as enzymes and G-Protein Coupled Receptor (GPCR). Among the numerous ways we could improve our methodologies to maximize the potential of developing new chemical entities on PPI targets, is the fundamental question of what type of compounds should we use to identify the first hits and among which chemical space should we navigate to optimize them to the drug candidate stage. In this review article, we cover different aspects on PPI but with the aim to gain some insights into the specific nature of the chemical space of PPI inhibitors. We describe the work of different groups to highlight such properties and discuss their respective approach. We finally discuss a case study in which we describe the properties of a set of 115 PPI inhibitors that we compare to a reference set of 1730 enzyme inhibitors. This case study highlights interesting properties such as the unfortunate price that still needs to be paid by PPI inhibitors in terms of molecular weight, hydrophobicity, and aromaticity in order to reach a critical level of activity. But it also shows that not all PPI targets are equivalent, and that some PPI targets can demonstrate a better druggability by illustrating the better drug likeness of their associated inhibitors.

The protein-protein interaction (PPI) between p53 and its negative regulator MDM2 comprises one of the most important and intensely studied PPI's involved in preventing the initiation of cancer. The interaction between p53 and MDM2 is conformation-based and is tightly regulated on multiple levels. Due to the Angstrom level structural insight there is a reasonable understanding of the structural requirements needed for a molecule to bind to MDM2 and successfully inhibit the p53/MDM2 interaction. The current review summarizes the binding characteristics of the different disclosed small molecules for inhibition of MDM2 with a co-crystal structure. Synthetic access to these compounds as well as their derivatives are described in detail.

This systematic review describes successful examples of small-molecule inhibitors of protein-protein interactions, and compares their binding strategies to those employed by the natural protein partners. It extends and updates an earlier survey of this type (Fry DC, Curr Prot Pep Sci 2008; 9: 240-7). From analysis of these systems, common themes and lessons are presented that may assist future drug discovery efforts involving targets in this class. One encouraging finding is that a wide scope appears to be allowed at these sites in terms of binding strategies and chemotypes, which suggests that the outlook for finding small-molecule protein-protein inhibitors is favorable.

Whilst fragment-based screening has found significant utility in aiding the discovery of high quality hits against a range of targets, the use of this technology in the protein-protein interaction inhibitor field is very much in its infancy. This review aims to highlight the key technologies used to identify fragment hits, such as NMR, SPR, X-ray crystallography and biochemical screening, the fragmentbased protein-protein interaction case studies reported to date and, more importantly, the potential of this methodology in unearthing high quality hit molecules in this critical area of drug discovery. In addition, we also discuss some of the key aspects of fragment library design, the composition of a high quality library and suggest ways in which future, more structurally diverse fragments which occupy different regions of chemical space to the vast majority of current fragment libraries may be selected.

The growing perception that diseases are often consequences of multiple molecular abnormalities rather than being the result of a single defect highlights the importance of network-centric view in therapeutic approaches. Protein interaction networks may contribute to understanding of disease, assist in drug design and discovery. Here, we review some recent advances in disease-associated protein interaction networks taking a structural approach. We first describe structural aspects of protein-protein interactions and properties of protein interfaces as related to drug design; we address protein interactions in a network perspective; in particular, we illustrate how integrating protein interfaces onto interaction networks can guide the identification of selective drug targets or drugs targeting multiple proteins in a network.

Diet has a high relevance in health. Hypertension is a major risk factor for cardiovascular diseases and has an important impact on public health, and consequently on countries economy. Scientific research gathered strong evidence about the role of several dietary factors either in etiology or in treatment/prevention of these diseases. Peptides from different food matrices have been studied, and indicated as compounds with particular interest in the context of hypertension. The classical approach involves the identification of peptides with an in vitro ACE inhibitory activity and the assumption that the observed in vivo effects are due to this enzyme blockade. However, in some cases the potency of ACE blockade does not correlate with the antihypertensive activity in vivo. This paper reviews the current literature that identifies mechanisms of action, other than ACE inhibition, that might explain antihypertensive effects of biologically active peptides from different food sources.

Vasopressin (AVP) secretion and release are regulated by a number of central nervous system sites that receive peripheral signals from the osmoreceptors and baroreceptors. Aim of this paper is to review anatomical pathways and neurotransmitters involved as well as drugs affecting AVP secretion.

PKC- is a serine/threonine specific protein kinase and its activation depends upon the concentration of diacylglycerol (DAG) and phospholipids (phosphatidylserine). PKC- phosphorylates a variety of proteins that are known to be involved in the diverse cellular signaling pathways. It is predominantly expressed in the T-cells and localized in the center of immunological synapse upon T-cell receptor (TCR) and CD28 signaling. Activation of PKC- leads to the activation of various transcription factors in the nuclei of T-cells, e.g. NF-κB, NFAT, c-Jun, c-Fos and AP-1 that further control the proliferation and differentiation of T-cells. Defective T-cell activation in turn leads to the aberrant expression of apoptosis related proteins that cause the poor T-cell survival. Researchers have found that T-cells deficient in PKC- exhibit reduced interleukin-2 (IL-2) production. Apart from this role on IL-2 expression, it also plays crucial roles in the proliferation, differentiation and survival of the T-cells, which make it an attractive therapeutic target for a variety of immunological and T-cell mediated diseases. Hence, new molecules capable of modulating the expression or biological activity of PKC- are being developed and tested for their potential as novel therapy for several T-cells mediated disease conditions such as multiple sclerosis, rheumatoid arthritis, asthma, inflammatory bowel disease and organ transplantation, etc. In the present review, we tried to integrate the recent discoveries on PKC- including its pharmacology and therapeutic potential, along with brief update on its inhibitor molecules.