Current Topics in Medicinal Chemistry - Volume 16, Issue 9, 2016

Compared with the increasing and widespread bacterial resistance to clinical medicines and the urgent need for cures of intractable diseases, there is a dramatic decline in the numbers of drugs reaching the market or clinical trials. Accordingly, it has become imperative to discover more rational and efficient strategies to design and develop novel drugs. Structure-based drug design/discovery (SBDD) is one of the computer-aided methods, by which novel drugs are designed or discovered based on the knowledge of 3D structures of the relevant specific targets. During the past few decades, the great potentials and success of SBDD have been seen in the field of drug discovery. In this review, we present an overview of the key mechanisms of SBDD, the frequently used computer programs in SBDD and the reported successful cases. Finally, several typical design processes of lead components from SBDD are also highlighted in detail, such as the discovery of inhibitors of G protein-coupled receptors (GPCRs), antibacterial drugs, and anti-cancer drugs.

Phosphodiesterases (PDEs) catalyze the hydrolysis of cAMP and cGMP, thereby regulating the cyclic nucleotide signalling pathways and biological responses. PDEs inhibitors can be used clinically for treatment of several diseases including central nervous system disorders, erectile dysfunction, pulmonary hypertension, acute refractory cardiac failure, and inflammatory diseases such as chronic obstructive pulmonary disease. However, the unfavourable risk-benefit ratio and side-effect profiles of non-selective PDEs inhibitors have impeded their therapeutic success and therefore spurred the pharmaceutical industry to develop family-selective PDE inhibitors. Given the recent remarkable advances in structure-based drug design, this review will summarize developments and achievements in structure-based search, design and optimization of PDEs inhibitors, and highlight the challenges that need to be addressed.

Aberrant epigenetic control is a common phenomenon in tumour progression. The epigenetic modifications such as DNA methylation, histone modification and nucleosome remodelling are involved in the regulation of many biological processes, alteration in which can result into tumourogenesis. Histone acetylation is often associated with gene expression; however deacetylated histones generally results in gene suppression. This whole reversible process is mediated by Histone acetyltranferase and Histone deacetylases (HDACs) respectively. HDACs perform the deacetylation of histones in nucleosomes, which intervenes changes in chromatin remodelling, prompting regulation of gene expression. HDACs likewise direct the acetylation status of various other non-histone substrates that includes oncogenes and tumour silencing proteins. As HDAC inhibition induces various tumour cells to enter apoptosis and consequently cell cycle arrest therefore, a large number of HDAC inhibitors have been reported to develop as a new class of anti-cancer agents. Apart from the two existing FDA approved HDAC inhibitors- Varinostat and Depsipetide, recently a new drug Farydak has been approved by the FDA for the treatment of multiple myeloma which thus validated the use of HDAC inhibitors for the treatment of cancer. Also, several other HDAC inhibitors are undergoing clinical trials. Here, we have reviewed the current status of structure based computational studies that has helped to rationalize the successful identification of HDAC inhibitors. The objective of the present review is to provide an overview of contribution of structure-based computational studies that have helped in identifying HDAC inhibitors with an emphasis on the perspectives of its insight, current status, advances and future opportunities as well as the evolving efforts to characterize the structural dynamics of HDACs.

The cysteine biosynthetic pathway is of fundamental importance for the growth, survival, and pathogenicity of the many pathogens. This pathway is present in many species but is absent in mammals. The ability of pathogens to counteract the oxidative defences of a host is critical for the survival of these pathogens during their long latent phases, especially in anaerobic pathogens such as Entamoeba histolytica, Leishmania donovani, Trichomonas vaginalis, and Salmonella typhimurium. All of these organisms rely on the de novo cysteine biosynthetic pathway to assimilate sulphur and maintain a ready supply of cysteine. The de novo cysteine biosynthetic pathway, on account of its being important for the survival of pathogens and at the same time being absent in mammals, is an important drug target for diseases such as amoebiasis, trichomoniasis & tuberculosis. Cysteine biosynthesis is catalysed by two enzymes: serine acetyl transferase (SAT) followed by O-acetylserine sulfhydrylase (OASS). OASS is well studied, and with the availability of crystal structures of this enzyme in different conformations, it is a suitable template for structure-based inhibitor development. Moreover, OASS is highly conserved, both structurally and sequence-wise, among the above-mentioned organisms. There have been several reports of inhibitor screening and development against this enzyme from different organisms such as Salmonella typhimurium, Mycobacterium tuberculosis and Entamoeba histolytica. All of these inhibitors have been reported to display micromolar to nanomolar binding affinities for the open conformation of the enzyme. In this review, we highlight the structural similarities of this enzyme in different organisms and the attempts for inhibitor development so far. We also propose that the intermediate state of the enzyme may be the ideal target for the design of effective highaffinity inhibitors.

The loss of effectiveness of current antibiotics caused by the development of drug resistance has become a severe threat to public health. Current widely used antibiotics are surprisingly targeted at a few bacterial functions - cell wall, DNA, RNA, and protein biosynthesis - and resistance to them is widespread and well identified. There is therefore great interest in the discovery of novel drugs and therapies to tackle antimicrobial resistance, in particular drugs that target other essential processes for bacterial survival. In the past few years a great deal of effort has been focused on the discovery of new inhibitors of the enzymes involved in the biosynthesis of aromatic amino acids, also known as the shikimic acid pathway, in which chorismic acid is synthesized. The latter compound is the synthetic precursor of L-Phe, L-Tyr, L-Phe, and other important aromatic metabolites. These enzymes are recognized as attractive targets for the development of new antibacterial agents because they are essential in important pathogenic bacteria, such as Mycobacterium tuberculosis and Helicobacter pylori, but do not have any counterpart in human cells. This review is focused on two key enzymes of this pathway, shikimate kinase and type II dehydroquinase. An overview of the use of structure-based design and computational studies for the discovery of selective inhibitors of these enzymes will be provided. A detailed view of the structural changes caused by these inhibitors in the catalytic arrangement of these enzymes, which are responsible for the inhibition of their activity, is described.

The highly persistent nature of Mycobacterium tuberculosis can be attributed to its lipophilic cell wall which acts as a major barrier in the process of drug discovery against tuberculosis. Glutamine synthetase plays a major role in nitrogen metabolism and cell wall biosynthesis of pathogenic mycobacteria. The current review focuses on the structural and functional aspects of Mtb glutamine synthetase and an overview of its reported inhibitors till date. Also in the present study, we employed a computational structure based drug design protocol for identifying novel inhibitors against Mtb glutamine synthetase (MtbGS). A total of 12 hits were identified based on e-pharmacophore related search and virtual screening, which were further tested for their in vitro MtbGS inhibitory activity. Three compounds (compound 6, 1 and 12) were found with IC50 less than 5 μM, of which compound 6 being top active with IC50 of 2.124 μM. Differential scanning fluorimetry studies were employed so as to measure the thermal stability of the protein complexed with the most active compound. Also the protein complexes with top three active compounds were subjected for molecular dynamics simulations to study their binding pattern and stabilization effect. The solvation free energies were also determined for these compounds, undertaking free energy perturbation studies, which can be used further for lead optimization in the process of anti-tubercular drug discovery targeting Mtb glutamine synthetase.

Human serum albumin (HSA) is the most abundant protein in the plasma. HSA plays a central role in drug pharmacokinetics because most drugs bound to HSA are delivered to their target organ/tissues. The prodrug strategies have shown great promise for improving the activity and selectivity of drugs. Designing prodrugs based on special HSA residues, such as Cys34 and Lys residues, has been extensively studied. Therefore, this review provides an overview of the development of nonsteroidal anti-inflammatory and anticancer prodrugs based on these special residues. In conclusion, this review may guide the rational design and development of new prodrugs for future clinical applications.

Ligand- and structure-based drug design approaches complement phenotypic and target screens, respectively, and are the two major frameworks for guiding early-stage drug discovery efforts. Since the beginning of this century, the advent of the genomic era has presented researchers with a myriad of high throughput biological data (parts lists and their interaction networks) to address efficacy and toxicity, augmenting the traditional ligand- and structure-based approaches. This data rich era has also presented us with challenges related to integrating and analyzing these multi-platform and multi-dimensional datasets and translating them into viable hypotheses. Hence in the present paper, we review these existing approaches to drug discovery research and argue the case for a new systems biology based approach. We present the basic principles and the foundational arguments/underlying assumptions of the systems biology based approaches to drug design. Also discussed are systems biology data types (key entities, their attributes and their relationships with each other, and data models/representations), software and tools used for both retrospective and prospective analysis, and the hypotheses that can be inferred. In addition, we summarize some of the existing resources for a systems biology based drug discovery paradigm (open TG-GATEs, DrugMatrix, CMap and LINCs) in terms of their strengths and limitations.

Biopolymer-based nanostructures or microstructures can be fabricated with different compositions, structures, and properties so that colloidal delivery systems can be tailored for specific applications. These structures can be assembled using various approaches, including electrospinning, coacervation, nanoprecipitation, injection, layer-by-layer deposition, and/or gelation. A major application of biopolymer-based particles is to encapsulate, protect, and release active molecules in the agricultural, food, supplements, personal care, and pharmaceutical sectors. The inherent variability and complexity of biopolymers (proteins and polysaccharides) often makes it challenging to produce particles with well-defined physicochemical and functional attributes. In this review, we discuss the properties of biopolymers, common particle fabrication methods, and some of the major challenges and opportunities associated with developing biopolymer-based particles for application as food-grade delivery systems.

The development in sequencing technologies over the past few decades have increased the pace of decoding genetic and functional information present in the genomes of pathogenic microorganisms. The knowledge obtained through sequencing projects facilitated the identification of genes that codes for virulence factors. A major portion of genomes of pathogenic of bacteria contains genes which are classified as “hypothetical or uncharacterized”. Due to unavailability of precise information about the functionality of these genes, the pathogenic mechanisms utilized by varieties of microorganisms are not fully understood. This respective class of proteins draws a significant interest of pharmaceutical research as they have potential to provide new clues regarding the development of novel therapeutics particularly against the multidrug resistant strains of bacteria. The in silico identification of putative drug and vaccine targets in the set of uncharacterized proteins through comparative and subtractive genome analyses facilitates the increase usability and efficiency of the present drugs. The functional annotation of these characterized target proteins can uncover varieties of biochemical pathways important for the survival and pathogenesis of bacteria. This review focuses on the current protocols available for identification and functional annotations of these uncharacterized potential therapeutic targets.