Functional genomics approaches for the identification and validation of antifungal drug targets

So far, antifungal drug discovery seems to have benefited little from the enormous advances in the field of genomics in the last decade. Although it has become clear that traditional drug screening is not delivering the long-awaited novel potent antifungals, little has been reported on efforts to use novel genome-based methodologies in the quest for new drugs acting on human pathogenic fungi. Although the market for a novel systemic and even topical broad-spectrum antifungal appears considerable, many large pharmaceutical companies have decided to scale back their activities in antifungal drug discovery. Here we report on some of the recent advances in genomics-based technologies that will allow us not only to identify and validate novel drug targets but hopefully also to discover active therapeutic agents. Novel drug targets have already been found by 'en masse' gene inactivation strategies (e.g. using antisense RNA inhibition). In addition, genome expression profiling using DNA microarrays helps to assign gene function but also to understand better the mechanism of action of known drugs (e.g. itraconazole) and to elucidate how new drug candidates work. No doubt, we have a long way to go just to catch up with the advances made in other therapeutic areas, but all tools are at hand to derive practical benefits from the genomics revolution. The next few years should prove a very exciting time in the history of antifungal drug discovery.     

Natural products – antifungal agents derived from plants

A new spectrum of human fungal infections is increasing due to increased cancer, AIDS, and immunocompromised patients. The increased use of antifungal agents also resulted in the development of resistance to the present drugs. It makes necessary to discover new classes of antifungal compounds to cure fungal infections. Plants are rich source of bioactive secondary metabolites of wide variety such as tannins, terpenoids, saponins, alkaloids, flavonoids, and other compounds, reported to have in vitro antifungal properties. Since the plant kingdom provides a useful source of lead compounds of novel structure, a wide-scale investigation of species from the tropics has been considered. Therefore, the research on natural products and compounds derived from natural products has accelerated in recent years due to their importance in drug discovery. A series of molecules with antifungal activity against different strains of fungus have been found in plants, which are of great importance to humans. These molecules may be used directly or considered as a precursor for developing better molecules. This review attempts to summarize the current status of important antifungal compounds from plants.     

Emerging trends in high-throughput screening

Novel technologies are emerging for high-throughput screening, driven by the needs and fine-tuning of established drug discovery activities, as well as by the emergence of novel target classes resulting from the deciphering of the human genome. Disciplines other than biology have now entered the screening scene, as bioinformatics, micro-technology and analytics provide powerful methodologies and applications that were not previously suitable for high-throughput screening. Many of these will move high-throughput screening from a numbers game to a content- and information-based approach to identify leads for novel disease targets.     

Genomic and genetic approaches for the identification of antifungal drug targets

Understanding how novel antifungal compounds work in target cells is useful not only in facilitating the discovery of new drugs but also new tools that can be used for further exploration of the targeted biological pathways and their regulation. Various genomic and genetic technologies have been developed in the model yeast Saccharomyces cerevisiae, and have been successfully used to identify drug target pathways. This review discusses the methods developed for some of these technologies, and how they have been used to evaluate the cellular pathways affected by a variety of therapeutic drugs and inhibitors. The advantages and disadvantages of each method are considered, and new advances are highlighted where applicable. The investigation of the mechanism of action of new antifungal compounds will undoubtedly lead to the development of new antifungal therapies targeting new fungal pathways that are more specific and less toxic than currently available antifungal drugs.     

New lead structures in antifungal drug discovery

During the past two decades, the incidence of invasive fungal infections has been increasing dramatically. Clinical available antifungal agents have several drawbacks such as limited potency and spectrum, drug related toxicity, non-optimal pharmacokinetics, and severe resistance. There is an emergent need to develop new antifungal drugs with novel chemical structures and novel mechanism of action. This review will focus on the most significant achievements in the discovery of antifungal lead structures within last few years. In particular, we pay more attention on the structure-activity relationship of antifungal leads and provide perspectives for future antifungal drug discovery.     

Emerging biological therapies for the treatment of myelodysplastic syndromes

Introduction: No drug has resulted in a survival advantage in patients with lower-risk myelodysplastic syndromes (MDS). While hypomethylating agents (HMA) have revolutionized treatment options for patients with higher-risk MDS, the prognosis remains dismal after HMA treatment failure. Novel effective therapies are urgently needed especially after HMA failure.

Areas covered: This review covers the current approach to disease prognostication and risk-adaptive therapy, as well as novel therapeutic approaches. We discuss the recent advancements in the understanding of MDS disease biology as a basis of targeted drug development. Several classes of novel agents are reviewed including drugs targeting dysregulated epigenetic control mechanisms, signaling pathways, abnormal splicing, as well as agents that target the immune system and the MDS bone marrow niche.

Expert opinion: Significant advancements in the understanding of the underlying biology of MDS are only starting to be translated into novel treatment options for MDS. Epigenetic therapy has shown significant clinical activity with HMA but the results of clinical trials combining HMAs with histone deacetylase inhibitors (HDACi) have been disappointing to date. Similarly, targeting several aberrant pathways in MDS has not resulted in significant improvements in therapy. Future therapies will focus both on synergic combination of existing drugs as well as novel agents targeting dysregulated immune responses and abnormal RNA splicing in MDS.     

Current advances in antifungal targets and drug development

Fungi are one of the most neglected pathogens apparent from the fact that the Amphotericin B, a polyene antibiotic, discovered way back in 1956 is still used as a "gold standard" for antifungal therapy. Past two decades have witnessed a dramatic rise in the incidences of life threatening systemic fungal infections. This can be ascribed to the increase in the number of immuno-compromised patients due to rise in HIV infected population, cancer chemotherapy and indiscriminate use of antibiotics. Majority of clinically used antifungals suffer from various drawbacks in terms of toxicity, efficacy and cost, and their frequent use has led to the emergence of resistant strains. Hence, there is a great demand for novel antifungals belonging to wide range of structural classes, selectively acting on novel targets with fewer side effects. This article aims at reviewing recent efforts made towards discovering novel antifungal drug targets and investigational molecules acting on them.     

抗真菌药物研究与开发的新进展

由于系统性真菌感染的死亡率较高,而现有抗真菌药物的数量少且具有一定毒性,以及由耐药性菌株引起的感染数量逐渐增加等因素,使得人们迫切需要更好的抗真菌治疗。近几十年来,研究人员一直在积极寻求替代的新型抗真菌药物,尽管新的潜在目标已被报道,但将这些研究成果从实验室转移到临床研究是一个漫长的过程,而且大多数药物都无法应用于患者。在本篇综述中,讨论了新型抗真菌药物的研发进展,主要从针对新靶点的药物设计,药物再利用,抗真菌肽为代表的生物制药,以及计算机辅助药物设计等几个方面进行阐述。     

Heat shock protein inhibitors for the treatment of fungal infections

Invasive fungal infections are a leading cause of mortality, especially in immunocompromised patients. Therapy is made difficult by the limited number of antifungal agents currently available which mostly target ergosterol in fungal cell membranes. The paucity of targets allows the development of cross resistance to all drugs with a common target. This highlights the need to develop new therapeutic strategies for fungal disease including agents with novel mechanisms of action. Heat shock protein 90 stabilizes calcineurin which regulates response to stress, allowing for calcineurin dependent stress responses required to survive exposure to antifungal drugs. Heat shock protein 90 inhibition abrogates calcineurin dependent stress responses, changing fungistatic drugs to fungicidal. Targeting a highly conserved protein that has a vital role in many cellular signaling pathways, reduces the potential for emergence of resistance to heat shock proteins inhibitors. This article will review recent patents in novel heat shock protein inhibitor therapy, such as efungumab, which diminish the emergence of antifungal drug resistance and enable greater efficacy of existing antifungals.     

Zebrafish: from disease modeling to drug discovery

The study of zebrafish, a leading model organism for developmental biology, is rapidly expanding to include human disease. Zebrafish models based on known disease mechanisms have been developed in several therapeutic areas, including blood diseases, diabetes, muscular dystrophy, neurodegenerative disease, angiogenesis and lipid metabolism. This review summarizes recent progress in disease model development, and outlines the potential of zebrafish to contribute to drug discovery through the identification of novel drug targets, validation of those targets and screening for new therapeutic compounds.     

Fabrication of a drug delivery system that enhances antifungal drug corneal penetration

Fungal keratitis (FK) remains a severe eye disease, and effective therapies are limited by drug shortages and critical ocular barriers. Despite the high antifungal potency and broad spectrum of econazole, its strong irritant and insolubility in water hinder its ocular application. We designed and fabricated a new drug delivery system based on a polymeric vector for the ocular antifungal application of econazole. This novel system integrates the advantages of its constituent units and exhibits superior comprehensive performance. Using the new system, drug content was significantly increased more than 600 folds. The results of in vivo and in vitro experiments demonstrated that the econazole-loaded formulation exhibited significantly enhanced corneal penetration after a single topical ocular administration, excellent antifungal activity, and good tolerance in rabbits. Drug concentrations and ocular relative bioavailability in the cornea were 59- and 29-time greater than those in the control group, respectively. Following the topical administration of one eye drop (50?μL of 0.3% w/v econazole) in fungus-infected rabbits, a high concentration of antimycotic drugs in the cornea and aqueous humor was sustained and effective for 4?h. The mechanism of corneal penetration was also explored using dual fluorescent labeling. This novel drug delivery system is a promising therapeutic approach for oculomycosis and could serve as a candidate strategy for use with various hydrophobic drugs to overcome barriers in the treatment of many other ocular diseases.     

Section Review: Anti-infectives: The potential of novel antifungal drugs for the treatment of disease in the immunocompromised host

There is a growing need for new antifungal drugs to complement those already in clinical use. Analogues and new formulations of existing drugs continue to give improvements in chemotherapy, but this account concentrates on possible new areas of development. These have been inspired chiefly by results from screening for new natural products. The most promising are three groups of antibiotics targeting the fungal cell walls. The echinocandins and related lipopeptides and their semisynthetic derivatives, and the papulacandin glycolipids are potent specific inhibitors of β-(1–3)-glucan synthesis. Pradimicins and benanomicins bind to mannan in the cell wall, but then target the cell membrane, disrupting its function. A wide range of other novel antifungal natural products, some with very unusual structures, are currently under critical investigation. The possible selective antifungal activity of members of the families of immunosuppressive drugs, cyclosporins and rapamycins, is worthy of further study. In a different approach, much current research in the metabolism and molecular biology of pathogenic fungi is directed ultimately at rational design of specific antifungal agents, but at present this is a hope for the future.     

The structural modification of natural products for novel drug discovery

Introduction: Throughout history, natural products (NPs) have provided a rich source of compounds that have wide applications in the fields of medicine, health sciences, pharmacy and biology. Although naturally active substances are good lead compounds for the discovery of new drugs, most of them suffer from various deficiencies or shortcomings, such as complex structures, poor stability and solubility. Therefore, structural modification of NPs is needed to develop novel compounds with specific properties.

Areas covered: This article presents an overview on the structural modifications of NPs in drug development. The application of multiple classes of NPs to the treatment of conditions such as cancers, infection, Alzheimer’s and diabetes are discussed. This article also reveals that modification of NPs is a versatile approach to explore their mode of actions, which may lead to the discovery of novel drugs.

Expert opinion: NPs are usually described by structural diversity and complexity. The use of isolated NPs as scaffolds for modification is a good approach to drug discovery and development. Despite many limitations associated with NPs, the total synthesis, semisynthetic modification, SAR-based modification, sometimes even a single atom alteration, may lead to the discovery of a novel drug.     

Hepatotoxicity of antifungal agents

Antifungal agents have been implicated in numerous cases of hepatotoxicity throughout the past few decades. Hepatotoxic reactions to antifungal agents range from slight, asymptomatic abnormalities in liver function tests to potentially fatal fulminant hepatic failure. Clinically significant hepatic injury resulting from antifungal therapy most commonly manifests as acute hepatocellular, cholestatic or mixed hepatocellular-cholestatic reactions. In general, reactions usually resolve on cessation of therapy, but some antifungal agents may induce chronic liver damage. This review will summarize the hepatotoxicity profiles of the major classes of antifungal agents and will provide recommendations for drug monitoring in order to minimize the risk of hepatotoxicity.     

Novel molecular targets for antimalarial drug development

Malaria with one million deaths and about 500 million new cases reported annually is a challenge to drug therapy and discovery. As current antimalarial therapeutics become increasingly ineffective because of parasitic resistance, there exists an urgent need to develop and pursue new therapeutic strategies. Antimalarial drug development can follow several strategies, ranging from minor modifications of existing agents to the design of novel agents that act against new targets. Recent advances in our knowledge of parasite biology as well as the availability of the genome sequence provide a wide range of novel targets for drug design. Several promising targets for drug intervention have been revealed in recent years. This review discusses novel molecular targets of the malaria parasite available to the drug discovery scientist.     

Fungal pathogens research: novel and improved molecular approaches for the discovery of antifungal drug targets

With the rise of fungal infection incidence amongst the patient population, the importance of developing new antifungal drug targets is higher than ever. This review mainly focuses on the three most prevalent fungal pathogens, Candida, Aspergillus and Cryptococcus, and on the most recent progresses in molecular research that contribute to a better understanding of the pathogen itself, but also its host and the interaction with its host. We consider the progress made in comparative genomics following the huge effort of fungal genome sequence projects undertaken in the last few years. We focus not only on currently used mammalian animal models such as mice, but also on novel non-mammalian models, such as the nematode worm Caenorhabditis elegans and the fruit fly Drosophila melanogaster, which offer useful tools in the area of the innate immune response to fungal infections. In addition we relate to the recent genomic and proteomic studies and focus on the use of these approaches in in vivo experiments in the pathogen itself as well as in the host. Finally, we describe the latest targeted mutagenesis strategy available in C. albicans and the use of RNA interference in both Cryptococcus neoformans and A. fumigatus. Our aim is not to give an exhaustive list of all new strategies but rather to give an overview of what will contribute most to the identification of new antifungal drug targets and the establishment of novel antifungal strategies.     

Novel drug targets in malaria parasite with potential to yield antimalarial drugs with long useful therapeutic lives

The status of chemotherapy as the main strategy in malaria control is rapidly being eroded by development of drug resistant Plasmodia, causing malaria to be dubbed a "re-emerging disease". To counter this misfortune, there is an urgent need to develop novel antimalarial drugs capable of delaying resistance, or circumventing it altogether. Mode of action of antimalarial drugs, inter alia, has a bearing on their useful therapeutic lives (UTLs), with single target drugs having short UTLs compared with drugs which possess pleiotropic action. Quinolines and artemisinins are the two classes of drugs with pleiotropic action and subsequently long UTLs. All other antimalarials are single-target drugs, and have been rendered ineffective within 1 to 5 years of their introduction for clinical use. This strongly underlines the need for development of new antimalarial therapies possessing long UTLs. The present review explores novel drug targets within the malaria parasite that may be exploited in the search for novel drugs that possess long and UTLs.     

Recent developments in the chemotherapy of malaria

Malaria remains a major infectious disease and is worsening in some areas, partly because of the spread of resistance to established antimalarial drugs. New drugs are urgently needed to combat the protozoan parasite, Plasmodium. This review covers new developments, including artemisinin derivatives, synthetic peroxides, folate pathway inhibitors, primaquine analogs and proteinase inhibitors. However, few of these agents are in clinical trials and many are derived from chemical classes already used extensively against malaria. The emerging understanding of parasite biology and new technological developments in drug discovery offer hope of improvement, but this will require increased interest from the pharmaceutical industry.