Progress in drug research. Fortschritte der Arzneimittelforschung. Progres des recherches pharmaceutiques最新文献

Plants produce complex small molecules - natural products - that exhibit anticancer, antimalarial and antimicrobial activity. These molecules play a key role in human medicine. However, plants typically produce these compounds in low quantities, and harvesting plant natural products is frequently expensive, time-consuming and environmentally damaging. Plant cell culture provides a renewable, easily scalable source of plant material. In this chapter we discuss the successes and pitfalls associated with natural product production in plant cell cultures.

Fingolimod (FTY720) represents the first in a new class of immune-modulators whose target is sphingosine-1-phosphate (S1P) receptors. It was first identified by researchers at Kyoto University and Yoshitomi Pharmaceutical as a chemical derivative of the ascomycete metabolite ISP-1 (myriocin). Unlike its natural product parent, FTY720 does not interfere with sphingolipid biosynthesis. Instead, its best characterized mechanism of action upon in vivo phosphorylation, leading to the active principle FTY720-P, is the rapid and reversible inhibition of lymphocyte egress from peripheral lymph nodes. As a consequence of S1P1 receptor internalization, tissue-damaging T-cells can not recirculate and infiltrate sites of inflammation such as the central nervous system (CNS). Furthermore, FTY720-P modulation of S1P receptor signaling also enhances endothelial barrier function. Due to its mode of action, FTY720 effectively prevents transplant rejection and is active in various autoimmune disease models. The most striking efficacy is in the multiple sclerosis (MS) model of experimental autoimmune encephalomyelitis, which has now been confirmed in the clinic. FTY720 demonstrated promising results in Phase II trials and recently entered Phase III in patients with relapsing MS. Emerging evidence suggests that its efficacy in the CNS extends beyond immunomodulation to encompass other aspects of MS pathophysiology, including an influence on the blood-brain-barrier and glial repair mechanisms that could ultimately contribute to restoration of nerve function. FTY720 may represent a potent new therapeutic modality in MS, combined with the benefit of oral administration.

This chapter is designed to demonstrate that compounds derived from nature are still in the forefront of drug discovery in diseases such as microbial and parasitic infections, carcinomas of many types and control of cholesterol/lipids in man. In each disease area we have provided short discussions of past, present and future agents, in general only considering compounds currently in clinical Phase II or later, that were/are derived from nature's chemical skeletons. Finishing with a discussion of the current and evolving role(s) of microbes (bacteria and fungi) in the production of old and new agents ostensibly produced by higher organisms.

Drugs developed from microbial natural products are in the fundaments of modern pharmaceutical companies. Despite decades of research, all evidences suggest that there must remain many interesting natural molecules with potential therapeutic application yet to be discovered. Any efforts to successfully exploit the chemical diversity of microbial secondary metabolites need to rely heavily on a good understanding of microbial diversity, being the working hypothesis that maximizing biological diversity is the key strategy to maximizing chemical diversity. This chapter presents an overview of diverse topics related with this basic principle, always in relation with the discovery of novel secondary metabolites. The types of microorganisms more frequently used for natural products discovery are briefly reviewed, as well as the differences between terrestrial and marine habitats as sources of bioactive secondary metabolite producers. The concepts about microbial diversity as applied to prokaryotes have evolved in the last years, but recent data suggest the existence of true biogeographic patterns of bacterial diversity, which are also discussed. Special attention is dedicated to the existing strategies to exploit the microbial diversity that is not easy to tackle by conventional approaches. This refers explicitly to the current attempts to isolate and cultivate the previously uncultured bacteria, including the application of high throughput techniques. Likewise, the advances of microbial molecular biology has allowed the development of metagenomic approaches, i.e., the expression of biosynthetic pathways directly obtained from environmental DNA and cloned in a suitable host, as another way of accessing microbial genetic resources. Also, approaches relying on the genomics of metabolite producers are reviewed.

The ongoing exponential growth of DNA sequence data will lead to the discovery of many natural-product biosynthesis pathways by genome mining for which no actual product has been characterised. In many cases, these clusters remain silent under laboratory conditions. New technologies based on genetic engineering are available to induce silent genes. Heterologous expression of a silent gene cluster under the control of defined promoters can be applied. Alternatively, promoters of biosynthesis genes within the genome can be exchanged by defined promoters. Most promising, however, is the activation of pathway-specific regulatory genes, which was recently demonstrated. Such regulatory genes are present in many secondary metabolite gene clusters. This approach is rendered feasible by the fact that all of the genes encoding the large number of enzymes required for the synthesis of a typical secondary metabolite are clustered and that in some cases, a single regulator controls the expression of all members of a gene cluster to a certain extent. The advantage of this technique is that only a small gene needs to be handled, and that an ectopic integration is sufficient, bypassing all limitations of homologous recombination. Most conveniently, this strategy can trigger the concerted expression of all pathway genes. The vast amount of DNA sequences in the public database represents only the beginning of this new genomics era. The activation of these gene clusters by genetic engineering will lead to the discovery of many so far unknown products and therefore represents a novel avenue to drug discovery.

Artemisinin-based Combination Therapies (ACT) are recommended by the World Health Organization (WHO) to treat especially multidrug resistant forms of malaria, as currently used medications have become increasingly ineffective. In this chapter, the discovery of artemisinin from Traditional Chinese Medicine and its further development to ACT are reviewed. It is highlighted how the complex supply chain to the naturally occurring endoperoxide artemisinin, required to produce ACT-based drugs, was established; thus addressing the significant therapeutic needs and high demands for the medication.

In this survey the impact of the virtual screening concept is discussed in the field of drug discovery from nature. Confronted by a steadily increasing number of secondary metabolites and a growing number of molecular targets relevant in the therapy of human disorders, the huge amount of information needs to be handled. Virtual screening filtering experiments already showed great promise for dealing with large libraries of potential bioactive molecules. It can be utilized for browsing databases for molecules fitting either an established pharmacophore model or a three dimensional (3D) structure of a macromolecular target. However, for the discovery of natural lead candidates the application of this in silico tool has so far almost been neglected. There are several reasons for that. One concerns the scarce availability of natural product (NP) 3D databases in contrast to synthetic libraries; another reason is the problematic compatibility of NPs with modern robotized high throughput screening (HTS) technologies. Further arguments deal with the incalculable availability of pure natural compounds and their often too complex chemistry. Thus research in this field is time-consuming, highly complex, expensive and ineffective. Nevertheless, naturally derived compounds are among the most favorable source of drug candidates. A more rational and economic search for new lead structures from nature must therefore be a priority in order to overcome these problems. Here we demonstrate some basic principles, requirements and limitations of virtual screening strategies and support their applicability in NP research with already performed studies. A sensible exploitation of the molecular diversity of secondary metabolites however asks for virtual screening concepts that are interfaced with well-established strategies from classical pharmacognosy that are used in an effort to maximize their efficacy in drug discovery. Such integrated virtual screening workflows are outlined here and shall help to motivate NP researchers to dare a step towards this powerful in silico tool.

Microbes have been good to us. They have given us thousands of valuable products with novel structures and activities. In nature, they only produce tiny amounts of these secondary metabolic products as a matter of survival. Thus, these metabolites are not overproduced in nature, but they must be overproduced in the pharmaceutical industry. Genetic manipulations are used in industry to obtain strains that produce hundreds or thousands of times more than that produced by the originally isolated strain. These strain improvement programs traditionally employ mutagenesis followed by screening or selection; this is known as 'brute-force' technology. Today, they are supplemented by modern strategic technologies developed via advances in molecular biology, recombinant DNA technology, and genetics. The progress in strain improvement has increased fermentation productivity and decreased costs tremendously. These genetic programs also serve other goals such as the elimination of undesirable products or analogs, discovery of new antibiotics, and deciphering of biosynthetic pathways.

Natural products have played a critical role in the identification of numerous medicines. Synthetic organic chemistry and combinatorial chemistry strategies such as diversity-oriented synthesis (DOS) have enabled the synthesis of natural product-like compounds. The combination of these approaches has both improved the desired biological properties of natural products as well as the identification of novel compounds. Diversity concepts and strategies to access novel compounds inspired by natural products will be reviewed.

Natural products have historically been a rich source of lead molecules in drug discovery. However, natural products have been de-emphasized as high throughput screening resources in the recent past, in part because of difficulties in obtaining high quality natural products screening libraries, or in applying modern screening assays to these libraries. In addition, natural products programs based on screening of extract libraries, bioassay-guided isolation, structure elucidation and subsequent production scale-up are challenged to meet the rapid cycle times that are characteristic of the modern HTS approach. Fortunately, new technologies in mass spectrometry, NMR and other spectroscopic techniques can greatly facilitate the first components of the process - namely the efficient creation of high-quality natural products libraries, bimolecular target or cell-based screening, and early hit characterization. The success of any high throughput screening campaign is dependent on the quality of the chemical library. The construction and maintenance of a high quality natural products library, whether based on microbial, plant, marine or other sources is a costly endeavor. The library itself may be composed of samples that are themselves mixtures - such as crude extracts, semi-pure mixtures or single purified natural products. Each of these library designs carries with it distinctive advantages and disadvantages. Crude extract libraries have lower resource requirements for sample preparation, but high requirements for identification of the bioactive constituents. Pre-fractionated libraries can be an effective strategy to alleviate interferences encountered with crude libraries, and may shorten the time needed to identify the active principle. Purified natural product libraries require substantial resources for preparation, but offer the advantage that the hit detection process is reduced to that of synthetic single component libraries. Whether the natural products library consists of crude or partially fractionated mixtures, the library contents should be profiled to identify the known components present - a process known as dereplication. The use of mass spectrometry and HPLC-mass spectrometry together with spectral databases is a powerful tool in the chemometric profiling of bio-sources for natural product production. High throughput, high sensitivity flow NMR is an emerging tool in this area as well. Whether by cell based or biomolecular target based assays, screening of natural product extract libraries continues to furnish novel lead molecules for further drug development, despite challenges in the analysis and prioritization of natural products hits. Spectroscopic techniques are now being used to directly screen natural product and synthetic libraries. Mass spectrometry in the form of methods such as ESI-ICRFTMS, and FACS-MS as well as NMR methods such as SAR by NMR and STD-NMR have been utilized to effectively screen molecular libraries. Over