Stephan Hüttel - Academia.edu (original) (raw)

Papers by Stephan Hüttel

Antibiotics

The reassessment of known but neglected natural compounds is a vital strategy for providing novel... more The reassessment of known but neglected natural compounds is a vital strategy for providing novel lead structures urgently needed to overcome antimicrobial resistance. Scaffolds with resistance-breaking properties represent the most promising candidates for a successful translation into future therapeutics. Our study focuses on chelocardin, a member of the atypical tetracyclines, and its bioengineered derivative amidochelocardin, both showing broad-spectrum antibacterial activity within the ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) panel. Further lead development of chelocardins requires extensive biological and chemical profiling to achieve favorable pharmaceutical properties and efficacy. This study shows that both molecules possess resistance-breaking properties enabling the escape from most common tetracycline resistance mechanisms. Further, we show that these compounds a...

PLOS Neglected Tropical Diseases

Current efforts to eliminate the neglected tropical diseases onchocerciasis and lymphatic filaria... more Current efforts to eliminate the neglected tropical diseases onchocerciasis and lymphatic filariasis, caused by the filarial nematodes Onchocerca volvulus and Wuchereria bancrofti or Brugia spp., respectively, are hampered by lack of a short-course macrofilaricidal–adult-worm killing–treatment. Anti-wolbachial antibiotics, e.g. doxycycline, target the essential Wolbachia endosymbionts of filariae and are a safe prototype adult-worm-sterilizing and macrofilaricidal regimen, in contrast to standard treatments with ivermectin or diethylcarbamazine, which mainly target the microfilariae. However, treatment regimens of 4–5 weeks necessary for doxycycline and contraindications limit its use. Therefore, we tested the preclinical anti-Wolbachia drug candidate Corallopyronin A (CorA) for in vivo efficacy during initial and chronic filarial infections in the Litomosoides sigmodontis rodent model. CorA treatment for 14 days beginning immediately after infection cleared >90% of Wolbachia end...

European Journal of Medicinal Chemistry

To address the global challenge of emerging antimicrobial resistance, the hitherto most successfu... more To address the global challenge of emerging antimicrobial resistance, the hitherto most successful strategy to new antibiotics has been the optimization of validated natural products; most of these efforts rely on semisynthesis. Herein, we report the semisynthetic modification of amidochelocardin, an atypical tetracycline obtained via genetic engineering of the chelocardin producer strain. We report modifications at C4, C7, C10, and C11 by the application of methylation, acylation, electrophilic substitution, and oxidative C-C coupling reactions. The antibacterial activity of the reaction products was tested against a panel of Gram-positive and Gram-negative0 pathogens. The emerging structure-activity relationships (SARs) revealed that positions C7 and C10 are favorable anchor points for the semisynthesis of optimized derivatives. The observed SAR was different from that known for tetracyclines, which underlines the pronounced differences between the two compound classes.

Chemical Science

The concept of targeted drug conjugates has been successfully translated to clinical practice in ... more The concept of targeted drug conjugates has been successfully translated to clinical practice in oncology. Whereas the majority of cytotoxic effectors in drug conjugates are directed against either DNA or...

ACS Chemical Biology

Supplemental Information 10 Table S 1: Sequence homologies of tilivalline biosynthetic gene clust... more Supplemental Information 10 Table S 1: Sequence homologies of tilivalline biosynthetic gene cluster found in K. oxytoca (ID 11 1976911) compared to the Schneditz et al. cluster and conferred from BLAST.

Angewandte Chemie (International ed. in English), Jan 9, 2018

Vioprolides are a promising class of anticancer and antifungal lead compounds produced by the myx... more Vioprolides are a promising class of anticancer and antifungal lead compounds produced by the myxobacterium Cystobacter violaceus Cb vi35. Previously nothing had been reported about their biosynthesis, including the origin of the unusual 4-methylazetidinecarboxylic acid (MAZ) moiety. We describe the vioprolide biosynthetic gene cluster and solve the production obstacle by expression in three heterologous hosts. Starting from unstable production in the wild type at the single-digit mg L scale, we developed a stable host that eventually allowed for yields of up to half a gram per liter in fermenters. Gene inactivations coupled with isotope feeding studies identified an S-adenosylmethionine (SAM)-dependent enzyme and a methyltransferase as being responsible for the generation of the MAZ building block by a proposed mechanism unprecedented in bacteria. Furthermore, nonnatural vioprolide derivatives were generated via rational genetic engineering.

Molecules (Basel, Switzerland), Jan 9, 2017

Extracts from an endophytic fungus isolated from the roots of the Algerian plantshowed prominent ... more Extracts from an endophytic fungus isolated from the roots of the Algerian plantshowed prominent antimicrobial activity in a screening for novel antibiotics. The producer organism was identified asby means of morphological studies and molecular phylogenetic methods. Studies on the secondary metabolite production of this strain in various culture media revealed that the major components from shake flasks were massarilactones D () and H () as well as two new furanone derivatives for which we propose the trivial names (5)--gregatin B () and graminin D (). Scale-up of the fermentation in a 10 L bioreactor yielded massarilactone D and several further metabolites. Among those were three new anthranilic acid derivatives (-), two known anthranilic acid analogues (and) and three cyclopeptides (-). Their structures were elucidated on the basis of extensive spectroscopic analysis (1D- and 2D-NMR), high-resolution mass spectrometry (HRESIMS), and the application of the modified Mosher's met...

Microbial Cell Factories

Background: Recently, the discovery of the elansolids, a group of macrolides, was reported. The m... more Background: Recently, the discovery of the elansolids, a group of macrolides, was reported. The molecules show activity against methicillin-resistant Staphylococcus aureus as well as other gram-positive organisms. This fact renders those substances a promising starting point for future chemical development. The active atropisomers A1/A2 are formed by macrolactonization of the biosynthesis product A3 but are prone to ring opening and subsequent formation of several unwanted side products. Recently it could be shown that addition of different nucleophiles to culture extracts of Chitinophaga sancti enable the formation of new stable elansolid derivatives. Furthermore, addition of such a nucleophile directly into the culture led exclusively to formation of a single active elansolid derivative. Due to low product yields, methods for production of gram amounts of these molecules have to be established to enable further development of this promising compound class. Results: Production of elansolid A2 by C. sancti was enabled using a synthetic medium with sucrose as carbon source to a final concentration of 18.9 mg L −1. A fed-batch fermentation was ensued that resulted in an elansolid A2 concentration of 55.3 mg L −1. When using glucose as carbon source in a fed-batch fermentation only 34.4 mg L −1 elansolid A2 but 223.1 mg L −1 elansolid C1 were produced. This finding was not unexpected since elansolids A1/A2 and A3 have been reported to easily react with nucleophiles like anthranilic acid, a precursor of tryptophan biosynthesis. Due to the fact that nucleophiles can be incorporated in vivo, a fed-batch cultivation under identical conditions, with addition of anthranilic acid was carried out and lead to almost exclusive formation of elansolid C1 (257.5 mg L −1). Conclusion: Reproducible elansolid A2 and C1 production is feasible in different synthetic media at relatively high concentrations that will allow further investigation and semi-synthetic optimization. The feeding of anthranilic acid enables the exclusive production of the stable elansolid derivative C1, which reduces product loss by unspecific reactions and eases downstream processing. This derivative shows activity in the same range as the elansolids A1/A2. Hence, the method can possibly serve as a model-process for incorporation of other nucleophiles and biotechnological production of specifically designed molecules.

Angewandte Chemie (International ed. in English), Jan 20, 2017

Antibiotic discovery and development is challenging as chemical scaffolds of synthetic origin oft... more Antibiotic discovery and development is challenging as chemical scaffolds of synthetic origin often lack the required pharmaceutical properties, and the discovery of novel ones from natural sources is tedious. Herein we report the discovery of new cystobactamids with a significantly improved antibacterial profile through detailed screening of myxobacterial producer strains. Some of these new derivatives display antibacterial activities in the sub µgmL-1 range against Gram-negative pathogens, including clinical isolates of Klebsiella oxytoca, Pseudomonas aeruginosa, and fluoroquinolone-resistant Enterobacteriaceae, which were not observed for previously reported cystobactamids. Our findings provide structure-activity relationships and show how pathogen resistance can be overcome by natural scaffold diversity. The most promising derivative 861-2 was prepared by total synthesis, enabling further chemical optimization of this privileged scaffold.

Scientific Reports, 2017

Tumour-associated macrophages mainly comprise immunosuppressive M2 phenotypes that promote tumour... more Tumour-associated macrophages mainly comprise immunosuppressive M2 phenotypes that promote tumour progression besides anti-tumoural M1 subsets. Selective depletion or reprogramming of M2 may represent an innovative anti-cancer strategy. The actin cytoskeleton is central for cellular homeostasis and is targeted for anti-cancer chemotherapy. Here, we show that targeting G-actin nucleation using chondramide A (ChA) predominantly depletes human M2 while promoting the tumoursuppressive M1 phenotype. ChA reduced the viability of M2, with minor effects on M1, but increased tumour necrosis factor (TNF)α release from M1. Interestingly, ChA caused rapid disruption of dynamic F-actin filaments and polymerization of G-actin, followed by reduction of cell size, binucleation and cell division, without cellular collapse. In M1, but not in M2, ChA caused marked activation of SAPK/JNK and NFκB, with slight or no effects on Akt, STAT-1/-3, ERK-1/2, and p38 MAPK, seemingly accounting for the better survival of M1 and TNFα secretion. In a microfluidically-supported human tumour biochip model, circulating ChA-treated M1 markedly reduced tumour cell viability through enhanced release of TNFα. Together, ChA may cause an anti-tumoural microenvironment by depletion of M2 and activation of M1, suggesting induction of G-actin nucleation as potential strategy to target tumour-associated macrophages in addition to neoplastic cells.

Current Topics in Microbiology and Immunology, 2016

Natural products continue to be a predominant source for new anti-infective agents. Research at t... more Natural products continue to be a predominant source for new anti-infective agents. Research at the Helmholtz Institute for Pharmaceutical Research Saarland (HIPS) and the Helmholtz Centre for Infection Research (HZI) is dedicated to the development of new lead structures against infectious diseases and, in particular, new antibiotics against hard-to-treat and multidrug-resistant bacterial pathogens. In this chapter, we introduce some of the concepts currently being employed in the field of antibiotic discovery. In particular, we will exemplarily illustrate three approaches: (1) Current sources for novel compounds are mainly soil-dwelling bacteria. In the course of our antimicrobial discovery program, a biodiverse collection of myxobacterial strains has been established and screened for antibiotic activities. Based on this effort, one successful example is presented in this chapter: Antibacterial cystobactamids were discovered and their molecular target, the DNA gyrase, was identified soon after the analysis of myxobacterial self-resistance making use of the information found in the respective biosynthesis gene cluster. (2) Besides our focus on novel natural products, we also apply strategies to further develop either neglected drugs or widely used antibiotics for which development of resistance in the clinical setting is an issue: Antimycobacterial griselimycins were first described in the 1960s but their development and use in tuberculosis therapy was not further pursued. We show how a griselimycin derivative with improved pharmacokinetic properties and enhanced potency against Mycobacterium tuberculosis revealed and validated a novel target for antibacterial therapy, the DNA sliding clamp. (3) In a third approach, biosynthetic engineering was used to modify and optimize natural products regarding their pharmaceutical properties and their production scale: The atypical tetracycline chelocardin is a natural product scaffold that was modified to yield a more potent derivative exhibiting activity against multidrug-resistant pathogens. This was achieved by genetic engineering of the producer strain and the resulting compound is now subject to further optimization by medicinal chemistry approaches.

Analytical chemistry, Jan 4, 2014

Tandem mass spectrometry is a widely applied and highly sensitive technique for the discovery and... more Tandem mass spectrometry is a widely applied and highly sensitive technique for the discovery and characterization of microbial natural products such as secondary metabolites from myxobacteria. Here, a data mining workflow based on MS/MS precursor lists targeting only signals related to bacterial metabolism is established using LC-MS data of crude extracts from shaking flask fermentations. The devised method is not biased toward specific compound classes or structural features and is capable of increasing the information content of LC-MS/MS analyses by directing fragmentation events to signals of interest. The approach is thus contrary to typical auto-MS(2) setups where precursor ions are usually selected according to signal intensity, which is regarded as a drawback for metabolite discovery applications when samples contain many overlapping signals and the most intense signals do not necessarily represent compounds of interest. In line with this, the method described here achieves ...

BIOspektrum, 2013

ABSTRACT Natural products from myxobacteria continue to be invaluable sources for novel bioactive... more ABSTRACT Natural products from myxobacteria continue to be invaluable sources for novel bioactive leads. Although screening for such compounds from the most abundant representatives has already been achieved, myxobacteria still bear an enormous potential, as novel species and even novel genera and families are continuously discovered. State-of-the-art analytical tools within a comprehensive discovery pipeline greatly facilitate the identification of potential lead structures from these organisms.

Angewandte Chemie International Edition, 2014

The development of new antibiotics faces a severe crisis inter alia owing to a lack of innovative... more The development of new antibiotics faces a severe crisis inter alia owing to a lack of innovative chemical scaffolds with activities against Gram-negative and multiresistant pathogens. Herein, we report highly potent novel antibacterial compounds, the myxobacteria-derived cystobactamids 1-3, which were isolated from Cystobacter sp. and show minimum inhibitory concentrations in the low μg mL(-1) range. We describe the isolation and structure elucidation of three congeners as well as the identification and annotation of their biosynthetic gene cluster. By studying the self-resistance mechanism in the natural producer organism, the molecular targets were identified as bacterial type IIa topoisomerases. As quinolones are largely exhausted as a template for new type II topoisomerase inhibitors, the cystobactamids offer exciting alternatives to generate novel antibiotics using medicinal chemistry and biosynthetic engineering.

Angewandte Chemie, 2014

ABSTRACT Die Entwicklung neuer Antibiotika befindet sich in einer ernsthaften Krise, die unter an... more ABSTRACT Die Entwicklung neuer Antibiotika befindet sich in einer ernsthaften Krise, die unter anderem dadurch begründet ist, dass kaum innovative chemische Grundstrukturen mit Aktivität gegen Gram-negative und multiresistente Bakterien gefunden werden. Hier berichten wir über die Entdeckung neuer hochwirksamer Antibiotika aus Myxobakterien, den Cystobactamiden 1–3, die aus Cystobacter sp. isoliert wurden und minimale Hemmkonzentrationen im niedrigen μg mL−1-Bereich aufweisen. Wir beschreiben die Aufreinigung und Strukturaufklärung von drei Derivaten und die Identifizierung und Annotation des dazugehörigen Biosynthesegenclusters. Die molekularen Zielstrukturen der Cystobactamide konnten über die Analyse des Eigenresistenzmechanismus des Produzentenstammes als bakterielle Typ-IIa-Topoisomerasen identifiziert werden. Da die Optimierungsmöglichkeiten von Chinolonen als Basis für neue Inhibitoren der Typ-II-Topoisomerasen weitgehend ausgereizt sind, erscheinen die Cystobactamide als hochinteressante Alternativen, die durch Medizinalchemie und biosynthetisches Engineering der Entwicklung neuartiger Antibiotika dienen können.