Confronting the challenges of natural product-based antifungal discovery

Starting with the discovery of penicillin, the pharmaceutical industry has relied extensively on natural products (NPs) as an unparalleled source of bioactive small molecules suitable for antibiotic development. However, the discovery of structurally novel and chemically tractable NPs with suitable pharmacological properties as antibiotic leads has waned in recent decades. Today, the repetitive "rediscovery" of previously known NP classes with limited antibiotic lead potential dominates most industrial efforts. This limited productivity, exacerbated by the significant financial and resource requirements of such activities, has led to a broad de-emphasis of NP research by most pharmaceutical companies, including most recently Merck. Here we review our strategies--both technological and philosophical--in addressing current antifungal discovery bottlenecks in target identification and validation and how such efforts may improve NP-based antimicrobial discoveries when aligned with NP screening and dereplication.     

A concise enantioselective synthesis of antimalarial febrifugine alkaloids

Reaction of (S)-2-(tert-butyldiphenylsilyloxy)-5-(mesyloxy)pentanal with hydroxylamine in allyl alcohol brought about simultaneous 1,3-dipolar cycloaddition of the resulting nitrone to allyl alcohol to give three diastereoisomeric adducts, from which (+)-febrifugine and (+)-isofebrifugine, potent antimalarial alkaloids, were synthesized.     

Microbial synthesis of natural,semisynthetic, and new-to-nature tetrahydroisoquinoline alkaloids

The tetrahydroisoquinoline (THIQ) moiety forms the backbone of several natural, synthetic, and semisynthetic drugs approved for the treatment of cancer, pain, gout, and various neurodegenerative diseases. Plants synthesize up to 3000 THIQ structures constituting the benzylisoquinoline, phenethylisoquinoline, Amaryllidaceae, and ipecac alkaloid classes. This review outlines the ongoing surge in activity surrounding the production of natural, semisynthetic, and new-to-nature THIQ metabolites in engineered microbial systems. These efforts have accelerated THIQ pathway discovery and engineering, unlocked new domains of chemical space, and spurred the development of highly productive microbial strains synthesizing unprecedented yields of THIQ precursors. Consolidating these advancements into a single bioprocess holds potential to outcompete crop-based manufacturing and total synthesis for the production of high-value THIQ pharmaceuticals at scale.     

anti-Tuberculosis natural products: synthesis and biological evaluation of pyridoacridine alkaloids related to ascididemin

There is an urgent need for novel therapeutics possessing new modes of action to treat tuberculosis (TB) infections. In this study we report on the synthesis and biological evaluation of a series of pyrido[2,3,4-kl]acridin-6-one alkaloids related to the anti-TB (MIC 0.35 μM) but cytotoxic (IC₅₀ <0.14 μM) marine natural product ascididemin (1). The most interesting compounds identified were 21 and 24, which were found to inhibit the growth of Mycobacterium tuberculosis (Mtb) H₃₇Rv with MIC 2.0 μM, but with negligible cytotoxicity towards Vero and P388 cells (IC₅₀>25 μM). Another analogue (10) was evaluated against a range of singly-drug-resistant strains of Mtb and was found to exhibit no cross-resistance. These results suggest that the pyrido[2,3,4-kl]acridin-6-one skeleton may provide a useful scaffold for future studies directed towards possible anti-TB drugs.     

Fatty Acid synthesis as a target for antimalarial drug discovery

In biological systems, fatty acids can be synthesized by two related, but distinct de novo fatty acid synthase (FAS) pathways. Human cells rely on a type I FAS whereas plants, bacteria and other microorganisms contain type II FAS pathways. This difference exposes the type II FAS enzymes as potential targets for anti-microbial drugs that have little to no side effects in the human host. A number of inhibitors of type II FAS enzymes have been discovered - many of which have anti-bacterial activity. Extensive biochemical and structural studies have shed light on how these compounds inhibit their target enzymes, laying the foundation for the design of inhibitors with increased potency. Recent work has shown that malaria parasites do not contain a type I FAS and rely solely on a type II FAS for the de novo biosynthesis of fatty acids. The malaria FAS enzymes are therefore an exciting source of new drug targets, and are being actively exploited by several drug discovery efforts. Rapid progress has been made, largely due to the vast body of mechanistic and structural information about type II FAS enzymes from bacteria and the availability of inhibitors. Ongoing antimalarial drug discovery projects will be described in this review as well as background information about the well-studied bacterial type II FAS enzymes.     

Microwave-assisted,rapid synthesis of 2-vinylquinolines and evaluation of their antimalarial activity

A rapid and efficient synthesis of 2-vinylquinolines via trifluoromethanesulfonamide-mediated olefination of 2-methylquinoline and aldehyde under microwave irradiation is reported. Biological evaluation of these scaffolds demonstrates that 2-vinylquinolines 3x-3z possess excellent antimalarial activities against chloroquine-resistant Dd2 strain of Plasmodium falciparum (IC₅₀ < 100 nM).     

Highly diastereoselective synthesis and NMR characterization of natural indolizidine alkaloids

A diastereoselective, general and versatile access to disubstituted indolizidines is described. The nmr conformational analysis permits us to establish preferred conformations.     

Microwave assisted synthesis,docking and antimalarial evaluation of hybrid PABA-substituted 1,3,5-triazine derivatives

A series of novel PABA-substituted 1,3,5-triazine derivatives were developed via microwave assisted synthesis and subsequently tested for antimalarial activity against chloroquine sensitive 3D7 strain of Plasmodium falciparum using chloroquine as standard. Antimalarial screening result showed that synthesized compounds exhibited IC₅₀ in the range of 4.46 to 79.72 μg mL⁻¹. Among the tested compounds, 4c and 4f showed significant antimalarial activity with low binding energies (BE) -172.32 and 160.41 kcal mol⁻¹ via interacting with Arg122 through the involvement of COOH of the phenyl linked to 1,3,5-triazine. In conclusion, these core scaffolds can be used for future antimalarial drug development.     

Divergent total synthesis of the natural antimalarial marinoquinolines A, B, C, E and unnatural analogues

A new synthetic route to marinoquinolines was developed, allowing the synthesis of several structurally related compounds from a common key intermediate. Four natural marinoquinolines (A, B, C and E) and nine unnatural new analogues were prepared by this strategy, which features a Heck-Matsuda reaction in pure water and the Pictet-Spengler reaction as key steps.     

Structure and total synthesis of (-)-myrionidine and (-)-schoberine, antimalarial alkaloids from Myrioneuron nutans

Two new alkaloids, (5S,9S,10R)-myrionidine (1) and (5S,9S,10R,13S)-myrionamide (2), along with the known schoberine (3), were isolated from the leaves of Myrioneuron nutans (Rubiaceae), and their structures were determined from spectral analysis, including mass spectrometry and 2D NMR. The total asymmetric syntheses of (-)-myrionidine (1), (-)-schoberine (3), their enantiomers as well as their 9-epimers derivatives were performed, allowing the determination of their absolute configuration together with that of myrionamide (2). (-)-Myrionidine (1) and its synthetic enantiomer (18) showed a significant antimalarial activity on Plasmodium falciparum.     

Design,synthesis and in vitro evaluation of tetrahydropyrimidine-isatin hybrids as potential antitubercular and antimalarial agents

A series of 5-substituted-3-[{5-(6-methyl-2-oxo/thioxo-4-phenyl-1,2,3,4-tetrahydropyrimidin-5-yl)-1,3,4-oxadiazol-2-yl}-imino] -1,3-dihydro-2H-indol-2-one were synthesized,characterized and screened for their anti-tubercular and antimalarial activity.     

The synthesis of heliotridine and related alkaloids

An efficient synthesis of heliotridine from readily available retronecine was accomplished by nucleophilic displacement of the 7-mesylate, in the 7-mesyl-9-benzoate of retronecine with various cesium carboxylates in DMF, followed by hydrolysis. The synthetic procedure also permits the ready synthesis of a number of naturally occurring pyrrolizidine alkaloids possessing dissimilar acyl groups at C-7 and C-9.     

The facile solid-phase synthesis of cholesterol-based polyamine lipids

A facile solid-phase methodology for the production of cholesterol-based polyamines useful in mediating nucleic acid delivery for gene therapy is described. The methodology is compatible with a range of polyamines producing a library of lipids in excellent yields (>87%) and purity.     

The role of in vitro ADME assays in antimalarial drug discovery and development

The high level of attrition of drugs in clinical development has led pharmaceutical companies to increase the efficiency of their lead identification and development through techniques such as combinatorial chemistry and high-throughput (HTP) screening. Since the major reasons for clinical drug candidate failure other than efficacy are pharmacokinetics and toxicity, attention has been focused on assessing properties such as metabolic stability, drug-drug interactions (DDI), and absorption earlier in the drug discovery process. Animal studies are simply too labor-intensive and expensive to use for evaluating every hit, so it has been necessary to develop and implement higher throughput in vitro ADME screens to manage the large number of compounds of interest. The antimalarial drug development program at the Walter Reed Army Institute of Research, Division of Experimental Therapeutics (WRAIR/ET) has adopted this paradigm in its search for a long-term prophylactic for the prevention of malaria. The overarching goal of this program is to develop new, long half-life, orally bioavailable compounds with potent intrinsic activity against liver- and blood-stage parasites. From the WRAIR HTP antimalarial screen, numerous compounds are regularly identified with potent activity. These hits are now immediately evaluated using a panel of in vitro ADME screens to identify and predict compounds that will meet our specific treatment criteria. In this review, the WRAIR ADME screening program for antimalarial drugs is described as well as how we have implemented it to predict the ADME properties of small molecule for the identification of promising drug candidates.     

The discovery, isolation, elucidation of structure, and synthesis of antamanide

Antamanide is the name we have given to a constituent of the fungus Amanita phalloides. This substance counteracts the lethal action of the Amanita toxins phalloidine and α-amanatine if administered to the white mouse before, or simultaneously with, the poisons. Its concentration in the fungus is, however, so low that the toxic action of the latter predominates. Antamanide is a cyclic decapeptide formed from the L -amino acids alanine, phenylalanine, proline, and valine in the molar ratio 1:4:4:1. The amino-acid sequence was determined by a combination of gas chromatography and mass spectrometry. The structural formula assigned was confirmed by synthesis according to a classic route of peptide chemistry.     

Synthesis of flavonoid analogues as scaffolds for natural product-based combinatorial libraries

The design and synthesis of flavonoid analogues as combinatorial scaffolds is reported. Using commercially available materials, we synthesized chalcones with fluoro and carboxy groups. Nitration of these compounds generated highly functionalized flavonoid scaffolds with an o-fluoronitrobenzene template. Subsequent cyclizations of these chalcones resulted in the formation of several flavone and flavonone scaffolds. One of the flavonones was chosen as the scaffold to synthesize flavonoid derivatives on the solid phase. A series of flavonoid derivatives were obtained in high yields, which demonstrates that these highly functionalized scaffolds can be used in the synthesis of natural product-based combinatorial libraries for drug discovery.     

Design, synthesis and in vitro antimalarial activity of spiroperoxides

Several spiroperoxy antimalarial compounds were designed and synthesized using the hydrogen peroxide in UHP (urea-H₂O₂ complex) as the source of the peroxy bond. Incorporation of the H₂O₂ into the organic molecule framework through ketal exchange reaction in the present cases was greatly facilitated by the potential to form a five- or six-membered cyclic hemiketal due to the presence of a hydroxyl group γ or δ to the ketone carbonyl group. When the electron-withdrawing group in the Michael acceptor was a nitro group, the closure of the peroxy ring occurred readily under the hydroxidation conditions. Presence of a benzene ring fused to the peroxy ring effectively reduced the degrees of freedom in the transition state for the ring-closure step and made the otherwise very difficult seven-membered 1,2-dioxepane rather easy to form through the intramolecular Michael addition.