Design, synthesis, and biological evaluation of platensimycin analogues with varying degrees of molecular complexity

Morphing Natural Product Platensimycin via Heck, Sonogashira, and One-Pot Sonogashira/Cycloaddition Reactions to Produce Antibiotics with In Vivo Activity

Type II fatty acid synthases are promising drug targets against major bacterial pathogens. Platensimycin (PTM) is a potent inhibitor against β-ketoacyl-[acyl carrier protein] synthase II (FabF) and β-ketoacyl-[acyl carrier protein] synthase I (FabB), while the poor pharmacokinetics has prevented its further development. In this work, thirty-two PTM derivatives were rapidly prepared via Heck, Sonogashira, and one-pot Sonogashira/cycloaddition cascade reactions based on the Gram-scale synthesis of 6-iodo PTM (4). About half of the synthesized compounds were approximately equipotent to PTM against the tested Staphylococcus aureus strains. Among them, the representative compounds 4, A4, and B8 exhibited different plasma protein binding affinity or stability in the human hepatic microsome assay and showed improved in vivo efficacy over PTM in a mouse peritonitis model. In addition, A4 was also effective in an S. aureus-infected skin mouse model. Our study not only significantly expands the known PTM derivatives with improved antibacterial activities in vivo, but showcased that C–C cross-coupling reactions are useful tools to functionalize natural product drug leads.

An Experimental Toolbox for Structure-Based Hit Discovery for P. aeruginosa FabF, a Promising Target for Antibiotics

FabF (3-oxoacyl-[acyl-carrier-protein] synthase 2), which catalyses the rate limiting condensation reaction in the fatty acid synthesis II pathway, is an attractive target for new antibiotics. Here, we focus on FabF from P. aeruginosa (PaFabF) as antibiotics against this pathogen are urgently needed. To facilitate exploration of this target we have set up an experimental toolbox consisting of binding assays using bio-layer interferometry (BLI) as well as saturation transfer difference (STD) and WaterLOGSY NMR in addition to robust conditions for structure determination. The suitability of the toolbox to support structure-based design of FabF inhibitors was demonstrated through the validation of hits obtained from virtual screening. Screening a library of almost 5 million compounds resulted in 6 compounds for which binding into the malonyl-binding site of FabF was shown. For one of the hits, the crystal structure in complex with PaFabF was determined. Based on the obtained binding mode, analogues were designed and synthesised, but affinity could not be improved. This work has laid the foundation for structure-based exploration of PaFabF.

Perspectives from nearly five decades of total synthesis of natural products and their analogues for biology and medicine

Covering: 1970 to 2020By definition total synthesis is the art and science of making the molecules of living Nature in the laboratory, and by extension, their analogues. Although obvious, its application to the synthesis of molecules for biology and medicine was not always the purpose of total synthesis. In recent years, however, the field has acquired momentum as its power to reach higher molecular complexity and diversity is increasing, and as the demand for rare bioactive natural products and their analogues is expanding due to their recognised potential to facilitate biology and drug discovery and development. Today this component of total synthesis endeavors is considered highly desirable, and could be part of interdisciplinary academic and/or industrial partnerships, providing further inspiration and momentum to the field. In this review we provide a brief historical background of the emergence of the field of total synthesis as it relates to making molecules for biology and medicine. We then discuss specific examples of this practice from our laboratories as they developed over the years. The review ends with a conclusion and future perspectives for natural products chemistry and its applications to biology and medicine and other added-value contributions to science and society.

Reductive Aromatization of Quinols with B2 pin2 as Deoxidizing Agent

We have demonstrated B₂ pin₂ as superior deoxidizing agent for the reductive deoxygenation of quinol derivatives under basic conditions. A wide range of highly functionalized phenols were obtained in good yields including a complex drug molecule, which revealed the high functional group tolerance of this protocol.

Iron-Catalyzed Reductive Vinylation of Tertiary Alkyl Oxalates with Activated Vinyl Halides

We present herein a rare and efficient method for the creation of vinylated all carbon quaternary centers via Fe-catalyzed cross-electrophile coupling of vinyl halides with tertiary alkyl methyl oxalates. The reaction displays excellent functional group tolerance and broad substrate scope, which allows cascade radical cyclization and vinylation to afford complex bicyclic and spiral structural motifs. The reaction proceeds via tertiary alkyl radicals, and the putative vinyl-Br/Fe complexation appears to be crucial for activating the alkene and enabling a possibly concerted radical addition/C-Fe forming process.

A continuous-flow approach for the multi-gram scale synthesis of C2-alkyl- or β-amino functionalized 1,3-dicarbonyl derivatives and ondansetron drug using 1,3-dicarbonyls

Continuous flow application has been developed for the gram-scale synthesis of C2-alkyl- or β-amino functionalized 1,3-dicarbonyl derivatives and ondansetron drug.

Late-Stage Functionalization of Platensimycin Leading to Multiple Analogues with Improved Antibacterial Activity in Vitro and in Vivo

Bacterial fatty acid synthases are promising antibacterial targets against multidrug-resistant pathogens. Platensimycin (PTM) is a potent FabB/FabF inhibitor, while its poor pharmacokinetics hampers the clinical development. In this study, a focused library of PTM derivatives was prepared through thiolysis of PTM oxirane (1), followed by various C-C cross-coupling reactions in high yields. Antibacterial screening of these compounds in vitro yielded multiple hits with improved anti-Staphylococcus activities over PTM. Among them, compounds A1, A3, A17, and A28 exhibited improved antibacterial activities over PTM against methicillin-resistant Staphylococcus aureus (MRSA) in a mouse peritonitis model. Compound A28 was further shown to be effective against MRSA infection in a mouse wound model, in comparison to mupirocin. Therefore, the facile preparation and screening of these PTM derivatives, together with their potent antibacterial activities in vivo, suggest a promising strategy to improve the antibacterial activity and pharmacokinetic properties of PTM.

Evaluation of Platensimycin and Platensimycin-Inspired Thioether Analogues against Methicillin-Resistant Staphylococcus aureus in Topical and Systemic Infection Mouse Models

Staphylococcus aureus is one of the most common pathogens causing hospital-acquired and community-acquired infections. Methicillin-resistant S. aureus (MRSA)-formed biofilms in wounds are difficult to treat with conventional antibiotics. By targeting FabB/FabF of bacterial fatty acid synthases, platensimycin (PTM) was discovered to act as a promising natural antibiotic against MRSA infections. In this study, PTM and its previously synthesized sulfur-Michael derivative PTM-2t could reduce over 95% biofilm formation by S. aureus ATCC 29213 when used at 2 μg/mL in vitro. Topical application of ointments containing PTM or PTM-2t (2 × 4 mg/day/mouse) was successfully used to treat MRSA infections in a BABL/c mouse burn wound model. As a potential prodrug lead, PTM-2t showed improved in vivo efficacy in a mouse peritonitis model compared with PTM. Our study suggests that PTM and its analogue may be used topically or locally to treat bacterial infections. In addition, the use of prodrug strategies might be instrumental to improve the poor pharmacokinetic properties of PTM.

Stereoselective functionalization of platensimycin and platencin by sulfa-Michael/aldol reactions

Bioinspired sulfa-Michael/aldol cascade reactions have been developed for the semisynthesis of sulfur-containing heterocyclic derivatives of platensimycin and platencin, with three newly formed contiguous stereogenic centers. Density functional theory calculations revealed the mechanism for the stereochemistry control. This method was used in a synthesis of a platensimycin thiophene analogue with potent antibacterial activities against Staphylococcus aureus.

Semisynthesis of Platensimycin Derivatives with Antibiotic Activities in Mice via Suzuki-Miyaura Cross-Coupling Reactions

Platensimycin (PTM), originally isolated from soil bacteria Streptomyces platensis, is a potent FabF inhibitor against many Gram-positive pathogens, such as methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci. However, the further clinical development of PTM is hampered by its poor pharmacokinetic properties. In this study, 20 PTM derivatives were prepared by Suzuki-Miyaura cross-coupling reactions catalyzed by Pd (0)/C. Compared to PTM, 6-pyrenyl PTM (6t) showed improved antibacterial activity against MRSA in a mouse peritonitis model. Our results support the strategy to target the essential fatty acid synthases in major pathogens, in order to discover and develop new generations of antibiotics.

Semisynthesis and Biological Evaluation of Platensimycin Analogues with Varying Aminobenzoic Acids

Platensimycin (PTM) is an excellent natural product drug lead against various gram-positive pathogens, including methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci. In this study, twenty PTM derivatives with varying aminobenzoic acids were semisynthesized. In contrast to all the previous reported inactive aminobenzaote analogues, a few of them showed moderate antibacterial activities against S. aureus. Our study suggested that modification of the conserved aminobenzoic acid remains a viable approach to diversify the PTM scaffold.

Biosynthesis of thiocarboxylic acid-containing natural products

Thiocarboxylic acid-containing natural products are rare and their biosynthesis and biological significance remain unknown. Thioplatensimycin (thioPTM) and thioplatencin (thioPTN), thiocarboxylic acid congeners of the antibacterial natural products platensimycin (PTM) and platencin (PTN), were recently discovered. Here we report the biosynthetic origin of the thiocarboxylic acid moiety in thioPTM and thioPTN. We identify a thioacid cassette encoding two proteins, PtmA3 and PtmU4, responsible for carboxylate activation by coenzyme A and sulfur transfer, respectively. ThioPTM and thioPTN bind tightly to β-ketoacyl-ACP synthase II (FabF) and retain strong antibacterial activities. Density functional theory calculations of binding and solvation free energies suggest thioPTM and thioPTN bind to FabF more favorably than PTM and PTN. Additionally, thioacid cassettes are prevalent in the genomes of bacteria, implicating that thiocarboxylic acid-containing natural products are underappreciated. These results suggest that thiocarboxylic acid, as an alternative pharmacophore, and thiocarboxylic acid-containing natural products may be considered for future drug discovery.

Thioplatensimycin (thioPTM) and thioplatencin (thioPTN) are recently discovered thiocarboxylic acid congeners of the antibacterial compounds PTM and PTN. Here, the authors identify a thioacid cassette encoding PtmA3 and PtmU4 that are responsible for carboxylate activation and sulfur transfer, respectively.

Recent advances in ruthenium-based olefin metathesis

Ruthenium-based olefin metathesis catalysts, known for their functional group tolerance and broad applicability in organic synthesis and polymer science, continue to evolve as an enabling technology in these areas. A discussion of recent mechanistic investigations is followed by an overview of selected applications.

The semi-synthesis, biological evaluation and docking analysis of the oxime, hydrazine and hydrazide derivatives of platensimycin

A dozen oxime, hydrazine and hydrazide derivatives of platensimycin (PTM) analogues were synthesized, some of which showed strong antibacterial activities and were shown to be stable under the bioassay conditions. Docking analysis revealed that they have certain new interactions with β-ketoacyl-[acyl carrier protein] synthase II (FabF), suggesting that Schiff base formation on its terpene scaffold is an effective strategy to diversify PTM structure.

Biomimetic Stereoselective Sulfa-Michael Addition Leads to Platensimycin and Platencin Sulfur Analogues against Methicillin-Resistant Staphylococcus aureus

Several sulfur-containing platensimycin (PTM) and platencin (PTN) analogues, with activities comparable to the parent natural products, have recently been discovered from microorganisms, implying a biomimetic route to diversify the PTM and PTN scaffolds for structure-activity relationship study. We present here a substrate-directed and scaleable semisynthetic strategy to make PTM and PTN sulfur analogues with excellent diasteroselectivity, without using any chiral catalysts. Most of the sulfur analogues showed strong activities against clinical Staphylococcus aureus isolates, with minimum inhibitory concentrations of 0.5-2 μg mL^-1. Density functional theory calculations were in agreement with the observed selectivity for these analogues and suggest that the conformation restraints of the terpene cages of PTM and PTN on the transition states determine the si-face attack selectivity.

In vivo instability of platensimycin and platencin: Synthesis and biological evaluation of urea- and carbamate-platensimycin

Platensimycin (PTM) and platencin (PTN), two natural products and promising drug leads that target bacterial and mammalian fatty acid synthases, are known to have unfavorable pharmacokinetic properties. It is not clear, however, what the metabolic fates of PTM and PTN are and no efforts have been reported to address this key roadblock in the development of these compounds as viable drug options. Here we describe the pharmacokinetics of PTM and PTN, and reveal rapid renal clearance as the primary metabolic liability with three additional sites of chemical liability: (i) amide hydrolysis, (ii) glucuronidation, and (iii) oxidation. We determined that hydrolysis is a viable clearance mechanism in vivo and synthesized two PTM analogues to address in vivo hydrolysis. Urea- and carbamate-PTM analogues showed no detectable hydrolysis in vivo, at the expense of antibacterial activity, with no further improvement in systemic exposure. The antibacterial sulfur-containing analogues PTM D1 and PTM ML14 showed significant decreases in renal clearance. These studies set the stage for continued generation of PTM and PTN analogues in an effort to improve their pharmacokinetics while retaining or improving their biological activities.

A Facile Semi-Synthetic Approach towards Halogen-Substituted Aminobenzoic Acid Analogues of Platensimycin

Platensimycin (PTM), produced by several strains of Streptomyces platensis, is a promising drug lead for infectious diseases and diabetes. The recent pilot-scale production of PTM from S. platensis SB12026 has set the stage for the facile semi-synthesis of a focused library of PTM analogues. In this study, gram-quantity of platensic acid (PTMA) was prepared by the sulfuric acid-catalyzed ethanolysis of PTM, followed by a mild hydrolysis in aqueous lithium hydroxide. Three PTMA esters were also obtained in near quantitative yields in a single step, suggesting a facile route to make PTMA aliphatic esters. 1-[Bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxid hexafluorophosphate (HATU)-catalyzed coupling of PTMA and 33 aminobenzoates resulted in the synthesis of 28 substituted aminobenzoate analogues of PTM, among which 26 of them were reported for the first time. Several of the PTM analogues showed weak antibacterial activity against methicillin-resistant Staphylococcus aureus. Our study supported the potential utility to integrate natural product biosynthetic and semi-synthetic approaches for structure diversification.

Biosynthetic Origin of the Ether Ring in Platensimycin

Platensimycin (PTM) and platencin (PTN) are highly functionalized bacterial diterpenoid natural products that target bacterial and mammalian fatty acid synthases. PTM and PTN feature varying diterpene-derived ketolides that are linked to the same 3-amino-2,4-dihydroxybenzoic acid moiety. As a result, PTM is a selective inhibitor for FabF/FabB, while PTN is a dual inhibitor of FabF/FabB and FabH. We previously determined that the PTM cassette, consisting of five genes found in the ptm, but not ptn, gene cluster, partitions the biosynthesis of the PTM and PTN diterpene-derived ketolides. We now report investigation of the PTM cassette through the construction of diterpene production systems in E. coli and genetic manipulation in the PTM-PTN dual overproducer Streptomyces platensis SB12029, revealing two genes, ptmT3 and ptmO5, that are responsible for the biosynthetic divergence between the PTM and PTN diterpene-derived ketolides. PtmT3, a type I diterpene synthase, was determined to be a (16R)-ent-kauran-16-ol synthase, the first of its kind found in bacteria. PtmO5, a cytochrome P450 monooxygenase, is proposed to catalyze the formation of the characteristic 11S,16S-ether ring found in PTM. Inactivation of ptmO5 in SB12029 afforded the ΔptmO5 mutant SB12036 that accumulated nine PTM and PTN congeners, seven of which were new, including seven 11-deoxy-16R-hydroxy-PTM congeners. The two fully processed PTM analogues showed antibacterial activities, albeit lower than that of PTM, indicating that the ether ring, or minimally the stereochemistry of the hydroxyl group at C-16, is crucial for the activity of PTM.

Platensimycin and platencin: Inspirations for chemistry, biology, enzymology, and medicine

Natural products have served as the main source of drugs and drug leads, and natural products produced by microorganisms are one of the most prevalent sources of clinical antibiotics. Their unparalleled structural and chemical diversities provide a basis to investigate fundamental biological processes while providing access to a tremendous amount of chemical space. There is a pressing need for novel antibiotics with new mode of actions to combat the growing challenge of multidrug resistant pathogens. This review begins with the pioneering discovery and biological activities of platensimycin (PTM) and platencin (PTN), two antibacterial natural products isolated from Streptomyces platensis. The elucidation of their unique biochemical mode of action, structure-activity relationships, and pharmacokinetics is presented to highlight key aspects of their biological activities. It then presents an overview of how microbial genomics has impacted the field of PTM and PTN and revealed paradigm-shifting discoveries in terpenoid biosynthesis, fatty acid metabolism, and antibiotic and antidiabetic therapies. It concludes with a discussion covering the future perspectives of PTM and PTN in regard to natural products discovery, bacterial diterpenoid biosynthesis, and the pharmaceutical promise of PTM and PTN as antibiotics and for the treatment of metabolic disorders. PTM and PTN have inspired new discoveries in chemistry, biology, enzymology, and medicine and will undoubtedly continue to do so.

A Mutasynthetic Library of Platensimycin and Platencin Analogues

Inactivation of ptmB1, ptmB2, ptmT2, or ptmC in Streptomyces platensis SB12029, a platensimycin (PTM) and platencin (PTN) overproducer, revealed that PTM and PTN biosynthesis features two distinct moieties that are individually constructed and convergently coupled to afford PTM and PTN. A focused library of PTM and PTN analogues was generated by mutasynthesis in the ΔptmB1 mutant S. platensis SB12032. Of the 34 aryl variants tested, 18 were incorporated with high titers.

A genetically amenable platensimycin- and platencin-overproducer as a platform for biosynthetic explorations: a showcase of PtmO4, a long-chain acyl-CoA dehydrogenase

Platensimycin (PTM) and platencin (PTN) are members of a new class of promising drug leads that target bacterial and mammalian fatty acid synthases. We previously cloned and sequenced the PTM and PTN gene clusters, discovered six additional PTM-PTN dual producing strains, and demonstrated the dramatic overproduction of PTM and PTN by inactivating the pathway-specific regulators ptmR1 or ptnR1 in five different strains. Our ability to utilize these PTM-PTN dual overproducing strains was limited by their lack of genetic amenability. Here we report the construction of Streptomyces platensis SB12029, a genetically amenable, in-frame ΔptmR1 dual PTM-PTN overproducing strain. To highlight the potential of this strain for future PTM and PTN biosynthetic studies, we created the ΔptmR1 ΔptmO4 double mutant S. platensis SB12030. Fourteen PTM and PTN congeners, ten of which were new, were isolated from SB12030, shedding new insights into PTM and PTN biosynthesis. PtmO4, a long-chain acyl-CoA dehydrogenase, is strongly implicated to catalyze β-oxidation of the diterpenoid intermediates into the PTM and PTN scaffolds. SB12029 sets the stage for future biosynthetic and bioengineering studies of the PTM and PTN family of natural products.

Review of Platensimycin and Platencin: Inhibitors of β-Ketoacyl-acyl Carrier Protein (ACP) Synthase III (FabH)

Platensimycin and platencin were successively discovered from the strain Streptomyces platensis through systematic screening. These natural products have been defined as promising agents for fighting multidrug resistance in bacteria by targeting type II fatty acid synthesis with slightly different mechanisms. Bioactivity studies have shown that platensimycin and platencin offer great potential to inhibit many resistant bacteria with no cross-resistance or toxicity observed in vivo. This review summarizes the general information on platensimycin and platencin, including antibacterial and self-resistant mechanisms. Furthermore, the total synthesis pathways of platensimycin and platencin and their analogues from recent studies are presented.

Antibiotics and bacterial resistance in the 21st century

Dangerous, antibiotic resistant bacteria have been observed with increasing frequency over the past several decades. In this review the factors that have been linked to this phenomenon are addressed. Profiles of bacterial species that are deemed to be particularly concerning at the present time are illustrated. Factors including economic impact, intrinsic and acquired drug resistance, morbidity and mortality rates, and means of infection are taken into account. Synchronously with the waxing of bacterial resistance there has been waning antibiotic development. The approaches that scientists are employing in the pursuit of new antibacterial agents are briefly described. The standings of established antibiotic classes as well as potentially emerging classes are assessed with an emphasis on molecules that have been clinically approved or are in advanced stages of development. Historical perspectives, mechanisms of action and resistance, spectrum of activity, and preeminent members of each class are discussed.

The use of platensimycin and platencin to fight antibiotic resistance

Infectious diseases are known as one of the most life-threatening disabilities worldwide. Approximately 13 million deaths related to infectious diseases are reported each year. The only way to combat infectious diseases is by chemotherapy using antimicrobial agents and antibiotics. However, due to uncontrolled and unnecessary use of antibiotics in particular, surviving bacteria have evolved resistance against several antibiotics. Emergence of multidrug resistance in bacteria over the past several decades has resulted in one of the most important clinical health problems in modern medicine. For instance, approximately 440,000 new cases of multidrug-resistant tuberculosis are reported every year leading to the deaths of 150,000 people worldwide. Management of multidrug resistance requires understanding its molecular basis and the evolution and dissemination of resistance; development of new antibiotic compounds in place of traditional antibiotics; and innovative strategies for extending the life of antibiotic molecules. Researchers have begun to develop new antimicrobials for overcoming this important problem. Recently, platensimycin - isolated from extracts of Streptomyces platensis - and its analog platencin have been defined as promising agents for fighting multidrug resistance. In vitro and in vivo studies have shown that these new antimicrobials have great potential to inhibit methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci, and penicillin-resistant Streptococcus pneumoniae by targeting type II fatty acid synthesis in bacteria. Showing strong efficacy without any observed in vivo toxicity increases the significance of these antimicrobial agents for their use in humans. However, at the present time, clinical trials are insufficient and require more research. The strong antibacterial efficacies of platensimycin and platencin may be established in clinical trials and their use in humans for coping with multidrug resistance may be allowed in the foreseeable future.

A carbohydrate-based approach for the total synthesis of (-)-dinemasone B, (+)-4a-epi-dinemasone B, (-)-7-epi-dinemasone B, and (+)-4a,7-Di-epi-dinemasone B

(-)-Dinemasone B was isolated by Krohn and co-workers from a culture of the endophytic fungus Dinemasporium strigosum and has shown promising antimicrobial activity. Described herein is the first total synthesis of (-)-dinemasone B, (+)-4a-epi-dinemasone B, (-)-7-epi-dinemasone B, and (+)-4a,7-di-epi-dinemasone B. Their absolute configurations were also determined. The developed synthesis features a stereoselective reduction of C-glycosidic ketone, lactonization, and E-olefination of aldehyde starting from D-glucose.

Inhibitors of fatty acid synthesis in prokaryotes and eukaryotes as anti-infective, anticancer and anti-obesity drugs

There is a large range of diseases, such diabetes and cancer, which are connected to abnormal fatty acid metabolism in human cells. Therefore, inhibitors of human fatty acid synthase have great potential to manage or treat these diseases. In prokaryotes, fatty acid synthesis is important for signaling, as well as providing starting materials for the synthesis of phospholipids, which are required for the formation of the cell membrane. Recently, there has been renewed interest in the development of new molecules that target bacterial fatty acid synthases for the treatment of bacterial diseases. In this review, we look at the differences and similarities between fatty acid synthesis in humans and bacteria and highlight various small molecules that have been shown to inhibit either the mammalian or bacterial fatty acid synthase or both.

Constructing molecular complexity and diversity: total synthesis of natural products of biological and medicinal importance

The advent of organic synthesis and the understanding of the molecule as they occurred in the nineteenth century and were refined in the twentieth century constitute two of the most profound scientific developments of all time. These discoveries set in motion a revolution that shaped the landscape of the molecular sciences and changed the world. Organic synthesis played a major role in this revolution through its ability to construct the molecules of the living world and others like them whose primary element is carbon. Although the early beginnings of organic synthesis came about serendipitously, organic chemists quickly recognized its potential and moved decisively to advance and exploit it in myriad ways for the benefit of mankind. Indeed, from the early days of the synthesis of urea and the construction of the first carbon-carbon bond, the art of organic synthesis improved to impressively high levels of sophistication. Through its practice, today chemists can synthesize organic molecules--natural and designed--of all types of structural motifs and for all intents and purposes. The endeavor of constructing natural products--the organic molecules of nature--is justly called both a creative art and an exact science. Often called simply total synthesis, the replication of nature's molecules in the laboratory reflects and symbolizes the state of the art of synthesis in general. In the last few decades a surge in total synthesis endeavors around the world led to a remarkable collection of achievements that covers a wide ranging landscape of molecular complexity and diversity. In this article, we present highlights of some of our contributions in the field of total synthesis of natural products of biological and medicinal importance. For perspective, we also provide a listing of selected examples of additional natural products synthesized in other laboratories around the world over the last few years.

Platensimycin and platencin: promising antibiotics for future application in human medicine

Platensimycin and platencin are novel antibiotics produced by Streptomyces platensis. They are potent and non-toxic natural products active against Gram-positive pathogens, including antibiotic-resistant strains and Mycobacterium tuberculosis. They were isolated using an intriguing target-based whole-cell antisense differential sensitivity assay as inhibitors of fatty acid biosynthesis of type II. This type of biosynthesis is not present in humans. Platensimycin inhibits the elongation-condensing enzyme FabF, whereas platencin inhibits both FabF and FabH. For these antibiotics to become successful drugs, their pharmacokinetics must be improved. They have too high a rate of clearance in the body, yielding a low degree of systematic exposure. They work well when administered by continuous infusion, but this is not a useful method of delivery to patients. The two antibiotics and many analogs have been prepared by chemical synthesis. Natural congeners have also been obtained from the producing actinomycete. However, none of these molecules are as active as platensimycin and platencin. Using tools of rational metabolic engineering, superior strains have been produced making hundreds of times more antibiotic than the natural strains.