Total Synthesis and Biological Evaluation of Tiancimycins A and B, Yangpumicin A, and Related Anthraquinone-Fused Enediyne Antitumor Antibiotics

Collective total synthesis of chartreusin derivatives and bioactivity investigations

Capitalizing on Hauser annulation and Yu glycosylation, the chemical synthesis of chartreusin-type aromatic polycyclic polyketide glycosides has been investigated, culminating in the successful establishment of chemical approaches toward chartreusin derivatives with intricate chemical structures but promising bioactivities. Based on the chemical synthesis strategy, the first and collective chemical syntheses of chartreusin, D329C, and elsamicins A and B have been accomplished. The chemical strategy was featured by two complementary routes to secure chartarin 10-O-monosaccharide glycosides, the key intermediates in chartreusin derivative synthesis, as well as the highly stereoselective construction of the difficult glycosidic linkages. Through the synthetic investigations, viable donors and acceptors of 3-C-methyl-branched sugars were determined for the first time. Moreover, facilitated by the established chemical synthetic strategy, the cytotoxic activities of chartreusin derivatives against human cancer cell lines were assessed and profound antineoplastic effects for chartreusin and elsamicins A and B were recorded. Based on RNA-seq analysis, the underlying working mechanisms against ES-2 cells were investigated, and the appended sugar chain-determined function mechanisms were disclosed.

The exploration of high production of tiancimycins in Streptomyces sp. CB03234-S revealed potential influences of universal stress proteins on secondary metabolisms of streptomycetes

BACKGROUND

Universal stress proteins (USPs) are prevalent in various bacteria to cope with different adverse stresses, while their possible effects on secondary metabolisms of hosts are unclear. Tiancimycins (TNMs) are ten-membered endiynes possessing excellent potential for development of anticancer antibody-drug conjugates. During our efforts to improve TNMs titer, a high-producing strain Streptomyces sp. CB03234-S had been obtained and its possible high yield mechanism is being continuously explored to further enhance TNMs production.

RESULTS

In this work, the whole-genome resequencing and analysis results revealed a notable 583 kb terminal deletion containing 8 highly expressed usp genes in the genome of CB03234-S. The individual complementation of lost USPs in CB03234-S all showed differential effects on secondary metabolism, especially TNMs production. Among them, the overexpression of USP3 increased TNMs titer from 12.8 ± 0.2 to 31.1 ± 2.3 mg/L, while the overexpression of USP8 significantly reduced TNMs titer to only 1.0 ± 0.1 mg/L, but activated the production of porphyrin-type compounds. Subsequent genetic manipulations on USP3/USP8 orthologs in Streptomyces. coelicolor A3(2) and Streptomyces sp. CB00271 also presented clear effects on the secondary metabolisms of hosts. Further sequence similarity network analysis and Streptomyces-based pan‑genomic analysis suggested that the USP3/USP8 orthologs are widely distributed across Streptomyces.

CONCLUSION

Our studies shed light on the potential effects of USPs on secondary metabolisms of streptomycetes for the first time, and USPs could become novel targets for exploring and exploiting natural products in streptomycetes.

Genomic Comparisons Revealed the Key Genotypes of Streptomyces sp. CB03234-GS26 to Optimize Its Growth and Relevant Production of Tiancimycins

Strain robustness and titer improvement are major challenges faced in the industrial development of natural products from Streptomyces. Tiancimycins (TNMs) produced by Streptomyces sp. CB03234 are promising anticancer payloads for antibody-drug conjugates, but further development is severely limited by the low titer of TNMs. Despite many efforts to generate various TNMs overproducers, the mechanisms underlying high TNMs production remain to be explored. Herein, genome resequencing and genomic comparisons of different TNMs overproducers were conducted to explore the unique genotypes in CB03234-GS26. Four target genes were selected for further bioinformatic analyses and genetic validations. The results indicated that the inactivation of histidine ammonia-lyase (HAL) showed the most significant effect by blocking the intracellular degradation of histidine to facilitate relevant enzymatic catalysis and thus improve the production of TNMs. Additionally, the potassium/proton antiporter (P/PA) was crucial for intracellular pH homeostasis, and its deficiency severely impaired the alkaline tolerance of the cells. Subsequent pan-genomic analysis suggested that HAL and P/PA are core enzymes that are highly conserved in Streptomyces. Therefore, HAL and P/PA represented novel targets to regulate secondary metabolism and enhance strain robustness and could become potential synthetic biological modules to facilitate development of natural products and strain improvement in Streptomyces.

Natural-source payloads used in the conjugated drugs architecture for cancer therapy: Recent advances and future directions

Drug conjugates are obtained from tumor-located vectors connected to cytotoxic agents via linkers, which are designed to deliver hyper-toxic payloads directly to targeted cancer cells. These drug conjugates include antibody-drug conjugates (ADCs), peptide-drug conjugates (PDCs), small molecule-drug conjugates (SMDCs), nucleic acid aptamer-drug conjugates (ApDCs), and virus-like drug conjugate (VDCs), which show great therapeutic value in the clinic. Drug conjugates consist of a targeting carrier, a linker, and a payload. Payloads are key therapy components. Cytotoxic molecules and their derivatives derived from natural products are commonly used in the payload portion of conjugates. The ideal payload should have sufficient toxicity, stability, coupling sites, and the ability to be released under specific conditions to kill tumor cells. Microtubule protein inhibitors, DNA damage agents, and RNA inhibitors are common cytotoxic molecules. Among these conjugates, cytotoxic molecules of natural origin are summarized based on their mechanism of action, conformational relationships, and the discovery of new derivatives. This paper also mentions some cytotoxic molecules that have the potential to be payloads. It also summarizes the latest technologies and novel conjugates developed in recent years to overcome the shortcomings of ADCs, PDCs, SMDCs, ApDCs, and VDCs. In addition, this paper summarizes the clinical trials conducted on conjugates of these cytotoxic molecules over the last five years. It provides a reference for designing and developing safer and more efficient conjugates.

Enantioselective Synthesis of Sealutomicin C

The sealutomicins are a family of anthraquinone antibiotics featuring an enediyne (sealutomicin A) or Bergman-cyclized aromatic ring (sealutomicins B-D). Herein we report the development of an enantioselective organocatalytic method for the synthesis of dihydroquinolines and the use of the developed method in the total synthesis of sealutomicin C which features a transannular cyclization of an aryllithium onto a γ-lactone as a second key step.

The occurrence and bioactivity of tetrahydronaphthoquinoline-diones (THNQ-dione)

Natural products have been important in the discovery of new drugs, but their use is limited due to issues with accessibility and synthesis. Tetrahydronaphthoquinoline-dione (THNQ-dione) is a key structural feature found in several natural and synthetic compounds that exhibit notable biological properties. The unique properties of THNQ-diones can be attributed to the fusion of tetrahydroquinoline and anthraquinone moieties. These alkaloids are synthesised through various biosynthetic pathways, leading to diverse structures and bioactivities. Despite their significance, THNQ-diones have not been extensively covered in the review literature, highlighting the importance of this article in discussing their natural occurrence and biological activities. This article explores the distribution of THNQ-dione alkaloids in different organisms and their potential as a source of novel bioactive natural products.

DNA Interaction and Cleavage Modes of Anthraquinone-Fused Enediynes: A Study on Tiancimycins, Yangpumicins, and Their Semisynthetic Analogues

Dynemicin A has been the sole prototypical anthraquinone-fused enediyne (AFE) explored since its discovery in 1989. This study investigates the distinct DNA binding and cleavage mechanisms of emerging AFEs, represented by tiancimycins and yangpumicins, along with semisynthetic analogues. Our findings reveal their potent cytotoxicity against various tumor cell lines, while 18-methoxy tiancimycin A treatment could significantly suppress breast tumor growth with minimal toxicity. One of the most potent AFEs, i.e., tiancimycin A, preferentially targets DNA sequences 5'-ATT, 5'-CTT, 5'-GAA, 5'-GAT, and 5'-TTA. Molecular dynamics simulations suggest that emerging AFEs intercalate deeper into AT-rich DNA base pairs compared to dynemicin A. Importantly, tiancimycin A may equilibrate between insertional and intercalative modes without deintercalation, enabling selective cleavage of T and A bases. This study underscores how subtle structural variations among AFEs significantly influence their DNA recognition and cleavage, facilitating future design of novel AFEs as potent and selective payloads for antibody-drug conjugates.

Cofactorless oxygenases guide anthraquinone-fused enediyne biosynthesis

The anthraquinone-fused enediynes (AFEs) combine an anthraquinone moiety and a ten-membered enediyne core capable of generating a cytotoxic diradical species. AFE cyclization is triggered by opening the F-ring epoxide, which is also the site of the most structural diversity. Previous studies of tiancimycin A, a heavily modified AFE, have revealed a cryptic aldehyde blocking installation of the epoxide, and no unassigned oxidases could be predicted within the tnm biosynthetic gene cluster. Here we identify two consecutively acting cofactorless oxygenases derived from methyltransferase and α/β-hydrolase protein folds, TnmJ and TnmK2, respectively, that are responsible for F-ring tailoring in tiancimycin biosynthesis by comparative genomics. Further biochemical and structural characterizations reveal that the electron-rich AFE anthraquinone moiety assists in catalyzing deformylation, epoxidation and oxidative ring cleavage without exogenous cofactors. These enzymes therefore fill important knowledge gaps for the biosynthesis of this class of molecules and the underappreciated family of cofactorless oxygenases.

Dynemicin A Derivatives as Potential Cancer Chemotherapeutics by Mutasynthesis

The enediyne antitumor antibiotics have remarkable structures and exhibit potent DNA cleavage properties that have inspired continued interest as cancer therapeutics. Their complex structures and high reactivity, however, pose formidable challenges to their production and development in the clinic. We report here proof-of-concept studies using a mutasynthesis strategy to combine chemical synthesis of select modifications to a key iodoanthracene-γ-thiolactone intermediate in the biosynthesis of dynemicin A and all other known anthraquinone-fused enediynes (AFEs). By chemical complementation of a mutant bacterial producer that is incapable of synthesizing this essential building block, we show that derivatives of dynemicin can be prepared substituted in the A-ring of the anthraquinone motif. In the absence of competition from native production of this intermediate, the most efficient utilization of these externally-supplied structural analogues for precursor-directed biosynthesis becomes possible. To achieve this goal, we describe the required Δorf15 blocked mutant and a general synthetic route to a library of iodoanthracene structural variants. Their successful incorporation opens the door to enhancing DNA binding and tuning the bioreductive activation of the modified enediynes for DNA cleavage.

Characterization of the Antibacterial Activity of Quinone-Based Compounds Originating from the Alnumycin Biosynthetic Gene Cluster of a Streptomyces Isolate

Bacteria of the genus Streptomyces produce various specialized metabolites. Single biosynthetic gene clusters (BGCs) can give rise to different products that can vary in terms of their biological activities. For example, for alnumycin and the shunt product K115, antimicrobial activity was described, while no antimicrobial activity was detected for the shunt product 1,6-dihydro 8-propylanthraquinone. To investigate the antibacterial activity of 1,6-dihydro 8-propylanthraquinone, we produced alnumycin and 1,6-dihydro 8-propylanthraquinone from a Streptomyces isolate containing the alnumycin BGC. The strain was cultivated in liquid glycerol-nitrate-casein medium (GN), and both compounds were isolated using an activity and mass spectrometry-guided purification. The structures were validated via nuclear magnetic resonance (NMR) spectroscopy. A minimal inhibitory concentration (MIC) test revealed that 1,6-dihydro 8-propylanthraquinone exhibits antimicrobial activity against E. coli ΔtolC, B. subtilis, an S. aureus type strain, and a vancomycin intermediate-resistance S. aureus strain (VISA). Activity of 1,6-dihydro 8-propylanthraquinone against E. coli ΔtolC was approximately 10-fold higher than that of alnumycin. We were unable to confirm gyrase inhibition for either compound and believe that the modes of action of both compounds are worth reinvestigating.

Normal and Aberrant Methyltransferase Activities Give Insights into the Final Steps of Dynemicin A Biosynthesis

The naturally occurring enediynes are notable for their complex structures, potent DNA cleaving ability, and emerging usefulness in cancer chemotherapy. They can be classified into three distinct structural families, but all are thought to originate from a common linear C15-heptaene. Dynemicin A (DYN) is the paradigm member of anthraquinone-fused enediynes, one of the three main classes and exceptional among them for derivation of both its enediyne and anthraquinone portions from this same early biosynthetic building block. Evidence is growing about how two structurally dissimilar, but biosynthetically related, intermediates combine in two heterodimerization reactions to create a nitrogen-containing C30-coupled product. We report here deletions of two genes that encode biosynthetic proteins that are annotated as S-adenosylmethionine (SAM)-dependent methyltransferases. While one, DynO6, is indeed the required O-methyltransferase implicated long ago in the first studies of DYN biosynthesis, the other, DynA5, functions in an unanticipated manner in the post-heterodimerization events that complete the biosynthesis of DYN. Despite its removal from the genome of Micromonospora chersina, the ΔdynA5 strain retains the ability to synthesize DYN, albeit in reduced titers, accompanied by two unusual co-metabolites. We link the appearance of these unexpected structures to a substantial and contradictory body of other recent experimental data to advance a biogenetic rationale for the downstream steps that lead to the final formation of DYN. A sequence of product-forming transformations that is in line with new and existing experimental results is proposed and supported by a model reaction that also encompasses the formation of the crucial epoxide essential for the activation of DYN for DNA cleavage.

Application of a Biocatalytic Strategy for the Preparation of Tiancimycin-Based Antibody-Drug Conjugates Revealing Key Insights into Structure-Activity Relationships

Antibody-drug conjugates (ADCs) are cancer chemotherapeutics that utilize a monoclonal antibody (mAb)-based delivery system, a cytotoxic payload, and a chemical linker. ADC payloads must be strategically functionalized to allow linker attachment without perturbing the potency required for ADC efficacy. We previously developed a biocatalytic system for the precise functionalization of tiancimycin (TNM)-based payloads. The TNMs are anthraquinone-fused enediynes (AFEs) and have yet to be translated into the clinic. Herein, we report the translation of biocatalytically functionalized TNMs into ADCs in combination with the dual-variable domain (DVD)-mAb platform. The DVD enables both site-specific conjugation and a plug-and-play modularity for antigen-targeting specificity. We evaluated three linker chemistries in terms of TNM-based ADC potency and antigen selectivity, demonstrating a trade-off between potency and selectivity. This represents the first application of AFE-based payloads to DVDs for ADC development, a workflow that is generalizable to further advance AFE-based ADCs for multiple cancer types.

Functionalized 10-Membered Aza- and Oxaenediynes through the Nicholas Reaction

The scope and limitations of the Nicholas-type cyclization for the synthesis of 10-membered benzothiophene-fused heterocyclic enediynes with different functionalities were investigated. Although the Nicholas cyclization through oxygen could be carried out in the presence of an ester group, the final oxaenediyne was unstable under storage. Among the N-type Nicholas reactions, cyclization via an arenesulfonamide functional group followed by mild Co-deprotection was found to be the most promising, yielding 10-membered azaendiynes in high overall yields. By contrast, the Nicholas cyclization through the acylated nitrogen atom did not give the desired 10-membered cycle. It resulted in the formation of a pyrroline ring, whereas cyclization via an alkylated amino group resulted in a poor yield of the target 10-membered enediyne. The acylated 4-aminobenzenesulfonamide nucleophilic group was found to be the most convenient for the synthesis of functionalized 10-membered enediynes bearing a clickable function, such as a terminal triple bond. All the synthesized cyclic enediynes exhibited moderate activity against lung carcinoma NCI-H460 cells and had a minimal effect on lung epithelial-like WI-26 VA4 cells and are therefore promising compounds in the search for novel antitumor agents that can be converted into conjugates with tumor-targeting ligands.

Micromonospora: A Prolific Source of Bioactive Secondary Metabolites with Therapeutic Potential

Micromonospora, one of the most important actinomycetes genera, is well-known as the treasure trove of bioactive secondary metabolites (SMs). Herein, together with an in-depth genomic analysis of the reported Micromonospora strains, all SMs from this genus are comprehensively summarized, containing structural features, bioactive properties, and mode of actions as well as their biosynthetic and chemical synthesis pathways. The perspective enables a detailed view of Micromonospora-derived SMs, which will enrich the chemical diversity of natural products and inspire new drug discovery in the pharmaceutical industry.

Nucleophilic Addition of Enolates to 1,4-Dehydrobenzene Diradicals Derived from Enediynes: Synthesis of Functionalized Aromatics

Alkylation of aromatics and formation of a new C-C bond is usually achieved by the electrophilic attack of an activated carbon species on an electron-rich aromatic ring. Herein, we report an alternative method for alkylation of aromatics via nucleophilic addition of enolates of active methylene compounds to 1,4-dehydrobenzene diradicals derived from enediynes cyclodec-1,5-diyne-3-ene, benzo[3,4]-cyclodec-1,5-diyne-3-ene, and cyclohexeno[3,4]-cyclodec-1,5-diyne-3-ene. The benzo-substituted enediyne produces slightly higher yields of alkylation products than do the other two enediynes, but the differences are not substantial. The reaction produces a new C-C bonded aromatic alkylation product, which allows the construction of complex polyfunctional structures in a few steps. Moreover, this reaction provides solely C-arylated products, and no O-arylation products were observed.

Liposome-Encapsulated Tiancimycin A Is Active against Melanoma and Metastatic Breast Tumors: The Effect of cRGD Modification of the Liposomal Carrier and Tiancimycin A Dose on Drug Activity and Toxicity

Enediyne natural products, including neocarzinostatin and calicheamicin γ¹, are used in the form of a copolymer or antibody-drug conjugate to treat hepatomas and leukemia. Tiancimycin (TNM) A is a novel anthraquinone-fused enediyne that can rapidly and completely kill tumor cells. Herein, we encapsulated TNM A in liposomes (Lip-TNM A) and cyclic arginine-glycine-aspartate (cRGD)-functionalized liposomes (cRGD-Lip-TNM A) and demonstrated its antitumor activity using mouse xenografts. Because TNM A causes rapid DNA damage, cell cycle arrest, and apoptosis, these nanoparticles exhibited potent cytotoxicity against multiple tumor cells for 8 h. In B16-F10 and KPL-4 xenografts, both nanoparticles showed superior potency over doxorubicin and trastuzumab. However, cRGD-Lip-TNM A reduced the tumor weight more significantly than Lip-TNM A in B16-F10 xenografts, in which the α_vβ₃-integrin receptors are significantly overexpressed in this melanoma. Lip-TNM A was slightly more active than cRGD-Lip-TNM A against KPL-4 xenografts, which probably reflected the difference of their in vivo fate in this mouse model. In a highly metastatic 4T1 tumor model, cRGD-Lip-TNM A reduced tumor metastasis induced by losartan, a tumor microenvironment-remodeling agent. These findings suggest that targeted delivery of enediynes with unique modes of action may enable more effective translation of anticancer nanomedicines.

A review on hydroxy anthraquinones from bacteria: crosstalk's of structures and biological activities

Anthraquinones (AQ), unveiling large structural diversity, among polyketides demonstrate a wide range of applications. The hydroxy anthraquinones (HAQ), a group of anthraquinone derivatives, are secondary metabolites produced by bacteria and eukaryotes. Plant-based HAQ are well-studied unlike bacterial HAQ and applied as herbal medicine for centuries. Bacteria are known to synthesize a wide variety of structurally diversified HAQ through polyketide pathways using polyketide synthases (I, II & III) principally through polyketide synthase-II. The actinobacteria especially the genus Streptomyces and Micromonospora represent a rich source of HAQ, however novel HAQ are reported from the rare actinobacteria genera (Salinospora, Actinoplanes, Amycoloptosis, Verrucosispora, Xenorhabdus, and Photorhabdus. Though several reviews are available on AQ produced by plants and fungi, however none on bacterial AQ. The current review focused on sources of bacterial HAQ and their structural diversity and biological activities along with toxicity and side effects.

Functional Characterization of Cytochrome P450 Hydroxylase YpmL in Yangpumicin A Biosynthesis and Its Application for Anthraquinone-Fused Enediyne Structural Diversification

Comparative analyses of four anthraquinone-fused enediyne biosynthetic gene clusters (BGCs) identified YpmL as a cytochrome P450 enzyme unique to the yangpumicin (YPM) BGC. In vitro characterization of YpmL established it as a hydroxylase, catalyzing C-6 hydroxylation in YPM A biosynthesis. In vivo application of YpmL enabled engineered production of four new tiancimycin analogues (14-17). Evaluation of their cytotoxicity against selected human cancer cell lines shed new insights into the enediyne structure-activity relationship.

Recent total syntheses of anthraquinone-based natural products

The scientific community has found deep interest in anthraquinone-based compounds due to their therapeutic properties and challenging structural elements. Various architecturally beautiful natural products have been successfully synthesized in recent decades utilizing two main strategies: either an early-stage synthesis of the anthraquinone and further elongation of the system, or a late-stage introduction of the anthraquinone ring moiety. Select syntheses of complex anthraquinone monomers and dimers within the past 20 years are described with an emphasis on the retrosynthetic disconnections that shape the anthraquinone-installation strategy.

Directed ortho and Remote Metalation-Suzuki-Miyaura Cross Coupling Route to Azafluorenol Core Liquid Crystals

Two new smectic C* mesogens containing a hexyloxy side chain and an azafluorenone (3a) or azafluorenol (3b) core were synthesized using a combined directed ortho metalation-directed remote metalation-Suzuki-Miyaura cross-coupling strategy. 3b was formed in 10 steps and 25% overall yield based on starting benzamide 1a. 3a forms a nematic phase, while 3b forms a smectic A phase. The large temperature range of the smectic phase for the azafluorenol 3b is indicative of mesophase stabilization by intermolecular hydrogen bonding between the hydroxyl group and pyridine nitrogen of neighboring 3b molecules.

Anthraquinone-fused enediynes: discovery, biosynthesis and development

Covering: up to the end of July, 2021Anthraquinone-fused enediynes (AFEs) are a subfamily of enediyne natural products. Dynemicin A (DYN A), the first member of the AFE family, was discovered more than thirty years ago. Subsequently, extensive studies have been reported on the mode of action and the interactions of AFEs with DNA using DYN A as a model. However, progress in the discovery, biosynthesis and clinical development of AFEs has been limited for a long time. In the past five years, four new AFEs have been discovered and significant progress has been made in the biosynthesis of AFEs, especially on the biogenesis of the anthraquinone moiety and their tailoring steps. Moreover, the streamlined total synthesis of AFEs and their analogues boosts the preparation of AFE-based linker-drugs, thus enabling the development of AFE-based antibody-drug conjugates (ADCs). This review summarizes the discovery, mechanism of action, biosynthesis, total synthesis and preclinical studies of AFEs.

Cyclohexeno[3,4]cyclodec-1,5-diyne-3-ene: A Convenient Enediyne

Enediynes are widely studied to understand their cycloaromatization and the trapping of the resulting p-dehydrobenzene diradical. However, few model substrates are known, and they are hard to synthesize and difficult to handle. Herein we report cyclohexeno[3,4]cyclodec-1,5-diyne-3-ene as a convenient model for studying the reactivity of enediynes. It can be easily synthesized from 1,2-diethynylcyclohexene and 1,4-diiodobutane. It is a solid that is stable at room temperature. In solution the p-dehydrobenzene diradical derived from its cycloaromatization can be trapped by nucleophiles. The rate-limiting step is the cyclization, which is slightly slower than that of the parent cyclodec-1,5-diyne-3-ene but faster than that of its benzo analogue, consistent with the distances between the reacting carbon atoms.

Yield improvement of enediyne yangpumicins in Micromonospora yangpuensis through ribosome engineering and fermentation optimization

Yangpumicins (YPMs), for example, YPM A, F, and G, are newly discovered enediynes from Micromonospora yangpuensis DSM 45577, which could be exploited as promising payloads of antibody-drug conjugates. However, the low yield of YPMs in the wild-type strain (∼1 mg L^-1 ) significantly hampers their further drug development. In this study, a combined ribosome engineering and fermentation optimization strategy has been used for yield improvement of YPMs. One gentamicin-resistant M. yangpuensis DSM 45577 strain (MY-G-1) showed higher YPMs production (7.4 ± 1.0 mg L^-1 ), while it exhibits delayed sporulation and slender mycelium under scanning electron microscopy. Whole genome re-sequencing of MY-G-1 reveals several deletion and single nucleotide polymorphism mutations, which were confirmed by PCR and DNA sequencing. Further Box-Behnken experiment and regression analysis determined that the optimal medium concentrations of soluble starch, D-mannitol, and pharmamedia for YPMs production in shaking flasks (10.0 ± 0.8 mg L^-1 ). Finally, the total titer of YPM A/F/G in MY-G-1 reached to 15.0 ± 2.5 mg L^-1 in 3 L fermenters, which was about 11-fold higher than the original titer of 1.3 ± 0.3 mg L^-1 in wild-type strain. Our study may be instrumental to develop YPMs into a clinical anticancer drug, and inspire the use of these multifaceted strategies for yield improvement in Micromonospora species. GRAPHICAL ABSTRACT LAY SUMMARY: ???

A carrier-free nanoparticle with dual NIR/acid responsiveness by co-assembly of enediyne and IR820 for combined PTT/chemotherapy

Combined photothermal therapy/chemotherapy by co-delivery of a photosensitizer (PS) and a chemotherapeutic drug has demonstrated great potential for cancer treatment. The intrinsic drawbacks of traditional drug delivery systems (DDSs), such as tedious synthetic procedures, side effects originated from the carrier materials, low loading efficiency, and uncontrolled drug release, however, have impaired their further advancement. On the other hand, enediyne antibiotics are highly cytotoxic toward cancer cells through the generation of lethal carbon radicals via thermal-induced cyclization, endowing them with great potential to achieve enhanced synergistic anticancer performance by incorporation with the photothermal effect of PS. To this end, a carrier-free and NIR/acid dual-responsive DDS was constructed for combined photothermal therapy/chemotherapy. The facile co-assembly of maleimide-based enediyne and PS IR820 was achieved in aqueous solution to give nanoparticles (EICN) with a hydrodynamic diameter of 90 nm and high stability. In vitro study confirmed the acid/NIR dual-responsive degradation and drug release, free radical generation and DNA-cleaving ability of EICN, which was accomplished by the corporation of enediyne and IR820 moieties. Further tests on HeLa cells verified the excellent synergistic anticancer performance of EICN including the improved cellular uptake, NIR-enhanced drug release, DNA damage and histone deacetylase inhibitor capacity. Overall, this carrier-free DDS with dual acid/NIR-responsivity would potentially provide new insights for the development of combined photothermal/chemotherapy.

Pathway Retrofitting Yields Insights into the Biosynthesis of Anthraquinone-Fused Enediynes

Anthraquinone-fused enediynes (AQEs) are renowned for their distinctive molecular architecture, reactive enediyne warhead, and potent anticancer activity. Although the first members of AQEs, i.e., dynemicins, were discovered three decades ago, how their nitrogen-containing carbon skeleton is synthesized by microbial producers remains largely a mystery. In this study, we showed that the recently discovered sungeidine pathway is a "degenerative" AQE pathway that contains upstream enzymes for AQE biosynthesis. Retrofitting the sungeidine pathway with genes from the dynemicin pathway not only restored the biosynthesis of the AQE skeleton but also produced a series of novel compounds likely as the cycloaromatized derivatives of chemically unstable biosynthetic intermediates. The results suggest a cascade of highly surprising biosynthetic steps leading to the formation of the anthraquinone moiety, the hallmark C8-C9 linkage via alkyl-aryl cross-coupling, and the characteristic epoxide functionality. The findings provide unprecedented insights into the biosynthesis of AQEs and pave the way for examining these intriguing biosynthetic enzymes.

Synthesis of natural products containing highly strained trans-fused bicyclo[3.3.0]octane: historical overview and future prospects

Owing to high strain energy, molecules with trans-fused bicyclo[3.3.0]octane ring systems are very difficult to synthesize, and there are very few approaches to access them. Recently, a number of natural products with such ring systems have been made by the synthetic community. However, there has been no review in this field before. This review provides a systematic and comprehensive discussion on the synthesis of natural products containing trans-fused bicyclo[3.3.0]octanes and the historical context of this work. The prospects for future research in this field are also discussed. Covering the literature before 2021, this review aims to offer a helpful reference for total synthesis of highly strained natural products containing trans-fused bicyclo[3.3.0]octane ring systems.

Applications of the Horner–Wadsworth–Emmons Olefination in Modern Natural Product Synthesis

The Horner–Wadsworth–Emmons (HWE) reaction is one of the most reliable olefination reaction and can be broadly applied in organic chemistry and natural product synthesis with excellent selectivity. Within the last few years HWE reaction conditions have been optimized and new reagents developed to overcome challenges in the total syntheses of natural products. This review highlights the application of HWE olefinations in total syntheses of structurally different natural products covering 2015 to 2020. Applied HWE reagents and reactions conditions are highlighted to support future synthetic approaches and serve as guideline to find the best HWE conditions for the most complicated natural products.

1 Introduction and Historical Background

2 Applications of HWE

2.1 Cyclization by HWE Reactions

2.2.1 Formation of Medium- to Larger-Sized Rings

2.2.2 Formation of Small- to Medium-Sized Rings

2.3 Synthesis of α,β-Unsaturated Carbonyl Groups

2.4 Synthesis of Substituted C=C Bonds

2.5 Late-Stage Modifications by HWE Reactions

2.6 HWE Reactions on Solid Supports

2.7 Synthesis of Poly-Conjugated C=C Bonds

2.8 HWE-Mediated Coupling of Larger Building Blocks

2.9 Miscellaneous

3 Summary and Outlook

Cationic polymer synergizing with chemotherapeutics and re-purposing antibiotics against cancer cells

Chemotherapy is one of the most effective treatments for cancer. However, toxicity and the development of drug resistance have become the major hurdles to the commonly used chemotherapeutics such as doxorubicin and paclitaxel. Antibiotics have also been used as anti-cancer drugs due to their anti-proliferative and cytotoxic effects. However, these anti-tumor antibiotics like ciprofloxacin face the similar resistance and toxicity issues. In this study, we used a quaternary ammonium-functionalized cationic polycarbonate to synergize with the existing chemotherapeutics and re-purpose antibiotics to address the resistance and toxicity issues. When used in combination with the drugs, the cationic polymer induced 2-3 fold more damage in the cancer cell membrane within 2 hours, thus enhancing the uptake of chemotherapeutics up to 2.5 fold more into the breast, liver and even chemotherapeutics-resistant cancer cells. On the other hand, the chemotherapeutics increased the cellular uptake of polymer. The combined effects resulted in 3-10 fold reduction in IC₅₀ of chemotherapy drugs and yielded therapeutic synergy at a clinically-relevant concentration range of drugs when treating multiple types of cancer cells, while the use of guanidinium-functionalized polymer capable of membrane translocation did not lead to a synergistic effect. Thus, the quaternary ammonium-functionalized cationic polymer can increase the therapeutic efficacies of existing drugs, mitigating toxicities by lowering required dosage and circumventing drug resistance via its membrane disruption mechanism. The findings of this study provide insights into designing future anticancer therapy.

Challenges and Opportunities to Develop Enediyne Natural Products as Payloads for Antibody-Drug Conjugates

Calicheamicin, the payload of the antibody-drug-conjugates (ADCs) gemtuzumab ozogamicin (Mylotarg^®) and inotuzumab ozogamicin (Besponsa^®), belongs to the class of enediyne natural products. Since the isolation and structural determination of the neocarzinostatin chromophore in 1985, the enediynes have attracted considerable attention for their value as DNA damaging agents in cancer chemotherapy. Due to their non-discriminatory cytotoxicity towards both cancer and healthy cells, the clinical utilization of enediyne natural products relies on conjugation to an appropriate delivery system, such as an antibody. Here we review the current landscape of enediynes as payloads of first-generation and next-generation ADCs.

Copper(i)-catalyzed regioselective Ullmann-type coupling of primary carbamates and 5-substituted-1,2,3-triiodobenzenes: facile synthesis of 2,3-diiodinated N-aryl carbamates

Mild, efficient, and unprecedented synthesis of 2,3-diiodinated N-aryl carbamates via highly regioselective Ullmann-type cross-coupling of 5-substituted-1,2,3-triiodobenzene and carbamate.

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.

Photocatalytic Umpolung of N- and O-substituted alkenes for the synthesis of 1,2-amino alcohols and diols

We report an organophotocatalytic 1,2-oxyalkynylation of ene-carbamates and enol ethers using Ethynyl BenziodoXolones (EBXs). 1-Alkynyl-1,2-amino alcohols and diols were obtained in up to 89% yield. Photocatalytic formation of radical cations led to Umpolung of the innate reactivity of the alkenes, enabling addition of a nucleophilic benzoate followed by radical alkynylation.

Photocatalytic Umpolung with organic dyes overcoming the innate nucleophilicity of enecarbamates and enol ethers for oxyalkynylation with EBX reagents to access 1-alkynyl-1,2-amino alcohols and diols.

Characterization of TnmH as an O-Methyltransferase Revealing Insights into Tiancimycin Biosynthesis and Enabling a Biocatalytic Strategy To Prepare Antibody-Tiancimycin Conjugates

The enediynes are among the most cytotoxic molecules known, and their use as anticancer drugs has been successfully demonstrated by targeted delivery. Clinical advancement of the anthraquinone-fused enediynes has been hindered by their low titers and lack of functional groups to enable the preparation of antibody-drug conjugates (ADCs). Here we report biochemical and structural characterization of TnmH from the tiancimycin (TNM) biosynthetic pathway, revealing that (i) TnmH catalyzes regiospecific methylation at the C-7 hydroxyl group, (ii) TnmH exhibits broad substrate promiscuity toward hydroxyanthraquinones and S-alkylated SAM analogues and catalyzes efficient installation of reactive alkyl handles, (iii) the X-ray crystal structure of TnmH provides the molecular basis to account for its broad substrate promiscuity, and (iv) TnmH as a biocatalyst enables the development of novel conjugation strategies to prepare antibody-TNM conjugates. These findings should greatly facilitate the construction and evaluation of antibody-TNM conjugates as next-generation ADCs for targeted chemotherapy.

Intramolecular Nicholas Reactions in the Synthesis of Heteroenediynes Fused to Indole, Triazole, and Isocoumarin

The applicability of an intramolecular Nicholas reaction for the preparation of 10-membered O- and N-enediynes fused to indole, 1,2,3-triazole, and isocoumarin was investigated. The general approach to acyclic enediyne precursors fused to heterocycles includes inter- and intramolecular buta-1,3-diyne cyclizations with the formation of iodoethynylheterocycles, followed by Sonogashira coupling. The nature of both a heterocycle and a nucleophilic group affects the possibility of a 10-membered ring closure by the Nicholas reaction. Among oxacycles, an isocoumarin-fused enediyne was obtained. In the case of O-enediyne annulated with indole, instead of the formation of a 10-membered cycle, BF₃-promoted addition of an OH-group to the proximal triple bond at the C3 position afforded dihydrofuryl-substituted indole. For 1,2,3-triazole-fused analogues, using NH-Ts as a nucleophilic functional group allowed obtaining 10-membered azaenediyne, while the substrate with a hydroxyl group gave only traces of the desired 10-membered oxacycle. An improved method for the deprotection of Co-complexes of cyclic enediynes using tetrabutylammonium fluoride in an acetone/water mixture and the investigation of the 10-membered enediynes' reactivity in the Bergman cyclization are also reported. In the solid state, all synthesized iodoethynylheterocycles were found to be involved in halogen bond (XB) formation with either O or N atoms as XB acceptors.