The interplay between mutasynthesis and semisynthesis: generation and evaluation of an ansamitocin library

Combinatorial Biosynthesis of 3-O-Carbamoylmaytansinol by Rational Engineering of the Tailoring Steps of Ansamitocins

Currently, most maytansine-containing antibody-drug conjugates (ADCs) in clinical trials are prepared with DM1 or DM4, which in turn is synthesized mainly from ansamitocin P-3 (AP-3), a bacterial maytansinoid, isolated from Actinosynnema pretiosum. However, due to the high self-toxicity of AP-3 to A. pretiosum, the yield of AP-3 has been difficult to improve. Herein, a new maytansinoid with much lower self-toxicity to A. pretiosum, 3-O-carbamoylmaytansinol (CAM, 3), was designed and generated by introducing the 3-O-carbamoyltransferase gene asc21b together with the N-methyltransferase genes from exogenous maytansinoid gene clusters into the 3-O-acyltransferase gene (asm19) deleted mutant HGF052. Meanwhile, two new shunt products, 20-O-demethyl-19-dechloro-N-demethyl-4,5-desepoxy-CAM (4) and 20-O-demethyl-N-demethyl-4,5-desepoxy-CAM (5) were identified from the recombinant strain. Furthermore, by screening of liquid fermentation media, overexpression of bottleneck tailoring enzymes and the pathway-specific activator, the titer of CAM reached 498 mg/L in the engineered strain. Since the 3-O-carbamoyl group of CAM can be removed by chemical cleavage as AP-3 to produce maytansinol, our work suggests that CAM may be a promising alternative to AP-3 in the future development of ADCs.

Biotransformation-coupled mutasynthesis for the generation of novel pristinamycin derivatives by engineering the phenylglycine residue

Streptogramins are the last line of defense antimicrobials with pristinamycin as a representative substance used as therapeutics against highly resistant pathogenic bacteria. However, the emergence of (multi)drug-resistant pathogens renders these valuable antibiotics useless; making it necessary to derivatize compounds for new compound characteristics, which is often difficult by chemical de novo synthesis due to the complex nature of the molecules. An alternative to substance derivatization is mutasynthesis. Herein, we report about a mutasynthesis approach, targeting the phenylglycine (Phg) residue for substance derivatization, a pivotal component of streptogramin antibiotics. Mutasynthesis with halogenated Phg(-like) derivatives altogether led to the production of two new derivatized natural compounds, as there are 6-chloropristinamycin I and 6-fluoropristinamycin I based on LC-MS/MS analysis. 6-Chloropristinamycin I and 6-fluoropristinamycin I were isolated by preparative HPLC, structurally confirmed using NMR spectroscopy and tested for antimicrobial bioactivity. In a whole-cell biotransformation approach using an engineered E. coli BL21(DE3) pET28-hmo/pACYC-bcd-gdh strain, Phg derivatives were generated fermentatively. Supplementation with the E. coli biotransformation fermentation broth containing 4-fluorophenylglycine to the pristinamycin mutasynthesis strain resulted in the production of 6-fluoropristinamycin I, demonstrating an advanced level of mutasynthesis.

New insights into the anticancer therapeutic potential of maytansine and its derivatives

Maytansine is a pharmacologically active 19-membered ansamacrolide derived from various medicinal plants and microorganisms. Among the most studied pharmacological activities of maytansine over the past few decades are anticancer and anti-bacterial effects. The anticancer mechanism of action is primarily mediated through interaction with the tubulin thereby inhibiting the assembly of microtubules. This ultimately leads to decreased stability of microtubule dynamics and cause cell cycle arrest, resulting in apoptosis. Despite its potent pharmacological effects, the therapeutic applications of maytansine in clinical medicine are quite limited due to its non-selective cytotoxicity. To overcome these limitations, several derivatives have been designed and developed mostly by modifying the parent structural skeleton of maytansine. These structural derivatives exhibit improved pharmacological activities as compared to maytansine. The present review provides a valuable insight into maytansine and its synthetic derivatives as anticancer agents.

Cascade Transformation of the Ansamycin Benzoquinone Core into Benzoxazole Influencing Anticancer Activity and Selectivity

The metal-free cascade transformation of geldanamycin benzoquinone core is proposed at relatively mild conditions. This approach yields new benzoxazole ansamycin antibiotics and enables their functionalization in an atom-economic manner, irrespective of the type of amine used. The analysis of the heterocyclization course reveals the dependence of its rate on the nature of the para-substituent within the benzylamine moiety (EDG/EWG) and the strength of the base. The reduction of the ansamycin core enables an increase in anticancer potency and selectivity.

Modifications of geldanamycin via CuAAC altering affinity to chaperone protein Hsp90 and cytotoxicity

Functionalization of alkyne (1) and azide (2) derivatives of geldanamycin (GDM) via dipolar cycloaddition CuAAC yielded 35 new congeners (3-37) with C(17)-triazole arms bearing caps of different nature (basic vs. acidic, hydrophilic vs. hydrophobic). Confrontation of biological data (anticancer activity vs. toxicity in normal cells) with lipophilicity (clogP), dissociation constants (K_d) of complexes with Hsp90 and binding modes to Hsp90 revealed SAR in specific subgroups of GDM derivatives. The most potent GDM congeners 14-16, bearing C(17)-triazole-benzyl-halogen arms exhibited the most optimal clogP values of 2.7-3.1 at favourable binding to Hsp90 (K_dHsp90 at μM level). The anticancer activity of 14-16 (IC₅₀ = 0.23-0.41 μM) is higher than those of GDM (IC₅₀ = 0.58-0.64 μM) and actinomycin D (ActD, IC₅₀ = 0.62-0.71 μM) in SKBR-3, SKOV-3 and PC-3 cell lines, with a comparable cytotoxicity in healthy cells. The relationship between structure and attractive anticancer potency (IC₅₀ = 0.53-0.74 μM) is also observed for congeners with C(17)-triazole-saccharide or C(17)-triazole-unsaturated arms. In the former, the absolute configuration at C(4) (ᴅ-glucose vs. ᴅ-galactose) whereas in the latter the length of the unsaturated arm influences the cytotoxic effects due to different binding strength (K_d, ΔE) and modes with Hsp90. Among all triazole congeners of GDM that are biologically attractive and exhibit lower toxicity in normal cells than GDM and ActD, the derivative 22, bearing the C(17)-triazole-cinnamyl arm, shows the lowest K_d (Hsp90), optimal clogP = 2.82, the best pro-apoptotic properties in SKBR-3 and SKOV-3 and the best selectivity indices (SI). For the most potent GDM derivatives with C(17)-triazole arm, the docking studies have suggested the importance of the intermolecular stabilization between the arm and the D57 or Y61 of Hsp90.

Structural diversity and biological relevance of benzenoid and atypical ansamycins and their congeners

Covering: 2011 to 2021The structural division of ansamycins, including those of atypical cores and different lengths of the ansa chains, is presented. Recently discovered benzenoid and atypical ansamycin scaffolds are presented in relation to their natural source and biosynthetic routes realized in bacteria as well as their muta and semisynthetic modifications influencing biological properties. To better understand the structure-activity relationships among benzenoid ansamycins structural aspects together with mechanisms of action regarding different targets in cells, are discussed. The most promising directions for structural optimizations of benzenoid ansamycins, characterized by predominant anticancer properties, were discussed in view of their potential medical and pharmaceutical applications. The bibliography of the review covers mainly years from 2011 to 2021.

Recombineering-Mediated Genome Editing in Burkholderiales Strains

Red/ET recombineering is primarily mediated by the E. coli recombinase pair Redα/Redβ from λ phage or RecE/RecT from Rac prophage, which is applied in E. coli and also closely related Gram-negative bacteria for efficient genome editing. However, some distant bacterial species like Burkholderiales strains require host-specific Redα/Redβ recombinase pair for highly efficient genome editing. A pair of recombinases Redαβ7029 from the Burkholderiales strain DSM 7029, recently identified as Schlegelella brevitalea, were identified for efficient genetic manipulation in the native strain and several other Burkholderiales strains. In this chapter, we describe a detailed protocol for genome engineering in Burkholderiales strains via the Redγ-Redαβ7029 recombineering and Cre/loxP site-specific recombination.

GenoChemetic Strategy for Derivatization of the Violacein Natural Product Scaffold

Natural products and their analogues are often challenging to synthesize due to their complex scaffolds and embedded functional groups. Solely relying on engineering the biosynthesis of natural products may lead to limited compound diversity. Integrating synthetic biology with synthetic chemistry allows rapid access to much more diverse portfolios of xenobiotic compounds, which may accelerate the discovery of new therapeutics. As a proof-of-concept, by supplementing an Escherichia coli strain expressing the violacein biosynthesis pathway with 5-bromo-tryptophan in vitro or tryptophan 7-halogenase RebH in vivo, six halogenated analogues of violacein or deoxyviolacein were generated, demonstrating the promiscuity of the violacein biosynthesis pathway. Furthermore, 20 new derivatives were generated from 5-brominated violacein analogues via the Suzuki-Miyaura cross-coupling reaction directly using the crude extract without prior purification. Herein we demonstrate a flexible and rapid approach to access a diverse chemical space that can be applied to a wide range of natural product scaffolds.

Anticancer activity and toxicity of new quaternary ammonium geldanamycin derivative salts and their mixtures with potentiators

Geldanamycin (GDM) has been modified by different type neutral/acidic/basic substituents (1–7) and by quinuclidine motif (8), transformed into ammonium salts (9–13) at C(17). These compounds have been characterised by spectroscopic and x-ray methods. Derivative 8 shows better potency than GDM in MCF-7, MDA-MB-231, A549 and HeLa (IC₅₀s = 0.09–1.06 µM). Transformation of 8 into salts 9–13 reduces toxicity (by 11-fold) at attractive potency, e.g. MCF-7 cell line (IC₅₀∼2 µM). Our studies show that higher water solubility contributes to lower toxicity of salts than GDM in healthy CCD39Lu and HDF cells. The use of 13 mixtures with potentiators PEI and DOX enhanced anticancer effects from IC₅₀∼2 µM to IC₅₀∼0.5 µM in SKBR-3, SKOV-3, and PC-3 cancer cells, relative to 13. Docking studies showed that complexes between quinuclidine-bearing 8–13 and Hsp90 are stabilised by extra hydrophobic interactions between the C(17)-arms and K58 or Y61 of Hsp90.

First Ring-Expanded Maytansin Lactone Accessed by a New Mutasynthetic Variant

A multiblocked mutant strain (ΔAHBA and Δasm12, asm21) of Actinosynnema pretiosum, the producer of the highly toxic maytansinoid ansamitocin, has been used for the mutasynthetic production of new proansamitocin derivatives. The use of mutant strains that are blocked in the biosynthesis of an early building block as well as in the expression of two tailoring enzymes broadens the scope of chemo-biosynthetic access to new maytansinoids. Remarkably, a ring-expanded macrolactone derived from ansamitocin was created for the first time.

Synthesis of Titanium Complexes Supported by Carbinolamide- and Amide-Containing Ligands Derived from Ti(NMe2)4-Mediated Selective Amidations of Carbonyl Groups

An efficient strategy for the syntheses of a series of titanium complexes has been developed. This protocol features the employment of Ti(NMe₂)₄ both as the metal center to trigger the deprotonation of the ligands and as an amine source to proceed the amidation reactions of carbonyl functionalities of the ligands. Treatment of Ti(NMe₂)₄ with a ligand HL1 (HL1 = 2,2'-(((2-hydroxybenzyl)azanediyl)bis(ethane-2,1-diyl))bis(isoindoline-1,3-dione) results in the formation of Ti(L1')(NMe₂) (1) (H₃L1' = N1-(2-((2-(1-(dimethylamino)-1-hydroxy-3-oxoisoindolin-2-yl)ethyl)(2-hydroxybenzyl)amino)ethyl)-N2,N2-dimethylphthalamide). One important feature regarding the synthesis of 1 is the occurrence of the in situ metal-ligand reaction between Ti(NMe₂)₄ and HL1, leading to the simultaneous formations of carbinolamide and amide scaffolds. Another prominent feature in terms of the preparation of 1 is the achievement of the selective ring-opening reaction of one of the two phthalimide units of the HL1 ligand, affording carbinolamide and amide functionalities within one ligand set. The developed methodology characterizes an ample substrate scope. The selective amidation reactions of the carbonyl groups have been realized for a series of analogous ligands HL2-HL7. Density functional theory calculations were employed to disclose the mechanisms for the formation of 1-7, and the details for the selective ring-opening reactions of the phthalimide unit were uncovered.

Semisynthesis of Plant-Derived Englerin A Enabled by Microbe Engineering of Guaia-6,10(14)-diene as Building Block

Herein, we report a short semisynthesis of the potent transient receptor potential canonical (TRPC) channel agonist englerin A (EA) and the related guaianes oxyphyllol and orientalol E. The guaia-6,10(14)-diene starting material was systematically engineered in Escherichia coli and Saccharomyces cerevisiae using the CRISPR/Cas9 system and was produced with high titers. The potentially scalable approach combines the advantages of synthetic biology and chemical synthesis providing an efficient and economical method for producing EA and analogues.

Identification of the Bacterial Maytansinoid Gene Cluster asc Provides Insights into the Post-PKS Modifications of Ansacarbamitocin Biosynthesis

A new biosynthetic gene cluster for the bacterial maytansinoids, ansacarbamitocins (ASCs), was identified in Amycolatopsis alba DSM 44262. The post-PKS modifications of ASCs were elucidated on the basis of bioinformatics analysis. Specific gene disruption and heterologous expression led to the isolation of seven new bacterial maytansinoids. The 3'-O-methyltransferase and 3-O-carbamyltransferase involved in bacterial maytansinoid biosynthesis were identified for the first time. The new bacterial maytansinoids 7 and 13 showed strong antitumor activities against four human cancer cell lines.

Naphthomycin-derived macrolactams with two new carbon skeletons from endophytic Streptomyces

A cytotoxic ansamycin class of natural products with two new carbon skeletons was isolated and characterized from endophytic Streptomyces.

New secondary metabolites from an engineering mutant of endophytic Streptomyces sp. CS

In previous work, a series of bioactive natural products had been isolated from the plant endophytic Streptomyces sp. CS, which was isolated from Maytenus hookeri. To mine new active metabolites, we describe introducing an alien carbamoyltransferase (asm21) gene into the strain CS by conjugal transfer. As a result, three recombinatorial mutants named CS/asm21-1, CS/asm21-2 and CS/asm21-4 were successfully constructed. Three mutants and wild type CS were cultured on solid medium, and the extracts were detected and analyzed by liquid chromatography-mass spectrometry (LC-MS). The LC-MS profiles showed several unknown peaks that were present in the spectra of extracts of the CS/asm21-4 cultured on oatmeal solid medium. Then, three new naphthomycins O-Q (1-3), a new macrolide hookerolide (4) as well as nine known compounds were obtained from the solid cultured medium. Their structures were identified by spectra data. These new compounds showed moderate antimicrobial activities.

Heat Shock Proteins Revisited: Using a Mutasynthetically Generated Reblastatin Library to Compare the Inhibition of Human and Leishmania Hsp90s

Thirteen new reblastatin derivatives, with alkynyl, amino and fluoro substituents on the aromatic ring, were prepared by a chemo-biosynthetic approach using an AHBA(-) mutant strain of Streptomyces hygroscopicus, the geldanamycin producer. The inhibitory potencies of these mutaproducts and of an extended library of natural products and derivatives were probed with purified heat shock proteins (Hsps), obtained from Leishmania braziliensis (LbHsp90) as well as from human sources (HsHsp90). We determined the activities of potential inhibitors by means of a displacement assay in which fluorescence-labelled ATP competes for the ATP binding sites of Hsps in the presence of the inhibitor in question. The results were compared with those of cell-based assays and, in selected cases, of isothermal titration calorimetry (ITC) measurements. In essence, reblastatin derivatives are also able to bind effectively to the ATP-binding site of LbHsp90, and for selected derivatives, moderate differences in binding to LbHsp90 and HsHsp90 were encountered. This work demonstrates that parasitic heat shock proteins can be developed as potential pharmaceutical targets.

Evaluation of the Synthetic Potential of an AHBA Knockout Mutant of the Rifamycin Producer Amycolatopsis mediterranei

Supplementing an AHBA(-) mutant strain of Amycolatopsis mediterranei, the rifamycin producer, with a series of benzoic acid derivatives yielded new tetraketides containing different phenyl groups. These mutasynthetic studies revealed unique reductive properties of A. mediterranei towards nitro- and azidoarenes, leading to the corresponding anilines. In selected cases, the yields of mutaproducts (fermentation products isolated after feeding bacteria with chemically prepared analogs of natural building blocks) obtained are in a range (up to 118 mg L(-1)) that renders them useful as chiral building blocks for further synthetic endeavors. The configuration of the stereogenic centers at C6 and C7 was determined to be 6R,7S for one representative tetraketide. Importantly, processing beyond the tetraketide stage is not always blocked when the formation of the bicyclic naphthalene precursor cannot occur. This was proven by formation of a bromo undecaketide, an observation that has implications regarding the evolutionary development of rifamycin biosynthesis.

Bioengineering and semisynthesis of an optimized cyclophilin inhibitor for treatment of chronic viral infection

Inhibition of host-encoded targets, such as the cyclophilins, provides an opportunity to generate potent high barrier to resistance antivirals for the treatment of a broad range of viral diseases. However, many host-targeted agents are natural products, which can be difficult to optimize using synthetic chemistry alone. We describe the orthogonal combination of bioengineering and semisynthetic chemistry to optimize the drug-like properties of sanglifehrin A, a known cyclophilin inhibitor of mixed nonribosomal peptide/polyketide origin, to generate the drug candidate NVP018 (formerly BC556). NVP018 is a potent inhibitor of hepatitis B virus, hepatitis C virus (HCV), and HIV-1 replication, shows minimal inhibition of major drug transporters, and has a high barrier to generation of both HCV and HIV-1 resistance.

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Optimization and preclinical analysis of a bacterial natural product

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Combination of bioengineering and semisynthetic chemistry

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Preclinical analysis revealing potent antiviral activity

New ansamycin derivatives generated by simultaneous mutasynthesis

The conversion from triene- to diene-typed ansamycins is clarified step by step in Streptomyces seoulensis IFB-A01. Such an intertype convertibility is adopted to establish for the first time the simultaneous mutasynthesis of both types of C17-benzene ansamycins (C17BAs). Three of the newly generated unnatural compounds showed potent cytotoxicity.

New, non-quinone fluorogeldanamycin derivatives strongly inhibit Hsp90

Streptomyces hygroscopicus is a natural producer of geldanamycin. Mutasynthetic supplementation of an AHBA-blocked mutant with all possible monofluoro 3-aminobenzoic acids provided new fluorogeldanamycins. These showed strong antiproliferative activity and inhibitory effects on human heat shock protein Hsp90. Binding to Hsp90 in the low nanomolar range was determined from molecular modelling, AFM analysis and by calorimetric studies.

Gold-catalyzed oxycyclization of allenic carbamates: expeditious synthesis of 1,3-oxazin-2-ones

A combined experimental and computational study on regioselective gold-catalyzed synthetic routes to 1,3-oxazinan-2-ones (kinetically controlled products) and 1,3-oxazin-2-one derivatives (thermodynamically favored) from easily accessible allenic carbamates has been carried out.

In vitro chemoenzymatic and in vivo biocatalytic syntheses of new beauvericin analogues

New beauvericins have been synthesized using the nonribosomal peptide synthetase BbBEAS from the entomopathogenic fungus Beauveria bassiana. Chemical diversity was generated by in vitro chemoenzymatic and in vivo whole cell biocatalytic syntheses using either a B. bassiana mutant or an E. coli strain expressing the bbBeas gene.