Herma C.
Pierre
a,
Chiraz Soumia M.
Amrine
ab,
Michael G.
Doyle
a,
Amrita
Salvi
c,
Huzefa A.
Raja
a,
Jonathan R.
Chekan
a,
Andrew C.
Huntsman
d,
James R.
Fuchs
d,
Kebin
Liu
ef,
Joanna E.
Burdette
c,
Cedric J.
Pearce
g and
Nicholas H.
Oberlies
*a
aDepartment of Chemistry and Biochemistry, University of North Carolina at Greensboro, P.O. Box 26170, Greensboro, North Carolina 27402, USA. E-mail: nicholas_oberlies@uncg.edu
bDepartment of Physical and Earth Sciences. Arkansas Tech University, 1701 N. Boulder Ave., Russellville, Arkansas 72801, USA
cDepartment of Pharmaceutical Sciences, University of Illinois at Chicago, 900 S. Ashland Ave (M/C 870), Chicago, Illinois 60607, USA
dDivision of Medicinal Chemistry and Pharmacognosy, College of Pharmacy, Ohio State University, 500 W. 12th Ave., Columbus, Ohio 43210, USA
eDepartment of Biochemistry and Molecular Biology and the Georgia Cancer Center, Medical College of Georgia, Augusta, GA 30912, USA
fCharlie Norwood Veterans Affairs Medical Center, Augusta, GA 30904, USA
gMycosynthetix, Inc., Hillsborough, NC 27278, USA
First published on 17th April 2024
Covering: 1970 through June of 2023
Verticillins are epipolythiodioxopiperazine (ETP) alkaloids, many of which possess potent, nanomolar-level cytotoxicity against a variety of cancer cell lines. Over the last decade, their in vivo activity and mode of action have been explored in detail. Notably, recent studies have indicated that these compounds may be selective inhibitors of histone methyltransferases (HMTases) that alter the epigenome and modify targets that play a crucial role in apoptosis, altering immune cell recognition, and generating reactive oxygen species. Verticillin A (1) was the first of 27 analogues reported from fungal cultures since 1970. Subsequent genome sequencing identified the biosynthetic gene cluster responsible for producing verticillins, allowing a putative pathway to be proposed. Further, molecular sequencing played a pivotal role in clarifying the taxonomic characterization of verticillin-producing fungi, suggesting that most producing strains belong to the genus Clonostachys (i.e., Bionectria), Bionectriaceae. Recent studies have explored the total synthesis of these molecules and the generation of analogues via both semisynthetic and precursor-directed biosynthetic approaches. In addition, nanoparticles have been used to deliver these molecules, which, like many natural products, possess challenging solubility profiles. This review summarizes over 50 years of chemical and biological research on this class of fungal metabolites and offers insights and suggestions on future opportunities to push these compounds into pre-clinical and clinical development.
With over 15 years of research on cytotoxic fungal metabolites,18,19 representing the evaluation of several thousand fungi and the isolation and elucidation of >700 fungal metabolites, the verticillins represent an area of emphasis for our team, including scaled production,20 analogue development,21,22 drug delivery,23,24 and in vitro and in vivo evaluation against a range of cancer models.22–30 While several reviews have been published on specific aspects of the epipolythiodioxopiperazine (ETP) alkaloids,31–36 as a structural class, none of these have focused on the verticillins. Given that research on these compounds now spans over 50 years, a comprehensive summary was timely, particularly with the growing number of publications on members of this class over the last decade.20–30,37–46
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Fig. 1 The basic structure of dimeric epipolythiodioxopiperazine (ETP) alkaloids, illustrating their biosynthetic origins. The color coding is used to illustrate the building blocks of these molecules. In red is tryptophan (Trp), common to all three dimeric structural classes. For verticillins, the “blue” substituents dictate the appendages at C-3 and C-3′. Unlike chaetocins and leptosins, the constituents at these positions vary and are typically derived from alanine (Ala), threonine (Thr), and in some rare cases, serine (Ser). The N-methylation, common to all three classes, likely derives from S-adenosylmethionine. For chaetocins and leptosins, biosynthesis often uses Ser (purple) and/or Val (green). Moreover, in these two classes, the sulfur bridge is often greater than two atoms (n = 2–4), whereas in the verticillins, the bridge is typically, but not always, a disulfide.33 |
Compound | Year | Molecular formula | Molecular weight (Da) | Fungus |
---|---|---|---|---|
a Recent studies have suggested that the taxonomic identity of this fungus may be different from Verticillium.63,64 b The name of this fungus has been changed to Clonostachys rosea.64,65 c Based on the current rules of fungal nomenclature,66,67 it is more appropriate to refer to fungi in the genus Bionectria as Clonostachys.64,68 | ||||
Verticillin A (1)49,51 | 1970 | C30H28N6O6S4 | 696.83 | Verticillium sp.a |
Verticillin B (2)53 | 1973 | C30H28N6O7S4 | 712.83 | Verticillium sp.a |
Verticillin C (3)53 | 1973 | C30H28N6O7S5 | 744.89 | Verticillium sp.a |
Sch 52900 (4)54 | 1995 | C31H30N6O7S4 | 726.86 | Gliocladium sp. |
Sch 52901 (5)54 | 1995 | C31H30N6O6S4 | 710.86 | Gliocladium sp. |
Verticillin D (6)50 | 1999 | C32H32N6O8S4 | 756.88 | Gliocladium catenulatum |
Verticillin E (7)50 | 1999 | C32H28N6O8S4 | 752.85 | Gliocladium catenulatum |
Verticillin F (8)50 | 1999 | C32H30N6O8S4 | 754.87 | Gliocladium catenulatum |
11′-Deoxyverticillin A (9)55 | 1999 | C30H28N6O5S4 | 680.83 | Penicillium sp. |
11,11′-Dideoxyverticillin A (10)55 | 1999 | C30H28N6O4S4 | 664.83 | Penicillium sp. |
Gliocladin A (11)56 | 2004 | C24H24N4O3S2 | 480.60 | Gliocladium sp. |
Gliocladin B (12)56 | 2004 | C24H24N4O2S2 | 464.60 | Gliocladium sp. |
Gliocladin C (13)56 | 2004 | C22H16N4O3 | 384.40 | Gliocladium sp. |
Gliocladine A (14)57 | 2005 | C30H28N6O6S5 | 728.89 | Gliocladium roseum |
Gliocladine B (15)57 | 2005 | C30H28N6O6S6 | 760.95 | Gliocladium roseum |
Gliocladine C (16)57 | 2005 | C23H20N4O3S2 | 464.56 | Gliocladium roseum |
Gliocladine D (17)57 | 2005 | C23H20N4O3S3 | 496.62 | Gliocladium roseum |
Gliocladine E (18)57 | 2005 | C23H20N4O3S4 | 528.68 | Gliocladium roseum |
Bionectin A (19)58 | 2006 | C22H18N4O3S2 | 450.53 | Bionectria byssicola |
Bionectin B (20)58 | 2006 | C24H22N4O4S2 | 494.58 | Bionectria byssicola |
Bionectin C (21)58 | 2006 | C24H24N4O3S2 | 480.60 | Bionectria byssicola |
Glioclatine (22)59 | 2006 | C23H20N4O2S2 | 448.56 | Gliocladium roseum |
Verticillin G (23)60 | 2007 | C30H28N6O7S4 | 712.83 | Bionectria byssicola |
Gliocladicillin A (24)61 | 2009 | C31H30N6O5S4 | 694.86 | Gliocladium sp. |
Gliocladicillin B (25)61 | 2009 | C31H30N6O4S4 | 678.86 | Gliocladium sp. |
Gliocladicillin C (26)62 | 2009 | C32H32N6O7S4 | 740.88 | Gliocladium sp. |
Verticillin H (27)29 | 2012 | C32H32N6O6S4 | 724.88 | Bionectria sp.c |
The pairs of five membered rings in verticillins are cis-fused, and the hydroxy groups at the 11 and 11′ positions add stability to the molecule by forming a hydrogen-bonding network. Indeed, Liu et al.48 reported the crystal structure of verticillin A (1), demonstrating that the O–H⋯O interactions between molecules facilitated the packing of crystals. Most verticillins are ether and methanol insoluble and precipitate as an off-white/yellow powder. The semisynthetic acetate analogues, with the location unspecified in verticillin A acetate49 or a triacetate analogue of verticillin D,50 are reported to have increased aqueous solubility. The structural diversity observed with the dimeric ETP alkaloids stems from differences in the biosynthetic gene clusters used to create these groups. While tryptophan (Trp) is a key unit in all verticillins, alanine (Ala), serine (Ser), or threonine (Thr) are believed to be the other building blocks used in the non-ribosomal peptide synthases (NRPS) for the biosynthesis of these fungal metabolites (see Section 5).33,34
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Fig. 2 Structures of verticillin analogues (2–27) discovered after A (1) as of June 2023 listed in chronological order and the biosynthetically related ETP alkaloid, gliotoxin (28). Note, while gliocladin A (11)56 and bionectin C (21)58 appear to have the same structure, they were reported in two separate publications with slightly different NMR data. |
No other analogues were reported for >20 years until Sch 52900 (4) and Sch 52901 (5) were disclosed by Schering-Plough (Fig. 2);54 while these trivial names may be confusing, they were based on the first three letters of the institute and a sequential serial number, a common practice at that time. Compounds 4 and 5 were isolated from a Gliocladium sp., and their structures were characterized by spectral methods and comparisons to the data for 1, including the use of UV, IR, MS, and 1H- and 13C-NMR data.54 In a separate study about sclerotium survival in soil, reported in 1999,50 a related species, G. catenulatum,64 was investigated after noticing that damage was caused by this mycoparasite to its host. To do so, this fungus was isolated from Aspergillus flavus sclerotia, which had been buried by the research group for two years in a cornfield,50 and after culturing and extracting, three new verticillins [verticillin D (6), verticillin E (7) and verticillin F (8)] were identified. In the same year, another team, who focused on exploring marine sources for fungal metabolites, isolated a Penicillium sp. from Caribbean green algae, Avrainvillea longicaulis. The cytotoxic extract was purified to yield 11′-deoxyverticillin A (9) and 11,11′-dideoxyverticillin A (10), which were both characterized by spectral methods.55 It could be argued that 10 is an outlier in the verticillin group, since it differs from verticillin A (1) by the absence of the hydroxy groups, technically classifying it to both verticillin and chaetocin groups (Fig. 1).33 However, due to the total synthesis of 10 (see Section 6), we consider it a key member of the verticillin structural class.
Gliocladin A (11), gliocladin B (12), and gliocladin C (13) were reported in 2004 from a Gliocladium sp. isolated from a marine environment.56 The disulfide bridge, which is a key determinant in most other analogues, has been reduced (with both sulfurs methylated) in 11 and 12, and while the sulfurs are absent in 13, it is the only isolated verticillin analogue with a trioxopiperazine moiety (Fig. 2). One year later, five new verticillin analogues [gliocladine A (14), gliocladine B (15), gliocladine C (16), gliocladine D (17), and gliocladine E (18)] were reported from submerged wood collected from fresh water G. roseum57 (=C. rosea).64 Notably, 14 and 17 have three sulfurs in the bridge across the dioxopiperazine moiety, while 15 and 18 possess four sulfur atoms. Bionectin A (19), bionectin B (20), and bionectin C (21) were isolated from Bionectria byssicola (=Clonostachys byssicola)64 by Zheng et al.58 in the course of a project screening microbial sources for antibacterial leads. It appears that gliocladin A (11)56 and bionectin C (21)58 were published with identical structures; however, since there are slight differences between their reported NMR data, further studies are warranted. Bionectin A (19) and B (20) are structurally related to gliocladine C (16), D (17) and E (18), as all of these are missing a dioxopiperazine ring, making them monomeric ETPs, which are rare compared to the dimers or pseudo-dimers observed with most verticillins. However, we believe that these monomeric analogues still belong to the verticillin group (Fig. 1), since they are observed in fungi that produce the more typical verticillins. In addition, it is believed that all of these compounds are biosynthesized via similar pathways.33,69,70
To close out the list of 27 verticillin analogues, glioclatine (22) was isolated from G. roseum (=C. rosea)64 grown on wheat medium,59 and verticillin G (23) was reported from Bionectra byssicola60 (=Clonostachys byssicola).64 In a study targeting compounds with anticancer activities, Chen et al.61 isolated and characterized the structures of two new dimeric ETPs from a Gliocladium sp., specifically gliocladicillin A (24) and gliocladicillin B (25).61 Gliocladicillin C (26) was isolated by the same research group, but was only published in a Chinese patent.62 Finally, Figueroa et al.29 isolated verticillin H (27), along with six other verticillins (1, 4–5, 9, 24, and 26), after a bioactivity-directed fractionation of extracts of solid phase cultures of Bionectria sp. (=Clonostachys sp.).64 All 27 verticillin analogues reported through June 2023 have been summarized, including both the structures (Fig. 2) and the source organisms (Table 1).
Insights: There are three aspects of the structures of verticillins that are important to clarify. First, their structures are often drawn in two different ways, as shown for verticillin A (1; Fig. 3). It was suggested to us by a colleague71 that the version on the left is more correct, as it defines the absolute configuration of positions 10b and 10b′ in a non-ambiguous manner. Alternatively, the center representation of the molecule has the dash between these two asymmetric centers, and even ChemDraw will warn that this makes the configuration of those positions ambiguous. Minato and colleagues, in their third and final manuscript on the verticillins,53 also drew 1 as shown on the right side of Fig. 3, as they were able to propose its absolute configuration based on CD data. Those assignments were confirmed via X-ray crystallography in 2006,48 establishing the configuration of 1 as 3S, 5aR, 10bS, 11S, 11aS, 3′S, 5a′R, 10b′S, 11′S, 11a′S. For those with a deep interest in the structure of verticillins, we encourage their examination in three dimensions (e.g., see graphical abstract). The elongated bond between 10b and 10b′, while convenient for drawing purposes, does not allow one to see how tightly woven those complex ring systems are. Moreover, in two dimensions, it appears that the disulfide bridges are on the same face of the molecule, but in three dimensions, it is apparent that they are anti to each other.
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Fig. 3 Various structural drawings of verticillin A (1). Verticillin A (1) can be represented several different ways. The representation shown in the middle is seen widely in literature and was how Minato and colleagues first presented the molecule.51 However, the dash line between the two halves of the molecule (i.e., 10b to 10b′) is problematic, as it is connecting two asymmetric centers. Alternatively, the drawing of the molecule on the left displays the same information (i.e. 3S, 5aR, 10bS, 11S, 11aS, 3′S, 5a′R, 10b′S, 11′S, 11a′S) in a non-ambiguous manner. Interestingly, Minato and colleagues subsequently drew 1 as shown on the right,53 where they defined the absolute configuration via circular dichroism; these assignments were confirmed via X-ray crystallography in 2006.48 |
Additionally, there is some ambiguity in the literature as to how to number the positions of the atoms throughout the structure of verticillins. For many of the analogues, including 1, the molecule is symmetrical, and thus, the numbering of the top half vs. the bottom half does not matter. However, there are several analogues where there is a difference between those two halves. When the first two asymmetric analogues (2 and 3) were described by Minato and colleagues,53 they did not propose a numbering scheme (likely because they were not assigning NMR spectroscopic data in 1973). However, when Sch 52900 (4) and Sch 52901 (5) were described in 1995,54 those authors assigned the NMR data to distinct positions, and in doing so, they used the prime designation (i.e., 10b′) for the half of the molecule that was different from verticillin A (1). For example, in 5, one half of the molecule has a methyl at the 3-position (i.e., identical to 1), whereas, the other half has an ethyl at the 3′-position. Other scientists have taken a different approach, where the half of the molecule that has the greatest number of atoms is numbered first.22 There are likely pros and cons to both approaches.
Finally, when describing the disulfide-bridged piperazine moiety, the terms dioxopiperazine (or epipolythiodioxopiperazine) and diketopiperazine (or epipolythiodiketopiperazine) seem to be used interchangeably. In fact, in the seminal publications by Minato and colleagues, all of which focused on verticillin A (1), they used the term diketopiperazine49 in their first report and dioxopiperazine51,53 in the latter two. Looking through modern literature, both terms are used interchangeably, although within the biosynthesis community, the term diketopiperazine is probably used more frequently. In discussing this with a colleague who is a journal editor,72 we believe that dioxopiperazine is technically more apropos, as the ‘dioxo’ essentially states that there are two carbonyls attached to the piperazine ring. Obviously, diketo basically implies the same designation, but the confusion is that it also implies ketone carbonyls, whereas the resulting carbonyls are amides. Regardless, both terms are well entrenched in the literature, and it is probably best to simply recognize this fact. Throughout this review, we utilize the terms epipolythiodioxopiperazine or dioxopiperazine.
As summarized in Table 1, it is apparent that the first three verticillins (i.e., compounds 1–3) were the only ones isolated from Verticillium sp.49,51,53 A 2011 study by Dirk et al.63 investigated the genes that were known to be responsible for the biosynthesis of verticillin-type compounds. Interestingly, their study was performed on V. dahliae, and the authors were unable to detect any genes responsible for the biosynthesis of verticillins. In addition, extraction of a culture of Verticillium sp. also did not yield any verticillins, which led to uncertainties about the ability of this fungal genus to be a producer of these compounds. The authors suggested that the original isolation of verticillin A (1)49 was from a different genus, the identity of which may have been obfuscated; we concur with their explanation. The ability of Clonostachys sp., the fungus which biosynthesizes most of the verticillins, to be a mycoparasite of Verticillium63 suggests the possibility that the original fungal culture reported in the 1970 paper49 was misidentified as Verticillium sp. This is especially true since genera such as Clonostachys, Gliocladium, and Verticillium all have a “verticillate” arrangement of phialides, which means that they are formed in a whorl and have been referred to as Verticillium-like anamorphs (Fig. 4).74 In our laboratory, we have identified several different strains that biosynthesize verticillins, including Clonostachys spp. and C. rogersoniana20 using molecular sequence data.73
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Fig. 4 Two microscopic images of the phialides of Clonostachys rogersoniana (strain MSX59553). This fungus biosynthesizes the following verticillins: verticillin A (1), Sch 52900 (4), Sch 52901 (5), verticillin D (6), 11′-deoxyverticillin A (9), gliocladicillin C (26), and verticillin H (27).20 The morphology of the phialides (which have a ‘flask shape’) are characteristic to species of the Clonostachys, Gliocladium, and Verticillium and can cause confusion for non-experts. As such, it is conceivable that fungi used in previous natural products chemistry studies that led to the isolation of some of the analogues shown in Fig. 2 and Table 1, many of which were identified based on morphology, were misidentified. Scale bars = (A) 10 μm and (B) 20 μm. |
Finally, there are reports of verticillins being isolated from Bionectria byssicola58 and Bionectria sp.,29 the latter of which was published by some of the authors of this review. The use of the name Bionectria is being phased out due to the adoption of One Fungus = One Name,66,67 in accord with recent rules for pleomorphic fungi in the International Code of Nomenclature for Algae, Fungi, and Plants.75 For pleomorphic names (i.e., where a fungus has a distinct sexual vs. asexual state) in the family Bionectriaceae, Rossman et al.68 proposed the use of the asexual morph (Clonostachys), since it was described first, rather than the sexual morph (Bionectria). Thus, going forward, it is more appropriate to term such fungi as being in the genus Clonostachys (Bionectriaceae).
Insights: The use of trivial names for compounds, which are often derived from the taxonomic nomenclature of the organism, can be problematic and lead to confusion in the literature. As noted by others,76,77 this may be particularly true for microbial products. The various names of the verticillin analogues (Table 1 and Fig. 2) are a glaring illustration of this. While it might be tempting to rename some of these into a consistent format, there is not really a means to do so. Moreover, since most of the pharmacological literature (see Sections 7 and 8) is based on studies of verticillin A (1), it is prudent to keep its trivial name intact.
The feeding studies were validated with the recent discovery39 of the biosynthetic gene cluster responsible for producing 11′-deoxyverticillin A (9). Previous work identified that an NRPS was responsible for assembling the key dioxopiperazine intermediate in the biosynthesis of chaetocin,82 an ETP structurally related to the verticillins (Fig. 1). Primers based on this NRPS were screened against a genomic fosmid library of the 11′-deoxyverticillin A (9) producer Clonostachys rogersoniana, and a putative biosynthetic gene cluster was identified. This was validated when the NRPS (verP) was knocked out and 11′-deoxyverticillin A (9) production was abolished.39 Closer examination of the nearby genes (Fig. 5A) revealed striking parallels to the biosynthetic gene cluster of gliotoxin (28), an iconic monomeric ETP. The gliotoxin biosynthetic pathway has been thoroughly investigated for many years,70,83–90 making it the prototype for this class of fungal metabolites. Using these results from gliotoxin, it is possible to develop a reasonable hypothesis for the production of the verticillins (Fig. 5B). Importantly, the following proposal is simply based on analogy to gliotoxin and has not been validated. The 11′-deoxyverticillin A (9) pathway is anticipated to begin with VerP, an NRPS that catalyzes the condensation of L-Trp and L-Ala to form the dioxopiperazine skeleton. This is likely followed by bis-hydroxylation catalyzed by the cytochrome P450 (CYP450) monooxygenase VerC. The glutathione-S-transferase VerG can then catalyze the addition of two glutathione moieties, from which the sulfur observed in the verticillins is derived. It is anticipated that VerG catalyzes the nucleophilic attack of the non-enzymatically dehydrated product of the VerC reaction.84 The two glutathiones are proposed to be processed and trimmed by a series of three enzymes: a cyclo-γ-glutamyl-cyclotransferase (VerK), a dipeptidase (VerJ), and a pyridoxal 5′-phosphate (PLP)-dependent C–S bond lyase (VerI).
At this point, the obvious similarities to gliotoxin end. Unlike gliotoxin (28), the verticillins are dimeric ETPs and require different biosynthetic steps. In gliotoxin (28), a CYP450 (GliF) was shown to catalyze N-heterocyclization, possibly through an epoxide intermediate, to form a pyrrolidine ring.90 In the verticillin cluster, the two remaining CYP450s in the pathway, VerL and VerB, do not have clear sequence similarity to GliF. Moreover, GliF installs a hydroxy group as part of its cyclization mechanism. Several verticillin family members, such as 11′-deoxyverticillin A (6), lack hydroxy groups at this position. Therefore, a different biosynthetic route is likely. One possibility is found in the dimerization of dioxopiperazines in both bacteria and fungi. It has been shown that in those cases, a CYP450 both cyclizes the indole to form the pyrroloindoline ring and joins two monomers together to form the dimeric scaffold.91–94 If this route is operational in verticillin biosynthesis, one of the two remaining CYP450s, VerB or VerL, could fulfill this role (Fig. 5C). The remaining CYP450 may serve as a monooxygenase and hydroxylate the C11 and/or C11′ positions. An N-methyltransferase, possibly VerN, is also required, which could function on the monomers either prior to or after dimerization. Finally, the free thiols need to be oxidized to the disulfide, either before or after dimerization. VerT is expected to catalyze this reaction, but the timing is not clear.
Enzyme reconstitution has not been completed for the ver cluster; however, thorough knockout studies of the proposed genes support this biosynthetic route.39 Specifically, disruption of verP, verT, verL, verM, verN, verI, verJ, verG, and verB all abolished 11′-deoxyverticillin A (6) production in C. rogersoniana. Knockout of verK lowered production of 11′- deoxyverticillin A (6) significantly, suggesting it is not completely essential. Finally, the disruption of verA lowered the observed levels of 11′-deoxyverticillin A (6), which is consistent with its anticipated role as an ABC transporter to export verticillin from the cell. Recently an ETP-like cluster from the verticillin D (6) producer, C. rosea, was identified that closely matched the ver cluster,95 further supporting this biosynthetic hypothesis.
Insights: Gene knockouts have firmly established the gene cluster responsible for the biosynthesis of 11′-deoxyverticillin A (6) in C. rogersoniana. While in vitro enzyme reconstitution has not been accomplished for any of the proposed enzymes, their similarity to the biosynthetic enzymes of gliotoxin (28) allows for a portion of the biosynthetic route to be proposed. However, the later stages of the biosynthesis, such as the dimerization and hydroxylation, are unclear and will deviate from the gliotoxin pathway. Ultimately, further studies are needed to uncover the dimerization process in ETPs. Additionally, one would predict that a better understanding of verticillin biosynthesis could lead to heterologous expression, a feat that has not been completed to date, both to generate analogues and ameliorate supply from the native host.
The synthesis of the first prototypical verticillin compound was reported in 2009 by Movassaghi.69 His synthesis of (+)-11,11′-dideoxyverticillin A (16), possessing both the disulfide bridge and bis-pyrroloindoline core, set the precedent for C10b-C10b′ bond formation between the vicinal quaternary centers via a biomimetic reductive radical dimerization (Scheme 1). Utilizing a similar strategy, Movassaghi100 and Sodeoka101 separately reported the synthesis of (+)-chaetocin the following year. The installation of the disulfide bridge in each of these syntheses hinged on the generation of a similar oxidized dioxopiperazine intermediate accessed either pre- (Sodeoka) or post- (Movassaghi) reductive radical dimerization. These strategies, along with other methods for the introduction of the disulfide bridges onto dioxopiperazines, have been reviewed elsewhere.102,103
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Scheme 1 Conceptual strategies for formation of bis-pyrroloindoline containing compounds via (A) reductive or (B) oxidative transformations. |
While access to 11,11′-dihydroxy containing bis-pyrroloindoline ETP natural products has yet to be established via chemical synthesis, the Movassaghi lab has reported the synthesis of the monomeric pyrroloindolines (+)-bionectin A (19) and (+)-bionectin C (21) (Scheme 2), which employed an intramolecular Friedel–Crafts reaction to introduce the indole ring at the C3-position of the bis-pyrroloindoline system (step h, Scheme 2). This approach also took advantage of their streamlined access to erythro-β-hydroxy-L-Trp as a precursor.104 Their gram-scale synthesis of this precursor may serve as a stepping stone towards achieving the synthesis of the bis-pyrroloindoline natural products. Additional syntheses of related structures, including (+)-gliocladin B (12) and (+)-gliocladin C (13)105 have also been reported and reviewed103 by Movassaghi. More recently, their lab has extended their synthetic efforts via generation of ETP scaffolds containing appended azide moieties that can be utilized for the synthesis of chemical probes through conjugation reactions.106
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Scheme 2 Total synthesis of (+)-bionectins A (19) and C (21) by Coste et al.104 Conditions: (a) TiCl(OEt)3, NEt3, CH2Cl2, 0 °C, 81% (58% desired diastereomer); (b) TBSOTf, 2,6-lutidine, CH2Cl2, 0 °C, 72%; (c) 2 N HCl, THF, 81%; (d) N-Boc-sarcosine, EDC·HCl, HOBt, CH2Cl2, 23 °C, 98%; (e) TFA, CH2Cl2, 23 °C; AcOH, morpholine, t-BuOH, 80 °C, 97%; (f) Br2, MeCN, 0 °C; anisole, 94%, 9![]() ![]() ![]() ![]() |
In addition to the total synthesis of members of this class, precursor-directed biosynthesis and semisynthesis experiments have also been explored to generate structural analogues (Fig. 6). This approach is of note for the more complex dimeric members of the class. For example, the Oberlies group has demonstrated that fluorinated derivatives of Sch 52901 (5), verticillin A (1), and verticillin H (27) can be generated at the C9 (and C9′) positions resulting in mono- and di-fluorinated analogues.22 This was accomplished via incorporation of 5-F-DL-Trp during a 28 day fermentation process, producing a ratio of non-, mono-, and di-fluorinated products of approximately 200:
20
:
1. Importantly, in most cases, the fluorinated analogues were equipotent to what was observed with the parent molecules.22
Semisynthesis has largely been limited to the introduction of acetyl groups during the isolation and structural elucidation of the verticillins and related natural products.50,51,53,107–109 In many of these cases, these natural products possessed hydroxy groups at not only the C11/C11′ positions, but also at C13/C13′ (i.e., compound 6 in Fig. 2). Notably, treatment of compounds of this type with excess acetic anhydride resulted in the formation of triacetylated, rather than tetraacetylated products. This result was explained for verticillin D (6)50 as resulting from a “conformational bias” of the C11/C11′ positions that would only permit reaction with one of these “centrally” located alcohol moieties. This bias was confirmed in later semi-synthetic studies by Oberlies and Fuchs,21 who synthesized a series of ester, carbonate, carbamate, and sulfonate analogues of verticillin A (1) and verticillin H (27; Fig. 6). This latter study was unique in that it was both facilitated by a media study to increase verticillin production (i.e., 10 mg of 1 per 10 g oatmeal fungal culture)20 to generate the quantities of the natural products required for this type of work and demonstrated that functionalization with a wide range of groups at the C11 position did not appreciably impact their activity against a variety of cancer cell lines. However, a key point is that high biological activity (with the exception of autophagy110) correlated to the presence of a disulfide bridge across C-3/C-11a, and it was possible to carry out these reactions while keeping that bridge intact. Indeed, removal of the sulfur atoms or reduction of the sulfur bridge diminished/abolished the activity of verticillins.58,111 These observations are critically important, as modifications to the structure likely need to preserve the disulfide bridge.
Insights: The total synthesis of molecules in this class achieved by Overman, Movassaghi, and Sodeoka demonstrate impressive creativity and problem-solving to establish the complex architecture and substitution patterns found in these natural products. These syntheses benefit in large part from methodology developed for the construction of the broader class of tryptamine-derived bis-pyrrolidinoindoline natural products112 like chimonanthine. While they have not yet been specifically utilized for members of the verticillin class, numerous other methods, including oxidative radical coupling strategies (Scheme 1B),113–115 have been developed for the rapid construction of the pyrroloindoline ring systems and installation of the key C10b–C10b′ vicinal quaternary centers from readily available starting materials. More recently, methods have also been described that provide enantioselective access to these tricyclic systems,116–119 including molecules like (+)-WIN 64821 and (−)-ditryptophenaline that also possess fused dioxopiperazine rings, but lack the sulfur bridges found in the verticillins. One of the major synthetic challenges remaining is the synthesis of dimeric verticillin derivatives containing C11 and C11′ hydroxy groups. This means that the total synthesis of verticillin A (1), the prototypical molecule of this class, has not yet been realized. In addition, scalability of the more complex natural products and their analogues for therapeutic development has not yet been demonstrated, although the Movassaghi lab reported the production of several of the monomeric azide probes on 100–200 mg scale.106 In addition to total synthesis, other strategies that promote analogue development and subsequent biological studies, including precursor feeding and semisynthesis, could provide a path toward pharmaceutical development of the verticillins. To do so will require more innovations in the scaled production of verticillins, possibly on the industrial scale. Clearly, there is additional room for compound development in this area, including structure–activity relationship studies and potential drug formulation, so as to advance these compounds into the clinic.
Cancer type | Xenograft | Dosing regimen | Ref. |
---|---|---|---|
Ovarian cancer (OVCAR8-RFP) | Intraperitoneal | Intraperitoneal injection twice weekly for 4 weeks | 30 |
Verticillin A (0.25 mg kg−1 body weight) | |||
Verticillin D (0.25 mg kg−1 body weight) | |||
Ovarian cancer (OVCAR8-RFP) | Intraperitoneal | Intraperitoneal injection every 2 days for 14 days | 23 |
0.5 mg kg−1 of verticillin A encapsulated nanoparticles (eNP-VA) | |||
Pancreatic cancer (PANC02-H7, UN-KC-6141) | Orthotopic | Every 2 days for 10 days | 123 |
0.5 mg kg−1 body weight | |||
Soft tissue sarcoma: malignant peripheral nerve sheath tumor (MPNST) (MPNST724) | Sub-cutaneous | Intraperitoneal injection every other day | 27 |
(1) 0.25 mg kg−1 body weight | |||
(2) 0.5 mg kg−1 body weight | |||
Colon carcinoma (SW620-5FU-R) | Sub-cutaneous | Intravenous injection on days 5, 7, 9, 11 and 13 | 28 |
1 mg kg−1 body weight | |||
Colon carcinoma (HepG2, SW620) | Sub-cutaneous | Intravenous injection every 2 days for 14 days | 25 |
HepG2 tumor: 1 mg kg−1 and 2 mg kg−1 body weight | |||
SW620 tumor: 0.125 mg kg−1 body weight |
In recent years, target identification studies have shown that verticillin A (1) is a histone methyltransferase (HMTase) inhibitor with selective activity towards G9a, GLP, SUV39H1, SUV39H2, MLL1, and NSD2 methyltransferases (Fig. 7).28,37 The highest selectivity of verticillin A (1) was observed against G9a, SUV39H1 and SUV39H2 methyltransferases with IC50 values of 0.54, 0.57, and 0.48 μM, respectively. Paschall et al.28 discerned the selective role of verticillin A (1) in inhibiting histone H3 lysine 9 (H3K9) trimethylation (H3K9me3) by inhibiting SUV39H1 and SUV39H2 enzyme activity. H3K9me3 was shown to be responsible for Fas transcription silencing, which results in a loss of its expression in human colon carcinoma cells.28,120 Treatment with verticillin A (1) showed a decrease in H3K9me3 deposition levels in the FAS promoter region with an increase of histone H3 lysine 9 acetylation (H3K9ac), which indicates active transcriptional chromatin, thereby enabling Fas expression.28 The ligand of Fas (i.e., Fas-L), present on the surface of cytotoxic T lymphocytes (CTL) when bound to Fas, initiates an immunological reaction that leads to apoptosis.121,122 Modification of H3K9me3 deposition by verticillin A (1) can impact gene expression, and, in certain cancer types, one of the targets is the re-expression of Fas that increases tumor cell sensitivity to CTL FasL-induced cytotoxicity, resulting in tumor growth suppression.28
More recently, Kaweesa et al.30 compared the in vitro and in vivo cytotoxicity of verticillin A (1) and verticillin D (6). The authors found that both compounds exhibited cytotoxicity and induced apoptosis in HGSOC cell lines OVCAR4 and OVCAR8 at nanomolar concentrations. The authors performed formulation studies to monitor bioavailability and achieve tolerable drug delivery for future pharmacokinetic studies. Both verticillin A (1) and verticillin D (6) reduced tumor burden in OVCAR8 xenografts at the same dose as was previously tested. Using this optimized formulation, verticillin dosing strategies that are effective in vivo were expanded from expansile nanoparticles to the use of improved formulations that likely increase solubility. Unfortunately, verticillin D (6) exhibited significant liver toxicity in mice,30 and as such, it may require additional formulation development. However, this was the first in vivo study to explore analogues of verticillin A (1), and this seems to be an area where further investigations are warranted.
In a study on soft tissue sarcoma (STS),27 which is known to be therapeutically challenging due to genetic and histological heterogeneity, verticillin A (1) showed potential in inhibiting malignant peripheral nerve sheath tumor (MPNST) and leiomyosarcoma (LMS) growth by inducing apoptosis, a finding that was consistent with the work of Liu et al.25 noted above. In vitro experiments demonstrated that verticillin A (1) inhibited colony formation in STS cells, while inducing apoptosis. Mice xenografted with MPNST724 revealed tumor growth inhibition and significant reduction of tumor volumes and weights when treated with verticillin A (1) at a dose of 0.25 or 0.50 mg kg−1 of body weight.27
Insights: In addition to exploring the in vivo efficacy of verticillin A (1), all the above studies were mechanistic in their approach and identified verticillin A (1) as an epigenetic modifier in various tumor models (Fig. 7). In general, most studies examine verticillin A (1) at a dose of about 1 mg kg−1, as summarized in Table 2. The narrow therapeutic range of verticillins is an area that warrants further study. For example, the study by Salvi et al.23 showed that a nanoparticle formulation was beneficial, as it resulted in a statistically significant minimization of tumor burden while simultaneously being non-toxic, especially to the liver. Later, Kaweesa et al.30 found that formulation was sufficient to dose verticillin A (1) effectively without toxicity. This suggests that metering the dose of verticillins may provide an avenue to circumvent toxicity concerns, and thus, improve the therapeutic index. In addition, with only one exception,30 all of these in vivo studies were carried out with verticillin A (1). The growing availability of both natural and semisynthetic analogues of the verticillins opens up the opportunity for further in vivo experimentation. Obviously, the ultimate goal is to uncover a verticillin-type molecule that can be formulated effectively, that retains potent anticancer activity, and yet that has minimal toxicity, particularly to the liver.
In colon cancer, verticillin A (1) was found to selectively inhibit HMTases leading to a decrease in H3K9me3 levels.28 Building upon this work, metastasis-related genes were investigated in colon cancer cells to identify additional signaling pathways changed by verticillin A (1) mediated epigenetic marks.41,42 After determining the cytotoxicity of 1 against human colon cancer cell lines DLD1 and RKO, as well as the murine colon cancer cell line CT26 (IC50 values of 0.90, 0.31, and 0.18 μM, respectively), the authors found that verticillin A (1) inhibited the migration and invasion ability of these cell lines.41,42 Subsequently, using qRT-PCR, key metastasis-associated genes were identified, including MET, CDH1, PLAU, RHOA, and RHOC. Among these genes, MET and PLAU were significantly downregulated when treated with verticillin A (1). Further results showed that verticillin A (1) selectively decreased C-Met protein levels in DLD1, RKO, and CT26 cells. These findings were substantiated further when it was determined that verticillin A (1) suppressed C-Met at the transcriptional level.41,42 Collectively, these results suggest that C-Met (c-mesenchymal–epithelial transition factor) is a biological target of verticillin A (1) in human colon carcinomas.
Lu et al.45 demonstrated that verticillin A (1) in human gastric (AGS) and cervical cancer (HeLa) cells inhibits migration by targeting C-Met and its downstream FAK/Src (focal adhesion kinase-steroid receptor cofactor) signaling pathways. Specifically, verticillin A (1) represses the expression of C-Met protein and inhibits HGF-induced C-Met phosphorylation, the latter of which results in the suppression of C-Met downstream of FAK/Src signaling. These findings are significant, as the C-Met/FAK/SRc signaling pathway is associated with cancer cell proliferation and invasion.128,129 Importantly, verticillin A (1) displayed cytotoxic activity against AGS and HeLa cells with IC50 values of 69.89 and 319.5 nM at 24 h and 47.59 and 233.9 nM at 48 h, respectively.45
A separate study44 demonstrated that verticillin A (1) decreased trimethylation at H3K9 at the G6pd promoter in tumor-specific 2/20 CTLs (cytotoxic T lymphocytes) co-cultured with mesothelioma AB1 tumor cells (i.e., T-cells exposed to the tumor microenvironment). This, in turn, significantly increased the expression of G6pd in the tumor-specific CTLs. The tumor microenvironment is hostile towards antitumor immune responses and likely responds to T-cells, in particular, by inducing T-cell exhaustion.130,131 Thus, these findings are significant, since the activation of G6pd was found to enhance the acetyl-CoA/H3K9ac pathway to reverse CTL exhaustion and subsequently increase CTL lytic function in tumor cell lysis in vitro and in vivo. These findings suggest that G6pd is a potential biological target in reversing immune suppression in cancer immunotherapy. The changes observed in this study in methylation are consistent with inhibition of HMTase as the target.
In the 1990s, Chu et al. showed that Sch 52900 (4), Sch 52901 (5), and verticillin A (1) inhibited serum-stimulated transcription of human c-fos promoter.54 These verticillin analogues showed potent activity in the fos/lac Z reporter gene assay with in vitro IC50 values of 1.5, 1.8, and 0.5 μM, respectively. Thus, verticillin A (1) exerted antitumor activity by inhibiting the activation of at least two signaling pathways involved in c-fos induction.54 Sch 52900 (4) demonstrated the ability to induce differentiation of 50–69% of HL-60 cells (human promyelocytic cells) at low concentrations (6.8–13.6 nM). Sch 52900 (4) caused induction of the cell cycle inhibitor p21WAF and inhibition of the extracellular signal regulated kinase (ERK) that led to cellular apoptosis and subsequent growth arrest.132 The ultimate result of these verticillins (i.e., compounds 1, 4 and 5) triggering cell death via apoptosis is consistent with the more recent mechanistic data obtained in other cancer cell lines, such as colon, pancreatic, and ovarian.
Several structurally related verticillins have also been studied. The compound 11,11′-dideoxyverticillin A (10) was demonstrated to have inhibitory activity towards epidermal growth factor receptor (EGFR), vascular endothelial growth factor receptor-1/fms-like tyrosine kinase-1 (VEGFR-1/FLT-1), and human epidermal growth factor receptor-2 (HER2/ErbB-2), being selectively more potent against EGFR and VEGFR-1 with IC50 values of 0.136 and 1.645 nM, respectively.133 11′-Deoxyverticillin A (9) was shown to induce autophagy in HCT116 human colon carcinoma cells leading to apoptotic cell death.38 Verticillins have also been shown to cause cell cycle G2/M phase arrest in HCT-116 colon cancer cells using in vitro and in vivo experiments.134 Similarly, gliocladicillins A (24) and B (25) are strong anti-proliferative and pro-apoptotic agents and are demonstrated to inhibit proliferation of cancer cell lines HeLa, HepG2, and MCF-7 by cell cycle blockage in the G2/M phase.61 Moreover, 11,11′-dideoxyverticillin A (10) has an anti-angiogenic effect135 and inhibits proliferation of HUVECs (human umbilical vein endothelial cells) with IC50 values of 0.17 and 0.39 μM for VEGF (vascular endothelial growth factor) stimulated cells and serum-stimulated cells, respectively.134
Insights: The cellular target for most of the in vitro studies on verticillins were not clearly defined; however, a common theme is that verticillins induce cytotoxicity and apoptosis in cancer cells. These studies were performed prior to identifying the role of verticillins in modifying histones, and evaluating these studies in light of current knowledge could be informative. For example it would be interesting to pursue whether any of the targets (such as BIM, c-Met, c-fos, etc.) undergo transcription changes via verticillin-mediated epigenetic modifications.
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