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Journal ArticleDOI

Diversity through semisynthesis: the chemistry and biological activity of semisynthetic epothilone derivatives

01 Jan 2011-Molecular Diversity (Springer Netherlands)-Vol. 15, Iss: 2, pp 383-399
TL;DR: The current review provides a comprehensive overview on the chemical transformations that have been investigated for the major epothilones A and B as starting materials, and it discusses the biological activity of the resulting products.
Abstract: Epothilones are myxobacterial natural products that inhibit human cancer cell growth through the stabil- ization of cellular microtubules (i.e., a "taxol-like" mech- anism of action). They have proven to be highly productive lead structures for anticancer drug discovery, with at least seven epothilone-type agents having entered clinical trials in humans over the last several years. SAR studies on epothil- ones have included a large number of fully synthetic ana- logs and semisynthetic derivatives. Previous reviews on the chemistryandbiologyofepothiloneshavemostlyfocusedon analogs thatwereobtainedbydenovochemical synthesis.In contrast, the current review provides a comprehensive over- view on the chemical transformations that have been investi- gatedforthemajorepothilonesAandBasstartingmaterials, and it discusses the biological activity of the resulting prod- ucts. Many semisynthetic epothilone derivatives have been found to exhibit potent effects on human cancer cell growth and several of these have been advanced to the stage of clini-

Summary (3 min read)

Introduction

  • MSA can be divided into two distinct functional classes, namely compounds that inhibit the assembly of soluble tubulin into microtubule polymers ("tubulin polymerization inhibitors") and those that promote the assembly of tubulin heterodimers into microtubule polymers and stabilize microtubules ("microtubule stabilizers") [4] .
  • After the elucidation of taxol's mode of action in 1979 [6] , it took more than a decade before other microtubulestabilizing agents with non-taxol-like structures were discovered.
  • Even for the natural product taxol [38] , the sustained supply of sufficient quantities of drug material for clinical use could only be secured for some time through the development of a semisynthetic production process from another natural product, namely 10-deacetylbaccatin III [39, 40] .
  • This will facilitate the comparison of the biological effects of related structural changes.

Modifications of the epoxide moiety

  • Modifications of the epoxide moiety have been an important trait of the semisynthetic work on epothilones from the very beginning, which is unsurprising in light of the multitude of transformations that are conceivable for an oxirane ring and the potential for further elaboration of the initial reaction products.
  • In contrast to the above acetonides, cis and trans diols 9 and 5 did not show any appreciable biological activity (IC 50 's for cancer cell growth inhibition >1 µM) [42, 43] .
  • As illustrated in Scheme 4, 12 was obtained through nucleophilic ring-opening of the epoxide moiety in Epo A with azide anion (to produce 11) and subsequent reduction of the azide group under Staudinger conditions (Ph 3 P/H 2 O) [48] .
  • These findings re-confirmed the notion that the oxirane ring system in epothilones merely serves to stabilize the proper bioactive conformation of the macrocyclic skeleton rather than acting as a reactive electrophile or a hydrogen bond acceptor.
  • Several of these derivatives show antiproliferative activities that are comparable with or even superior to that of Epo A [45] .

Modifications of the ester moiety

  • One of the most obvious modifications of the epothilone scaffold is the hydrolysis of the ester bond to produce the corresponding seco acid.
  • This transformation has been achieved by treatment of Epo A with NaOH/MeOH, which gave seco acid 25 in 65% yield (Scheme 8); however, ester hydrolysis was also accompanied by the retro aldol cleavage of the C3-C4 bond, thus leading to 23% of the retro aldol product 26 (Scheme 8) [58] .
  • While the situation is more complex in vivo and the stability of Epo B is much higher in human than in rodent plasma, their findings on the reduced activity of epothilones in the presence of mouse plasma have led the BMS group to pursue lactambased epothilone analogs as metabolically more stable alternatives to the natural macrolactones [41, 59] .
  • Using this chemistry, the BMS group has also developed an efficient one-pot process for the conversion of Epo B into lactam derivative 34, which involves the above Pd(0)-catalyzed ring-opening reaction, reduction of the azide with trimethyl phosphine and macrolactam formation with EDCI/HOBt [59] ; this provides the desired lactam in 23% overall yield.
  • In spite of its limited effects against highly multidrug-resistant cell lines in vitro, intriguingly, the compound has also been found to be superior to Taxol in Taxol -resistant tumor models [62] .

Modifications in the C2-C8 region

  • Semisynthesis-based modifications in the C2-C8 region have involved transformations of all functional groups present in this sector of the epothilone structure, including the hydroxyl groups at C3 and C7 as well as the keto group at C5.
  • After TES protection of 37 and 38, to give 39 and 40, 1,4-addition of cyanide ion produced a ca. 1/1 mixture of 3S and 3R isomers, which were separated and then deprotected with acetic acid individually to provide 41 and 42 (as well as their corresponding 3R isomers).
  • The in vitro activity of analogs 37/41 and 38/42 is less than one order of magnitude lower than that of Epo A and B, respectively; for 38 and 42 their in vitro antiproliferative activity is thus comparable with taxol [44] .
  • No oxidation of the 7-OH group takes place under these conditions, but the oxidized product is obtained as its C7-methylthiomethyl (MTM) ether 44 (30% yield; Scheme 11).

Side chain modifications

  • The C15 side chain of epothilones has been targeted for semisynthesis in a number of different ways that include modifications (or replacement) of the heterocycle, the vinyl linker between the heterocycle and the macrolactone ring, or both.
  • Both Epo E and F have been elaborated into different C21-modified derivatives [66] and C20 substituents of limited size (but still larger than the natural methyl group) have been found compatible with potent antiproliferative activity; more bulky substituents result in a substantial loss in potency.
  • ABJ879 (55) has demonstrated potent antitumor activity in experimental animal models [69] , where it produced transient regressions and inhibition of tumor growth of slowgrowing (NCI H-596 lung adenocarcinomas, HT-29 colon tumors) as well as fast-growing, difficult-to-treat tumors (NCI H-460 large cell lung tumors).
  • In contrast, olefination reactions involving the C16 keto group proved to be highly problematic.
  • In order to enable the replacement of the thiazole ring by other aryl moieties, Höfle and co-workers have developed an alternative strategy for the construction of the aryl-vinyl part of the epothilone side chain from ketone 59 (Scheme 17).

Replacement of the C13-O16 segment

  • Semisynthetic epothilone analogs have also been prepared via intermediates that were obtained through the degradative removal of the entire C13-O16 segment (including the pendant side chain), which were then (re)elaborated into modified versions of the original structure.
  • Treatment of the crude keto aldehyde thus obtained with K 2 CO 3 led to facile elimination of the C1-C12 segment as the free carboxylic acid, which was converted to ester 77 with TMS-diazomethane.
  • Oxidation of 82 with NaClO 2 gave the mono-ester of a dicarboxylic acid, which could be coupled with amines 83 or 84, to give the C12-C13 amide-based analogs 85 and 86, respectively, after ester saponification, Yamaguchi-type macrolactonization and final deprotection with CF 3 COOH (Scheme 20) [41] .
  • Unfortunately, none of the derivatives 85-88 showed any significant tubulin-polymerizing or growth-inhibitory activity.

Conclusions

  • The work discussed in this review article has defined the chemistry associated with the epothilone molecular framework in significant detail.
  • Most importantly, these efforts have led to the discovery of three derivatives that have been advanced to clinical studies in humans.
  • Thus, notwithstanding the wealth of fully synthetic analogs that have been prepared over the last 15 years and many of which exhibit very attractive biological profiles (at least in vitro), semisynthesis so far has had the more profound impact on the clinical advancement of the epothilone class of microtubule stabilizers than total synthesis.
  • At the same time, it is clear that the potential of semisynthesis for the creation of new structurally unique epothilone analogs is far from being exhausted.
  • It is unclear at this point, whether those groups with a sufficient supply of natural epothilones (BMS, GBF (now Helmholtz Centre for Infection Research), Novartis) continue to be active in the area of epothilone semisynthesis.

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Diversity through semisynthesis
The chemistry and biological activity of semisynthetic
epothilone derivatives
Review Article
Author(s):
Altmann, Karl-Heinz; Gaugaz, Fabienne Z.; Schiess, Raphael
Publication date:
2011-05
Permanent link:
https://doi.org/10.3929/ethz-b-000035789
Rights / license:
In Copyright - Non-Commercial Use Permitted
Originally published in:
Molecular diversity 15(2), https://doi.org/10.1007/s11030-010-9291-0
This page was generated automatically upon download from the ETH Zurich Research Collection.
For more information, please consult the Terms of use.

Mol Divers (2011) 15:383–399
DOI 10.1007/s11030-010-9291-0
COMPREHENSIVE REVIEW
Diversity through semisynthesis: the chemistry and biological
activity of semisynthetic epothilone derivatives
Karl-Heinz Altmann · Fabienne Z. Gaugaz ·
Raphael Schiess
Received: 6 July 2010 / Accepted: 25 October 2010 / Published online: 1 January 2011
© Springer Science+Business Media B.V. 2010
Abstract Epothilones are myxobacterial natural products
that inhibit human cancer cell growth through the stabil-
ization of cellular microtubules (i.e., a “taxol-like” mech-
anism of action). They have proven to be highly productive
lead structures for anticancer drug discovery, with at least
seven epothilone-type agents having entered clinical trials in
humans over the last several years. SAR studies on epothil-
ones have included a large number of fully synthetic ana-
logs and semisynthetic derivatives. Previous reviews on the
chemistry and biology of epothilones have mostly focused on
analogs that were obtained by de novo chemical synthesis. In
contrast, the current review provides a comprehensive over-
view on the chemical transformations that have been investi-
gated for the major epothilones A and B as starting materials,
and it discusses the biological activity of the resulting prod-
ucts. Many semisynthetic epothilone derivatives have been
found to exhibit potent effects on human cancer cell growth
and several of these have been advanced to the stage of clini-
cal development. This includes the epothilone B lactam ixab-
epilone (Ixempra
), which has been approved by the FDA
for the treatment of advanced and metastatic breast cancer.
K.-H. Altmann (
B
)
Department of Chemistry and Applied Biosciences,
Institute of Pharmaceutical Sciences, Swiss Federal Institute
of Technology (ETH) Zürich, HCI H405, Wolfgang-Pauli-Str. 10,
8093 Zürich, Switzerland
e-mail: karl-heinz.altmann@pharma.ethz.ch
F. Z. Gaugaz · R. Schiess
Department of Chemistry and Applied Biosciences,
Institute of Pharmaceutical Sciences, Swiss Federal Institute
of Technology (ETH) Zürich, Wolfgang-Pauli-Str. 10,
8093 Zürich, Switzerland
Keywords Cancer · Drug discovery · Epothilones ·
Microtubules · Natural products · SAR · Semisynthesis ·
Review
Introduction
Microtubule-interacting agents (MSA) are an important class
of antitumor agents [1], which are in use for the treatment of
a variety of cancers, either as single agents or as part of com-
bination chemotherapy [2,3]. MSA can be divided into two
distinct functional classes, namely compounds that inhibit
the assembly of soluble tubulin into microtubule polymers
(“tubulin polymerization inhibitors”) and those that promote
the assembly of tubulin heterodimers into microtubule poly-
mers and stabilize microtubules (“microtubule stabilizers”)
[4]. Among microtubule stabilizers the natural product taxol
(paclitaxel; Taxol
) and its semisynthetic analog docetaxel
(Taxotere
) (Fig. 1) are an important part of today’s arma-
mentarium for the pharmacotherapy of cancer [5].
After the elucidation of taxol’s mode of action in 1979
[6], it took more than a decade before other microtubule-
stabilizing agents with non-taxol-like structures were dis-
covered. Most prominent among these new microtubule
stabilizers are the epothilones, which are bacteria-derived
macrolides whose microtubule-stabilizing properties were
discovered in 1996 by a group at Merck Research Laborato-
ries [7]; the compounds themselves, however, had been first
isolated 9 years earlier from the myxobacterium Sorangium
cellulosum Sc 90 by Reichenbach and Höfle (Fig. 2)[8,9].
The major products originally isolated by Reichenbach
and Höfle were epothilone A and epothilone B (Epo A and B),
but numerous other members of this natural products family
have subsequently been obtained as minor components from
fermentations of myxobacteria [10]. Based on their unusual
123

384 Mol Divers (2011) 15:383–399
O
O
OH
OH
O
O
O
O
O
O
O
O
NH
OH
O
O
OH
OH
OH
O
O
O
O
O
O
O
NH
OH
O
O
Taxol (Paclitaxel; Taxol
®
) Docetaxel (Taxotere
®
)
Fig. 1 Molecular structures of taxol and docetaxel
O
OH
O
OH
O
N
S
O
R
13
9
11
12 13
15
17
21
26
20
19
R = H: Epothilone A
R = CH
3
: Epothilone B
Fig. 2 Molecular structures of epothilones A and B
NH
OH
O
OH
O
N
S
O
Ixabepilone (Ixempra )
®
Fig. 3 Molecular structure of the anticancer drug ixabepilone
mechanism of action (at the time of its discovery), Epo A and
B were quickly adopted as attractive targets for total synthe-
sis and, more importantly, as lead structures for anticancer
drug discovery.
Epothilone-based drug discovery research was addition-
ally triggered by the fact that epothilones, in contrast to
taxol, can inhibit the growth of multidrug-resistant cancer cell
lines at concentrations similar to or only slightly higher than
those required against drug-sensitive cancer cells, [7,1113]
including cells whose taxol resistance is mediated by spe-
cific tubulin mutations [13,14]. Epo B and a number of its
analogs have been demonstrated to possess potent in vivo
antitumor activity and at least seven epothilone-type com-
pounds have entered clinical evaluation in humans (although
several of these are not anymore under development). One
of these compounds, the epothilone B lactam BMS-247550,
was approved for clinical use in humans in 2007 (ixabepi-
lone, Ixempra
), (Fig. 3)[15].
As indicated above, epothilones have been attractive tar-
gets for total chemical synthesis and numerous syntheses of
Epo A and B have been successfully completed (for reviews
cf. [1622]). At the same time, the methodology developed
in the course of those studies has been exploited for the
synthesis of a host of synthetic analogs for SAR studies
(reviewed in [16,17,20,2327]), which highlights the differ-
ence in structural complexity (and, consequently, in synthetic
accessibility) between epothilone-type structures and taxol.
Beyond the investigation of fully synthetic analogs, however,
important aspects of the epothilone SAR (structure–activity
relationship) have also been derived from numerous semi-
synthetic epothilone analogs and ixabepilone (Fig. 3), the
only epothilone that has reached the approval stage so far, in
fact is a semisynthetic derivative of Epo B.
Semisynthetic derivatives of natural products hold a
prominent place in natural product-based drug discovery in
virtually all disease areas [28,29]; due to the structural
complexity of many biologically active natural products
[30], the chemical derivatization of material isolated from
natural sources often represents the only practical means
to explore structure–activity relationships and to produce
analogs with improved biological and/or pharmaceutical
properties. In cancer treatment, important natural product
derivatives include compounds such as etoposide or tenipo-
side (derived from podophyllotoxin) [3133], irinotecan and
topotecan (derived from camptothecin) [ 34 36 ], or doce-
taxel (derived from 10-deacetylbaccatin III) [5,37 ]. Even
for the natural product taxol [38], the sustained supply of
sufficient quantities of drug material for clinical use could
only be secured for some time through the development of a
semisynthetic production process from another natural prod-
uct, namely 10-deacetylbaccatin III [39,40]. Thus, it is not
surprising that semisynthetic approaches have also featured
prominently in the elucidation of the SAR of epothilones and
in the discovery of a number of clinical development candi-
dates. In fact, out of the seven epothilones that have entered
clinical evaluation in humans so far ( including the natural
product Epo B), only one is produced by total chemical syn-
thesis. This bias towards semisynthesis reflects the technical
(fewer chemical steps) and economic (cost of goods) advan-
tages still associated with natural product derivatization.
Obviously, the most fundamental provision for the genera-
tion of semisynthetic derivatives of a natural product is a suf-
ficient supply of the natural product starting material itself.
Fermentation processes are characterized by their own com-
plexities; thus, in the case of epothilones, only few groups
have had access to fermentatively produced starting materi-
als to perform semisynthetic work. Thus, most semisynthetic
epothilone derivatives reported in the literature originate
either from the Höfle group at the former “Gesellschaft für
Biotechnologische Forschung” in Braunschweig, Germany
(GBF, now “Helmholtz Centre for Infection Research”), one
of the discoverers of epothilones, or the group at Bristol–
Myers–Squibb (BMS), either by themselves or in collabo-
ration [8,41]. Semisynthetic work on epothilones, although
more limited in scope, has also been reported by groups at
Novartis, Kosan, and, most recently, our own group at the
ETH Zürich.
123

Mol Divers (2011) 15:383–399 385
This review will provide an overview on the semisynthetic
work that has been reported for epothilones in the public lit-
erature over the last 15 years. The discussion will address
both, the organic chemistry of the system, as well as the
most important aspects of the biological activity and SAR of
these derivatives, and it will be structured according to the
location of groups of modifications in the overall epothilone
structural framework. This will facilitate the comparison of
the biological effects of related structural changes. However,
it is important to note that biological data (i.e., tubulin poly-
merization, in vitro and in vivo antiproliferative activity) may
not always be directly comparable when originating from
different laboratories, due to differences in the experimental
conditions employed.
Semisynthesis and SAR studies
Modifications of the epoxide moiety
Modifications of the epoxide moiety have been an important
trait of the semisynthetic work on epothilones from the very
beginning, which is unsurprising in light of the multitude
of transformations that are conceivable for an oxirane ring
and the potential for further elaboration of the initial reaction
products. The earliest contributions to this area stem from the
GBF group and involved the transformation of epothilones
A, B, and C (vide infra) into a variety of C12/C13-modified
derivatives [23]. Thus, treatment of Epo A with HCl in THF
(aq) or with 1M HCl gave chlorohydrins 1 and 2 in 60–80%
overall yield in a ca. 2–4:1 ratio (in favor of the C12-chloro
isomer 1; Scheme 1)[42].
The corresponding bromo- and iodohydrins were obtained
with bromine or iodine in CCl
4
/CHCl
3
, respectively, with a
ca. 3:1 preference for the C12-halo regioisomer in both cases
[42]. Preferential (but not completely selective) opening of
the epoxide moiety at position 12 upon treatment of Epo A
with different nucleophiles (HCl, MgBr
2
· Et
2
O, NaI/TMS-I,
LiN
3
, Mg(OMe)
2
) has also been reported by the Novartis
group [43]; in contrast, the reaction of Epo A with MgBr
2
·
Et
2
OinCH
2
Cl
2
at 20
Cto5
C leads to the C13-bromo
isomer preferentially with less than 2% of the C12-regio-
isomer being formed [44,45](vide infra). Due to the greater
stability of the C12 over the C13 carbocation in S
N
1-type
reactions, treatment of Epo B with HCl gave chlorohydrin 7
as the only regioisomer in >80% yield (Scheme 2)[42].
Treatment of Epo A with a non-nucleophilic Brønsted acid
such as TFA led to rearranged products 3 and 4 exclusively
(85% combined yield), when acetone was used as the solvent
(Scheme 1)[42]. In contrast, exposure of Epo A or B to non-
nucleophilic acids in the presence of water gave diols 5/6
(Scheme 1) and 8 (Scheme 2), respectively. As for halohy-
drin formation, nucleophilic attack of the epoxide moiety in
Epo A occurs at position 12 preferentially, leading to isomer
O
OH
O
OH
O
N
S
OH
Cl
O
OH
O
OH
O
N
S
Cl
OH
O
OH
O
OH
O
N
S
OH
OH
O
OH
O
OH
O
N
S
O
O
O
O O
O
OH
OH
H
H
S
N
Epo A
1
b)
c)
2
+
a)
12
13
15R: 3
15S: 4
12S, 13S: 5
12R, 13R: 6
12S, 13S : 5a
12R, 13R: 6a
d)
15
Scheme 1 a THF/HCl (aq) or 1 M HCl, RT, 20 min, 60–80%, 1:2, 2:1–4:1. b 0.65 M CF
3
COOH, acetone, 50
C, 10 h, 50% (3) and 35% (4).
c 0.65 M CF
3
COOH, H
2
O, 23
C, 48 h, 55% (5) and 15% (6). dp-TsOH, acetone, 28% (5a) and 15% (6a). (Yields for 5a and 6a are from [43])
123

386 Mol Divers (2011) 15:383–399
O
OH
O
OH
O
N
S
OH
Cl
O
OH
O
OH
O
N
S
OH
OH
Epo B
7
a)
b)
8
Scheme 2 a THF/HCl (aq) or 1 M HCl, RT, 20 min, >80%. b CF
3
COOH/H
2
O, 23
CorH
2
SO
4
/H
2
O/THF, 60
C, 75% (H
2
SO
4
) or 45%
(CF
3
COOH)
O
OH
O
OH
O
N
S
O
OH
O
OH
O
N
S
OH
OH
O
OH
O OH
O
N
S
O
O
Epo C
a) b)
12
13
12R, 13S: 9
12S, 13R: 10
12R, 13S: 9a
12S, 13R: 10a
12
13
Scheme 3 a OsO
4
cat., NMO, t-BuOH, THF/H
2
O, RT, 75 min, 62%, 9:10, 2:1 (inseparable mixture). b acetone, p-TsOH, RT, 2 h, 90% (for
separable mixture of isomers)
5 as the major (but not the only) product; with Epo B diol
8 is the only isomer formed. The rearranged products 3 and
4 show substantially lower antiproliferative activity against
human cancer cells than Epo A [42].
The GBF group has also used OsO
4
-catalyzed dihydroxy-
lation of fermentatively produced Epo C (12,13-deoxyepo-
thilone A) to prepare cis-diols 9 and 10 (Scheme 3); these
compounds were subsequently converted into acetonides 9a
and 10a (as were diols 5 and 6; Scheme 1)[42](seealso
[43]). Acetonides 5a/6a and 9a/10a have been independently
reported by the Novartis group [43], which has also investi-
gated the biological activity of these analogs.
Interestingly, the acetonides derived from 13S diols 5
and 9 (i.e., 5a and 9a) proved to be only 10–15-fold less
potent antiproliferative agents than Epo A against the human
cervical carcinoma cell line KB-31 and its P-gp-expressing
KB-8511 subline (IC
50
values of 23 nM (10 nM) and 30 nM
(17 nM), respectively), while the respective diastereoisomers
6a and 10a were found to be 30–100-fold less potent [43];
likewise, Sefkow et al. [42] have reported 5a to have similar
antiproliferative activity as Epo C against the L929 mouse
fibroblast cell line. These data suggest that for a tetrahedral
geometry at C12 and C13 the size of the ring fused to the
C12–C13 single bond can be significantly increased with-
out substantial loss in biological potency (which does not
seem to be the case for analogs with a planar geometry of
the C12–C13 bond [43,46]). In addition, the data for 9a and
5a also illustrate that, given the proper absolute stereochem-
istry at C12 and C13, activity is retained even upon moving
from a cis-toatrans-fused system; this is in line with data
obtained for a number of synthetic C12,C13-trans epothil-
ones A [27]. It should be noted, however, that the absolute
configuration of compounds 5a and 9a (or the respective
diastereoisomers) has not been rigorously established in the
literature, and it is simply inferred from a comparison of the
biological data with those obtained for Epo A/epi-Epo A (the
inactive 12S,13R-isomer of Epo A) and 12S,13S/12R,13R-
trans-Epo A, respectively.
In contrast to the above acetonides, cis and trans diols 9
and 5 did not show any appreciable biological activity (IC
50
’s
for cancer cell growth inhibition >1 µM) [42,43]. Interest-
ingly, however, the azido alcohol obtained through epoxide
ring opening with NaN
3
at position C12 (i.e., (12R, 13S)-
12-azido-13-hydroxy-12,13-dihydro-Epo C (11), Scheme 4)
was found to be significantly more potent (e.g., IC
50
’s of
11 against the human cervix cancer cell lines KB-31 and
KB-8511 are 61 and 64 nM, respectively) [43]. This indi-
cates that the loss in activity for the above diols cannot be
simply ascribed to increased conformational flexibility. How-
ever, the interpretation of changes in cellular activity is not
straightforward, as they may be caused by a combination of
changes in target affinity, cellular penetration, and metabolic
stability.
Building on the above findings on the potent activity
of acetonides 5a and 9a, we have recently studied bicy-
clic epothilones 1315 (Scheme 4) and a series of related
analogs [48], in order to delineate the biological effects
of other 5-membered heterocycles fused to C12–C13 in
a non-planar arrangement and, in particular, to assess the
impact of substituents on the 5-membered ring. The synthe-
sis of Epo A-derived oxazolines 1315 was based on amino
alcohol 12 as the central intermediate (Scheme 4). As illus-
trated in Scheme 4, 12 was obtained through nucleophilic
ring-opening of the epoxide moiety in Epo A with azide anion
123

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TL;DR: Evaluating the pharmacophoric characteristics of promising compounds against NS2B/NS3pro reported in the past 10 years found several groups were associated with biological activity against dengue, including nitro, catechol, halogen and ammonium quaternaries, which seem to be potential pharmacophores that can be further investigated to treat d Dengue infections.
Abstract: Dengue virus (DENV) infection can lead to a wide range of clinical manifestations, including fatal hemorrhagic complications. There is a need to find effective pharmacotherapies to treat this disease due to the lack of specific immunotherapies and antiviral drugs. That said, the DENV NS2B/NS3pro protease complex is essential in both the viral multiplication cycle and in disease pathogenesis, and is considered a promising target for new antiviral therapies. Here, we performed a systematic review to evaluate the pharmacophoric characteristics of promising compounds against NS2B/NS3pro reported in the past 10 years. Online searches in the PUBMED/MEDLINE and SCOPUS databases resulted in 165 articles. Eight studies, which evaluated 3,384,268 molecules exhibiting protease inhibition activity, were included in this review. These studies evaluated anti-dengue activity in vitro and the IC50 and EC50 values were provided. Most compounds exhibited non-competitive inhibition. Cytotoxicity was evaluated in BHK-21, Vero, and LLC-MK2 cells, and the CC50 values obtained ranged from < 1.0 to 780.5 µM. Several groups were associated with biological activity against dengue, including nitro, catechol, halogen and ammonium quaternaries. Thus, these groups seem to be potential pharmacophores that can be further investigated to treat dengue infections.

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15 Feb 2013
TL;DR: The results imply that the paclitaxel binding site, which was thought to be exclusive for microtubule-stabilizing drugs may also bind micro Tubuline-destabilizing agents.
Abstract: Purpose: Vinca alkaloids and taxanes are tubuline inhibitors in cancer therapy, whose value is restricted by drug resistance and severe side effects. Novel classes of microtubule-stabilizing agents have been developed, including the myxobacterial epothilones, disorazoles and tubulysins. Methods: The binding of two disorazoles (A1, C3) and five tubulysins (pretubulysin, desmethyltubulysin, tubulysins A, B and D) to tubulin was investigated an in silico molecular docking approach. The results were compared with six epothilones (A-F) as well as vinblastine and paclitaxel as control drugs. Results: Epothilones, disorazoles and tubulysins bound to both the vinblastine and paclitaxel binding sites of tubulin. Tubulysins and disorazoles bound with weaker affinity to tubulins as epothilones did. Epothilones showed binding energies close to that of vinblastine at the vinblastine pharmacophore. Conclusions: These results imply that the paclitaxel binding site, which was thought to be exclusive for microtubule-stabilizing drugs may also bind microtubule-destabilizing agents. This may be explained by the fact that the binding site of paclitaxel extends over regions that are relatively close to those of vinblastine.

12 citations


Cites background from "Diversity through semisynthesis: th..."

  • ...Epothilones are myxobacterial natural products, but many fully synthetic analogues and semisynthetic derivatives have been described after their initial discovery (9)....

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Journal ArticleDOI
TL;DR: Two syntheses of the C(7)–C(16)-fragment 41 of epothilone D 2 were developed that were based on tin(IV) bromide mediated reactions of 5,6-difunctionalised hex-2-enylstannanes with aldehydes.
Abstract: Two syntheses of the C(7)–C(16)-fragment 41 of epothilone D 2 were developed that were based on tin(IV) bromide mediated reactions of 5,6-difunctionalised hex-2-enylstannanes with aldehydes. In the first synthesis, (5S)-6-tert-butyldimethylsilyloxy-5-hydroxy-2-methylhex-2-enyl(tributyl)stannane 20 was reacted with (E)-but-2-enal to give (2S,7R,4Z,8E)-1-tert-butyldimethylsilyloxy-5-methyldeca-4,8-diene-2,7-diol 26 containing ca. 20% of its (7S)-epimer. Following desilylation, the crystalline (2S,7R)-triol 32 was protected as its acetonide 33 and esterified to give the (4-methoxybenzyloxy)acetate 34. An Ireland–Claisen rearrangement of this ester gave methyl (2R,3S,10S,4E,7Z)-3,7-dimethyl-10,11-(dimethylmethylene)dioxy-2-(4-methoxybenzyloxy)undeca-4,7-dienoate 35 that was converted into (2S,9S,6Z)-2,6-dimethyl-9,10-(dimethylmethylene)dioxydec-6-en-1-ol 41 by regioselective alkene manipulation, ester reduction and cleavage of the resulting terminal diol 40 with a reductive work-up. The second synthesis involved the tin(IV) bromide mediated reaction between the stannane 20 and (3S)-4-(4-methoxybenzyloxy)-3-methylbutanal 44 that gave (2S,7S,9S,4Z)-1-tert-butyldimethylsilyloxy-5,9-dimethyl-10-(4-methoxybenzyloxy)dec-4-ene-2,7-diol 45 containing ca. 20% of its (7R)-epimer. After desilylation and protection of the vicinal diol as its acetonide 46, a Barton–McCombie reductive removal of the remaining hydroxyl group gave the (2S,9S,6Z)-2,6-dimethyl-9,10-(dimethylmethylene)dioxydec-6-en-1-ol 41 after oxidative removal of the PMB-ether. The first of these syntheses uses just one chiral starting material, but the second is shorter and more convergent. It was therefore modified by the use of (5S)-6-tert-butyldimethylsilyloxy-5-(2-trimethylsilylethoxy)methoxy-2-methylhex-2-enyl(tributyl)stannane 49 that reacted with (3S)-4-(4-methoxybenzyloxy)-3-methylbutanal 44 to give a 50:50 mixture of the C(4)-epimers of (2S,9S,6Z)-10-tert-butyldimethylsilyloxy-1-(4-methoxybenzyloxy)-2,6-dimethyl-9-(2-trimethylsilylethoxy)methoxydec-6-en-4-ol 50 with high fidelity for formation of the (Z)-alkene. Following the Barton–McCombie deoxygenation, the product 52 was taken through to (2S,9S,6Z,10E)-2,6,10-trimethyl-11-(2-methyl-1,3-thiazol-4-yl)-9-(2-trimethylsilylethoxy)methoxyundeca-6,10-dienal 59 that corresponded to the fully functionalised C(7)–C(17) fragment of epothilone D 2. A precedented stereoselective aldol condensation followed by O-protection, selective deprotection, oxidation and macrocyclisation then gave the macrolide 71 that was deprotected to complete a synthesis of epothilone D 2. Finally regio- and stereo-selective epoxidation gave epothilone B 1.

11 citations

Book ChapterDOI
01 Jan 2020
TL;DR: This article briefly review advances pertaining to the heterologous expression of various biosynthetic gene clusters in myxobacteria by highlighting theMyxobacterial model strain Myxococcus xanthus and the recent technical advances concerning the myXobacterial production platform.
Abstract: Myxobacteria are prolific producers of natural products exhibiting diverse chemical structures, biological properties and complex biosynthetic machineries. The immense potential of myxobacteria as “natural product factories” is still underexploited and holds auspicious perspectives for biosynthetic engineering applications. In this article, we briefly review advances pertaining to the heterologous expression of various biosynthetic gene clusters in myxobacteria by highlighting the myxobacterial model strain Myxococcus xanthus. We will focus on the employed biosynthetic logic of the heterologously expressed biosynthetic pathways and the recent technical advances concerning the myxobacterial production platform.

10 citations

Journal ArticleDOI
TL;DR: In this article, the complex scaffold afforded through optimized fermentation was used as a feedstock for semisynthetic efforts designed to explore the reactivity of the C11 and C11' hydroxy substituents.
Abstract: The verticillins, a class of epipolythiodioxopiperazine alkaloids (ETPs) first described 50 years ago with the discovery of verticillin A (1), have gained attention due to their potent activity against cancer cells, noted both in vitro and in vivo. In this study, the complex scaffold afforded through optimized fermentation was used as a feedstock for semisynthetic efforts designed to explore the reactivity of the C11 and C11' hydroxy substituents. Functionality introduced at these positions would be expected to impact not only the potency but also the pharmacokinetic properties of the resulting compound. With this in mind, verticillin H (2) was used as a starting material to generate nine semisynthetic analogues (4-12) containing a variety of ester, carbonate, carbamate, and sulfonate moieties. Likewise, verticillin A succinate (13) was synthesized from 1 to demonstrate the successful application of this strategy to other ETPs. The synthesized compounds and their corresponding starting materials (i.e., 1 and 2) were screened for activity against a panel of melanoma, breast, and ovarian cancer cell lines: MDA-MB-435, MDA-MB-231, and OVCAR3. All analogues retained IC50 values in the nanomolar range, comparable to, and in some cases more potent than, the parent compounds.

9 citations

References
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Journal ArticleDOI

3,761 citations


"Diversity through semisynthesis: th..." refers background in this paper

  • ...Even for the natural product taxol [38], the sustained supply of sufficient quantities of drug material for clinical use could only be secured for some time through the development of a semisynthetic production process from another natural product, namely 10-deacetylbaccatin III [39,40]....

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Journal ArticleDOI
22 Feb 1979-Nature
TL;DR: It is reported here that taxol acts as a promoter of calf brain microtubule assembly in vitro, in contrast to plant products such as colchicine and podophyllotoxin, which inhibit assembly.
Abstract: TAXOL (Fig. 1) was isolated from the plant Taxus brevifolia (western yew) by Wani et al., who reported that the molecule has antitumour activity in several experimental systems1. In our laboratory we have found that taxol, a low molecular weight neutral compound, completely inhibits division of exponentially growing HeLa cells at low concentrations of drug (0.25 µM) that have no significant effects on DNA, RNA or protein synthesis during a 4-h incubation with the cells. HeLa cells incubated with taxol for 20 h are blocked in late G2 and/or M (ref. 2). We report here that taxol acts as a promoter of calf brain microtubule assembly in vitro, in contrast to plant products such as colchicine and podophyllotoxin, which inhibit assembly. Taxol decreases the lag time for microtubule assembly and shifts the equilibrium for assembly in favour of the microtubule, thereby decreasing the critical concentration of tubulin required for assembly. Microtubules polymerised in the presence of taxol are resistant to depolymerisation by cold (4 °C) and CaCl2 (4 mM).

3,430 citations


"Diversity through semisynthesis: th..." refers background in this paper

  • ...After the elucidation of taxol’s mode of action in 1979 [6], it took more than a decade before other microtubulestabilizing agents with non-taxol-like structures were discovered....

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Journal ArticleDOI
TL;DR: To continue to be competitive with other drug discovery methods, natural product research needs to continually improve the speed of the screening, isolation, and structure elucidation processes, as well addressing the suitability of screens for natural product extracts and dealing with issues involved with large-scale compound supply.
Abstract: Although traditionally natural products have played an important role in drug discovery, in the past few years most Big Pharma companies have either terminated or considerably scaled down their natural product operations. This is despite a significant number of natural product-derived drugs being ranked in the top 35 worldwide selling ethical drugs in 2000, 2001, and 2002. There were 15 new natural product-derived drugs launched from 2000 to 2003, as well as 15 natural product-derived compounds in Phase III clinical trials or registration at the end of 2003. Recently, there has been a renewed interest in natural product research due to the failure of alternative drug discovery methods to deliver many lead compounds in key therapeutic areas such as immunosuppression, anti-infectives, and metabolic diseases. To continue to be competitive with other drug discovery methods, natural product research needs to continually improve the speed of the screening, isolation, and structure elucidation processes, as well addressing the suitability of screens for natural product extracts and dealing with issues involved with large-scale compound supply.

1,264 citations


"Diversity through semisynthesis: th..." refers background in this paper

  • ...Semisynthetic derivatives of natural products hold a prominent place in natural product-based drug discovery in virtually all disease areas [28,29]; due to the structural complexity of many biologically active natural products [30], the chemical derivatization of material isolated from natural sources often represents the only practical means to explore structure–activity relationships and to produce analogs with improved biological and/or pharmaceutical properties....

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Journal Article
TL;DR: Epothilones represent a novel structural class of compounds, the first to be described since the original discovery ofTaxol, which not only mimic the biological effects of taxol but also appear to bind to the same microtubule-binding site as taxol.
Abstract: Tubulin polymerization into microtubules is a dynamic process, with the equilibrium between growth and shrinkage being essential for many cellular processes. The antineoplastic agent taxol hyperstabilizes polymerized microtubules, leading to mitotic arrest and cytotoxicity in proliferating cells. Using a sensitive filtration-calorimetric assay to detect microtubule nucleating activity, we have identified epothilones A and B as compounds that possess all the biological effects of taxol both in vitro and in cultured cells. The epothilones are equipotent and exhibit kinetics similar to taxol in inducing tubulin polymerization into microtubules in vitro (filtration, light scattering, sedimentation, and electron microscopy) and in producing enhanced microtubule stability and bundling in cultured cells. Furthermore, these 16-membered macrolides are competitive inhibitors of [3H]taxol binding, exhibiting a 50% inhibitory concentration almost identical to that of taxol in displacement competition assays. Epothilones also cause cell cycle arrest at the G2-M transition leading to cytotoxicity, similar to taxol. In contrast to taxol, epothilones retain a much greater toxicity against P-glycoprotein-expressing multiple drug resistant cells. Epothilones, therefore, represent a novel structural class of compounds, the first to be described since the original discovery of taxol, which not only mimic the biological effects of taxol but also appear to bind to the same microtubule-binding site as taxol.

1,188 citations


"Diversity through semisynthesis: th..." refers background in this paper

  • ...Most prominent among these new microtubule stabilizers are the epothilones, which are bacteria-derived macrolides whose microtubule-stabilizing properties were discovered in 1996 by a group at Merck Research Laboratories [7]; the compounds themselves, however, had been first isolated 9 years earlier from the myxobacterium Sorangium cellulosum Sc 90 by Reichenbach and Höfle (Fig....

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Journal ArticleDOI
TL;DR: Results identify residues β270 and β364 as important modulators of paclitaxel’s interaction with tubulin as well as acquired mutations in the M40 isotype at nucleotide 810 (T → G; Phe270 → Val) in 1A9PTX10 cells and nucleotide 1092 (G → A; Ala364 → Thr) in1A 9PTX22 cells.

708 citations


"Diversity through semisynthesis: th..." refers background in this paper

  • ...Epothilone-based drug discovery research was additionally triggered by the fact that epothilones, in contrast to taxol, can inhibit the growth of multidrug-resistant cancer cell lines at concentrations similar to or only slightly higher than those required against drug-sensitive cancer cells, [7,11–13] including cells whose taxol resistance is mediated by specific tubulin mutations [13,14]....

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Frequently Asked Questions (15)
Q1. What have the authors contributed in "The chemistry and biological activity of semisynthetic epothilone derivatives" ?

In contrast, the current review provides a comprehensive overview on the chemical transformations that have been investigated for the major epothilones A and B as starting materials, and it discusses the biological activity of the resulting products. 

While 59 could be converted to methylene derivative 60 in a modest overall yield of 15% (after deprotection), all attempts to re-introduce the natural thiazole side chain or to create a phenyl-based Epo A analog using Wittig-type chemistry were unsuccessful [47,71,75]. 

The earliest contributions to this area stem from the GBF group and involved the transformation of epothilonesA, B, and C (vide infra) into a variety of C12/C13-modified derivatives [23]. 

Most prominent among these new microtubule stabilizers are the epothilones, which are bacteria-derived macrolides whose microtubule-stabilizing properties were discovered in 1996 by a group at Merck Research Laboratories [7]; the compounds themselves, however, had been first isolated 9 years earlier from the myxobacterium Sorangium cellulosum Sc 90 by Reichenbach and Höfle (Fig. 2) [8,9]. 

The resulting N-unsubstituted 12,13-aziridinyl-Epo A 21 has been converted into a series of N-substituted derivatives via alkylation, acylation, carbamoylation, or sulfonylation. 

While not accessible by base treatment and subsequent electrophilic quenching, C21-substituted epothilone derivatives can nevertheless be obtained through semisynthesis in a very efficient manner. 

Employing PLE-catalyzed hydrolysis of the lactone group and subsequent cleavage of the C12/C13 double bond by ozonolysis, the BMS group was able to establish the controlled degradation of Epo C into ester 82 (Scheme 20). 

Selective oxidation of the hydroxyl group on C3 in Epo A is more difficult and could only be accomplished in very moderate yield with a mixture of dimethylsulfide and dibenzoylperoxide [47]. 

The phenyl-substituted oxazoline 13 was found to inhibit human cancer cell growth in vitro with IC50 values around 20 nM [48]; thus, the activity of this analog is within a 10- fold range of the activity of Epo A and it is comparable with the activity of cyclic acetals 5a and 9a (vide supra). 

It is, therefore, unclear to what extent (if at all) the enhanced cellular activity of 14 (over 13) is a result of higher affinity interactions with the tubulin/microtubule system (possibly through H-bond formation between the pyridine nitrogen and a donor group on the protein). 

In addition to cyclopropyl-epothilones, the BMS group has also devised a strategy for the conversion of Epo A to a whole range of analogs incorporating a (substituted) aziridine ring in place of the epoxide moiety [45]. 

Bis-substitution of the oxygen-replacing carbon in cyclopropyl-Epo B by bulky bromine substituents leads to reduced activity, but the resulting analog is still more potent than Epo D against the HCT-116 cell line (IC50 against HCT116 of 3.8 nM versus 6.5 nM for Epo D [49]). 

The first implementation of this concept was reported by the GBF group, who used ring-opening olefin metathesis (ROM) with ethylene for the conversion of Epo C into the ring-opened product 69 in 73% yield (employing Grubbs II catalyst) [73] (Scheme 18). 

This bias towards semisynthesis reflects the technical (fewer chemical steps) and economic (cost of goods) advantages still associated with natural product derivatization. 

Epo B and a number of its analogs have been demonstrated to possess potent in vivo antitumor activity and at least seven epothilone-type compounds have entered clinical evaluation in humans (although several of these are not anymore under development).