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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-

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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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"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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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).

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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.

1,166 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
Mark S. Butler1Institutions (1)
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.

1,165 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 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.
Abstract: Acquired resistance to paclitaxel can be mediated by P-glycoprotein or by alterations involving tubulin. We report two paclitaxel-resistant sublines derived from 1A9 human ovarian carcinoma cells. Single-step paclitaxel selection with verapamil yielded two clones that are resistant to paclitaxel and collaterally sensitive to vinblastine. The resistant sublines are not paclitaxel-dependent, and resistance remained stable after 3 years of drug-free culture. All cell lines accumulate [3H]paclitaxel equally, and no MDR-1 mRNA was detected by polymerase chain reaction following reverse transcription. Total tubulin content is similar, but the polymerized fraction increased in parental but not in resistant cells following the paclitaxel addition. Purified tubulin from parental cells demonstrated paclitaxel-driven increased polymerization, in contrast to resistant cell tubulin, which did not polymerize under identical conditions. In contrast, epothilone B, an agent to which the resistant cells retained sensitivity, increased assembly. Comparable expression of beta-tubulin isotypes was found in parental and resistant cells, with predominant expression of the M40 and beta2 isotypes. Sequence analysis demonstrated acquired mutations in the M40 isotype at nucleotide 810 (T --> G; Phe270 --> Val) in 1A9PTX10 cells and nucleotide 1092 (G --> A; Ala364 --> Thr) in 1A9PTX22 cells. These results identify residues beta270 and beta364 as important modulators of paclitaxel's interaction with tubulin.

690 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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20213
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