Design and total synthesis of a superior family of epothilone analogues, which eliminate xenograft tumors to a nonrelapsable state.

Full text of DOI:10.1002/anie.200352361

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Communications
Molecular editing of epothilone B (isolated from a myxobacterium)
led to the discovery of a 26-trifluoro analogue that is remarkably active
in treating xenograft tumors in nude mice. These findings underscore
the potential of directed multistep total synthesis in the quest for new
drugs, as discussed by S. J. Danishefsky and co-workers on the
following pages.
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Epothilones: Synthesis and Activity
already been advanced to late phase I and phase II clinical
trials, and additional structural variants are being made ready
to be approved for investigation.[11]
Design and Total Synthesis of a Superior Family of
Epothilone Analogues, which Eliminate
Xenograft Tumors to a Nonrelapsable State**
A governing concept in our laboratory had been the
notion that the 12,13-oxido linkage of the epothilones is a site
of non-tumor-selective toxicity.[12] Therefore, this linkage was
“edited”, initially through chemical synthesis, to provide
12,13-desoxyEpoB(dEpoB, 2) and related desoxy conge-
ners.[13,14] Although it is significantly less cytotoxic than
epothilone B(EpoB, 1) itself, in xenograft models dEpoBis
roughly equipotent with paclitaxel (taxol)[15] and taxotere, the
currently leading tubulin-directed anticancer drugs. dEpoB
benefits from a much broader therapeutic index in xenograft
models than taxoids or other epothilones, which contain
epoxides. The major advantage of dEpoBover paclitaxel in
Ting-Chao Chou, Huajin Dong, Alexey Rivkin,
Fumihiko Yoshimura, Ana E. Gabarda,
Young Shin Cho, William P. Tong, and
Samuel J. Danishefsky*
The epothilone macrolides, first discovered by Höfle et al. in
fermentation broths, have elicited a great deal of interest as
potential agents in cancer chemotherapy.[1–8] A key paper of
Bollag et al.[9] seeded the now multi-
center epothilone initiative. This disclo-
sure revealed the cellular target of the
epothilones to be microtubule stabiliza-
tion, which is the same mode of action
as the established, clinically useful tax-
oids exhibit. Moreover, the epothilones
were shown to manifest virtual imper-
viousness to the defenses of otherwise
multidrug-resistant (MDR) cells.[10] As
susceptibility to disablement by MDR
cells is one of the main liabilities of the
taxoid drugs, the robustness of the epothilones in this regard is
of particular significance. Following extensive multidiscipli-
nary research into the biology, chemistry, pharmacology,
toxicology, and biosynthesis of the structurally new epothi-
lones, three agents, including compound 2 (see below), have
these nude-mouse xenograft models is its particularly dra-
matic activity against resistant tumors. dEpoBhas been
entered into human clinical trials.[11]
In a second-generation 12,13-desoxyepothilone drug can-
didate, we hoped to recover some of the loss of potency that
had been sustained by deletion of the oxido linkage from
EpoB. For further development, we also required a second-
generation epothilone to exhibit a clinically and readily
exploitable therapeutic index, and in particular to lead to
complete and nonreversible cures in xenograft models. This
standard is seldom, if ever, set in the search for new anticancer
drugs. We also hoped to produce an agent type of greater
pharmacostability in humans than dEpoB, which has a
potentially vulnerable macrolide (cyclic ester) linkage.
By a combination of chemical synthesis, molecular
modeling, and spectroscopic analysis, we discovered that the
introduction of an E-9,10 double bond (see compound 4)
leads to an approximately 10-fold enhancement of drug
potency in xenograft experiments with drug-resistant MX-1
tumors.[16] It was apparent from the correlation of in vitro and
in vivo experiments focused on MX-1 tumor types that 4 is
inherently more cytotoxic than 1. A second contributing
factor is that the lactone functionality in the 9,10-dehydro
series is significantly more stable in mouse and human plasma
than that in the 9,10-saturated congeners. The sum of these
two complementary effects was to render 4 capable of
complete tumor suppression in a variety of xenografts at
3 mgkgꢀ1, in contrast to the 30 mgkgꢀ1 dose required for 2.
However, along with this much-enhanced antitumor
activity in 4, significant non-tumor-specific toxicity was
observed, which sorely complicated the full eradication of
its tumor targets. As with other potent chemotherapeutic
[*] Prof. S. J. Danishefsky, Dr. A. Rivkin, Dr. F. Yoshimura,
Dr. A. E. Gabarda, Dr. Y. S. Cho
Laboratory for Bioorganic Chemistry
Sloan-Kettering Institute for Cancer Research
1275 York Avenue, New York, NY 10021 (USA)
Fax: (+1)212-772-8691
E-mail: s-danishefsky@ski.mskcc.org
and
Department of Chemistry, Columbia University
Havemeyer Hall, 3000 Broadway, New York, NY 10027 (USA)
Dr. T.-C. Chou, Dr. H. Dong
Preclinical Pharmacology Core Facility
Sloan-Kettering Institute for Cancer Research
1275 York Avenue, New York, NY 10021 (USA)
Dr. W. P. Tong
Analytical Pharmacology Core Facility
Sloan-Kettering Institute for Cancer Research
1275 York Avenue, New York, NY 10021 (USA)
[**] This work was supported by the National Institutes of Health (CA-
28824). F.Y. is a Uehara Memorial Foundation Fellow. A.R. is a NIH
Cancer Pharmacology Fellow (T32-CA62948). A.E.G. is a MSKCC
Experimental Therapeutics Center Fellow. We thank Sylvi Rusli
(NMR Core Facility, CA-02848) and Anna Dudkina for mass-spectral
analyses. We thank Dr. Klaus Gerth and Dr. Rolf Müller of GBF,
Germany for the picture of Sorangium cellulos that is part of the
frontispiece design.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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agents, easily perceptible tumors reappeared in some fraction
of the animals upon suspension of treatment. Accordingly,
fully synthetic 4 does not, at least at present, qualify as having
a highly favorable effective therapeutic index and causing
elimination of tumors to a nonrelapsing state, as we had
aimed for in a second-generation drug candidate.
relapsed. Extended observation following suspension of
treatment with 20 mgkgꢀ1 dosages of 5 showed a long-term
absence of tumors until day 27, at which point 2/4 mice
relapsed. Remarkably, treatment with dosages of 5 of
30 mgkgꢀ1 resulted in complete tumor disappearance and
the absence of any relapse for over two months after
suspension of treatment.
These findings directed our attention to the consequences
of substituting the three hydrogen atoms of the 26-methyl
group of 4 with three fluorine atoms. We anticipated that
incorporation of fluorine atoms at this position could lead to
improved stability of the C12–C13 double bond toward
oxidation.[17] Previous experience had pointed to some
attenuation of cytotoxicity when polar groups were placed
near the C12–C13 double bond.[16] Herein we report on the
total synthesis and antitumor activity of 9,10-dehydro-26-
trifluoroepothilones, with particular emphasis on the unique
biological activity of the parent structure 5.
When the dose of agent 5 was lowered to 10 mgkgꢀ1
(Q2Dx12 = administered every other day, 12 doses in total),
nine doses were required for tumor disappearance to be
observed (Figure 1a). As an added challenge, chemothera-
peutic treatment was delayed until the size of the tumor
reached 0.5 g (~ 2.3% of body weight). Treatment with
dosages of 25 mgkgꢀ1 (Q2Dx7 = administered every other
day, 7 doses in total) of 5 then caused all four of the mouse
tumors to disappear. By contrast, dosages of 30 mgkgꢀ1
(Q2Dx8) of dEpoBwere required to induce the disappear-
ance of tumors in 3/4 mice. Furthermore, the apparent tumor
disappearance that occurred following treatment with dEpoB
was subject to relapse with time (Figure 1b).
The fact that agent 5 completely suppressed the growth of
the human-mammary-carcinoma MX-1 xenografts, shrank
the tumors, and eliminated the tumors for as long as 64 days is
impressive. Moreover, following the cures by 5 (20 mgkgꢀ1 or
30 mgkgꢀ1 Q2Dx6, six-hour intravenous infusion, Table 2),
body weight returned to pretreatment control level within 12–
18 days after the suspension of
The therapeutic efficacy of dEpoB(30 mgkg ꢀ1), paclitaxel
(20 mgkgꢀ1), and F3-deH-dEpoB( 5, 20 and 30 mgkgꢀ1
)
against human-mammary-carcinoma MX-1 xenografts was
closely studied in terms of tumor disappearance and relapse
(Table 1). Each dose group consisted of four or more nude
mice. “Body weight” refers to total body weight minus tumor
weight. Tumor disappearance was observed for all three
compounds. On the 10th day after suspension of treatment, 5/
10 (dEpoB), 2/7 (paclitaxel), and 0/4 (compound 5) mice
treatment. This finding suggests a
Table 1: Therapeutic effect of dEpoB (2), paclitaxel, and F3-deH-dEpoB (5) against an MX-1 xenograft in
lack of vital-organ damage. Most
remarkably, at curative low
nude mice.[a]
a
Drug
Dosage
Change in body weight [%][b]
Tumor-free after
treatment period
Tumor
dosage of 10 mgkgꢀ1, Q2Dx12 (Fig-
ure 1a), the maximal decrease in
body weight was only 12%, with a
gain in body weight of 6% during
the last three doses. The body
weight recovered to the pretreat-
ment control level only three days
after the cessation of treatment.
Table 2 shows that the animals
could survive body-weight losses of
as much as 27%.
[mgkgꢀ1
]
day 4
day 8
reappearance[c]
2
30
20
20
30
ꢀ25.3ꢁ2.1
ꢀ23.9ꢁ3.7
ꢀ22.4ꢁ0.6
ꢀ27.1ꢁ2.7
ꢀ9.1ꢁ4.1
ꢀ8.7ꢁ0.7
ꢀ7.3ꢁ0.7
ꢀ17.4ꢁ5.5
10/10
7/7
4/4
5/10
2/7
paclitaxel
5
5
0/4[d]
0/4[e]
4/4
[a] Human-mammary-carcinoma MX-1 xenograft tissue (50 mg) was implanted subcutaneously on
day 0. The treatment (Q2Dx6 6-h intravenous infusion) was started on day 8 and stopped on day 18.
[b] Change in body weight on day 4 and day 8 of treatment, measured after administration of the drug.
[c] Number of mice in sample in which tumor reappeared on the 10th day after the end of the treatment
period. [d] Detectable tumor reappearance in 2/4 mice on the 27th day after treatment was stopped. No
further tumor reappearance between the 28th and 64th days after treatment was stopped. [e] No tumor
reappearance during 64 days after treatment was stopped, at which point the experiment was
terminated.
Table 2: Profile of dEpoB derivatives.
Compound
IC50 [nm][a]
Maximal drop
in body weight
without death
[%]
Stability half-life
Solubility
in water
Lipophilicity
octanol/water
partition
Therapeutic
Relative
therapeutic
index
Mouse plasma
[min]
Human-liver S9
fraction [h]
dose regimen
[mgmLꢀ1
]
[mgkgꢀ1 [b]
]
(POW)
at MTD[c]
1
2
4
3
5
0.53ꢁ0.2
5.6ꢁ2.8
0.90ꢁ0.40
9.3ꢁ5.2
3.2ꢁ0.3
15
32
29
22
33
57
15.8
n.d.
9.4
27
8
20
n.d.
4.4
3.3
4.1
3.3
0.6–0.8
25–30
3–4
15–20
10–30
+++
46ꢁ7
1.0ꢁ0.1
4.9ꢁ0.7
1.6ꢁ0.4
10.5ꢁ2.3
++++
++++
++
84ꢁ6
66ꢁ7
212ꢁ88
+++++
[a] IC50 values are for CCRF-CEM leukemic cells. Values are the range of two experiments. All values are obtained from eight time points. n.d. =not
determined. [b] Therapeutic dose regimen for Q2D 6-h intravenous infusion. [c] Graded relative therapeutic index (TI) at MTD (maximal tolerated
dose): + Tumor growth suppressed by 25–50%. ++ Tumor growth suppressed by 50–100%. +++ Tumor shrinkage but no tumor disappearance.
++++ Tumor disappearance in some or all nude mice with slow body-weight recovery and/or with relapse in some mice within one week after
treatment was stopped. +++++ Tumor disappearance in all nude mice with rapid body-weight recovery and/or without relapse. The therapeutic
effects of epothilones against human xenografts in nude mice, such as MX-1, were studied in references [15] and [27].
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Figure 1. Chemotherapeutic effect against human tumor xenografts in nude mice. Tumor tissue (40–50 mg) was implanted subcutaneously on
day 0. Treatment was started when tumor size reached about 100 mm3 or larger, as indicated. All treatments, indicated by arrows, were carried
out by six-hour intravenous infusion into the tail vein with a minicatheter and programmable pump as described previously.[7,14] Each dose group
consisted of four or more mice. “Body weight” refers to the total body weight minus the tumor weight, with the assumption that 1 mm3 of tumor
tissue has a weight of 1 mg. a) Mammary-carcinoma MX-1 xenograft treated with a low dose of 5 (10 mgkgꢀ1) relative to the experiments in
Table 1 (20 mgkgꢀ1 and 30 mgkgꢀ1). b) MX-1 large xenografts (500 mm3) treated with 5 (25 mgkgꢀ1) and dEpoB (30 mgkgꢀ1). c) Slow-growing
A549 lung-carcinoma xenograft treated with 5 (25 mgkgꢀ1) and dEpoB (30 mgkgꢀ1). d) A549/taxol (44-fold resistance to paclitaxel in vitro) xeno-
graft treated with 5 (20 mgkgꢀ1) and 4 (4 mgkgꢀ1). The treatment with deH-dEpoB on day 28 was skipped as a result of marked and rapid
decreases in body weight. D=day.
The therapeutic safety margin observed in this work is
remarkably broad for a curative therapeutic agent against
cancer. The therapeutic efficacy of 5 against human-lung-
carcinoma xenograft (A549) and the paclitaxel-resistant
human-lung-carcinoma A549/taxol xenografts was also eval-
uated (Figure 1c,d). The slow-growing lung-carcinoma xeno-
grafts A549 were treated with 5 (25 mgkgꢀ1, Q2Dx6, twice,
eight days apart), which resulted in 99.5% tumor suppression
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with the eventual complete eradication of all four tumors
after two more doses (Figure 1c). Interestingly, the body
weight of the mice decreased as much as 35% without any
lethality, and suspension of the treatment led to rapid body-
weight recovery to near the pretreatment control level
(Figure 1c). In contrast,
a parallel study with dEpoB
(30 mgkgꢀ1, Q2Dx6) resulted in 97.6% tumor suppression
but led to no tumor eradication. In an additional study of 5
(dosage: 20 mgkgꢀ1) against A549/taxol-resistant xenografts
(Figure 1d), tumor growth was totally suppressed, and the
tumor was eventually reduced by 24.4% of the pretreatment
control. During this study, the maximal body weight
decreased by 24%. However, upon suspension of drug
treatment the body weight returned to 90% of the pretreat-
ment control. In a comparison study with (E)-9,10-dehydro-
dEpoB( 4, 4 mgkgꢀ1), tumor growth was suppressed by
41.6%.
Scheme 1. Synthesis of the acyl sector 11. Reagents and conditions: a) 1. LDA,
tert-butyl acetate, THF, 80%; 2. Dess–Martin periodinane, 74%; b) Noyori cata-
lyst (10 mol%), MeOH/HCl, H2, 1200 psi, 80%; c) 1. TESCl, imidazole, 77%;
2. Zn, AcOH, THF, 99%; 3. TBSOTf, 2,6-lutidine, 82%; for remaining steps, see
We wanted to assess the factors responsible for the
remarkable therapeutic index of compound 5. The pertinent reference [16]. Troc=trichloroethoxycarbonyl, LDA=lithium diisopropylamide,
TES=triethylsilyl, TBS=tert-butyldimethylsilyl.
data for this analysis in conjunction with corresponding data
for closely related congeners are provided in Table 2. Awhole
order of magnitude is lost in moving from EpoB(1) to dEpoB
(2) in terms of inherent cytotoxicity. About 60% of this loss is
restored in the case of 9,10-dehydro-dEpoB( 4). Some of this
inherent cytotoxicity is forfeited by the inclusion of the CF3
group in 5, which is ~ 1.8 times as cytotoxic as the benchmark
compound 2, at least in the cell.
Among the 12,13-dehydroepothilones, 5 exhibited by far
the highest stability in mouse plasma and was also the most
stable in human liver S9 plasma. We also observed that in the
two sets of 12,13-dehydro isomers, the 26-trifluoromethyl
substituent gives rise to decreased lypophilicity and somewhat
increased water solubility. It would appear that the excellent
therapeutic activity of 5 arises from improvements in serum
stability and bioavailability.
All of the agents 2–5 were initially discovered through
total synthesis. A practical synthesis of 2 has been described
previously.[13,14] First-generation discovery-level routes to 4
and 5 have also been described.[16,18–20] Selective reduction of
the C9–C10 double bond of 5 afforded 3. The remarkable
results obtained from the xenograft studies described above
for 5, which is currently the most promising compound,
clearly called for its advancement to detailed toxicology and
pharmacokinetic studies in higher animals, and from there, if
appropriate, to human clinical trials. Such prospects com-
pletely altered the nature of the synthetic challenge from the
preparation of probe samples to the preparation of multigram
quantities of these new epothilone derivatives. We carried out
a major revamp of our previous routes, which were initially
conceived and demonstrated in a discovery setting. In
particular, our new protocols allowed major simplifications
in the stereospecific elaboration of the C3 and C26 centers.
epimers to the ketone 9, which was then subjected to a highly
successful Noyori reduction[21] under the conditions shown.
The C3 alcohol product 10 was converted into the acid 11 in a
few additional simple steps, as shown.
A new, straightforward synthesis, which can be scaled up
readily, has also been developed for 17 (Scheme 2). The
synthesis starts with the reaction of the commerically
available trifluorinated b-ketoester 12 with allyl indium
bromide. The key step in the synthesis is the regio- and
stereospecific dehydration of the resulting tertiary alcohol to
produce 13 in 65% yield from 12. We surmise that the
stereoselectivity observed in this reaction arises from a
“dipolar effect”, whereby a trans orientation of the strongly
electron-withdrawing CF3 and CO2Et groups is favored, with
respect to the double bond that is forming. The required
iodide 14 was obtained in two steps from 13. Alkylation of the
Scheme 2. Synthesis of the alkyl sector 17. Reagents and conditions:
a) 1. Allyl bromide, In, THF/H2O (3:1), 488C, 85%; 2. SOCl2, pyridine,
558C, 77%; b) 1. DIBAL-H, CH2Cl2, ꢀ788C!RT, 99%; 2. I2, PPh3, imid-
azole, CH2Cl2, 74%; c) 1. LiHMDS, THF, ꢀ788C!RT; 2. HOAc/THF/
H2O (3:1:1), 81% for two steps; d) 1. AlMe3, MeONHMe, THF, 08C!
RT, 97%; 2. MeMgBr, THF, 08C, 53% (73% based on recovered start-
ing material). DIBAL-H=diisobutylaluminium hydride, HMDS=hexa-
methyldisilazide.
Aldehyde
8
was prepared as described previously
(Scheme 1).[16] In the new synthesis, the stereocenters C6,
C7, and C8 are derived from the readily available ketone 6
and aldehyde 7. Condensation of the aldehyde 8 with tert-
butyl acetate, as shown, afforded an aldol-like product. As the
condensation is not diastereomerically controlled, it was
followed by oxidation of the resulting 1:1 mixture of C3
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previously reported lithium enolate of 15 with iodide 14 in
This study serves to underscore the potential applicability
of directed total synthesis, even in a multistep setting, in the
quest for new substances of material clinical benefit. The
current research environment tends to favor recourse to
massive numbers of compounds for screening, in preference
to smaller numbers of more carefully crafted, hypothesis-
driven candidate structures. However, there is much to be
learnt from natural products, and these warrant close and
continuing study.
THF, followed by removal of the silyl protecting group,
afforded 16 in 81% yield and with high diastereoselectivity
(d.r. > 25:1). Compound 16 was advanced in two steps to 17 as
shown.
With 11 and 17 in hand, the route to 5 was clear based on
chemistry first developed in our discovery phase.[16] The key
ring-closing metathesis reaction of 18 was carried out in
toluene in the presence of the second-generation Grubbs
catalyst (Scheme 3).[22–26] The reaction afforded the trans
Received: July 11, 2003 [Z52361]
Published Online: September 11, 2003
Please note: Minor changes have been made to this manuscript since
its publication in Angewandte Early View. The Editor.
Keywords: antitumor agents · conformation analysis ·
.
epothilones · natural products · structure–activity relationships
[1] G. Höfle, N. Bedorf, K. Gerth, H. Reichenbach (GBF), DE-B
4138042, 1993.
[2] G. Höfle, N. Bedorf, H. Steinmetz, D. Schumburg, K. Gerth, H.
Reichenbach, Angew. Chem. 1996, 108, 1671; Angew. Chem. Int.
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[3] K. Gerth, N. Bedorf, G. Höfle, H. Irschik, H. Reichenbach, J.
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[4] K. C. Nicolaou, A. RitzØn, K. Namoto, Chem. Commun. 2001,
17, 1523.
[5] K. H. Altmann, M. Wartmann, T. O'Reilly, Biochim. Biophys.
Acta 2000, 1470, M79.
Scheme 3. Final steps of the synthesis of 5. Reagents and conditions:
a) EDCI, DMAP, CH2Cl2, 11, 08C!RT, 86% based on 11; b) second-
generation Grubbs catalyst, toluene, 1108C, 20 min, 71%;
c) 1. KHMDS, 20, THF, ꢀ788C!ꢀ208C, 70%; 2. HF·pyridine, THF,
98%. EDCI=1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydro-
chloride, DMAP=4-(dimethylamino)pyridine.
[6] C. R. Harris, S. J. Danishefsky, J. Org. Chem. 1999, 64, 8434.
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[8] G. A. Orr, S. B. Horwitz, Drug Discovery Today 2001, 6, 1153.
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[10] T.-C. Chou, X. G. Zhang, C. R. Harris, S. D. Kuduk, A. Balog,
K. A. Savin, J. R. Bertino, S. J. Danishefsky, Proc. Natl. Acad.
Sci. USA 1998, 95, 9642.
isomer 19 exclusively in 71% yield. Installation of the thiazole
moiety, as shown in Scheme 3, was followed by removal of the
two silyl protecting groups with HF·pyridine to give 5.
Compound 5 was converted into 3 in high yield by reduction
of the C9–C10 double bond. Gram quantities of these
structurally novel epothilones have been prepared by total
synthesis in our laboratory.
At the moment there are no grounds on which to argue
that the strikingly superior performance of 5 arises from
factors other than the significantly improved pharmacokinetic
and bioavailability features that were designed into the
molecule through the medium of chemical synthesis. How-
ever, the possibility that the results reflect new (or enhanced)
drug–target interactions is the object of continuing study. New
areas of analogue synthesis are accessible through permuta-
tions of the late-stage olefination (e.g. 19 + 20) by the use of
other phosphonates. In this way, new design features that
result in enhanced pharmaceutical properties are being
explored further.
[11] For more information about clinical trials of dEpoB, visit:
www.kosan.com.
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Danishefsky, J. Am. Chem. Soc. 1999, 121, 7050.
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[15] T.-C. Chou, X. G. Zhang, C. R. Harris, S. D. Kuduk, A. Balog,
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[16] A. Rivkin, F. Yoshimura, A. E. Gabarda, T.-C. Chou, H. Dong,
W. P. Tong, S. J. Danishefsky, J. Am. Chem. Soc. 2003, 125, 2899.
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It is important to keep in mind that the ultimate purpose
of the chemotherapeutic arm of cancer research is to provide
clinically valuable treatment for patients with neoplastic
diseases. Only progression to clinical trials can establish the
value of any of the new epothilone agents with regard to this
central goal.
[21] R. Noyori, T. Ohkuma, M. Kitamura, H. Takaya, N. Sayo, H.
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