Design, synthesis, and biological properties of highly potent epothilone B analogues

Article abstract of DOI:10.1002/anie.200351819

A number of rationally designed epoxide and cyclopropane epothilone B (1) analogues with substituted side chains were prepared and their biological activities were evaluated against a series of human cancer cell lines. The cyclopropane analogue 2 with a m

Full text of DOI:10.1002/anie.200351819

Angewandte
Chemie
Epothilone Analogues
Design, Synthesis, and Biological Properties of
Highly Potent Epothilone B Analogues**
K. C. Nicolaou,* Pradip K. Sasmal, Gerasimos Rassias,
Mali Venkat Reddy, Karl-Heinz Altmann,
Markus Wartmann, Aurora O'Brate, and
Paraskevi Giannakakou
Owing to their potent cytotoxicity against tumor cells,
including taxol (paclitaxel)-resistant cell lines, the epothilones
(for example, epothilone A (1) and epothilone B (2))[1]
continue to be the focus of intense chemical, biological, and
clinical research efforts around the world.[2,3] Following the
findings that cyclopropane-,[4] methylsulfanylthiazole-,[2d,5]
and pyridine-[6]containing epothilone B derivatives (e.g. 3[5a]
and 5,[6]) exhibit outstanding biological profiles as potential
antitumor agents, we directed our attention toward the
synthesis and evaluation of a small designed library of
epothilone B analogues whose members are characterized
by such structural motifs.Herein we report the details of these
synthetic and biological investigations, which culminated in
the discovery of 12,13-cis-cyclopropane methylsulfanyl epo-
thilone B (4) as an extremely potent epothilone B analogue.
The design of the present focused epothilone library was
based on the current knowledge of structure–activity relation-
ships (SAR), specifically the facts that: 1) epothilone B (2) is
considerably more potent than epothilone A (1), 2) a meth-
ylsulfanyl replacement for the methyl group on the thiazole
moiety enhances the potency,[2d,5] 3) a heterocycle (e.g.
pyridine)[6] replacement for the thiazole ring needs to
maintain the proper position (adjacent to the point of
[*] Prof. Dr. K. C. Nicolaou, Dr. P. K. Sasmal, Dr. G. Rassias,
Dr. M. V. Reddy
Department of Chemistry and
The Skaggs Institute for Chemical Biology
The Scripps Research Institute
10550 NorthTorrey Pines Road, La Jolla, CA 92037 (USA)
Fax: (+1)858-784-2469
E-mail: kcn@scripps.edu
and
Department of Chemistry and Biochemistry
University of California, San Diego
9500 Gilman Drive, La Jolla, California 92093 (USA)
Dr. K.-H. Altmann
Corporate Research, Novartis Pharma AG
4002 Basel (Switzerland)
Dr. M. Wartmann
Oncology Business Unit, Novartis Pharma AG
4002 Basel (Switzerland)
A. O'Brate, Dr. P. Giannakakou
Winship Cancer Institute, Emory University School of Medicine
Atlanta, GA 30322 (USA)
[**] We thank Dr. D. H. Huang and Dr. G. Siuzdak for NMR spectro-
scopic and mass spectrometric assistance, respectively. Financial
support for this work was provided by The Skaggs Institute for
Chemical Biology, the National Institutes of Health (USA), and
Novartis Pharma.
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DOI: 10.1002/anie.200351819
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Communications
attachment to the mainframe) for the nitrogen for biological
ambient temperature, leading directly to the desired epothi-
lones 7–16 in the indicated yields.The required aromatic
stannanes were prepared as summarized in Scheme 2.Thus,
for the thiazole compounds 20a–d, the commercially avail-
able 2,4-dibromothiazole (18) was treated with the corre-
sponding thiol in the presence of NaH, leading first to the
intermediate sulfides 19a–d through replacement of the more
reactive 2-bromide substituent.Subsequent coupling of these
substrates with Me3SnSnMe3 in the presence of [Pd(PPh3)4] in
toluene at 1008C then gave the desired products 20a–d by
reaction of the second bromide residue.The pyridyl stannanes
22a–d were similarly synthesized from the
activity, and 4) a cyclopropane ring can replace the epoxide
moiety without loss of activity.[4] From these considerations,
epothilones 4, 6, and 7–16 were considered as prime
candidates for chemical synthesis and biological evaluation.
The designed epothilone analogues 7–16 were synthesized
in a convergent manner from vinyl iodide 17[7] and the
corresponding aromatic stannanes as shown in Scheme 1.A
Stille-type coupling of 17 with the appropriate stannanes
20a–d, 22a–d, 23, and 24 (Scheme 2) was carried out in the
presence of [PdCl2(MeCN)2], CuI, and AsPh3 in DMF at
readily available 2-bromopyridines 21a,b,[8]
c,[5a] and d,[9] respectively, through a metal–
halogen exchange (nBuLi) followed by
quenching of the resulting 2-lithio deriva-
tives[10] with nBu3SnCl.Stannanes 23[11] and
24[12] were prepared from the respective hal-
ides according to the literature procedures.
The synthesis of cyclopropane epothilones
4 and 6 required the key aldehyde 39, which
was constructed from nerol (25) as shown in
Scheme 1. Synthesis of 7–16. Reagents and conditions: a) [PdCl2(MeCN)2] (0.5 equiv), CuI
(2.0 equiv), AsPh3 (1.0 equiv), 20a–d, 22a–d, 23–24 (2.5 equiv), DMF, 258C, 1–3 h, 41–80%.
DMF=N,N-dimethylformamide.
Scheme 3.Thus, Charrette asymmetric cyclo-
propanation[4a,5a,13] of 25 in the presence of
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ligand 26 furnished cyclopropane alcohol 27 in 80%
yield and 95% ee.The hydroxy group in 27 was protected
as a benzyl ether (NaH, BnBr, 100%) and the resulting
product was subjected to ozonolysis (O3, then NaBH4),
leading to primary alcohol 28 (83% yield).This alcohol
was converted into the corresponding iodide 29 through
mesylation (MsCl, Et3N) and subsequent displacement
of the intermediate mesylate with NaI (91% overall
yield).Alkylation of ( ꢀ)-SAMP hydrazone 30 with
iodide 29 according to the procedure of Enders[14]
under the influence of LDA proceeded smoothly to
afford hydrazone 31 (87% yield), whose cleavage (MeI,
then aqueous HCl) led to aldehyde 32 (91% yield).The
crucial aldol reaction between ketone 33[15] and aldehyde
32 in the presence of LDA proceeded smoothly and
stereoselectively in THF/Et2O (1:1) at ꢀ788C to afford
the desired hydroxy ketone 34 in 80% yield (d.r. > 14:1).
Protection of the secondary alcohol in 34 as a silyl ether
(TBSOTf, 2,6-lutidine) followed by selective removal of
the primary TBS group (HF·py) furnished primary
Scheme 2. Preparation of 20a–d and 22a–d. Reagents and conditions:
a) NaH (3.0 equiv), RSH (3.0 equiv), iPrOH, 258C, 24 h, 70–81%;
b) (Me3Sn)2 (5.0–10.0 equiv), [Pd(PPh3)4] (5 mol%), toluene, 1008C, 1–3 h,
71–88%; c) nBuLi (1.1 equiv), diethyl ether, ꢀ788C, 1 h; then nBu3SnCl
(1.2 equiv), ꢀ78!258C, 1 h, 49–62%.
Scheme 3. Construction of 39. Reagents and conditions: a) 80% yield, 95% ee;[4a,5a,13] b) NaH (1.5 equiv), BnBr (1.2 equiv), DMF, 0!258C, 12 h,
100%; c) O3, CH2Cl2/MeOH (4:1), ꢀ788C; then NaBH4 (3.0 equiv), ꢀ78!258C, 1 h, 83%; d) MsCl (1.3 equiv), Et3N (1.5 equiv), CH2Cl2, 258C,
1 h; e) NaI (3.0 equiv), acetone, 25 8C, 12 h(91% for two steps); f) LDA (1.4 equiv), 30 (1.3 equiv), THF, 08C, 6 h; then 29, ꢀ98!ꢀ108C, 14 h,
87%; g) MeI, reflux, 3 h; h) HCl (3 n)/pentane (1:1), 258C, 3 h(91% for two steps); i) LDA (2.4 equiv), 33 (2.3 equiv), THF/Et2O (1:1), ꢀ788C,
1 h; then ꢀ408C, 0.5 h; then 32 at ꢀ788C, 5 min, 80%; j) TBSOTf (1.5 equiv), 2,6-lutidine (2.0 equiv), CH2Cl2, ꢀ208C, 1 h; k) HF·py
(1.8 mLmmolꢀ1), pyridine/THF (1:2), 08C, 8 h(86% for two steps); l) (COCl) 2 (1.2 equiv), DMSO (2.0 equiv), CH2Cl2, ꢀ788C, 5 min; then 35
(1.0 equiv), 20 min; then Et3N (3.0 equiv), ꢀ78!08C; m) NaClO2 (5.0 equiv), NaH2PO4 (3.0 equiv), 2-methyl-2-butene (75 equiv), tBuOH/THF/
H2O (4:2:1), 258C, 1 h; n) TMSE-OH (4.0 equiv), EDC (1.5 equiv), DMAP (0.1 equiv), DMF, 25 8C, 12 h(73% for three steps); o) Pd(OH) 2/C
(10 wt%; 10%), H2, EtOH/EtOAc (1:1), 258C, 2 h, 89%; p) (COCl)2 (1.2 equiv), DMSO (2.0 equiv), CH2Cl2, ꢀ788C, 5 min; then 37 (1.0 equiv),
20 min; then Et3N (3.0 equiv), ꢀ78!08C, 99%; q) MeOCH2PPh3Cl (3.0 equiv), nBuLi (2.8 equiv), THF, 08C, 1 h; then 38, ꢀ78!08C, 2 h, 79%;
r) PPTS (10.0 equiv), dioxane/water (9:1), 708C, 12 h, 81%. Bn=benzyl, Ms=methanesulfonyl, LDA=lithium diisopropyl amide, TBS=tert-butyl-
dimethylsilyl, Tf=trifluoromethanesulfonyl, py=pyridine, DMSO=dimethyl sulfoxide, TMSE=2-(trimethylsilyl)ethyl, EDC=1-(3-dimethylamino-
propyl)-3-ethylcarbodiimide, DMAP=4-dimethylaminopyridine, PPTS=pyridinium para-toluene sulfonate.
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Table 1: Selected data for 4.
alcohol 35 (86% overall yield).The latter compound was then
4: Rf =0.19 (silica gel, EtOAc/hexanes 3:7); [a]2D0 =ꢀ19.3 (c=0.14,
CH2Cl2); IR (film): n˜max =3484 br, 2932, 1729, 1459, 1375, 1249, 1043,
oxidized, and the resulting carboxylic acid was protected as a
TMSE ester to afford 36 in 73% overall yield.Hydrogenolysis
of the benzyl group within 36 (H2, 10% Pd(OH)2/C) led to
alcohol 37 (89% yield).Swern oxidation of 37 led to the
corresponding aldehyde 38 (99% yield).Homologation of 38
through Wittig olefination (MeOCH2PPh3Cl, nBuLi, 79%
yield) followed by acid hydrolysis (PPTS, 81% yield) of the
resulting enol ether led to the targeted aldehyde 39.
1
982, 733 cmꢀ1; H NMR (400 MHz, CDCl3): d=6.97 (s, 1H), 6.47 (s,
1H), 5.25 (dd, J=7.1, 5.7 Hz, 1H), 4.04 (dd, J=8.1, 3.0 Hz, 1H), 3.91
(dd, J=4.1, 4.1 Hz, 1H), 3.23 (m, 1H), 2.69 (s, 3H), 2.52 (dd, J=14.9,
8.4 Hz, 1H), 2.46 (dd, J=14.9, 2.6 Hz, 1H), 2.11 (s, 3H), 2.04 (dd,
J=14.5, 4.0 Hz, 1H), 1.72–1.66 (m, 1H), 1.62–1.44 (m, 4H), 1.36 (s,
3H), 1.35–1.22 (m, 2H), 1.17 (d, J=7.5 Hz, 3H), 1.16 (s, 3H), 1.15–1.04
(m, 1H), 0.99 (d, J=7.0 Hz, 3H), 0.97 (s, 3H), 0.48 (m, 1H), 0.40 (dd,
J=8.8, 3.9 Hz, 1H), ꢀ0.11 ppm (br t, J=4.6 Hz, 1H); 13C NMR
(100 MHz, CDCl3): d=221.5, 171.1, 165.7, 152.9, 138.6, 120.1, 116.2,
82.0, 73.8, 73.2, 52.0, 42.9, 39.4, 36.5, 35.0, 33.2, 31.6, 24.6, 23.5, 22.54,
22.49, 21.1, 20.8, 19.4, 17.4, 16.8, 15.0, 13.2 ppm; FTMS (MALDI): m/z
calcd for C28H44NO5S2: 538.2655, found: 538.2632 [MH+]
Following on our previously developed strategy[4a] toward
epothilone analogues, we subjected aldehyde 39 to a Nozaki–
Hiyama–Kishi coupling[16] reaction with vinyl iodides 40a[4a]
and 40b[5a] followed by treatment with TBAF to afford the
corresponding secondary alcohols 41 and 43 as mixtures (ca.
1:1) of the two epimers (at C15) in an unoptimized combined
yield of 42–45% (Scheme 4).These mixtures were then
cyclized under Yamaguchi conditions[17] (2,4,6-trichloroben-
zoyl chloride, Et3N, DMAP, toluene, 0!758C) to afford the
desired 15S 16-membered lactones 42 (33% yield) and 44
(32% yield) together with their 15R epimers (ca.1:1 ratio,
chromatographically separated, silica gel).[18] Finally the TBS
groups were removed from 42 and 44 by the action of TFA,
leading to epothilones 6 (48% yield) and 4 (Table 1, 71%
yield) (unoptimized yields) as shown in Scheme 4.
The biological activities of the synthesized epothilones
were evaluated through cell-growth-inhibition assays (cyto-
toxicity assays).Cytotoxicity was first evaluated in a set of
ovarian carcinoma cell lines, including a parental cell line
(lA9) and three drug-resistant cell lines, namely the taxol-
resistant cell lines lA9/PTX10 and lA9/PTX22[19] and the
epothilone-resistant cell line 1A9/A8.[20] These resistant cell
lines harbor distinct acquired b-tubulin mutations that affect
drug–tubulin interaction and result in impaired taxane and
epothilone-driven tubulin polymerization.The results of
these biological investigations are summarized in Table 2.
Further cytotoxicity studies were carried out on a set of
human epidermoid cancer cell lines, including a parent cell
line (KB-31) and a taxol-resistant (due to Pgp overexpres-
sion) cell line (KB-8511).The results of these studies are
summarized in Table 3.
There is a general agreement in the relative potency of the
substituted epothilone B analogues against the 1A9 human
ovarian and the KB-31 human epidermoid cancer cells.
Collectively, the results of these cytotoxicity assays revealed
interesting information in terms of structure–activity relation-
ships within the epothilone family.First, compounds 4 and 6,
in which the C12–C13 epoxide moiety is replaced by a
cyclopropane ring, are the two most potent compounds of all
the epothilone B analogues presented herein.This result
reaffirms that the C12–C13 epoxide moiety is not necessary
for biological activity, as previously noted.[4] Compound 4 is
six times more active than the parent
epothilone B (2) against the 1A9 human
ovarian carcinoma cells (Table 2), which
further confirms that the replacement of
the methyl group on the thiazole side
chain with a methylsulfanyl group leads to
increased activity.This result is in agree-
ment with our previous data on a similar
substitution in epothilone B without
replacement of the C12–C13 epoxide
(i.e. compound 3).[5a] The latter compound
(3) was about twice as active as the parent
epothilone B, whereas compound 4 is
sixfold more potent than epothilone B.
This result makes compound 4 the most
active epothilone B analogue against the
1A9 cell line synthesized to date and
suggests that replacement of the epoxide
by a cyclopropane moiety together with
the replacement of the methyl substituent
Scheme 4. Synthesis of 4 and 6. Reagents and conditions: a) CrCl2 (10.0 equiv), NiCl2 (0.2 equiv),
4-tBuPy (30 equiv), 40a or 40b (3.0 equiv), DMSO, 258C, 24 h; b) TBAF (2.0 equiv), THF, 25 8C, 2 h,
42% yield for two steps (41) or 45% yield for two steps (43); c) Et3N (6.0 equiv), 2,4,6-trichlorobenzoyl
chloride (2.4 equiv), 41 or 43, THF, 08C, 1 h; then DMAP (2.2 equiv), toluene, 75 8C, 3 h, 33% (42) or
32% (44); d) TFA/CH2Cl2 (20% v/v), 258C, 3 h, 48% (6) or 71% (4). TBAF=tetra-n-butylammonium
fluoride, TFA=trifloroacetic acid.
on the thiazole moiety with a methylsul-
fanyl group act synergistically, leading to
the observed enhancement of biological
activity.Interestingly, substitution of the
methyl group of the thiazole ring with
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Table 2: Cytotoxicity of epothilones 4, 6, and 8–16 against 1A9 human carcinoma cells and b-tubulin
mutant cell lines selected withtaxol or epothilone A.
which a pyrimidine side chain with
[a]
a methylsulfanyl substituent has
replaced the thiazole side chain of
the parent compound.
Compound
Cell Line
IC50
1A9
IC50
A8
PTX10
PTX22
IC50
RR
3.3
38.7
10.7
7.6
23.5
41.4
5.3
RR
IC50
RR
Varying degrees of cross-resist-
ance are obtained with the substi-
tuted epothilone B analogues
against the taxol- and epothilone-
resistant human ovarian carcinoma
sublines (Table 2) ranging from 3-
to 41-fold.These results suggest
that the location of the tubulin
mutations in these lines affects
differentially the binding of each
of the analogues to tubulin.More-
over, and in agreement with the
original observations with the nat-
urally occurring epothilones A and
B, none of the epothilone B ana-
logues tested herein appears to be
a good substrate for the drug-efflux
pump P-glycoprotein (Pgp).This is
evident by the lack of cross-resist-
ance of each of these analogues to
the Pgp-expressing cell line KB-
8511 (Table 3).In contrast, we have
taxol
EpoA
EpoB
3[b]
4
6
8
9
10
11
12
13
14
15
16
3.0ꢁ0.4
10.1ꢁ2.9
91.0ꢁ10.0
6.5ꢁ0.9
89.7ꢁ9.0
34.2ꢁ2.0
3.1ꢁ0.5
29.5
14.5
5.2
1.5
6.5
13.2
4.6
5.6
4.7
5.8
5.1
10.5
3.7
15.1
5.0
53.4ꢁ26.5
8.7ꢁ2.2
0.8ꢁ0.5
0.25ꢁ0.17
0.6ꢁ0.5
1.3ꢁ1.1
3.8ꢁ0.3
5.2ꢁ0.8
2.9ꢁ1.3
0.3ꢁ0.1
3.2ꢁ0.3
2.1ꢁ1.9
6.6ꢁ2.6
9.6ꢁ1.3
9.6ꢁ1.0
17.6
3.7
1.3
1.5
5.9
5.3
1.1
1.2
1.4
0.5
1.0
5.8
2.0
2.2
1.1
2.4ꢁ0.6
0.6ꢁ0.3
0.17ꢁ0.8
0.1ꢁ0.0
0.3ꢁ0.1
3.5ꢁ0.7
4.4ꢁ2.4
2.1ꢁ0.8
0.7ꢁ0.2
3.2ꢁ0.1
0.4ꢁ0.1
3.3ꢁ0.2
4.3ꢁ0.4
8.6ꢁ1.2
1.3ꢁ0.65
2.4ꢁ1.1
0.26ꢁ0.11
0.7ꢁ0.3
10.4ꢁ2.4
18.4ꢁ1.4
42.9ꢁ5.1
16.0ꢁ5.5
11.1ꢁ1.0
31.9ꢁ3.1
11.6ꢁ6.7
27.7ꢁ3.2
83.0ꢁ2.0
32.3ꢁ2.7
3.3ꢁ1.2
16.1ꢁ2.1
24.7ꢁ4.9
9.8ꢁ1.4
9.7
7.6
16.6
10.0
31.7
8.3
19.2
3.8
3.9ꢁ0.4
16.1ꢁ4.1
3.9ꢁ1.1
12.2ꢁ7.4
65.3ꢁ11.9
42.9ꢁ10.3
[a] The antiproliferative effects of the tested compounds against the parental 1A9 and the taxol- and
epothilone-selected drug-resistant clones (PTX10, PTX22, and A8, respectively) were assessed in a 72 h
growth-inhibition assay using the SRB (sulforhodamine-B) assay.[22] IC50 values for eachcompound are
given in nm and represent the mean of three independent experiments ꢁstandard error of the mean.
Relative resistance (RR) is calculated as an IC50 value for each resistant subline divided by that for the
parental cell line (1A9). [b] Results taken from ref. [5a].
previously shown that taxol—a known Pgp substrate—was
214-fold less active against KB-8511 cells than against their
parent counterpart, non-Pgp-expressing KB-31 cells.[5a]
In conclusion, we have constructed a number of rationally
designed epoxide and cyclopropane epothilone B analogues
with substituted side chains and evaluated their biological
activities against a series of human cancer cell lines.Among
the several bioactive analogues, the novel cyclopropyl epo-
thilone B analogue 4 with a methylsulfanyl thiazole ring
stands out as the most potent.This compound is six times
more active than the naturally occurring epothilone B (2) and
appears to be, together with its oxygen counterpart 3, the
most potent epothilone B analogue synthesized to date.Our
previous structure–activity relationship studies,[2c,4b,5a]
together with the data presented herein, reconfirm that the
epoxide oxygen atom is not required for biological activity
within this class of small molecules and that the lipophilic
methylsulfanyl group on the thiazole moiety considerably
enhances the potency of these compounds.As epothilone 4
lacks the relatively reactive epoxide function of 2 and 3, it
may prove to be advantageous over the latter compounds
with regard to stability and side effects and, therefore, may
present a unique opportunity for clinical development.[23]
Table 3: Cytotoxicity (IC50) of selected epothilones against the human
epidermoid cell lines KB-31 and KB-8511.[a]
Compound
KB-31
KB-8511
EpoB[b]
3[b]
4
6
8
0.19
0.11
0.20
0.44
3.04
10.0
1.16
0.72
0.54
4.87
8.38
9.01
0.12
0.07
0.12
0.29
2.67
6.73
1.28
0.55
0.41
3.24
7.37
11.65
9
10
11
13
14
15
16
[a] The antiproliferative effects of the tested compounds were assessed
in two human epidermoid cancer cell lines, including a parent cell line
(KB-31) and a taxol-resistant (due to Pgp-overexpression) cell line (KB-
8511). IC50 values are given in nm. [b] Results taken from ref. [5a].
larger moieties (compounds 7–10)[21] led to diminished bio-
logical activity relative to epothilone B (Tables 2 and 3).
Among the epothilone B analogues with a side chain at C5
of the pyridine substituent, 11–13 and 15, the methylsulfanyl
analogue 13 is the most potent, followed by the bromo-
substituted derivative 11 and the chloro-substituted system
12.When the methylsulfanyl group is relocated from C5 of the
pyridine ring (13) to C6 (14), loss of activity occurs as the IC50
value drops from 0.4 nm (for 13) to 3. 3 nm (for 14) (Table 2).
Furthermore, replacement of the methylsulfanyl group at the
C5 of the pyridine ring (13) with a trifloromethyl group (15)
results in a tenfold loss of activity.Finally, the least active of
the synthesized epothilone B analogues is compound 16 in
Received: May 6, 2003 [Z51819]
Keywords: antitumor agents · epothilones · structure–activity
.
relationships · sulfur · total synthesis
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