Anticancer Agents - American Chemical Society

Chapter 8

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Synthesis and Biological Activity of Epothilones Ulrich Klar, Werner Skuballa, Bernd Buchmann, Wolfgang Schwede, Thomas Bunte, Jens Hoffmann, and Rosemarie B. Lichtner Research Laboratories of ScheringAG,Müllerstrasse 170, D-13342 Berlin, Germany

The total synthesis and biological activity of epothilone analogs are described. Selected SAR data indicate the possibility to improve activity and selectivity by structural modifications. The new compounds may help to elucidate the therapeutic potential of this class of anticancer drugs.

Introduction

The discovery that the novel natural product class of epothilones parallels the biological activity of paclitaxel (PT) regarding its action on the tubulin system has stimulated intensive research activities in chemistry, pharmacology, and medicine. Epothilones, which have been isolated from myxobacterial strain sorangium cellulosum and characterized by the groups of Reichenbach and Hôfle (l),stabilize microtubules by inhibiting their depolymerization, analogous to PT (2). As a consequence, cell cycle is blocked in G2/M phase driving the cell into apoptosis.

© 2001 American Chemical Society In Anticancer Agents; Ojima, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

131

132


Epothilone Λ (R=H) Epothilone B (R=CH$

Epothilone C (R=H) Epothilone D (R^CH^

In contrast to PT, epothilones display their antiproliferative effects in so called multi-drug-resistant (MDR) cells in vitro and in vivo (3). This has been demonstrated for epothilone D (epo D) very impressively by the group of Danishefsky with a variety of xenograft models (4, 5, 6, 7). Epothilone Β (epo B), which is more potent than PT in vitro and in vivo displays significant toxicity at therapeutically relevant doses (7). Thus, an epothilone analog possessing an improved therapeutic window would be favorable.

Syntheses of Epothilone Β and D Analogs Due to the reduced structural complexity of epothilones compared to the taxoids, total syntheses offer the opportunity for extensive structural modifications most of which cannot be performed using a partial synthetic approach. Like other groups we made use of a highly convergent strategy using three modular building blocks designated as A, Β and C which represent ring carbons 1 to 6, 7 to 12, and 13 to 15 respectively (8, 9). c

Β 7 0H %

A As a prerequisite, each module should offer the potential for flexible structural modifications as well as a large scale production with high optical purity.

In Anticancer Agents; Ojima, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

133


Synthesis of Building Block A (Cl - C6) (-)-Pantolactone (1) was chosen as a readily available optically active starting material which possesses epothilone carbons 2 to 5 including the gerninal dimethyl moiety at C-4 (Scheme 1). The hydroxyl group was protected as a tetrahydropyranyl (THP) ether (2) and the lactone was reduced to the lactol. After elongation by carbon atom 1 in a Wittig reaction, the remaining primary hydroxyl group was protected as t-butyldiphenylsilyl ether (TBDPS) and the THP ether was cleaved to yield allylic alcohol 4. Olefin 4 was then subjected to a hydroboration-oxidation sequence giving 1,3-diol 5 along with minor amounts of stereochemically pure 1,2-diol 5a resulting from a Markovnikov hydration (10). The 1,3-diol 5 was then converted into acetonide 6. Removal of the silyl ether, Swern-oxidation and subsequent Grignard reaction followed by an oxidation of the resulting alcohol epimers gave ketones 8a to 8k. If desired, the acetonide in 8 can be converted into the bis-t-butyldimethylsilyl ether (e.g. 30). In most cases, a better stereoselectivity in the aldol reaction is observed using the acetonide (Scheme 4). This chiral pool synthesis of building block A is characterized by the facts that • no racemization occurs during synthesis; • all intermediates are chemically stable; • the sequence can be scaled up; • there is flexibility for synthetic modifications at C-5.

Synthesis of Building Block B (C7 - C12) Starting from 1,4-butynediol, triflate 10 was reacted with the Evans oxazolidinone 11 to yield the acyloxazolidinone 12 with high diastereoselectivity (Scheme 2). Transesterification followed by hydrogénation and reduction yielded the primary alcohol 14 in enantiomerically pure form. To introduce the methyl group at C-12, protecting group manipulation to 15 was followed by oxidation and subsequent reaction with methyllithium to afford key alcohol 16. By oxidation to ketone 18 afragmentsuitable for the connection to building block C was generated. In order to connect to building block A, alcohol 16 was converted to aldehyde 17 by silylation, cleavage of the tetrahydropyranyl ether and subsequent oxidation.

• • • •

This synthesis of building block Β is characterized by the features that all intermediates are chemically stable and in particular crystalline; the sequence can be scaled up; the chiral auxiliary can be recycled; there exists high flexibility for synthetic modifications (e.g. at C-8, C-10 to C-12). In Anticancer Agents; Ojima, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

134

Scheme 1 1) DIB A H , toluene, DHP, PPTS,

jHPQ

C H C 1 , rt, 98% 2

-70°C to -20°C, 94%

i t

^—Ο

2

2) Ph P=CH,, 3

T H F , 0°C to rt, 78%

1)TBDPSC1, imid., 1) B H j T H F , T H F , rt


D M F , rt, 85% 2) EtOH, PPTS,

2) H 0 / NaOH, 68%

>TBDPS

2

2

50°C, 95%

1 ) B u N F , THF, 4

Me C(OMe) , CSA, 2

50°C, 99%

2

>TBDPS

C H C 1 , rt, 89% 2

2

1) R-MgBr, E t 0 , rt, 83%

2) Swera-ox., crude

2

1

2) T P A P , N M O , 4A MS, CH C1 , 2

2

rt, 96%

8

R

χ

a-d

-[CH ] -CH

e,f

-[CH ] -Ph

g,h

-[CH ] -CH=CH,

l

2

2

X

1,2

X

2

1,2

X

'l

-[CH ] -C=CTMS

k

-[CH ] -CH=C(CH,)

:

2

0,1,2,3

3

2

X

X

2

'

3

1


135

Scheme 2

AjpN O

1) TBDMSC1, imid., D M F , 0°C to rt, 62%

HO

y

n

ο ο

TBDMSO

9

I Downloaded by COLUMBIA UNIV on September 12, 2012 | http://pubs.acs.org Publication Date: August 24, 2001 | doi: 10.1021/bk-2001-0796.ch008

OH

2) T f 0 , 2,6-lutidinc,

Tff)

2

toluene, -40°C

LiN(SiMe ) , THF, 3

2

-60°C to rt, 47% 10

1) H / P d - C , 2

EtOAc, rt, 96% 2) D I B A H , toluene, COOEt

-40°C,91%


136


Synthesis of Building Block C (C13 - C17) As described for building block A, the chiral pool was also used for preparation of building block C (Scheme 3). Starting with L(-)Malic acid, (2S)hydroxybutyrolactone 20 was prepared in 4 steps according to the literature (11). After protection of the hydroxy 1 group as t-butyldiphenylsilyl ether, addition of methyllithium at -70°C afforded 21 in equilibrium with its open chain isomer. Silylation of the primary hydroxyl function gave methyl ketone 22. The thiazole moiety was introduced by a Horner-Wittig reaction. Afterwards, the t-butyldimethylsilyl ether was removed under acidic conditions and the primary alcohol 24 converted to phosphonium salt 25 via the corresponding iodide. Scheme 3

N

1) B H T H F , THF, 0°C

HOOC

H00C13 H O O C

1) ( C F - C O ) , 0 , rt 3

^

'•

15 Ξ OH

1

?

MeOOC^)

2) MeOH, rt, 95%

1

2) Dowex, toluene, reflux, crude

OH

19

L(-)-MaIic acid 1) TBDPSC1, imid., DMF.rt, 49% from 1 W 2) M e L i , THF, -70°C, 96%

OH

Me

?0

TBDMSC1, imid.,

HO

D M F , rt, 72%

OTBDPS

21

20

!-OEt OEt

OTBDMS

XJ

BuLi, THF,

OTBDPS

-78°C to rt, 83%

22

OTBDMS

~SXJV

hoac

thf h

- ' -°--

rt, 92%

OTBDPS

23

OH

1) I , PPh , imid., 2

3

CH,Cl„rt,91% 2) PPh , EtN'Pr , 3

OTBDPS

2

85°C, 80%

24


137 The synthesis of building block C is characterized by the facts that • no racemization occurs during synthesis; • all intermediates are chemically stable; • the sequence can be scaled up; • there is flexibility for modifications in the side chain (C-16 and aryl).


Construction of the Framework

With all three fragments in hand two routes can be followed to complete the carbon skeleton as depicted in Figure 1. By this strategy the most valuable building block A or C may be introduced last.

A +B i=>

A-B

i=£> +C

most valuable fragment

0

OPG

+ A B +C c=>

B-C

i=>

PGO

OPGO

A-B-C P G : protecting group

Figure 1 Although this general sequence is well established in several laboratories, the total synthesis of compounds 35 to 37 in which the methylene groups at C-9 to C-l 1 are replaced by a phenyl ring is shown as an example (Scheme 4). The modified building block Β is synthesized in a straight-forward manner similar to the one described above. Wittig reaction between ketone 27 (12) and the ylide generated from salt 26 afforded the B-C-fragment 28 as a nearly 1:1 mixture of E/Z-isomers. The tetrahydropyranyl ether was cleaved and the alcohol oxidized to aldehyde 29 which was coupled with building block A (30) in an aldol reaction to yield 31 with high selectivity along with minor amounts of its diastereoisomer 32 which was removed by chromatography. The stereochemistry was assigned in analogy to the synthesis of the natural product epo B. The newly formed secondary alcohol was protected and the primary silyl ether selectively removed under mild acidic conditions to give 33 which was oxidized in two steps to the



138 corresponding carboxylic acid (34). At this stage the Z/E-isomers 34-Z and 34E were separated very easily by chromatography. The double bond configuration can be assigned undoubtedly by NOE experiments. Next, the allylic alcohol was liberated and the crude hydroxy acid was subjected to Yamaguchi cyclization conditions (13). The remaining protecting groups were removed to yield the modified epo D analog 35-Z. Epoxidation of the double bond afforded the α-epoxide 36-Z with high stereoselectivity along with minor amounts of stereoisomer 37-Z. Starting from acid 34-E the corresponding double bond isomer 35-E was obtained. In contrast to the Z-series, the epoxidation to 36-E and 37-E was less selective (structures not shown). Compared with their natural counterparts, the biological activity of analogs 35 to 37 was reduced. Scheme 4



Scheme 4. Continued.


140

Structure Activity Relationships Following the above strategy we have synthesized a wide range of analogs of epo Β and epo D, most of which cannot be generated by partial synthesis. In this section, some selected aspects regarding structure-activity-relationships will be discussed (14).


Structure and MDR-Sensitivity One major advantage of epothilones compared with other established anticancer drugs is their ability to overcome MDR. Nevertheless, a closer investigation reveals that this does not apply for all analogs. In the course of our drug-finding program we have synthesized compounds bearing N-aryl-oxides in the side chain.

.OH

Ar 1:

Ar2:

α

No

I C [nM] NCI/ 38 Ar MCF7 ADR a* Arl 0.6 6.0 b Arl 8.0 1.0 c Arl 3.8 40 d Ar2 0.8 0.8 e Ar2 2.0 2.0

MDR - Sensitivity

"OFF"^>

No

compound

5 0

R6 CH C

4

3

2 5 C H CH C H 2

5

3

2

4

R, R'

H

5

CH , CH CH , CH CH -CH 3

3

3

3

2

-α O"

2

CH3, CH3 CH3, CH3

ic

5

0

39

Ar

MCF7

a b c d e

Ar3 Ar3 Ar3 Ar4 Ar4

5.0 4.0 3.0 9.0 8.0

>: 38a = epo Β Figure 2


[nM] NCI/ ADR >100 >100 >100 >100 >100


141 As demonstrated in Figure 2, their activities against MCF-7 cells cover a range of 3 to 9 nM compared to 0.6 to 3.8 nM for the corresponding desoxy heterocycles. While the latter ones are also active against the MDR cell line NCI/ADR (0.8 to 8.0 nM) the N-oxides show no inhibition of cell proliferation up to 100 nM. Thus, the introduction of charges leads to a loss in sensitivity against MDR-positive tumor cells. Interestingly, even an increase in polarity e.g. by replacing the 12-methyl by a hydroxymethylene group or the exchange of the lactone moiety in epo D by a lactam (15) leads to this effect (data not shown) indicating that these analogs represent a better substrate for the efflux pump P-glycoprotein (P-gp). To address the hypothesis that overcoming P-gp efflux might also be responsible for enhanced toxicity observed for epo B, the following in vivo experiment was performed. Nude mice bearing a human colon carcinoma (LS 174T) were treated with equi-efficient doses of epo Β and its corresponding N oxide. As shown in Figure 3, similar toxicity was observed at therapeutically relevant doses for both compounds indicating that overcoming P-gp efflux may not be responsible for the narrow therapeutic window seen for epo Β in this experiment.

(Sensitive to MDR-cells) dose [mg/kg/i.p.] 0.07 0.14 0.28

T/C [%] 110 104 49

(Insensitive to MDR-cells) dose [mg/kg/i.p.] 0.4 0.8 1.6

Tox 0/8 0/8 3/8

T/C [%] 106 92 36

Tox 0/8 0/8 4/8

Figure 3

Improvement of Activity While epo Β possesses very impressive antiproliferative effects in vitro the potency can be further improved by simply replacing the methyl at carbon 6 by


142


an ethyl group. Compared to epo Β this analog displayed at least a 4-fold enhanced overall activity in a panel of tumor cell lines listed in Figure 4.

Figure 4

Bioactive Conformation of the Side Chain For a better understanding of published as well as our own SAR-data we were also interested in getting information about the bioactive conformation of the epothilone side chain. It was already known that removal or shortening of the side chain resulted in a dramatic loss in activity (16). To understand if the position of nitrogen in the heterocycle is important and if the bioactive conformation is more likely represented by conformer I or II, only three analogs needed to be synthesized (Figure 5):


143 •

•


•

In the first analog (40) the methylthiazole was replaced by 2-pyridyl. The in vitro activity of this compound is similar or even improved, compared to epo D. In the second analog (41) a 3-pyridyl moiety is incorporated as heterocycle leading to a significant decrease in activity indicating that the position of nitrogen is crucial. In the third analog (42) the double bond of the side chain is incorporated into a phenyl ring leading to a fixed conformation I. The high biological activity of this compound which is superior to epo D reveals that the bioactive conformation seems to be well described by conformer I.

Figure 5

Improvement in Activity and Profile In general, the benzothiazole, as an epothilone side chain equivalent, turned out to enhance activity in vitro. Compared to epo D the corresponding analog was 5-, 8- and 14-fold more active on A 431, MCF-7 and NCI/ADR cell lines respectively (Figure 6). Similar results were obtained with the epo Β analog 43 showing an 1-, 2- and 9-fold improvement. More interesting, the activity towards the MDR cell line was improved for both analogs. This is demonstrated by the defined selectivity ratio (IC -MCF7 : IC -NCI/ADR) which increases for the olefin compound from 0.46 to 0.78 and for the epoxide from 0.17 to 0.75, respectively. 50

50


144 IC50 [nM]

IC50 [nM] 1


ιοαοο,

Κ* » 0.46

42

looo

0

1

_

K* ' 0.17

'

Epo D

43

"Ratio" is defined as ICc -MCF7 n

:

0

Epo Β

IC< -NCI/ADR n

Figure 6

Radiolabeled Epothilone Β and D Analogs Data from the literature (7) as well as our own experience suggested that the olefin epo D possesses a broader therapeutic window compared to the epoxide epo Β in vivo, the latter being about 10-times more potent in vitro. To study possible differences (e.g. at the cellular level) between these compounds, we decided to synthesize some closely related analogs of epo D and epo B, which can be labelled at the very last step of the synthesis.

Synthesis and In Vitro Characterization The synthesis of our target compounds 44 and 46 followed the sequence fragment BC + fragment A as already described before. In these analogs the sterically less hindered mono substituted double bond was hydrogenated or tritiated (17) with high regioselectivity providing 45 and 47 (Scheme 5).


145


Scheme 5

Table 1 Compound 44 45 epo D 46 47 epo Β

MCF-7 [nM] 17 38 19 1.2 3.4 0.6

NCI/ADR [nM] 41 76 50 3.8 4.1 3.5


146 The in vitro characterization revealed that compound 44 was equipotent to epo D while 45 showed a slightly reduced activity in MCF-7 and NCI/ADR cells (Table 1). The epoxides 46 and 47 are equipotent to epo Β in the MDR cell line, while their activity was only slightly reduced in MCF-7 cells. Thus, compounds 45 and 47 represent appropriate tools for further studies.

Indications for a Different Mode of Action Between Epothilone Β and D The labelled analogs 45-T and 47-T were used to study their cellular distribution. As can be seen from Figure 7 (right panel) the total uptake of epoxide 47 is about 4-fold higher compared to olefin 45. Looking to the relative cellular distribution (left panel) nearly all of olefin 45 is detected in the protein fraction of the cytosol as is to be expected for a compound binding to tubulin. A qualitatively different picture is obtained for epoxide 47 which was also found in the cell nucleus. Studies are ongoing to figure out whether these results translate to the different activities/selectivities seen between epo D and epo B.


2

2

Figure 7

In conclusion, epothilones represent both structurally and mechanistically a truly novel entity among the tubulin active compounds. Analogs obtained by total syntheses may help to elucidate the therapeutic potential of this class of anti cancer drugs.


147

References 1 2


3 4

5

6

7 8 9 10 11 12 13 14 15 16

17

Gerth, Κ.; Bedorf, N.; Höfle, G.; Irschik, H.; Reichenbach, H. J. Antibiotics 1996, 49, 560-563. Bollag, D. M . ; McQueney, P. Α.; Zhu, J.; Hensens, O.; Koupal, L.; Liesch, J.; Goetz, M.; Lazarides, E.; Woods, C. M . Cancer Res. 1995, 55, 23252333. Altmann, K.-H.; Wartmann, M.; O'Reilly, T. Biochim. Biophys. Acta 2000, 1470, M79-M91. Chou, T.-C.; Zhang, X.-G.; Balog, Α.; Su, D.-S.; Meng, D.; Savin, Κ.; Bertino, J. R.; Danishefsky, S. J. Proc. Natl. Acad. Sci. 1998, 95, 96429647. Chou, T.-C.; Zhang, X.-G.; Harris, C. R.; Kuduk, S. D; Balog, Α.; Savin, Κ. Α.; Bertino, J. R.; Danishefsky, S. J. Proc. Natl. Acad. Sci. 1998, 95, 15798-15802. Sepp-Lorenzino, L.; Balog, Α.; Su, D.-S.; Meng, D.; Timaul, N.; Scher, H. I.; Danishefsky, S. J.; Rosen, N. Prostate Cancer Prostatic Dis. 1999, 2, 4152. Harris, C. R.; Danishefsky, S. J. J. Org. Chem. 1999, 64, 8434-8456. Mulzer, J. MonatsheftefürChemie 2000, 131, 205-238. Nicolaou, K. C.; Roschangar, F.; Vourloumis, D. Angew. Chem. Int. Ed. Engl. 1998, 37, 2014-2045. A similar observation has been recently described for 3,4-dialkoxy-1alkenes: Jung, M . E.; Karama, U. Tetrahedron Lett. 1999, 40, 7907-7910. Wünsch, B.; Dieckmann, H.; Höfner, G. Liebigs Ann. Chem. 1993, 12731278. Ketone 27 was synthesized using a similar methodology which is described in detail in WO 00/49020. Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M . Bull. Chem. Soc. Jpn. 1979, 52, 1989-1993. In vitro cell proliferation assays were performed according to: Kueng, W.; Silber E.; Eppenberger U. Analyt. Biochem. 1989, 182, 6-19. Stachel, S. J.; Chappell, M . D.; Lee, C. B.; Danishefsky, S. J. Org. Lett. 2000, 2, 1637-1639. Su, D.-S.; Balog, Α.; Meng, D.; Bertinato, P.; Danishefsky, S. J.; Zheng, Y.-H.; Chou, T.-C.; He, L.; Horwitz, S. B. Angew. Chem. Int. Ed. Engl. 1997, 36, 2093-2096. We thank Dr. Jürgen Gay, Research Laboratories of Schering AG, for the tritiation experiments.


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