Design, synthesis and cytotoxic activities of naphthyl analogues of combretastatin A-4

Article abstract of DOI:10.1016/S0960-894X(00)00506-0

The 3,4,5-trimethoxyphenyl and 3-hydroxy-4-methoxyphenyl rings of combretastatin A-4 are deemed optimal for its activity as antimitotic agent. The replacement of either one by a naphthalene ring results in compounds with a potency comparable to that of the parent compound. These results show that the naphthalene ring is a good surrogate for the 3,4,5-trimethoxyphenyl or the 3-hydroxy-4-methoxyphenyl rings of combretastatin A-4 and that neither of them is essential for the antitumor activity. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0960-894X(00)00506-0

Bioorganic & Medicinal Chemistry Letters 10 (2000) 2549±2551
Design, Synthesis and Cytotoxic Activities of Naphthyl
Analogues of Combretastatin A-4
Ana B. S. Maya,a Benedicto del Rey,a Rafael Pelaez Lamamie de Clairac,a
Esther Caballero,a Isabel Barasoain,b Jose Manuel Andreub and Manuel Medardea,*
a
Â
Laboratorio de Quõmica Organica y Farmaceutica, Facultad de Farmacia, Universidad de Salamanca, Campus Miguel de Unamuno,
Â
Â
37007-Salamanca, Spain
Â
Centro de Investigaciones Biologicas, CSIC, Velazquez 144, 28006 Madrid, Spain
b
Received 18 July 2000; revised 1 September 2000; accepted 6 September 2000
AbstractÐThe 3,4,5-trimethoxyphenyl and 3-hydroxy-4-methoxyphenyl rings of combretastatin A-4 are deemed optimal for its
activity as antimitotic agent. The replacement of either one by a naphthalene ring results in compounds with a potency comparable
to that of the parent compound. These results show that the naphthalene ring is a good surrogate for the 3,4,5-trimethoxyphenyl or
the 3-hydroxy-4-methoxyphenyl rings of combretastatin A-4 and that neither of them is essential for the antitumor activity. # 2000
Elsevier Science Ltd. All rights reserved.
The microtubule system of eukaryotic cells is an
appealing target for the design of potential anticancer
agents. A variety of compounds, including colchicin-
oids, lignans and combretastatins inhibit tubulin poly-
merization by binding to a common site (Fig. 1).
Combretastatin A-4 is reported to be one of the most
potent antimitotic agents so far described and to
strongly inhibit the polymerization of tubulin.1
that the trends in potency for the substituents on the
aromatic rings are maintained when going from the less
potent dihydrocombretastatins to the cis-stilbenes.6 In
this paper we describe the synthesis and some pre-
liminary biological data for some new compounds in
this family displaying a high potency.
The naphthyl combretastatins were synthesized follow-
ing the classical Wittig methodology. The 3,4,5-tri-
methoxybenzaldehyde and 2-naphthalenecarbaldehyde
were converted in good overall yields into the corre-
sponding phosphonium salts by sequential reduction
with NaBH4, substitution of the resulting benzylic alco-
hol with PBr3 and subsequent displacement of the bro-
mide with PPh3. The corresponding ylides obtained by
treatment with nBuLi were reacted with the required
aldehydes (the phenolic hydroxyls protected when
necessary as the ethoxymethyl ethers) to yield roughly
1:1 mixtures of the E and Z isomers, which were sepa-
rated by chromatography. The stereochemistries of the
isomers were unambiguously established by means of
NOE and the value of the J coupling between the ole-
phinic protons when available. These data are in agree-
ment with the mutual shielding observed in the 1H
NMR spectra for several aromatic hydrogens on both
rings when the stereochemistry holds them in a spatial
disposition required for such an e€ect to be seen
(Scheme 1).7
The structure±activity analysis of the combretastatins
has shown that the optimal bridge between the two
phenyl rings is a cis ethylene. The 3,4,5-trimethoxy-
phenyl and the 3-hydroxy-4-methoxyphenyl rings or
others structurally close are present in almost all of the
more potent combretastatin analogues and have thus
been considered as responsible for the activity.2 The
rationale for the substitution of the polyoxygenated
rings by the nonpolar naphthalene is a previous QSAR
for such compounds, which indicated a good correlation
between the cytotoxic activity and the hydrophobicity
values of the functional groups on ring B of com-
bretastatins,3 and our previous experience: compounds
with the naphthalene substitution and either a dihydro-
combretastatin skeleton or conformationally restricted
analogues bearing an indole bridge (Fig. 2) display
activities close to those of the traditional substitutions
of combretastatins.4,5 The SARs have also indicated
The cytotoxic activities against four di€erent tumor cell
lines were measured as previously described8 and are
*Corresponding author. Tel.: +34-923-294528; fax: +34-923-294515;
e-mail: medarde@gugu.usal.es
0960-894X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00506-0

2550
A. B. S. Maya et al. / Bioorg. Med. Chem. Lett. 10 (2000) 2549±2551
summarized in Table 1. The cis isomers are more potent
than the corresponding trans compounds, as usually
observed for the combretastatins. The activities of the
dihydro-compounds, previously obtained and taken as
models for the synthesis of these more potent analogues,
are close to or higher than those displayed by the trans
isomers and two orders of magnitude lower than those
of the cis analogues. Such an observation is in agree-
ment with previous results on the activity of similar
series of compounds and with the lack of `pre-organi-
zation' of the hydrogenated products, when compared
to the cis-stilbenes, and further supports our initial
hypothesis that the trends of the activities reported for
the di€erent substituents on both series of compounds
are parallel. The indole-bridged compound 3, despite its
conformational rigidity did not improve the potency,
suggesting that the combination of three extended aro-
matic rings might be too bulky in order to achieve a
strong interaction with its target.
The naphthyl-combretastatins 4a and 5a display a
cytotoxic potency similar to each other and to that of
Figure 1. Colchicine-site ligands.
Figure 2. Described naphthyl analogues.
Scheme 1. (i) NaBH4/MeOH (>95%); (ii) PBr3/Et2O (65±85%); (iii) PPh3/toluene (>95%); (iv) nBuLi, 78 ꢀC and then 2-naphthalenecarbalde-
hyde (>90%); (v) nBuLi, 78 ꢀC and then 3-ethoxymethoxy-4-methoxybenzaldehyde (90%); (vi) HCl/MeOH (70%).

A. B. S. Maya et al. / Bioorg. Med. Chem. Lett. 10 (2000) 2549±2551
Table 1. Cytotoxic potencies of compounds synthesized expressed as
the log(IC50) M
2551
Acknowledgements
Financial support came from Spanish DGICYT
(SAF98-0103). R.P.L.C. thanks the Spanish Ministerio
de Educacion y Ciencia (MEC) for his `contrato de
reincorporacion'. Biological evaluation was supplied by
Instituto Biomar S.L. (Leon, Spain).
Compound
P388
A-549
HT29
MEL-28
Combretastatin A-4
8.5
5.3
6.5
5.9
7.8
6.4
7.4
5.4
5.4
8.5
5.3
6.5
5.9
7.8
6.4
7.4
5.4
5.4
7.5
5.0
6.5
5.9
7.8
6.4
7.4
5.4
5.4
8.5
5.3
6.5
5.9
7.8
6.4
7.4
5.4
5.4
14
24
35
4a
4b
5a
5b
6a
References and Notes
1. Shi, Q.; Chen, K.; Morris-Natschke, S. L.; Lee, K. H. Curr.
Pharm. Design 1998, 4, 219.
2. Pettit, G. R.; Singh, S. B.; Boyd, M. R.; Hamel, E.; Pettit, R.
K.; Schmidt, J. M.; Hogan, F. J. Med. Chem. 1995, 38, 1666.
3. Nandy, P.; Banerjee, S.; Gao, H.; Hui, M. B. V.; Lien, E. J.
Pharm. Res. 1991, 8, 776.
4. Medarde, M.; Pelaez Lamamie de Clairac, R.; Ramos, A.
C.; Caballero, E.; Lopez, J. L.; Gravalos, M. D.; San Feli-
ciano, A. Bioorg. Med. Chem. Lett. 1995, 5, 229.
combretastatin A-4. This result implies that the naph-
thalene ring is a good surrogate for either the tri-
methoxyphenyl or the 3-hydroxy-4-methoxyphenyl
rings of combretastatin A-4 and that neither one is
required for the activity. The capability of the naphtha-
lene ring for substituting for either ring suggests that the
interaction with tubulin is hydrophobic in nature and
that some polar interactions can be lost at the expense
of lower entropic loss upon binding. The lower potency
of bis-naphthyl analogue 6a indicates that the whole
substitution by hydrophobic naphthalene moieties is
detrimental for the activity.
5. Medarde, M.; Ramos, A. C.; Caballero, E.; Pelaez Lama-
mie de Clairac, R.; Lopez, J. L.; Gravalos, D. G.; San Feli-
ciano, A. Bioorg. Med. Chem. Lett. 1999, 9, 2303.
6. (a) Lin, C. M.; Singh, S. B.; Chu, P. S.; Dempcy, R. O.;
Schmidt, J. M.; Pettit, G. R.; Hamel, E. Mol. Pharmacol.
1988, 34, 200. (b) Cushman, M.; Nagarathnam, D.; Gopal, D.;
Chakraborti, A. K.; Lin, C. M.; Hamel, E. J. Med. Chem.
1991, 34, 2579.
7. For example, for compound 4a J1,2=12 Hz; NOE:{1}: 2, 10
and 30; {2}: 1 and 200/600; {200/600}: 2, C300/C500-OMe and 30. d
(1H): 7.78 (1H, sa, H10); 7.73±7.78 (2H, m, H60/H70); 7.68 (1H,
d, J=8.8, H40); 7.42±7.46 (3H, m, H30/H50/H80); 6.74 (1H, d,
J=12.0, H1); 6.59 (1H, d, J=12.0, H2); 6.53 (2H, s, H200/
H600); 3.84 (3H, s, 400-OMe); 3.59 (6H, s, 300/500-OMe). d (13C):
153.5 (C, C300/C500); 137.5 (C, C400); 135.0 (C, C20); 133.6 (C,
C8a0); 132.6 (C, C4a0); 132.6 (C, C100); 130.5 (CH, C2); 129.9
(CH, C1); 128.2 (CH, C70); 127.9 (CH, C10); 127.7 (CH, C60);
127.5 (CH, C30); 127.1 (CH, C40); 126.2 (CH, C80); 126.1 (CH,
C50); 106.3 (CH, C200/C600); 61.0 (CH3-O-C400); 56.2 (CH3-O-
C300/500).
In order to con®rm that the activities of these com-
pounds are due to interactions with the microtubule
system, in vitro studies on their e€ect on cellular
microtubules of leukemic cells have been performed.9
The preliminary results show that the tested compounds
disrupt the microtubule system, 4a being roughly 40
times more potent than 4b but 5 times less than model
combretastatin A-4.
The new compounds and cytotoxicity results are of
interest for establishing the SAR of the combretastatins
and the design of new antimitotic agents based on
naphthalene systems that could inhibit tubulin poly-
merization. Further studies are under way to better
characterize the interactions of these naphthylcom-
bretastatins with tubulin.
8. San Feliciano, A.; Medarde, M.; Pelaez Lamamie de
Clairac, R.; Lopez, J. L.; Puebla, P.; Gravalos, M. D. G.;
Ruiz, P.; Garcõa de Quesada, M. T. Arch. Pharm. (Weinheim)
1993, 326, 421.
9. Evangelio, J. A.; Abal, M.; Barasoain, I.; Souto, A.; Lillo,
M. P.; Acuna, A. U.; Amat-Guerri, F.; Andreu, J. M. Cell
Motil. Cytoskeleton 1998, 39, 73.