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R. Silvestri - Indole nucleus as a selected pharmacophore for the design of novel highly potent anti-viral agents active against HIV-1
drawing substituent(s) at position(s) 4 and 5 of the
indole is currently in progress.
Inhibitors of Tubulin Polymerization.
Microtubules (MTs) are essential cytoskeletal polymers
that are made of repeating α, β-tubulin (TB) heterodimers
and are present in all eukaryotes. The crucial
role of MTs in vital cellular functions for tumor
cells, including mitosis, motility and cell-cell contacts,
has made of MT interfering agents (MIAs) one of the
best classes of cancer chemotherapeutic drugs available
to date. A large number of chemically diverse
compounds are able to bind TB or MTs and inhibit
proliferation by acting on the mitotic spindle. Some of
these compounds (vinca alkaloids, colchicine) inhibit
MT polymerization, whereas others (taxanes) stabilize
MTs. While drugs that act on the vinca and taxane
sites have well-established roles in the treatment
of human cancers, the therapeutic potential of the
colchicine site in cancer treatment has yet to be realized.
Recently, we have reported arylthioindoles
(ATIs) as a new class of potent inhibitors of tubulin
polymerization and the growth of some carcinoma
cells that bind to colchicine site on β-tubulin close to
its interface with α-tubulin. Fist generation ATIs
were characterized by (i) an ester function at position
2 of the indole nucleus; (ii) a 3-arylthio group; (iii) an
appropriate substituent on the indole nucleus.
Focusing our attention on amino derivatives related to
combretastatin A-4, we designed and synthesized second
generation of ATI derivatives, bearing a hydrogen
atom or a methyl group at position 2 of the indole
nucleus. New ATIs were synthesized by a two-step
procedure. O-Ethyl-S-(3,4,5-trimethoxyphenyl) carbonodithioate
was transformed into 3,4,5trimethoxythiophenol
by heating at 65 °C in aqueous
ethanol in the presence of sodium hydroxide. This
mixture was made acidic with 6 N HCl and treated at
25°C with the appropriate indole while adding dropwise
an aqueous iodine-potassium iodide solution. The
new compounds strongly inhibited tubulin polymerization,
with activity in the low micromolar range,
comparable to the effects of colchicine and combretastatin
A-4. Derivatives bearing a halogen atom or a
small alkyl or ether group at position 5 of the indole
nucleus, were also potent inhibitors of MCF-7
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cell growth. For example, 5-bromo-3-[(3,4,5trimethoxyphenyl)thio]-1H-indole
showed tubulin
polymerization IC 50 = 1.6 µM and growth of MCF-
7 cells IC 50 = 43 nM, and inhibited [ 3 H]colchicine
binding at 65%. Cell cycle analysis revealed an accumulation
of HeLa cells in the G2/M phase at 24 h and
polyploidization at 48 h. At 24 h, inhibition of tubulin
polymerization had not caused extensive apoptosis,
suggesting that impaired cell viability might occur
through a “mitotic catastrophe”. Molecular modelling
studies showed that, despite the absence of the ester
moiety, the second generation ATIs bind into the
colchicine site in the same orientation as the first generation
ones. Binding to β-tubulin involved formation
of a hydrogen bond between the indole and Thr179
and positioning of the trimethoxy phenyl group in a
hydrophobic pocket near Cys241. The synthesis of
new derivatives obtained by bioisosteric replacement
of the sulfur bridge with either carbonyl or methylene,
and some related groups, and the indole nucleus
with differently substituted pyrrole and imidazole
rings is currently in progress.
Selected publications
La Regina G, Coluccia A, Piscitelli F, Bergamini A,
Sinistro A, Cavazza A, Maga G, Samuele A, Zanoli S,
Novellino E, Artico M, Silvestri R. Indolyl aryl sulfones
as HIV-1 non-nucleoside reverse transcriptase
inhibitors: role of two halogen atoms at the indole
ring in developing new analogues with improved
antiviral activity. J Med Chem. 2007, 50:5034-8.
La Regina G, Edler MC, Brancale A, Kandil S,
Coluccia A, Piscitelli F, Hamel E, De Martino G,
Matesanz R, Díaz JF, Scovassi AI, Prosperi E,
Lavecchia A, Novellino E, Artico M, Silvestri R.
Arylthioindole inhibitors of tubulin polymerization.
3. Biological evaluation, structure-activity relationships
and molecular modeling studies. J Med Chem.
2007, 50: 2865-74.
Ragno R, Coluccia A, La Regina G, Silvestri R.
Indolyl aryl sulphones as HIV-1 reverse transcriptase
inhibitors: docking and 3D QSAR studies. Exp
Opin Drug Discovery 2007, 2:87-114.