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Mini-Reviews in Medicinal Chemistry, 2011, 11, 000-000 1
1389-5575/11 $58.00+.00 © 2011 Bentham Science Publishers Ltd.
Synthesis and Cytotoxic Activity of New -Carboline Derivatives
A. Peduto*, V. More, P. de Caprariis, M. Festa, A. Capasso, S. Piacente, L. De Martino, V. De Feo
and R. Filosa
Dipartimento di Scienze Farmaceutiche e Biomediche, Università di Salerno, Via Ponte don Melillo, 84084 Fisciano
(SA), Itlay
Abstract. On the basis of harmine and 1-methoxy-canthin-6-one chemical structures, a series of novel 1,4-disubstituted
and 1,4,9-trisubstituted -carbolines and tetracyclic derivatives were designed and synthesized. Cytotoxic activities of
these compounds in vitro were investigated in a human tumor cell line panel. Almost all compounds demonstrated
interesting cytotoxic activities in particular against prostate cancer cells PC-3 with IC50 in the low micromolar range.
Compound X was found to be the most potent one with IC50 value of 8.0 M; this suggests further studies with models of
prostate cancer.
Keywords: -carbolines, 1-methoxycanthin-6-one, prostate cancer.
INTRODUCTION
Natural products have historically and continually been
investigated for promising new leads in pharmaceutical
development. The -carboline alkaloids, that possess a
common tricyclic pyrido[3,4-b]indole ring structure [1,2]
have attracted attention regarding several aspects of
medicinal chemistry. They were originally isolated from
Peganum harmala (Zygophillaceae, Syrian Rue), which is
being used as a traditional herbal drug as an emmenagogue
and abortifacient in the Middle East and North Africa [3]. In
the Amazon basin plants containing -carbolines were
widely used as hallucinogenic drinks or snuffs. Besides, the
extracts of the seeds of Peganum harmala have been
traditionally used for hundreds of years to treat the
alimentary tract cancers and malaria in Northwest China [4].
These compounds possess a wide diversity of important
biochemical effects and pharmacological properties.
Numerous previous reports investigated the effects of -
carboline alkaloids on the central nervous system (CNS),
such as their affinity with benzodiazepine receptors (BZRs),
5-HT2A and 5-HT2C receptors [5-7]. Recent interest in these
alkaloids has been focused on their potent antitumor activity.
Several investigations [8-20] on the synthesis of a variety of
-carboline derivatives and the evaluation of their antitumor
activities unraveled that -carbolines demonstrated potent
antitumor activities and the activ ity was correlated to both
the planarity of the molecu le and th e presence of the ring
substituents. Recently it is discovered that -carboline
derivatives may function their antitumor activity through
multiple mechanisms, such as intercalating into DNA [21],
inhibiting Topoisomerase I and II [22], CDK [23-24], and
IKK (IkB kinase complex) [25].
*Address correspondence to this author at the Dipartimento di Scienze
Farmaceutiche, Università di Salerno, Via Ponte don Melillo, 84084
Fisciano (SA), Italy; Tel: +39-(0)89-969398; Fax: +39-(0)89-969602;
E-mail: apeduto@unisa.it
Harmine (Fig. (1)) is the most representative -carboline
alkaloid endowed with antitumor properties. It showed high
cytotoxicity both in vitro against different human tumor cell
lines and in vivo in mice bearing both Lewis lung cancer and
Sarcoma 180. It exhibited remarkable DNA intercalation
capacity and significant Topo I inhibition activity [14,17].
Tetracyclic systems in which another ring has been fused
to the -carboline nucleus have also shown promising
antitumor activity. In particular, 1-methoxycanthin-6-one
(Fig. (1)), a natural product isolated from the medicinal plant
Ailanthus altissima, showed interesting antiproliferative
properties, suppressing the growth of a panel of human
tumor cell lines, including epiderimoid carcinoma of the
nasopharynx (KB), ileocecal carcinoma (HCT-8), renal
cancer (CAK-1), breast cancer (MCF-7) and melanoma (SK-
MEL-2) [26].
As part of our ongoing efforts, we recently explored the
effects of 1-methoxycanthin-6-one on apoptosis in human
leukemia (Jurkat), thyroid carcinoma (ARO and NPA), and
hepatocellular carcinoma (HuH7) cell lines.
We demonstrated that 1-methoxy-canthin-6-one induced
apoptosis via a JNK-dependent mechanism. Furthermore, the
compound synergized with human recombinant tumor
necrosis factor (TNF)–related apoptosis-inducing ligand
(hrTRAIL) in apoptosis induction [27].
Inspired by these results, in the present investigation, we
designed and synthesized novel 1,4-disubstituted and 1,4,9-
trisubstituted -carbolines and 1-methoxycanthin-6-one
derivatives (Fig. (2)). The purpose of this study is to evaluate
the antiproliferative properties of these compounds in order
to acquire more information about the structural
requirements for the possible improvement of the cytotoxic
potential and to elucidate SARs between substituent
properties and antitumor activities.
2 Mini-Reviews in Medicinal Chemistry, 2011, Vol. 11, No. 6 Peduto et al.
Fig. (1). Chemical structure of harmine and 1-methoxycanthin-6-one.
Fig. (2). Chemical structure of 1,4-disubstituted and 1,4,9-trisubstituted -carbolines and tetracyclic derivatives.
RESULTS AND DISCUSSION
Chemistry
The preparation of derivatives I-IV is summarized in
Scheme 1. Treatment of indole-2-carboxylic acid (1) with a
solution of methyllithium in ether afforded 2-acetylindole (2)
in very good yield. 2 was converted to the glycine derivative
(3) via reductive amination, using sodium triacetoxyboro-
hydride as reducing agent in presence of triethylamine and
acetic acid. Ethyl 2-[1-(1H-indol-2-yl)ethylamino]acetate 3
was treated with ethylformate and formic acid to furnish the
key intermediate 4. Cyclization of 4 with methanesulfonic
acid gave intermediate 5 in excellent yield. 4-Methoxy-1-
methyl--carboline (I) was obtained in a one-step
conversion. This reaction was achieved by treatment of 5
with 2,2-dimethoxypropane, dehydrogenation of
intermed iate with chloranil, followed by ready methanolysis
of N-formyl pyridinium intermediate. Methylation of the N-9
position of compound I has been achieved by the action of
sodium hydride in a mixture of anhydrous DMF/THF
followed by addition of methyl iodide, affording compound
II in good yield. 1-Benzylidine substituted -carboline III
was readily prepared by reaction of 1,9-dimethyl-4-methoxy-
-carboline II with benzaldehyde in refluxing acetic
anhydride.
Treating 4 with methanesulfonic acid 70% in water we
obtained compound IV in which cyclization and
deformylation were accomplished simultaneously.
Compounds V, VI, VII, VIII, and 1-methoxycanthin-6-
one (IX) were prepared according to the literature
procedures [28, 29]. 3-benzyl-1-methoxycanthin-6-onium
bromide (X) was synthesized from 1-methoxycanthin-6-one
by the simple quaternarization with benzyl bromide in
refluxing ethyl acetate (Scheme 2).
Antiproliferative Activity
The cytotoxic potential of all synthesized compounds
was evaluated in vitro against a panel of human tumor cell
lines. The tumor cell line panel consisted of human T cell
lymphoblast-like cells (Jurkat), human breast cancer (MCF-
7), human colon carcinoma (HT-29), human lung
adenocarcinoma epithelial (A549), human prostate cancer
(PC3), human melanoma (M14), human anaplastic thyroid
carcinoma (ARO), and human glioblastoma (T98G). IC50
N
N
O
O
N
NH
O
Harmine 1-Methoxy- canthin- -6-one
VI
IR=H
II R=CH
3
IX X
VII R=COOCH
3
VIII R=CHO
V
N
N
O
N
N
O
III
R
IV
NH
NH
O
N
NH
O
N
NH
O
ON
NH
O
R
N
N
O
O
N
N
O
O
Synthesis and Cytotoxic Activity of New -Carboline Derivatives Mini-Reviews in Medicinal Chemistry, 2011, Vol. 11, No. 6 3
values for I-X and harmine (for comparison) are reported in
Table 1.
In this study we evaluated cytotoxic effects of -
carboline derivatives and compared with harmine, a natural
alkaloid highly cytotoxic against human tumor cell lines. In
addition, 1-methoxycanthin-6-one (IX) was tested for
cytotoxicity against a panel of several tumor cells. Our
experiments confirmed the cytotoxic effect of 1
methoxycanthin-6-one (IX) and, for the first time, we
demonstrated the potential cytotoxic activity of compounds
I-VIII and X in different cancer cell lines. From the above
data, the following conclusions were drawn.
Scheme 1. Synthesis of compounds I-IV.
Scheme 2. Synthesis of compound X.
a
23
5
IV
1
Reagents and conditions: a) MeLi 1.6M in ether,THF, rt, o.n.,84%; b)glycine ethyl ester hydrochloride, Na BH(OAc)
3,
TE A, AcO H, CH
2
Cl
2,
rt, 22 h,
89% ; c) HCO
2
Et, HCOOH, rt , 12 h, 94%; d) Me SO
3
H, 70°C, 1h, 96%; e)Me
2
C( O Me )
2,
p - Ts O H , be nz e n e; t he n p - ch lo r an il , rt , 12 h , 50 % ; f ) , CH
3
I,
NaH, THF-DMF, rt, 30', 80%; g) (CH
3
CO
2
)O , C
6
H
5
CHO, ref lux, 24h, 67%; h) MeSO
3
H70%in H
2
O, 70°C, 1h, 95%.
NH
O
OH NH
O
b
NH
NH
O
O
4
NH
N
O
O
CH
O
h
c
NH
NH
O
d
NCHO
NH
O
N
NH
O
I
efN
N
O
II
gN
N
O
II I
IX X
N
N
O
O
N
N
O
O
a
Reagents and conditions: a) BnBr, AcOEt, ref lux, 7h, 67%.
Br
4 Mini-Reviews in Medicinal Chemistry, 2011, Vol. 11, No. 6 Peduto et al.
The shift of methoxy group from position 7 to 4 (harmine
vs I) led to an increase of activity against HT-29 and A54 9
cell lines while no significant activities were detected in the
other cell lines. The introduction of methyl group into
position-9 of compound I led to compound II which
demonstrated an improvement of activity to all tumor cell
lines. When methyl group in position 1 was replaced with
benzylidine substituent there is a loss of activity.
Compounds V and VI, with no substituent at position-1,
were inactive to all tumor cell lines. Interestingly, the
compound bearing a carboxyaldehyde substituent in
position-1 (VIII) of -carboline ring system displayed a
strong anti-proliferative effect against almost all tumor cell
lines. In addition, the compound with a carboxylate (VII)
substituent in position-1 was inactive to all tumor cell lines
except to PC-3. Tetrahydro--carboline derivative IV
showed good activity only against PC-3 and ARO cells.
As almost of the synthesized compounds of the present
study showed interesting anticancer activity especially
against PC-3 and ARO cell lines, oral bioavailability was
considered to play an important role for the development of
bioactive molecu les as therapeu tic agents. Therefore, a
computational study for prediction of ADME properties of
the molecules was performed by determination of
lipophilicity, topological polar surface area (TPSA),
absorption (% ABS) and simple molecular descriptors used
by Lipinski in formulating his ‘‘rule of five’’[30].
Calculations were performed using Molinspiration online
property calculation toolkit [31]. Table 2 represents a
calculated percentage of absorption (% ABS), topological
polar surface area (TPSA) and Lipinski parameters of the
synthesized compounds. Percentage of absorption (% ABS)
was estimated using the equation: % ABS = 109 - 0.345 x
TPSA, according to Zhao et al. [32]. TPSA was also
calculated using Molinspiration online property calculation
toolkit [31] according to the fragment-based method of Ertl
et al. [33]. Polar surface area, together with lipophilicity, is
an important property of a molecule in transport across
biological membranes. Too high TPSA values give rise to a
poor bioavailability and absorption of a drug. According to
the above criterions, calculated percentages of absorption for
compounds ranged between 87 and 99%.
As a whole, the present results showed that all the
compounds studied, except V and VI, showed a very
selective activity against prostate can cer cells PC-3 at IC50
values nearly to 20 μM. In particular, X demonstrated an
important anti-proliferative effect at low concentration (IC50
8 M). Moreover the most active compounds II, VIII, IX
and X appear to be suitable as leads for further anticancer
molecules development efforts on the fact their size and
chemical properties are appropriate to classify them as drug-
like compounds as they follow all the Lipinski’s rule of 5.
CONCLUSION
In conclusion, a number of novel -carboline derivatives
described in this paper proved to be selective and potent
agent against prostate cancer. This important first step will
allow us to determine preliminary relationship between
structure and cytotoxic activities. The in vi tro experiments
revealed that the shift of methoxy group in position 4 of -
carboline ring of harmine led to enhanced cytotoxic activity,
substituents at position-1 were essential for high activity
towards specific tumor types. From the above data we have
also demonstrated that substitution at the 3-position of 1-
methoxycanthin-6-one with benzyl increased activity against
most of the cell lines tested.
Ongoing studies will probe the mechanism of action of
these new derivatives but the data presented in this paper
already suggest that several derivatives are potential
Table 1. In Vitro Antitumor Activity of Derivatives I-X
IC50 (M)a
Compounds
M14 MCF-7 HT-29 A549 PC-3 Jurkat ARO T98G
harmine n.s.b n.s.b 79.4±0.9 n.s.b 22±0.8 n.s.b 26.5±0.7 35±1.1
I n.s.b n.s.b 50±1.5 80±0.5 20±1.2 n.s.b 50±0.8 n.s.b
II 36±0.6 32±0.1 27±0.2 50±0.4 22±1.2 65±0.8 30±0.5 40±1.2
III n.s.b n.s.b n.s.b n.s.b 60±0.9 n.s.b n.s.b n.s.b
IV n.s.b n.s.b n.s.b n.s.b 24±0.8 n.s.b 18±0.6 n.s.b
V n.s.b n.s.b n.s.b n.s.b n.s.b n.s.b 64±1.2 n.s.b
VI n.s.b n.s.b n.s.b n.s.b n.s.b n.s.b n.s.b n.s.b
VII n.s.b n.s.b n.s.b n.s.b 50±0.6 n.s.b n.s.b n.s.b
VIII 38.9±0.1 35±0.45 90±0.3 45±1.2 25±0.2 28.8±0.9 39±0.4 n.s.b
IX 50±1.1 50±0.3 31±0.1 n.s.b 26±0.1 50±0.2 15±0.6 90±0.7
X 37±0.3 46±0.75 72±0.4 n.s.b 8±0.9 n.s.b 28±1.0 n.s.b
aIC50= compound concentratio n required to inhibit tumor cell proliferation by 50%; bn.s. not significant. Data represent the mean values of three independent determinations
p
erformed in tri
p
licate.
Synthesis and Cytotoxic Activity of New -Carboline Derivatives Mini-Reviews in Medicinal Chemistry, 2011, Vol. 11, No. 6 5
candidates for clinical development with models of prostate
cancer.
SUPPLEMENTARY MATERIAL
Supplementary material is available on the publishers
Web site along with the published article.
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Cpd % ABS TPSA n-ROTB n-OHNH do nors
n-ON acceptors mi LogP Mol. Wt n violations
harmine 95 37.92 1 1 3 2.626 212.25 0
I 95 37.92 1 1 3 2.796 212.25 0
II 99 27.06 1 0 3 2.864 226.28 0
III 99 27.06 3 0 3 5.192 314.39 1
IV 93 44.89 0 2 3 1.829 200.24 0
V 96 37.92 1 1 3 2.575 198.23 0
VI 92 50.48 1 1 4 0.943 214.22 0
VII 87 64.22 3 1 5 2.646 256.26 0
VIII 90 54.99 2 1 4 2.604 226.24 0
IX 94 43.61 1 0 4 2.891 250.26 0
X 97 34.61 3 0 4 0.816 341.39 0
a % ABS: Percentage of absorption, TPSA: topological polar surface area, n-ON: number of hydrogen bond acceptors, n-OHNH: number of hydrogen bond donors, n-ROTB: number
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