Photoswitching off the Antiproliferative Activity of Combretastatin A-4 Analogues

Article abstract of DOI:10.1021/acs.orglett.9b03780

The photostability and antiproliferative activity of combretastatin A-4 (CA-4) analogues against human epidermoid carcinoma cells A-431 were studied. For the first time, it was shown that UV or sunlight irradiation of furanone analogues of CA-4 results in

Full text of DOI:10.1021/acs.orglett.9b03780

Letter
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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Photoswitching off the Antiproliferative Activity of Combretastatin
A‑4 Analogues
Anton V. Yadykov,† Alexander M. Scherbakov,‡ Victoria V. Trofimova,§ Andrey G. Lvov,†
Ashot I. Markosyan,∥ Igor V. Zavarzin,† and Valerii Z. Shirinian*,†
†N. D. Zelinsky Institute of Organic Chemistry, RAS, 47, Leninsky prosp., 119991 Moscow, Russian Federation
‡N. N. Blokhin National Medical Research Center of Oncology, Kashirskoye sh. 24, 115522 Moscow, Russian Federation
§D.I. Mendeleev University of Chemical Technology of Russia, Moscow 125047, Russian Federation
∥Scientific Technological Center of Organic and Pharmaceutical Chemistry, NAS RA, Yerevan 0014, Republic of Armenia
S
* Supporting Information
ABSTRACT: The photostability and antiproliferative activity
of combretastatin A-4 (CA-4) analogues against human
epidermoid carcinoma cells A-431 were studied. For the first
time, it was shown that UV or sunlight irradiation of furanone
analogues of CA-4 results in a photorearrangement giving
products with relatively low antiproliferative activity. The
observed ability of this series CA-4 to the photodegradation
can be used for the design of a new class of drug candidates
with high selectivity to cancer cells.
iarylethenes occupy an important place among the
good yields (Scheme S1, Table 1). Photochemical studies of
Dphotoswitchable compounds, which are extensively these compounds showed that as opposed to photochromic
diarylethenes,22 they undergo an irreversible photorearrange-
ment giving naphthalene derivatives (Scheme 1).23 The
photoirradiation of CA-4 analogues was carried out in N-
methyl-2-pyrrolidone (NMP) (see Scheme S2). The photo-
rearrangement of these diarylethenes provides naphthalene
derivatives with different functional groups (Table 1).
Evaluation of the effectiveness of this phototransformation
was carried out using UV and NMR spectroscopy. The spectral
properties of the initial analogues of CA-4 and the products of
photochemical reactions were studied in acetonitrile upon
irradiation at 365 nm (for UV−vis spectra of 1a−g and 2a−g,
see Figures S1−S15). The dynamics of spectral alterations
under irradiation of diarylethenes solutions are characterized
by the presence of two isosbestic points that is indicative of a
high rate of a 1,9-sigmatropic rearrangement with subsequent
oxazole ring opening and the absence of the contribution of
intermediates to the measured stationary absorption spectra.24
Therefore, the absorption spectra measured under irradiation
of diarylethenes 1a−g are superpositions of solely two forms,
namely, initial 1 and the product 2 (Scheme 1).
studied for the design of smart materials and devices.1,2 On
the other hand, CA-4 analogues, being diarylethene derivatives,
are of interest as biologically active compounds acting as
inhibitors of tubulin polymerization and suppressing micro-
tubule formation.3−5 CA-4 analogues occupy a special place
among the ligands capable of binding to the colchicine-binding
site of tubulin and inhibiting proliferation of cancer cells.6−8
Due to their simple structures and high biological activity,
these compounds are extensively studied as effective anticancer
drug candidates.9−11 Some CA-4 derivatives, such as CA-4P,
ombrabulin, and podophyllotoxin, and also several compounds
of the colchicine and allocolchicine series, are currently in
clinical trials.4,12
Despite considerable research on the anticancer activity of
CA-4 analogues,13−15 the photostability of these compounds
(resistance to UV light or sunlight) is poorly known,16,17
although this property is an important factor in drug design.
Since heterocyclic analogues of CA-4 have attracted great
interest and are potential drug candidates, the aim of this work
is to study the photodegradation of new heterocyclic CA-4
analogues of the furanone series18−20 and to evaluate their
antiproliferative activity before and after exposure to UV light
or sunlight.
The high efficiency of the photorearrangement was
1
demonstrated by H NMR monitoring. Figure 1 presents the
results of monitoring for diarylethene 1c. The formation of
other byproducts associated with the photocyclization/
Furanone analogues of CA-4 (1a−g) comprising a 3,4,5-
trimethoxyphenyl moiety as the ring A and various five-
membered heterocycles or 4-methoxyphenyl residue as the
ring B were synthesized according to the known method21 in
Received: October 24, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03780
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Organic Letters
Letter
Table 1. Structures and Yields of CA-4 Analogues and Their
Photoproducts
Figure 1. 1H NMR monitoring of the photoirradiation of diarylethene
1c.
appears at δ 2.80 ppm with the disappearance of the starting
diarylethene at δ 1.92 ppm.
1
The H NMR monitoring of 1a upon irradiation with UV
light was also carried out (see Figure S16). As opposed to the
heterocyclic analogues, furanone 1a, comprising a 4-methox-
yphenyl residue, gives three photocyclization products upon
UV irradiation: phenanthrene derivative 2a′ and two E/Z
isomers of photorearrangement product 2a′′ (Scheme 2).25
Scheme 2. Photoreaction of Diarylethene 1a
The photoreaction affords phenanthrene derivative 2a′ as
the major product that is formed via the Mallory reaction
(tandem photocyclization/oxidation reaction, Figure S16).
The formation of the phenanthrene derivative is confirmed by
the presence of a characteristic doublet at δ 9.03 ppm with a
small spin−spin coupling constant (2.4 Hz). Two isomeric
products E/Z-2a′′ are generated via a tandem reaction
involving the photocyclization of the hexatriene system
followed by a rearrangement and benzene ring opening.
These compounds are characterized by spin−spin coupling
constants of olefinic protons (16.0 and 12.5 Hz for the E and Z
isomers, respectively, Figure 3). It should be noted that the
maximum formation of butadiene derivatives 2a′′ (Z- and E-
isomers) is observed when the reaction mixture is irradiated for
2 h; further irradiation leads to the disappearance of these
compounds and only phenanthrene 2a′ remains (Figure S16).
Thus, the photochemical studies of CA-4 analogues of the
furanone series showed that the compounds containing five-
membered heterocycles as the ring B are very sensitive to UV
light and undergo a rearrangement giving naphthalene
derivatives. Diarylethenes comprising the 4-methoxyphenyl
moiety as the ring B undergo either a tandem photo-
cyclization/oxidation reaction (Mallory reaction) giving a
phenanthrene 2a′ or a tandem photocyclization/rearrange-
ment reaction accompanied by the benzene ring opening to
form an E/Z isomer mixture of butadienes 2a′′. For a
comparative evaluation of the antiproliferative activity of CA-4
Scheme 1. Photorearrangement of CA-4 Analogues
oxidation or photocyclization/elimination was virtually not
observed, as can be clearly seen by following the changes in the
position of a signal of methoxy groups. As can be seen in
Figure 1, the signal of methyl group in the naphthalene system
B
DOI: 10.1021/acs.orglett.9b03780
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
analogues and their photoproducts, analytically pure samples
were prepared.
For a more detailed study and dynamic evaluation of
biological activity, we selected two compounds (1a and 1c)
displaying activity in the submicromolar concentration range.
To determine the dependence of antiproliferative activity of
these compounds on UV irradiation, we evaluated the activity
before and after irradiation for 2 and 4 h. Irradiation was
performed before treatment of A-431 cells. As can be seen in
Figure 2, in the concentration range under study, these
Combretastatin A-4 analogues and their photoproducts were
examined for their effect on viability of human epidermoid
carcinoma cells A-431. The activity was evaluated in the
micromolar concentration range from 0.7 to 15 μM. Six pairs
of compounds (CA-4 analogues and their photoproducts)
were tested (Table 2). Two CA-4 analogues, compounds 1a
Table 2. Effect of the Synthesized Compounds on A-431
Cell Viability (72 h Growth in the Presence of Compounds)
cell viability (15 μM
treatment) (%)
entry
compd
IC50 (μM)
DMSO (vehicle
control)
99
1
2
3
4
5
6
7
8
1a
<0.7
>15
>15
<0.7
>15
>15
>15
>15
>15
>15
9
60
81
10
77
41
77
44
11
22
82
74
57
25
1b
2b
1c
2c
1d
2d
1e
2e
1f
2f
1g
2g
cisplatin
9
10
11
12
13
14
>15
>15
>15
6.3 0.9
Figure 2. Effect of compounds 1a (A) and 1c (B) on the viability of
the A-431 cells (72 h cell growth in the presence of compounds), w/o
UV, with UV (λ = 365 nm) for 2 or 4 h.
and 1c, exhibited high activity against A-431 cells (IC50 of
these compounds were less than 0.7 μM). These compounds
were selected for more detailed investigation. Both compounds
contain the 4-methoxyphenyl moiety as the ring B, which is
apparently responsible for their high activity.26 The anti-
proliferative properties of 1a and 1c are much better compared
to the reference cisplatin, and the latter shows an IC50 of about
6 μM. Naphthalene 2c obtained from compound 1c via a
photoreaction exerted a weak effect on cell viability and, when
used at a concentration of 15 μM, insignificantly inhibited the
cell growth. A similar, but less pronounced, decrease in activity
was observed for photoproduct 2d as compared to starting 1d.
On the contrary, photoproduct 2e proved to be more active
that the starting compound. CA-4 analogue 1f exhibited high
activity, and the cell viability in the presence of this compound
at a concentration of 15 μM was about 20%. Its photoproduct
2f showed much lower antiproliferative activity. Interesting
results were obtained for 1b. Thus, the replacement of the
peripheral 4-methoxyphenyl substituent in compound 1c by
the phenyl group led to a significant loss in the activity of both
diarylethene 1b and its photoproduct 2b. These compounds
did not show a pronounced antiproliferative effect on A-431
cells; at a concentration of 15 μM, they induced less than 40%
cell death. Compounds 1g and 2g also did not show significant
antiproliferative effects. In this pair, photoproduct 2g is more
active than the starting diarylethene 2g. Additional assays were
performed for more active compounds.
compounds did not exhibit inhibitory activity against A-431
cell growth after 2 h irradiation. The UV irradiation of the
compounds 1a and 1c for 4 h also led to the loss of
antiproliferative activity. As can be seen in Figure 2 and Table
S1, these compounds did not exhibit inhibitory activity against
A-431 cell growth after 2 h irradiation. The UV irradiation of
the compounds 1a and 1c for 4 h also led to the loss of
antiproliferative activity.
Since light sensitivity of biologically active compounds is an
important factor in the efficient drug design,16,27,28 we have
studied the sunlight-induced photodegradation of two diary-
lethenes. Solutions of CA-4 analogues in DMSO-d6 were
monitored by 1H NMR spectroscopy upon exposure to
sunlight (Figure 3, the experiment was performed at the
latitude of Moscow at room temperature in April 2019).
The exposure to sunlight was found to induce processes
similar to those caused by UV irradiation. Furanone 1a gives
three products, including the phenanthrene analogue. The only
difference is the reaction time. As expected, upon exposure to
sunlight, the reaction is not completed even within 4 days,
whereas under UV irradiation, the reaction proceeds within 4 h
(compare Figure 3 with Figure S16). A similar pattern was
observed for diarylethene 1c (Figure S17). The complete
conversion giving a photocyclization/rearrangement product
was achieved also after irradiation for 4 days.
Thus, the photobiological studies demonstrated that CA-4
analogues containing the 3,4,5-trimethylphenyl group as the
ring A and different five-membered heterocycles as the ring B
are highly susceptible to UV irradiation and undergo
photodegradation. Most photodegradation products exhibit
relatively low activity compared to the starting diarylethenes.
To summarize, we have synthesized combretastatin A-4
analogues of the furanone series comprising a 3,4,5-
trimethoxyphenyl derivative as the ring A and various five-
membered heterocycles as the ring B. Photochemical and
antiproliferative properties of the synthesized compounds and
C
DOI: 10.1021/acs.orglett.9b03780
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Organic Letters
Letter
ACKNOWLEDGMENTS
■
This work was financially supported by the Russian
Foundation for Basic Research (Grant No. 18-53-05019) and
the State Science Commission, Ministry of Education and
Science, Republic of Armenia (Grant No. 18RF-083) in the
Collaborative Scientific Programs framework. The authors are
grateful to Dr. Oxana Ryabaya for A-431 cells culture protocol.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b03780.
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Detailed experimental procedures, compound character-
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: shir@ioc.ac.ru
ORCID
Andrey G. Lvov: 0000-0003-2951-2651
Valerii Z. Shirinian: 0000-0001-9480-3565
Notes
The authors declare no competing financial interest.
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