Potent radiosensitizing agents: 5-Methylselenyl- and 5-phenylselenyl- methyl-2′-deoxyuridine

Article abstract of DOI:10.1016/j.bmcl.2010.12.102

This Letter describes the novel radiosensitizing agents based on nucleoside base modification. In addition to the known 5-phenylselenide derivative, 5-methylselenide modified thymidine, which has a van der Waals radius smaller than the phenyl group, was newly synthesized. The similar monomer activity of 5-methylselenide derivative under oxidation condition was confirmed by NMR experiments. The cytotoxicity tests and radiosensitizing experiments of both compounds were carried out using the H460 lung cancer cell line. Both the 5-phenylselenide and the 5-methylselenide derivatives showed a relatively low toxicity to the cells. However, in combination with γ-radiolysis, both exerted good radiosensitizing effects to the lung cancer cell lines in vitro. This result confirms that 5-methylselenide modified thymidine could be a useful candidate as a potential radiosensitizing agent in vivo.

Full text of DOI:10.1016/j.bmcl.2010.12.102

Bioorganic & Medicinal Chemistry Letters 21 (2011) 1151–1154
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Potent radiosensitizing agents: 5-Methylselenyl- and
5-phenylselenyl-methyl-20-deoxyuridine
Ambadas B. Rode a, Byeong Mo Kim b, Seon Hwa Park b, In Seok Hong a, , Sung Hee Hong b,
⇑
⇑
a Department of Chemistry, College of Natural Science, Kongju National University, 182 Shinkwan-dong, Gongju, Chungnam 314-701, Republic of Korea
b Division of Radiation Cancer Research, Korea Institute of Radiological & Medical Sciences, 215-4 Gongneung-Dong, Nowon-Gu, Seoul 139-706, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
This Letter describes the novel radiosensitizing agents based on nucleoside base modification. In addition
to the known 5-phenylselenide derivative, 5-methylselenide modified thymidine, which has a van der
Waals radius smaller than the phenyl group, was newly synthesized. The similar monomer activity of
5-methylselenide derivative under oxidation condition was confirmed by NMR experiments. The cytotox-
icity tests and radiosensitizing experiments of both compounds were carried out using the H460 lung
cancer cell line. Both the 5-phenylselenide and the 5-methylselenide derivatives showed a relatively
Received 7 November 2010
Revised 18 December 2010
Accepted 21 December 2010
Available online 25 December 2010
Keywords:
low toxicity to the cells. However, in combination with c-radiolysis, both exerted good radiosensitizing
Radiosensitizing agents
Modified thymidine derivatives
Interstrand cross-links
Cytotoxicity
effects to the lung cancer cell lines in vitro. This result confirms that 5-methylselenide modified thymi-
dine could be a useful candidate as a potential radiosensitizing agent in vivo.
Ó 2011 Elsevier Ltd. All rights reserved.
c
-Radiolysis
Radiation therapy has been the mainstay of nonsurgical treat-
methide intermediate.26–31 Moreover, they suggested a modified
nucleotide triphosphate 2 that can be incorporated in DNA by
DNA polymerases, which produces ICLs with the opposing strand
ment of cancer for over a century.1 Drugs that affect nucleoside
and nucleotide metabolism are among the most effective and most
widely used agents to sensitize tumor cells in radiotherapy.
These include halogenated thymidine analogs, 5-bromo-20-deoxy-
uridine2–4 and 5-iodo-20-deoxyuridine,5–7 which were the first
nucleoside anti-metabolites shown to enhance ionizing radiation-
induced cell killing.2 More recently, several other nucleoside
anti-metabolites have been shown to have radiosensitizing proper-
ties in vitro, which have been translated to clinical success for
drugs such as fluorouracil,8–10 5-fluoro-20-deoxyuridine,11–13
20,20-difluoro-20-deoxycytidine.14–16
dA when DNA containing it is exposed to
c-radiolysis under
O2-deficient conditions (Fig. 1).28
In this Letter we report the radiosensitizing effects of 5-methyl-
selenyl- and 5-phenylselenyl-methyl-20-deoxyuridine nucleosides
in vitro. To function as a radiosensitizer, a modified nucleoside
should be incorporated into the cellular DNA in the replication
step. The modified nucleoside drugs, which are structurally similar
to the natural nucleosides, act as a good substrate for natural en-
zymes such as DNA polymerase and kinase. The methyl group
has a van der Waals radius smaller than the phenyl group, so we
expected that replacement of the phenyl group with methyl in
phenylselenide derivative 1 would produce methylselenide 3
Interstrand cross-links (ICLs) are among the most toxic DNA le-
sions and have been suggested as the primary mechanism for the
cytotoxic activity of many clinical antitumor drugs, such as nitro-
gen mustards,17–19 platinum agents,20,21 and mitomycin C.22,23 In
addition, interstrand cross-linking agents such as mitomycin C
and cisplatin have been used in combination with radiotherapy
for the purpose of radiosensitization.24,25 Therefore, molecules that
produce ICLs in the hybridized DNA upon
useful as a novel radiosensitizing agent.
c-radiolysis might be
The Greenberg group showed phenylselenide derivative 1 pro-
duces an ICL with the opposing dA upon UV photolysis through
5-(20-deoxyuridinyl) methyl radical or upon oxidation through
⇑
Corresponding authors. Tel.: +82 41 850 8498; fax: +82 41 856 8613.
E-mail addresses: ishong@kongju.ac.kr (I.S. Hong), gobrian@kcch.re.kr (S.H.
Figure 1. Structures of 5-phenylselenyl-methyl-20-deoxyuridine and its nucleotide
Hong).
triphosphate.
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.12.102

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A. B. Rode et al. / Bioorg. Med. Chem. Lett. 21 (2011) 1151–1154
which the electrophilic methide-type intermediate 5 was gener-
ated by the spontaneous allylic selenoxide rearrangement (Scheme
1).32 Finally, water attacked the reactive methide intermediate to
produce a stable 5-hydroxymethyl 20-deoxyuridine (HO-mdU, 6),
which was confirmed by independently synthesized authentic
HO-mdU.33
In contrast to the phenylselenoxide derivative of 1 that was not
detected in NMR because of a fast allylic selenoxide rearrange-
ment,29 the methylselenoxide derivative 4, which was confirmed
by the presence of the diastereomeric H6 proton, was detected in
the NMR spectrum (Fig. 2B). This means that the rate of rearrange-
ment of the methyselenoxide derivative is much slower than that
of the phenylselenoxide derivative. The rate of rearrangement of
the methylselenoxide 4 was determined by absorption spectros-
copy33 and was (1.2 0.01) ꢀ 10ꢁ4 sꢁ1 (t1/2 = 95 min), which was
almost 50 times slower than the phenylselenoxide derivative
((6.1 0.8) ꢀ 10ꢁ3 sꢁ1, t1/2 = 1.9 min).31
Scheme 1. Decomposition of 3 under oxidative condition.
After further reaction time, the methide-type intermediate 5
was generated and confirmed by the presence of vinyl hydrogens
(Ha d 6.45, 6.41 ppm and Hb d 6.09, 6.06 ppm) and upfield shift of
diastereomeric H6 hydrogen (d 5.88, 5.85 ppm) as shown in Figure
2B and c and was also fully characterized by 13C, COSY, HMBC, and
HSQC experiments.33 Surprisingly, intermediate 5 was at least five
times more stable in physiological conditions (Fig. 2D) compared
to the methide-type intermediate of 1, which was converted to 6
during 1 day incubation.29 As a result, the slow rearrangement of
methylselenoxide 4 and the stability of methide-type intermediate
5 are the distinctive differences between the methylselenide 3 and
the phenylselenide 1. These properties of 3 suggest that there may
which would then be a good substrate for mammalian thymidine
kinase and DNA polymerase.
Phenylselenide 1 was easily converted to the methide-type
intermediate under oxidative conditions such as chemical oxi-
dants, singlet oxygen,
c
-radiolysis.29 Methylselenide derivative 3
was first synthesized from the similar synthetic method of phenyl-
selenide 1.33 The reactivity of methylselenide 3 under oxidative
condition was checked by NMR experiments. The methylselenide
derivative 3 was also easily oxidized to the methylselenoxide
derivative 4 after treatment of a chemical oxidant, NaIO4, after
Figure 2. 1H NMR analysis of the reaction of 3 (10 mM) with NaIO4 (10 mM) in a deuterated phosphate buffer (10 mM, pH 7.0, 25 °C). (A) Before NaIO4 addition, (B) 10 min
after NaIO4 addition, (C) 30 h after NaIO4 addition, and (D) 5 days after NaIO4 addition.

A. B. Rode et al. / Bioorg. Med. Chem. Lett. 21 (2011) 1151–1154
1153
Figure 3. Cell viability curves of H460 cancer cells in the presence of modified
nucleoside: (A) dose ranges of 0–500 M of phenylselenide 1, (B) dose ranges of 0–
100 M of methylselenide 3.
l
Figure 4. Radiosensitizing effects of 1 and 3 upon
forming fraction curves of H460 cancer cells exposed to 2, 4, 6 or 8 Gy of
c
-ionizing radiation. Colony
l
c-radiation
with or without dose concentrations: (A) 300 M of 1 and (B) 50 M of 3.
l
l
be some differences in the cross-linking efficiency, the cytotoxicity
and the radiosensitizing effect between 1 and 3.
The c-irradiation of the DNA duplex containing phenylselenide
Next, the cytotoxicity tests of phenylselenide 1 and methyl sel-
enide 3 derivatives were carried out to optimize the reaction con-
dition for the radiosensitizing experiments. Figure 3 shows that
1 induces interstrand cross-links by the methide intermediate pro-
duced through a H2O2 oxidation-rearrangement pathway.28 Based
on this observation, our experimental results of the NaIO4 reaction
with the methylselenide 3 suggest that the formation of double-
strand breaks during nucleotide excision repair of ICLs is a possible
key source of the radiosensitizing effect of 1 and 3.
methylselenide 3 was more toxic (72 h, IC50 = 100
lM) than the
phenylselenide 1 (72 h, IC50 = 500
l
M).33 Detailed studies to reveal
a mechanism of single effect to the cytotoxicity of 1 and 3 were in
progress.
The NaIO4 oxidation reaction of 3 shows that the methide 5 pro-
duced is more stable than the methide produced from 1 with
NaIO4. This observation led us to the conclusion that the stability
of methide 5 makes the methylselenide 3 more bioavailable in cells
than the less stable methide of 1 to produce ICLs which resulted in
more efficient radiosensitization by 3 than 1.
In conclusion, novel radiosensitizing agents based on nucleoside
base modification were developed in vitro. Methylselenide 3 and
phenylselenide derivative 1 could be useful candidates as a radio-
sensitizing agents in vivo because a low toxicity of the drug itself
Finally, the radiosensitizing efficiency of these agents was as-
sessed with a clonogenic assay. The colony forming fraction curves
were obtained after exposure of H460 cells with 2, 4, 6 or 8 Gy of
c-radiation with a IC20 concentration dose of 1 (300
lM) and a
IC20 concentration dose of 3 (50 M) (Fig. 4). The colony forming
l
fractions showed that the dose enhancement ratio (DER) of the
phenylselenide analogue 1 was 1.12 and that of the methylselenide
nucleoside 3 was 1.2 suggesting that both agents have radiosensi-
tizing effects with increasing radiation doses compared to radia-
tion alone (Fig. 4).33

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A. B. Rode et al. / Bioorg. Med. Chem. Lett. 21 (2011) 1151–1154
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This research was supported by the Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science and Technol-
ogy (2009-0071513). We are also grateful for support from the
National Nuclear R&D Program of the Ministry of Education, Sci-
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2010.12.102.
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