Novel methylselenoesters as antiproliferative agents

Article abstract of DOI:10.3390/molecules22081288

Selenium (Se) compounds are potential therapeutic agents in cancer. Importantly, the biological effects of Se compounds are exerted by their metabolites, with methylselenol (CH₃SeH) being one of the key executors. In this study, we developed a new series of methylselenoesters with different scaffolds aiming to modulate the release of CH₃SeH. The fifteen compounds follow Lipinski’s Rule of Five and with exception of compounds 1 and 14, present better drug-likeness values than the positive control methylseleninic acid. The compounds were evaluated to determine their radical scavenging activity. Compound 11 reduced both DPPH and ABTS radicals. The cytotoxicity of the compounds was evaluated in a panel of five cancer cell lines (prostate, colon and lung carcinoma, mammary adenocarcinoma and chronic myelogenous leukemia) and two non-malignant (lung and mammary epithelial) cell lines. Ten compounds had GI₅₀ values below 10 μM at 72 h in four cancer cell lines. Compounds 5 and 15 were chosen for further characterization of their mechanism of action in the mammary adenocarcinoma cell line due to their similarity with methylseleninic acid. Both compounds induced G₂/M arrest whereas cell death was partially executed by caspases. The reduction and metabolism were also investigated, and both compounds were shown to be substrates for redox active enzyme thioredoxin reductase.

Full text of DOI:10.3390/molecules22081288

molecules
Article
Novel Methylselenoesters as Antiproliferative Agents
Nuria Díaz-Argelich 1,2,3, Ignacio Encío 4 ID , Daniel Plano 1,2 ID , Aristi P. Fernandes 3,
Juan Antonio Palop 1,2 and Carmen Sanmartín 1,2,
*
1
Department of Organic and Pharmaceutical Chemistry, Faculty of Pharmacy and Nutrition,
University of Navarra, Irunlarrea 1, E-31008 Pamplona, Spain; ndiaz@alumni.unav.es (N.D.-A.);
dplano@unav.es (D.P.); jpalop@unav.es (J.A.P.)
2
3
4
Oncology and Hematology Section, IdiSNA, Navarra Institute for Health Research, Irunlarrea 3,
E-31008 Pamplona, Spain
Division of Biochemistry, Department of Medical Biochemistry and Biophysics (MBB), Karolinska Institutet,
SE-171 77 Stockholm, Sweden; Aristi.Fernandes@ki.se
Department of Health Sciences, Public University of Navarra, Avda. Barañain s/n, E-31008 Pamplona, Spain;
ignacio.encio@unavarra.es
*
Correspondence: sanmartin@unav.es; Tel.: +34-948-425-600
Received: 29 June 2017; Accepted: 28 July 2017; Published: 2 August 2017
Abstract: Selenium (Se) compounds are potential therapeutic agents in cancer. Importantly, the
biological effects of Se compounds are exerted by their metabolites, with methylselenol (CH3SeH)
being one of the key executors. In this study, we developed a new series of methylselenoesters
with different scaffolds aiming to modulate the release of CH3SeH. The fifteen compounds follow
Lipinski’s Rule of Five and with exception of compounds 1 and 14, present better drug-likeness values
than the positive control methylseleninic acid. The compounds were evaluated to determine their
radical scavenging activity. Compound 11 reduced both DPPH and ABTS radicals. The cytotoxicity of
the compounds was evaluated in a panel of five cancer cell lines (prostate, colon and lung carcinoma,
mammary adenocarcinoma and chronic myelogenous leukemia) and two non-malignant (lung and
mammary epithelial) cell lines. Ten compounds had GI50 values below 10
µM at 72 h in four
cancer cell lines. Compounds and 15 were chosen for further characterization of their mechanism
5
of action in the mammary adenocarcinoma cell line due to their similarity with methylseleninic
acid. Both compounds induced G2/M arrest whereas cell death was partially executed by caspases.
The reduction and metabolism were also investigated, and both compounds were shown to be
substrates for redox active enzyme thioredoxin reductase.
Keywords: methylselenoester; methylselenol release; cytotoxicity; cell cycle arrest; cell death;
thioredoxin reductase
1. Introduction
Cancer is one of the leading causes of mortality and morbidity worldwide. Current chemotherapy
is not completely satisfactory [
years, scientific evidence has backed the rationale for studying the mechanism of selenium-containing
compounds as cancer therapeutic agents [ ]. The role of Se is highly versatile due to multiple factors
that determine its activity. It has been described both as a chemopreventive and cytotoxic agent [ ] on
1] and the development of new drugs is an urgent need. In the last few
2
2–4
account of the dual role as antioxidant and pro-oxidant, which is highly dependent on dose, chemical
species and the redox state of the cell. Chemotherapeutic activity of Se compounds is mainly based in a
multi-targeting effect which triggers complex cascades of death signaling and inhibits tumor formation
and metastasis in animal models [
2,5–7]. Because of this, the utility of Se compounds for the treatment
of tumoral diseases and even drug-resistant cancers offers an interesting pursuit.
Molecules 2017, 22, 1288; doi:10.3390/molecules22081288
www.mdpi.com/journal/molecules

Molecules 2017, 22, 1288
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However, Se activity relies in multiple factors such as chemical form, dose and metabolism.
Results from in vitro studies, animal experiments and clinical trials suggest that the biological activities
of Se are dependent on the type and the nature of metabolites derived from the Se derivative [8,9].
Metabolites, and not original molecules, seem to be the ultimate executors of Se biological activity
for some Se compounds. Thus, understanding the metabolic routes of Se compounds is highly
necessary [10,11]. Se biochemistry encompasses a complex net of interrelated intermediates that
converge in two main metabolites: methylselenol (CH3SeH) and hydrogen selenide (H2Se). Among all
Se metabolites, it is largely accepted that CH3SeH stands out as a key executor for Se anticancer
activity [3].
The volatile nature and high reactivity of CH3SeH obliges to the use of precursors with the ability
to release it through hydrolysis, chemical reactions or cellular metabolism. Different approaches have
been investigated. Methylselenocysteine requires the action of a ß-lyase to release the metabolite, but
it has shown some promising results, especially in vivo [2,12,13]. The prodrug/enzyme system of
selenomethionine/L-methioninase is considered to be less potent since they are quite inert and present
lower redox activity [14]. Methylseleninic acid (MSA) is a synthetic molecule which has been broadly
studied both in vivo and in vitro [15–17] and it is considered one of the best CH3SeH precursors [14].
In addition to these classical precursors, our group has been synthesizing during the last decade
several methylseleno derivatives [18
–22]. These compounds exhibited significant anticancer activity
compared to other alkyl derivatives against a broad spectrum of cancer cells both in vitro and in vivo
.
Moreover, we demonstrated that Se-methylselenourea derivatives were able to release CH3SeH in
aqueous systems [23]. Recently, we identified a novel series of selenoester derivatives in which the
presence of a methylselenoester moiety substantially improved the antiproliferative activity of the
corresponding selenoglycolic acids [24]. Continuing with this work, herein we propose an extended
series of molecules bearing this functionality. The methylselenoester entity can be easily attacked by
a nucleophile such as water, as one possible mechanism to deliver CH3SeH. The release of the key
metabolite in aqueous medium could therefore be modulated through the chemical features of the
core of the molecule.
Using a fragment-based approach, different aromatic or heteroaromatic rings were selected to
ensure enough chemical diversity to either hinder or facilitate the hydrolysis. In addition, we chose
fragments that are present in compounds which have been reported active as anticancer agents by
our group [19] and the literature: thienyl [25
pyrazinyl [32], pyridyl [33], thiazolyl [34], benzo[b]thienyl [35], quinolyl and phenylquinolyl [36
,
26], isoxazolyl [27
,
28], furyl [29], chromonyl [30
,
31],
38
–
]
or acridinyl [39,40] as well as substituted aromatic rings (Figure 1).
Figure 1. Fragment-based design of the novel methylselenoesters.

Molecules 2017, 22, 1288
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Fifteen new compounds were synthesized and their ability to release CH3SeH in a modulated
way was investigated. In addition, given the redox-modulating properties of Se compounds and based
on previous antioxidant results of our work [24,41], the radical scavenging activity was also evaluated.
All the compounds were screened against a panel of five cancer cell lines and two non-malignant
cell lines. Moreover, two compounds were chosen to further perform mechanistic studies, including
modulation of cell cycle, cell death evaluation and interaction with redox active enzymes.
2. Results and Discussion
2.1. Chemistry
Based on our previous research, we designed a new series of methylselenoesters. The selenoester
moiety was chosen to facilitate a nucleophilic attack (i.e., water) resulting in the release of the
key metabolite CH3SeH. Diverse active aromatic and heteroaromatic fragments were linked to the
methylselenoester in order to provide different hydrolysis modulation, by hindering or facilitating the
reaction. Bifunctionalized molecules bearing two methylselenoester groups were also synthesized to
analyze if the cytotoxicity could be enhanced.
A novel series of methylselenoesters was synthesized according to previously reported synthetic
routes with few modifications [24
,
42
,43]. The reaction of selenium powder and sodium borohydride
in water (for compounds
4
,
6
,
8
,
9
,
11
,
14 and 15) or ethanol (for compounds 10 12 and 13
1
–
3
,
5
,
7
,
,
)
yielded sodium hydrogen selenide, which acts as a nucleophile. Substitution of the corresponding acyl
chloride led to the sodium aroyl or heteroaroyl selenide, which was further methylated by an excess of
methyl iodide (Scheme 1). Structures of the newly synthesized compounds are shown in Table 1.
Scheme 1. Synthesis of the methylselenoesters. Synthetic procedure and proposed reaction mechanism
for the formation of the methylselenoesters.
Even though acyl chlorides are usually hydrolyzed in aqueous solvents, the compounds might
be formed in water due to the fact that NaHSe is a superior nucleophile compared to water and
reacts faster. Compounds 1–3, 5, 7, 10, 12 and 13, however, could not be obtained in these conditions.
Changing the solvent to absolute ethanol not only improved the solubility of the acyl chlorides,
but enabled the formation of these compounds. For example, hydrolysis of extremely reactive acyl
chlorides before reacting with NaHSe (i.e., the 4-nitrophenyl derivative) might be avoided in absolute
ethanol. In addition, ethanol is less polar than water resulting in enhanced nucleophility of NaHSe,
thus facilitating the nucleophilic substitution in more deactivated molecules, for instance in those with
electron donors (i.e., the 4-methoxyphenyl derivative).
The purity of the newly synthesized compounds was assessed by thin layer chromatography
(TLC) and elemental analysis. The compounds were purified mainly by recrystallization in different
solvents or by column chromatography using methylene chloride as the eluent. Compounds 3, 8, 10
and 12 presented a troublesome purification with poor yields (<20%).
Structures were confirmed by infrared spectroscopy (IR), 1H-NMR, 13C-NMR and mass
spectrometry (MS). Regarding IR, a characteristic strong peak corresponding to the carbonyl group
appeared at 1620–1681 cm−1 whereas peaks corresponding to the methyl group (C–Haliph) appeared at

Molecules 2017, 22, 1288
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2918–2981 cm−1. In 1H-NMR, two microsatellites showing coupling of 77Se with 1H (JSe–H = 5.2–5.6)
were characteristic of the methyl group, whose chemical shift appeared at 2.28–2.64 ppm. MS data
revealed that the base peak in almost all the molecules was the ion resulting from a 95-weight fragment
loss, corresponding to CH3SeH. Interestingly, the base peak of the quinoline derivatives and 10 had
δ
9
also lost the carbonyl group, even though compound 10 had an almost equal abundant ion (96%)
which conserved it. For compound 13, the two ions with and without the carbonyl group coexisted at
almost the same abundance (100% and 92%). Compound 11 was the only molecule where the base
peak did not correspond to the loss of CH3SeH but to a more fragmented ion. Compound 15 showed
an interesting behavior. The base peak corresponded to the ion resulting from losing only one CH3SeH
group. This did not happen with the other bifunctionalized molecule, compound 14, whose more
stable ion had lost both CH3SeH moieties in addition to the cyano group.
Table 1. Structure of the novel synthesized methylselenoesters.
General Structure
O
SeCH3
R
a
Solvent b
Ref.
1
R
Ref.
8
R
Solvent
O
O
O2N
EtOH
H2O
H3CO
2
3
4
EtOH
EtOH
H2O
9
H2O
N
N
10
EtOH
N
S
N
O
S
Br
5
EtOH
11
H2O
Cl
O
N
S
O
N
N
6
7
H2O
12
13
EtOH
EtOH
EtOH
S
N
O
O
General structure
H3CSe
SeCH3
R
Ref.
R
Solvent
Ref.
15
R
Solvent
O
14
H2O
H2O
CN
The wavy bond indicates the attachment position for the methylselenoester moiety. a Compound reference; b Solvent
used as reaction medium.

Molecules 2017, 22, 1288
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2.2. Methylselenol Release Studies
The new compounds were designed as CH3SeH precursors. The chemical features of the
selenoester functional group facilitate a nucleophilic attack (i.e., with water) which would deliver the
CH3SeH residue. In order to verify if CH3SeH could be released from the molecules by hydrolysis and
if the rate differed among the compounds, the reaction of CH3SeH with 5,50-dithiobis(2-nitrobenzoic
acid) (DTNB or Ellman’s reagent) in aqueous environment was monitored [23,44]. Briefly, when the
disulfide bridge of DTNB reacts with thiols or selenols, 2-nitro-5-thiobenzoate (TNB2−) is released and
the mixed disulfide or selenylsulfide is formed. Each released CH3SeH produces one yellow TNB2−
species, whose absorbance can be quantified at 412 nm (Figure 2A).
Figure 2. CH3SeH release rates are dependent on the chemical features of the molecule. Released
CH3SeH was quantified through reaction with Ellman’s reagent (DTNB). (
CH3SeH and DTNB; (
) Quantification of TNB2− at different times. The compounds were tested at
L of 100 mM phosphate buffer (1 mM EDTA, pH = 8). DTNB concentration was also
A) Proposed reaction between
B
100
100
µ
M in 200 µ
µM. Absorbance was followed at 412 nm over 72 h. A cysteine standard curve (7.5–150 µM) was
used to calculate the concentration of TNB2− over time. Compounds showing less than 15% hydrolysis
at 72 h are not included, except for compound 5 as example. Error bars indicate SD of triplicates.
Results showed that the hydrolysis rate in these conditions varied among the compounds, as
intended in the design of the molecules (Figure 2B). A fast release was observed for compounds
and 11, which were completely hydrolyzed within the first two hours of incubation. CH3SeH was
completely released from compound at 24 h whereas a more sustained release corresponded, in this
order, to compounds 15 , and . However, the remaining compounds were hardly hydrolyzed (<15%)
6
3
,
1
9
at 72 h. Unfortunately, compound 12 could not be tested due to reproducibility issues caused by lack
of solubility in the assay conditions.
Results from this assay might not be predictive of the behavior of the compounds in other
biological matrixes due to different testing conditions. In addition, other mechanisms rather than
hydrolysis could lead to the release of methylselenol in the cell culture. However, we demonstrate that
the chemical features are diverse enough to differentially modulate the lability of the carbonyl-selenium
bond in this particular setting.
2.3. Theoretical Calculations of Molecular Properties
In order to provide a theoretical prediction of the drug-likeness properties of the new
methylselenoesters, the freely accessible Molinspiration and Osiris DataWarrior programs were employed.

Molecules 2017, 22, 1288
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2.3.1. Molinspiration Calculations
Molinspiration calculations predict large values of logP, fitting all the compounds the
recommended range of 0 logP 5 (Table 2). In fact, most of the derivatives present a logP value
≤
≤
around the mean value of this range, which can be considered as the optimal relation between
hydrophilicity and lypophilicity. Also, the theoretical values obtained are in accordance with the
expected fact that structures with 3 aromatic rings (compounds 10 and 13) should be the most lypophilic
derivatives, showing higher logP values.
Table 2. Theoretical calculations of molecular properties based in Molinspiration calculations.
Molinspiration Calculations
Compound milogP a
MW c
nOHNH e
Nrotb g
Volume
PSA b
nON d
nViolations f
1
2
3
4
5
6
7
8
9
2.39
2.48
0.96
2.33
2.93
1.15
3.63
2.32
2.41
4.55
3.33
2.23
4.19
2.60
2.41
0.60
62.90
26.30
42.85
17.07
17.07
43.10
17.07
35.54
29.96
29.96
47.28
42.85
29.96
57.93
47.28
37.30
244.11
229.14
201.09
205.14
239.59
190.09
255.20
243.12
250.16
326.26
346.04
283.21
300.22
345.12
310.07
127.00
4
2
3
1
1
3
1
3
2
2
3
3
2
3
3
2
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
3
3
2
2
2
2
2
2
2
3
2
3
2
4
4
0
166.41
168.62
134.77
133.79
147.33
120.49
177.78
167.01
182.91
254.32
205.51
196.89
226.91
218.97
183.68
68.78
10
11
12
13
14
15
MSA
a
e
b
c
d
Octanol/water partition coefficient; Molecular Polar Surface Area; Molecular weight; H-bond acceptors;
H-bond donors; f Number of violations of Lipinski’s Rule of Five; g Number of rotatable bonds.
Furthermore, polar surface area (PSA) values equal to or greater than 140 are expected to exhibit
poor intestinal absorption. All the values predicted are significatively below this threshold, pointing
towards excellent intestinal absorption (Table 2).
To summarize, each of the fifteen selenoesters obtained shows good to excellent values for the
calculated molecular descriptors and fulfills every facet of the Lipinski’s Rule of Five. This fact suggests
that these CH3SeH precursors may show bioavailability, metabolic stability and transport properties
comparable to known drugs.
2.3.2. Osiris DataWarrior Calculations
Toxicity risks predicted by using Osiris-based calculations are shown in Table 3. Nine out of the
fifteen compounds showed no toxicity for the four toxic parameters predicted. However, although the
drug-likeness scores are low for all of the compounds, we hypothesized that this might be due to the
presence of the Se atom, which is treated by the program as a hazardous element. Nevertheless, a wide
range for this drug-likeness score can be found among the Se compounds synthesized. Noteworthy,
thirteen compounds exhibit better drug-likeness values than the reference MSA. Compounds 12
, 4, 7,
15 and 5, in this order, present the five highest values predicted.

Molecules 2017, 22, 1288
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Table 3. Predicted toxicity of the compounds according to Osiris DataWarrior-based calculations.
Toxicity Level
Osiris Calculations
Compound
Mutagenic
Tumorigenic
Irritant
RE a
DL b
−10.80
−5.34
−5.25
−1.96
−2.77
−5.52
−2.67
−2.91
−5.25
−5.25
−4.86
−0.85
−5.25
−9.53
−2.76
−5.69
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
MSA
a
Reproductive effects; b Drug-likeness.
High risk;
Mild risk;
No risk.
2.4. Biological Studies
2.4.1. Radical Scavenging Activity of the New Methylselenoesters
Se compounds have a dual role as pro-oxidant and antioxidant, depending on the dose and the
chemical species. In fact, the chemopreventive activity of Se is believed to be due to its antioxidant
features through incorporation into selenoproteins. Aiming to have a first approach on the redox
properties, we analyzed the radical scavenging capability of the novel methylselenoesters. First, the
reduction of 1,1-diphenyl-2-picrylhydrazyl (DPPH) was measured. Compounds were tested at 100 µM
up to 3 h and ascorbic acid was included as a positive control. Except for compound 11, which showed
similar activity to ascorbic acid after 1 h incubation, none of the compounds demonstrated radical
scavenging properties (Figure 3A,B). To validate the antioxidant properties of compound 11, the
reduction of another radical, 2,2-azinobis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS+.), was also
tested. The radical scavenging activity of compound 11 towards this radical was similar to ascorbic
acid at 100
µM (Figure 3C) but at lower concentrations, ascorbic acid demonstrated greater antioxidant
properties than compound 11.
Figure 3. Cont.

Molecules 2017, 22, 1288
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Figure 3. Compound 11 shows radical scavenging activity against DPPH. and ABTS+. radicals.
) DPPH. radical scavenging activity of the methylselenoesters and ascorbic acid (Asc. acid) (100
M)
after 3 h incubation. Results represent the mean SEM of at least three independent assays performed
in triplicate; (
) DPPH. radical scavenging activity of compound 11 at different times. Results represent
the mean SEM of at least three independent assays performed in triplicate; (
) ABTS+. radical
(A
µ
±
B
±
C
scavenging activity of compound 11 at different concentrations after 6 min incubation. Results represent
the mean ± SD of quadruplicates. * p < 0.05, ** p < 0.01 with respect to ascorbic acid.
2.4.2. Cytotoxic Activity of the Novel Methylselenoesters
The novel compounds were tested against a panel of different cancer lines: PC-3 (prostate
adenocarcinoma), MCF7 (mammary adenocarcinoma), HTB-54 (lung carcinoma), K-562 (chronic
myelogenous leukemia) and HT-29 (colorectal adenocarcinoma). Two non-malignant cell lines,
BEAS-2B (bronchial epithelial) and 184B5 (mammary epithelial) were also included to evaluate the
toxicity of the novel synthesized compounds. The antiproliferative activity was measured with
the [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] (MTT) assay after 72 h treatment.
Six concentrations ranging from 0.1 to 100 µM were tested. MSA is largely known as the main
CH3SeH putative precursor and was therefore included as positive control. The following parameters
were calculated: GI50 (concentration which reduces the growth by 50%), TGI (concentration completely
inhibiting growth) and LC50 (concentration that kills 50% of the cells). Results are summarized in Table 4.
Table 4. Cytotoxic activity of the novel methylselenoesters at 72 h. Average GI50, TGI and LC50
values are expressed in µM.
±
SD
Cell Line
PC-3 a
TGI
184B5 c
TGI
MCF7 b
TGI
No.
GI50
LC50
GI50
LC50
GI50
LC50
1
2
5.8 ± 0.7
7.3 ± 1.7
5.4 ± 1.1
5.2 ± 0.9
8.6 ± 1.7
4.5 ± 1.3
18.4 ± 4.5
5.7 ± 1.6
5.9 ± 0.5
4.4 ± 1.3
5.3 ± 1.1
16.9 ± 4.1
39.7 ± 5.1
7.2 ± 3.5
4.8 ± 0.8
4.4 ± 1.1
10.4 ± 2.3
45.1 ± 4.5
15.3 ± 1.7
19.4 ± 3.6
47.7 ± 2.9
7.7 ± 0.1
47.8 ± 3.9
11.0 ± 3.1
10.0 ± 1.9
9.9 ± 4.7
17.1 ± 5.0
50.9 ± 4.9
>100
75.7 ± 3.3
4.1 ± 0.5
4.1 ± 0.8
3.5 ± 0.8
5.3 ± 1.4
3.1 ± 0.9
3.5 ± 1.0
6.9 ± 1.4
4.6 ± 0.5
4.4 ± 0.8
5.6 ± 1.3
4.2 ± 0.7
4.4 ± 1.0
4.2 ± 1.8
4.9 ± 0.8
1.8 ± 0.9
1.5 ± 0.5
7.9 ± 0.9
9.3 ± 4.1
7.2 ± 1.0
22.8 ± 3.8
6.9 ± 1.1
8.8 ± 0.7
61.6 ± 4.1
9.2 ± 1.3
8.1 ± 1.2
51.3 ± 3.7
8.2 ± 1.1
8.1 ± 1.7
35.4 ± 4.7
8.4 ± 1.1
7.2 ± 1.9
5.4 ± 0.5
45.0 ± 4.6
74.8 ± 4.0
40.1 ± 4.4
89.6 ± 4.1
35.9 ± 4.4
71.9 ± 3.2
>100
3.4 ± 0.4
3.7 ± 0.8
3.1 ± 0.5
3.6 ± 0.8
3.5 ± 0.4
2.7 ± 0.2
3.9 ± 1.2
3.7 ± 0.3
3.7 ± 0.5
3.9 ± 0.7
3.3 ± 0.1
3.4 ± 0.3
6.6 ± 1.2
3.8 ± 0.7
3.5 ± 1.1
5.3 ± 1.2
6.4 ± 1.1
6.4 ± 1.5
6.6 ± 0.9
7.2 ± 1.4
5.9 ± 0.3
6.0 ± 0.4
6.9 ± 1.7
6.4 ± 0.2
6.2 ± 0.4
6.3 ± 0.5
6.7 ± 0.9
6.5 ± 0.8
26.8 ± 1.0
6.5 ± 0.6
6.4 ± 0.5
9.2 ± 1.3
9.3 ± 0.6
9.0 ± 2.1
17.6 ± 4.4
27.9 ± 4.6
8.3 ± 0.2
9.3 ± 2.0
9.9 ± 0.8
9.0 ± 0.4
8.6 ± 0.4
8.7 ± 0.4
14.5 ± 4.5
9.6 ± 2.8
65.0 ± 4.5
9.3 ± 0.7
9.2 ± 0.9
13.1 ± 1.3
>100
3
60.5 ± 4.3
62.0 ± 4.9
91.6 ± 3.9
61.5 ± 14.0
77.3 ± 4.5
96.0 ± 4.7
84.1 ± 8.9
70.9 ± 5.2
66.2 ± 2.4
85.0 ± 4.7
>100
4
5
6
7
8
68.7 ± 2.5
48.0 ± 7.5
>100
9
10
11
12
13
14
15
MSA
>100
56.2 ± 9.6
83.4 ± 4.1
54.4 ± 2.7
57.0 ± 4.0
9.4 ± 1.0
32.3 ± 3.7
8.4 ± 0.6
8.5 ± 2.4
68.7 ± 3.0
47.8 ± 4.4
47.6 ± 4.6

Molecules 2017, 22, 1288
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Table 4. Cont.
Cell Line
HT-29 e
TGI
BEAS-2B g
TGI
K-562 d
TGI
HTB-54 f
TGI
No.
GI50
LC50
GI50
LC50
GI50
LC50
GI50
LC50
1
2
3
4
5
6
7
8
9
50.1 ± 2.3
40.8 ± 4.8
30.2 ± 2.2
40.6 ± 4.5
38.7 ± 2.1
18.5 ± 7.1
29.2 ± 4.4
29.7 ± 4.7
39.5 ± 3.1
9.7 ± 4.9
85.3 ± 4.9
70.6 ± 5.9
59.1 ± 2.1
74.6 ± 4.9
67.0 ± 1.9
58.3 ± 3.5
58.8 ± 1.5
62.6 ± 3.9
72.6 ± 4.1
59.1 ± 3.5
>100
>100
n.a.
54.7 ± 0.7
70.6 ± 3.4
n.a.
>100
>100
5.0 ± 0.7
4.1 ± 0.1
4.7 ± 0.6
3.4 ± 0.2
3.6 ± 0.3
5.0 ± 0.8
3.7 ± 0.2
5.9 ± 0.8
3.4 ± 0.2
4.1 ± 0.3
4.9 ± 0.7
5.1 ± 0.1
6.0 ± 0.5
3.8 ± 0.1
3.0 ± 0.5
n.a.
9.3 ± 1.4
7.5 ± 1.1
9.1 ± 0.8
7.2 ± 1.2
6.3 ± 0.2
15.1 ± 6.7
7.4 ± 0.9
27.7 ± 4.5
7.0 ± 0.2
7.5 ± 0.7
9.6 ± 3
87.2 ± 5.0
50.0 ± 2.4
>100
22.5 ± 4.5
6.8 ± 1.7
6.2 ± 1.5
8.5 ± 3.1
4.9 ± 0.2
9.0 ± 2.8
17.5 ± 4.2
6.0 ± 0.9
7.8 ± 1.2
5.9 ± 1.1
9.4 ± 3.5
9.8 ± 5.1
n.a.
50.5 ± 5.1
29.7 ± 4.3
21.1 ±4.1
39.0 ± 3.1
9.3 ± 2.7
43.9 ± 3.9
55.4 ± 4.4
23.2 ± 2.6
33.0 ± 4.5
16.7 ± 2.3
52.3 ± 2.9
44.9 ± 3.7
n.a.
78.6 ± 2.9
70.0 ± 2.9
71.1 ± 3.8
73.9 ± 4.1
61.5 ± 3.2
82.0 ± 1.3
93.4 ± 4.8
77.1 ± 4.9
69.2 ± 2.7
64.4 ± 4.2
>100
3.8 ± 0.3
3.2 ± 0.5
3.6 ± 0.6
2.9 ± 0.3
2.7 ± 0.5
3.0 ± 0.9
3.9 ± 0.6
3.1 ± 0.3
4.1 ± 1.0
3.1 ± 0.5
2.4 ± 0.9
3.7 ± 1.3
3.8 ± 0.1
3.2 ± 0.7
2.6 ± 0.8
3.7 ± 0.7
6.4 ± 0.4
5.8 ± 0.6
6.1 ± 0.5
5.8 ± 0.7
5.3 ± 0.7
5.6 ± 0.7
6.8 ± 0.8
5.6 ± 0.6
7.1 ± 4.5
5.5 ± 0.4
5.0 ± 0.8
6.4 ± 1.3
6.4 ± 0.2
5.5 ± 0.5
5.6 ± 0.9
6.2 ± 1.2
9.0 ± 0.7
8.4 ± 1.0
8.6 ± 0.5
8.7 ± 2.1
8.0 ± 1.0
8.2 ± 0.5
9.8 ± 1.1
8.0 ± 1.1
15.3 ± 9.1
7.8 ± 0.5
7.6 ± 0.7
9.2 ± 1.0
8.9 ± 0.4
7.8 ± 0.3
8.6 ± 1.2
8.7 ± 1.8
87.9 ± 2.0
>100
47.5 ± 3.2
8.9 ± 0.2
90.8 ± 15.0
41.8 ± 4.5
81.1 ± 4.0
37.2 ± 5.0
35.0 ± 2.4
74.4 ± 4.2
>100
95.2 ± 2.2
98.1 ± 0.7
88.3 ± 3.3
95.4 ± 4.9
>100
>100
>100
>100
n.a.
79.5 ± 0.7
99.2 ± 5.3
n.a.
10
11
12
13
14
15
MSA
81.9 ± 4.0
82.0 ± 4.4
14.2 ± 6.6
16.5 ± 1.9
7.9 ± 0.7
6.6 ± 0.4
n.a.
80.5 ± 3.5
h
66.6 ± 0.1
n.a.
n.a.
29.9 ± 0.6
42.0 ± 3.8
n.a.
>100
3.8 ± 0.6
15.6 ± 4.3
3.5 ± 0.1
7.7 ± 1.0
46.0 ± 3.0
6.9 ± 0.2
66.5 ± 4.5
76.3 ± 1.1
19.9 ± 1.2
35.8 ± 5.0
n.a.
Prostate adenocarcinoma; b Mammary adenocarcinoma; c Non-malignant mammary epithelium; d Chronic myelogenous leukemia; e Colorectal adenocarcinoma; f Lung carcinoma;
a
g
Non-malignant bronchial epithelial; h Data not available.

Molecules 2017, 22, 1288
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As shown in the table, every compound was able to inhibit proliferation, with compounds
11 and 14 presenting GI50 values below 10 M in PC-3, MCF7, HT-29 and HTB-54, and greater than
M in K-562, which was the most resistant cell line. GI50 values greater than 10 M were also
found for compounds 12 and 13 towards the prostate cancer cell line, and for compounds and
15 against the lung cancer cell line, indicating that a higher dose of these compounds was needed
to inhibit proliferation. When comparing compounds and , we found that the inclusion of an
electron-withdrawing atom in the thiophene ring (compound 5) improved the activity in MCF7 (LC50
= 89.6 M, LC50 = 35.9 M, respectively) and HT-29 (LC50 = 47.5 M, LC50 = 8.9 M, respectively)
2–6,
8
–
µ
10
µ
µ
7
,
1, 7
4
5
µ
µ
µ
µ
but not in PC-3 or HTB-54. However, the presence of two methylseleno moieties did not particularly
enhance potency, as neither compound 14 nor 15 had significantly lower GI50, TGI or LC50 values than
the monofunctionalized molecules. Regarding TGI, higher values were generally found in HTB-54 and
PC-3 cell lines than in MCF7 and HT-29. On the contrary, low TGI values (<10
µM) were found for
compounds and 14 in HTB-54 and for compounds 10, and 15 in PC-3. Finally, when considering
5
6,
the LC50 parameter, all the compounds exhibited similar moderate potencies. In fact, the compounds
were more cytostatic than cytotoxic due to high LC50 values (>30 µM).
The similar behavior of the compounds might support a shared mechanism of action for all of
them. This led us to consider CH3SeH as the ultimate effector of the biological activity. However,
we could not establish a clear correlation between CH3SeH release rates due to hydrolysis in the
Ellmans’s assay and biological activity at the tested time period. The different conditions of both
experiments could be an explanation for this fact. Besides, cell culture could modify the release rates:
in addition to hydrolysis, other factors might be taking place, such as cell uptake before the molecules
are hydrolyzed; cell metabolism leading to release of methylselenol; or concomitant reactions of the
molecules with medium components. In fact, to evaluate if the acidic residue contributed to the activity
in case of hydrolysis in the cell culture, we tested the corresponding acids. Given that CH3SeH has
been largely characterized in mammary carcinoma [45,46], we chose this cell line (MCF7) for further
studies. Although the residues had been selected according to an active fragment-based approach,
the acidic forms were not toxic at 72 h (GI50 > 100
toxic activity.
µM), thus supporting CH3SeH as the effector of the
Regarding the non-malignant cell lines, the positive control MSA was found to be toxic. Consistently,
the new CH3SeH precursors were also toxic for BEAS-2B and 184B5. Their toxicity, however, was in the
same range as MSA, which has been broadly studied both in vitro and in vivo [7,47].
Taking into account all the data, and on the basis of similarity to MSA kinetic parameters in both
tumor MCF7 and non-malignant 184B5 cell lines, compounds
profile. In addition, compounds and 15 did not violate Lipinski’s Rule of Five, presented good
drug-likeness parameters and were among the five best compounds according to Osiris DataWarrior
predictions. For these reasons, compounds and 15 were selected for further characterization of their
mechanism of action on the MCF7 cell line.
5 and 15 exhibited the most similar
5
5
2.4.3. Compounds 5 and 15 Lead to Cell Cycle Arrest in G2/M Phase
Cell cycle arrest is the target of many anticancer drugs, as it is the first step to decrease cell
proliferation. To gain a better understanding of the mechanism of these compounds, the effect of
compounds
concentrations of both compounds for 24 h or with 15
control was treated with vehicle, and camptothecin (6
5
and 15 on cell cycle distribution was analyzed. MCF7 cells were treated with increasing
or 30 15 for different times. The negative
M) was used as a positive control. Samples
µ
M
µ
5
µM
were stained with propidium iodide using the Apo-Direct kit (BD Pharmingen, Madrid, Spain) and
processed by flow cytometry.
As shown in Figure 4, treatment with compounds
G2/M phase. This increase was dose-dependent at 24 h and evident for compound 15 when cells
were treated with 7 M or higher concentrations whereas it was observed for compound only from
15 M treatment. In the time-course analysis, we found an increase in G2/M phase after 8 h treatment
5 and 15 increased the percentage of cells in
µ
5
µ

Molecules 2017, 22, 1288
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for both compounds that was correlated with a significant decrease in the G0/G1 phase at 8 h for
compound and 16 h for compound 15. We conclude that these results indicate a cell cycle blockage
in G2/M phase.
5
Figure 4. Compounds
treated with different concentrations of compound
time periods. Cells were stained with propidium iodide with the Apo-Direct kit and analyzed by
flow cytometry. ( ) Quantification of cell cycle distribution after 24 h treatment with increasing
concentrations of compound ; ( ) Representative experiment of A; ( ) Quantification of cell cycle
distribution after 24 h treatment with increasing concentrations of compound 15; ( ) Representative
experiment of C; ( ) Cell cycle distribution after treatment with vehicle, 15 M of compound or 30
SEM of at least three independent
5
and 15 induce cell cycle arrest in G2/M phase in MCF7. MCF7 cells were
5,
15 or vehicle (control) for 24 h or for different
A
5
B
C
D
E
µ
5
µM
of compound 15 for different times. Results represent the mean
experiments performed in duplicate. * p < 0.05, ** p < 0.01.
±

Molecules 2017, 22, 1288
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2.4.4. Evaluation of Cell Death Mechanism Induced by Compounds 5 and 15
Induction of cell death is the major aim of antitumor drugs. We analyzed cell death progression
using the TUNEL technique (in the Apo-Direct kit), which is based on DNA fragmentation. MCF7
cells were treated with increasing concentrations of compounds
5 and 15 for 24 h or with 30 µM of
compound 15 or 15 M of compound for different times. The negative control was treated with
µ
5
vehicle, and camptothecin (6 µM) was included as a positive control.
Both compounds exhibited a dose-dependent effect at 24 h (Figure 5A), but only compound 15
caused cell death in a time-dependent manner (Figure 5B). Cell death was evident from only 4 h
treatment for both compounds. However, whereas cell death induced by compound 15 rose from 20%
of dead cells at 4 h up to 80% at 48 h, the highest values of cell death for compound
after 4 h treatment.
5 were observed
Figure 5. Cell death induction is dose and time-dependent for compound 15 in MCF7. MCF7 cells were
treated with vehicle (control) or increasing doses of compound or 15 for 24 h. For the time-course
experiment, MCF7 cells were treated with vehicle (control), compound (15 M) or compound 15
(30 M) for different times. Camptothecin (6 M) was included as positive control. Cells were stained
with the Apo-Direct kit (TUNEL assay) and analyzed by flow cytometry. ( ) Dose-dependent cell
death at 24 h; ( ) Time-course analysis of cell death. Results show the mean SEM of at least three
independent experiments performed in duplicate. * p < 0.05, ** p < 0.01.
5
5
µ
µ
µ
A
B
±
Se compounds induce different types of cell death, among which apoptosis and autophagy are
the most common. To further elucidate the pathway through which compounds and 15 trigger
cell death, we used Z-VAD-FMK, a pan-caspase inhibitor and wortmannin, a PI3K inhibitor which
blocks autophagy. MCF7 cells were pre-incubated for 1 h with 50 M of Z-VAD-FMK or 100 nM
wortmannin and co-incubated with compound (25 M), compound 15 (30 M) or vehicle (control)
for 48 h. Treated cells without inhibitors were also included.
5
µ
5
µ
µ
As shown in Figure 6, cell death induced by the novel methylselenoesters was not altered in the
presence of wortmannin, ruling out autophagy as a mechanism of action. On the other hand, cell
death was partially prevented when cells were treated in the presence of the pan-caspase inhibitor.
Cell death induced by compound
5 was decreased by 36%, whereas the total number of dead cells was
decreased by 14% in case of compound 15. These results indicate that caspases are at least partially
implicated in the cell death mechanism of these compounds. In fact, CH3SeH has been described to
induce caspase-mediated cell death [47,48].

Molecules 2017, 22, 1288
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Figure 6. Cell death induced by compounds
pre-incubated with 50 M of Z-VAD-FMK or 100 nM wortmannin for 1 h before treating cells with
compound (25 M), compound 15 (30 M) or vehicle (control) for 48 h. Cells were stained with the
Apo-Direct kit (TUNEL assay) and analyzed by flow cytometry. ( ) Cell death induced by compound
in the presence and absence of the inhibitors; ( ) Cell death induced by compound 15 in the presence
5 and 15 involves the caspases pathway. MCF7 cells were
µ
5
µ
µ
A
5
B
and absence of the inhibitors. Results show the mean
performed in duplicate, ** p < 0.01, *** p < 0.001.
±
SEM of at least three independent experiments
2.4.5. Compounds 5 and 15 are Substrates for Thioredoxin Reductase But not for the Glutathione-
Glutaredoxin System
Metabolism is crucial for Se compounds, as the biological activity is mainly exerted through
their metabolites. It has been described that Se compounds can be metabolized by redox active
enzymes [49
,
50]. Thus, in an attempt to refine possible metabolic pathways, we examined whether
compounds
5
and 15 interacted with thioredoxin reductase (TrxR) and/or the gluthatione-glutaredoxin
(GSH-Grx) system. We used MSA as control, as it is known to be reduced by both GSH and TrxR [51
Our results indicated that, while MSA was consistently reduced by both TrxR1 and GSH in
accordance with previous studies, compounds and 15 were substrates only for TrxR1 (Figure 7).
,52].
5
The reduction by TrxR1 is considerably more efficient compared the spontaneous hydrolysis, indicating
that this would facilitate the release of the active CH3SeH metabolite intracellularly.
Figure 7.
glutathione-glutaredoxin system. (
MSA as control by thioredoxin reductase. The reaction mixture contained 100 nM TrxR1, 227
and the corresponding amount of compound in TE buffer (20 mM Tris, 2 mM EDTA pH = 8); (
Compounds
5
and 15 are substrates for thioredoxin reductase but not for the
) NADPH consumption indicating reduction of compounds 15 or
M NADPH
) NADPH
A
5,
µ
B
consumption indicating reduction of MSA by glutathione in the presence or absence of glutaredoxin.
The reaction mixture contained the corresponding amount of compound, 0.1 M Tris, 2 mM EDTA pH = 8,
0.1 mg/mL BSA, 1 mM GSH, 200
µM NADPH, 0.008 OD/mL yeast GR and 1 µM hGrx1 when required.
Only results for MSA are shown, as compounds 5 and 15 were not reduced.

Molecules 2017, 22, 1288
14 of 23
3. Material and Methods
3.1. General Information
Melting points (m.p.) were determined with a FP82 + FP80 apparatus (Mettler, Greifensee,
Switzerland). The NMR spectra (1H/400 MHz and 13C/100 MHz, Supplementary Materials) were
recorded on a UltrashieldTM 400 spectrometer (Bruker, Rheinstetten, Germany). The samples were
dissolved in DMSO-d6 or CDCl3 and TMS was used as internal standard. IR spectra were obtained
on a FT-IR Nexus spectrophotometer (Thermo Nicolet, Madison, WI, USA) using KBr pellets for
solids or NaCl plates for oil compounds. Elemental analysis was performed on a CHN-900 Elemental
Analyzer (LECO, Saint Joseph, MI, USA). HRMS were recorded using an Accurate-Mass TOF LC/MS
6220 (Agilent Technologies, Santa Clara, CA, USA). Only data from compounds 3, 9, 10, 12 and 13
could be obtained due to poor volatilization of the remaining compounds. For TLC assays, Alugram
SIL G7UV254 sheets (Macherey-Nagel; Düren, Germany) were used. Column chromatography was
performed with silica gel 60 (Merck, Darmstadt, Germany). Chemicals were purchased from E. Merck,
Panreac Química S.A. (Montcada i Reixac, Barcelona, Spain), Sigma-Aldrich Quimica, S.A. (Madrid,
Spain) and Acros Organics (Janssen Pharmaceuticalaan, Geel, Belgium).
3.2. Chemistry
3.2.1. General Procedure
Following a described procedure [24,42,43] with a few modifications, sodium borohydride was
slowly added to a suspension of selenium powder in water at room temperature or in ethanol, N2
atmosphere and 0 ◦C, and stirred until the formation of the typical colorless solution of NaHSe. Then,
the corresponding aroyl or heteroaroyl chloride was added. Temperature and time of reaction varied
depending on the compounds. Methylation was achieved through the addition of methyl iodide
(in excess). Purification was performed by several washings, recrystallization in different solvents or
column chromatography. In those cases where the acyl chloride was not available, it was formed by
the reaction of the corresponding carboxylic acid with SOCl2 for 1–8 h at reflux. Solvent was removed
under vacuum by rotatory evaporation, and the product was then washed three times with dry toluene,
which was also eliminated by rotatory evaporation.
3.2.2. General Procedure for Compounds 4, 6, 8, 9, 11, 14 and 15
Sodium borohydride (4.15 mmol) was added to a suspension of powdered selenium (2 mmol) in
water at room temperature. Discoloration to a colorless solution indicated the formation of sodium
hydrogen selenide. Then, a solution of the acyl or heteroaroyl chloride in chloroform was added
◦
(2 mmol) and the mixture was stirred at 50 C for 1 h, unless stated otherwise. For the bifunctionalized
molecules (compounds 14 and 15), 8.3 mmol of sodium borohydride, 4 mmol of powdered selenium
and 2 mmol of the acyl chloride were used. Reaction was followed by TLC or IR. After filtering
insoluble salts, an excess of methyl iodide (1.5 mL) was added and the reaction was heated at the same
temperature (1 h, unless stated otherwise) until precipitation of the product or discoloration of the
aqueous phase. The solid was filtered or extracted with methylene chloride, washed with slightly basic
water and dried over Na2SO4. The solvent was eliminated under rotatory evaporation. Compounds
were purified through recrystallization in appropriate solvents or column chromatography using
methylene chloride as eluent (mobile phase).
Methyl 2-thiophencarboselenoate (
which was further purified by column chromatography. Yield: 26%. H-NMR (CDCl3):
4
). From 2-thiophencarbonyl chloride. A yellow oil was obtained
1
δ 2.42 (s, 3H,
–SeCH3), 7.15 (dd, 1H, H4, J4–3 = 3.9, J4–5 = 4.9 Hz), 7.68 (dd, 1H, H5, J5–3 = 1.2 Hz, J5–4 = 4.9 Hz), 7.82
ppm (dd, 1H, H3, J3–4 = 3.9, J3–5 = 1.2 Hz). 13C-NMR (CDCl3):
5.7 (–SeCH3), 128.3 (C4), 131.8 (C5),
133.3 (C3), 144.3 (C2), 185.5 ppm (–C=O). IR (KBr):
3102 (w, C–Harom), 2932 (C–Haliph), 1662 cm−1
δ
ν

Molecules 2017, 22, 1288
15 of 23
(–C=O). MS [m/z (% abundance)]: 206 (M+• +1, 6), 111 (100), 83 (11). Elemental analysis calculated for
C6H6OSSe (%): C: 35.13, H: 2.95; found: C: 35.20, H: 2.77.
Methyl 5-isoxazolecarboselenoate (
6
). From 5-isoxazolecarbonyl chloride. A brownish solid was obtained.
Yield: 49%; m.p.: 41–42 C. 1H-NMR (DMSO-d6):
2.42 (s, 3H, –SeCH3), 7.40 (d, 1H, H4, 4–3 = 2.0 Hz),
8.92 ppm (d, 1H, H3, J3–4 = 2.0 Hz). 13C-NMR (DMSO-d6):
6.0 (–SeCH3), 107.0 (C4), 153.3 (C5),
165.8 (C3), 183.2 ppm (–C=O). IR (KBr): 3158–3101 (w, C–Harom), 2931 (w, C–Haliph), 1693 (s, –CN),
◦
δ
J
δ
ν
1649 cm−1 (s, –C=O). MS [m/z (% abundance)]: 191 (M+• +1, 15), 96 (100), 68 (63) 113 (46), 40 (57).
Elemental analysis calculated for C5H5NO2Se (%): C: 31.60, H: 2.65, N: 7.37; found: C: 31.45, H: 3.03,
N: 7.27.
Methyl 1,3-benzodioxole-5-carboselenoate (
was obtained, which was purified through recrystallization from diethyl ether:hexane (1:1). Yield:
15%; m.p.: 48–49 ◦C. 1H-NMR (CDCl3):
2.36 (s, 3H, –SeCH3), 6.05 (s, 2H, –CH2–), 6.84 (d, 1H, H7,
7–6 = 8.2 Hz), 7.35 (s, 1H, H4), 7.55 ppm (d, 1H, H6, J6–7 = 8.2 Hz). 13C-NMR (CDCl3):
4.9 (–SeCH3),
101.7 (C2), 106.5 (C7), 107.8 (C4), 123.2 (C6), 133.2 (C5), 147.8 (C3a), 151.8 (C7a), 192.3 ppm (–C=O). IR
(KBr):
2918 (w, C–Haliph), 1655 (s, –C=O), 1036 (s, C–O–C sym) 925 cm−1 (s, C–O–C as). MS [m/z
8). From 1,3-benzodioxole-5-carboxylic acid. A yellow solid
δ
J
δ
ν
(% abundance)]: 149 (100), 121 (35), 65 (28), 91 (10). Elemental analysis calculated for C9H8O3Se (%):
C: 44.46, H: 3.32; found: C: 44.17, H: 3.91.
Methyl 3-quinolinecarboselenoate (
9
). From 3-quinolinecarboxylic acid. Conditions: 15 min reaction with
2.48
methyl iodide. A brown solid was obtained. Yield: 43%; m.p.: 74–75 ◦C. 1H-NMR (CDCl3):
δ
(s, 3H, –SeCH3), 7.66 (dd, 1H, H6, J6–5 = J6–7 = 7.5 Hz), 7.87 (dd, 1H, H7, J7–6 = 7.5 Hz, J7–8 = 7.7 Hz),
7.99 (d, 1H, H5, J5–6 = 7.5 Hz), 8.21 (d, 1H, H8, J8–7 = 7.7 Hz), 8.72 (s, 1H, H4), 9.35 ppm (s, 1H, H2).
13C-NMR (CDCl3): δ 6.0 (–SeCH3), 127.4 (C6), 128.5 (C4a), 129.5 (C3), 129.8 (C5), 132.0 (C8), 132.9 (C7),
136.9 (C4), 147.5 (C2), 149.9 (C8a), 193.5 ppm (–C=O). IR (KBr):
ν 3027 (w, C–Harom), 2955 (w, C–Haliph),
1671 (s, –C=O), 1616 cm−1 (s, –C=N). MS [m/z (% abundance)]: 156 (88), 128 (100), 101 (55), 75 (35).
Elemental analysis calculated for C11H9NOSe (%): C: 52.81, H: 3.63, N: 5.60; found: C: 53.01, H: 3.93,
N: 5.39. HRMS calculated for C11H10NOSe+ [M + H]+: 251.9922; found: 251.9978.
Methyl 6-bromochromone-2-carboselenoate (11). From 6-bromochromone-2-carboxylic acid. The solid was
◦
purified through column chromatography. A yellow solid was obtained. Yield: 27%; m.p.: 156–157 C.
1H-NMR (CDCl3):
δ
2.46 (s, 3H, –SeCH3), 6.93 (s, 1H, H3), 7.52 (d, 1H, H8, J8–7 = 8.9), 7.86 (dd, 1H, H7,
7–5 = 2.4, J7–8 = 8.9), 8.33–8.35 ppm (m, 1H, H5). 13C-NMR (CDCl3):
5.7 (–SeCH3), 101.0 (C3), 120.2
(C6), 120.9 (C8), 126.3 (C4a), 129.0 (C5), 138.2 (C7), 154.6 (C8a), 157.1 (C2), 177.2 (C4), 190.6 ppm (–C=O).
J
δ
IR (KBr):
ν
3039 (w, C–Harom), 2928 (w, C–Haliph), 1676 (s, –C=O), 1645 cm−1 (s, –COSe). MS [m/z (%
abundance)]: 346 (M+•, 30), 251 (69), 282 (24), 223 (36), 169 (100), 88 (41), 69 (57). Elemental analysis
calculated (%) for C11H7BrO3Se: C: 38.18, H: 2.04; found: C: 37.87, H: 2.21.
Dimethyl 5-cyano-1,3-benzenedicarboselenoate (14).
From 5-cyano-1,3-benzenedicarboxylic acid.
Conditions: 1.5 h reaction with NaHSe and 4 h reaction with methyl iodide. The solid was purified
◦
through column chromatography. A pink solid was obtained. Yield: 26%; m.p.: 167–168 C.
1H-NMR (CDCl3):
H2,
δ
2.51 (s, 6H, –SeCH3), 8.36 (d, 2H, H4 + H6,
J4–2 = J6–2 = 2.7), 8.57 ppm (d, 1H,
6.6 (–SeCH3), 114.7 (C5), 117.1 (–CN), 129.1 (C4), 134.5
J
2–4 = J2–6 = 2.7). 13C-NMR (CDCl3):
δ
(C2), 141.0 (C1 + C3), 192.9 ppm (–C=O). IR (KBr):
ν 3073 (w, C–Harom), 2935 (w, C–Haliph), 2229 (s,
–CN), 1670 cm−1 (s, –C=O). MS [m/z (% abundance)]: 252 (59), 129 (100), 101 (84). Elemental analysis
calculated for C11H9O2Se2N (%): C: 38.26, H: 2.61, N: 4.05; found: C: 38.36, H: 3.04, N: 3.98.
Dimethyl 2,5-furandicarboselenoate (15). From 2,5-furandicarboxylic acid. Conditions: 1.5 h reaction with
NaHSe and 3 h reaction with methyl iodide. The solid was purified through column chromatography.
A yellow solid was obtained. Yield: 30%; m.p.: 155–157 ◦C. 1H-NMR (CDCl3):
δ 2.43 (s, 6H, –SeCH3),
7.21 ppm (s, 2H, H3 + H4). 13C-NMR (CDCl3):
183.6 ppm (–C=O). IR (KBr):
δ
5.0 (–SeCH3), 115.0 (C3 + C4), 153.7 (C2 + C5),
−1
ν
2924 (w, C–Haliph), 1643 cm (s, –C=O). MS [m/z (% abundance)]: 312

Molecules 2017, 22, 1288
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(179, M+• +2), 217 (100), 189 (42), 133 (54), 94 (62), 66 (68). Elemental analysis calculated for C8H8O3Se2
(%): C: 30.97, H: 2.58; found: C: 31.09, H: 2.86.
3.2.3. General Procedure for Compounds 1–3, 5, 7, 10, 12 and 13
Under N2 atmosphere, absolute ethanol (10 mL) was added to a mixture of NaBH4 (2.15 mmol)
and selenium (2 mmol) cooled by an ice bath, with magnetic stirring. Although reaction of these
species occurs 1:1, a little excess of NaBH4 was added, due to the slow rate of decomposition of NaBH4
in this solvent reported by Klayman et al. [42] When the typical colorless solution of NaHSe was
achieved, the ice bath was removed and the following reactions were carried out at room temperature.
The acyl chloride was added and stirred for different amounts of time, depending on the reagents.
Reaction was followed by TLC or IR. Before adding an excess of methyl iodide (1.5 mL), the mixture
was filtered. After discoloration (20 min–1 h), the mixture was filtered, and ethanol was eliminated
with rotatory evaporation or the product was precipitated with water. Compounds were purified
through recrystallization from different solvents or column chromatography.
Methyl 4-nitrobenzoselenoate (
and 20 min reaction with methyl iodide. The compound was recrystallized from methylene chloride.
A yellow powder was obtained. Yield: 28%; m.p.: 78–79 ◦C. 1H-NMR (CDCl3):
2.48 (s, 3H, –SeCH3),
8.07 (d, 2H, H2 + H6, J2–3 = J6–5 = 8.9 Hz), 8.33 ppm (d, 2H, H3 + H5, J3–2 = J5–6 = 8.9 Hz). 13C-NMR
(CDCl3): 6.0 (–SeCH3), 124.1 (C3 + C5), 127.9 (C2 + C6), 143.3 (C1), 150.5 (C4), 193.7 ppm (–C=O). IR
(KBr)
1665 (s, –C=O), 1518, 1348, 850 cm−1 (–NO2 arom). MS [m/z (% abundance)]: 245 (15, M+• +1),
1). From 4-nitrobenzoyl chloride. Conditions: 10 min reaction with NaHSe
δ
δ
ν
150 (100), 120 (24), 104 (94), 92 (45), 76 (61). Elemental analysis calculated for C8H7NO3Se (%): C: 39.36,
H: 2.89, N: 5.74; found: C: 39.40, H: 3.17, N: 5.86.
Methyl 4-methoxybenzoselenoate (
2). From 4-methoxybenzoyl chloride. Conditions: 1 h reaction
with NaHSe and 20 min with methyl iodide. The compound was recrystallized from methylene
◦
1
chloride. A grayish powder was obtained. Yield: 23%; m.p.: 36–37 C. H-NMR (CDCl3):
(s, 3H, –SeCH3), 3.88 (s, 3H, –OCH3), 6.94 (d, 2H, H3 + H5, J3–2 = J5–6 = 8.9 Hz), 7.91 ppm (d, 2H,
H2 + H6, J2–3 = J6–5 = 8.9 Hz). 13C-NMR (CDCl3):
5.0 (–SeCH3), 55.6 (–OCH3), 114.0 (C3 + C5), 129.4
δ 2.38
δ
(
C2 + C6), 132−.01(C1), 164.0 (C4), 193.0 ppm (–C=O). IR (KBr):
ν 2933–2839 (w, C–Haliph), 1652 (–C=O),
1264–1025 cm (–OCH3). MS [m/z (% abundance)]: 230 (M+• +1, 4), 135 (100), 107 (14), 92 (38), 77 (39),
63 (25). Elemental analysis calculated for C9H10O2Se (%): C: 47.18, H: 4.40; found: C: 47.38, H: 4.63.
Methyl pyrazinecarboselenoate (3). From pyrazinecarboxylic acid. Conditions: 30 min reaction
with NaHSe and 30 min reaction with methyl iodide. The compound was recrystallized from
hexane:methylene chloride (1:1). A yellow solid was obtained. Yield: 14%; m.p.: 48–49 ◦C. 1H-NMR
(DMSO-d6):
9.03 ppm (d, 1H, H3, J3–5 = 1.5 Hz). 13C-NMR (DMSO-d6):
(C5), 150.9 (C3), 198.0 ppm (–C=O). IR (KBr):
δ
2.28 (s, 3H, –SeCH3), 8.84 (dd, 1H, H5, J5–6 = 2.4, J5–3 = 1.5 Hz), 9.01 (d, 1H, H6,
4.9 (–CH3), 140.0 (C2), 145.8 (C6), 147.2
2922 (w, C–Haliph), 1681 cm−1 (s, –C=O). MS [m/z (%
J6–5 = 2.4),
δ
ν
abundance)]: 202 (M+• +1, 37), 191 (45), 121 (61), 107 (100), 99 (95), 79 (80), 69 (61). Elemental analysis
calculated for C6H6N2OSe (%): C: 35.84, H: 3.01, N: 13.93; found: C: 36.02, H: 3.31, N: 13.61. HRMS
calculated for C6H7N2OSe+ [M + H]+ : 202.9718; found: 202.9778.
Methyl 3-chlorothiophen-2-carboselenoate (5). From 3-chlorothiophen-2-carboxylic acid. Conditions:
20 min reaction with NaHSe and 20 min reaction with methyl iodide. The compound was recrystallized
from methylene chloride. A brownish powder was obtained. Yield: 31%; m.p.: 35–36 ◦C. 1H-NMR
(DMSO-d6):
13C-NMR (DMSO-d6):
IR (KBr):
δ
2.35 (s, 3H, –SeCH3), 7.33 (d, 1H, H4, J4–5 = 5.45 Hz), 8.10 ppm (d, 1H, H5, J5–4 = 5.45 Hz).
δ
6.9 (–SeCH3), 127.9 (C4), 131.6C2), 134.7 (C5), 137.2 (C3), 184.3 ppm (–C=O).
ν
3096 (w, C–Harom), 2938 (w, C–Haliph), 1620 cm−1 (s, –C=O). MS [m/z (% abundance)]: 240
(M+• +1, 5), 211 (31), 145 (100), 43 (35). Elemental analysis calculated for C6H5ClOSSe (%): C: 30.08,
H: 2.10; found: C: 30.35, H: 2.22.

Molecules 2017, 22, 1288
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Methyl benzo[b]thiophene-2-carboselenoate (
7
). From benzo[b]thiophene-2-carboxylic acid. Conditions:
30 min reaction with NaHSe and 20 min reaction with methyl iodide. The compound was precipitated
◦
1
with water and recrystallized from methylene chloride. Yield: 35%; m.p.: 65–67 C. H-NMR
(DMSO-d6): 2.41 (s, 3H, –SeCH3), 7.50 (ddd, 1H, H6, J6–7 = 8.0, J6–4 = 0.7 Hz), 7.57 (ddd, 1H,
H5, J5–4 = 8.0, J5–7 = 1.2 Hz), 8.08 (d, 1H, H4, J4–6 = 0.7 Hz, J4–5 = 8.0), 8.09 (d, 1H, H7, J7–5 = 1.2 Hz),
8.41 ppm (s, 1H, H3). 13C-NMR (DMSO-d6):
6.3(–SeCH3), 124.1 (C7), 126.4 (C5), 127.4 (C4), 128.9 (C6),
δ
δ
130.6 (C3), 139.3 (C3a), 141.8 (C7a), 143.0 (C2), 187.6 ppm (–C=O). IR (KBr):
ν 3064 (w, C–Harom), 2925
(w, C–Haliph), 1657 cm−1 (–C=O). MS [m/z (% abundance)]: 256 (M+• +1, 6), 161 (100), 133 (42), 89 (65).
Elemental analysis calculated for C10H8OSSe (%): C: 47.06, H: 3.16; found: C: 47.32, H: 3.19.
Methyl 2-phenyl-4-quinolinecarboselenoate (10). From 2-phenyl-4-quinolinecarboxylic acid. Conditions:
45 minute reaction with NaHSe cooled in the ice bath and 1 h reaction with methyl iodide at
room temperature. The compound was precipitated with water and purified through column
chromatography using methylene chloride as eluent. A white solid was obtained. Yield: 11%; m.p.:
119–120 ◦C. 1H-NMR (CDCl3):
(t, 1H, H4’, J4’–3’ = J4’–5’ = 7.7 Hz), 8.23–8.25 (m, 2H, H6 + H7), 8.24 (s, 1H, H3), 8.33–8.41 ppm (m,
2H, H5 + H8). 13C-NMR (CDCl3):
7.3 (–SeCH3), 117.9 (C3), 121.4 (C6), 125.3 (C5), 128.2 (C2’ + C6’),
128.8 (C3’ + C5’), 129.5 (C4’), 129.8 (C4a), 130.7 (C8), 131.3 (C7), 138.2 (C1’), 146.3 (C4), 148.7 (C8a), 157.3
δ 2.57 (s, 3H, –SeCH3), 7.54–7.68 (m, 4H, H2’ + H3’ + H5’ + H6’), 7.84
δ
(C2), 197.1 ppm (–C=O). IR (KBr):
ν
3056 (w, C–Harom), 2922 (w, C–Haliph), 1685 cm−1 (s, –C=O). MS
[m/z (% abundance)]: 327 (M+• +1, 5), 232 (96), 204 (100), 75 (33). Elemental analysis calculated for
C17H13NOSe (%): C: 62.58, H: 4.02, N: 4.29; found: C: 62.58, H: 3.89, N: 4.28. HRMS calculated for
C17H14NOSe+ [M + H]+: 328.0235; found: 328.0251.
Methyl 2-(4-pyridyl)thiazole-4-carboselenoate (12).
From 2-(4-pyridyl)thiazole-4-carboxylic acid.
Conditions: 30 minute reaction with NaHSe cooled on an ice bath and 30 minute reaction with
methyl iodide at room temperature. The compound was recrystallized from methylene chloride
and diethyl ether (1:1). A white solid was obtained. Yield: 20%. 1H-NMR (CDCl3):
δ
2.40 (s, 3H,
–SeCH3), 8.00 (d, 2H, H2’ + H6’, J2’–3’ = J6’–5’ = 6.2 Hz), 8.20 (s, 1H, H5), 8.82 ppm (d, 2H, H3’ + H5’,
J3’–2’ = J5’–6’ = 6.2 Hz). 13C-NMR (CDCl3):
5.3 (–SeCH3), 121.4 (C2’ + C6’), 122.7 (C5), 141.1 (C1’), 149.6
(C3’ + C5’), 156.5 (C4), 164.9 (C2), 189.7 ppm (–C=O). IR (KBr): 3134 (w, C–Harom), 2919 (w, C–Haliph),
δ
ν
1665 (s, C=O), 1132 cm−1 (s, thiazole ring vibration). MS [m/z (% abundance)]: 284 (M+• +1, 21), 189
(84), 156 (100), 128 (77), 57 (91), 69 (59). Elemental analysis calculated for C10H8N2OSSe: C: 42.41, H:
2.85, N: 9.89; found: C: 42.14, H: 3.34, N: 9.84. HRMS calculated for C10H9N2OSSe+ [M + H]+: 284.9595;
found: 284.9578.
Methyl 9-acridinecarboselenoate (13). From 9-acridinecarboxylic acid. Before adding the chloride to the
reaction, it was dissolved in dry chloroform, treated with triethylamine (1:1) for 20 min to eliminate
the hydrochloride and then used for reaction without further treatment. Conditions: 25 min reaction
with NaHSe and 25 min reaction with methyl iodide. Th◦e compound was recrystallized from hexane.
An orange solid was obtained. Yield: 23%; m.p.: 132–134 C. 1H-NMR (CDCl3):
δ 2.64 (s, 3H, –SeCH3),
7.58–7.63 ppm (dd, 2H, H2 + H7, J2–3 = J7–6 = 7.7, J2–1 = J7–8 = 8.7), 7.79–7.85 (dd, 2H, H3 + H6,
J
J
3–2 = J6–7 = 7.7; J3–4 = J6–5 = 8.8), 8.09 (d, 2H, H1 + H8, J1–2 = J8–7 = 8.7), 8.31 ppm (d, 2H, H4 + H5,
4–3 = J5–6 = 8.8). 13C-NMR (CDCl3):
6.9 (–SeCH3), 120.2 (C8a + C9a), 120.2 (C1 + C8), 124.6 (C2 + C7),
2971–2925
δ
126.9 (C4 + C5), 129.2 (C3 + C6), 130.7 (C9), 148.1 (C4a + C10a), 198.4 ppm (–C=O). IR (KBr):
ν
(w, C–Haliph), 1673 cm−1 (s, –C=O). MS [m/z (% abundance)]: 301 (M+• +1, 4), 206 (100), 178 (93), 151
(31). Elemental analysis calculated for C15H11NOSe (%): C: 60.01, H: 3.69; N: 4.67; found: C: 59.83,
H: 3.97, N: 4.38. HRMS calculated for C15H12NOSe+ [M + H]+: 302.0078; found: 302.0091.
3.3. Methylselenol Release
A stock solution of the compounds was prepared in anhydrous DMSO (10 mM). The compounds
were placed in a 96-well plate in a final concentration of 100
µM. Reaction started after adding 200
µL
of an aqueous 100 mM Na2HPO4 buffer (1 mM EDTA, pH = 8), containing 100
µM of DTNB from a

Molecules 2017, 22, 1288
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freshly prepared 10 mM ethanolic stock, which were added just before starting the reaction. The DTNB
concentration was quantified with a cysteine standard curve (7.5–100 M). Blanks for the compounds
µ
in the absence of DTNB were also included as well as a blank to measure the slow but spontaneous
hydrolysis of DTNB, which was further subtracted from all the values. Absorbance was read at 412 nm
in a FLUOstar Omega microplate reader (BMG LabTech, Ortenberg, Germany). Results represent
means of triplicate values ± SD.
3.4. Theoretical Calculations of Molecular Properties
The drug-likeness score along with the TPSA values and the properties described in Lipinski’s
Rule of Five [molecular weight (MW)
≤
500 Da, logP
≤
5, H-bond donors (nOHNH)
≤
5 and H-bond
acceptors (nON) 10] were calculated using the freely download version of Osiris DataWarrior
≤
v.4.5.2 [53] and the online available Molinspiration [54] property calculation programs, respectively.
Likewise, the toxicity risks (mutagenic, tumorigenic, irritant and reproductive effects) were obtained
by Osiris DataWarrior program and are labeled in different colours (green for no risk, orange for mild
risk and red for high risk).
3.5. Biology
3.5.1. Radical Scavenging Assays
DPPH assay
The assay was performed following a described protocol [41], with few modifications. Briefly, a
stock solution of DPPH· (2,2-diphenyl-1-picrylhydrazyl, Sigma Aldrich, Madrid, Spain) in methanol
(0.04 mg/mL) was freshly prepared. Absorbance of the stock solution was adjusted at 0.8
at 516 nm for each experiment. Due to solubility issues in methanol, a stock solution of each
compound (10 mM) was prepared in DMSO. Compounds were tested at a final concentration of
±
0.02
100
µM. The negative control contained the same amount of DMSO to avoid interferences. The reaction
was incubated in a 2 mL microtube in the dark and 300
µL were seeded in a 96-well plate at the
determined times, to avoid erratic measures due to methanol evaporation if incubated all the time in
the plate. Discolouration to the yellowish reduced form was followed at 516 nm in a FLUOstar Omega
(BMG LabTech) plate reader. Ascorbic acid was used as a positive control.
Percentage of scavenged DPPH· was calculated as follows:
Acontrol − Asample
% o f scavenged DPPH· = 100 ×
Acontrol
with Acontrol being the absorbance of the negative control (only DMSO) and Asample the absorbance of
each tested compound. The experiment was performed three times in quadruplicate.
ABTS assay
ABTS assay was performed with a colorimetric assay [55]. Briefly, ABTS was dissolved (1 mg/mL)
in a 2.45 mM potassium persulfate solution and kept overnight in the dark at room temperature
to generate the radical ABTS+·. For the reaction, the mixture was diluted with 50% ethanol to an
absorbance of 0.700
concentrations. The negative control had the same amount of DMSO. Ascorbic acid was used as a
positive control. The reaction was started after adding 180 L of the solution. Absorbance was read
±
0.02 at 741 nm. The compounds were dissolved in DMSO and tested at different
µ
with a FLUOstar Omega (BMG LabTech) plate reader after 6 min incubation in the dark. The percentage
of scavenged ABTS+· was calculated with the following formula:
Acontrol − Asample
% o f scavenged ABTS+· = 100 ×
Acontrol

Molecules 2017, 22, 1288
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where Acontrol corresponds to the absorbance of the negative control (DMSO only) and Asample is the
absorbance of the tested compounds. The experiment was performed in quadruplicate.
3.5.2. Cell Culture
All cells were purchased from the American Type Culture Collection (ATCC, Barcelona,
Spain). HT-29 (colorectal adenocarcinoma), HTB-54 (grade III lung carcinoma), MCF7 (mammary
adenocarcinoma), PC-3 (grade IV prostate adenocarcinoma) and K-562 (chronic myelogenous leukemia)
were cultured in RPMI (Gibco, Madrid, Spain), 10% FBS (Gibco), 100 units/mL penicillin and
100
(Gibco), 10% FBS, 100 units/mL penicillin and 100
gland) cell line was cultured in DMEM:F12 supplemented with 5% FBS, 1
Spain), 100 nM hydrocortisone (Sigma-Aldrich), 2 mM sodium pyruvate (Lonza), 20 ng/mL EGF
µ
g/mL streptomycin (Gibco). BEAS-2B cell line (normal epithelial lung) was cultured in DMEM
g/mL streptomycin. 184B5 (normal mammary
ITS (Lonza, Barcelona,
µ
×
(Sigma-Aldrich), 0.3 nM trans-retinoic acid (Sigma-Aldrich), 100 units/mL penicillin and 100 µg/mL
streptomycin. Cells were cultured at 37 ◦C under 5% CO2.
3.5.3. Viability Assay
Cell viability after treatment was assessed by the well-known 3-(4,5-dimethyl-thiazol-2-yl)-2,5-
diphenyltetrazolium bromide (MTT, Sigma-Aldrich) assay [21]. Briefly, 10,000 cells were seeded in
96-well plates, treated and incubated for 72 h. Dilutions of the compounds in cell medium were freshly
prepared from a 0.01 M stock in DMSO. After treatment, 50
µL of MTT solution in PBS (2 mg/mL)
were added and cells were incubated for 4 h. Medium was removed and 150
µL of DMSO was added
to dissolve the formed formazan crystals. Absorbance was read at 550 nm in a microplate absorbance
reader (Sunrise, Tecan, Männedorf, Switzerland).
3.5.4. Cell Cycle and Cell Death Analysis
3
×
106 or 2
×
106 MCF7 cells were seeded in 25 cm2 flasks for treatments up to 24 h or 48 h,
respectively. DMSO (vehicle) or the dissolved compounds were added 24 h after seeding and cells
were incubated for different times at 37 ◦C under 5% CO2. Cell cycle and cell death were analyzed
simultaneously with the Apo-Direct kit (BD Pharmingen, Madrid, Spain), following the manufacturer’s
protocol. Briefly, cells were fixed in 1% paraformaldehyde for 45 min at 0 ◦C, washed with PBS and
incubated for at least 30 min in 70% ethanol on ice. Cells were then stained both with FITC-dUTP (1 h,
37 ◦C) and propidium iodide (30 min, room temperature) and analyzed by flow cytometry using a
Coulter Epics XL cytometer (Beckman Coulter, Brea, CA, USA).
3.5.5. Caspase and Autophagy Inhibitors Assay
2
×
106 MCF7 cells were seeded in 25 cm2 flasks. After 24 h, cells were pre-treated with 50
pan-caspase inhibitor Z-VAD-FMK (BD Pharmingen) or 100 nM wortmannin (Santa Cruz Biotechnology,
Heidelberg, Germany) for 1 h before co-incubating them with 25 M of compound , 30 M of
µM of the
µ
5
µ
compound 15 or vehicle (maximum amount of DMSO used in the treatments) for 48 h. Cells were
collected and processed with the Apo-Direct kit as described previously.
3.5.6. Enzymatic Assays
Thioredoxin Reductase Activity Assay
The assay was performed as previously described [56], but in a final volume of 200
µL in a
96-well plate. Reaction started after addition of 133 L of TE buffer (20 mM Tris, 2 mM EDTA, pH = 8),
µ
0.227 mM NADPH (freshly prepared) to different concentrations of compounds solved in DMSO
and TrxR1 (final concentration 100 nM). NADPH consumption was followed measuring absorbance
decrease at 340 nm in a VersaMax microplate reader (Molecular Devices, Sunnyvale, CA, USA) over

Molecules 2017, 22, 1288
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45 min. A background sample with DMSO without the compounds was included and subtracted from
the values.
Glutaredoxin/Glutathione Assay
The assay was performed as previously described [57], with minor modifications and adapted
for a 96-well plate. A mixture of 0.1 M Tris pH 8, 2 mM EDTA pH = 8, 0.1 mg/mL BSA, 1 mM
GSH, 200
Sigma-Aldrich). Reactions were performed with 1
when corresponding and different concentrations of the compounds. A 100
and the final volume was adjusted to 110 L per well. Consumption of NADPH was monitored at
µ
M NADPH, and 0.008 OD/mL yeast GR was prepared (all reagents were purchased from
M hGrx1 (IMCO Corporation, Stockholm, Sweden)
L mixture was added,
µ
µ
µ
A340 on a VersaMax microplate reader (Molecular Devices) for 45 min.
3.5.7. Statistics
The statistical analyses were performed with a Mann-Whitney test or a Wilcoxon test for the
inhibitors assay. Statistical significance was calculated using GraphPad Prism 6.01. (* p < 0.05,
** p < 0.01, *** p < 0.001).
4. Conclusions
A novel series of 15 methylselenoesters has been synthesized. Differences in the hydrolytic
release of CH3SeH in aqueous medium indicated that the chemical features of the core of the molecule
modulated the lability of the carbonyl-Se bond. In the preliminary assessment of redox properties, only
compound 11 presented radical scavenging activity. The compounds were able to inhibit proliferation
in different cancer cell lines and their toxicity towards non-malignant cell lines was in the same range
as MSA. Compounds
5 and 15 arrested cell cycle in G2/M phase in MCF7 cell line. Although both
compounds induced cell death in a dose-dependent manner, only compound 15 was time-dependent.
Pre-treatment of the cells with a pan-caspase inhibitor partially prevented cell death, suggesting that
the caspase pathway is implicated in their mechanism. Even though further research is needed, results
suggest that these compounds might be good precursors of CH3SeH, one of the key metabolites in Se
anticancerous activity and might be of great interest as potential antitumor agents.
Supplementary Materials: Supplementary materials are available online. 1H- and 13C-NMR of all the new
synthesized compounds.
Acknowledgments: The authors express their gratitude to the Plan de Investigación de la Universidad de Navarra,
PIUNA (Ref 2014 26), “la Caixa” and “CAN” Foundations for financial support for the project. The research
−
leading to these results has also received funding from “la Caixa” Banking Foundation and from the Asociación de
Amigos de la Universidad de Navarra, to whom Nuria Díaz-Argelich wishes to expresses her gratitude. We thank
Elena González-Peñas and Ángel Irigoyen for technical support with the HRMS experiments.
Author Contributions: Carmen Sanmartín, Ignacio Encío and Juan Antonio Palop conceived and designed the
experiments; Aristi P. Fernandes conceived and designed the enzyme kinetics experiment, contributed with
the material and analyzed the data; Nuria Díaz-Argelich performed the experiments; Daniel Plano performed
the theoretical calculations and wrote that section; Carmen Sanmartín, Ignacio Encío, Juan Antonio Palop and
Nuria Díaz-Argelich analyzed the data; Nuria Díaz-Argelich wrote the paper and all the authors read, commented
and approved the final manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds 1–15 reported in this paper are available from the authors.
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