In vitro assessment of antimicrobial, antioxidant, and cytotoxic properties of saccharin–tetrazolyl and –thiadiazolyl derivatives: The simple dependence of the ph value on antimicrobial activity

Article abstract of DOI:10.3390/ph12040167

The antimicrobial, antioxidant, and cytotoxic activities of a series of saccharin–tetrazolyl and –thiadiazolyl analogs were examined. The assessment of the antimicrobial properties of the referred-to molecules was completed through an evaluation of minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values against Gram-positive and Gram-negative bacteria and yeasts. Scrutiny of the MIC and MBC values of the compounds at pH 4.0, 7.0, and 9.0 against four Gram-positive strains revealed high values for both the MIC and MBC at pH 4.0 (ranging from 0.98 to 125 μg/mL) and moderate values at pH 7.0 and 9.0, exposing strong antimicrobial activities in an acidic medium. An antioxidant activity analysis of the molecules was performed by using the DPPH (2,2-diphenyl-1-picrylhydrazyl) method, which showed high activity for the TSMT (N-(1-methyl-2H-tetrazol-5-yl)-N-(1,1-dioxo-1,2-benzisothiazol-3-yl) amine, 7) derivative (90.29% compared to a butylated hydroxytoluene positive control of 61.96%). Besides, the general toxicity of the saccharin analogs was evaluated in an Artemia salina model, which displayed insignificant toxicity values. In turn, upon an assessment of cell viability, all of the compounds were found to be nontoxic in range concentrations of 0–100 μg/mL in H7PX glioma cells. The tested molecules have inspiring antimicrobial and antioxidant properties that represent potential core structures in the design of new drugs for the treatment of infectious diseases.

Full text of DOI:10.3390/ph12040167

pharmaceuticals
Article
In Vitro Assessment of Antimicrobial, Antioxidant,
and Cytotoxic Properties of Saccharin–Tetrazolyl and
–Thiadiazolyl Derivatives: The Simple Dependence
of the pH Value on Antimicrobial Activity
Luís M. T. Frija 1,*, Epole Ntungwe 2 , Przemysław Sitarek 3, Joana M. Andrade 2,
4
4
, Lília Cabral 5, M. Lurdes S. Cristiano 5, Patrícia Rijo 2,6,
*
´
Monika Toma , Tomasz Sliwin´ski
and Armando J. L. Pombeiro 1
1
Centro de Química Estrutural (CQE), Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais,
1049-001 Lisboa, Portugal; pombeiro@ist.utl.pt
2
CBIOS—Research Center for Health Sciences & Technologies, ULusófona de Humanidades e Tecnologias,
Campo Grande 376, 1749-024 Lisboa, Portugal; ntungweepolengolle@yahoo.com (E.N.);
p5319@ulusofona.pt (J.M.A.)
3
4
5
6
Department of Biology and Pharmaceutical Botany, Medical University of Lodz, Muszyn´skiego Street 1,
90-151 Łódz´, Poland; przemyslaw.sitarek@umed.lodz.pl
Laboratory of Medical Genetics, Faculty of Biology and Environmental Protection, University of Lodz,
´
90-151 Lodz, Poland; monika.toma@biol.uni.lodz.pl (M.T.); tomasz.sliwinski@biol.uni.lodz.pl (T.S.)
Department of Chemistry and Pharmacy (FCT) and Center of Marine Sciences (CCMar), Universidade do
Algarve, P-8005-039 Faro, Portugal; liliacabral80@gmail.com (L.C.); mcristi@ualg.pt (M.L.S.C.)
iMed.ULisboa - Research Institute for Medicines, Faculdade de Farmácia, Universidade de Lisboa, Av. Prof.
Gama Pinto, 1649-003 Lisboa, Portugal
*
Correspondence: luisfrija@tecnico.ulisboa.pt (L.M.T.F.); patricia.rijo@ulusofona.pt (P.R.)
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 4 October 2019; Accepted: 7 November 2019; Published: 12 November 2019
Abstract: The antimicrobial, antioxidant, and cytotoxic activities of a series of saccharin–tetrazolyl and
–thiadiazolyl analogs were examined. The assessment of the antimicrobial properties of the referred-to
molecules was completed through an evaluation of minimum inhibitory concentration (MIC) and
minimum bactericidal concentration (MBC) values against Gram-positive and Gram-negative bacteria
and yeasts. Scrutiny of the MIC and MBC values of the compounds at pH 4.0, 7.0, and 9.0 against
four Gram-positive strains revealed high values for both the MIC and MBC at pH 4.0 (ranging
from 0.98 to 125 µg/mL) and moderate values at pH 7.0 and 9.0, exposing strong antimicrobial
activities in an acidic medium. An antioxidant activity analysis of the molecules was performed by
using the DPPH (2,2-diphenyl-1-picrylhydrazyl) method, which showed high activity for the TSMT
(N-(1-methyl-2H-tetrazol-5-yl)-N-(1,1-dioxo-1,2-benzisothiazol-3-yl) amine, 7) derivative (90.29%
compared to a butylated hydroxytoluene positive control of 61.96%). Besides, the general toxicity of
the saccharin analogs was evaluated in an Artemia salina model, which displayed insignificant toxicity
values. In turn, upon an assessment of cell viability, all of the compounds were found to be nontoxic
in range concentrations of 0–100 µg/mL in H7PX glioma cells. The tested molecules have inspiring
antimicrobial and antioxidant properties that represent potential core structures in the design of new
drugs for the treatment of infectious diseases.
Keywords: saccharin; tetrazole; 1,3,4-thiadiazole; H7PX glioma cells; antimicrobial screening;
antioxidant capacity
Pharmaceuticals 2019, 12, 167; doi:10.3390/ph12040167
www.mdpi.com/journal/pharmaceuticals

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1. Introduction
In 1878, 1,2-benzisothiazole-3-one 1,1-dioxide (
1
, Figure 1), commercially known as
saccharin, was discovered accidentally by Fahlberg during an investigation of the oxidation of
o-toluenesulfonamide [1,2]: it was published by Remsen and Fahlberg one year later [ ]. For more
3
than a century, saccharin has been commonly used as a noncaloric artificial sweetener in the
form of its water-soluble salts (mainly sodium, ammonium, and calcium), and it is the principal
sweetening component of diabetic diets. For about three decades (since reports on carcinogenicity
in laboratory animals were published), the debate on its toxicity to humans has not reached
a consensus [
in vitro biological activity [
4
–
6
]. Numerous N-substituted derivatives of saccharin have been assessed for
10]. For example, first-row transition metal saccharinates as well as
7–
dioxovanadium(VI), dioxouranium(VI), and cerium(IV) saccharinates have been classified as protease
inhibitors, and several metal(II) saccharinates have displayed superoxide dismutase-like activity [11].
Besides, structure–activity relationship studies have shown that the saccharin scaffold is an effective
element for the development of inhibitors of human leukocyte clastase (HLE), cathepsin G (Cat G),
and proteinase 3 (PR3), as well as antimycobacterium and central nervous system agents [9,12–14].
Recently, different saccharin-based antagonists have been recognized for their interferon-signaling
pathways, showing antitumor activity through the inhibition of cancer-related isoforms in humans [15].
It should also be noted that the first non-benzoannelated 4-amino-2,3-dihydroisothiazole 1,1-dioxide,
which lacks a 3-oxo group, has been described and shows anti-HIV-1 activity. Additionally, saccharin
and isothiazolyl derivatives have been used in agriculture as herbicides, fungicides, and pesticides [16].
Figure 1. Structures of 1,2-benzisothiazole-3-one 1,1-dioxide (1, saccharin), 1H-tetrazole (2), and
1,3,4-thiadiazole (3).
Tetrazole (CN4H2) and its derivatives have attracted much attention as well due to their practical
applications. The tetrazolic acid fragment –CN4H has acidity similar to the carboxylic acid group –CO2H
and is almost allosteric with it, but it is metabolically more stable at the physiologic pH [17]. Hence,
synthetic methodologies leading to the replacement of –CO2H groups by –CN4H groups in biologically
active molecules are of major relevance [18]. Indeed, the number of patent claims and publications
related to medicinal uses of tetrazolyl derivatives continues to grow rapidly and cover a wide range
of applications: tetrazoles have been found, for instance, in compounds with antihypertensive,
antiasthmatic, antitubercular, antimalarial, and antibiotic activity [19
–22]. Several tetrazole derivatives
have shown potential as anticonvulsants and anticancer and anti-HIV-1 drugs [23
–
25]. Tetrazoles have
also had important applications in agriculture as plant growth regulators, herbicides, fungicides [26],
and stabilizers in photography and photoimaging [27]. Due to the high enthalpy of formation, tetrazole
decomposition results in the liberation of two nitrogen molecules and a significant amount of energy.
Therefore, several tetrazole derivatives have been explored as explosives, propellant components for
missiles, and gas generators for airbags (applicable to the automobile industry) [28]. In addition, various
tetrazole-based compounds have good coordination properties and are able to form stable complexes
with several metal ions. This ability is successfully used in analytical chemistry for the removal of heavy
metal ions from liquids and in chemical systems formulated for metal protection against corrosion [29].
Many physical, chemical, physicochemical, and biological properties of tetrazoles are closely related
to their ability to behave as acids and bases. In the tetrazole ring, the four nitrogen atoms connected

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in succession are able to be involved in proteolytic processes. This heterocyclic system is unusual in
structure and unique in terms of acid–base characteristics.
In line with what is mentioned above for saccharin and tetrazole derivatives, the 1,3,4-thiadiazole
scaffold represents an important class of core structures that are of great interest mainly because
of their various biological activities and respective therapeutic applications. The 1,3,4-thiadiazole
ring is a very weak base, possesses relatively high aromaticity, and is moderately stable in aqueous
acid solutions although it is vulnerable to ring cleavage with an aqueous base [30]. Besides, this
heterocyclic ring is very electron-deficient due to the electron-withdrawing effect of the nitrogen
atoms and is relatively inert toward electrophilic substitution but susceptible to nucleophilic attack,
whereas when substitutions are introduced into the 20 or 50 position of this ring, it is highly
activated and readily reacts to produce varied derivatives [31]. To some extent, these specific
properties lead to the application of 1,3,4-thiadiazole derivatives in pharmaceutical, agricultural,
and material chemistry. Therefore, several 1,3,4-thiadiazole-based compounds display a broad
spectrum of biological activities, such as antimicrobial [32], antituberculosis [33], antioxidant [34],
anti-inflammatory [35], anticonvulsant [36], antidepressant, anxiolytic [37], antihypertensive [38],
anticancer [39], and antifungal activity [40]. The most prominent thiadiazole derivative is possibly the
acetazolamide [N-(5-sulfamoyl-1,3,4-thiadiazol-2-yl)acetamide], a very well-known carbonic anhydrase
inhibitor that is used in the treatment of glaucoma [41], high-altitude illness [42], epileptic seizures [43],
idiopathic intracranial hypertension [44], hemiplegic migraine [45], cystinuria [46], obstructive sleep
apnea [47], and congenital myasthenic syndromes [48].
The search for new molecular entities, whether natural or synthetic, with relevant pharmacological
activities for effective applications in medical practices continues to be a hot topic in health science as
well as in science as a whole. This permanent search is well documented in the immense scientific
literature and is mainly supported by the fact that many pathogenic organisms are able to develop
mechanisms of resistance to high-activity medicines in their early lives [49–54]. In this context,
and given our prior interest in the synthesis, reactivity, and bioactivity of tetrazole and thiazole
derivatives, our collaborative interest was piqued by the great potential of these heterocycles for
medical applications. Herein, we report on the antimicrobial, antioxidant, and cytotoxic activities of
four mixed-azole compounds of benzisothiazole–tetrazolyl and benzisothiazole–thiadiazolyl.
2. Results and Discussion
2.1. Chemistry
The 3-chloro-1,2-benzisothiazole 1,1-dioxide (
synthesis of the studied molecules, was first prepared through the halogenation of saccharin (
previously described (Scheme 1 (A)) [55]. The three saccharin–tetrazolyl analogs (TS ( ), 2MTS (
and TSMT ( )) were synthetized through a combination of the amino-tetrazoles 4a, and 4b with
following a nucleophilic substitution reaction of the chloride anion by the amine functionality (Scheme 1
(B)). Similarly, compound (MTSB) was prepared by coupling and 5-methyl-1,3,4-thiadiazole-2-thiol
) (Scheme 1 (C)). All of the reactions proceeded smoothly under experimental protocols originally
developed by us [56–59], affording crystalline products in reasonable to very good yields.
2
), one of the strategic building blocks for the
), as
),
1
5
6
7
3
,
2,
9
2
(8

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Scheme 1. Synthesis of saccharin–tetrazolyl (TS, 2MTS, and TSMT) and saccharin–thiadiazolyl
(MTSB) derivatives.
2.2. Biological Assays
2.2.1. Antimicrobial Activity
The assessment of the antibacterial properties of compounds TSMT, MTSB, TS, and 2MTS was
determined through minimum inhibitory concentration (MIC) and minimum bactericidal concentration
(MBC) values against Gram-positive (Staphylococcus aureus and Enterococcus faecalis), Gram-negative
(Pseudomonas aeruginosa and Escherichia coli), and yeast (Saccharomyces cerevisiae and Candida albicans)
strains obtained from the American Type Culture Collection (ATCC) (Table 1). The MIC value
corresponding to the lowest concentration at which no visible growth was observed was assessed
by the microdilution method [60]. For MBC evaluation, the bacterial suspension in the wells was
homogenized, serially diluted, spread in triplicate on appropriate medium, and incubated at 37 ◦C. All
compounds and respective positive controls were tested at the same concentration of 500 µg/mL.

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Table 1. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC)
values of TSMT, MTSB, TS, and 2MTS (obtained through the microdilution method against Gram-positive
bacteria, Gram-negative bacteria, and yeast strains (in µM)).
E. faecalis
MIC MBC
S. aureus
MIC MBC
P. aeruginosa
E. coli
S. cerevisiae
C. albicans
Sample
MIC
MBC
MIC
MBC
MIC
MBC
MIC
MBC
TSMT
MTSB
TS
236.5 1892.1 118.3 1892.1 118.3
219.0 1752.1 109.5 1752.1 109.5
249.8 1998.1 124.9 1998.1 124.9
236.5 1892.1 118.3 1892.1 118.3
946.0
876.1
999.0
946.0
236.5
219.0
249.8
236.5
473.0
438.0
499.5
473.0
473.0
438.0
499.5
473.0
946.0
876.1
999.0
946.0
236.5 1892.1
219.0 1752.1
249.8 1998.1
236.5 1892.1
2MTS
Positivecontrol
1.95
VAN
nt
1.95
VAN
nt
0.977
NOR
nt
0.488
nt
15.6
nt
7.81
nt
NOR
NYS
NYS
VAN: vancomycin; NOR: norfloxacin; NYS: nystatin; nt: not tested. Data are the median values of at least
three replicates.
The results of the antimicrobial assay showed that the compounds tested were bacteriostatic.
This was concluded because the MBC values were much higher than the MIC values (see Table 1).
The compounds were more active against the Gram-negative P. aeruginosa and the Gram-positive S.
aureus bacteria. However, it seemed that the MIC and MBC values were more similar against E. coli.
Additionally, antimicrobial tests were performed at different pH values (pH 4.0, 7.0, and 9.0), also
using the microdilution method (see Tables 2–4). The microorganisms used in these tests were chosen
based on the initial results from the antimicrobial screening of Gram-positive bacteria and comprised
four Gram-positive strains, namely S. aureus CIP6538, E. faecalis ATCC 29212, methicillin-resistant S.
aureus CIP106760 (MRSA), and vancomycin-resistant E. faecalis ATCC51299 (VRE).
Table 2. MIC and MBC values of TSMT, MTSB, TS, and 2MTS (obtained through the microdilution
method against Gram-positive bacteria at pH 4.0).
E. faecalis ATCC51299
(VRE)
S. aureus CIP106760
(MRSA)
S. aureus CIP6538
E. faecalis 29212
Sample
MIC
MBC
MIC
MBC
MIC
MBC
MIC
MBC
TSMT
MTSB
TS
473.0
13.7
62.5
29.6
1.95
1892.1
109.5
499.5
236.5
nt
236.5
13.7
31.2
14.8
1.95
1892.1
109.5
249.8
118.3
nt
236.5
3.42
3.90
3.70
0.977
473.0
1.71
1.95
1.85
nt
236.5
3.42
7.80
473.0
0.488
1892.1
27.4
62.5
1892.1
nt
2MTS
Positive control
VAN
VAN
NOR
NOR
VAN: vancomycin; NOR: norfloxacin; NYS: nystatin; nt: not tested. Data are the median values of at least
three replicates.
Table 3. MIC and MBC values of TSMT, MTSB, TS, and 2MTS (obtained through the microdilution
method against Gram-positive bacteria at pH 7.0).
E. faecalis ATCC51299
(VRE)
S. aureus CIP106760
(MRSA)
S. aureus CIP6538
E. faecalis 29212
Sample
MIC
MBC
MIC
MBC
MIC
MBC
MIC
MBC
TSMT
MTSB
TS
473.0
438.0
499.5
473.0
1.95
1892.1
1752.1
1998.1
1892.1
nt
236.5
219.0
249.8
236.5
1.95
1892.1
1752.1
1998.1
1892.1
nt
473.0
438.0
499.5
473.0
0.977
1892.1
1752.1
1998.1
1892.1
nt
946.0
876.1
999.0
946.0
0.488
1892.1
1752.1
1998.1
1892.1
nt
2MTS
Positive control
VAN
VAN
NOR
NOR
VAN: vancomycin; NOR: norfloxacin; NYS: nystatin; nt: not tested. Data are the median values of at least
three replicates.

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Table 4. MIC and MBC values of TSMT, MTSB, TS, and 2MTS (obtained through the microdilution
method against Gram-positive bacteria at pH 9.0).
E. faecalis ATCC51299
(VRE)
S. aureus CIP106760
(MRSA)
S. aureus CIP6538
E. faecalis 29212
Sample
MIC
MBC
MIC
MBC
MIC
MBC
MIC
MBC
TSMT
MTSB
TS
946.0
438.0
499.5
473.0
1.95
1892.1
1752.1
1998.1
1892.1
nt
473.0
219.0
499.5
473.0
1.95
1892.1
1752.1
1998.1
1892.1
nt
473.0
438.0
999.0
473.0
0.977
1892.1
1752.1
1998.1
1892.1
nt
473.0
438.0
499.5
473.0
0.488
1892.1
1752.1
1998.1
1892.1
nt
2MTS
Positive control
VAN
VAN
NOR
NOR
VAN: vancomycin; NOR: norfloxacin; NYS: nystatin; nt: not tested. Data are the median values of at least
three replicates.
The results of the antimicrobial assays at the different pH values showed that lowering the medium
pH to 4.0 had a positive effect on the compounds MTSB, TS, and 2MTS against the methicillin-resistant
S. aureus CIP106760 (MRSA). Overall, it was attested that MTSB at pH 4.0 was the most active
derivative against all Gram-positive strains. It should be noted that MTSB was the sole compound
comprising the thiadiazole function, and presumably, the different activity must be correlated with
the type of heterocycle ring. The results for pH 7.0 and 9.0 did not provide better results than those
previously obtained.
In terms of pH-dependent antimicrobial mechanisms, it should be emphasized that several
antimicrobial peptides (AMPs) have increasingly been reported as potent antibiotics that utilize
pH-dependent antimicrobial mechanisms [61]. Some of these antibiotics display high pH optima
related to their antimicrobial activity and show activity against microbes that present low pH optima,
which reflects the acidic pH generally found at their sites of action, namely the skin. This effect should
be comparable to our compounds and could be the explanation for the high antimicrobial activity of
our compounds at low pH. Several pH-dependent AMPs and other antimicrobial proteins have been
developed for medical purposes and have successfully gone through clinical trials, namely kappacins,
LL-37, histatins, lactoferrin, and their derivatives. The major examples of the therapeutic applications
of these antimicrobial compounds include wound healing as well as the treatment of multiple infections.
Generally, such applications involve topical administration, a source of novel biologically active agents
that could aid in the fulfilment of the urgent need for alternatives to conventional antibiotics, helping
to avert a return to the pre-antibiotic era: our compounds could be a key development.
2.2.2. Antioxidant Activity
The antioxidant activity of the studied compounds was evaluated using a DPPH assay, which
evaluates the potential of test samples to quench DPPH radicals via hydrogen-donating ability. The
antioxidant agents convert DPPH into a stable diamagnetic molecule, 1-1diphenyl-2-picryl hydrazine,
through electron or hydrogen transfers. A color change from purple to yellow indicates the increasing
radical scavenging activity of the test compounds. Herein, it was observed that TSMT and MTSB
derivatives possessed a high free radical scavenging ability (see Figure 2). These two molecules had the
highest reducing power, indicating that they were good electron/hydrogen donors and could prevent
oxidative stress.

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100.00
90.29
*
80.00
60.00
40.00
20.00
0.00
61.96
42.05
*
**
9.28
**
4.73
BHT
TSMT
MTSB
2MTS
TS
Figure 2. Antioxidant activity tested at a concentration of 10
calculated from three independent experiments and compared to butylated hydroxytoluene (BHT)
µ
g/mL. The mean value
±
SD was
(∗ p < 0.05; ∗∗ p < 0.001).
2.2.3. Brine Shrimp Lethality Bioassay (General Toxicity)
The Artemia salina test is a known, simple, fast, and low-cost test and was used in this investigation
to check the general toxicity of the compounds. As a general rule, it was observed that all of the
compounds exhibited low toxicity in the Artemia salina model (Figure 3). Nevertheless, it should
be emphasized that the MTSB and TSMT derivatives presented with higher toxicity, making them
potential lead therapeutic agents, and thus they must further be tested using different cell- and
microorganism-based assays.
40.00
**
**
35.00
30.00
25.00
20.00
15.00
10.00
5.00
31.35
30.99
15.23
15.25
11.06
8.59
0.00
Salt
DMSO 1:100 2MTS
in salt Vial
TS
TSMT
MTSB
Figure 3. General toxicity screening at a concentration of 10 ppm using the Artemia salina test. The mean
value ± SD was calculated from three independent experiments and compared to salt (∗∗ p < 0.001).
2.2.4. Cell Viability after Treatment with MTSB, TS, TSMS, and 2MTS in H7PX Glioma Cells (IV Grade)
In the course of this study, H7PX cells were treated with a concentration range of 0–100 µg/mL
of the four compounds (MTSB, TS, TSMS, and 2MTS) over 24 h, after which the percentage of cell
viability was determined by an MTT assay. It was demonstrated that none of the tested compounds

Pharmaceuticals 2019, 12, 167
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reduced the viability of human H7PX cells in the stated range of concentration (0–100
µg/mL) after
24 h (Figure 4). None of the compounds’ IC50 values were reached. Similar effects were observed with
48 h of incubation (data not shown).
120
90
60
MTSB
0.39
TS
TSMT
6.25
2MTS
30
0
0
1.56
25
100
Compound concentration (µg/mL)
Figure 4. Effect of MTSB, TS, TSMT, and 2MTS treatment on the viability of H7PX glioma cells after
24 h of incubation (data are reported as means ± SD of three determinations).
3. Experimental Section
3.1. Chemistry
3.1.1. General
Unless indicated otherwise, solvents and starting materials were obtained from Sigma.
All chemicals used were of reagent grade without further purification before use. Column
chromatography was performed using silica gel 60 MN, and aluminum-backed silica gel Merck
60 F254 plates were used for analytical thin-layer chromatography (TLC). Melting points were recorded
and are uncorrected. 1H and 13C NMR spectra were recorded at room temperature on a Bruker
Avance II 400 (UltraShield
™
Magnet, Billerica, MA, USA) spectrometer operating at 400 MHz (1H) and
101 MHz (13C). The chemical shifts are reported in ppm using TMS (tetramethylsilane) as an internal
standard. Carbon, hydrogen, and nitrogen elemental analyses were carried out by the Microanalytical
Service of the Instituto Superior Té
cnico—University of Lisbon. FT-IR spectra (4000–400 cm–1) were
recorded on a VERTEX 70 (Bruker, Billerica, MA, USA) spectrometer using KBr pellets. Mass spectra
were obtained on a VG 7070E mass spectrometer through electron ionization (EI) at 70 eV.
3.1.2. Synthetic Protocols
The synthesis of 3-chloro-1,2-benzisothiazole-1,1-dioxide (
4a), 1-methyl-(1H)-tetrazole-5-amine (4b), N-(1H-tetrazol-5-yl)-N-(1,1-dioxo-1,2-benzisothiazol-3-yl)
amine ( ) (TS), N-(2-methyl-2H-tetrazol-5-yl)-N-(1,1-dioxo-1,2-benzisothiazol-3-yl) amine (
(2MTS), N-(1-methyl-2H-tetrazol-5-yl)-N-(1,1-dioxo-1,2-benzisothiazol-3-yl) amine ( ) (TSMT), and
) (MTSB) was carried out
2), 2-methyl-(2H)-tetrazole-5-amine
(
5
6)
7
3-[(5-methyl-1,3,4-thiadiazol-2-yl)sulfanyl]-1,2-benzisothiazole 1,1-dioxide (
as previously described [55,56,58,59].
9

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3.2. Biologic Activities
3.2.1. Antioxidant Activity (DPPH Method)
The antioxidant activity of all the compounds was measured by the DPPH method, as described by
Rijo et al. [62]. Accordingly, a mixture containing 10 L of sample and 990 L of DPPH solution (0.002%
µ
µ
in methanol) was incubated for 30 min at room temperature followed by absorbance measurements
at 517 nm against the corresponding blank sample. The antioxidant activity of each compound was
calculated using Equation (1). AA denotes the antioxidant activity, ADPPH is the absorption of DPPH
against the blank, and Asample represents the absorption of the compound or control against the blank.
All tests were carried out in triplicate at a sample concentration of 10 µg/mL. The reference standard
used for this procedure was butylated hydroxytoluene (BHT) in the same conditions as the samples:
ADPPH − ASample
AA =
× 100%,
(1)
ADPPH
3.2.2. Brine Shrimp Lethality Bioassay (General Toxicity)
The general toxicity of the compounds was evaluated by the use of a test of lethality to Artemia
salina (brine shrimp) [63]. Concentrations of 10 ppm of each sample were tested. The number of dead
larvae was recorded after 24 h and was used to calculate the lethal concentration (%) according to
Equation (2) (where TotalA. salina = the total number of larvae in the assay and AliveA. salina = the number
of alive A. salina larvae in the assay):
TotalA. salina − AliveA. salina
Letal concentration =
× 100%,
(2)
TotalA. salina
3.2.3. Cell Culture
An H7PX primary glioblastoma cell line was cultivated in DMEM (Biowest, Nuaoll
é, France)
supplemented with 10% FBS (Euroclone, Pero, Italy), 100 IU/mL penicillin (Sigma-Aldrich, Saint Louis,
MO, USA), 100 µg/mL streptomycin (Sigma-Aldrich, Saint Louis, MO, USA) and 50 µg/mL gentamicin
(Biowest, Nuaollé, France) in a humidified atmosphere (5% CO2, 37 ◦C).
3.2.4. In Vitro Cell Viability by MTT Assay
An MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) assay was employed to
measure the viability of H7PX cells (glioma cells in IV grade derived from patient) treated with different
concentrations of TS (
5
), 2MTS (
6
), TSMT (
7
), or MTSB (
9
). Cells were seeded at 1
×
104 cells per well in
96-well culture plates and were left overnight before treatments for attachment. Subsequently, the cells
were incubated for 24 h with all compounds over a range of concentrations: 0.0 (control), 0.39, 0.78,
1.56, 3.13, 6.25, 12.5, 25, 50, and 100
µ
g/mL. Following this, the cells were incubated with 0.5 mg/mL of
MTT at 37 ◦C for 1.5 h. After that, the MTT was carefully removed, and DMSO (100
µL) was added to
each well and vortexed at low speed for 5 min to fully dissolve the formazan crystals. Absorbance
was measured at 570 nm with a reference at 630 nm using a Bio-Tek Synergy HT Microplate Reader
(Bio-Tek Instruments, Winooski, VT, USA). All experiments were repeated in triplicate. Cell viability
was expressed as a percentage relative to the untreated cells, which was defined as 100%. The study
was approved by the Ethical Commission of the Medical University of Lodz, and informed consent
was obtained from the patients (Nr. RNN/194/12/KE).
3.2.5. Antimicrobial Activity
Microorganisms and growth conditions: The strains used in this study comprised Staphylococcus
aureus (ATCC 25,923 and CIP 106760), Enterococcus faecalis (ATCC 51,299 and ATCC29212), Escherichia
coli ATCC 25922, Pseudomonas aeruginosa ATCC 27,853, and the yeasts Candida albicans ATCC 10,231

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and Saccharomyces cerevisiae ATCC 2601. All bacteria were grown at 37 ◦C in Mueller–Hinton broth
(Biokar Diagnostics, Beauvais, France), and the yeasts were grown in Sabouraud dextrose agar (Biokar
Diagnostics, Allone, France).
Microdilution method (MIC determination): The minimum inhibitory concentrations (MICs) of
the compounds (dissolved in the respective pH aqueous solutions) were evaluated using a twofold
serial broth microdilution assay (CLSI, 2011) in Mueller–Hinton broth (MHB, Biokar Diagnostics,
Beauvais, France). Overnight cultures were diluted in MHB with increasing concentrations of each
compound (in µM). Vancomycin (VAN), norfloxacin (NOR), and nystatin (NYS) were used as positive
controls for Gram-positive bacteria, Gram-negative bacteria, and yeasts, respectively. The negative
control for the aqueous solutions at different pH values showed no inhibition growth. The cultures
were incubated for 24 h at 37 ◦C, and Optical Density at 620 nm was measured using a Microplate
Reader (Thermo Scientific Multiskan FC, Loughborough, UK). Assays were carried out in triplicate for
each tested microorganism.
Minimum bactericidal concentration (MBC) assessment: To define the minimum bactericidal
concentration (MBC) for each set of wells in the MIC determination, a loopful of agar was collected from
the wells without any growth and inoculated on sterile Mueller–Hilton medium broth (for bacteria)
through streaking. Plates inoculated with bacteria were incubated at 37 ◦C for 24 h. After incubation,
the lowest concentration was noted as the MBC (for bacteria) at which no visible growth was observed.
3.2.6. Statistical Analysis
The values in this study are expressed as means
±
SD. The Shapiro–Wilk test was used for
verification of the normality of the data. Statistical differences were determined by one-way ANOVA.
The results were analyzed using STATISTICA 12.0 software (StatSoft, Tulsa, OK, USA). Differences of
p < 0.05 were considered statistically significant.
4. Conclusions
The antimicrobial, antioxidant, and cytotoxic activities of three saccharin–tetrazolyl (TS, TSMT,
and 2MTS) derivatives and one saccharin–thiadiazolyl (MTSB) derivative were addressed throughout
this investigation. The antimicrobial activity of the synthesized compounds was evaluated against a
series of Gram-positive and Gram-negative bacteria and yeast strains. An evaluation of the MIC and
MBC values of the four derivatives was completed at pH 4.0, 7.0, and 9.0 against four Gram-positive
strains (S. aureus, E. faecalis, S. aureus (MRSA), and E. faecalis (VRE)), showing high values for the MIC
and MBC at pH 4.0 (ranging from 3.42 to 473.0 µM). It was attested that the derivative MTSB, the
sole compound comprising the thiadiazole function, was the most active against all of the considered
Gram-positive strains at pH 4.0.
In addition, the antioxidant activity of the compounds (calculated by using the DPPH method)
was the highest value for TSMT (90.29% compared to the BHT positive control of 61.96%). Finally, we
demonstrated for the first time that the TS, TSMT, 2MTS, and MTSB compounds did not show in vitro
cytotoxic effects on H7PX glioma cells.
The present study exposed the influence of the pH of the medium on the antimicrobial activity
of the TS, TSMT, 2MTS, and MTSB compounds, which is similar to well-described antimicrobial
peptide antibiotics. Therefore, the use of these kinds of molecules to produce new antibiotics should
be considered in the future, although further studies are needed to confirm this hypothesis.
Author Contributions: L.M.T.F. and P.R. designed the study, analyzed the data, and wrote the paper; E.N.,
´
P.S., J.M.A., M.T., and T.S. performed the biological assays; L.M.T.F. and L.C. performed the synthesis and
characterization of the compounds; M.L.S.C. and A.J.L.P. were responsible for supervision and reviewing the draft.
Funding: This work was partially supported by the Foundation for Science and Technology (FCT), Portugal
((UID/QUI/00100/2019) and (UID/MULTI/04326/2019 – CCMAR)). L.M.T.F. expresses gratitude to FCT for the post
doc fellowship (SFRH/BPD/99851/2014) and work contract n◦ IST-ID/115/2018.
Conflicts of Interest: The authors declare no conflicts of interest.

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References
1.
Schulze, B.; Illgen, K. Isothiazol-1,1-dioxide—Vom Süßstoff zum chiralen Auxiliar in der stereoselektiven
Synthese. J. Prakt. Chem. 1997, 339, 1–14. [CrossRef]
2.
3.
Ellis, J.W.J. Overview of sweeteners. Chem. Educ. 1995, 72, 671–675. [CrossRef]
Remsen, I.; Fahlberg, C. On the oxidation of substitution products of aromatic hydrocarbons. IV. On the
oxidation of orthotoluenesulphonamide. J. Am. Chem. Soc. 1879, 1, 426–438. [CrossRef]
Price, M.J.; Biava, G.C.; Oser, L.B.; Vogin, E.E.; Steinfeld, J.; Ley, L.H. Bladder tumors in rats fed
cyclohexylamine or high doses of a mixture of cyclamate and saccharin. Science 1970, 167, 1131–1132.
[CrossRef] [PubMed]
4.
5.
Masui, T.; Mann, M.A.; Borgeson, D.C.; Garland, M.E.; Okamura, T.; Fujii, H.; Pelling, C.J.; Cohen, M.S.
Sequencing analysis of HA-RAS, KI-RAS, and N-RAS genes in rat urinary-bladder tumors induced by
N-[4-(5-Nitro-2-furyl)-2-thiazolyl]formamide (FANFT) and sodium saccharin. Terat. Carcin. Mutagen. 1993
13, 225–233. [CrossRef] [PubMed]
,
6.
7.
Garland, M.E.; Sakata, T.; Fisher, M.J.; Masui, T.; Cohen, M.S. Influences of Diet and Strain on the Proliferative
Effect on the Rat Urinary Bladder Induced by Sodium Saccharin. Cancer Res. 1989, 49, 3789–3794. [PubMed]
Groutas, W.C.; Houser-Archield, N.; Chong, L.S.; Venkataraman, R.; Epp, J.B.; Huang, H.; McClenahan, J.J.
Efficient inhibition of human-leukocyte elastase and cathepsin-G by saccharin derivatives. J. Med. Chem.
1993, 36, 3178–3181. [CrossRef] [PubMed]
8.
9.
Groutas, W.C.; Chong, L.S.; Venkataraman, R.; Kuang, R.; Epp, J.B.; Houser-Archield, N.; Huang, H.;
Hoydal, R.J. Amino acid-derivative phthalimide and saccharin derivatives as inhibitors of human leukocyte
elastase, cathepsin G, and proteinase 3. Arch. Biochem. Biophys. 1996, 332, 335–340. [CrossRef] [PubMed]
Groutas, W.C.; Epp, J.B.; Venkataraman, R.; Kuang, R.; Truong, T.M.; McClenahan, J.J.; Prakash, O. Design,
synthesis, and in vitro inhibitory activity toward human leukocyte elastase, cathepsin G, and proteinase 3 of
saccharin-derived sulfones and congeners. Bioorg. Med. Chem. 1996, 4, 1393–1400. [CrossRef]
10. Elghamry, I.; Youssef, M.M.; Al-Omair, M.A.; Elsawy, H. Synthesis, antimicrobial, DNA cleavage and
antioxidant activities of tricyclic sultams derived from saccharin. Eur. J. Med. Chem. 2017, 139, 107–113.
[CrossRef] [PubMed]
11. Apella, M.C.; Totaro, R.; Baran, E.J. Determination of superoxide dismutase-like activity in some divalent
metal saccharinates. Biol. Trace Elem. Res. 1993, 37, 293–299. [CrossRef] [PubMed]
12. Guenther, U.; Wrigge, H.; Theuerkauf, N.; Boettcher, M.F.; Wensing, G.; Zinserling, J.; Putensen, C.;
Hoeft, A. Repinotan, a selective 5-HT1A-R-agonist, antagonizes morphine-induced ventilator depression in
anesthetized rats. Anesth. Analg. 2010, 111, 901–907. [PubMed]
13. Malinka, W.; Ryng, S.; Sieklucka-Dziuba, M.; Rajtar, G.; Gownial, A.; Kleinrok, Z.
2-Substituted-3-oxoisothiazolo[5,4-b]pyridines as potential central nervous system and antimycobacterial
agents. Farmaco 1998, 53, 504–512. [CrossRef]
14. Malinka, W.; Ryng, S.; Sieklucka-Dziuba, M.; Rajtar, G.; Gownial, A.; Kleinrok, Z. Synthesis and preliminary
screening of derivatives of 2-(4-arylpiperazine-1-ylalkyl)-3-oxoisothiazolo[5,4,b]pyridines as CNS and
antimycobacterial agents. Pharmazie 2000, 55, 416–425. [PubMed]
15. Csakai, A.; Smith, C.; Davis, E.; Martinko, A.; Coulp, S.; Yin, H. Saccharin derivatives as inhibitors of
interferon-mediated inflammation. J. Med. Chem. 2014, 57, 5348–5355. [CrossRef] [PubMed]
16. Fischer, R.; Kretschik, O.; Schenke, T.; Schenkel, R.; Wiedemann, J.; Erdelen, C.; Loesel, P.; Drewes, M.W.;
Feucht, D.; Andersch, W.W. Ger. Offen. DE 1999244668. 1999. Chem. Abstr. 2001, 134, 4932.
17. Singh, H.; Chawla, A.S.; Kapoor, V.K.; Paul, D.; Malhotra, R.K. Medicinal chemistry of tetrazoles. Prog. Med.
Chem. 1980, 17, 151–183. [PubMed]
18. Noda, K.; Saad, Y.; Kinoshita, A.; Boyle, T.P.; Graham, R.M.; Husain, A.; Karnik, S.S. Tetrazole and carboxylate
receptor antagonists bind to the same subsite by different mechanisms. J. Biol. Chem. 1995, 270, 2284–2289.
[CrossRef] [PubMed]
19. Mavromoustakos, T.; Kolocouris, A.; Zervou, M.; Roumelioti, P.; Matsoukas, J.; Weisemann, R. An effort to
understand the molecular basis of hypertension through the study of conformational analysis of Losartan
and Sarmesin using a combination of nuclear magnetic resonance spectroscopy and theoretical calculations.
J. Med. Chem. 1999, 42, 1714–1722. [CrossRef] [PubMed]

Pharmaceuticals 2019, 12, 167
12 of 14
20. Toney, J.H.; Fitzgerald, P.M.D.; Grover-Sharma, N.; Olson, S.H.; May, W.J.; Sundelof, J.G.; Vanderwall, D.E.;
Cleary, K.A.; Grant, S.K.; Wu, J.K.; et al. Antibiotic sensitization using biphenyl tetrazoles as potent inhibitors
of Bacteroides fragilis metallo-β-lactamase. Chem. Biol. 1998, 5, 185–196. [CrossRef]
21. Gao, C.; Chang, L.; Xu, Z.; Yan, X.-F.; Ding, C.; Zhao, F.; Wu, X.; Feng, L.-S. Recent advances of tetrazole
derivatives as potential anti-tubercular and anti-malarial agents. Eur. J. Med. Chem. 2019, 163, 404. [CrossRef]
[PubMed]
22. Hashimoto, Y.; Ohashi, R.; Kurosawa, Y.; Minami, K.; Kaji, H.; Hayashida, K.; Narita, H.; Murata, S.
Pharmacologic Profile of TA-606, a Novel Angiotensin II-Receptor Antagonist in the Rat. J. Cardiovasc.
Pharmacol. 1998, 31, 568–575. [CrossRef] [PubMed]
23. Desarro, A.; Ammendola, D.; Zappala, M.; Grasso, S.; Desarro, G.B. Relationship between Structure and
Convulsant Properties of Some b-Lactam Antibiotics following Intracerebroventricular Microinjection in
Rats. Antimicrob. Agents Chemother. 1995, 39, 232–237. [CrossRef] [PubMed]
24. Tamura, Y.; Watanabe, F.; Nakatani, T.; Yasui, K.; Fuji, M.; Komurasaki, T.; Tsuzuki, H.; Maekawa, R.;
Yoshioka, T.; Kawada, K.; et al. Highly Selective and Orally Active Inhibitors of Type IV Collagenase (MMP-9
and MMP-2): N-Sulfonylamino Acid Derivatives. J. Med. Chem. 1998, 41, 640–649. [CrossRef] [PubMed]
25. Abell, A.D.; Foulds, G.J. Synthesis of a cis-conformationally restricted peptide bond isostere and its application
to the inhibition of the HIV-1 protease. J. Chem. Soc. Perkin Trans. 1997, 1, 2475–2482. [CrossRef]
26. Sandmann, G.; Schneider, C.; Boger, P. A New Non-Radioactive Assay of Phytoene Desaturase to Evaluate
Bleaching Herbicides. Z. Naturforsch. C 1996, 51, 534–538. [CrossRef] [PubMed]
27. Koldobskii, G.I.; Ostrovskii, V.A.; Poplavskii, V.S. Advances in tetrazole chemistry. Khim. Geterotsikl. Soedin.
1981, 10, 1299.
28. Ostrovskii, V.A.; Pevzner, M.S.; Kofman, T.P.; Shcherbinin, M.B.; Tselinskii, I.V. Targets in Heterocyclic Systems.
Chemistry and Properties; Attanasi, O.A., Spinelli, D., Eds.; Societa Chimica Italiana: Rome, Italy, 1999; Volume
3, p. 467.
29. Moore, D.S.; Robinson, S.D. Catenated Nitrogen Ligands Part II. Transition Metal Derivatives of Triazoles,
Tetrazoles, Pentazoles, and Hexazine. Adv. Inorg. Chem. 1988, 32, 171–239.
30. Balaban, A.T.; Oniciu, D.C.; Katritzky, A.R. Aromaticity as a cornerstone of heterocyclic chemistry. Chem.
Rev. 2004, 104, 2777–2812. [CrossRef] [PubMed]
31. Hu, Y.; Li, C.-Y.; Wang, X.-M.; Yang, Y.-H.; Zhu, H.-L. 1,3,4-Thiadiazole: Synthesis, reactions, and applications
in medicinal, agricultural, and materials chemistry. Chem. Rev. 2014, 114, 5572–5610. [CrossRef] [PubMed]
32. Almajan, G.L.; Barbuceanu, S.F.; Bancescu, G.; Saramet, I.; Saramet, G.; Draghici, C. Synthesis and
antimicrobial evaluation of some fused heterocyclic [1,2,4]triazolo[3,4-b][1,3,4]thiadiazole derivatives. Eur. J.
Med. Chem. 2010, 45, 6139–6146. [CrossRef] [PubMed]
33. Kolavi, G.; Hegde, V.; Khazi, I.; Gadad, P. Synthesis and evaluation of antitubercular activity of
imidazo[2,1-b][1,3,4]thiadiazole derivatives. Bioorg. Med. Chem. 2006, 14, 3069–3080. [CrossRef] [PubMed]
34. Khan, I.; Ali, S.; Hameed, S.; Rama, N.H.; Hussain, M.T.; Wadood, A.; Uddin, R.; Ul-Haq, Z.; Khan, A.; Ali, S.;
et al. Synthesis, antioxidant activities and urease inhibition of some new 1,2,4-triazole and 1,3,4-thiadiazole
derivatives. Eur. J. Med. Chem. 2010, 45, 5200–5207. [CrossRef] [PubMed]
35. Hafez, H.N.; Hegab, M.I.; Ahmed-Farag, I.S.; El-Gazzar, A.B.A. A facile regioselective synthesis of
novel spiro-thioxanthene and spiro-xanthene-90,2-[1,3,4]thiadiazole derivatives as potential analgesic and
anti-inflammatory agents. Bioorg. Med. Chem. Lett. 2008, 18, 4538–4543. [CrossRef] [PubMed]
36. Jatav, V.; Mishra, P.; Kashaw, S.; Stables, J.P. CNS depressant and anticonvulsant activities of some novel
3-[5-substituted 1,3,4-thiadiazole-2-yl]-2-styryl quinazoline-4(3H)-ones. Eur. J. Med. Chem. 2008, 43,
1945–1954. [CrossRef] [PubMed]
37. Clerici, F.; Pocar, D.; Guido, M.; Loche, A.; Perlini, V.; Brufani, M. Synthesis of
2-amino-5-sulfanyl-1,3,4-thiadiazole derivatives and evaluation of their antidepressant and anxiolytic
activity. J. Med. Chem. 2001, 44, 931–936. [CrossRef] [PubMed]
38. Hasui, T.; Matsunaga, N.; Ora, T.; Ohyabu, N.; Nishigaki, N.; Imura, Y.; Igata, Y.; Matsui, H.; Motoyaji, T.;
Tanaka, T.; et al. Identification of benzoxazin-3-one derivatives as novel, potent, and selective nonsteroidal
mineralocorticoid receptor antagonists. J. Med. Chem. 2011, 54, 8616–8631. [CrossRef] [PubMed]
39. Noolvi, M.N.; Patel, H.M.; Singh, N.; Gadad, A.K.; Cameotra, S.S.; Badiger, A. Synthesis and anticancer
evaluation of novel 2-cyclopropylimidazo[2,1-b][1,3,4]-thiadiazole derivatives. Eur. J. Med. Chem. 2011, 46,
4411–4418. [CrossRef] [PubMed]

Pharmaceuticals 2019, 12, 167
13 of 14
40. Liu, X.H.; Shi, Y.X.; Ma, Y.; Zhang, C.Y.; Dong, W.L.; Pan, L.; Wang, B.L.; Li, B.J.; Li, Z.M. Synthesis, antifungal
activities and 3D-QSAR study of N-(5-substituted-1,3,4-thiadiazol-2-yl)cyclopropanecarboxamides. Eur. J.
Med. Chem. 2009, 44, 2782–2786. [CrossRef] [PubMed]
41. Kaur, I.P.; Smitha, R.; Aggarwal, D.; Kapil, M. Acetazolamide: Future perspective in topical glaucoma
therapeutics. Int. J. Pharm. 2002, 248, 1–14. [CrossRef]
42. Luks, A.M.; McIntosh, S.E.; Grissom, C.K.; Auerbach, P.S.; Rodway, G.W.; Schoene, R.B.; Zafren, K.;
Hackett, P.H. Wilderness Medical Society consensus guidelines for the prevention and treatment of acute
altitude illness. Wilderness Environ. Med. 2010, 21, 146–155. [CrossRef] [PubMed]
43. Wolf, P. Acute drug administration in epilepsy: A review. CNS Neurosci. Ther. 2011, 17, 442–448. [CrossRef]
[PubMed]
44. Rangwala, L.M.; Liu, G.T. Pediatric idiopathic intracranial hypertension. Surv. Ophthalmol. 2007, 52, 597–617.
[CrossRef] [PubMed]
45. Russell, M.B.; Ducros, A. Sporadic and familial hemiplegic migraine: Pathophysiological mechanisms,
clinical characteristics, diagnosis, and management. Lancet Neurol. 2011, 10, 457–470. [CrossRef]
46. Tiselius, H.G. New horizons in the management of patients with cystinuria. Curr. Opin. Urol. 2010, 20,
169–173. [CrossRef] [PubMed]
47. Jalandhara, N.B.; Patel, A.; Arora, R.R.; Jalandhara, P. Obstructive sleep apnea: A cardiopulmonary
perspective and medical therapeutics. Am. J. Ther. 2009, 16, 257–263. [CrossRef] [PubMed]
48. Schara, U.; Lochmuller, H. Therapeutic strategies in congenital myasthenic syndromes. Neurotherapeutics
2008, 5, 542–547. [CrossRef] [PubMed]
49. Superti-Furga, G.; Cochran, J.; Crews, C.M.; Frye, S.; Neubauer, G.; Prinjha, R.; Shokat, K. Where is the Future
of Drug Discovery for Cancer? Cell 2017, 168, 564–565.
50. Gulçin, I. Antioxidant activity of food constituents: An overview. Arch. Toxicol. 2012, 86, 345–391. [CrossRef]
[PubMed]
51. Walker, B.; Barrett, S.; Polasky, S.; Galaz, V.; Folke, C.; Engstrom, G.; Ackerman, F.; Arrow, K.; Carpenter, S.;
Chopra, K.; et al. Environment. Looming global-scale failures and missing institutions. Science 2009, 325,
1345–1346. [CrossRef] [PubMed]
52. D’Costa, V.M.; King, C.E.; Kalan, L.; Morar, M.; Sung, W.W.; Schwarz, C.; Froese, D.; Zazula, G.; Calmels, F.;
Debruyne, R.; et al. Antibiotic resistance is ancient. Nature 2011, 477, 457. [CrossRef] [PubMed]
53. Andersson, D.I.; Hughes, D. Antibiotic resistance and its cost: Is it possible to reverse resistance? Nat. Rev.
Microbiol. 2010, 8, 260. [CrossRef] [PubMed]
54. Cassir, N.; Rolain, J.M.; Brouqui, P. A new strategy to fight antimicrobial resistance: The revival of old
antibiotics. Front. Microbiol. 2014, 5, 551. [CrossRef] [PubMed]
55. Brigas, A.F.; Fonseca, C.S.C.; Johnstone, R.A.W. Preparation of 3-Chloro-1,2-Benzisothiazole 1,1-Dioxide
(Pseudo-Saccharyl Chloride). J. Chem. Res. 2002, 6, 299–300. [CrossRef]
56. Frija, L.M.T.; Alegria, E.C.B.A.; Sutradhar, M.; Cristiano, M.L.S.; Ismael, A.; Kopylovich, M.N.; Pombeiro, A.J.L.
Copper(II) and cobalt(II) tetrazole-saccharinate complexes as effective catalysts for oxidation of secondary
alcohols. J. Mol. Catal. A Chem. 2016, 425, 283–290. [CrossRef]
57. Frija, L.M.T.; Fausto, R.; Loureiro, R.M.S.S.; Cristiano, M.L.S. Synthesis and structure of novel
benzisothiazole-tetrazolyl derivatives for potential application as nitrogen ligands. J. Mol. Catal.
Chem. 2009, 305, 142–146. [CrossRef]
A
58. Ismael, A.; Paix
methyltetrazole-saccharinates. J. Mol. Struct. 2012, 1023, 128–142. [CrossRef]
59. Cabral, L.; Br s, E.; Henriques, M.; Marques, C.; Frija, L.M.T.; Barreira, L.; Paix
ão, J.A.; Fausto, R.; Cristiano, M.L.S. Molecular structure of nitrogen-linked
á
ão, J.A.;
Fausto, R.; Cristiano, M.L.S. Synthesis, Structure, and Cytotoxicity of a New Sulphanyl-Bridged
Thiadiazolyl-Saccharinate Conjugate: The Relevance of S···N Interaction. Chem. Eur. J. 2018, 24, 3251–3262.
[CrossRef] [PubMed]
60. Rijo, P.; Duarte, A.; Francisco, A.P.; Semedo-Lemsaddek, T.; Simões, M.F. In vitro antimicrobial activity of
royleanone derivatives against Gram-positive bacterial pathogens. Phytother. Res. 2014, 8, 76–81. [CrossRef]
[PubMed]
61. Malik, E.; Dennison, S.R.; Harris, F.; Phoenix, D.A. pH Dependent Antimicrobial Peptides and Proteins, Their
Mechanisms of Action and Potential as Therapeutic Agents. Pharmaceuticals 2016, 9, 67. [CrossRef] [PubMed]

Pharmaceuticals 2019, 12, 167
14 of 14
62. Rijo, P.; Falé, P.L.; Serralheiro, M.L.; Sim
õ
es, M.F.; Gomes, A.; Reis, C. Optimization of medicinal plant
extraction methods and their encapsulation through extrusion technology. Measurement 2014, 58, 249–255.
[CrossRef]
63. Alanís-Garza, B.A.; González-González, G.M.; Salazar-Aranda, R.; Waksman de Torres, N.;
Rivas-Galindo, V.M. Screening of antifungal activity of plants from the northeast of Mexico. J. Ethnopharmacol.
2007, 114, 468–471. [CrossRef] [PubMed]
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