Synthesis and evaluation of novel 1,3,4-thiadiazole–fluoroquinolone hybrids as antibacterial, antituberculosis, and anticancer agents

Article abstract of DOI:10.3906/kim-1710-35

A series of 5-substituted-1,3,4-thiadiazole-based fluoroquinolone derivatives were designed as potential antibacterial and anticancer agents using a molecular hybridization approach. The target compounds 16–25 were synthesized by reacting the correspondin

Full text of DOI:10.3906/kim-1710-35

Turk J Chem
Turkish Journal of Chemistry
(2018) 42: 839 – 858
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http://journals.tubitak.gov.tr/chem/
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c
⃝ TUBITAK
doi:10.3906/kim-1710-35
Research Article
Synthesis and evaluation of novel 1,3,4-thiadiazole–fluoroquinolone hybrids as
antibacterial, antituberculosis, and anticancer agents
1
1
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2
Aslı DEMIRCI , Kaan G¨okc¸e KARAYEL , Esra TATAR , Sinem OKTEM OKULLU ,
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2
Nihan UNUBOL , Pakize Neslihan TAS¸LI , Zu¨htu¨ Tanıl KOCAGOZ ,
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2
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Fikrettin S¸AHIN , Ilkay KUC¸ UKGUZEL
1,∗
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Department of Pharmaceutical Chemistry, Faculty of Pharmacy, Marmara University, Istanbul, Turkey
2
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Department of Medical Microbiology, School of Medicine, Acıbadem University, Istanbul, Turkey
3
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Department of Genetics and Bioengineering, Faculty of Engineering, Yeditepe University, Istanbul, Turkey
Received: 16.10.2017
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Accepted/Published Online: 02.04.2018
•
Final Version: 01.06.2018
Abstract: A series of 5-substituted-1,3,4-thiadiazole-based fluoroquinolone derivatives were designed as potential an-
tibacterial and anticancer agents using a molecular hybridization approach. The target compounds 16–25 were syn-
thesized by reacting the corresponding N -(5-substituted-1,3,4-thiadiazol-2-yl)-2-chloroacetamides with ciprofloxacin or
norfloxacin. The purity and identity of the synthesized compounds were determined by the use of chromatographic
and spectral techniques (NMR, IR, MS, etc.) besides elemental analysis. Antibacterial, antituberculosis, and anti-
cancer activity of the target compounds were evaluated against selected strains and cancer cell lines. Compound 20
was appreciated as the most active agent representing antibacterial activity against Escherichia coli and Staphylococcus
aureus with MIC values of 4 µg/mL and 2 µg/mL, respectively. Amongst the synthesized fluoroquinolone derivatives,
compounds 19 and 20 were found to have modest antitubercular activity with 8 µg/mL MIC values for each. Most
potent derivative, compound 20 was docked against Staphylococcus aureus and Mycobacterium tuberculosis DNA gyrase
enzymes to visualize the possible conformation of the compound. Additionally, anticancer activities of target compounds
were evaluated on seven different cancer cell lines.
Key words: Fluoroquinolones, 1,3,4-thiadiazoles, antibacterials, tuberculosis, DNA gyrase, molecular modeling, cyto-
toxicity
1. Introduction
Fluoroquinolones (FQs) are commonly used antibacterial agents that have been shown to possess a broad
spectrum of antibacterial activity, great potency, and good oral bioavailability, as well as low side effects.1
Moreover, the World Health Organization (WHO) approves FQs as second-line antituberculosis agents.2 Despite
the remarkable clinical success of FQs, new fluoroquinolone containing medicinal agents are needed immediately
owing to increasing resistance against commonly prescribed antibacterials3,4 since resistance is a growing
problem for treatment.3 Furthermore, the anticancer activity of FQs is a partly new and promising area for
these agents.4
∗Correspondence: ikucukguzel@marmara.edu.tr
The research was presented at the International Multidisciplinary Symposium on Drug Research and Development on 28–30
November 2013, Antalya, Turkey, and the 50th International Conference on Medicinal Chemistry (50th RICT) on 2–4 July 2014,
Rouen, Normandy, France.
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Nalidixic acid is the first of the quinolone antibacterial agents, although technically it is a naphthyri-
dine structure-containing compound, not quinolone.5 It was discovered as the synthetic by-product of the
antimalarial agent chloroquine and indicated to have antibacterial activity towards gram-negative bacteria, 5
decades ago.5 Later, the quinolone ring has had many different modifications. The first one was the introduc-
tion of a fluorine atom to the sixth position of the quinolone ring; thereafter several fluoroquinolone-bearing
antibacterial agents, namely norfloxacin, ciprofloxacin, levofloxacin, moxifloxacin, gatifloxacin, etc., have been
discovered and gone into use in the clinic.6
FQs are most commonly prescribed broad spectrum antibacterial agents that are used against respiratory
tract infections, urinary tract infections, gastrointestinal infections, and sexually transmitted diseases.7 More-
over, tuberculosis therapy is a relatively new indication of FQs approved by the WHO.2 Ofloxacin, levofloxacin,
gatifloxacin, and moxifloxacin were demonstrated to show activity against M. tuberculosis;8,9 meanwhile, studies
to generate new ones are going on globally.10−14
FQ-containing agents possess antibacterial activity by inhibiting bacterial type II topoisomerase enzymes,
also known as DNA gyrases. They preferably inhibit topoIV enzymes of gram-positive microorganisms and
DNA gyrases of gram-negative microorganisms. Beyond, FQs inhibit the ParC domain of TopoII and the GyrA
domain of DNA gyrase.15 Topoisomerases are well established essential enzymes for bacterial survival, DNA
transcription, replication, and DNA repair. They are defined as crucial enzymes for every movement of DNA
in cells16 so that inhibitors of topoisomerases are widely accepted rational candidates for antibacterial and
anticancer agents.17
A breakthrough development concerning FQs and tuberculosis was the publication of the M. tuberculosis
DNA gyrase crystal structure, in 2016.18 Crystal structures of DNA gyrase enzymes were established in 1997.19
Furthermore, co-crystals of DNA gyrases, derived from different species such as S. pneumoniae and S. aureus,
and FQs have been published recently.20−22
While FQs have been known as antibacterial agents for decades, there are several studies proposing
FQs may be used for anticancer therapy. Although Hussy et al. showed prokaryotic topoisomerases are more
responsive to FQs after they were first introduced into the clinic,23 subsequent studies found that human topoi-
somerases could be inhibited by FQs and could be attractive targets for FQs during anticancer chemotherapy.
In this manner, ciprofloxacin was found to affect cell proliferation of transitional cell carcinoma of the bladder.24
Ciprofloxacin and ofloxacin were shown to enhance cytotoxicity of doxorubicin against bladder cancer.25
Ciprofloxacin was screened to induce apoptosis in colon carcinoma cell lines time and dose dependently.26
It was shown to be cytotoxic against ovarian cancer27 and lung cancer28 and was shown to inhibit prolifer-
ation of human lymphoidal cells by inducing apoptosis pathways.29 Ciprofloxacin was also shown to have an
antiproliferative and apoptosis-inducing effect on prostate cancer cells.30 In addition, ciprofloxacin, norfloxacin,
enoxacin, and levofloxacin were shown to inhibit growth of nonsmall lung cancer cells in a concentration- and
time-dependent manner.31 It was important to see ciprofloxacin and moxifloxacin both inhibit topoI activity32
after knowing moxifloxacin inhibits human topoII, the considerable target for anticancer chemotherapy.33 On
the other hand, human breast cancer cells were screened to accumulate enoxacin at G2 /M phase when they
were exposed to this agent,34 whilst colon cancer cell accumulated ciprofloxacin at the S phase when they were
exposed to ciprofloxacin.35
Modifying the already known FQ-containing agents is a widely used approach for drug discovery.
To date, some studies have been performed to discover potent antibacterial agents,36−44 antimycobacterial
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agents,10−14,45,46 and anticancer agents.47−53 Since it was already shown that the substituents at C7 deter-
mine the power and the preferential action target of the FQ, that is, the greater or lesser affinity for topoiso-
merase IV or gyrase,54 we modified the piperazine ring at C7 of the FQ core by adding a 1,3,4-thiadiazole ring
through –CH2 –CO– linker to gain some insights into this existing relationship. Based on the above findings,
several fluoroquinolone derivatives containing 1,3,4-thiadiazole moiety differing in the structure of substituents
at C5 position have been designed, synthesized, and evaluated for their antibacterial, antimycobacterial, and
anticancer activity.
2. Results and discussion
2.1. Chemistry
The synthetic route to achieve target molecules is shown in the Scheme. For this aim, selected aldehydes were
converted to thiosemicarbazones 1–5 by reacting with thiosemicarbazide. 2-Amino-1,3,4-thiadiazole derivatives
6–10 were obtained by oxidative cyclization of thiosemicarbazones in the presence of ferric chloride. 2-Amino-
1,3,4-thiadiazole derivatives 6–10 were converted to 2-chloro-N -(heteroaryl/alkyl)acetamide derivatives 11–15
by using α-chloroacetyl chloride in the presence of TEA. Finally, 2-chloro-N -(5-substituted-1,3,4-thiadiazole-2-
yl)acetamide derivatives 11–15 and excess amount of ciprofloxacin/norfloxacin were reacted to yield the target
compounds 16–25. Following the isolation process, the crude products were crystallized from appropriate
solvents. Purity of the synthesized compounds was checked by TLC and HPLC, and their structures were
confirmed by IR, 1 H NMR, 13 C NMR, and mass spectral data besides elemental analysis.
16 (R1 : cyclohexyl; R2 : ethyl); 17 (R1 : 4-fluorophenyl; R2 : ethyl); 18 (R1 : 2-chlorophenyl; R2 : ethyl); 19 (R1 :
4-chlorophenyl; R2 : ethyl); 20 (R1 : 2,4-dichlorophenyl; R2 : ethyl); 21 (R1 : cyclohexyl; R2 : cyclopropyl); 22 (R1 :
4-fluorophenyl; R2 : cyclopropyl); 23 (R1 : 2-chlorophenyl; R2 : cyclopropyl); 24 (R1 : 4-chlorophenyl; R2 : cyclopropyl);
25 (R1 : 2,4-dichlorophenyl; R2 : cyclopropyl).
Scheme. Synthetic route for target compounds 16–25. Reagents and conditions: i.R1 -CHO, EtOH, g.AcOH, reflux;
ii. FeCl3 , EtOH, reflux; iii. ClCH2 COCl, DCM, TEA; iv. DMF, NaHCO3 .
In the FTIR spectra, N–H, C=N, and C=S stretching bands of thiosemicarbazone derivatives 1–5 were
observed at 3444–3245, 1612–1587, and 1386–1244 cm−1 absorption values, respectively. After cyclization
of thiosemicarbazones, C=N stretching bands of 1,3,4-thiadiazole rings 5–10 were detected at 1558–1587
cm−1 , while N–H stretches of amines were seen at 3288–3173 cm−1 . Stretching bands of C=O groups were
monitored at 1701–1709 cm−1 values, which demonstrated the formation of chloracetamide derivatives 11–15.55
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FTIR spectral data of the target compounds 16–25 showed 3607–3194, 3302–3182, 1732–1712, 1705–1681, and
1631–1624 cm−1 stretches, which were attributed to O–H, N–H, carboxylic acid, amide, and ketone groups,
respectively.56
In the analysis of the 1 H NMR spectra of the target compounds, protons belonging to piperazinyl
moiety were observed at 2.76–3.54 ppm as multiplets.57 Phenyl protons of compounds 17–20 and 21–25 were
observed at 7.35–8.14 ppm. Cyclohexyl protons of compounds 16 and 21 were detected at 1.20–2.10 and
3.05 ppm. Cyclopropyl protons of compounds 21–25 were determined at 1.19–1.32 ppm. In the 1 H NMR
spectra of the target compounds, there were no peaks attributable to NH protons of N -[5-substituted-1,3,4-
thiadiazol-2-yl]acetamide residues since they were expected to be observed at around 9.28–11.10 ppm.47 It
was observed that the mentioned NH protons were exchanged with deuterium from DMSO-d6 . Meanwhile,
carboxylic acid protons were observed at 15.23–15.39 ppm. Methylene protons of N -[5-substituted-1,3,4-
thiadiazol-2-yl]acetamide residues were detected at 2.49–2.52 ppm. When we analyzed the 1 H NMR spectra
of the final compounds 16–25, we identified H2 , H5 , and H8 protons of the quinolone ring at 8.66–8.96, 7.60–
7.91, and 7.19–7.92 ppm, respectively. Quinolone H5 and H8 protons coupled with fluorine atoms at the 6th
position of the ring. Coupling constants were calculated for quinolone ring H5 and H8 protons as J = 6.6–13.5
Hz and J = 3.0–7.5 Hz, respectively. For the compounds 16–25, 1 H NMR results were consistent with the
literature.41,47,56−60
Furthermore, in the 13 C NMR spectra of the selected compounds, carboxylic acid carbons displayed
resonances at 166–167 ppm. Piperazine carbons were observed at 50–53 ppm, whilst conjugated ketone carbons
were observed at 176–177 ppm. Other quinolone carbons were identified at 106–154 ppm. C2 and C5 carbon
signals of 1,3,4-thiadiazole rings were detected at 158 and 160 ppm. Carbons that belong to acetylamino residue
were determined at 169 ppm. Moreover, phenyl carbons were observed at 128–164 ppm. Similar to the 1 H
NMR data, the 13 C NMR spectra also showed correlation with the literature.58−60
Similar to the 1 H NMR findings, quinolone ring C5 , C6 , C7 , and C4a carbons interacted with fluorine
atoms at the 6th position in the 13 C NMR spectra of the final compounds. In order of coupling constants for
C6 , C5 , C7 , and C4a carbons were calculated as J = 247.5 Hz, J = 22.5–27.0 Hz, J = 9.0–10.5 Hz, and J
= 7.5 Hz. These interactions are found to be consistent with the literature.60
Low-resolution ESI mass spectra of compounds 16–25 were recorded in either positive or negative
ionization mode and confirmed their molecular weights. The LC-MS/MS (ESI) analysis of the synthesized
compounds gave correct molecular ion peaks corresponding to [M+H]+ in positive ionization and [M–H]− in
negative ionization mode in each case. All ESI negative LC-MS/MS analysis data revealed [M–H]− m/z values
with 100% relative abundance, even as positive LC-MS/MS analysis data displayed [M+H]+ , [M+Na]+ , and
[M+K]+ m/z values of the target compounds with different relative abundances.
Of the synthesized fluoroquinolone–thiadiazole hybrids, compound 24 has been synthesized by a different
method.61 As the melting point of this compound was different from the reported one, we presented full
structural characterization data for this compound, including 13 C NMR and elemental analysis, which is not
reported in the above mentioned study.61
2.2. Prediction of drug-likeness and ADME properties of compounds 16–25
Pharmacokinetic properties, which are specified as ADME (absorption, delivery, metabolism, elimination) and
toxicity, are vital in the process of generating a new drug candidate. Desired physicochemical properties of
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pharmacologically active drugs were summarized by Lipinski.62 Historical analyses of physicochemical properties
of orally available marketed drugs that reached Phase II clinical trials demonstrated that 90% of them had fewer
than five hydrogen bond donors and fewer than ten hydrogen bond acceptors. Their molecular masses were
less than 500 Daltons, whereas log P values were scaled less than five. Listed properties then were stated as
“Lipinski’s rule of five”.62,63
Correlated to improvement in computational sciences, in silico programs can help to predict drug-
gability of a small molecule. Physicochemical properties and ADME criteria can be estimated by these
programs. Thus, the Molinspiration online calculation toolkit was used to predict drug-likeness aspects
(http://www.molinspiration.com/services/properties.html). With the help of this technology, total polar surface
area (TPSA), absorption% (ABS%), and Lipinski parameters were calculated as shown in Table 1.
Table 1. Drug-likeness properties* of compounds 16–25.
Compound MW
Vol
TPSA ABS% nROTB nON nOHNH miLogP nviol
16
17
18
19
20
21
22
23
24
25
Nor
Cip
541.65 471.61 120.66 67.37
554.58 457.95 120.66 67.37
571.03 466.55 120.66 67.37
571.03 466.55 120.66 67.37
605.48 480.09 120.66 67.37
554.65 477.83 120.66 67.37
566.59 464.18 120.66 67.37
583.04 472.78 120.66 67.37
583.04 472.78 120.66 67.37
617.49 486.32 120.66 67.37
7
7
7
7
7
7
7
7
7
7
3
3
10
10
10
10
10
10
10
10
10
10
6
2
2
2
2
2
2
2
2
2
2
2
2
1.41
1.88
2.34
2.39
3.00
1.40
1.86
2.33
2.38
2.98
-0.69
-0.70
1
1
1
1
1
1
1
1
1
1
0
0
319.33 279.26 74.57
331.35 285.46 74.57
83.27
83.27
6
*Nor: norloxacin, Cip: ciprofloxacin, MW: molecular weight, Vol: volume, TPSA: total polar surface area, ABS%:
absorption%, nROTB: number of rotatable bonds, nOHNH: number of hydrogen bond donors, nON: number of
hydrogen bond acceptors, miLogP: molinspiration partition coefficient n-octanol and water, nviol: number of violations.
Oral bioavailability is a desirable feature for drug candidates.64 Due to the poor pharmacokinetic profiles,
about 30% of oral drugs are eliminated in the area of drug development.65 Log P calculation gives us an idea
about oral bioavailability relevant to absorption, solubility, and permeability. A drug candidate should be
neither too hydrophilic to cross the gastrointestinal wall nor too lipophilic to be absorbed. According to Log P
calculation results, synthesized compounds 16–25 do not exceed the lipophilicity limitation.
The equation 109 – (0.345 × TPSA) = ABS% gives predicted percentage absorption.66 Calculated
absorption percentages of compounds 16–25 offered average results close to 70%. TPSA was calculated with
the help of the Molinspiration online property calculation toolkit using the parameters originally proposed by
Ertl et al.67 Similar to lipophilicity, polar surface area is substantial for drug candidates to cross biological
membranes. Too high TPSA results in poor absorption and bioavailability.64
Numbers of hydrogen bond donors and hydrogen bond acceptors for compounds 16–25 amounted in the
range of Lipinski’s rule of five. Due to Molinspiration analyses, only one violation was molecular weight, which
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is not an important problem, since there are pharmacologically effective marketed and FDA approved molecules
that have molecular weight over 500 Da, for example bedaquiline offering tuberculosis treatment.68
2.3. Osiris calculations/prediction of toxicity, solubility, drug-likeness, and drug score for com-
pounds 16-25
Potential toxicity, solubility, drug-like properties, and drug scores of the synthesized compounds 16–25 were
estimated by Osiris Property Explorer (http://www.organic-chemistry.org/prog/peo/). Table 2 represents
possibilities of mutagenicity, tumorigenicity, irritation, and reproductive toxicity of target compounds depending
upon this predictor tool. Compounds 16–25 do not possess these undesirable features, according to the Osiris
calculation. Calculated drug score for a lead molecule is expected to be over 0.5. According to this claim we
may propose our candidates are close to being good candidates.
Table 2. Osiris calculations* for compounds 16–25.
Toxicity risks
Compound
cLogP Sol
MW
TPSA DL
DS
Mut Tum Irrit Rep
16
17
18
19
20
21
22
23
24
25
Nor
Cip
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
■■
1.51
1.30
1.81
1.81
2.41
1.64
1.42
1.93
1.93
2.54
–1.65
–1.53
–4.74 542.0 147.2
–4.59 554.0 147.2
–5.01 570.0 147.2
–5.01 570.0 147.2
–5.74 604.0 147.2
–5.21 554.0 147.2
–5.05 566.0 147.2
–5.47 582.0 147.2
2.30 0.51
7.73 0.53
7.69 0.48
7.99 0.48
8.14 0.39
2.06 0.45
7.45 0.48
7.41 0.43
7.71 0.43
7.66 0.35
2.24 0.86
2.07 0.82
5.47
582.0 147.2
–6.21 616.0 147.2
–2.96 319.0 72.88
–3.32 331.0 72.88
*Nor: norloxacin, Cip: ciprofloxacin, ■: nontoxic, ■: slightly toxic, ■: highly toxic, Mut: mutagenicity, Tum:
tumorigenicity, Irrit: irritation, Rep: reproductive, cLogP: partition coefficient n-octanol and water, Sol: solubility,
MW: molecular weight, TPSA: total polar surface area, Sol: solubility, DL: drug likeness, DS: drug score.
2.4. Biological studies
2.4.1. Antimicrobial activity
Antibacterial activity of compounds 16–25 was tested against Staphylococcus aureus, Escherichia coli, and
Candida albicans. Antimicrobial activities were determined as minimal inhibitory concentrations (MICs) and
minimum bactericidal or (fungicidal) concentrations (MBCs or MFCs) by microwell dilution method and
designated in Table 3. Since lower MIC values were observed with reference drugs, it might be predicted
that introduction of 2-(heteroarylamino)-2-oxoethyl moiety at the N -4 position of the piperazine ring causes
diminution in antibacterial potency.
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Table 3. Antimicrobial activity results of compounds 16–25.
Compound
MIC (µg/mL)
MBC/MFC (µg/mL)
S. aureus E. coli C. albicans S. aureus E. coli C. albicans
16
256
256
128
512
4
512
512
128
256
2
512
512
64
64
256
512
512
64
256
64
-
512
512
512
512
8
512
512
512
512
4
512
512
256
128
512
512
512
128
256
128
-
17
18
19
20
21
128
256
256
128
128
0.5
512
512
512
256
256
0.06
0.008
-
256
512
512
512
512
-
512
512
512
512
512
-
22
23
24
25
Norfloxacin
Ciprofloxacin 0.125
Fluconazole
-
-
-
-
-
1
-
-
-
Compound 20 bearing a 2,4-dichlorophenyl moiety at the R1 position and an ethyl group at the
R2 position was appreciated as the most potent compound, representing MIC values of 4 µg/mL and 2
µg/mL against E. coli and S. aureus, respectively. Thus, compound 20 drastically differs from other designed
fluoroquinolones, depending on antibacterial activity results. The lack of antifungal activity observed with
compound 20 clearly shows the selectivity of the antibacterial activity of this compound. Another noteworthy
feature of this compound is that it has the highest Log P value amongst compounds 16–25, which might indicate
the influence of lipophilicity.
2.4.2. Antituberculosis activity
Target compounds 16–25 were initially screened for their in vitro antituberculosis activity against M. tuber-
culosis H37 Rv strain. The minimal inhibitory concentration vs. M. tuberculosis H37 Rv was determined by
a broth microdilution method in the range of 8–64 µg/mL (Table 4). Antituberculosis activity results of the
compounds were compared to those of ciprofloxacin and norfloxacin as reference drugs.69,70
Norfloxacin-derived compounds 19 and 20 carrying 4-chlorophenyl and 2,4-dichlorophenyl substituents on
the 1,3,4-thiadiazole ring scored the best results with 8 µg/mL MIC value against M. tuberculosis. Compounds
21 and 22 with ciprofloxacin core and cyclohexyl and 4-fluorophenyl substituents on the 1,3,4-thiadiazole ring
were found to show MIC values of 16 and 32 µg/mL respectively. Other target compounds are considered as
weakly active against M. tuberculosis H37 Rv strain with the same MIC value of 64 µg/mL.
It has also been reported that MICs inhibiting 50% and 90% of the M. tuberculosis isolates for norfloxacin
were 4 and 8 µg/mL, while MIC50 and MIC90 values for ciprofloxacin were 0.5 and 1 µg/mL.71
According to the results that we were able to experimentally observe it might be concluded that the
presence of a bulky group at the N -4 position of the piperazine ring of either norfloxacin or ciprofloxacin
decreases the antituberculosis activity.
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Table 4. Antituberculosis activity and cytotoxicity results of compounds 16–25.
Selectivity
index*
Compound
MIC (µg/mL)
Cytotoxicity-IC50 (µg/mL)
M. tuberculosis H37Rv VERO L929
16
17
18
19
20
21
22
23
24
25
64
64
64
8
192
160
265
274
236
125
280
130
221
311
311
375
191
259
561
433
469
85
3.0
2.5
4.1
34.3
29.5
7.8
8
16
32
64
64
64
318
146
96
8.8
2.0
3.5
540
250
128
4.9
69
70
Ciprofloxacin 0.5
Norfloxacin 2.0
622.0
187.5
*Selectivity index was calculated as SI = IC50 (V ERO)/MIC
(Mtb)
2.4.3. Anticancer activity
Cytotoxic properties of the synthesized compounds 16–25 were tested against A579 (lung cancer), PC3 (prostate
cancer), and SK MEL1 (melanoma) cell lines. Cell viability was measured by the MTS assay. However, no
significant activity was observed against the mentioned cancer cell lines. Percentage viability results of the cell
lines exposed to reference drugs and synthesized compounds 16–25 are presented in Table 5.
Table 5. Percentage viability of the cell lines exposed to compounds 16–25 at 10 µM.
Compound A549 MRC5 PC3
PNT1 SK MEL 1 HACAT HEK 293
16
17
18
19
20
21
22
23
24
25
91.6
91.3
91.4
93.3
93.6
81.0
91.8
91.3
88.1
94.7
91.8
98.9
88.5
94.8
84.5
63.2
87.0
95.4
83.9
75.2
108.3 102.1
110.6 104.5
106.9 103.2
107.4 109.6
107.2 100.7
105.8 103.2
104.6 101.1
106.6 106.1
105.3 103.7
103.6 99.5
102.2
100.4
99.9
76.8
95.4
86.8
94.8
90.8
78.4
86.0
79.0
67.1
86.5
71.8
73.6
67.8
68.3
67.4
66.8
60.1
59.8
72.4
69.7
100.7
102.7
98.7
105.4
104.9
99.9
104.8
2.5. Molecular modeling studies
Molecular docking studies concerning the synthesized compounds were performed to simulate potential inhibition
profiles of related bacterial and mycobacterial targets. M. tuberculosis DNA gyrase enzyme and S. aureus DNA
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gyrase enzyme were used for docking studies (Figure 1). Synthesized compounds as ligands revealed promising
results according to docking calculations. Binding affinity (kcal/mol) of each compound was calculated and all
docked poses were evaluated.
Figure 1. DNA Gyrase ciprofloxacin binding site. A. M. tuberculosis DNA gyrase (PDB code: 5BTC) B. S. aureus
DNA gyrase (PDB code: 2XCT).
In order to evaluate the accuracy of our docking studies, the co-crystallized structure of ciprofloxacin
was re-docked first into both DNA gyrase active sites with RMSD, being 0.586 and 0.840 values (Figure 2).
Newly synthesized compounds were docked afterwards. Molecular docking studies showed that all synthesized
fluoroquinolone derivatives adopt a similar binding mode in both DNA gyrase enzymes as already known
fluoroquinolone derivative compounds.
Figure 2. A. Superimposition of re-docked ciprofloxacin (gray) into M. tuberculosis DNA gyrase on the co-crystallized
one (green). B. Superimposition of re-docked ciprofloxacin (white) into S. aureus DNA gyrase on the co-crystallized one
(orange).
Mycobacterium tuberculosis DNA gyrase Ser90, Arg128, Arg482, Gly483, Thr500, and Glu501 amino acid
˚
residues were detected within 4 A area of the docked pose of compound 20. The carboxylate group of compound
20 forms a hydrogen bond with Arg128 residue. Interactions with Ser90, Arg482, Gly483, Thr500, and Glu501
amino acid residues were also observed. Possible conformation of compound 20 in M. tuberculosis DNA gyrase
is presented in Figure 3A. The Mg2+ and oxygen atom of compound 20 quinolone ring interact, and water
molecules of the crystal structure and carboxylate group of compound 20 interact as well.
Docking studies with Staphylococcus aureus DNA gyrase indicate that compound 20 makes interactions
with Arg458, Asp 1083, Ser1084, and Arg1122 amino acid residues. Interactions with Mg2+ and oxygen atom
of compound 20 quinolone ring were also observed. Interactions between water molecules of crystal structure
and carboxylate group of compound 20 were detected as well. Staphylococcus aureus DNA gyrase amino acid
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˚
Figure 3. Possible conformation of compound 20 in complex with DNA gyrase residues around the ligand within 4 A
distance. A. M. tuberculosis DNA gyrase. B. S. aureus DNA gyrase.
˚
residues and DNA coil within the 4 A area of the docked pose of compound 20 are shown in Figure 3B.
˚
Residues of M. tuberculosis DNA gyrase and residues of S. aureus DNA gyrase around 4 A of compound
20 docked poses are presented in Table 6.
˚
Table 6. Residues around compound 20 within 4 A distance at DNA gyrase binding site.
Protein
Residues
DNA, H2O molecules, Mg2+
Ser90, Arg128, Arg482, Gly483, Thr500, Glu501
DNA, H2O molecules, Mg2+
Arg458, Asp 1083, Ser1084, Arg1122
M. tuberculosis DNA gyrase
S. aureus DNA gyrase
In conclusion, amongst newly synthesized thiadiazole–fluoroquinolone hybrids, only one representative
(compound 20) exhibited significant antibacterial activity towards S. aureus and E. coli. This unpredictable
activity could be attributed to poor solubility of the compounds. This study also revealed two active fluoro-
quinolone derivatives (compounds 19 and 20) against M. tuberculosis H37 Rv. Docking studies showed that
compounds 16–25 are capable of binding DNA-gyrase B enzyme of S. aureus and M. tuberculosis. Further
studies on newer fluoroquinolones with better solubility are in progress.
3. Experimental
3.1. Chemistry
All solvents and reagents were obtained from commercial sources and used without further purification. The
purity of the compounds was confirmed by thin-layer chromatography (TLC) performed on Merck silica gel 60
F254 aluminum sheets (Merck, Darmstadt, Germany), using developing systems: S1 : petroleum ether/ethyl
acetate (50:50 v/v) and S2 : chloroform/methanol/acetic acid (93:5:2 v/v/v). Spots were detected under UV
light at λ = 254 and 366 nm. All melting points were determined using a Kleinfeld SMP-II basic model point
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apparatus and are uncorrected. Elemental analyses were obtained using a Leco CHNS-932 and are consistent
with the assigned structures. ESI positive and ESI negative ionization (low resolution) mass spectra of the
synthesized compounds were obtained using an AB SCIEX API 2000 LC-MS/MS instrument. FT-infrared
spectra were recorded on a Shimadzu FT-IR Affinity-1 and data are expressed in wavenumbers ? (cm−1). 1 H
and 13 C NMR spectra were recorded on a Bruker AVANCE DPX at 150, 300, and 600 MHz. The chemical
shifts were expressed in δ (ppm) downfield from tetramethylsilane (TMS) using DMSO-d6 as solvent.
The high-performance liquid chromatographic system consisted of an Agilent 1100 series instrument
equipped with a quaternary solvent delivery system and a model Agilent series G1315 B photodiode array
detector. A Rheodyne syringe loading sample injector with 50 µL sample loop was used for the injection
of the analytes. Chromatographic data were collected and processed using Agilent ChemStation plus soft-
ware. The separation was performed at ambient temperature by using a reversed phase ACE C18 (100 ×
4 mm; 5 µm particle size) column. All experiments were performed in gradient mode. The mobile phase
was prepared by mixing pH 4.50 phosphate buffer containing 0.1% TEA and acetonitrile (75:25 v/v during
0–2 min, 50:50 v/v during 2–4 min, 0:100 v/v during 4–12 min, 75:25 v/v during 12–15 min) and filtered
through a 0.45-µm pore filter and subsequently degassed by ultrasonication, prior to use. Solvent delivery
was employed at a flow rate of 1.0 mL/min. Detection of the analytes was carried out at 210, 230, 254, and
280 nm. SMILES codes were generated from the structures using the ACD/ChemSketch freeware version 12.0
molecular editor (http://www.acdlabs.com) and then pharmacokinetic properties were calculated using Molin-
spiration and Osiris web tools (http://www.molinspiration.com/services/properties.html, http://www.organic-
chemistry.org/prog/peo/). The calculated log P values for the compounds are given in Tables 4 and 5.
General procedure for the synthesis of hydrazinecarbothioamides 1–5
Ethanolic solution of thiosemicarbazide (30 mmol) was heated under reflux with various aromatic alde-
hydes (30 mmol) in the presence of a few drops of acetic acid. The crude products 1–5 precipitated on cooling
were filtered and crystallized from ethanol.
2-(Cyclohexylmethylidene)hydrazinecarbothioamide ( 1)
Yield 90%. mp 90 ◦ C (EtOH) (lit. 84–86 ◦ C).72
2-(4-Fluorobenzylidene)hydrazinecarbothioamide ( 2)
Yield 69%. mp 195–196 ◦ C (EtOH) (lit. 197–198 ◦ C).73
2-(2-Chlorobenzylidene)hydrazinecarbothioamide (3)
Yield 75%. mp 221 ◦ C (EtOH) (lit. 220 ◦ C).55
2-(4-Chlorobenzylidene)hydrazinecarbothioamide (4)
Yield 72%. mp 216 ◦ C (EtOH) (lit. 217–220 ◦ C).74
2-(2,4-Dichlorobenzylidene)hydrazinecarbothioamide (5)
Yield 62%. mp 240 ◦ C (EtOH) (lit. 240 ◦ C).55
General procedure for the synthesis of 1,3,4-thiadiazol-2-amines 6–10
Compounds 1–5 (1 mmol) were dissolved in ethanol and ethanolic ferric chloride solution (4 mmol) was
added. The reaction mixtures were heated under reflux for 16–20 h. The mixtures were neutralized using
ammonia solution, filtered and washed with water, dried, and crystallized from ethanol to obtain compounds
6–10.
5-Cyclohexyl-1,3,4-thiadiazol-2-amine ( 6)
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Yield 77%. mp 237 ◦ C (EtOH) (lit. 238–240 ◦ C).75
5-(4-Fluorophenyl)-1,3,4-thiadiazol-2-amine ( 7)
Yield 65%. mp 235 ◦ C (EtOH) (lit. 240 ◦ C).76
5-(2-Chlorophenyl)-1,3,4-thiadiazol-2-amine ( 8)
Yield 58%. mp 192 ◦ C (EtOH) (lit. 190–192 ◦ C).55
5-(4-Chlorophenyl)-1,3,4-thiadiazol-2-amine ( 9)
Yield 68%. mp 225 ◦ C (EtOH) (lit. 230 ◦ C).76
5-(2,4-Dichlorophenyl)-1,3,4-thiadiazol-2-amine ( 10)
Yield 45%. mp 232 ◦ C (EtOH) (lit. 229 ◦ C).55
General procedure for the synthesis of 2-chloro-N-(heteroaryl/alkyl)acetamides 11–15
Compounds 6–10 (5 mmol) were dissolved in DCM and TEA (6 mmol) was added to the reaction
mixtures. α-Chloroacetyl chloride (10 mmol) was slowly added to the reaction mixtures. The reaction mixtures
were heated for 2 h under reflux. The reaction was checked with TLC. The crude products were filtered, dried,
and crystallized from 1,4-dioxane to obtain products 11–15.
2-Chloro-N-[5-(cyclohexyl)-1,3,4-thiadiazol-2-yl]acetamide (11)
Yield 64%. mp 218 ◦ C .77 TLC Rf: 0.58 (S1). HPLC tR (min): 6.1. IR (cm−1): 3182 (N–H str), 1701
(amide C=O), 1566 (C=N str).
2-Chloro-N-[5-(4-fluorophenyl)-1,3,4-thiadiazol-2-yl]acetamide (12)
Yield 72%. mp 252 ◦ C (lit. 252 ◦ C).76 TLC Rf: 0.68 (S1). HPLC tR (min): 5.8. IR (cm−1): 3182
(N–H str), 1705 (amide C=O), 1567 (C=N str).
2-Chloro-N-[5-(2-chlorophenyl)-1,3,4-thiadiazol-2-yl]acetamide (13)
Yield 61%. mp 215–217 ◦ C (lit. 215–217 ◦ C).55 TLC Rf: 0.66 (S1). HPLC tR (min): 6.1. IR (cm−1):
3192 (N–H str), 1709 (amide C=O), 1575 (C=N str).
2-Chloro-N-[5-(4-chlorophenyl)-1,3,4-thiadiazol-2-yl]acetamide (14)
Yield 72%. mp 251 ◦ C (lit. 251 ◦ C).76 TLC Rf: 0.70 (S1). HPLC tR (min): 6.3. IR (cm−1): 3180
(N–H str), 1705 (amide C=O), 1554 (C=N str).
2-Chloro-N-[5-(2,4-dichlorophenyl)-1,3,4-thiadiazol-2-yl]acetamide (15)
Yield 28%. mp 248–250 ◦ C (lit. 248–250 ◦ C).55 TLC Rf: 0.75 (S1). HPLC tR (min): 10.4. IR (cm−1):
3174 (N–H str), 1708 (amide C=O), 1581 (C=N str).
General procedure for the synthesis of fluoroquinolone derivatives 16–25
Compounds 11–15 (1 mmol) and norfloxacin/ciprofloxacin (1.5 mmol) were dissolved in DMF. The
reaction mixtures were stirred at room temperature in the presence of NaHCO3 (1.5 mmol) for 24 h. The crude
products were filtered, dried, and crystallized from appropriate solvent to obtain final products 16–25.
1-Ethyl-6-fluoro-7-[4-(2-{[5-(cyclohexyl)-1,3,4-thiadiazol-2-yl]amino }-2-oxo-ethyl)piperazine-1-yl]-4-oxo-
1,4-dihydroquinoline-3-carboxylic acid (16)
Yield 57%; mp 256 ◦ C; TLC Rf: 0.54 (S1); HPLC tR (min): 6.3; IR (cm−1): 3198 (O–H and N–H str),
1732 (c. acid C=O str), 1681 (amide C=O str), 1627 (ketone C=O str), 1510, 1477 (C=N str, N–H b); LC/MS
ESI− m/z (%): 541.30 ([M–H]− , 100); LC/MS ESI+ m/z (%): 581.07 ([M+K]+ , 85), 565.14 ([M+Na]+ , 100),
543.33 ([M+H]+ , 37); 1 H NMR δ ppm (300 MHz, DMSO-d6): 1.20–2.10 (m, 13H, 5 × CH2 for cyclohexyl
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and methyl), 2.51 (s, 2H, –COCH2 –), 2.76 (m, 4H, piperazine H3 , H5), 3.01–3.08 (m, 1H, cyclohexyl –CH–),
3.45 (m, 4H, piperazine H2 , H6), 4.59 (q, 2H, –CH2 CH3), 7.19 (s, 1H, quinolone H8), 7.90 (d, 1H, J =
13.2 Hz, quinolone H5), 8.95 (s, 1H, quinolone H2), 15.36 (bs, 1H, –COOH); Elemental analysis, Calcd. for
C27 H32 FN5 O4 S.H2 O: C 55.70; H 5.93; N 14.99; S 5.72. Found: C 56.29; H 5.78; N 15.00; S 5.49.
1-Ethyl-6-fluoro-7-[4-(2-{[5-(4-fluorophenyl)-1,3,4-thiadiazol-2-yl]amino }-2-oxo-ethyl)piperazine-1-yl]-4-
oxo-1,4-dihydroquinoline-3-carboxylic acid (17)
Yield 51%. mp 245 ◦ C (dec.). TLC Rf: 0.75 (S1). HPLC tR (min): 6.2. IR (cm−1): 3288 (O–H and
N–H str), 1697 (c. acid C=O str, amide C=O str), 1625 (ketone C=O str), 1558, 1447 (C=N str, N–H b).
LC/MS ESI− m/z (%): 553.24 ([M–H]− , 100.) LC/MS ESI+ m/z (%): 593.04 ([M+K]+ , 100 LC), 577.03
([M+Na]+ , 28), 555.14 ([M+H]+ , 11). 1 H NMR δ ppm (300 MHz, DMSO-d6): 1.43 (t, J = 7.0 Hz, 3H,
–CH2 CH3), 2.51 (s, 2H, –COCH2 –), 2.79 (m, 4H, piperazine H3 , H5), 3.51 (m, 4H, piperazine H2 , H6),
4.60 (q, 2H, –CH2 CH3), 7.20 (d, J = 7.2 Hz, 1H, quinolone H8), 7.36–7.41 (m, 2H, Ar H3’, H5’), 7.89 (d,
1H, J = 13.2 Hz, quinolone H5), 7.99–8.04 (m, 2H, Ar H2’, H6’), 8.96 (s, 1H, quinolone H2), 15.39 (bs, 1H,
–COOH). 13 C NMR δ ppm (150 MHz, DMSO-d6): 14.83 (–N–CH2 CH3), 49.73 (–N–CH2 CH3), 52.65 and
52.36 (piperazine C3 , C5), 52.50 and 52.56 (piperazine C2 , C6), 60.20 (1,3,4-thiadiazole–NH–CO–CH2 –),
106.44 (quinolone C8), 107.54 (quinolone C3), 111.66 (quinolone C5 , J = 27.0 Hz), 119.75 (quinolone C4a
,
J = 7.5 Hz), 116.94, 117.09, 127.26, 129.69, 129.85, and 164.64 (phenyl C), 137.67 (quinolone C8a), 145.94
(quinolone C7 , J = 9.0 Hz), 149.01 (quinolone C2), 153.38 (quinolone C6 , J = 247.5 Hz), 158.29 (1,3,4-
thiadiazole C2), 162.15 (1,3,4-thiadiazole C5), 166.59 (–COOH), 169.24 (amide C=O), 176.63 (quinolone C4
=O). Elemental analysis, Calcd. for C26 H24 F2 N6 O4 S.3/2H2 O: C 55.12; H 4.51; N 14.83; S 5.66. Found: C
55.87; H 4.65; N 15.01; S 5.75.
1-Ethyl-6-fluoro-7-[4-(2-{[5-(2-chlorophenyl)-1,3,4-thiadiazol-2-yl]amino }-2-oxo-ethyl)piperazine-1-yl]-4-
oxo-1,4-dihydroquinoline-3-carboxylic acid (18)
Yield 62%. mp 272 ◦ C (dec.). TLC Rf: 0.54 (S1). HPLC tR (min): 6.3. IR (cm−1): 3452 (O–H str),
3182 (N–H str), 1728 (c. acid C=O str), 1701 (amide C=O str), 1624 (ketone C=O str), 1554, 1447 (C=N
str, N–H b). LC/MS ESI− m/z (%): 569.11 ([M–H]− , 100). LC/MS ESI+ m/z (%): 609.08 ([M+K]+ , 100),
593.07 ([M+Na]+ , 42), 571.02 ([M+H]+ , 9). 1 H NMR δ ppm (300 MHz, DMSO-d6): 1.43 (t, J = 7.2 Hz,
3H, –CH2 CH3), 2.52 (s, 2H, –COCH2 –), 2.80 (m, 4H, piperazine H3 , H5), 3.52 (m, 4H, piperazine H2 ,
H6), 4.60 (q, 2H, –CH2 CH3), 7.20 (d, J = 7.2 Hz, 1H, quinolone H8), 7.51–7.61 (m, 2H, Ar H4’, H5’),
7.68–7.71 (m, 1H, Ar H6’), 7.93 (d, 1H, J = 13.2 Hz, quinolone H5), 8.10–8.14 (m, 1H, Ar H3’), 8.96 (s, 1H,
quinolone H2), 15.38 (s, 1H, COOH). 13 C NMR δ ppm (150 MHz, DMSO-d6): 14.82 (–N–CH2 CH3), 49.75
(–N–CH2 CH3), 49.94 and 49.96 (piperazine C3 , C5), 52.48 (piperazine C2 , C6), 60.16 (1,3,4-thiadiazole–
NH–CO–CH2 –), 106.39 (quinolone C8), 107.54 (quinolone C3), 111.65 (quinolone C5 , J = 22.5 Hz), 119.75
(quinolone C4a , J = 7.5 Hz), 128.38, 129.42, 131.08, 131.35, 131.53, and 132.33 (phenyl C), 137.68 (quinolone
C8a), 145.95 (quinolone C7 , J = 9.0 Hz), 149.01 (quinolone C2), 153.37 (quinolone C6 , J = 247.5 Hz),
158.29 (1,3,4-thiadiazole C2), 160.21 (1,3,4-thiadiazole C5), 166.60 (–COOH), 169.41 (amide C=O), 176.64
(quinolone C4 =O). Elemental analysis, Calcd. for C26 H24 ClFN6 O4 S.H2 O: C 53.01; H 4.45; N 14.27; S 5.44.
Found: C 53.34; H 4.66; N 14.19; S 5.37.
1-Ethyl-6-fluoro-7-[4-(2-{[5-(4-chlorophenyl)-1,3,4-thiadiazole-2-yl]amino }-2-oxo-ethyl)piperazine-1-yl]-4-
oxo-1,4-dihydroquinoline-3-carboxylic acid (19)
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Yield 23%. mp 273 ◦ C (dec.). TLC Rf: 0.53 (S1). HPLC tR (min): 6.5. IR (cm−1): 3282 (O–H and
N–H str), 1728 (c. acid C=O str), 1697 (amide C=O str), 1627 (ketone C=O str), 1498, 1475 (C=N str, N–H
b). LC/MS ESI− m/z (%): 569.22 ([M–H]− , 100). LC/MS ESI+ m/z (%): 609.10 ([M+K]+ , 67), 593.09
([M+Na]+ , 100), 571.21 ([M+H]+ , 11). 1 H NMR δ ppm (300 MHz, DMSO-d6): 1.42 (t, J = 7.0 Hz, 3H,
–CH2 CH3), 2.51 (s, 2H, –COCH2 –), 2.79 (m, 4H, piperazine H3 , H5), 3.51 (m, 4H, piperazine H2 , H6),
4.60 (q, 2H, –CH2 CH3), 7.20 (d, J = 7.2 Hz, 1H, quinolone H8), 7.60 (d, 2H, J = 8.7 Hz, Ar H3’, H5’),
7.91 (d, 1H, J = 13.5 Hz, quinolone H5), 7.96 (d, 2H, J = 8.7 Hz, Ar H2’, H6’), 8.95 (s, 1H, quinolone H2),
15.35 (bs, 1H, –COOH). Elemental analysis, Calcd. for C26 H24 ClFN6 O4 S: C 54.69; H 4.24; N 14.72; S 5.62.
Found: C 54.59; H 4.47; N 14.56; S 5.46.
1-Ethyl-6-fluoro-7-[4-(2-{[5-(2,4-dichlorophenyl)-1,3,4-thiadiazol-2-yl]amino }-2-oxo-ethyl)piperazine-1-yl]-
4-oxo-1,4-dihydroquinoline-3-carboxylic acid (20)
Yield 40%. mp 258 ◦ C (dec.). TLC Rf: 0.55 (S1). HPLC tR (min): 6.8. IR (cm−1): 3282 (O–H and
N–H str), 1728 (c. acid C=O str), 1710 (amide C=O str), 1622 (ketone C=O str), 1552, 1469 (C=N str, N–H
b). LC/MS ESI− m/z (%): 603.07 ([M–H]− , 100). LC/MS ESI+ m/z (%): 626.97 ([M+Na]+ , 100), 605.09
([M+H]+ , 40). 1 H NMR δ ppm (600 MHz, DMSO-d6) δ ppm: 1.42 (t, J = 7.2 Hz, 3H, –CH2 CH3), 2.50
(s, 2H, –COCH2 –, DMSO), 2.79 (m, 4H, piperazine H3 , H5), 3.52 (m, 4H, piperazine H2 , H6), 4.59 (q, 2H,
–CH2 CH3), 7.20 (d, J = 3.0 Hz, 1H, quinolone H8), 7.63 (d, J = 4.2 Hz, 1H, Ar H5’), 7.90 (s, 1H, Ar H3’),
7.94 (d, J = 6.6 Hz, 1H, quinolone H5), 8.14 (d, 1H, J = 4.8 Hz, Ar H6’), 8.96 (s, 1H, quinolone H2), 15.38
(bs, 1H, –COOH). Elemental analysis, Calcd. for C26 H23 Cl2 FN6 O4 S.3/2H2 O: C 49.37; H 4.14; N 13.29; S
5.07. Found: C 49.10; H 4.31; N 12.92; S 4.65.
1-Cyclopropyl-6-fluoro-7-[4-(2-{[5-(cyclohexyl)-1,3,4-thiadiazol-2-yl]amino }-2-oxo-ethyl)piperazine-1-yl]-
4-oxo-1,4-dihydroquinoline-3-carboxylic acid (21)
Yield 59%. mp 255–258 ◦ C (dec.). TLC Rf: 0.82 (S1). HPLC tR (min): 6.3. IR (cm−1): 3452 (O–H
and N–H str), 1722 (c. acid C=O str), 1701 (amide C=O str), 1627 (ketone C=O str), 1558, 1506 (C=N str,
N–H b). LC/MS ESI− m/z (%): 553.34 ([M–H]− , 100). LC/MS ESI+ m/z (%): 593.12 ([M+K]+ , 100),
577.21 ([M+Na]+ , 67), 555.18 ([M+H]+ , 57). 1 H NMR δ ppm (300 MHz, DMSO-d6): 1.13–2.10 (m, 14H,
5 × CH2 for cyclohexyl, 2 × CH2 for cyclopropyl), 2.51 (s, 2H, –COCH2 –), 2.90 (m, 4H, piperazine H3 ,
H5), 3.05 (m, 1H, cyclohexyl –CH–), 3.45 (m, 4H, piperazine H2 , H6), 3.83 (m, 1H, cyclopropyl –CH–), 7.58
(d, J = 7.2 Hz, 1H, quinolone H8), 7.89 (d, J = 13.2 Hz, 1H, quinolone H5), 8.66 (s, 1H, quinolone H2).
Elemental analysis, Calcd. for C27 H31 FN6 O4 S: C 58.47; H 5.63; N 15.15; S 5.78. Found: C 58.31; H 5.75; N
15.02; S 5.58.
1-Cyclopropyl-6-fluoro-7-[4-(2-{[5-(4-fluorophenyl)-1,3,4-thiadiazol-2-yl]amino }-2-oxo-ethyl)piperazine-1-
yl]-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (22)
Yield 41%. mp 245 ◦ C (dec.). TLC 0.75 (S1). HPLC tR (min): 6.2. IR (cm−1): 3288 (O–H and
N–H str), 1722 (c. acid C=O str), 1701 (amide C=O str), 1627 (ketone C=O str), 1558, 1506 (C=N str, N–H
b). LC/MS ESI− m/z (%): 565.23 ([M–H]− , 100). LC/MS ESI+ m/z (%): 605.07 ([M+K]+ , 100), 589.15
([M+Na]+ , 57), 567.22 ([M+H]+ , 32). 1 H NMR δ ppm (300 MHz, DMSO-d6): 1.19 (s, 2H, cyclopropyl
–CH2 –), 1.32 (d, J = 6.2 Hz, 2H, cyclopropyl –CH2 –), 2.51 (s, 2H, –COCH2 –), 2.81 (m, 4H, piperazine H3 ,
H5), 3.51 (m, 4H, piperazine H2 , H6), 3.83 (m, 1H, cyclopropyl –CH–), 7.35–7.41 (m, 2H, Ar H3’, H5’),
7.58 (d, J = 7.5 Hz, 1H, quinolone H8), 7.90 (d, J = 13.2 Hz, 1H, quinolone H5), 7.99–8.04 (m, 2H, Ar H2’,
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H6’), 8.86 (s, 1H, quinolone H2). 13 C NMR δ ppm (150 MHz, DMSO-d6): 8.04 (–N–CH(CH2)2), 36.34
(–N–CH(CH2)2), 49.84 and 49.87 (piperazine C3 , C5), 52.49 (piperazine C2 , C6), 60.20 (1,3,4-thiadiazole–
NH–CO–CH2 –), 106.91 (quinolone C8), 107.20 (quinolone C3), 111.42 (quinolone C5 , J = 24.0 Hz), 119.05
(quinolone C4a , J = 7.5 Hz), 116.85, 117.00, 127.26, 129.71, 129.77, and 164.64 (phenyl C), 139.64 (quinolone
C8a), 145.65 (quinolone C7 , J = 10.5 Hz), 148.47 (quinolone C2), 153.50 (quinolone C6 , J = 247.5 Hz),
158.58 (1,3,4-thiadiazole C2), 162.99 (1,3,4-thiadiazole C5), 166.41 (–COOH), 169.38 (amide C=O), 176.82
(quinolone C4 =O). Elemental analysis, Calcd. for C27 H24 F2 N6 O4 S.1/2H2 O: C 56.34; H 4.38; N 14.60; S
5.57. Found: C 56.37; H 4.29; N 14.44; S 5.17.
1-Cyclopropyl-6-fluoro-7-[4-(2-{[5-(2-chlorophenyl)-1,3,4-thiadiazol-2-yl]amino }-2-oxo-ethyl)piperazine-1-
yl]-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (23)
Yield 32%. mp 265–275 ◦ C (dec.). TLC Rf: 0.75 (S1). HPLC tR (min): 6.3. IR (cm−1): 3190 (O–H
and N–H str), 1712 (c. acid C=O str), 1693 (amide C=O str), 1624 (ketone C=O str), 1543, 1446 (C=N str,
N–H b). LC/MS ESI− m/z (%): 581.10 ([M–H]− , 100). LC/MS ESI+ m/z (%): 621.03 ([M+K]+ , 57), 605.09
([M+Na]+ , 100), 583.13 ([M+H]+ , 54). 1 H NMR δ ppm (300 MHz, DMSO-d6): 1.19 (s, 2H, cyclopropyl
–CH2 –), 1.32 (d, J = 6.0 Hz, 2H, cyclopropyl –CH2 –), 2.51 (s, 2H, –COCH2 –), 2.81 (m, 4H, piperazine H3 ,
H5), 3.53 (m, 4H, piperazine H2 , H6), 3.83 (m, 1H, cyclopropyl –CH–), 7.51–7.60 (m, 3H, Ar H4’, H5’,
quinolone H8), 7.68–7.71 (m, 1H, Ar H6’), 7.90 (d, J = 13.2 Hz, 1H, quinolone H5), 8.10–8.14 (m, 1H, Ar
H3’), 8.66 (s, 1H, quinolone H2), 15.23 (bs, 1H, –COOH). 13 C NMR δ ppm (150 MHz, DMSO-d6): 8.03 (–N–
CH(CH2)2), 36.33 (–N–CH(CH2)2), 49.86 and 49.89 (piperazine C3 , C5), 52.46 (piperazine C2 , C6), 60.14
(1,3,4-thiadiazole–NH–CO–CH2 –), 106.87 (quinolone C8), 107.19 (quinolone C3), 111.40 (quinolone C5 , J
= 24.0 Hz), 119.05 (quinolone C4a , J = 7.5 Hz), 129.37, 129.40, 131.08, 131.33, 131.53, and 132.33 (phenyl C),
139.64 (quinolone C8a), 145.64 (quinolone C7 , J = 10.5 Hz), 148.44 (quinolone C2), 153.49 (quinolone C6 ,
J = 247.5 Hz), 158.30 (1,3,4-thiadiazole C2), 160.15 (1,3,4-thiadiazole C5), 166.41 (–COOH), 169.38 (amide
C=O), 176.82 (quinolone C4 =O). Elemental analysis, Calcd. for C27 H24 ClFN6 O4 S.3/2H2 O: C 53.95; H
4.36; N 13.98; S 5.33. Found: C 54.42; H 4.52; N 14.19; S 5.43.
1-Cyclopropyl-6-fluoro-7-[4-(2-{[5-(4-chlorophenyl)-1,3,4-thiadiazol-2-yl]amino }-2-oxo-ethyl)piperazine-1-
yl]-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (24)
Yield 43%. mp 272 ◦ C (dec.) (lit. 330–333 ◦ C).61 TLC Rf: 0.74 (S1). HPLC tR (min): 6.6. IR (cm−1):
3607 (O–H str), 3302 (N–H str), 1728 (c. acid C=O str), 1701 (amide C=O str), 1627 (ketone C=O str), 1554,
1447 (C=N str, N–H b). LC/MS ESI− m/z (%): 581.10 ([M–H]− , 100). LC/MS ESI+ m/z (%): 621.07
([M+K]+ , 100), 605.13 ([M+Na]+ , 47), 583.15 ([M+H]+ , 43). 1 H NMR δ ppm (300 MHz, DMSO-d6): 1.19
(s, 2H, cyclopropyl –CH2 –), 1.32 (d, J = 6.2 Hz, 2H, cyclopropyl –CH2 –), 2.51 (s, 2H, –COCH2 –), 2.81 (m,
4H, piperazine H3 , H5), 3.52 (m, 4H, piperazine H2 , H6), 3.83 (m, 1H, cyclopropyl –CH–), 7.57–7.59 (m,
3H, quinolone H8 , Ar H2’, H6’) 7.91 (d, J = 13.2 Hz, 1H, quinolone H5), 7.96 (d, 2H, J = 8.7 Hz, Ar H3’,
H5’), 8.67 (s, 1H, quinolone H2), 15.24 (bs, 1H, –COOH). 13 C NMR δ ppm (150 MHz, DMSO-d6): 8.03
(–N–CH(CH2)2), 36.26 and 36.34 (–N–CH(CH2)2), 49.82 and 49.85 (piperazine C3 , C5), 52.48 (piperazine
C2 , C6), 60.20 (1,3,4-thiadiazole–NH–CO–CH2 –), 106.90 (quinolone C8), 107.18, (quinolone C3), 111.43
(quinolone C5 , J = 22.5 Hz), 119.07 (quinolone C4a ,J = 7.5 Hz), 129.09, 129.46, 129.91, and 135.68 (phenyl
C), 139.67 (quinolone C8a), 145.66 (quinolone C7 , J = 10.5 Hz), 148.48 (quinolone C2), 153.50 (quinolone
C6 , J = 247.5 Hz), 158.79 (1,3,4-thiadiazole C3), 162.82 (1,3,4-thiadiazole C5), 166.47 (–COOH), 169.31
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(amide C=O), 176.84 (quinolone C4 =O). Elemental analysis, Calcd. for C27 H24 ClFN6 O4 S.1/2H2 O: C
54.77; H 4.26; N 14.19; S 5.42. Found: C 54.38; H 4.43; N 14.14; S 5.30.
1-Cyclopropyl-6-fluoro-7-[4-(2-{[5-(2,4-dichlorophenyl)-1,3,4-thiadiazol-2-yl]amino }-2-oxo-ethyl)piperazine-
1-yl]-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (25)
Yield 46%. mp 252 ◦ C (dec.). TLC Rf: 0.78 (S1). HPLC tR (min): 6.9. IR (cm−1): 3288 (O–
H and N–H str), 1730 (c. acid C=O str), 1701 (amide C=O str), 1627 (ketone C=O str), 1487 (C=N str,
N–H b). LC/MS ESI− m/z (%): 615.17 ([M–H]− , 100). LC/MS ESI+ m/z (%): 639.18 ([M+Na]+ , 100),
617.09 ([M+H]+ , 48). 1 H NMR δ ppm (600 MHz, DMSO-d6): 1.19 (m, 2H, cyclopropyl –CH2 –), 1.32 (m,
2H, cyclopropyl –CH2 –), 2.50 (m, 2H, –COCH2 –, DMSO), 2.81 (m, 4H, piperazine H3 , H5), 3.54 (m, 4H,
piperazine H2 , H6), 3.83 (m, 1H, cyclopropyl –CH–), 7.60-7.92 (m, 5H, quinolone H5 , quinolone H8 , Ar
H3’, Ar H5’, Ar H6’), 8.67 (s, 1H, quinolone H2), 15.24 (s, 1H, –COOH). Elemental analysis, Calcd. for
C27 H23 Cl2 FN6 O4 S.3/2H2 O: C 50.32; H 4.07; N 13.04; S 4.98. Found: C 50.43; H 4.09; N 13.00; S 5.20.
3.2. Biological studies
3.3. Antimicrobial activity
The antimicrobial activity of the compounds was tested against E. coli (ATCC 25922), S. aureus (ATCC 25923),
and C. albicans (ATCC 10231). The minimal inhibitory concentration (MIC) and minimum bactericidal or
(fungicidal) concentration (MBC or MFC) for E. coli (ATCC 25922), S. aureus (ATCC 25923), and C. albicans
(ATCC 10231) were determined by a microbroth dilution method depicted below.78
For MIC determination, the compounds were dissolved in dimethyl sulfoxide and serial twofold dilutions
were done in Luria-Bertani (LB) broth. Microorganisms were suspended in LB broth to match the turbidity of
0.5 McFarland (1.5 × 108 cfu/mL) and 1/10 dilution was prepared from this suspension and used as inoculum.
The tested final concentrations ranged between 512 and 0.5 µg/mL. To make sure that dimethyl sulfoxide did
not show any inhibitory activity, controls prepared with serial dilutions of dimethyl sulfoxide were also tested.
The tubes were incubated at 37 ◦ C for 24 h and then examined for turbidity. MIC was determined if turbidity
was observed in the positive control tube containing no compound and no turbidity in the negative control tube
containing no microorganism.
After MIC determination, aliquots of 10 µL from all tubes in which no visible bacterial growth was
observed were inoculated in agar plates for determination of MBC. The plates were then incubated overnight at
37 ◦ C. MBC was identified as the lowest concentration of the compound that completely eliminated the growth
of the microorganism. Ciprofloxacin, norfloxacin, and fluconazole were used as the positive sensitivity reference
standard for bacteria and yeast.
The antimicrobial activity study was carried out at the Department of Medical Microbiology, School of
˙
Medicine, Acıbadem University, Istanbul, Turkey.
3.3.1. Antituberculosis activity
Antimycobacterial activity of the synthesized compounds was tested against M. tuberculosis H37 RV strain.
For the MIC determination, the compounds were dissolved in dimethyl sulfoxide and serial twofold dilutions
were done in Middlebrook 7H9 Broth containing glycerol. Microorganisms were suspended in Middlebrook 7H9
Broth to match the turbidity of 0.5 McFarland (1.5 × 108 cfu/mL) and 1/10 dilution was prepared from this
suspension and used as inoculum. The tested final concentrations ranged between 512 and 0.5 µg/mL. To
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make sure that dimethyl sulfoxide did not show any inhibitory activity, controls prepared with serial dilutions
of dimethyl sulfoxide were also tested. The tubes were incubated at 37 ◦ C for 24 h and then examined for
turbidity. MIC was determined if turbidity was observed in the positive control tube containing no compound
and no turbidity in the negative control tube containing no microorganism.79−81 Isoniazid and rifampicine were
used as the positive sensitivity reference standard for mycobacteria.
The antimycobacterial activity study was performed at the Department of Medical Microbiology, School
˙
of Medicine, Acıbadem University, Istanbul, Turkey.
3.3.2. Anticancer activity
A549, MRC5, PC3, PNT1, SK MEL 1, HACAT, and HEK 293 cell lines were grown in Dulbecco’s Modified
Eagle’s Medium (DMEM) supplemented with 10% (v/v) fetal bovine serum (FBS) and 1% (v/v) PSA (Invitro-
gen, Gibco, UK). After sufficient confluence was achieved (about ∼70–80), cells were trypsinized using 0.25%
(v/v) trypsin/EDTA (Invitrogen, Gibco, UK) and seeded on a T-75 flask (Zelkultur Flaschen, Switzerland).
The cells were maintained at 37 ◦ C and 5% CO2 in a humidified incubator.
Next 50 µM, 100 µM, 150 µM, 200 µM, and 250 µM concentrations of compounds and references were
prepared in DMEM. Cell lines were seeded at a concentration of 5000 cells/well onto 96-well plates (BIOFIL;
TCP, Switzerland). The following day, cells were treated with different concentrations of synthesized compounds
and references besides 20% (v/v) DMSO as lethal dose (positive control). Cell viability was measured by the
MTS assay (CellTiter96 Aqueous One Solution, Promega, UK) according to the manufacturer’s instructions.
After incubating the cells in the presence of pluronics for 24, 48, and 72 h, 10 µL of MTS reagent was added to
the growth medium followed by further incubation for 2 h. Thereafter, the absorbance at 490 nm was measured
by an ELISA plate reader (BioTek Instruments, Inc., Winooski, VT, USA).
The anticancer activity studies were carried out at the Department of Genetics and Bioengineering,
˙
Faculty of Engineering and Architecture, Yeditepe University, Istanbul, Turkey.
3.4. Docking studies
Since DNA gyrase enzyme was selected as target, M. tuberculosis DNA gyrase in complex with ciprofloxacin
(PDB ID: 5BTC) and S. aureus DNA gyrase in complex with ciprofloxacin (PDB ID: 2XCT) were obtained
from the Protein Data Bank.
Biovia Discovery Studio Visualizer and MGLTools software were used to prepare data before docking.
DNA, Mg2+ , and conserved water molecules within the receptor were kept during calculations. Gasteiger
charges were assigned to the ligands and the receptors. AutoDock Vina docking software was used to calculate
binding affinities (kcal/mol) of each compound.
Acknowledgment
This work was supported by the Research Fund of Marmara University, project number: SAG-C-YLP-130313-
0062, between 13 March 2013 and 13 March 2014.
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