Thiazole compounds with activity against Cryptococcus gattii and Cryptococcus neoformans in vitro

Article abstract of DOI:10.1016/j.ejmech.2015.07.032

Human cryptococcosis can occur as a primary or opportunistic infection and develop as an acute, subacute, or chronic, systemic infection involving different host organs. We evaluated the antifungal activity of thirteen compounds against Cryptococcus gattii and Cryptococcus neoformans in vitro, by assessing the toxicity of the compounds showing the greatest antifungal activity in VERO cells and murine macrophages. From these results, four compounds were considered promising for further studies because they displayed low cytotoxicity and significant antifungal activity. The heterocyclic compounds 1b, 1c, 1d, and 1m have antifungal activity levels between that of amphotericin B and fluconazole in vitro. The death curve of Cryptococcus spp. treated with these four compounds was similar to the curve obtained for amphotericin B, in that we observed a significant reduction in cell viability within the first 24 h of treatment. Additionally, we found that there was no effect when these compounds were combined with amphotericin and fluconazole, except for 1c, which antagonized the effect of amphotericin B against C. gattii, also reflected in the reduction of the post-antifungal effect (PAFE); however, this interaction did not alter the ergosterol content. The results shown in this paper reveal the discovery of novel thiazole compounds, which are easy to synthesize, and with potentially exhibit antifungal activity, and display low cytotoxicity in normal mammalian cells. These compounds can be used as prototypes for the design of new antifungal drugs against C. gattii and C. neoformans.

Full text of DOI:10.1016/j.ejmech.2015.07.032

Accepted Manuscript
Thiazole compounds with activity against Cryptococcus gattii and C. neoformans in
vitro
Nívea Pereira de Sá, Cleudiomar Inácio Lino, Nayara Cristina Fonseca, Beatriz
Martins Borelli, Jonas Pereira Ramos, Elaine Maria Souza-Fagundes, Carlos Augusto
Rosa, Daniel Assis Santos, Renata Barbosa de Oliveira, Susana Johann
PII:
S0223-5234(15)30164-1
10.1016/j.ejmech.2015.07.032
EJMECH 8016
DOI:
Reference:
To appear in: European Journal of Medicinal Chemistry
Received Date: 17 March 2015
Revised Date: 8 June 2015
Accepted Date: 16 July 2015
Please cite this article as: N. Pereira de Sá, C.I. Lino, N.C. Fonseca, B.M. Borelli, J.P. Ramos, E.M.
Souza-Fagundes, C.A. Rosa, D.A. Santos, R. Barbosa de Oliveira, S. Johann, Thiazole compounds with
activity against Cryptococcus gattii and C. neoformans in vitro, European Journal of Medicinal Chemistry
(2015), doi: 10.1016/j.ejmech.2015.07.032.
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Graphical Abstract
Anti- Cryptococcus
activity
OH
S
N
N
N
H3CO
H
1c
MIC 90
C. neoformans 0.9 µM
C. gattii 1.4 µM
MIC 90
C. neoformans 3.2 µM
C. gattii 3.1 µM
MIC 90 1d
C. neoformans 2.2 µM
C. gattii 1.9 µM
1m
C. neoformans 3.5 µM
C. gattii 4.2 µM

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Thiazole compounds with activity against Cryptococcus gattii and C. neoformans in vitro
Nívea Pereira de Sá1#, Cleudiomar Inácio Lino2#, Nayara Cristina Fonseca2, Beatriz Martins Borelli1,
Jonas Pereira Ramos3, Elaine Maria Souza-Fagundes3, Carlos Augusto Rosa1, Daniel Assis Santos1, Renata
Barbosa de Oliveira2, Susana Johann1*.
1Departamento de Microbiologia, Instituto de Ciências Biológicas, Universidade Federal de Minas
Gerais, Belo Horizonte, MG, Brazil.
2Departamento de Produtos Farmacêuticos, Faculdade de Farmácia da UFMG, Universidade Federal
de Minas Gerais, Belo Horizonte, MG, Brazil.
3 Departamento de Fisiologia e Biofísica, Instituto de Ciências Biológicas, Universidade Federal de
Minas Gerais, Belo Horizonte, MG, Brazil.
*Corresponding author:
Susana Johann, Departamento de Microbiologia, Instituto de Ciências Biológicas, Universidade
Federal de Minas Gerais, Av. Antônio Carlos, 6627, PO Box 486, 31270-901, Belo Horizonte, MG, Brazil.
Tel.: +55 3349 7700; Fax: +55 31 3295 3115; Email: sjohann@icb.ufmg.br
#These authors contributed equally to this work.

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ABSTRACT
Human cryptococcosis can occur as a primary or opportunistic infection and develop as an acute,
subacute, or chronic, systemic infection involving different host organs. We evaluated the antifungal activity
of thirteen compounds against Cryptococcus gattii and C. neoformans in vitro, by assessing the toxicity of
the compounds showing the greatest antifungal activity in VERO cells and murine macrophages. From these
results, four compounds were considered promising for further studies because they displayed low
cytotoxicity and significant antifungal activity. The heterocyclic compounds 1b, 1c, 1d, and 1m have
antifungal activity levels between that of amphotericin B and fluconazole in vitro. The death curve of
Cryptococcus spp. treated with these four compounds was similar to the curve obtained for amphotericin B,
in that we observed a significant reduction in cell viability within the first 24 h of treatment. Additionally, we
found that there was no effect when these compounds were combined with amphotericin and fluconazole,
except for 1c, which antagonized the effect of amphotericin B against C. gattii, also reflected in the reduction
of the post-antifungal effect (PAFE); however, this interaction did not alter the ergosterol content. The results
shown in this paper reveal the discovery of novel thiazole compounds, which are easy to synthesize, and with
potentially exhibit antifungal activity, and display low cytotoxicity in normal mammalian cells. These
compounds can be used as prototypes for the design of new antifungal drugs against C. gattii and C.
neoformans.
Keywords: Cryptococcus, antifungal, cytotoxicity, thiazole.

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INTRODUCTION
Cryptococcosis is an opportunistic or primary fungal infection caused by yeasts of the genus
Cryptococcus. These encapsulated yeasts can infect many animals including humans, leading to severe
clinical conditions such as meningoencephalitis [1]. The species Cryptococcus neoformans and C. gattii are
the main etiological agents of cryptococcosis, which is usually acquired by the inhalation of basidiospores or
desiccated environmental yeast [2]. The opportunistic disease, which is associated with immunosuppression,
is mainly caused by C. neoformans, while the primary infection occurs mainly in immunocompetent hosts
infected with C. gattii [3, 4].
After infection by the inhalation of infectious seedlings, Cryptococcus spp. can progress into a
regressive form and develop into extrapulmonary foci, and these foci may be reactivated years after infection
[5]. Both the opportunistic and primary forms of cryptococcosis are capable of causing meningoencephalitis
with severe and fatal outcomes, and may present no obvious lung lesions, fungemia, or secondary foci in
various organs such as the skin, bone, and kidney [6, 7].
Estimates show that one million cases of cryptococcosis occur annually, and mortality rates can reach 60%
[8, 6]. In Brazil, cryptococcosis is a fungal disease with the highest mortality rate in seropositive individuals
[9]. In sub-Saharan Africa, the mortality rate in individuals with AIDS can exceed 90%. In this region,
azoles, especially fluconazole, are used as a primary therapy for cryptococcosis, and amphotericin B and
flucytosine are usually not used due to their high cost and the difficulty of administering and monitoring
treatment [10]. Thus, fluconazole remains the only therapeutic option for most patients in sub-Saharan
Africa, which has led to an increased rate of resistance against fluconazole treatment for Cryptococcus spp.
and resulted in treatment failures [11, 12]. In Brazil, cutaneous and systemic forms of cryptococcosis
treatment are used. The systemic therapeutic regimen of choice is amphotericin B, as deoxycholate or in lipid
formulation, for 6 weeks in the acute phase of the disease, followed by fluconazole in the consolidation
phase. In pulmonary infections or when symptoms are considered clinically mild, fluconazole or itraconazole

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treatment is used for 6 to 12 months [13]. The increased incidence of invasive fungal infections is faced with
a limited arsenal of treatment options. In addition, one of the biggest problems has been the increasing
emergence of fungal resistance to the antifungal agents. Azoles drugs are the most widely used class in
medical practice and since its creation has been to increase the resistance [14]. Di Santo [14] also showed
new compounds are being studied with potent activity against C. neoformans; as A-192411.29, zofimarin and
R-135853. A-192411.29 is a novel compound derived by total synthesis from the structural template of the
echinocandins. Zofimarin is a sordarin analogue. The R-135853 is a modification of zofimarin, with
antifungal activity against C. neoformans including fluconazole-resistant strains.
Although treatments already exist for the fungal infections of clinical interest, the search for new
drugs is necessary because the increase emergence of resistance, the number of these available antifungal
drugs is limited, and they cause serious and toxic side effects [15]. Thus, there is a real need for new
therapeutic options for the treatment of cryptococcosis. With this goal, and based on the wide range of
biological activities exhibited by thiazole heterocycles [16, 17, 18], we report here the synthesis and
screening of thiazole heterocyclic compounds (Figure 1 and 2) with therapeutic potential against
cryptococcosis. Some of these molecules have shown promising antifungal activity in vitro. To the best of
our knowledge, six of the synthesized heterocycles have not been previously published (1a, 1c, 1d, 1k, 1l,
and 1m).
MATERIALS AND METHODS
1. Chemistry
Melting points were determined on a Microchemical MQAPF 301 apparatus and are reported
uncorrected. FT-IR spectra were recorded using a Perkin Elmer Spectrum One infrared spectrometer and
absorptions are reported as wave numbers (cm-1). All NMR spectra were recorded on a Bruker Avance DPX
200 spectrometer (200 MHz). Chemical shifts are given in d (ppm) scale and J values are given in Hz. All
reagents of analytical grade were obtained from commercial suppliers and used without previous purification.
The thiosemicarbazones (3a-i) and their cyclic derivatives (1a-m) were synthesized according to

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methodology previously described in the literature [17, 18, 19]. The synthesis of compounds 1b and 1f [15],
1e [20], 1g [21] 1h [22], 1i [23], and 1j [24] was previously reported in the literature. The logarithm of the
compounds’ partition coefficient (ClogP) was calculated using the ALOGPS 2.1 program
(http://www.vcclab.org/lab/alogps/).
(E)-2-[(2-benzylidenecyclohexylidene)hydrazinyl]-4-phenylthiazole (1a). Obtained as a pale yellow
1
solid, yield 66%; mp 164.2-166.8 ºC. H NMR (200 MHz, CDCl3), δ/ppm: 8.71 (1H, s), 7.75 (2H, m), 7.39
(9 H, m), 6.77 (1 H, s), 2.73-2.69 (4H, m), 1.83-1.66 (4H, m); 13C NMR (50 MHz, CDCl3), δ/ppm: 169.62,
158-29, 143.52, 136.50, 134.60, 129-9; 129-83, 129.59, 129.25, 128.19, 127.56, 125.61, 101.75, 28.64 (C-2),
28.37 (C-4), 23.79 (C-6), 22.71 (C-5); HRMS (m/z) 360.1535 [M+H]+, calcd 360.1529 C22H22N3S+.
(E)-2-[(2-benzylidenecyclohexylidene)hydrazinyl]-4-(chlorophenyl)thiazole (1k). Obtained as a pale
1
yellow solid, yield 95%; mp 180.1-181.4 ºC; H NMR (200 MHz, DMSO-d6), δ/ppm: 7.85 (2H, d, J = 8.0
13
Hz), 7.45 (2H, d; J = 8,0 Hz), 7.35 (6H, m); 7.05 (1H, s), 2.65 (4H, m), 1.67-1.56 (4H, m); C NMR (50
MHz, DMSO-d6), δ/ppm: 170.07, 136.69, 135.85, 132.10, 129.66, 128.70, 128.36, 127.30, 126.81, 104.96,
28.54, 27.54, 23.72, 22.50; HRMS (m/z) 394.1142 [M+H]+, calcd 394.1139 C22H21ClN3S+.
(E)-2-[(2-benzylidenecyclohexylidene)hydrazinyl]-4-(methoxyphenyl)thiazole (1l). Obtained as a
1
pale yellow solid, yield 79%; mp 155.9-156.9 ºC; H NMR (200 MHz, CDCl3), δ/ppm: 8.33 (1H, s), 7.69
(2H, m), 7.39-7.21 (6 H, m), 6.98 (2 H, m), 6.62 (1H, s), 3.83 (3H, s), 2.74-2.69 (4H, m), 1.83-1.66 (4H, m);
13C NMR (50 MHz, CDCl3), δ/ppm: 169.48, 160.55, 158.03, 143.34, 136.53, 134.62, 129.83, 128.18, 127,5
(C-4’’); 127.08, 114.61, 99.67, 55.34, 28.65, 28.28, 23.78, 22.70.
(E)-3-[2-(4-(4-methoxyphenyl)thiazol-2-yl)hydrazono)]butan-1-ol (1c). Obtained as a pale yellow
solid, yield 88%; mp 189.2-190.4 ºC; 1H NMR (200 MHz, DMSO-d6), δ/ppm: 7.71 (2H, d, J = 8.0 Hz), 7.17
(1H, s), 7.01 (2H, d, J = 8.0 Hz), 6.11 (1H, s), 3.78 (3H, s), 3.65 (2H, t, J = 8.0 Hz), 2.47 (2H, d J = 8.0 Hz),
2.05 (3H, s); 13C NMR (50 MHz, DMSO-d6), δ/ppm: 169.59, 159.56, 157.12, 144.52, 127.32, 124.14,
114.24, 102.05, 55.22, 55.32, 41.38, 17.17; HRMS (m/z) 392.1119 [M+H]+, calcd 392.1114 C14H18N3O2S+.

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(E)-2-[2-(cyclohexylmethylene)hydrazinyl)]-4-phenylthiazole (1d). Obtained as a pale yellow solid,
yield 76%; mp 138.3 – 140.3 ºC; HRMS (m/z) 286.1372 [M+H]+, calcd 286.1372 C16H20N3S+.
(E)-2-[2-(cyclohexylmethylene)hydrazinyl)]-4-)4-methoxyphenyl)thiazole (1m). Obtained as a pale
yellow solid, yield 96%; mp 146.3 – 148.9 ºC; 1H NMR (200 MHz, DMSO-d6), δ/ppm: 7.74 (2H, d, J = 8.0
Hz), 7.26 (1H, d, J = 6.0 Hz), 7.00 (1H, s), 6.96 (2H, d, J = 8.0 Hz), 3.76 (3H, s), 2.20 (1H, broad s), 1.73-
13
1.66 (5H, m), 1.27-1.21 (5H, m); C NMR (50 MHz, DMSO-d6), δ/ppm: 168.58, 158.73, 149.95, 149.43,
127.59, 126.82, 113.89, 100.77, 55.07, 39.92, 29.85, 25.56, 25.00.
2. In vitro cytotoxicity assay
2.1. Cell lines
The VERO (African green monkey kidney cells) lineage was used as a model of normal cells and was
kindly provided by Dr. Erna Kroon (Universidade Federal de Minas Gerais, UFMG). This lineage was
maintained in the logarithmic phase of growth in D-MEM supplemented with 100 IU/mL penicillin and 100
ꢀg/mL streptomycin enriched with 5% fetal bovine serum. VERO cells were maintained at 37°C in a
humidified incubator with 5% CO2 and 95% air. The medium was changed twice weekly, and the cells were
regularly examined and used until 20 passages.
2.2. Peritoneal macrophage isolation and cell culture
Male C57BL/6 mice (20–22g) were obtained from the Biotery Center (CEBIO) of the Institute of
Biological Sciences, UFMG. Animal experiments were conducted in accordance with current ethical
regulations on animal research (221/2013). Mice were randomized and housed in plastic cages in groups of
five per cage. The animals were maintained under standard laboratory conditions at a temperature of 25 ±
2ºC and a photoperiod of 12 h, and received standard mouse chow and water ad libitum.
Peritoneal macrophages were obtained from 6 to 8-week-old C57BL/6 mice. Mice were
intraperitoneally injected with 2 mL of 3% sodium thioglycollate (Amresco, Solon, OH, USA). After 72 h,
mice were euthanized and dipped in 70°GL alcohol for disinfection. The peritoneal membrane was exposed
and 3 mL of ice-cold PBS was injected into the peritoneal cavity to collect macrophages. Peritoneal lavage

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was obtained and centrifuged (10 min, 150 × g, 4°C). The cell pellet was suspended in RPMI (Sigma, St.
Louis, MO, USA) with 10% fetal bovine serum (Cultilab, Campinas, Brazil), 100 U/mL penicillin, and 100
ꢀg/mL streptomycin (Sigma, St. Louis, USA), and cell density was determined by counting in a Neubauer
chamber. The macrophages of each mouse were seeded (1 x 105 cells/well) in 96-well plates and incubated
for 12 h at 37°C in an incubator with 5% CO2.
2.3. Evaluation of the cytotoxic effects against VERO cells and murine macrophages
VERO cells at 1 x 104 cells per well and murine macrophages at 1 x 105 cells/well were pre-incubated
in 96-well plates for 24 h at 37°C to allow cells to adapt prior to the addition of the test compounds. All
experimental compounds and amphotericin B (Sigma, St Louis, USA) were dissolved in DMSO prior to
dilution, with the exception of fluconazole (Sigma, St Louis, USA), which was dissolved in sterile deionized
water. The half-maximal inhibitory concentration (IC50) was determined over a range of concentrations (eight
nonserial dilutions: from 100 to 0.0128 ꢀM). VERO cells and peritoneal macrophages were incubated with
the compounds or 0.5% DMSO as a control in a 5% CO2/95% air-humidified atmosphere at 37ºC for 48 h.
Cell viability was estimated by measuring the rate of the mitochondrial reduction of tetrazolium–dye, 3-(4,5-
dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) (Amresco, Solon, USA).
Controls included drug-containing medium (background) and drug-free complete medium. Drug-free
complete medium was used as a control (blank) and was treated in the same way as the drug-containing
media. Results were expressed as a percentage of inhibition of cell viability compared to the 0.5% DMSO
control and were calculated as follows: % inhibition of cell viability (%) = 100 - (mean OD treated – mean
OD background)/(mean OD untreated culture, i.e. 0.5% DMSO – mean OD blank wells) x 100. Interactions
between compounds and media were estimated on the basis of variations between drug-containing media and
drug-free media to avoid false-positives or false-negatives [27]. All samples were tested in triplicate in two
independent experiments. The cytotoxicity of fluconazole and amphotericin B was evaluated under the same
experimental conditions as the positive controls.
2.4. Selectivity index determination

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The IC50 and minimum inhibitory concentration (MIC) data were used to calculate the selectivity
index (SI) of each compound as was done previously by Protopopova et al. [28]. The SI was determined by
the ratio of the IC50 value in VERO cells or murine macrophages to the IC50 value in MIC and calculated as
SI = IC50/MIC.
2.5. Statistical analysis
The IC50 value for cytotoxicity activity was determined using Prism 5.0® (GraphPad Software Inc.).
Data were presented as median and 95% confidence interval. Statistical differences between the treatments
and the control were evaluated by an analysis of variance (ANOVA) test. Statistical differences between the
treatments and the control were evaluated by an ANOVA test followed by a Bonferroni test (P > 0.05).
3. Determining the antifungal activity
3.1. Fungal strains and inoculum quantification
Cryptococcus gattii (ATCC24065, ATCC32608) and C. neoformans (ATCC24067, ATCC28957,
ATCC62066) were obtained from the Culture Collection of the University of Georgia (Atlanta, GA, USA).
Ten isolates of C. gattii (L135/03, L28/02, 23/10993, 196L/03, 1913 ER, L27/01, LMM 818, L24/01 -
clinical isolates; 547/OTTI/94-PI-10, ICB 181 - environmental isolates), and nine clinical isolates of C.
neoformans (96806, 5396, LMM820, F10, 28JF, VM-MCMMPI, 27JF, WP – clinical isolates; CN31 -
environmental isolate), all from the culture collection of the Mycology Laboratory/Institute of Biological
Sciences - UFMG, were used in this study. All yeast strains were stored frozen at -80ºC.
Starting inoculates of Cryptococcus isolates were cultured onto Sabouraud dextrose agar (SDA,
Himedia, Mumbai, India) twice for 48 h at 35°C to ensure purity and viability. The inoculum was prepared
by picking distinct colonies that were suspended in 5 mL of sterile saline (0.85% NaCl), and the
transmittance was adjusted to 70% in the wavelength of 530 nm. Therefore, dilution was carried out in RPMI
medium supplemented with L-glutamine and buffered to pH 7.0 with 0.165 M morpholine propanesulfonic
acid (MOPS) (Sigma, St Louis, USA), adjusting the concentration of the yeast to 1-5 x103 cells/mL, in
accordance with M-27-A3 [29].

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3.2. Determination of minimum inhibitory and fungicidal concentration
Broth microdilution testing was performed in accordance with the guidelines in the CLSI document
M27-A3 [29]. Susceptibility was determined by the microbroth dilution method, which was performed in
sterile flat-bottom 96-well microplates (Difco Laboratories, Detroit, MI, USA). The compounds were
dissolved in DMSO (Vetec, RJ, Brazil) and diluted in synthetic RPMI medium. Later, serial dilutions were
performed in RPMI. The experimental compounds were tested at concentrations of 0.1 to 62.5 µM. As
growth and sterility controls, RPMI alone was used, without the addition of the compounds and solvent.
Fluconazole (0.97 to 125 µM) and amphotericin B (0.05 to 7.8 µM) were included as the positive antifungal
controls. The plates were incubated at 35°C for 48-72 h. The endpoints were determined visually by
comparison with the endpoints of the drug-free growth-control wells. The value of the minimum inhibitory
concentration (MIC) was defined as the lowest compound concentration (µM) at which the well was optically
clear. The minimal fungicidal concentration (MFC) of each compound tested was determined as described by
Espinel-Ingroff [30].
3.3. Time-kill curve procedures
The time-kill curve was determined for compounds against C. gattii and C. neoformans isolates. In
this experiment, the required time of contact between fungi and antifungal drug necessary to kill 100% of the
yeast was determined. Microplates were prepared in accordance with the methodology described for the MIC
assay containing the experimental substances and the antifungal control (amphotericin B and fluconazole)
only the concentrations used were equal to the MIC and twice the MIC in the RPMI medium. The plates were
then incubated at 35°C for 0, 6, 12, 24, 48, or 72 h. In sequence, we added MTT (5.0 mg/mL) to determine
the reduction in the metabolic cell activity. The plates were incubated at 35°C for 3 h, after which
isopropanol was added before the spectrophotometric reading was carried out at 490 nm. Culture controls
without antifungal drug were subjected to the same procedures at the same intervals. The results were

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confirmed by plating an amount of sample from each well on SDA for colony counting [31]. All kill curves
were performed in triplicate.
3.4. Checkerboard microtiter test
Eight serial two-fold dilutions of experimental compounds (0.11 to 15.6 ꢀM) and amphotericin B
(0.025 to 3.9 ꢀM) were prepared with the same solvent (DMSO) and dilutions of fluconazole (0.48 to 62.5
ꢀM) were prepared with water as in the MIC test. The checkerboard was prepared in a microtiter plate for
multiple combinations of two antimicrobial agents. Each row (x-axis) in the plate contained the same diluted
concentration as the first antimicrobial compound, while the concentration in each subsequent row was half
of this value. Similarly, each column (y-axis) in the plate contained the same diluted concentration as the
second antimicrobial compound, while the concentration in each subsequent column was half of this value.
The concentration of the drug combination at which growth was completely inhibited was taken as the
effective MIC for the combination. One hundred microliters of inoculate suspension was added to each well
and cultured for 48 to 72 h. Fractional inhibitory concentrations (FICs) were calculated as the MIC of the
combination of drugs divided by the MIC of drug alone. The same calculation was performed for all
experimental compounds. The FIC index (FICI) was calculated by adding both FICs, and this was interpreted
in the following manner: an effect was classified as synergistic when its value was ≤0.5, as no interaction
when the value fell within the 0.5–4.0 range, and as antagonistic when its value was >4.0 [32].
3.5. Ergosterol quantification
Ergosterol quantification in the fungal cell membrane was performed as described previously by
Arthington-Skaggs et al. [33] with modifications. We selected the L27/01 and 196/03 isolates of C. gattii as
representatives of antagonistic interactions observed between amphotericin B and compound 1c.
Approximately 25 mg of the fungal cell mass was transferred to polypropylene tubes. The yeasts were then
treated with 1c, amphotericin B, and fluconazole at concentrations equal to the MIC and with a combination
of amphotericin B and 1c (both at MIC) in YPD medium for 24 h under agitation at 37°C. A growth control
assay was also performed. After incubation, the tubes were centrifuged (Jouan, model BR4i) at 7000 rpm for

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10 min at 4°C, and the supernatant was removed. The cells were washed with sterile distilled water. The wet
weight of the cell pellet was determined. For the extraction of lipids, 3 mL ethanolic solution of 25%
potassium hydroxide was added to each cell mass, followed by agitation for 1 min. The tubes were incubated
in a water bath at 85°C for 1 h and then cooled at room temperature. A mixture of 1 mL sterile water and 3
mL n-heptane was added, followed by agitation in a vortex for 3 min. The supernatant was removed, and the
reading was performed using a spectrophotometer (Hitachi U-1100) at 282 and 230 nm. A calibration curve
with standard ergosterol (Sigma-Aldrich) was constructed and used to calculate the amount of ergosterol. The
ergosterol levels were expressed as a percentage of the ergosterol in the growth control.
3.6. Post-antifungal effect
The post-antifungal effect (PAFE) was determined for compound 1c in combination with
amphotericin B for isolate L27/01 of C. gattii and isolate CN31 of C. neoformans, which showed
antagonistic effect when treated with these combinations of drugs. We prepared 96-well plates in according
to the methodology described in the Checkerboard microtiter test (item 3.4) with inoculum of the 103
cells/mL. The concentrations tested for the drugs alone and in combination against C. gattii were 0.11 µM for
amphotericin B and 0.45 µM for 1c. For C. neoformans the concentrations were 0.05 µM for amphotericin B
and 0.9 µM for 1c. A growth control with 104 cells/mL was performed for the representative increase of 1
log10 of growth for each strain of the Cryptococcus spp.. The plates were incubated at 35°C for 1 h. After
incubation, the plates were centrifuged (Jouan, model BR4i) at 4000 rpm at 4°C for 30 minutes. The
supernatant was removed from each well, and the plates were washed twice with RPMI 1640 medium. A
volume of 100 µL was added and the plates were incubated at 35°C for 24 h. At different times, we added
MTT (5.0 mg/mL) to determine the reduction in the metabolic cell activity. The plates were incubated at
35°C for 3 h, after which isopropanol was added before the spectrophotometric reading that was carried out
at 490 nm. The PAFE was calculated as the time that each strain took to achieve the cell population
represented by the control wells (104 cells/mL), which is equivalent to an increase of 1 log10 in the yeast
number [31].

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RESULTS
Synthesis
Initially, 2-benzylidenecyclohexanone 2 was synthesized by an aldol reaction between benzaldehyde
and cyclohexanone, producing a 67% yield. Subsequently, the appropriate aldehyde or ketone, including 2,
were subjected to reaction with thiosemicarbazide to give the thiosemicarbazones 3a-i (51-97% yield) [19,
20], which were converted into the corresponding thiazole derivatives 1a-m (66-98% yield), according to the
procedure outlined by Dimmock et al. [21] (Scheme 1). To correlate lipophilicity with antifungal activity, the
logarithm of the compounds’ partition coefficient (ClogP) was calculated, and the values obtained ranged
from 2.51 to 6.91.
In vitro activity
Thirteen thiazole heterocyclic compounds were initially assayed for their MICs against C. neoformans
(ATCC24067) and C. gattii (ATCC24065). In this first test, 1e, 1f, 1g, 1i, and 1j showed MIC values >250
ꢀM (Table 1). This value was much higher than that of the other compounds tested, which had MICs from
0.45 to 15.6 ꢀM, which were values considered promising because they resemble the MICs of amphotericin
B and fluconazole. From these results, toxicity against VERO cells and murine macrophages was evaluated
only for 1a, 1b, 1c, 1d, 1k, 1l, and 1m (Table 2), which were the compounds that showed the best MIC
values in this study. These compounds did not affect the viability of VERO cells and murine macrophages at
concentrations up to 100 ꢀM, which is reflected by the IC50 values of greater than 100 ꢀM. The only
difference between the results of the two cytotoxicity assays was with amphotericin B, which in an assay
with macrophages showed an IC50 value of 53.37 ± 18.46 ꢀM, although VERO cell toxicity was not detected.
Considering the MIC values for C. neoformans (ATTCC24067) and C. gattii (ATCC24065) and the toxicity
results for VERO cells and murine macrophages, the SI was calculated. Compounds 1b, 1c, 1d, and 1m
showed the highest SI values ( ≥ 26), and, therefore, were selected for follow-up tests.

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Antifungal activity against twelve strains of C. gattii showed MIC values ranging from 1 to 3.4 ꢀM
for compound 1b, 0.9 to 1.7 ꢀM for 1c, 0.7 to 2.2 ꢀM for 1d, and 1.4 to 4.5 ꢀM for 1m. These MIC values
were smaller than that observed for fluconazole, (7.8 to 41.7 ꢀM). Amphotericin B exhibited MIC values
ranging from 0.04 to 0.14 ꢀM (Table 3). MIC values of compounds against strains of C. neoformans ranged
from 1.2 to 3.5 ꢀM for 1b, 0.5 to 1.2 ꢀM for 1c, 1.2 to 2.4 ꢀM for 1d, and 1.9 to 3.5 ꢀM for 1m. Concerning
amphotericin B and fluconazole, MIC values were 0.04 to 0.14 ꢀM and 3.2 to 31.2 ꢀM, respectively.
However, based on the Newman-Keuls multiple comparison test, the MIC50 and MIC90 values of compounds
did not differ significantly from amphotericin B. In contrast, there was a statistically significant difference
between the experimental compounds and fluconazole. The same was true for both species of Cryptococcus.
The activity of thiazole compounds was reflected in the death curve constructed for the average of all
isolates of C. gattii and C. neoformans tested (Figure 3). Compounds 1c and 1d and amphotericin B showed
about a 90% reduction in viable cells of C. gattii over 72 h, while this reduction ranged from 80-90% for
compounds 1b and 1m and fluconazole in the same period. Curves constructed for C. neoformans had similar
kinetic profiles between compounds and control drugs.
The average FICI for 1b, 1c, 1d, and 1m in combination with fluconazole showed values of
interaction indifferent to antagonism, yet the geometric mean for the twelve isolates demonstrated the
absence of an interaction (Table 4a and b). The combination with amphotericin B obtained similar results as
the combination with fluconazole, with the exception of 1c against C. gattii, which exhibited a geometric
mean of 4.29, which indicated antagonism (Table 5a and b). Ergosterol quantification was performed to
evaluate the influence of ergosterol content on the interaction between 1c and amphotericin B. We observed
that only the fluconazole treatment promoted a reduction in ergosterol content, while treatment with
amphotericin B and 1c alone or in combination did not significantly change the amount of ergosterol present
in C. gattii cells (Figure 4).
The PAFE found to C. gattii for combination between 1c and amphotericin B was equal to 2.15 h,
while for drugs alone was 2.65 h (amphotericin B) and 1.7 h (1c). The PAFE found for C. neoformans was

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2.1 h for the combination, 2.75 h (amphotericin B) and 2.8 h (1c). In both species the antagonism between
compound 1c and amphotericin B reflected in decreased of the PAFE when compared to the effect of these
drugs alone (Table 6).
DISCUSSION
Fungal infections are a major clinical problem and, compared to bacterial infections, limited
therapeutic agents are available for their treatment, among which only some are able to identify unique
targets that are not shared with human hosts [34]. Moreover, currently several cases of resistance to current
antifungals have been described [35, 13, 36].
The present work evaluated seven compounds previously described in the literature and six
unpublished compounds, of which three (1c, 1d and 1m) were found to be promising for the study of
cryptococcosis in an animal model. Of the seven known compounds, only 1b showed no cytotoxicity and
promising antifungal activity against C. gattii and C. neoformans. Furthermore, Barthi et al. [18] had already
reported the antifungal activity of 1b against Candida albicans, C. neoformans, and Aspergillus flavus.
Our group has focused on the discovery of fungicidal compounds with low toxicity. In this study, we
used VERO cells and murine macrophages as models of normal mammalian cells to evaluate the toxicity of
new compounds, and found that the compounds we tested were selective for cells of Cryptococcus spp., with
less toxicity against the mammalian cells used in this study. Compounds with high selectivity offer the
potential for safer therapy with fewer side effects and a mechanism to identify candidates for efficacy studies
in mice [37]. There are several reports in the literature that have used these cell types for assessing
cytotoxicity in the process of drug development [28, 37, 39, 40]. Magalhães et al. [37] tested new active
hydroxyaldimines against VERO cells and found a low toxicity and high selectivity of these compounds;
hydroxyaldimine 3A8 was more selective to Cryptococcus spp. than to VERO cells. From these findings, the
authors considered these compounds good candidates for in vivo studies.
We present four thiazole heterocyclic compounds (1b, 1c, 1d and 1m) that exhibited especially
excellent activity against Cryptococcus spp. and which showed no toxic effects in mammalian cells. These

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compounds have pharmaceutically acceptable compositions and are easily synthesized from commercially
available starting materials with good yields.
In the present study, we observed that the thiazole compounds were able to inhibit and kill the yeast,
showing promising activity in vitro against both C. gattii and C. neoformans. The death curves obtained for
these compounds were comparable to that observed for the drugs amphotericin B and fluconazole. It is also
interesting to note that there was a good correlation between the antifungal activity and ClogP. Ideally, a
good lead compound must have a ClogP of less than 5 for a drug to passively cross biological membranes
[38]. The most promising compounds 1b (ClogP = 3.88), 1c (ClogP = 2.51), 1d, and 1 m (ClogP = 5.24)
showed ClogP within or very close to this range (Table 2).
When testing the in vitro interactions between amphotericin B and fluconazole, we found that the
average CIFI values were indifferent to the combination or showed antagonism depending on the strain
tested; specifically, 1c with amphotericin B showed that the average CIFI of 11 isolates of C. gattii were
equal to 4.29 ꢀM, which is considered an antagonistic interaction. Some studies suggest that antagonism
between two or more drugs may occur because they compete for the same target, while no observable
interaction indicates that the drugs in combination have no adverse effect on the therapeutic response [41,
42]. Santos et al. [31] evaluated whether an interaction occurred between amphotericin B and fluconazole
against various isolates of C. gattii and noted the existence of an interaction between these drugs ranging
from synergism to antagonism, depending on the isolate and concentration tested. These authors also showed
that the antagonistic effects of these drugs could be related to the reduction of ergosterol by fluconazole,
leading to lower activity of amphotericin B when used in combination with fluconazole.
In the present study, the incidence of antagonism between the compound 1c and amphotericin B could
not be related to the reduced content of ergosterol in the fungal membrane, which was observed only in cells
treated with fluconazole. Generally, azoles interfere with the ergosterol biosynthesis pathway resulting in less
extracted ergosterol. In cells treated with 1c alone as well as in cells treated with the combination of
amphotericin B and 1c, no change in ergosterol biosynthesis was observed, which excludes the possibility

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that the antagonism observed was a result of the action of ergosterol in the fungal membrane, culminating in
the reduced action of amphotericin B.
The PAFE is a drug-related factor important for determination of the in vivo efficacy of antifungal
compounds. The PAFE refers to the suppression of fungal growth that persists following brief exposure of
organisms to an antifungal drug and subsequent removal of the drug [43]. The PAFE found for amphotericin
B is in agreement with the results of Santos et al. [31], who observed PAFE values from 1 to 3 h, with
average PAFE equal to 2.1 h.
CONCLUSION
In this study, we investigated thiazole heterocyclic compounds with potential antifungal activity
against Cryptococcus spp. that are easily synthesized and have low toxicity in mammalian cells. Thus, we
present important results primarily regarding the unpublished compounds 1c, 1d, and 1m and the known
compound 1b as promising candidates for in vivo studies concerning the treatment of cryptococcosis.
Acknowledgements
This work was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq),
Fundação de Amparo Pesquisa Estado de Minas Gerais (FAPEMIG), Coordenação de Aperfeiçoamento de
Pessoal de Nível Superior (CAPES) and Pró-Reitoria de Pesquisa da UFMG.
Conflicts of interest
All authors: None to declare.
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TABLE 1. Screening with 13 thiazole heterocyclic compounds to determine the minimum inhibitory
concentration (MIC) in ꢀM against Cryptococcus neoformans (ATCC24067) and Cryptococcus gattii
(ATCC24065).
C. neoformans
(ATCC24067)
ꢀM
C. gattii
(ATCC24065)
ꢀM
Compounds
15.6
15.6
1a
1.9
1.9
1b
0.45
0.9
1c
0.9
0.9
1d
>250
>250
>250
15.6
>250
>250
>250
15.6
1e
1f
1g
1h
>250
>250
7.8
>250
>250
7.8
1i
1j
1k
7.8
7.8
1l
1m
1.9
3.9
Amphotericin B
0.486
0.486

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TABLE 2. Cytotoxicity (IC50), selectivity index (SI), and ClogP in VERO cells and murine macrophages.
IC50VERO
µM
SI
IC50 Macrophage
µM
SI
CLogP
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
6.41
29
>100
6.41
29
6.31*
3.88*
2.51*
5.24*
5.25*
6.91*
6.47*
5.24*
0.58**
-0.66**
1a
>100
1b
111 a // 222 b
111
>100
222
1c
>100
111
1d
6.41
>100
6.41
1h
12.8
>100
12.8
1k
1l
12.8
>100
12.8
52.63a // 25.64b
15.74a // 7.87b
205.76
>100
52.63a // 25.64b
15.74a // 7.87b
109.81
1m
Fluconazole
Amphotericin B
>100
53.37±18.46
(*) Clog P values were calculated using the ALOGPS 2.1 program (http://www.vcclab.org/lab/alogps/), (**)
ClogP values were calculated using the XlogP3 program. http://www.vcclab.org/lab/alogps/ (programa
b
ALOGPS 2.1). SI, selectivity index (SI = IC50/MIC). (a), Cryptococcus neoformans (ATCC24067). ( ),
Cryptococcus gattii (ATCC24065).

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TABLE 3. Minimum inhibitory concentration (MIC) and minimum fungicidal concentration (MFC) against twelve Cryptococcus gattii strains
and twelve C. neoformans strains.
Isolates of
C. gattii
1c
1d
1b
1m
Amphotericin B
Fluconazole
MIC
MFC
MIC
MFC
MIC
MFC
MIC
MFC
MIC
MFC
MIC
MFC
(µM)
(µM)
(µM)
(µM)
(µM)
(µM)
(µM)
(µM)
(µM)
(µM)
(µM)
13.0
13.0
13.0
(µM)
0.9
1.4
1.4
1.9
3.9
3.9
1.5
1.4
1.9
3.9
1.9
7.8
1.5
2.2
2.9
1.9
3.9
3.9
1.9
2.9
2.6
3.9
3.9
3.9
0.07
0.14
0.08
0.2
0.9
0.1
31.3
62.5
ATCC 24065
ATCC 32608
23/10993
>62.5
1.4
1.7
3.9
3.9
1.9
1.9
7.8
7.8
2.7
1.9
3.9
7.8
4.2
2.6
7.8
3.9
0.08
0.04
0.1
0.1
26.0
13.0
>62.5
>62.5
L28/02
547/OTTI-97-PI-10
135L/03
0.9
1.4
0.8
0.7
1.4
1.4
0.9
1.4
1.4
3.9
3.9
1.9
1.9
3.9
3.9
1.9
3.9
3.9
1.9
1.4
1.0
0.7
1.4
2.2
1.4
1.4
1.9
3.9
3.9
1.9
3.9
3.9
3.9
7.8
3.9
7.8
3.1
2.2
1.5
1.0
1.7
3.4
1.0
1.7
3.1
3.9
7.8
1.9
1.9
7.8
3.9
7.8
3.9
7.8
2.6
2.9
2.2
1.4
2.9
4.5
2.2
2.6
4.2
3.9
3.9
3.9
1.9
3.9
7.8
3.9
3.9
7.8
0.14
0.04
0.08
0.12
0.04
0.14
0.08
0.08
0.14
0.5
0.2
0.1
0.9
0.1
0.2
0.1
0.2
0.9
7.8
26.0
41.7
10.4
13.0
20.8
13.0
13
31.3
>62.5
>62.5
62.5
196L/03
L27/01
ICB181
>62.5
>62.5
>62.5
>62.5
>62.5
L24/01
LMM 818
1913 ER
MIC50 / / MFC50
MIC90 / / MFC90
26

ACCEPTED MANUSCRIPT
Isolates of
1c
1d
1b
1m
Amphotericin
Fluconazole
C. neoformans
MIC
MFC
MIC
MFC
MIC
MFC
MIC
MFC
MIC
MFC
MIC
MFC
(µM)
(µM)
(µM)
(µM)
(µM)
(µM)
(µM)
(µM)
(µM)
(µM)
(µM)
(µM)
0.6
0.6
0.6
0.5
0.9
0.5
0.5
0.7
0.6
0.5
1.2
0.9
0.6
0.9
3.9
3.9
15.6
7.8
3.9
3.9
3.9
3.9
3.9
3.9
7.8
3.9
3.9
7.8
1.4
1.2
1.2
1.4
1.9
1.4
1.7
1.7
2.7
1.4
2.4
2.2
1.4
2.2
3.9
7.8
7.8
1.9
3.9
1.9
1.9
3.9
7.8
3.9
7.8
3.9
3.9
7.8
3.5
2.2
1.2
1.9
1.9
1.2
2.2
2.2
2.6
1.6
3.2
3.2
2.2
3.2
7.8
7.8
3.9
1.9
7.8
0.9
3.9
7.8
1.9
7.8
7.8
3.9
3.9
7.8
3.5
2.2
1.9
1.9
1.9
2.2
3.2
2.2
3.5
2.2
3.5
3.2
2.2
3.5
7.8
3.9
7.8
3.9
1.9
3.9
7.8
3.9
7.8
3.9
7.8
3.9
3.9
7.8
0.09
0.04
0.03
0.08
0.09
0.06
0.06
0.06
0.11
0.07
0.07
0.09
0.07
0.09
0.22
0.22
0.22
0.45
0.45
0.11
0.22
0.11
0.22
0.22
0.45
0.22
0.22
0.45
7.8
62.5
62.5
ATCC 24067
3.2
ATCC 28957
ATCC 62066
CN31
13.0
6.5
>62.5
31.25
>62.5
62.5
31.2
23.4
13.0
10.4
7.8
96806
5396
62.5
LMM 820
F10
>62.5
>62.6
15.6
28JF
11.7
10.4
15.6
10.4
23.4
VM-MCMMPI
27JF
>62.5
>62.5
>62.5
>62.5
WP
MIC50 / / MFC50
MIC90 / / MFC90

ACCEPTED MANUSCRIPT
TABLE 4. Fractional inhibitory concentration index values for compounds 1b, 1d, 1m, and 1c in combination with fluconazole against twelve
Cryptococcus gattii (a) strains and twelve Cryptococcus neoformans (b) strains. Ind., Indifferent; Ant., Antagonism.
(a)
1d
1c
1b
1m
FICI (µM) Interaction FICI (µM) Interaction FICI (µM) Interaction FICI (µM) Interaction
1.72
2.47
4.47
2.47
2.47
2.47
2.47
4.23
2.09
1.58
1.47
2.58
2.41
Ind.
Ind.
Ant.
Ind.
Ind.
Ind.
Ind.
Ant.
Ind.
Ind.
Ind.
Ind.
2.32
1.95
3.31
2.38
1.01
3.32
2.32
2.14
2.47
0.88
2.28
1.99
2.06
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
3.45
1.84
5.96
2.45
4.22
6.46
1.59
1.45
2.45
3.71
4.23
4.58
3.17
Ind.
Ind.
Ant.
Ind.
Ant.
Ant.
Ind.
Ind.
Ind.
Ind.
Ant.
Ant.
Ant.
Ind.
Ind.
Ind.
Ant.
Ant.
Ind.
Ind.
Ind.
Ind.
Ind.
Ant.
ATCC32608
196L/03
4.38
2.28
3.46
2.71
4.38
6.79
1.19
1.18
2.46
2.43
2.46
4.46
2.82
547/OTTI-97-PI-10
135L/03
L28/02
L24/01
LMM818
L27/01
1913ER
ATCC24065
23/10993
ICB181
Geometric mean

ACCEPTED MANUSCRIPT
(b)
1d
1c
1b
1m
FICI (µM) Interaction FICI (µM) Interaction FICI (µM) Interaction FICI (µM) Interaction
1.99
1.76
2.04
3.97
2.47
2.4
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ant.
Ant.
Ant.
Ant.
Ind.
Ant.
1.47
2.01
2.3
Ind.
Ind.
Ind.
Ant.
Ind.
Ind.
Ant.
Ant.
Ind.
Ind.
Ind.
Ant.
3.77
1.01
2.46
2.71
2.46
3.46
4.21
3.33
4.46
3.36
4.46
1.98
2.93
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ant.
Ind.
Ant.
Ind.
Ant.
Ind.
Ant.
Ind.
Ind.
Ind.
Ind.
Ind.
Ant.
Ant.
Ant.
Ind.
Ind.
Ind.
28JF
4.59
1.01
3.58
2.58
2.21
3.48
4.46
4.59
4.59
2.51
3.97
2.46
3.09
96806
ATCC62066
CN31
4.38
2.32
1.32
4.33
7.44
2.1
WP
5396
VM-MCMMPI
27JF
4.47
6.8
LMM820
ATCC28957
ATCC24067
F10
4.24
4.47
3.59
4.47
3.28
2.28
1.92
4.33
2.64
Geometric mean

ACCEPTED MANUSCRIPT
TABLE 5. Fractional inhibitory concentration index values for compounds 1b, 1d, 1m, and 1c in combination with amphotericin B against
twelve Cryptococcus gattii strains and twelve Cryptococcus neoformans strains. Ind., Indifferent; Ant., Antagonism.
1d
1c
Interaction
1b
1m
FICI (µM)
2.58
Interaction FICI(µM)
FICI (µM)
1.7
Interaction
Ind.
FICI(µM) Interaction
Ind.
Ind.
Ant.
Ind.
Ind.
Ant.
Ind.
Ind.
Ind.
Ind.
Ind.
Ant.
3.14
9.38
2.98
4.98
2.78
4.98
4.98
4.61
2.78
2.98
3.51
9.55
4.29
Ind.
Ant.
Ind.
Ant.
Ind.
Ant.
Ant.
Ant.
Ind.
Ind.
Ind.
Ant.
2.78
1.58
2.58
1.46
2.18
2.38
1.46
2.66
1.75
2.00
2.36
1.73
2.03
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
ATCC32608
196L/03
3.97
1.46
2.66
1.46
2.66
2.16
2.58
2.48
1.04
1.2
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
547/OTTI-97-PI-10
135L/03
4.74
2.67
1.34
L28/02
L24/01
4.87
2.58
1.34
LMM818
L27/01
1.47
1913ER
1.72
2.67
Ind.
Ind.
Ind.
ATCC24065
23/10993
2.18
1.28
1.81
ICB181
4.98
2.61
Geometric mean
1d
1c
Interaction FICI(µM) Interaction FICI (µM)
1b
1m
FICI(µM) Interaction
FICI (µM)
1.38
Interaction
Ind.
Ind.
2.58
2.78
2.58
4.98
2.78
1.58
1.58
2.67
2.58
1.98
2.78
2.47
2.50
Ind.
Ind.
Ind.
Ant.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
2.46
2.46
3.06
2.66
1.46
4.86
2.66
2.66
2.66
2.22
1.46
2.58
2.48
1.75
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
28JF
2.67
2.47
2.47
2.41
1.15
1.47
2.58
4.3
1.09
2.67
2.62
2.11
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ant.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
Ant.
Ind.
Ind.
Ind.
Ind.
Ind.
Ind.
1.46
2.58
2.78
1.46
2.07
2.16
2.78
2.78
2.07
2.36
1.46
2.08
96806
ATCC62066
CN31
WP
5396
VM-MCMMPI
27JF
LMM820
ATCC28957
ATCC24067
F10
Geometric mean