Synthesis and antimycobacterial activity of some isonicotinoylhydrazones

Article abstract of DOI:10.1016/S0223-5234(99)00124-5

A series of isonicotinoylhydrazones 2 were prepared by addition of some aryloxyacetonitriles with isonicotinoylhydrazine in basic medium. These compounds have been further reacted with pyridinecarboxaldehydes to give the corresponding pyridylmethyleneamino derivatives 3-5. The new synthesized hydrazones and their pyridylmethyleneamino derivatives were tested for their activity against mycobacteria, Gram-positive and Gram-negative bacteria. The cytotoxicity was also tested. Several compounds showed a good activity against Mycobacterium tuberculosis H37Rv and some isonycotinoylhydrazones 2 showed a moderate activity against a clinically isolated M. tuberculosis which was isoniazid resistant.

Full text of DOI:10.1016/S0223-5234(99)00124-5

Eur. J. Med. Chem. 34 (1999) 1071−1076
© 1999 Éditions scientifiques et médicales Elsevier SAS. All rights reserved
1071
Short communication
Synthesis and antimycobacterial activity of some isonicotinoylhydrazones
Maria T. Coccoa*, Cenzo Congiua, Valentina Onnisa, Maria C. Pusceddub, Maria L. Schivob,
Alessandro De Logub
aDipartimento di Tossicologia, Università di Cagliari, Viale A. Diaz 182 - 09126 Cagliari, Italy
bDipartimento di Scienze Chirurgiche e Trapianti d’Organo, Sezione di Microbiologia e Virologia, Università di Cagliari, Via Palabanda
14 - 09123 Cagliari, Italy
(Received 15 February 1999; accepted 8 June 1999)
Abstract – A series of isonicotinoylhydrazones 2 were prepared by addition of some aryloxyacetonitriles with isonicotinoylhydrazine in basic
medium. These compounds have been further reacted with pyridinecarboxaldehydes to give the corresponding pyridylmethyleneamino
derivatives 3–5. The new synthesized hydrazones and their pyridylmethyleneamino derivatives were tested for their activity against
mycobacteria, Gram-positive and Gram-negative bacteria. The cytotoxicity was also tested. Several compounds showed a good activity against
Mycobacterium tuberculosis H37Rv and some isonycotinoylhydrazones 2 showed a moderate activity against a clinically isolated M.
tuberculosis which was isoniazid resistant. © 1999 Éditions scientifiques et médicales Elsevier SAS
aminohydrazone derivatives / synthesis / antimycobacterial activity
1. Introduction
suggest that INH inhibits the biosynthesis of cell wall
mycolic acids, thereby making the mycobacteria suscep-
tible to reactive oxygen radicals and other environmental
factors [7]. INH is active against the Mycobacterium
complex (Mycobacterium tuberculosis, M. bovis, M.
africanum and M. microti) at MICs ranging from
0.025–0.05 µg/mL [8], while at a higher concentration
(MIC of 500 µg/mL) it inhibits the growth of other
microorganisms such as the opportunists Klebsiella, Ser-
ratia and Enterobacter.
For these reasons the antimycobacterial pharmacoph-
ore moiety of INH is introduced in several molecules to
improve their activity against Mycobacteria.
On the other hand aminohydrazone derivatives, struc-
turally correlated to INH, have been described for their in
vitro antimycobacterial activity and some of these com-
pounds exhibited inhibitory activity toward a human
strain of M. avium resistant to the primary drugs INH,
rifampicin and ofloxacin [9, 10]. As a part of our studies
on aminohydrazone derivatives [11, 12], we became
At present, tuberculosis is considered, by the World
Health Organisation, to be the most important chronic
communicable disease in the world [1, 2, 3]. Over the
past decade, tuberculosis has re-emerged both in indus-
trial and developing countries. The emergence of AIDS,
decline of socioeconomic standards and a reduced em-
phasis on tuberculosis control programs contribute to the
disease’s resurgence in industrialised countries [4]. In
most developing countries, although the disease has
always been endemic, its severity has increased because
of the global HIV endemic and extensive social restruc-
turing due to rapid industrialisation and conflicts.
Further contributing to the increased morbidity is the
emergence of new strains of M. tuberculosis resistant to
some or all current antitubercular drugs [5, 6]. Among
these, isoniazid (INH), an inexpensive and relatively safe
drug, continues to be well established for the treatment of
tuberculosis. The mechanism of action of INH, as well as
the mechanism conferring INH resistance, are complex
and not completely understood. However, several studies
interested in
a
new series of 2-amino-2-(iso-
nicotinoylhydrazono)ethyl aryl ethers, assuming that the
isonicotine hydrazonic moiety is an important pharma-
cophore to antimycobacterial activity. Here, we report the
*Correspondence and reprints

1072
Table I. Chemical and spectral data of compounds 2.
Compound
R
Yield
(%)
M.p. (°C)
IR (Nujol)
1H-NMR (DMSO-d6/TMS) δ (ppm)
(recryst. solvent) ν (cm–1
)
2a
H
77
174–176
(CH3CN)
3 410, 3 280,
3 020, 1 680,
1 590, 1 570
4.50 (s, 2H, CH2), 6.63 (s, 2H, NH2), 6.97–7.24 (m, 5H, Ar ), 7.72, 8.63
(m, 4H, Py), 10.11 (br s, 1H, NH)
2b
2c
2d
2e
2f
2-CH3
3-CH3
4-CH3
2-OCH3
3-OCH3
4-OCH3
2-Cl
74
68
66
72
99
68
68
64
99
77
178–180
(EtOH)
3 270, 3 310,
1 660, 1 605
2.17 (s, 3H, CH3), 4.51 (s, 2H, CH2), 6.59 (s, 2H, NH2), 6.82–7.10 (m,
4H, Ar), 7.73, 8.63 (m, 4H, Py), 10.13 (br s, 1H, NH)
162–164
(2-PrOH)
208–210
(CH3CN)
175–177
(EtOH)
158–160
(CH3CN)
193–195
(CH3CN)
188–190
(EtOH)
180–182
(EtOH)
236–238
(CH3CN)
228–230
3 170, 3 060,
1 680, 1 650
2.23 (s, 3H, CH3), 4.49 (s, 2H, CH2), 6.61 (s, 2H, NH2), 6.75–7.13 (m,
4H, Ar), 7.73, 8.60 (m, 4H, Py), 10.11 (br s, 1H, NH)
3 270, 3 080,
1 680, 1 650
2.17 (s, 3H, CH3), 4.46 (s, 2H, CH2), 6.60 (s, 2H, NH2), 6.72–7.06 (m,
4H, Ar), 7.72, 8.62 (m, 4H, Py), 10.09 (br s, 1H, NH)
3 260, 3 100,
3 020, 1 670
3.73 (s, 3H, CH3), 4.47 (s, 2H, CH2), 6.58–6.94 (m, 6H, Ar + NH2), 7.72,
8.63 (m, 4H, Py) 10.11 (s, 1H, NH)
3 270, 3 120,
1 675, 1 635
3.68 (s, 3H, CH3), 4.49 (s, 2H, CH2), 6.49–7.18 (m, 6H, Ar + NH2), 7.72,
8.61 (m, 4H, Py), 10.10 (brs, 1H,NH)
2g
2h
2i
3 360, 1 675,
1 590, 1 570
3.64 (s, 3H, CH3), 4.44 (s, 2H, CH2), 6.60 (s, 2H, NH2), 6.80–6.94 (m,
4H, Ar), 7.72, 8.62 (m, 4H, Py), 10.08 (br s, 1H, NH)
3 410, 3 320,
1 690, 1 620
4.58 (s, 2H, CH2), 6.62 (s, 2H, NH2), 6.94–7.38 (m, 4H, Ar), 7.72, 8.64
(m, 4H, Py), 10.16 (br s, 1H, NH)
3-Cl
3 410, 3 080,
1 680, 1 585
4.54 (s, 2H, CH2), 6.64 (s, 2H, NH2), 6.95–7.31 (m, 4H, Ar), 7.72, 8.63
(m, 4H, Py), 10.12 (br s, 1H, NH)
2j
4-Cl
3 240, 3 080,
1 680, 1 640
4.51 (s, 2H, CH2), 6.64 (s, 2H, NH2), 7.01–7.31 (m, 4H, Ar), 7.72, 8.63
(m, 4H, Py), 10.11 (br s, 1H, NH)
2k
4-NO2
3 360, 3 280,
3 160, 3 080,
1 680, 1 640
4.70 (s, 2H, CH2), 6.74 (s, 2H, NH2), 7.18–7.73 (m, 4H, Ar), 8.20, 8.63
(m, 4H, Py), 10.16 (br s, 1H, NH)
(EtOH)
2l
4-NHCOCH3 73
191–193
(EtOH)
3 220, 3 060,
1 660, 1 595
1.95 (s, 3H, CH3), 4.46 (s, 2H, CH2), 6.61 (s, 2H, NH2), 6.90–7.42 (m,
4H, Ar), 7.72, 8.62 (m, 4H, Py), 9.77 (s, 1H, NH), 10.09 (br s, 1H, NH).
synthesis of these aminohydrazone derivatives and the
evaluation of their activity against mycobacteria. Activity
against Gram-positive and Gram-negative bacteria as
well as their cytotoxicity were also evaluated.
3. Microbiology
Antimycobacterial activity was investigated against M.
tuberculosis H37Rv ATCC 25584, M. avium ATCC
19421 and M. Fortuitum ATCC 9820, and M. tuberculosis
resistant to isoniazid (INH-R) isolated from a patient with
an active clinical infection treated at the Università di
Cagliari. The antimicrobial activity of compounds 2–5
was also evaluated against Gram-positive and Gram-
negative bacteria isolated from clinical specimens and
Candida albicans ATCC E10931. At the same time, cell
cytotoxicity of all compounds was tested in vitro on Vero
cells.
2. Chemistry
Isonicotinoylhydrazones 2 and their pyridylmethylene-
amino derivatives 3, 4 and 5 described in this study are
shown in tables I and II, and a reaction sequence for their
preparation is outlined in figure 1. The starting aryloxy-
acetonitriles 1 were prepared according to known proce-
dures [13]. Aryloxyacetonitriles 1 were converted to hy-
drazones 2 by addition of isonicotinoylhydrazine in the
presence of catalytic amounts of sodium ethoxide in
anhydrous ethanolic solution.
4. Results and discussion
Heating compounds 2 with pyridinecarboxaldehyde in
ethanolic solution in the presence of piperidine afforded
pyridylmethyleneamino derivatives 3–5.
With the exception of compound 2f which showed
toxicity at concentrations higher than 62.5 µg/mL, the
tested compounds exhibited high values of maximum
non-toxic dose (MNTD) on Vero cells, ranging from
250 µg/mL for 4j, to 500–1 000 µg/mL for the other
members of the series.
The structures for all new obtained compounds 2–5
1
were determined by examining their IR and H-NMR
spectra as well as by elemental analyses (tables I and II).

1073
Table II. Chemical and spectral data of compounds 3, 4 and 5.
Compound
R
Pyr
Yield
(%)
M.p. (°C)
IR (Nujol)
1H-NMR (DMSO-d6 /TMS) δ (ppm)
(recryst. solvent) ν (cm–1
)
3a
4a
H
H
2-pyridyl 42
3-pyridyl 55
168–169
(Benzene)
164–165
3 050, 1 610, 4.74 (s, 2H, CH2), 6.64 (s, 1H, CH), 6.93–7.81 (m, 10H, Ar), 8.54
1 590, 1 540 (s, 1H, NH), 8.63 (m, 3H, Py)
3 180, 3 030, 4.81 (s, 2H, CH2), 6.75 (s, 1H, CH), 6.93–8.62 (m, 13H, Ar), 8.68
1 615, 1 600, (s, 1H, NH)
1 550
(2-PrOH)
5a
5b
4c
5c
4d
5d
4e
5e
4f
H
4-pyridyl 63
4-pyridyl 48
3-pyridyl 57
4-pyridyl 59
3-pyridyl 64
4-pyridyl 56
3-pyridyl 68
4-pyridyl 67
3-pyridyl 50
4-pyridyl 52
3-pyridyl 72
4-pyridyl 60
3-pyridyl 60
4-pyridyl 50
190–191
(Benzene)
162–163
(Toluene)
124–125
(Toluene)
174–175
(Toluene)
152–153
(Toluene)
159–160
(Toluene)
134–135
(Toluene)
129–130
(Toluene)
137–138
(Toluene)
157–158
(Toluene)
123–124
(Toluene)
129–130
(Toluene)
173–174
(Toluene)
184–185
3 070, 1 620, 4.80 (s, 2H, CH2), 6.69 (s, 1H, CH), 6.93–8.64 (m, 13H, Ar), 8.71
1 595
(s, 1H, NH)
2-CH3
3-CH3
3-CH3
4-CH3
4-CH3
2-OCH3
2-OCH3
3-OCH3
3-OCH3
4-OCH3
4-OCH3
2-Cl
3 360, 3 020, 2.13 (s, 3H, CH3), 4.81 (s, 2H, CH2), 6.70 (s, 1H, CH), 6.83–8.64
1 620, 1 590 (m, 13H, Ar + NH)
3 060, 1 610, 2.24 (s, 3H, CH3), 4.81 (s, 2H, CH2), 6.78 (s, 1H, CH), 6.80–8.68
1 540 (m, 13H, Ar + NH)
3 060, 1 620, 2.23 (s, 3H, CH3), 4.79 (s, 2H, CH2), 6.70 (s, 1H, CH), 6.78–8.65
1 540 (m, 12H, Ar), 8.71 (s, 1H, NH)
3 020, 1 620, 2.19 (s, 3H, CH3), 4.77 (s, 2H, CH2), 6.74 (s, 1H, CH), 6.86–8.74
1 600, 1 540 (m, 13H, Ar + NH)
3 060, 1 620, 2.22 (s, 3H, CH3), 4.78 (s, 2H, CH2), 6.70 (s, 1H, CH), 6.88–8.66
1 560 (m, 12H, Ar), 8.72 (s, 1H, NH)
3 080, 3 020, 3.74 (s, 3H, CH3), 4.80 (s, 2H, CH2), 6.79 (s, 1H, CH), 6.87–8.65
1 630, 1 590
(m, 13H, Ar + NH)
1 630, 1 600
3.72 (s, 3H, CH3), 4.76 (s, 2H, CH2), 6.71 (s, 1H, CH), 6.83–8.64
(m, 13H, Ar + NH)
3 160, 3 060, 3.66 (s, 3H, CH3), 4.79 (s, 2H, CH2), 6.51–9.13 (m, 14H, Ar +
1 640, 1 590 NH + CH)
5f
3 140, 3 080, 3.66 (s, 3H, CH3), 4.79 (s, 2H, CH2), 6.70–8.63 (m, 13H, Ar +
1 620, 1 600 CH), 8.71 (s, 1H, NH)
4g
5g
4h
5h
3 150, 3 020, 3.67 (s, 3H, CH3), 4.75 (s, 2H, CH2), 6.75 (s, 1H, CH), 6.83–8.64
1 620, 1 580 (m, 12H, Ar), 8.67 (s, 1H, NH)
3 100, 1 630, 3.65 (s, 3H, CH3), 4.73 (s, 2H, CH2), 6.68 (s, 1H, CH), 6.81–8.64
1 620, 1 595 (m, 12H, Ar), 8.69 (s, 1H, NH)
3 350, 1 630, 4.92 (s, 2H, CH2), 6.78 (s, 1H, CH), 6.95–8.65 (m, 13H, Ar + NH)
1 610, 1 550
2-Cl
3 340, 3 020, 4.90 (s, 2H, CH2), 6.71 (s, 1H, CH), 6.95–8.63 (m, 12H, Ar), 8.68
1 620, 1 590, (s, 1H, NH)
1 540
(Toluene)
4i
5i
4j
3-Cl
3-Cl
4-Cl
3-pyridyl 51
4-pyridyl 67
3-pyridyl 20
139–140
(Toluene)
139–140
(Toluene)
164–165
3 060, 1 620, 4.88 (s, 2H, CH2), 6.76 (s, 1H, CH), 6.96–8.73 (m, 13H, Ar + NH)
1 590, 1 550
3 060, 1 620, 4.88 (s, 2H, CH2), 6.72 (s, 1H, CH), 6.97–8.66 (m, 12H, Ar), 8.74
1 590, 1 540
(s, 1H, NH)
3 100, 3 020, 4.84 (s, 2H, CH2), 6.75 (s, 1H, CH), 7.00–8.62 (m, 12H, Ar), 8.67
1 630, 1 610, (s, 1H, NH)
1 550
(Ethyl acetate)
5j
4-Cl
4-pyridyl 51
3-pyridyl 65
4-pyridyl 62
159–160
(Toluene)
189–190
(2-PrOH)
184–185
(2-PrOH)
1 600, 1 550
4.82 (s, 2H, CH2), 6.69 (s, 1H, CH), 6.99–8.64 (m, 12H, Ar), 8.71
(s, 1H, NH)
4k
5k
4-NO2
4-NO2
1 620, 1 580, 5.04 (s, 2H, CH2), 6.79 (s, 1H, CH), 7.20–8.63 (m, 12H, Ar), 8.74
1 500 (s, 1H, NH)
1 600, 1 580, 5.02 (s, 2H, CH2), 6.73 (s, 1H, CH), 7.19–8.64 (m, 12H, Ar), 8.78
1 540
(s, 1H, NH)
The tested compounds showed activity against myco-
bacteria with MIC values ranging from 6.25 µg/mL to
over 100 µg/mL. MIC values as well as the results of
cytotoxicity assays are reported in table III. The most
active compounds against M. tuberculosis H37Rv were
2d, 2j and 2k (MIC 6.25 µg/ml). However, a significant
activity was also exhibited by compounds 2c, 2f, 2g and
2i (MIC 12.5 µg/mL) and 2b, 2h and 4a (MIC 25 µg/
mL). The most effective compounds in the inhibition of
growth of M. tuberculosis INH-R were 2g, 2j and 2k
(MIC 25 µg/mL). All the tested compounds showed a
higher activity against M. tuberculosis H37Rv than M.

1074
Figure 1. Synthetic pathways to compounds 2, 3, 4 and 5.
tuberculosis INH-R, with the exception of compounds 4j,
5g and 5j that were inactive against M. tuberculosis
H37Rv (MIC > 100 µg/mL) but showed a weak activity
against M. tuberculosis INH-R (MIC 50µg/mL). Isony-
cotinoylhydrazones 2g, 2j and 2k were the most active
compounds in the whole series, with potencies superior to
INH against M. tuberculosis INH-R. Although none of
the tested compounds exhibited a significant activity
against M. fortuitum, we observed a poor activity by 2g
and 2i (MIC 100 µg/mL) against M. avium.
Compounds 2–5 were inactive at inhibiting the growth
of the Gram-negative species tested. A weak activity
against Staphyloccocus aureus was shown by compounds
2h, 2i and 2j (MIC 100 µg/mL) and by compound 3a
against C. albicans (MIC 100 µg/mL).
5.1.1. 2-Amino-2-(isonicotinoylhydrazono)ethyl aryl
ethers 2
General Procedure: to a stirred solution of NaOEt
(0.005 mol) in dry EtOH (5 mL) the appropriate aryloxy-
acetonitrile 1 (0.05 mol) was added. The mixture was
stirred at room temperature for 1 h and after cooling to
0 °C, an ethanolic solution of INH (6.8 g, 0.05 mol) was
added dropwise. The resulting solution was stirred at the
Table III. Cytotoxicity and antimycobacterial activity of com-
pounds 2–5.
Com- MNTD
MIC (µg/mL)
pound (µg/mL)
Vero
cells
M. tuberculosis M. tuberculosis M. avium
H37Rv
INH-R
ATCC 19421
From a first examination of these results, it appears that
compounds 2, containing the NH2 group, show better
activity against M. tuberculosis H37Rv and M. tubercu-
losis INH-R with respect to derivatives 3, 4 and 5. Further
studies to acquire more information about structure-
activity relationships are in progress in our laboratories.
ATCC 25584
2b
2c
2d
2f
2g
2h
2i
2j
2k
2l
4a
4j
4k
5c
5g
5i
1 000
1 000
500
62.5
500
500
1 000
500
1 000
1 000
1 000
250
1 000
1 000
500
25
12.5
6.25
12.5
12.5
25
12.5
6.25
6.25
50
25
> 100
50
50
> 100
50
100
50
50
100
25
100
50
25
25
> 100
50
> 100
> 100
> 100
> 100
100
> 100
100
> 100
> 100
> 100
> 100
> 100
> 100
> 100
> 100
> 100
> 100
100
5. Experimental protocols
5.1. Chemistry
50
100
> 100
50
Melting points were determined on a Kofler hot stage
and are uncorrected. IR spectra were recorded on Nujol
mulls between salt plates in a Perkin-Elmer 398 spectro-
500
500
1 000
50
50
50
1
photometer. H-NMR spectra were recorded on a Varian
5j
INH
> 100
0.06
Unity 300 spectrometer. Elemental analyses were carried
out with a Carlo Erba Model 1106 Elemental Analyzer.

1075
same temperature for 2 h and then allowed to reach room
temperature and to stand overnight. The formed precipi-
tate was collected by filtration and recrystallised from the
solvent shown in table I.
and M. tuberculosis resistant to isoniazid (INH-R) iso-
lated from a patient with an active clinical infection. The
determination of MICs against mycobacteria were carried
out by the two-fold agar dilution method [17] using 7H11
agar (Difco Laboratories) containing compounds 2–5 at
concentrations that ranged between 100–0.19 µg/mL, on
which 100 µL of the test bacterial suspension were
spotted. Suspensions to be used for drug susceptibility
testing were prepared from 7H9 broth cultures supple-
mented with 10% OADC (oleic acid-albumin-dextrose-
catalase) enrichment (Difco Laboratories) and 0.05%
(v/v) Tween 80 to avoid clumping. Cells were then
washed, suspended in saline, shaken and sonicated in a
bath type ultrasonicator (output power 80 W) until
visibile clumps were disrupted (usually from 15–30 s).
Suspensions were then diluted in saline to a turbidity of
no. 1 McFarland (M. tuberculosis) or no. 0.5 McFarland
(M. avium and M. fortuitum) and then diluted to obtain
inocula of 3 × 105 cells per well of M. tuberculosis and
1.5 × 104 cells per well of M. avium and M. fortuitum.
The MICs of the compounds were determined after 7
(M. fortuitum) or 21 (slow growers) days of cultivation at
37 °C in a CO2 (5% CO2/95% humidified air) incubator.
5.1.2. 2-(Pyridylmethyleneamino)-2-(isonicotinoylhydra-
zono)ethyl aryl ethers 3, 4 and 5
General procedure: to a solution of compounds 2
(1.85 mmol) and 2-, 3- or 4-pyridinecarboxaldehyde
(1.85 mmol) in EtOH (15 mL), a few drops of piperidine
were added. After heating at reflux for 6 h the solvent was
removed in vacuo and the residue treated with H2O
(20 mL) and extracted with CHCl3 (3 × 10 mL). The
organic layers were dried (Na2SO4) and the solvent
evaporated to give the crude 3, 4 and 5 that were purified
by crystallisation from the solvent indicated in table II.
5.2. Microbiology
For the antimicrobial and cytotoxicity studies, com-
pounds were dissolved in DMSO at 10 mg/mL and kept
at –20 °C. The working solutions were prepared in the
same medium used for the tests. To avoid interference by
the solvent [14], the highest DMSO concentration was
1%. The MICs of various compounds against Gram-
positive, Gram-negative and C. albicans were determined
by a standard broth serial dilution method [15, 16].
5.2.2. Antifungal assay
For the evaluation of the antifungal activity, Candida
albicans ATCC E10931 was employed. Antifungal activ-
ity against C. Albicans ATCC E10231 was evaluated in
yeast extract peptone dextrose medium (Difco) [18].
5.2.1. Antibacterial assay
The antimicrobial activity of compounds 2–5 was
evaluated against six Gram-positive and five Gram-
negative species isolated at the Sezione di microbiologia
e virologia, Dipartimento di Scienze Chirurgiche e Tra-
pianti d’Organo, from clinical specimens obtained from
patients treated at the Università di Cagliari. In particular,
compounds 2–5 were tested against Staphylococcus au-
reus (isolated from urine), S. epidermidis (isolated from
urine by suprapubic aspirate), Streptococcus agalactiae
(isolated from vaginal swab), S. faecalis (isolated from
urine), Bacillus licheniformis (isolated from blood), B.
subtilis (isolated from eye swab), Escherichia coli (iso-
lated from urine), Pseudomonas aeruginosa (isolated
from urine), Salmonella typhi (isolated from faeces),
Proteus mirabilis (isolated from urine) and Klebsiella
pneumoniae (isolated from urine). Tests with Gram-
positive and Gram-negative bacteria were carried out in
Mueller Hinton broth (Difco). The compounds were
diluted in the test medium to obtain final concentrations
ranging from 100–0.19 µg/mL. Tubes were inoculated
with 1 × 105 cells/mL and were incubated at 37 °C for 18
or 24 h. The effects on the growth of mycobacteria was
investigated against M. tuberculosis H37Rv ATCC
25584, M. avium ATCC 19421, M. fortuitum ATCC 9820
5.2.3. In vitro cytotoxicity assay
Cell cytotoxicity of compounds 2–5 was tested in vitro
by two methods. In the first method, RPMI 1640 medium
(Gibco) with 2% foetal calf serum (FCS, Gibco) alone, or
RPMI 1640 with 2% FCS containing compounds at
concentrations ranging from 1 000–62.5 µg/mL, were
inoculated onto cultures of Vero cells in 6 well tissue
culture plates. The cells were observed daily for 6 days
for any sign of cell cytotoxicity compared with the
controls. In the second method, a cell viability assay
previously reported [19, 20] was used. Monolayers of
Vero cells in 96 multiwell plates were incubated with the
testing compounds at concentrations of 1 000–62.5 µg/
mL in RPMI 1640 with 5% FCS for 48 h and the medium
replaced with 50 µL of 1 mg/mL solution of 3-(4,5-
dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide
(MTT) in RPMI without phenol red. Cells were incubated
at 37 °C for 3 h, the untransformed MTT removed and
0.04 N HCl isopropanolic solution (50 µL) was added to
each well. After a few minutes at room temperature to
ensure that all crystals were dissolved, the plates were

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