Synthesis and antituberculosis activity of indole-pyridine derived hydrazides, hydrazide-hydrazones, and thiosemicarbazones

Article abstract of DOI:10.1016/j.bmcl.2015.12.049

We describe the design, synthesis, and in vitro antimycobacterial activity of a series of novel simple hybrid hydrazides and hydrazide-hydrazones combining indole and pyridine nuclei. The compounds are derivatives of 1-acetylindoxyl or substituted indole-3-carboxaldehydes tethered via a hydrazine group by simple C-N or double C=N bonds with 3- and 4-pyridines, 1-oxide 3- and 4-pyridine carbohydrazides. The most active of 15 compounds showed MICs values against an INH-sensitive strain of Mycobacterium tuberculosis H37Rv equal to that of INH (0.05-2 μg/mL). Five compounds demonstrated appreciable activity against the INH-resistant M. tuberculosis CN-40 clinical isolate (MICs: 2-5 μg/mL), providing justification for further in vivo studies.

Full text of DOI:10.1016/j.bmcl.2015.12.049

Accepted Manuscript
Synthesis and antituberculosis activity of indole-pyridine derived hydrazides,
hydrazide-hydrazones, and thiosemicarbazones
Valeriya Velezheva, Patrick Brennan, Pavel Ivanov, Albert Kornienko, Sergey
Lyubimov, Konstantin Kazarian, Boris Nikonenko, Konstantin Majorov,
Alexander Apt
PII:
S0960-894X(15)30362-0
DOI:
http://dx.doi.org/10.1016/j.bmcl.2015.12.049
BMCL 23410
Reference:
Bioorganic & Medicinal Chemistry Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
19 October 2015
10 December 2015
15 December 2015
Please cite this article as: Velezheva, V., Brennan, P., Ivanov, P., Kornienko, A., Lyubimov, S., Kazarian, K.,
Nikonenko, B., Majorov, K., Apt, A., Synthesis and antituberculosis activity of indole-pyridine derived hydrazides,
hydrazide-hydrazones, and thiosemicarbazones, Bioorganic & Medicinal Chemistry Letters (2015), doi: http://
dx.doi.org/10.1016/j.bmcl.2015.12.049
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1
Synthesis and antituberculosis activity of indole-pyridine derived hydrazides,
hydrazide-hydrazones, and thiosemicarbazones
Valeriya Velezhevaa*, Patrick Brennanb*, Pavel Ivanova, Albert Kornienkoa,
Sergey Lyubimova, Konstantin Kazarianc, Boris Nikonenkoc, Konstantin Majorovc,
and Alexander Aptc
aA.N.Nesmeyanov Institute of Organoelement Compounds Russian Academy of
Sciences, 28 Vavilov Str.,117813 Moscow, Russia; bDepartment of Microbiology,
Immunology and Pathology, Colorado State University, Fort Collins, Colorado
80523-1682 U.S.A.; cCentral TB Research Institute Russian Academy of Medical
Sciences, 2, Yauzskaya alley, 107564 Moscow, Russia
Abstract.
We describe the design, synthesis, and in vitro antimycobacterial activity of a series of novel
simple hybrid hydrazides and hydrazide-hydrazones combining indole and pyridine nuclei. The
compounds are derivatives of 1-acetylindoxyl or substituted indole-3-carboxaldehydes tethered
via a hydrazine group by simple C-N or double C=N bonds with 3- and 4-pyridines, 1-oxide 3-
and 4-pyridine carbohydrazides. The most active of 15 compounds showed MICs values against
an INH-sensitive strain of M. tuberculosis H37Rv equal to that of INH (0.05-2μg/mL). Five
compounds demonstrated appreciable activity against the INH-resistant M. tuberculosis CN-40
clinical isolate (MICs: 2-5 μg/mL), providing justification for further in vivo studies.
Tuberculosis (TB) remains a leading infectious killer in the world. Approximately one-third of
the world's population is thought to be infected with the causative agent Mycobacterium
tuberculosis. Every year about 8 million people develop active, contagious pulmonary TB and
more than 1.6 million people die of infection. M. tuberculosis rapidly develops genetic
resistance to several drugs; thus the anti-tuberculosis chemotherapy regimen recommended by
the WHO is based upon a cocktail of four/five first-line drugs, including isoniazid (INH).
Nevertheless, multidrug resistant (MDR) TB, extremely drug resistant (XDR) TB and TB-HIV
co-infections are rapidly spreading, making the discovery and development of new anti-TB drugs
lacking cross-resistance with those currently available an urgent need.1-5 A current trend for drug
design and development is the concept of multi-target “intramolecular” hybridization.6
According to this concept, new hybrid drugs should combine different pharmacophores acting at
more than one target providing an advantage when treatment is aimed at drug-resistant infectious
agents. Hydrazine-derived compounds constitute an important class of potential anti-TB drug
candidates, given that the hydrazine-based pharmacophore INH (isonicotinohydrazide) is an
invariable anti-tuberculosis pyridine-based medicine.5,7-9 However, the emergence of INH
resistant M. tuberculosis strains dictates the necessity of re-designing this old drug in order to
create analogues effective against INH resistant strains. To this end, many efforts have been
undertaken but without much success.10 On the other hand, recent studies of INH structural

2
analogs revealed high levels of activity both in vitro and in vivo against sensitive and resistant
strains of Mtb.11-18 Anti-resistant compounds were also found among direct inhibitors of the
clinically validated target of INH, the mycobacterial enoyl acyl carrier protein reductase, known
as InhA, in the mycolic acid synthetic pathway.19-21 Noteworthy, INH inhibits this enzyme after
preliminary fermentative activation, via an indirect mechanism, whereas some agents active
against INH-resistant strains act directly, which may be explained by the concept of allosteric
binding of inhibitor to the enzyme target.22-24 Meanwhile, a number of direct InhA inhibitors
possessing antimycobacterial activity were found in a series of indole-containing compounds,
21,25-27
being versatile privileged drug-like scaffolds.
Importantly, many known indole-based
drugs and bioactive molecules demonstrated an ability to inhibit novel targets, structurally
different from those currently known.28 In addition, indole agents were found recently to affect a
number of vital targets in M. tuberculosis, in particular as inhibitors of cell wall biosynthesis and
bacterial respiration (inhibitors of MmpL3, DprE1, CYP125A1, the M. tuberculosis protein
tyrosine phosphatase B, indoleamine 2,3-dioxygenase (IDO) enzymes, etc.).29-38 Based on this
background, we designed hybrid structures combining two redox-active heterocyclic scaffolds
with previously demonstrated anti-TB activity: hydrophilic electron-deficient pyridines and
lipophilic electron-rich indoles tethered through a hydrazine pharmacophore. We aimed to
synthesize a series of simple hydrazide/hydrazide-hydrazone hybrids from commercially
available precursors, anticipating establishment of hybrid molecules with significant
antimycobacterial activity due to synergistic effects.
In continuation of our studies of hydrazine-derived antimycobacterials39 a series of novel 1-
acetylindoxyl 1 and substituted indole-3-carboxaldehydes 7-12 derived hydrazides and
hydrazide-hydrazones (Schiff bases) as well as substituted indole-3-carboxaldehyde derived
thiosemicarbazones were synthesized (Schemes 1-4).

3
O
X
Y
NH
N
NHNH2
S
O
NH2
N
NH
N
O
K1-4
Ac
Y X
3-6
NH2NHCSNH2
2
O
X
C2H5OH / AcOH
C2H5OH / AcOH
N
N
Y
NH
HN
Ac
Ac
1
D7
3:X=N,Y=CH;
4:X=CH,Y=N;
N
5:X=N+-O-,Y=CH;
6:X=CH,Y=N+-O-;
A1-4
Ac
A1: X=N,Y=CH; A2: X=CH,Y=N;
A3: X=N+-O-,Y=CH; A4: X=CH,Y=N+-O-)
Scheme 1. Synthesis of hydrazides A1-4 and thiosemicarbazone D7
NHNH2
O
O
N
CHO
N
X
R1
R1
-
3,5 (X = N, N+-O )
H
R2
R2
C2H5OH
N
X
N
H
H
7-12
B1-9
7:R1=H, R2=H;
B1:R1=H, R2=H, X=N; B2:R1=H, R2=COOC2H5, X=N;
B3:R1=Cl, R2=COOC2H5, X=N; B4:R1=CH3, R2=COOC2H5, X=N;
B5:R1=Br, R2=COOC2H5, X=N; B6:R1=CH3, R2=COOH, X=N;
B7:R1=H, R2=H, X=N+-O-; B8:R1=Br, R2=COOC2H5, X=N+-O-;
B2:R1=H, R2=COOC2H5, X=N+-O-
8:R1=H, R2=COOC2H5;
9:R1=Cl, R2=COOC2H5;
10:R1=CH3, R2=COOC2H5;
11:R1=Br, R2=COOC2H5;
12:R1=CH3, R2=COOH;
Scheme 2. Synthesis of hydrazide-hydrazone B1-9.
NHNH2
O
O
N
CHO
R2
N
X
R1
R1
-
3,5 (X = N, N+-O )
H
R2
C2H5OH
N
X
N
H
H
7-12
B1-9
7:R1=H, R2=H;
B1:R1=H, R2=H, X=N; B2:R1=H, R2=COOC2H5, X=N;
B3:R1=Cl, R2=COOC2H5, X=N; B4:R1=CH3, R2=COOC2H5, X=N;
B5:R1=Br, R2=COOC2H5, X=N; B6:R1=CH3, R2=COOH, X=N;
B7:R1=H, R2=H, X=N+-O-; B8:R1=Br, R2=COOC2H5, X=N+-O-;
B2:R1=H, R2=COOC2H5, X=N+-O-
8:R1=H, R2=COOC2H5;
9:R1=Cl, R2=COOC2H5;
10:R1=CH3, R2=COOC2H5;
11:R1=Br, R2=COOC2H5;
12:R1=CH3, R2=COOH;

4
C1:R1=H, R2=H, X=N;
H
O
N
C2:R1=H, R2=COOC2H5, X=N;
C3:R1=Cl, R2=COOC2H5, X=N;
C4:R1=CH3,R2=COOC2H5, X=N;
C5:R1=Br, R2=COOC2H5, X=N;
C7:R1=H, R2=COOC2H5, X=N+-O-
N
R1
H
NaBH4
R2
C2H5OH
N
X
H
C1-5,7
O
B1-5,7
H
N
N
H
H2, 5% Pt/C, 8-10 atm
C2H5OH
CH3SO3H
NH
COOC2H5
N
H
(from B2)
C6
Scheme 3. Synthesis of hydrazides C1-7.
S
N
CHO
N
R1
R1
NH2
H
R2
R2
NH2NHCSNH2
N
N
H
C2H5OH / AcOH
H
10, 12-16
D1-6
D1:R1=CH3, R2=COOC2H5; D2:R1=CH3, R2=COOH; D3:R1=H, R2=COOH;
D4:R1=Cl, R2=COOH; D5:R1=OCH3, R2=COOH; D6:R1=OCH3, R2=COOC2H5
Scheme 4. Synthesis of thiosemicarbazones D1-6.
Combining indole and pyridine or 1-oxide pyridine nuclei as a single hybrid molecule was
performed via a hydrazine group involving a simple C-N or double C=N bond to generate
corresponding substituted hydrazides and hydrazide-hydrazones. Recent studies showed that the
N-oxidation of a pyridine nucleus mostly resulted in retained or increased minimum inhibitory
concentrations (MIC) against M. tuberculosis.40 Certain 1-oxide pyridine derivatives turned out
to be active against dormant strains of M. tuberculosis.41 This data provided the stimulus for us
to prepare a few N-oxidized pyridine hybrids as well. We also paid attention to the data
concerning strong antimycobacterial activity of known indole-based Schiff bases and
thiosemicarbazones.42-43 As reported earlier Schiff base B1 derived from INH and unsubstituted
indole-3-carboxaldehyde 7 had a MIC value of 0.0063 μg/mL against M. tuberculosis-H37Rv.44
Moreover, indole-3-carboxaldehyde thiosemicarbazone was reported as an anti-TB agent with in
vivo activity in mice.45 Isatin β-thiocarbohydrazone showed activity against drug resistant
mycobacterial strains.46 Recently, a novel anti-TB thiosemicarbazone drug, perchlozon, was
reported by a Russian group.47 Introduction of COOH or COOC2H5 groups at the position 2 of
the starting indole-3-carboxaldehydes 7 - 12, i.e. in the vicinity of the carbonyl group, allowed
attachment of some novel properties to target compounds to enable comparison with
unsubstituted 7.

5
All compounds synthesized were divided into A1-4, C1-7 hydrazide, B1-9 hydrazide-hydrazone and
thiosemicarbazone D1-6 series (Schemes 1-4, Table 1). Series A1-5 was comprised of so-called
iproniazid and INH analogs (isonicotinohydrazides), derivatives of pyridine-4-carbohydrazide 3
(INH), pyridine-3-carbohydrazide 4 (nicotinohydrazide), 1-oxide pyridine-4-carbohydrazide 5;
1-oxide pyridine-3-carbohydrazide 6 bearing an 1-acetyl-3-indolyl unit at the terminal N2
nitrogen of the hydrazine group. Series B1-9 comprised Schiff bases, derivatives of INH and 1-
oxide isonicotinohydrazide 5, bearing substituted 3-indolyl-methylidene unit at the N2 nitrogen.
Series C1-5,7-9 of iproniazid analogs comprised isonicotinohydrazide 3 and 1-oxide
isonicotinohydrazide 5 derivatives bearing substituted/unsubstituted 3-indolyl-methylene unit at
the terminal nitrogen of the hydrazine group. The saturated analog C6 comprised a fully reduced
pyridine ring.
The synthesis of the intermediate and target compounds was performed according to reactions
outlined in Schemes 1-4. Acetylindoxyl 1 was generally condensed under reflux with starting
hydrazides 3-6 to afford target hydrazides A1-4 instead of the corresponding isomeric
Knovenagel products K1-4. The NMR 1H and 13C spectra data indicate that aromatization of K1-4
occurs under reaction conditions due to the known imine-enamine and hydrazone–enhydrazine
tautomerism.48 All reactions were done in ethanol or acetic acid. Diacylated derivative A5 was
obtained by the treatment of A1 with 4-chlorobenzoylchloride according to a standard method.
The target hydrazide-hydrazones B2-9 were prepared by the reaction of 2-ethoxycarbonyl/2-
hydroxycarbonyl-indole-3-carboxaldehydes 7 - 12 with starting hydrazides 3, 5 under conditions
similar to those described above and resulted in the formation of the compounds A. The
hydrazides C1-5, 7 were prepared via a two-step approach including the reduction of the starting
compounds B1- 5, 7 by NaBH4 in C2H5OH. Hydrazide C6 with a fully reduced pyridine ring was
obtained by the reduction of hydrazone B2 by hydrogen at 10 atm in ethanol in the presence of
5% Pd/C as catalyst. Diacylated derivatives C8, 9 were obtained by the treatment of C2 with 4-
chlorophenylisocyanate and isonicotinoylchloride according standard methods.
Additionally, the condensation of 2-ethoxycarbonyl/hydroxycarbonyl-indole-3-carboxaldehydes
10, 12 - 16 and ketone 1 with thiosemicarbazide was conducted according to known methods by
heating the starting components in ethanol or acetic acid to afford a series of corresponding
thiosemicarbazones D1-6. Thiosemicarbazone D7 was reported by us earlier.49 The structure and
purity of the above indole derivatives were verified by 1H & 13C NMR spectroscopy, GC-MS
and MALDI-TOF mass spectrometry.

6
Table 1.
Compd
Starting Material
Melting
Point, ºC
Yield,
%
Product structure
Hydrazide/ Compd with
Thiosemi-
carbazide
C=O group
O
NH
228-231
(decomp.)
65
61
A1
A2
3
4
1
N
HN
N
Ac
O
208-211
(decomp.)
1
1
N
NH
HN
N
Ac
O
194-196
(decomp.)
57
A3
5
N
O
NH
HN
N
Ac
O
132-135
(decomp.)
46
50
A4
A5
6
1
N
NH
HN
N
O
Ac
O
C
N
O
N
210-213
A1
O
(decomp.)
Cl
N
H
NH
N
Cl
N
Ac
O
N
N
239-241
86
93
B1
B2
3
3
7
N
H
N
N
H
O
252-254
(decomp.)
8
9
N
H
COOC2H5
N
N
H
O
245-248
(267-268)
82
B3
3
N
N
H
Cl
COOC2H5
N
N
H

7
O
248-250
248-250
82
B4
3
10
N
N
H3C
H
COOC2H5
N
N
H
O
N
80
72
90
B5
B6
B7
3
3
5
11
12
7
N
H
Br
COOC2H5
N
N
H
O
N
>350
(decomp.)
N
H
H3C
COOH
N
N
H
O
N
305-307
(decomp.)
N
H
N
N
H
O
O
N
267-268
76
95
B8
B9
5
5
11
8
N
Br
H
COOC2H5
N
N
H
O
O
N
279-282
(decomp.)
N
H
COOC2H5
N
N
H
O
O
H
N
151-154
(decomp.)
89
82
C1
C2
B1
B2
-
-
N
H
N
N
H
O
H
N
201-204
(decomp.)
N
H
COOC2H5
N
N
H
O
H
N
203-205
171-173
196-198
95
75
80
C3
C4
C5
B3
B4
B5
-
-
-
N
H
Cl
COOC2H5
N
N
H
O
H
N
N
H3C
H
COOC2H5
N
N
H
O
H
N
N
Br
H
COOC2H5
N
N
H
O
H
N
201-203
(decomp.)
19
C6
B2
-
N
H
COOC2H5
NH
CH3SO3H
N
H

8
O
H
198-199
(decomp.)
54
65
C7
C8
B7
C2
-
N
N
H
COOC2H5
N
N
H
O
O
H
N
O
C
227-230
(decomp.)
N
Cl
O
N
N
H
COOC2H5
N
N
Cl
H
O
Cl
N
242-245
50
C9
C2
(decomp.)
O
O
N
N
HCl
N
H
COOC2H5
N
N
H
S
264-266
68
97
D1
D2
2
2
10
12
HN
N
NH2
H3C
COOC2H5
S
N
H
295-298
(decomp.)
HN
NH2
N
H3C
COOH
N
H
S
265-270
(decomp.)
90
D3
2
13
HN
N
NH2
COOH
N
H
S
302-305
(decomp.)
64
79
D4
D5
2
2
14
15
HN
N
NH2
Cl
COOH
N
H
S
270-273
(decomp.)
HN
N
NH2
H3CO
COOH
N
H
S
238-240
(decomp.)
93
90
D6
D7
2
2
16
1
HN
NH2
N
H3CO
COOC2H5
N
H
S
NH2
262-264
NH
N
N
Ac

9
Further, a series of A-C hybrid compounds and thiosemicarbazones D were tested for their in
vitro antimycobacterial activity against a laboratory strain M. tuberculosis H37Rv and against a
clinical isolate of INH-resistant M. tuberculosis with selective single INH resistance, designated
as CN-40. M. tuberculosis strain H37Rv (museum strain) was originally obtained from the
Institute Pasteur, Paris, France (a kind gift of G. Marchal). The CN-40 strain was isolated from a
TB patient at the TB Research Institute of the Russian Academy of Medical Sciences. MICs
against M. tuberculosis were determined by standard microdilution in microtubes using Dubos
medium containing 0.05% Tween 80 (Table 2).
Table 2.
Entry
MIC
µg/ml
against
H37Rv
0.2
20
2
2-5
NA*
0.1
0.15
0.1
0.05
NT
0.1
1
NT
2
0.1
0.2
MIC
µg/ml
against
CN-40
2-5
NT**
10
10
NT
10
2-5
2-5
2-5
NT
4-10
10
NT
10
10
10
10
Entry
MIC
µg/ml
against
H37Rv
0.2
MIC
µg/ml
against
CN-40
10
NT
NT
10
NT
NT
2-5
NT
NT
NT
NT
NT
NT
A1
A2
A3
A4
A5
B1
B2
B3
B4
B5
B6
B7
B8
B9
C1
C2
C3
C4
C5
C6
C7
C8
C9
D1
D2
D3
D4
D5
D6
D7
3
10
20
2
NA
NA
2
>10
>10
10
>10
>10
>10
0.06
5
20
>20
>20
>20
4
5
6
5.2
>10
0.2
* NA – inactive
** NT - nontested
The preliminary studies revealed that thirty of the compounds tested (twenty nine of which were
novel) were able to impair the growth of M.tuberculosis H37Rv to varying extents: (MIC= 0.05–
20μg/mL); among those fifteen and nine compounds have MICs values of 0.05–2μg/mL and
2.0-20, respectively, three compounds proved to be virtually inactive (MIC>20). The hydrazide-
hydrazone B1 was previously reported to have a MIC of 0.0063 μg/mL against M. tuberculosis-
H37Rv.44 However, when re-investigated in this present study we observed a MIC of 0.1 μg/mL
against this present strain of M. tuberculosis.

10
As can seen in Table 2, INH-containing hydrazide-hydrazones B and hydrazides A1, C showed
higher activity (except for A5, B8, C8, C9) than the corresponding N-oxide analogs of INH (A3,
B7, B9, C7), nicotinic acid-derived analogs (A2, A4), hydrazide C6 with fully reduced pyridine
ring, and all thiosemicarbazones except for the compound D1. The saturated hybrid A1 and
hydrazides with reduced double C=N bond (C1-C4,) had similar low values of MICs as compared
to their corresponding precursor hydrazide-hydrazones B1–B4. Hence, these studies have
identified seven lead molecules A1, B1–B4, C2, and C3 from all series of indole derivatives
investigated with a MIC of ≥0.2 μg/mL which approach the antituberculosis potential of INH.
The antimycobacterial potential of compounds A1, B1–B4, C2, and C3 was found to be better than
the standard drug ethambutol51 (MIC= 0.5μg/mL).
Against INH-resistant strain M. tuberculosis CN-40, all of the hybrids tested demonstrated MIC
values comparable to 20- or to 100-fold lower than those tested against the INH-sensitive M.
tuberculosis H37Rv (Table 2). In particular, the MICs of B1 - B4 against CN-40 were 10 μg/mL
for the first B1, and 2-5 μg/mL for B2-B4, respectively, compared to a MIC of 20 μg/mL
demonstrated by INH against the same strain (Table 2). For comparison, the MIC values of the
same compounds against M. tuberculosis H37Rv were 0.1, 0.15, 0.1, and 0.05 μg/mL,
respectively. In the case of CN-40 resistant to INH, MICs of certain target compounds (A1, B1 -
B4, D1, etc) (Table 2) were in the range of 2-10 μg/mL, that is, the MIC values were in a
different degree less than that of the reference drug INH against the same strain .
Concurrent with determination of MIC values, compounds with reasonably high anti-
mycobacterial activity (MIC=0.05-2) were also tested for cytotoxicity in macrophages at
concentrations less or equal to 400μg/ml or 10 times the MIC for M. tuberculosis H37Rv. This
allowed identifying the concentrations of “therapeutic gaps” for a few compounds with
reasonably high anti-mycobacterial activity. The viability and purity of peritoneal macrophages
isolated from C57BL/6 female mice exceeded 95%. To assess the cytotoxicity of tested
compounds on peritoneal macrophages, we used methods described earlier.50 Cytotoxic effects
of compounds was estimated by lactate dehydrogenase (LDH) release from macrophages, as a
measure of macrophage lysis, and was determined from the enzymatic activity of LDH in culture
supernatants using a CytoTox 96 kit (Promega, Madison, Wis.) as recommended by the
manufacturer. Per cent specific lysis was calculated according to the following formula: (A490 in
experimental wells - A490 after spontaneous release)/(A490 after total lysis - A490 after spontaneous
release) x100. The in vitro cytotoxicity to macrophages (IC50) was determined to establish a
selectivity index (SI) which was calculated by dividing the IC50 by the MIC; the IC50 value

11
represents the concentration of a compound that is required for 50% inhibition in vitro. Values of
IC50 and SI are presented in Table 3.
Table 3. IC50 and SI (against H37Rv)
Entry
A1
A4
B1
B2
B4
B6
B7
B9
C1
C7
3
IC50
50
17
40
16
15
25
30
18
15
16
>100
60
SI
250
3.4 - 8.5
400
106.6
300
250
30
9
150
8
>1666
11.5
5
Most of the compounds represented in the Table 3 (except for A4, B9, and C7) displayed no
evident cytotoxicity when administered at the 30-400 μg/mL MIC concentrations. Thus, the
hydrazide-hydrazone B4 was the most potent compound examined with MIC of 0.05 μg/mL and
with a high Selectivity Index (SI) of 300 and was approximately as active as INH against M.
tuberculosis H37Rv; however, unlike INH, B4 showed appreciable activity against the INH-
resistant M. tuberculosis CN-40 strain.
It should be noted that the synthetic design of Schiff bases B2-9 involves the presence of COOH
or COOC2H5 groups at the position 2 of the indole ring i.e., in the vicinity to the azomethyne
C=N group. Similar kinds of Schiff bases are generally considered as quite good ligands for
chelation with various metal ions including those from the nutrient medium of M. tuberculosis.
Schiff base metal complexes provide favorable conditions for the penetration into bacterial
cells.52 The process is apparently facilitated by increased lipophilicity of the indole nucleus. As
mentioned previously29 changes in penetration of a compound may produce changes in affinity
for the molecular target and also modulate molecular activity. Accordingly, a series of Schiff
base carrying metal complexes are found to be more active against certain M. tuberculosis strains
than the ligands themselves.53 Thus, it can be speculated that certain of the INH-indole hybrids
investigated by us to overcome INH resistance is connected with acquired synergism. A concept
of allosteric regulation of enzyme sites binding with the putative metal ligands may explain this
phenomenon.

12
In conclusion, numerous recent reports in the literature provide evidence of an indefatigable
search for new anti-TB agents among the isonicotinohydrazide/nicotinohydrazide-hydrazones
protected from undesirable N2-acetylation. The results described herein are promising and
suggest further optimization and mechanism of action studies of these nitrogen-nitrogen-
containing indole and INH hybrid analogs are warranted.
Acknowledgments
This work was supported partially by a grant (Grant RB 2-2032) from the U.S. Civilian and
Research and Development Foundation (CRDF) for the Independent States of the Former Soviet
Union, Arlington, Virginia, U.S.A. and the Russian Foundation for Basic Research (project №
14-04-01688).
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16
Synthesis and antituberculosis activity of indole-
pyridine derived hydrazides, hydrazide-hydrazones,
and thiosemicarbazones
Leave this area blank for abstract info.
Valeriya Velezheva, Patrick Brennan, Pavel Ivanov, Albert Kornienko, Sergey Lyubimov, Konstantin Kazarian,
Boris Nikonenko, Konstantin Majorov, and Alexander Apt
O
H
O
O
X, Y = N, N+-O-, CH
N
N
HN
N
N
X
R1
N
R1
H
H
Y
H
R2
R2
N
X
N
N
X
Against
H
H
Ac
MIC 0.2-5/2-10 µg/mL
M.tuberculosis
H37Rv/CN-40
MIC 0.05-2/2-10 µg/mL
MIC 0.1-10/10 µg/mL