Enoyl acyl carrier protein reductase (InhA) is an important target within
Mycobacterium tuberculosis (Mtb) for the first and second line antituberculosis
agents such as isoniazid (INH) and ethionamide (ETH). The main drawback in the
design of these compounds is the necessity of their activation that involves
mycobacterial enzymes: a catalase/peroxidase, KatG, and a flavine monooxygenase,
EthA. As a result, a quick resistance development towards these compounds can
appear due to mutations in the genes encoding the enzymes, katG and ethA, which
are considered one of the main causes of resistance development in Mtb strains.
Therefore, there is an emerging need in development of new InhA inhibitors as
antituberculosis agents active against INH- and ETH-resistant strains that do not
require activation by KatG and EthA. Many groups of such agents are already well
known: triclosane derivatives, diaryl ethers, pyrolidine and pyperazine derivatives,
pyrolidine carboxamides, and arylamide derivatives.
Among plant derived compounds trypthantrin (1, indolo[2,1-b]quinazoline-6,12-
dione), a tryptophan derived alkaloid, and its analogues have also been reported to
possess antimycobacterial activity with different potencies in vitro and in vivo.1-3
The analysis of 1 molecular docking into the binding site of Mbt InhA has
demonstrated a good level of affinity to the enyzme.4

A high level of inhibitory activity has also been reported for its analogue 2,
however, no further research data has appeared on its activity in vivo. Prompted by
all mentioned above, we report design and synthesis of a novel class of TRPN
analogues 3 and evaluation of their inhibitory activity against Mtb H37Rv, toxic
studies in animals of the most active compound, as well as molecular docking
studies in the binding site of InhA.
The obtained compounds were evaluated for their antimycobacterial activity. Ethyl
derivative (R = C2H5) appeared to have 32% inhibitory activity (MIC <6.25 μM),
thus stressing the importance of structural requirements needed for Mtb activity:
three cyclic substituted-quinazolin-4-one moiety, and substitution near to the centre
S, and an alkyl chain.
The highest activity (up to 100%) of propyl-derivative (R = C3H7) compared to its
ethyl homolog can be attributed to increased number of hydrophobic interactions
with InhA amino acids residues of the first. At the same time, longer electronwithdrawing
and electron-releasing groups in side chain of other derivatives had a
detrimental effect upon anti-Mtb activity. Incorporation of electron-donating group
as R gave a rise of activity, and 2-(pyrimidin-2-ylthio)-5H-[1,3,4]thiadiazolo[2,3-
b]quinazolin-5-one showed superior activity as compared to the compounds
electron-withdrawing groups. Lower indices of activity have been observed for
nitro- (8%) and fluoro- (7%) substituted derivatives, in comparison to methyl and
chloro-derivatives. Some activity has been shown for 1,3-dioxolane derivative, and
no activity for its methoxy analogue.
Thus, the most active against Mtb has been detected propyl-derivative, which has
also been shown to be active against C. albicans and E. faecalis.5
3D representations for the docking poses of the most active propyl derivati ve (left)
and non active p-methylacetophenone derivative (right) in the active site of InhA
presented below.

Weak hydrophobic interactions with Met199 (3.39Å), Gly96 (3.58Å) and Ile215
(3.59Å) occur in the case of the propyl derivative. Furthermore, these interactions
increased with the growth of alkyl chain length, the observation that could be made
while comparing the propyl and ethyl derivatives. At the same time, inactive pmethylacetophenone
derivative binds with receptor through the aromatic--aromatic

moiety (Gly96-3.34Å) and H--Aromatic interactions with Phe97 (3.71Å), Met98
(3.33 Å) and Tyr158 (3.56Å) and presents no hydrophobic interactions.
Electron density of the propyl derivtive is distributed both on ligand and amino acid
residues causing more effective donor-acceptor interaction. For the pmethylacetophenone
derivative, electron density is concentrated on residues and
absent on the ligand’s atoms.
REFERENCES
(1) A. M. Baker, William R.; Lester, Indolo [2, 1-Biquinazoline-6, 12-Dione Antibacterial
Compounds and Methods of Use Thereof, 1995, US Patent 5,441,955.
(2) L. A. Mitscher, W. Baker, Med. Res. Rev. 1998, 18, 363–374.
(3) J.-M. Hwang, T. Oh, T. Kaneko, A. M. Upton, S. G. Franzblau, Z. Ma, S.-N. Cho, P. Kim, J.
Nat. Prod. 2013, 76, 354–367.
(4) A. Tripathi, N. Wadia, D. Bindal, T. Jana, Indian J. Biochem. Biophys. 2012, 49, 435–441.
(5) S. Pogrebnoi, C. Chiriţă, V. Valica, F. Macaev, M. C. Chifiriuc, C. Kamerzan, L. Uncu,
Farmacia 2017, 65, 69–74.
Acknowledgements: the authors are grateful for the funding support from the Agency for Research and
Development of the Republic of Moldova under moldo-romanian bilateral project
16.80013.5007.05/Ro.