Pyrazinamide is a first-line TB drug, and its possible targets include FAS I,
QAPRTase, RpsA, PanD, Rv2783 (Table 1; Fig. 2). Pyrazinamide is a prodrug
which is converted by pyrazinamidase into the active form pyrazinoic acid [87, 88].
This drug inhibits the transport mechanism and energy pathway in Mtb, thereby
disrupting its growth. Pyrazinoic acid is able to inhibit the enzyme, fatty acid
synthase, in Mtb, which in turn inhibits the production of fatty acids. There is also
evidence of this inhibitor in disrupting the membrane potential and energy production, which are essential for Mtb survival [89].
2.6 Mycobacterial Drug Targets Involved in Replication
and Transcription
DNA gyrase is an essential ATP-dependent bacterial enzyme that acts by creating a
transient double-stranded DNA break, which relieves the strain caused by unwinding
of double-stranded DNA by helicase enzyme. DNA gyrase is found in all the bacterial
species and essential for DNA replication, transcription, and recombination processes
(Fig. 3). The enzyme exists as a heterotetramer consisting of two A subunits and two
B subunits (A2B2) [90]. In Mtb, it is proven that the inhibition of DNA gyrase results
in high anti-mycobacterial activity toward both actively replicating and nonreplicating, dormant bacilli, which is necessary for shortening the TB treatment [91].
The synthetic antimicrobial class, Fluoroquinolones, has been demonstrated to have
anti-tubercular activity by inhibiting to the mycobacterial DNA gyrase subunit A [92].
Fluoroquinolones are currently being used as second-line drugs for TB, and also these
drugs have been used in TB treatment to hamper the development of XDR-TB from
MDR-TB (Table 1; Fig. 2). Though the emergence of fluoroquinolone resistance
affects its use as second-line drugs [93], this has driven the interest in targeting the
DNA gyrase subunit B (GyrB). An approved anti-bacterial agent novobiocin was the
only approved GyrB inhibitor; however, it was withdrawn from the market due to
safety concerns [94]. A new class of compounds, aminobenzimidazole antibiotics
targeting the ATP-binding site of GyrB, has also been reported [95]. Chopra et al.
evaluated the biological activity of both aminobenzimidazole and novobiocin. The
drugs are demonstrated to be active against Mtb with minimum inhibitory concentration (MIC) of 1 and 4 mg/ml, respectively. Only aminobenzimidazole compound
exhibited a time-dependant mycobactericidal activity against both drug-sensitive and
drug-resistant bacilli, which also showed potent activity against non-replicating
persistent Mtb. Further, this compound significantly reduced the lung colony forming
unit counts in the mice TB model [95].
Shirude et al. identified aminopyrazinamides as novel and specific GyrB inhibitors that kill replicating and non-replicating Mtb bacilli through HTS of compounds. They solved the X-ray crystal structure of GyrB of M. smegmatis in
complex with one of the aminopyrazinamides. The aminopyrazinamides also
showed significant anti-mycobacterial activity under in vitro, intracellular, and
Impact of Target-Based Drug Design in Anti-bacterial …
321
QAPRTase, RpsA, PanD, Rv2783 (Table 1; Fig. 2). Pyrazinamide is a prodrug
which is converted by pyrazinamidase into the active form pyrazinoic acid [87, 88].
This drug inhibits the transport mechanism and energy pathway in Mtb, thereby
disrupting its growth. Pyrazinoic acid is able to inhibit the enzyme, fatty acid
synthase, in Mtb, which in turn inhibits the production of fatty acids. There is also
evidence of this inhibitor in disrupting the membrane potential and energy production, which are essential for Mtb survival [89].
2.6 Mycobacterial Drug Targets Involved in Replication
and Transcription
DNA gyrase is an essential ATP-dependent bacterial enzyme that acts by creating a
transient double-stranded DNA break, which relieves the strain caused by unwinding
of double-stranded DNA by helicase enzyme. DNA gyrase is found in all the bacterial
species and essential for DNA replication, transcription, and recombination processes
(Fig. 3). The enzyme exists as a heterotetramer consisting of two A subunits and two
B subunits (A2B2) [90]. In Mtb, it is proven that the inhibition of DNA gyrase results
in high anti-mycobacterial activity toward both actively replicating and nonreplicating, dormant bacilli, which is necessary for shortening the TB treatment [91].
The synthetic antimicrobial class, Fluoroquinolones, has been demonstrated to have
anti-tubercular activity by inhibiting to the mycobacterial DNA gyrase subunit A [92].
Fluoroquinolones are currently being used as second-line drugs for TB, and also these
drugs have been used in TB treatment to hamper the development of XDR-TB from
MDR-TB (Table 1; Fig. 2). Though the emergence of fluoroquinolone resistance
affects its use as second-line drugs [93], this has driven the interest in targeting the
DNA gyrase subunit B (GyrB). An approved anti-bacterial agent novobiocin was the
only approved GyrB inhibitor; however, it was withdrawn from the market due to
safety concerns [94]. A new class of compounds, aminobenzimidazole antibiotics
targeting the ATP-binding site of GyrB, has also been reported [95]. Chopra et al.
evaluated the biological activity of both aminobenzimidazole and novobiocin. The
drugs are demonstrated to be active against Mtb with minimum inhibitory concentration (MIC) of 1 and 4 mg/ml, respectively. Only aminobenzimidazole compound
exhibited a time-dependant mycobactericidal activity against both drug-sensitive and
drug-resistant bacilli, which also showed potent activity against non-replicating
persistent Mtb. Further, this compound significantly reduced the lung colony forming
unit counts in the mice TB model [95].
Shirude et al. identified aminopyrazinamides as novel and specific GyrB inhibitors that kill replicating and non-replicating Mtb bacilli through HTS of compounds. They solved the X-ray crystal structure of GyrB of M. smegmatis in
complex with one of the aminopyrazinamides. The aminopyrazinamides also
showed significant anti-mycobacterial activity under in vitro, intracellular, and
Impact of Target-Based Drug Design in Anti-bacterial …
321