Abstract

Tuberculosis (TB) continues to pose significant global health challenge, worsened by the rise of drug-resistant strains of Mycobacterium tuberculosis. Pyrazinamide (PZA) is a cornerstone in the treatment of TB, particularly for its ability to sterilize dormant bacilli. However, the increasing prevalence of PZA-resistant strains has necessitated the development of novel analogues to enhance treatment efficacy and overcome resistance. This review explores recent advancements in the design and synthesis of pyrazinamide analogues, focusing on their improved pharmacological profiles and mechanisms of action. Novel analogues have been developed to target specific molecular pathways, demonstrating potent anti-tubercular activity in preclinical studies. Structural modifications to the pyrazine ring and the introduction of various substituents have led to improved bioavailability, reduced toxicity, and enhanced activity against resistant strains. Furthermore, these analogues exhibit synergistic effects when used in combination with other anti-tubercular agents, potentially shortening treatment durations and improving patient outcomes. The review also highlights the challenges in translating these analogues from bench to bedside, emphasizing the need for comprehensive clinical evaluations to assess their safety and efficacy in humans. Overall, novel pyrazinamide analogues represent a promising avenue in TB drug development, offering potential solutions to combat the ongoing threat of drug-resistant tuberculosis.

Keywords

Introduction

Chronic granulomatous disease known as tuberculosis is caused by the bacterial infection of Mycobacterium tuberculosis and targets the myeloid cells. It is transmitted by inhalation of infected droplets with nuclei that contains viable microbes.[1] The rational development of more efficient TB vaccines and treatments can be aided by knowledge on interaction that exists between M. tuberculosis and the host immunity. Although the precise mechanism of action of pyrazinamide is unknown, it can be determined that, like INH, it is metabolized by an enzyme called pyrazinamidase, which is encoded by the pncA gene, inside the cell of mycobacterial strains to produce its active form pyrazinoic acid. This metabolite accumulates in acidic conditions and likely prevents the synthesis of mycolic acid by interfering with another fatty acid synthase. Moreover, pyrazinoic acid tends to interfere with the transportation function of the mycobacterial cell membrane. If pyrazinamide is utilized alone, it develops resistance and is primarily caused by mutations in the pncA gene.[2]The spread of M. tuberculosis to new vulnerable individuals is believed to rely on an elevated level of inflammation that leads to the disruption of the lung matrix and the development of lung cavities.[3] The clinical manifestation of the disease morphology is influenced by the immune system of the host. To improve the global situation, the World Health Organization (WHO) has established and increased the implementation of three major worldwide public health approaches: DOTS, Stop TB, and End TB.[4] Pyrazinamide is a first line anti-tubercular agent that was initially produced in 1936 but is widely used from 1972. It is a white solid powder. Chemically, it is a pyrazine derivative containing monocarboxylic acid amide group.[5] It is prepared by reacting o-phenylenediamine with glyoxal forms quinoxaline which is oxidized in presence of potassium permanganate forming pyrazine-2,3-dicarboxylic acid that is heated to decarboxylate and form pyrazine-2-carboxylic acid and is esterified by treating withy methanol or hydrochloric acid to form acetyl ester of pyrazin-2-carboxylic acid and treated with ammonia to form pyrazinamide. It is a prodrug of pyrazinoic acid for the treatment of tuberculosis. It is used in combination with isoniazid, rifampicin or ethambutol and streptomycin in the treatment of tuberculosis. In general, it is not advised for the treatment of latent TB.[6,7] When pyrazinamide penetrates into the M. tuberculosis granuloma, it is transformed to pyrazinoic acid, the active form, by the tubercular enzyme pyrazinamidase. The pyrazinoic acid that slowly seeps out transforms into the protonated acid conjugates in an acidic environment (pH of 5 to 6), which is assumed to diffuse back into the bacilli and aggregate. As a result, at acidic pH the accumulation of pyrazinoic acid is higher in bacillus than in the neutral pH.[8,9] Pyrazinoic acid appears to inhibit the production of coenzyme A. Pyrazinoic acid degrades aspartate decarboxylase (PanD) by weakly binding to it. Pyrazinamide's mode of action is unique in that it causes its target's destruction indirectly rather than directly by blocking its action.[10]

Pathophysiology of tuberculosis

Bacillus enters the lungs when M. tuberculosis is transmitted to a new host, thereafter macrophages consume them. The granuloma, the TB hallmark, forms as a result of the recruitment of more immune cells to block off infection-carrying macrophages. Though the spread of infection is prevented during this phase in the healthy individuals who remain infected, there is possibility that it may reactivate. When foamy macrophages undergo necrosis, the lipid composition is released, causes caseation. Caseum is the breakdown compromising the integrity of granuloma at its core. The bacillus begins to ooze out of macrophages and beneath the caseum layer during granuloma expansion. Granuloma ruptures and microbes are released into the airways when the reactivation arises when M. tb replicates and the number of bacteria rises to an extremely high level. Following expectoration, the bacilli spread the infection to other people by aerosol droplets that are infectious.[11]

Pyrazinamide based analogues

Sriram D et. al (2006) synthesized piperazine and quinoline incorporated pyrazinamide derivatives and studied them for their antitubercular activity. Compound 1 containing fluoro and cyclopropyl derivative showed potent antitubercular activity with MIC value of 0.2 µg/ml compared with reference drug pyrazinamide 12.5µg/ml. [12]

Dolezal M et.al (2008) synthesized substituted pyrazinamide derivatives and studied them for their antitubercular activity. Compound 2 containing trifluoromethylphenyl substitution showed potent antitubercular activity with MIC value of 3.13 µg/ml compared with reference drug pyrazinamide, fluconazole and diourene.[13]

Dolezal M et.al (2009) synthesized pyrazinamide derivatives incorporating various substituted benzene connected by amide linkage and studied them for their antitubercular activity. Compound 3 containing 4-iodo-3-methyl phenyl group showed potent antitubercular activity with MIC value of 2.0?mol/l compared with reference drug pyrazinamide with MIC value of 8 mg/l.[14]

Zitko J et.al (2011) synthesized pyrazinamide derivatives related compounds and studied them for their antitubercular activity. Compound 4a containing chloro substitution and compound 4b and 4c containing methylamino and octylamino substitution showed potent antitubercular activity with MIC value of 25µg/ml and 12.5µg/ml respectively compared to the reference compound pyrazinamide with MIC value of 12.5–25µg/ml.[15]

Servusova B et.al (2013) synthesized chloro pyrazinamide derivatives and studied them for their antitubercular activity. Compound 5 containing nitrophenyl group showed potent antitubercular activity with MIC value of 16µg/ml compared with reference drug isoniazid and pyrazinamide with MIC value of 1.56µg/ml and 12.5-25µg/ml respectively.[1

Zitko J et. al (2013) synthesized chloro pyrazinamide derivatives and studied them for their antitucular activity. The compound 6 containing hydroxybenzoic acid substitution showed potent antitubercular activity with MIC value of 3.13µg/ml with reference compound pyrazinamide and isoniazid with MIC value of 6.25–12.5µg/ml and 0.39–0.78µg/ml.[17]

Vanaskova et.al (2015) synthesized pyrazinamide analogs and studied them for their antitubercular activity. The compound 7e containing heptylamino substitution and 7f containing heptylamino along with chlorophenyl group showed potent antitubercular activity with MIC value of 5-10µg/ml compared with reference drug isoniazid and pyrazinamide with MIC value of 0.2-1.56µg/ml.[18]

Semelkova L et.al (2015) synthesized alkyl amino pyrazinamide derivatives and studied their antitubercular activity. The compound 8a, 8b, 8c containing hexyl, heptyl, octyl substitution showed potent antitubercular activity with MIC value of 25µg/ml compared with reference compound isoniazid with MIC value of 0.1-0.38µg/ml.[19]

Jandourek O et.al (2017) synthesized benzylamines substituted pyrazinamide derivatives and studied them for their antitubercular activity. Compound 9 containing tolyl group showed potent antitubercular activity with MIC value of 1.56 µg/ml compared with reference drug isoniazid and pyrazinamide with MIC value of 0.39µg/ml and 12.5µg/ml respectively.[20]

Semelkova L et.al (2017) synthesized series of benzyl pyrazinamide derivatives and studied them for their antitubercular activity. Compound 10a, 10b containing methylbenzyl and dichlorobenzyl substitution showed to be have potent antitubercular activity with MIC value of 12.5 µg/ml compared with reference drug isoniazid with MIC value of 0.2 µg/ml.[21]

Zhou S et.al (2017) synthesized pyrazinamide derivatives and studied them for their antitubercular activity. Pyrazine carboxylic acid was used as the starting material for the synthesis of the desired compounds through alkylation, acylation and amidation. Compound 11 containing morpholine ring showed potent antitubercular activity with MIC value of 8.0 µg/ml with bacteriostatic rate of 99.6% compared with reference drug pyrazinamide with MIC value of 12.5µg/ml with bacteriostatic rate of 99.2%.[21]

Zitko J et.al (2018) synthesized thiazole incorporated pyrazinamide derivatives and studied the compounds for their antitubercular activity. The compound 12 containing fluorophenyl group showed potent antitubercular activity with MIC value of 0.78?g ml and selectivity index of >21.5 compared with reference drug isoniazid having MIC value of 0.1-0.2 µg/ml.[22]

Gangarapu N R et.al (2019) synthesized aryl substituted pyrazinamide derivatives and studied them for their antitubercular activity. Compound 13 containing chlorophenyl and 2-chloro-5-fluoro phenyl group showed potent antitubercular activity with MIC value of 10?g/ml compared with reference drug rifampicin with MIC value of 40µg/ml and also carried out in-silico studies resulting that this compound showed good ADME.[23]

Srinivasarao et.al (2020) synthesized series of pyrazinamide derivatives and studied them for their antitubercular activity. Five compounds from the 1st series 14a, 14e, 14h, 14j, 14k containing phenyl, bromophenyl, o-methylphenyl substitution, 2-bromo-5-chlorophenyl, 4-bromo-2-chlorophenyl and compound 14m from 2nd series containing pyrimidine substitution are found to have potent antitubercular activity with IC50 values ranging between 1.35- 2.18µM compared with reference drug isoniazid 0.013µM.[24]

Zulqurnain M et. al (2023) synthesized pyrazinamide derivatives and studied them for their antitubercular activity. The compounds 15d, 15g containing chlorobenzyl and cycloheptane substitution showed potent antitubercular activity with MIC value of <6>

CONCLUSION:

An important development in the fight against tuberculosis (TB) is the creation of innovative pyrazinamide analogues as anti-tubercular drugs. Because of Pyrazinamide's (PZA) special sterilizing action against dormant bacteria and its capacity to reduce therapy duration, PZA has been a mainstay of TB treatment. Recent research has shown that the anti-mycobacterial qualities of these derivatives have been effectively increased by structural alterations such as the inclusion of additional side chains and heterocyclic groups. To completely understand the potential of these substances and apply the results into clinical practice, more investigation into the mechanism of action and clinical trials are essential. Prolonged investigation and refinement of these substances may be important in tackling the worldwide tuberculosis epidemic, ultimately leading to enhanced outcome for patients and advancements in public health.

ACKNOWLEDGEMENT:

The authors are thankful to Chanda Ranjan, Akshay C, Assistant Professor of Pharmaceutical Chemistry, The Oxford college of Pharmacy for their support and encouragement.

CONFLICTS OF INTEREST

The authors declare that there is no conflict of interest.

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