Chapter 17. Recent Advances in the Chemistry and Biology of Anti-mycobacterial Agents

Synthesis, characterization and in vitro biological evaluation of some novel 1,3,5-triazine-Schiff base conjugates as potential antimycobacterial agents

A series of some novel 1,3,5-triazine-Schiff base conjugates (1-32) have been synthesized and evaluated for their in vitro antimycobacterial activity against Mycobacterium tuberculosis H37Rv using Alamar Blue assay and the activity expressed as the minimum inhibitory concentration (MIC) in μg/mL. Compounds 4 (4-Methoxy-6-methyl-N-(3,4,5-trimethoxybenzylidene)-1,3,5-triazin-2-amine), 11 (4-Methoxy-6-methyl-N-(2-hydroxy-3-bromo-5-chloro-benzylidene)-1,3,5-triazin-2-amine) and 24 (4-Methoxy-6-methyl-N-(1-(2,5-dihydroxyphenyl)ethylidene)-1,3,5-triazin-2-amine) exhibited a significant activity at 3.125, 6.25 and 6.25μg/mL, respectively, when compared with the antitubercular drugs such as ethambutol (3.125μg/mL), pyrazinamide (6.25μg/mL) and streptomycin (6.25μg/mL) and it could be a potential starting point to develop new lead compounds in the fight against Mycobacterium tuberculosis H37Rv.

The treatment of tuberculosis: current status and future prospects

Although a vaccine and effective chemotherapy against tuberculosis (TB) have been available for more than half a century, TB was declared a global emergency in 1993. Current chemotherapeutic regimens are being undermined by lack of resources for proper implementation and control, and the emergence of multi-drug resistant strains of Mycobacterium tuberculosis. Several new chemotherapeutic agents are under development, mainly derived from existing anti-TB drugs or broad-spectrum antibiotics. New experimental agents include immunomodulants and drugs directed against novel cellular targets.

New drugs for tuberculosis

In the last ten years, an unexpected resurgence of tuberculosis (TB) has occurred in industrialised countries; contributing factors are likely to include the spread of HIV infection and increasing waves of immigration. Moreover, multidrug resistant (MDR) strains of Mycobacterium tuberculosis are emerging, rendering older therapies largely ineffective. One approach to circumvent this situation has been the addition of antimicrobials with some in vitro antituberculosis activity, but which are marketed for other infections (particularly some quinolones and the combination of oxicillin-clavulanic acid), to current therapeutic regimens. New drugs, possibly acting on novel targets, are urgently required, as are more specific vaccines.

Synthesis and anti-tubercular evaluation of 4-quinolylhydrazones

A series of 4-quinolylhydrazones were synthesized and tested against Mycobacterium tuberculosis H37Rv. Preparation of the title compounds was achieved by reaction of 4-quinolylhydrazine and aryl- or heteroaryl-carboxaldehyde. For the most of derivatives interesting antitubercular properties were showed; two compounds (3(2) and 3(25)), identified as the most active, were tested also against Mycobacterium avium.

Antimycobacterial activity of 5-arylidene aromatic derivatives of hydantoin

Various 5-(chlorobenzylidene)-2-isoniazido and 5-(chlorobenzylidene)-2-amino substituted derivatives of imidazoline-4-one were synthesized and evaluated in the primary assay for their antimycobacterial activity against Mycobacterium tuberculosis H37Rv. Eight of them exhibited > 90% inhibition in the primary screening at 12.5 microg/ml. For these primarily selected compounds the actual MIC and IC50 values were determined. Two of the isoniazid derivatives, for which MIC < 3.13 microg/ml and SI > 10, were selected for further screening and investigated for efficacy in vitro in a TB-infected macrophage model. The most promising compound, 5-(3-chlorobenzylidene)-2-(isonicotinoylhydrazino)-imidazoline-4-one, with activity in vitro comparable with rifampin (MIC = 0.8 microg/ml. SI > 78) was tested in vivo in the animal tuberculosis model but exhibited insignificant activity. For several compounds the primary screening of antimycobacterial activity against Mycobacterium avium (ATCC 25291) was conducted as well, but none of them demonstrated satisfactory activity.

Antimycobacterial pyrroles: synthesis, anti-Mycobacterium tuberculosis activity and QSAR studies

A number of known antifungal pyrrole derivatives and some newly synthesized compounds (5-33) were tested in vitro against Mycobacterium tuberculosis CIP 103471. The majority of tested compounds were efficient antimycobacterial agents showing MIC values ranging from 0.5 to 32 microg/mL. A 3-D-QSAR study has been performed on these pyrrole derivatives to correlate their chemical structures with their observed inhibiting activity against M. tuberculosis. Due to the absence of information on a putative receptor responsible for this activity, classical quantitative structure-activity relationships (QSAR) and comparative molecular field analysis (CoMFA) have been applied. A model able to well correlate the antimycobacterial activity with the chemical structures of pyrrole derivatives 5-33 has been developed which is potentially helpful in the design of novel and more potent antituberculosis agents. The combination of CoMFA with classical QSAR descriptors led to a better hybrid 3-D-QSAR model, that successfully explains the structure-activity relationships (r2 = 0.86) of the training set. A comparison between the QSAR, CoMFA and mixed QSAR-CoMFA models is also presented. The hybrid model is to be preferred, however, because of its lowest values of the average absolute error of prediction toward a limited external test set.

Tuberculosis: a search for novel therapy starting with natural products

The reemergence of tuberculosis and closely related diseases as significant public health problems is briefly reviewed with particular emphasis on the exacerbating role of AIDS and multiple drug resistance. Screening methods available for discovering new chemical entities active against resistant strains are discussed and their use in screening extracts and compounds for activity is illustrated with a number of newly discovered structures being presented. In particular, the properties of the potent and structurally novel indoloquinazolinone alkaloid, tryptanthrin, is described. Many analogs of this lead structure were synthesized by combinatorial and multiple parallel synthetic techniques and evaluated in vitro and in vivo for their potential in the chemotherapy of human infections.

Homologated aza analogs of arabinose as antimycobacterial agents

A series of hydrolytically-stable aza analogs of arabinofuranose was prepared and evaluated against Mycobacterium tuberculosis and M. avium. The compounds were designed to mimic the putative arabinose donor involved in biogenesis of the essential cell wall polysaccharide, arabinogalactan. Though most compounds displayed little activity in cell culture, one compound showed significant activity in infected macrophage models.