Antituberculosis: Synthesis and Antimycobacterial Activity of Novel Benzimidazole Derivatives

A total of seven novel benzimidazoles were synthesized by a 4-step reaction starting from 4-fluoro-3-nitrobenzoic acid under relatively mild reaction conditions. The synthesized compounds were screened for their antimycobacterial activity against M. tuberculosis H37Rv (MTB-H37Rv) and INH-resistant M. tuberculosis (INHR-MTB) strains using agar dilution method. Three of them displayed good activity with MIC of less than 0.2 μM. Compound ethyl 1-(2-(4-(4-(ethoxycarbonyl)-2-aminophenyl)piperazin-1-yl)ethyl)-2-(4-(5-(4-fluorophenyl)pyridin-3-ylphenyl-1H-benzo[d]imidazole-5-carboxylate (5g) was found to be the most active with MIC of 0.112 μM against MTB-H37Rv and 6.12 μM against INHR-MTB, respectively.


Introduction
Tuberculosis (TB) is the oldest documented infectious disease. It is the only disease which does not require any vector for transportation from one person to another [1]. The primary site of infection is the lungs, followed by dissemination via the circulatory and lymphatic system to secondary sites including the bones, joints, liver, and spleen.
In 2010, there were 8.8 million (range: 8.5-9.2 million) incident cases of TB, 1.1 million (range: 0.9-1.2 million) deaths from TB among HIV-negative people, and an additional 0.35 million (range: 0.32-0.39 million) deaths from HIV-associated TB [2]. The introduction of the first-line drugs like streptomycin, para-aminosalicylic acid, and isoniazid for treatment some 50 years ago has witnessed a remarkable decline in TB cases all over the world. The active TB is currently treated with a four-first-line-drug regimen comprising mainly isoniazid, rifampicin, pyrazinamide, and ethambutol for a period of at least 6 months [3,4]. However, the disease has been undergoing a resurgence in the last two decades driven by variety of changes in social, medical, and economic factors as well as M. tuberculosis resistance to the aforementioned drugs itself.
The resurgence of TB is now one of the most serious public health concerns worldwide. Despite its global impact on world health, TB is considered a neglected disease, and no new anti-TB therapeutics have been introduced into the market over the last half-century. The last drug with a new mechanism of action approved (rifampicin) was discovered in 1963 [5]. Therefore there is an urgent need for development of new drug leads to combat this chronic infectious disease.
The benzimidazole nucleus is of significant importance in medicinal chemistry research, and many benzimidazolecontaining compounds exhibit important biological properties such as antiviral [6], anti-inflammatory [7], and anti-HIV [8]. In the light of the affinity they display towards a variety of enzymes and protein receptors, medicinal chemists thus classify them as "privileged substructures" for drug design [9]. Recently, there has been reported work done on utilizing benzimidazole derivatives to counter TB with relatively good results [10][11][12], thus further reinforcing our belief that benzimidazole could potentially be a lead compound in our effort to discover new potent anti-TB agents. In the present paper, we wish to report the synthesis and antimycobacterial activity of novel 2-substituted benzimidazole derivatives.

General Procedure for the Preparation of Sodium Bisulfite
Adducts of 4-Substituted Benzaldehyde (4a-g). Appropriate benzaldehyde (10 mmol) was dissolved in ethanol (20 mL). Sodium metabisulfite (15 mmol) in 5 mL water was added in portion over 5 minutes. The reaction mixture was stirred at room temperature for 1 hour and subsequently stirred at 4 ∘ C overnight. The precipitate formed was filtered and dried to afford sodium bisulfite adducts (55%-90%).

Chemistry.
Our synthetic study into novel benzimidazoles started with 4-fluoro-3-nitro benzoic acid which was esterified in the presence of catalytic sulfuric acid in ethanol by refluxing for 8 hours to afford ethyl-4-fluoro-3-nitrobenzoate 1, in 75% yield. The ethylbenzoate 1 was then treated with amine and DIPEA in dry dichloromethane at room temperature to yield ethyl 3-amino-4-(4-(2-((4-(ethoxycarbonyl)-2-nitrophenyl)amino)ethyl)piperazin-1yl)benzoate 2, which was then reduced to 3 using ammonium formate and 10% Pd/C for 1 hour to give 70% yield. The phenylenediamine 3 was then refluxed with various substituted bisulfite adducts of aromatic aldehydes [14] in DMF overnight to afford benzimidazole derivatives 5a-g in moderate to good yields (63-90%). The structure of the novel benzimidazoles was confirmed by chromatographic and spectroscopic analysis. The mechanism for the formation of the novel benzimidazole derivatives is proposed and summarized in Scheme 1.  Table 3 with standard drug isoniazid as comparison.

Pharmacology.
We synthesized compounds with a wide range of substitution including compounds with electron-donating as well as electron-withdrawing groups. Generally, we found that electron-withdrawing group substituents at 4-position in the phenyl ring are important for good activities. Three compounds (5b, 5d, and 5g)  activity against MTB Mycobacterium tuberculosis. This clearly showed that the presence of electron-withdrawing group, especially halogen substitution at 4-position of the phenyl ring, caused marked improvement in antimycobacterial activity.
All the compounds were also tested for cytotoxicity (IC 50 ) in VERO cells at a concentration of 62.5 g/mL. After 72 h of exposure, viability was assessed on the basis of cellular conversion of MTT into a formazan product using the Promega CellTiter 96 nonradioactive cell proliferation assay according to the manufacturer's protocol. All of the active compounds were found to be nontoxic till 62.5 g/mL.

Conclusion
In conclusion, we have synthesized successfully a series of novel benzimidazole derivatives with good antimycobacterial properties against M. tuberculosis. Encouraged by the positive results we have reported here, further modification on the 4-position on the bisulfite adducts as well as quantitative structure-activity relationship (QSAR) is currently in progress in our laboratory. It is conceivable that derivatives showing antimycobacterial activity can be further modified to exhibit better potency than standard drugs.