Allium sativum Constituents Exhibit Anti-tubercular Activity In vitro and in RAW 264.7 Mouse Macrophage Cells Infected with Mycobacterium tuberculosis H37Rv

BACKGROUND: Long duration of treatment, side-effects of currently used anti-tubercular drugs and emergence of drug-resistant forms of Mycobacterium tuberculosis (MTB) warrants the need to develop new drugs to tackle the scourge of tuberculosis (TB). Garlic is an edible plant reported to have anti-tubercular activity. However, previous researches on anti-tubercular effect of garlic were focused mostly on preliminary in vitro screening. OBJECTIVE: To identify constituents responsible for anti-tubercular activity of thiosulfinate-derivative rich extract of garlic (GE) and to evaluate activity of the most active constituent in RAW 264.7 mouse macrophage cells infected with M. tuberculosis H37Rv (MTBH). MATERIALS AND METHODS: In the present study, we have isolated eight compounds from GE by flash chromatography. The isolated compounds were characterized by (1)H nuclear magnetic resonance spectroscopy, liquid chromatography-mass spectrometry and Fourier transform infrared spectroscopy. Individual isolates and GE were screened for activity against MTBH by Resazurin Microtitre Plate Assay (REMA). RESULTS: Anti-tubercular activity of GE was superior to that of isolates when evaluated by REMA, possibly due to synergism amongst the constituents of GE. Cytotoxicity of GE was evaluated in RAW 264.7 mouse macrophage cells and it was observed that GE had a favorable selectivity index (>10). Therefore, anti-tubercular activity of GE was further evaluated by intracellular macrophage infection model. GE demonstrated concentration-dependent activity in macrophages infected with MTBH. CONCLUSION: This is the first report on intracellular anti-tubercular activity of any extract of garlic or its components. Appreciable intracellular anti-tubercular activity of GE in macrophages combined with low cytotoxicity makes it a suitable candidate for further development as an anti-tubercular agent. SUMMARY: Thiosulfinate-derivative rich extract of Allium sativum showed better activity than its isolated constituents against Mycobacterium tuberculosis H37Rv.(MTBH) when evaluated by Resazurin Microtitre Plate Assay. The extract showed least cytotoxic potential against RAW 264.7 mouse macrophage cells as compared to rifampicin, isoniazid and ethambutol when evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. The extract had an appreciable selectivity index. Extract showed appreciable activity in RAW 264.7 mouse macrophage cells infected with MTBH, indicating its potential to be developed further as an anti-tubercular agent that can be administered as an adjunct to the existing anti-tubercular drug regimen. Abbreviations used: TB: Tuberculosis, MTB: Mycobacterium tuberculosis, MTBH: Mycobacterium tuberculosis H37Rv, GE: Thiosulfinate-derivative rich extract of garlic, REMA: Resazurin Microtitre Plate Assay, VD: Vinyldithiin, CFU: Colony forming unit, (1)H NMR: (1)H nuclear magnetic resonance spectroscopy, FT-IR: Fourier transform-infrared spectroscopy, LC-MS: Liquid chromatography-mass spectrometry, IC(50): Concentration required to inhibit the cells by 50%, ANOVA: Analysis of variance.