Evaluation of the MTT and MABA assays for rapid screening of the in vitro activity of synthetic chalcones against Mycobacterium tuberculosis.
Background: The chalcone scaffold (1,3-diaryl-2-propen-i-ones) has the advantage of easy chemical modification and has been shown to possess biological activity against a variety of organisms, including a wide range of anti-TB activity. The focus of this study was 2-fold: firstly, to compare the performance of the colorimetric MTT and MABA assays for screening synthetic chalcones, and secondly, to evaluate the activity of fluorinated and non-fluorinated chalcones against drug susceptible and resistant clinical strains of M. tuberculosis. Materials and methods: Twenty seven chalcones and chromenochalcones were screened against the laboratory strain M. tuberculosis H37Rv, using a microtitre plate MTT assay at 7 days. The MIC for 20 active compounds was subsequently determined using the MABA, MTT and the macroscopic broth assays at 7, 14 and 21 days, extracellular activity against clinical isolates of varying drug susceptibility patterns and genotypes using the MTT assay, intracellular activity in a macrophage model and eukaryotic cytotoxicity using Vero cells. Results and discussion: No significant difference in the MICs, or increase in the MICs was observed over time between the MABA (p = 0.209) and the MTT (p = 0.207) assays, in contrast to the gold standard, the macroscopic broth assay (p = 0.000). Fluorinated and non-fluorinated chalcones displayed moderate activity (32- 128 μg/mL) against MDR- and XDR-TB isolates, no significant activity against intracellular H37Rv and low selectivity for M. tuberculosis. The elevated MICs and lack of intracellular activity may be explained by the precipitation of the compounds indicating low solubility, with the exception of IV and XVI. Conclusions: The MTT assay is a more cost effective drug susceptibility testing method than the MABA assay for the rapid in vitro screening of the activity of chalcones against M. tuberculosis. Compound XIX and XI have the most potential for reformulation to improve their biological activity to yield a more potent drug candidate.