Background: Mycobacterium tuberculosis (Mtb) strains H37Ra and H37Rv are commonly used to study new and re-evaluate old antituberculous agents with respect to their pharmacodynamic effects in vitro. The differences in membrane proteins and, in particular, differences in carrier proteins between Mtb H37Ra and Mtb H37Rv may have an impact on antibiotic potency. The question of whether H37Ra can be used as a reliable surrogate for H37Rv and clinical strains has not been addressed sufficiently. The purpose of this study is to provide a full comparison of susceptibility data of the most common antituberculosis (TB) agents against both Mtb strains. Methods: In addition to a literature review, in vitro checkerboard susceptibility study was conducted comparing the in vitro minimum inhibitory concentration (MIC) of 16 common antituberculous drugs against H37Ra and H37Rv. Heifets–Sanchez TB agar drug susceptibility plates were utilized. Results: Half of the antibiotics demonstrated similar growth inhibition against both strains, while slightly differing MIC values were found for 7 of 16 drugs. With the exception of rifampicin, no marked difference in MIC against H37Ra and H37Rv was observed. Conclusion: While neither the attenuated (H37Ra) nor the virulent strain (H37Rv) is a clinical strain, both strains predicted MICs of clinical isolates equally well, when comparing the current in vitro results to clinical susceptibility data in the literature. H37Ra comes with the benefits of lower experimental costs and less administrative barriers including the requirement of a biosafety Level III environment.

In vitro and in vivo metabolism studies revealed that 2-alkylsulfanylimidazole ML3403 (4-(5-(4-fluorophenyl)-2-(methylthio)-1H-imidazol-4-yl)-N-(1-phenylethyl)pyridin-2-amine) undergoes rapid oxidation to the sulfoxide. Replacing the sulfur atom present in the two potent p38α mitogen-activated protein (MAP) kinase inhibitors ML3403 and LN950 (2-((5-(4-fluorophenyl)-4-(2-((3-methylbutan-2-yl)amino)pyridin-4-yl)-1H-imidazol-2-yl)thio)ethan-1-ol) by a methylene group resulted in 2-alkylimidazole derivatives 1 and 2, respectively, having a remarkably improved metabolic stability. The 2-alkylimidazole analogs 1 and 2 showed 20% and 10% biotransformation after 4 h of incubation with human liver microsomes, respectively. They display a 4-fold increased binding affinity towards the target kinase as well as similar in vitro potency and ex vivo efficacy relative to their 2-alkylsulfanylimidazole counterparts ML3403 and LN950. For example, 2-alkylimidazole 2, the analog of LN950, inhibits both the p38α MAP kinase as well as the LPS-stimulated tumor necrosis factor-α release from human whole blood in the low double-digit nanomolar range.