- Tuberculoѕiѕ (TB) iѕ a number one killer among treatable infectious diseases. It is caused by the pathogenic bacterium Mycobacterium tuberculoѕiѕ that primarily affects the lungѕ. It infects one third of the world population and kills approximately 2 million people each year (based on WHO report). Drug-reѕiѕtant TB haѕ become a very ѕeriouѕ problem in recent yearѕ in certain populationѕ. Therefore, the discovery and development of novel anti-tubercular drugs is a matter of emergency.
Our research goal focused on the discovery of new drug candidates that can be developed as new anti-tubercular drugs. We employed semi-synthetic approaches using naturally derived compounds, 27-O-demethylrifamycin SV (DMRSV), 27-O-demethyl-25-O-deacetylrifamycin SV (DMDARSV), and validoxylamine A, as scaffolds.
DMRSV and DMDARSV were obtained from a mutant strain of the rifamycin producing bacterium Amycolatopsis mediterranei S699, in which the methyltransferase gene within the rifamycin biosynthetic gene cluster has been inactivated. The compounds were modified by attaching a side chain diethylaminomethyl to the C-3 position. The products were tested for their activity against Mycobacterium smegmatis, B. subtilis, P. aeruginosa and S. aureus. However, the activity of DMRSV and DMDARSV derivatives were almost the same or less than that of 3-[(N,N-diethyl)-aminomethyl]-rifamycin SV.
Validoxylamine A is a degradation product of the antifungal agent validamycin A. Structurally, it mimics trehalose, which is found in many bacteria and fungi, but not in the vertebrates. In M. tuberculosis, trehalose-6,6'-dimycolate (TDM) and trehalose-6-monomycolate (TMM) as well as other trehalose esters are important components of the cell wall and play a role in pathogenicity.
Due to its similarity to trehalose, it is postulated that the ether derivatives of validoxylamine A could interfere with TMM and TDM synthesis and/or transport, as well as M. tuberculosis cell wall biosynthesis. To this end, we modified validoxylamine A by attaching various alkyl groups to the C-7 and C-7' positions of the compound, followed by reducing the C5-C6 double bond. The products were tested against M. smegmatis using agar diffusion and microdilution assays. The results showed that a number of validoxylamine A derivatives have significant antibacterial activity and an appropriate length of the side chain is critical for their activity. Compounds with alkyl chain range between C6 – C10 showed significant activity against M. smegmatis, whereas compounds with alkyl chains shorter than C6 or higher than C10 showed little or no activity. To confirm that this activity was not due to a detergent effect, we tested one of the active compounds against different strains of Gram (+) and Gram (-) bacteria, such as S. aureus, B. subtilis, and E. coli. The result showed that the active compound killed S. aureus and E. coli but not B. subtilis. Further testing against human cells (SF-295 glioma) revealed that the active compounds have no significant cytotoxicity. The results suggested that validoxylamine A derivatives may have potential to be developed as new anti- tubercular drugs with novel mechanism(s) of action.