Antimycobacterial activity of DNA intercalator inhibitors of Mycobacterium tuberculosis primase DnaG

Owing to the rise in drug resistance in tuberculosis combined with the global spread of its causative pathogen, Mycobacterium tuberculosis (Mtb), innovative anti mycobacterial agents are urgently needed. Recently, we developed a novel primase-pyrophosphatase assay and used it to discover inhibitors of an essential Mtb enzyme, primase DnaG (Mtb DnaG), a promising and unexplored potential target for novel antituberculosis chemotherapeutics. Doxorubicin, an anthracycline antibiotic used as an anticancer drug, was found to be a potent inhibitor of Mtb DnaG. In this study, we investigated both inhibition of Mtb DnaG and the inhibitory activity against in vitro growth of Mtb and M. smegmatis (Msm) by other anthracyclines, daunorubicin and idarubicin, as well as by less cytotoxic DNA intercalators: aloe-emodin, rhein and a mitoxantrone derivative. Generally, low-μM inhibition of Mtb DnaG by the anthracyclines was correlated with their low-μM minimum inhibitory concentrations. Aloe-emodin displayed threefold weaker potency than doxorubicin against Mtb DnaG and similar inhibition of Msm (but not Mtb) in the mid-μM range, whereas rhein (a close analog of aloe-emodin) and a di-glucosylated mitoxantrone derivative did not show significant inhibition of Mtb DnaG or antimycobacterial activity. Taken together, these observations strongly suggest that several clinically used anthracyclines and aloe-emodin target mycobacterial primase, setting the stage for a more extensive exploration of this enzyme as an antibacterial target.