Old drug ethacridine blocks key TB transporter MmpL3

Tuberculosis, caused by Mycobacterium tuberculosis (Mtb), remains a leading global health challenge, made worse by the rise of multidrug-resistant and extensively drug-resistant strains. The mycobacterial cell envelope is a formidable barrier to antibiotics, and researchers are increasingly looking to that structure for new ways to attack the bacterium. In work led by corresponding author Nitin Pal Kalia, investigators searched for existing drugs that could be repurposed to target an essential transporter in the cell envelope called MmpL3. MmpL3 is required to export trehalose monomycolate, a key building block used to assemble the mycobacterial cell wall. By focusing on this transporter, the team aimed to find a compound that would disrupt the bacterium’s ability to maintain its protective outer layers. Their strategy combined computer-based modeling with laboratory tests against different forms of Mtb, including drug-sensitive strains, resistant clinical isolates, non-replicating bacteria, and bacteria living inside host cells, to evaluate whether any known drug could be redirected to cripple MmpL3 and compromise the cell envelope.

The study identified the FDA-approved drug ethacridine as a strong inhibitor of MmpL3. Computational docking and molecular dynamics simulations indicated that ethacridine engages the MmpL3 binding pocket at residues that are also targeted by SQ109. In laboratory testing, ethacridine showed potent activity against both drug-sensitive and resistant Mtb isolates, with a minimum inhibitory concentration (MIC) of 1 μg/mL. The compound remained effective against non-replicating bacteria and in models of intracellular infection. Genetic and biochemical approaches supported MmpL3 as the relevant target: ethacridine-resistant mutants and overexpression strains altered sensitivity, and a spheroplast TMM-flipping assay confirmed disruption of trehalose monomycolate transport. The authors also report that the compound disrupts the membrane potential, and that flow-cytometry assays provided complementary information on bacterial responses, together building a consistent picture of how ethacridine impairs MmpL3 function and bacterial physiology.

These findings point to several important implications. First, because ethacridine is already FDA-approved, repurposing it as a tuberculosis therapy could in principle shorten the development timeline compared with a wholly new molecule. Second, the work highlights MmpL3 and the cell envelope as vulnerable targets for tackling both drug-sensitive and drug-resistant Mtb. Demonstrating activity against non-replicating bacteria and intracellular infection is especially relevant because these states are often difficult to treat with existing drugs. The study also establishes multiple lines of evidence — computational, genetic, biochemical, and physiological — that converge on MmpL3 as the target of ethacridine. While the abstract does not report clinical testing or safety data for TB treatment, the data support further preclinical and clinical exploration of ethacridine as a potential addition to the anti-tuberculosis toolkit, particularly where resistance undermines current regimens.