Discovery of repurposed drugs that disrupt intracellular replication of Burkholderia pseudomallei and Burkholderia mallei and protect against lethal aerosol infection

Burkholderia pseudomallei (Bp) and Burkholderia mallei (Bm) are the etiological agents of the fatal diseases melioidosis and glanders, respectively. No licensed vaccine is available to protect against these facultative intracellular bacteria and treatment of infection is difficult, requiring intensive and prolonged antimicrobial therapy with low success rates due to intrinsic resistance of Bp and Bm to most antibiotics. This, combined with concerns regarding their adversarial use as biological warfare agents, emphasize an immediate need to identify novel and effective countermeasures for the organisms. Herein, we developed an in vitro high-content imaging, high-throughput screening platform and tested a L ibrary of P harmacologically A ctive C ompounds (LOPAC) and the Repurposing, F ocused R escue, and A ccelerated ME dchem (ReFRAME) library of small molecules for bioactive compounds protecting against bacterial intracellular replication, which is a key pathogenicity trait of Bp and Bm. We first analyzed confocal microscopy images of Burkholderia -infected monolayers of Vero E6 cells and established a reproducible endpoint readout that quantifies cytopathic effects and the formation of plaques during infection. Following this, we screened ~14,800 small molecules and identified 170 unique structures providing ≥ 75% inhibition of intracellular replication of Bp and Bm with EC 50 values ranging from 1 pM to 9.99 µM. Of these hits, 23 were found to disrupt the intracellular replication of both organisms including 8 compounds targeting dihydrofolate reductase, 3 tetracycline-class antibiotics, an inhibitor of bacterial leucyl-tRNA synthetase (epetraborole), and 2 host-directed compounds targeting receptors on the surface of mammalian cells. Selected compounds were evaluated in vivo using a BALB/c mouse model of aerosol infection and we discovered that daily treatment with epetraborole (20 mg/kg) and the fluoroquinolone levofloxacin (5 mg/kg) completely protected mice against exposure to lethal doses of Bm and Bp , reducing bacterial burdens in the lungs and spleen to undetectable levels in all animals by day 14 post-challenge.