| Summary: | ABSTRACT Typhoid fever is caused primarily by the enteric microbe Salmonella enterica serovar Typhi and remains a major global health problem with approximately 14 million new infections and 136,000 fatalities annually. While there are antibiotic options available to treat the disease, the global increase in multidrug-resistant strains necessitates alternative therapeutic options. Host-targeted therapeutics present a promising anti-infective strategy against intracellular bacterial pathogens. A cell-based assay identified a compound that inhibits Salmonella proliferation in infected cells, 2-(3-hydroxypropyl)-1-(3-phenoxyphenyl)-1,2-dihydrochromeno[2,3-c]pyrrole-3,9-dione (KH-1), which is devoid of direct activity against Salmonella. The compound inhibits the growth of both antibiotic-sensitive and -resistant Salmonella strains inside macrophages and reduces lactate dehydrogenase (LDH) release from Salmonella-infected cells. Subsequent screening of KH-1 commercial analogs identified 2-(4-fluorobenzyl)-1-(3-phenoxyphenyl)-1,2-dihydrochromeno[2,3-c] pyrrole-3,9-dione (KH-1-2), which is more effective in controlling Salmonella growth inside macrophages. In vitro KH-1-2 treatment of Salmonella infection resulted in an 8- to 10-fold reduction in bacterial load in infected macrophages. In combination with suboptimal ciprofloxacin treatment, KH-1-2 further reduces Salmonella growth inside macrophages. The toxicity and efficacy of KH-1-2 in controlling Salmonella infection were examined in vivo using a mouse model of typhoid fever. No significant compound-related clinical signs and histological findings of the liver, spleen, or kidney were observed from uninfected mice that were intraperitoneally treated with KH-1-2. KH-1-2 significantly protected mice from a lethal dose of infection by an antibiotic-resistant Salmonella strain. Thus, our study provides support that this is a promising lead compound for the development of a novel host-targeted therapeutic agent to control typhoid fever. IMPORTANCE Salmonella spp. cause significant morbidity and mortality worldwide. Typhoidal spp. (e.g., S. Typhi) cause a systemic disease typically treated with antibiotics. However, growing antibiotic resistance is resulting in increased treatment failures. We screened a compound library for those that would reduce Salmonella-induced macrophage toxicity, identifying compound KH-1. KH-1 has no direct effects on the bacteria but limits Salmonella survival in macrophages and protects against lethal infection in a mouse model of typhoid fever. A suboptimal concentration of ciprofloxacin worked in conjunction with the compound to further decrease Salmonella survival in macrophages. An analog (KH-1-2) was identified that possessed increased activity in vitro in macrophages and in vivo against both antibiotic-sensitive and -resistant strains. Thus, we report the identification of a lead compound that may be a useful scaffold as a host-directed antimicrobial against typhoid fever.
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