Tolfenpyrad\nDerivatives Exhibit Potent <i>Francisella</i>-Specific Antibacterial\nActivity without Toxicity to Mammalian Cells <i>In Vitro</i>

Tularemia is a deadly disease caused by Francisella\ntularensis, an emerging intracellular bacterial pathogen\nthat can be disseminated rapidly through aerosols and vector-borne\ntransmission. Recent surveillance data demonstrate an increasing incidence\nin several countries. Although clinical isolates of <i>Francisella</i> strains are sensitive to currently used antibiotics, engineered\nor horizontal acquisition of antibiotic resistance is a constant threat\nto public health. Therefore, the identification of antibiotics that\ntarget previously undrugged pathways is required to safeguard human\nhealth. An environmental pesticide that is registered for use in multiple\ncountries, tolfenpyrad, shows promising activity to block <i>Francisella</i> growth; however, it is not a suitable antimicrobial\ncandidate for use <i>in vivo</i> due to potential toxicity\nin humans and other animals. In this study, we applied a structure–activity\nrelationship approach to tolfenpyrad to generate compounds with improved\nantibacterial activity and reduced toxicity. Through screening of\na library of derivatives, we identified analogs with improved therapeutic\nwindows compared with tolfenpyrad. Although structural diversity exists\namong these analogs, they inhibit the growth of <i>Francisella</i> species but not other Gram-negative or Gram-positive species. These\ncompounds block intramacrophage growth of F. novicida and pathogenesis in an <i>in vivo</i> arthropod model\nof infection. Although the biochemical activity of these drugs is\nunknown, they appear to target the same pathway as the parent molecule\nbecause F. novicida mutants that are\nresistant to tolfenpyrad are also resistant to its analogs. Taken\ntogether, these findings suggest that these tolfenpyrad-derived compounds\ncomprise a new class of <i>Francisella</i>-targeted antimicrobials\nand merit further evaluation and development.