L-Thyroxine and L-thyroxine-based Antimicrobials Against Streptococcus Pneumoniae and Other Gram-positive Bacteria

Objectives: The rise of antibiotic-resistant () poses a significant global health threat, urging the quest for novel antimicrobial solutions. We have discovered that the human hormone l-thyroxine has antibacterial properties. In order to explore its drugability we perform here the characterization of a series of l-thyroxine analogues and describe the structural determinants influencing their antibacterial efficacy.

Method: We performed a high-throughput screening of a library of compounds approved for use in humans, complemented with ITC assays on purified -flavodoxin, to pinpoint molecules binding to this protein. Antimicrobial susceptibility assays of the hit compound (l-thyroxine) as well as of 13 l-thyroxine analogues were done against a panel of Gram-positive and Gram-negative bacteria. Toxicity of compounds on HepG2 cells was also assessed. A combined structure-activity and computational docking analysis was carried out to uncover functional groups crucial for the antimicrobial potency of these compounds.

Results: Human l-thyroxine binds to -flavodoxin, forming a 1:1 complex of low micromolar . While l-thyroxine specifically inhibited growth, some derivatives displayed activity against other Gram-positive bacteria like and , while remaining inactive against Gram-negative pathogens. Neither l-thyroxine nor some selected derivatives exhibited toxicity to HepG2 cells.

Conclusions: l-thyroxine derivatives targeting bacterial flavodoxins represent a new and promising class of antimicrobials.