AR-709

AR-709 is an experimental, synthetic, low-molecular-weight diaminopyrimidine antibiotic originally developed by Arpida AG. It functions as a bacterial dihydrofolate reductase (DHFR) inhibitor, which disrupts the synthesis of bacterial DNA, RNA, and proteins. The drug was specifically designed to target respiratory tract infections, demonstrating potent in vitro activity against *Streptococcus pneumoniae*, including multidrug-resistant and trimethoprim-sulfamethoxazole-resistant strains. Clinical development reached phase 0 microdose studies to evaluate its pharmacokinetic profile and penetration into lung compartments, but further development appears to have stalled following Arpida's financial difficulties and the failure of its lead candidate, iclaprim.