Photoactivatable diazafluorenone-based ruthenium compounds as a new strategy to combat microbial resistance

Every year, millions of people around the world die due to infections caused by a growing number of resistant bacteria, making urgent the development of new antimicrobial compounds. One promising approach is the use of metal complexes, including photoreactive ones based on ruthenium polypyridine derivatives. Here, we synthesized three new ruthenium(II) complexes with the formulation [Ru(DFO)(phtpy-R)Cl](PF₆), where phtpy = 4'-phenyl-2,2': 6',2″-terpyridine; R = -F(MPD3), -Cl(MPD4), -thiophene(MPD5), and DFO = 4,5-diazafluoren-9-one, expanding our previous investigation with R = H and CH₃. Here, we studied their chemical and biochemical activities, focusing on antibacterial assays. These compounds showed capacity to photogenerate reactive oxygen species (ROS) such as singlet oxygen (¹O₂) using blue light (463 nm) for MPD3, MPD4, and MPD5 that had a Φ_Δ of 0.51, 0.48 and 0.49, respectively. In addition to that, they also photogenerated superoxide and hydroxyl radicals. Their antibacterial activities (bacteriostatic and bactericidal) showed a significant increase upon blue light irradiation (>32 times) up to 3.9 μg mL^-1. Interestingly, the combination of MPD5 with ampicillin showed a synergistic effect against Gram-negative bacteria, while the combination of MPD4 or MPD5 with tetracycline showed a synergistic effect against all bacterial strains, including resistant ones. Besides this, these compounds were similarly photoactive even upon red light irradiation (631 nm). These compounds exhibited photoactivity against biofilm formation, where MPD3 showed the greatest biomass reduction for E. coli (ca. 71% at 7.8 μg mL^-1). Altogether, these data provide exciting evidence for the potential use of these compounds as antimicrobial photodynamic therapeutic agents, enriching the arsenal of metallodrugs.