Abstract
Background and objective
Antimicrobial photodynamic therapy (aPDT) relies on activation of a photosensitizer by light in the presence of oxygen, leading to reactive oxygen species formation. Here, a phenalenone derivative series was examined to find green-light-responsive compounds for photodynamic inactivation of methicillin-sensitive and methicillin-resistant Staphylococcus aureus (MSSA and MRSA).Methods
Twelve phenalenone derivatives were first compared for visible absorption, photostability, macroscopic appearance in the assay medium, and relative singlet-oxygen generation using ABDA bleaching. Based on this screen, compounds 4d and 4m were taken forward for microbiological testing. Bacterial association, viable counts, residual metabolic activity, and intracellular ROS-related fluorescence were assessed by confocal microscopy, CFU enumeration, resazurin reduction, and DCFDA fluorescence, respectively. All microbiological efficacy experiments were performed with planktonic bacterial suspensions; biofilm activity was not evaluated in the present study.Results
All derivatives absorbed within 512.5-571.0 nm and retained 0.977-0.989 of their initial absorbance after 3 h of green-light irradiation. The ABDA-derived apparent ΦΔ estimates were 0.25-0.60. Compound 4d had the highest apparent ΦΔ estimate, showed no visible precipitation at 16 μM under the working conditions, and gave the most pronounced antibacterial response under 530-540 nm irradiation. Compound 4m showed similar photophysical behavior but left higher residual survival under lower-dose conditions. For both compounds, irradiation increased ROS-associated fluorescence and reduced viable counts and resazurin signals in a concentration- and fluence-dependent manner.Conclusions
Within this series, 4d gave the most favorable profile, combining the highest ABDA-derived apparent ΦΔ estimate, absence of visible precipitation at the screening concentration, and strong green-light-triggered activity against both MSSA and MRSA. Because the ABDA result was not validated by an orthogonal singlet-oxygen method, it is interpreted as a comparative screening estimate rather than a definitive absolute ΦΔ measurement. The present visual screen does not establish molecular aggregation state; further evaluation should prioritize quantitative aggregation/solubility measurements, biofilm experiments, formulation development, and mammalian-cell compatibility. Mammalian-cell cytotoxicity was not measured in the present study, so a therapeutic selectivity index cannot yet be calculated.