Photoactivated quercetin as an inhibitor of Porphyromonas gingivalis virulence factor Kgp and biofilms: Validation through in vitro assays, molecular docking, and ADMET profiling.

BACKGROUND: Porphyromonas gingivalis is a key pathogen in periodontitis, significantly contributing to biofilm formation and immune system evasion through virulence factors such as lysine-specific gingipain (Kgp). Kgp, a protease produced by P. gingivalis, plays a critical role in bacterial pathogenicity by facilitating biofilm development, evading host defenses, and acquiring nutrients through cleavage of host proteins at lysine residues. Traditional treatments, such as mechanical debridement (scaling and root planing), systemic or local antibiotics, and antiseptic mouthwashes, often fail to eliminate pathogens in deep periodontal pockets and may cause antimicrobial resistance and disruption of the microbiota. Photodynamic therapy (PDT) using natural photosensitizers like quercetin is emerging as a promising alternative by targeting the virulence factor Kgp to combat P. gingivalis.

MATERIALS AND METHODS: An integrated in silico and in vitro methodology was utilized. The in silico analysis included retrieval, modeling, and validation of the Kgp protein structure (PDB ID: 4RBM), followed by molecular docking with quercetin to evaluate its binding affinity. The ADMET properties of quercetin were assessed to determine its drug-likeness and safety profile. In vitro experiments involved determining the minimum biofilm inhibitory concentration (MBIC) of quercetin, the minimum biofilm inhibitory dose (MBID) of diode laser, and the combined efficacy of PDT. Lastly, gene expression of kgp was measured after treatment using quantitative real-time polymerase chain reaction (qRT-PCR).

RESULTS: In silico docking revealed a strong binding affinity between quercetin and Kgp (-7.5 kcal/mol). Quercetin exhibited drug-like characteristics and favorable pharmacokinetic properties. In vitro, quercetin inhibited P. gingivalis biofilms in a dose-dependent manner, with a MBIC of 1000 µg/mL. The diode laser alone significantly reduced biofilms after 8 min of exposure (206.4 J/cm²). PDT using sub-MBIC concentrations of quercetin (as low as 125 µg/mL) combined with sub-MBID light exposure (2 min, 51.6 J/cm²) markedly decreased kgp gene expression by 4.5-fold (P<0.05) without impacting biofilm biomass.

CONCLUSION: Quercetin-based PDT effectively inhibits P. gingivalis biofilms and downregulates a key virulence gene. Due to its natural origin, safety, and effectiveness, it holds promise as a targeted and biocompatible treatment option for managing periodontal disease. Further in vivo investigations are needed to confirm these findings.