Antimicrobial Photodynamic Therapy with the photosensitizer TONS504 eradicates Acanthamoeba

Microbial keratitis is a potential cause of corneal blindness. We investigated the amoebicidal efficacy of photodynamic antimicrobial therapy with a light-emitting diode as the light source and the cationic chlorin derivative TONS504 as the photosensitizer for the elimination of Acanthamoeba, a causative organism of corneal infection and blindness. Acanthamoeba keratitis remains a challenge to treat because of limited available treatments.

Acanthamoeba castellani 50370 was exposed to TONS504 at various concentrations (0, 1, or 10 mg/L for trophozoites; 0, 1, 10, or 20 mg/L for cysts), irradiated at various light energies (0, 10, or 30 J/cm² for trophozoites; 0, 30, or 60 J/cm² for cysts), and incubated at 26 °C for 3 h. Assessment of cell viability by trypan blue staining revealed that photodynamic antimicrobial therapy attenuated the survival of trophozoites and cysts dependent on TONS504 concentration and light energy.

Photodynamic antimicrobial therapy with 10 mg/L TONS504 and 30 J/cm² light energy suppressed trophozoite viability by 77%, and 20 mg/L TONS504 and 60 J/cm² light energy attenuated cyst survival by 42%. Staining with fluorescein isothiocyanate-conjugated annexin V and ethidium homodimer III revealed photodynamic antimicrobial therapy induced apoptosis and necrosis in trophozoites dependent upon the intensity of treatment, whereas apoptosis was the predominant form of cell death in cysts.

Photodynamic antimicrobial therapy with TONS504 warrants further investigation as a potential treatment modality for Acanthamoeba keratitis.