Long term follow-up of stiffening of rabbit corneas by WST11 using near infrared light

Abstract

Purpose: To evaluate the efficacy and safety of photochemical corneal stiffening by Pd- bacteriochlorin 13’-(2-sulfoethyl)-amide dipotassium salt (WST11) and near infrared (NIR) illumination, using rabbit eye models.

Methods: Corneas of living rabbits were pretreated topically with 2.5 mg/ml WST11 in saline or in 20% dextran T-500 for 20 minutes, and illuminated with a NIR diode laser (755 nm, 10 mW/cm2) for 30 minutes. Corneas of untreated fellow eyes served as controls. The rabbits were sacrificed after one month and 4 months. Corneal strips of 4 mm width were excised. Ultimate stress and Young’s modulus measurements were performed using a biomechanical tester. Comparative studies were performed with standard riboflavin/ultraviolet-A light (UVA) treatment. Histology, electron spin resonance, fluorescence microscopy and optical coherence tomography (OCT) were used to evaluate treatment effects.

Results: WST11/NIR treatment significantly increased corneal stiffness following treatment, compared to untreated contralateral eyes. After 1 month the ultimate stress and Young’s modulus of treated corneas increased by 113%, and 80% respectively. WST11 in 20% dextran T-500/NIR had similar stiffening results, but markedly reduced post-treatment edema and time of epithelial healing compared to WST11 without dextran. After 4 months, treated corneas maintained biomechanical stiffness values measured at 1 month post treatment. OCT demonstrated a demarcation line in the anterior third of the corneal stroma. Histology showed a reduction in the keratocyte population in the anterior half of the corneal stroma, without damage to the endothelium. Electron spin resonance demonstrated that WST11/NIR generates hydroxyl and superoxide radicals but no singlet oxygen in the cornea. These results, in particular when combined with additional optical spectroscopy techniques, suggest a novel mechanism for corneal stiffening that has not been so far anticipated in connection with current stiffening approaches.

Conclusions: Treatment of rabbit corneas, with the bacteriochlorophyll derivative WST11 and NIR illumination, increased their biomechanical strength through mechanism that does not involve singlet oxygen. WST11 in 20% dextran T-500/NIR treatment showed less adverse effects. This treatment may have a potential for clinical use in keratoconus and corneal ectasia.

Keywords: 484 cornea: stroma and keratocytes • 574 keratoconus • 647 photodynamic therapy