The Effect of Sodium Iodide on Stromal Loading, Distribution and Degradation of Riboflavin in a Rabbit Model of Transepithelial Corneal Crosslinking

Purpose To evaluate effects of sodium iodide (NaI) on riboflavin concentration in corneal stroma before and during ultraviolet A (UVA) light exposure using a novel transepithelial corneal collagen crosslinking (CXL) procedure (EpiSmart CXL system, CXL Ophthalmics, Encinitas CA). Methods Riboflavin solutions with NaI (Ribostat, CXL Ophthalmics, Encinitas CA) and without NaI were used for CXL in rabbits using EpiSmart. A pilot study determined sufficient riboflavin loading time. Four rabbits were dosed and monitored. Riboflavin fluorescence intensity was assessed from masked slit-lamp photos. A 12 min loading time was selected. Sixteen additional rabbits received the two formulae in contralateral eyes for CXL. Riboflavin uptake was assessed at 0, 10, 15, 20, 25, and 30 min of UVA exposure using a scale for riboflavin fluorescence previously validated against stromal concentration. Post sacrifice, corneal stromal samples were analyzed for concentrations of riboflavin and riboflavin 5ʹ-phosphate. Results Eyes dosed with NaI riboflavin had higher riboflavin grades compared to eyes dosed with the NaI-free riboflavin formulation immediately after riboflavin loading and persisting throughout UVA exposure, with significantly higher (P < 0.01 to < 0.05) riboflavin grades from 15 through 25 min of UVA exposure. Riboflavin grades decreased more slowly in eyes dosed with NaI riboflavin through 25 minutes of UVA exposure. Minor conjunctival irritation was noted with or without NaI. Conclusion The addition of NaI to riboflavin solution is associated with increased riboflavin concentration in corneal stroma throughout a clinically relevant time course of UVA exposure. This effect may be a combination of enhanced epithelial penetration and reduced riboflavin photodegradation and should enhance intrastromal crosslinking.