Non-invasive assessment of ocular pharmacokinetics using confocal Raman spectroscopy

A Laser Scanning Confocal Raman Spectroscopy (LSCRS) system was applied for the non-invasive quantification of the transport of a drug through the rabbit cornea in vivo. Employing LSCRS, the changes in the amplitude of a drug-specific Raman signal were assessed over time in the tearfilm and corneal epithelium of the living rabbit eye (n=6), after topical application of 25 μL·Trusopt 2%(TM). This allowed for quantification of pharmacokinetic variables. The effect of the drug on corneal hydration was also monitored. LSCRS demonstrated adequate sensitivity and reproducibility, for continuous real-time monitoring of the Trusopt concentration. Each concentration-time curve had a bi-phasic trend; the rapid initial phase (t<8 min.) corresponds to the nonproductive losses of Trusopt from the tears (k₁₀=0.24±0.04 min^{- 1}), and the slower later phase (t>20 min.) is the result of transfer of the drug from the corneal epithelium to the stroma (k₂₃=0.0047±0.0004 min^{- 1}). Drug absorption into the corneal epithelium occurred at a rate of k₁₂=0.034±0.006 min^-1. Trusopt caused an acute dehydrating effect, with a maximum decrease in corneal hydration of ~15% at ~60 min. following application of the drug. LSCRS has the specificity, sensitivity, reproducibility and spatial resolution for employment as a potentially valuable tool for the study of ocular pharmacokinetics.