Metabolism of fluorescein after intravenous administration

The measurement of plasma unbound (free) fluorescein is important in the study of blood-ocular barrier kinetics. The authors became concerned about the quantitative significance of the presumed glucuronide metabolite of fluorescein to the measurement of plasma fluorescence in diabetic and normal subjects. Fluorescein was given intravenously (14 mg/kg) to seven normal subjects and eight diabetic subjects. Plasma samples taken during 60 min were subjected to microfiltration, from which aliquots of ultrafiltrate were incubated with beta-glucuronidase. Samples were subjected to high-performance liquid chromatography, and fluorescence activity was measured in the eluent. All subjects showed an additional fluorescence peak to that of fluorescein in plasma and ultrafiltrate 5 min after fluorescein administration and increased thereafter. This additional peak was abolished by incubation of ultrafiltrate with beta-glucuronidase and resulted in a marked increase in fluorescence due to the liberation of fluorescein from its presumed glucuronide. There were no pharmacokinetic differences between normal and diabetic subjects in plasma-free fluorescein and fluorescein glucuronide pharmacokinetics or in their respective binding to plasma proteins. The glucuronide had only 4.5% of the fluorescence of fluorescein, but because more of the glucuronide was unbound (32%) compared with fluorescein (10%) and its concentration increased while that of fluorescein decreased, it constituted an increasing proportion of the fluorescence in the ultrafiltrate. At 60 min, 80% of the fluorescein was present as glucuronide and contributed 20% of the total fluorescence in the ultrafiltrate. Fluorescein-glucuronide is a potential source of variability in studies on blood-ocular barrier kinetics.