Ocular pharmacokinetics in rabbits using a novel dual probe microdialysis technique

Ocular infections involve delicate internal structures of the eye that ofte n require treatment with antimicrobial agents. A major constraint to the st udy of ocular drug absorption from systemic administration is the inaccessi bility of the vitreous for continuous serial sampling. A novel dual probe m icrodialysis technique has been employed in our laboratory, which will enab le the delineation of complete ocular pharmacokinetics of a drug. New Zeala nd albino rabbits weighing 2-2.5 kg were used. The animals were kept under anesthesia throughout the experiment. A concentric probe was implanted in t he vitreous chamber about 3 mm below the corneal scleral limbus. Simultaneo usly a linear probe was implanted in the anterior chamber across the cornea . Intraocular pressure (IOP) was measured using Schiotz tonometer. The tota l protein concentrations in the aqueous and vitreous humors were determined using the Bio-Rad protein assay method. The aqueous and vitreous eliminati on kinetics of fluorescein were studied after intravitreal and systemic adm inistrations over a period of 10 hr. Microdialysis technique was also compa red to the conventional direct sampling technique by determining the intrav itreal kinetics of fluorescein. Results suggest that IOP reverted to normal within 2 hr after the implantation of the probes. The increase in the vitr eal total protein concentration was not significantly different from the ba seline. The increase in the aqueous total protein concentration was less th an five times the basal concentration throughout the experiment. The blood- aqueous and blood-retinal barrier integrity was delineated by determining t he permeability index for fluorescein and were found to be 9.48 +/- 4.25 % and 1.99 +/- 0.66 % for the anterior and vitreous chamber, respectively. Th e rate constant of penetration of fluorescein into the anterior chamber was found to be 8.48 +/- 1.33 x 10(-2) min(-1), which was significantly higher than into the vitreous i.e. 4.34 +/- 2.82 x 10(-2) min(-1) The terminal el imination rate constant of fluorescein from the anterior chamber (1.48 +/- 0.79 x 10(-2) min(-1)) was found to be similar to that of the plasma termin al elimination rate constant (1.57 +/- 0.25 x 10(-2) min(-1)), but signific antly higher than from the vitreous (3.0 +/- 0.7 x 10(-3) min(-1)). The ter minal vitreal elimination rate constant of fluorescein after intravitreal a dministration was found to be similar by both microdialysis (3.98 +/- 0.6 x 10(-3) min(-1)) and direct sampling (4.38 +/- 1.4 x 10(-3) min(-1)) techni ques. In case of direct sampling technique the area under the vitreous conc entration-time curve was higher compared to that obtained by the microdialy sis technique, There was no breakdown of the blood ocular barriers as shown by a very small change in the intraocular fluid protein concentrations. Th is was also confirmed by the fluorescein kinetics, which were in accordance with the previous studies. IOP data suggests that the intraocular fluid dy namics were not affected and the animals stabilized within 2 hr after the i mplantation of the probes. Fluorescein data suggests that the vitreous comp artment is surrounded by a tighter barrier compared to the anterior chamber . This technique appears to be more sensitive. reproducible and requires on ly one animal for the determination of entire ocular pharmacokinetic profil e. (C) 2001 Academic Press.