Pharmacokinetics of Sirolimus Eye Drops Following Topical Ocular Administration in Rabbits.

Purpose: To evaluate the pharmacokinetics of sirolimus eye drops following topical instillation in rabbits. Methods: The study included 2 experiments. In single-dose pharmacokinetic study, rabbits received a single bilateral instillation of 0.05% sirolimus eye drops (0.5 mg/mL, 50 μL/eye). In repeat-dose pharmacokinetic study, 0.05% sirolimus eye drops (0.5 mg/mL, 50 μL/eye/time) were instilled into both eyes of rabbits four times a day for 6 consecutive days and one time on day 7. Whole blood, tears, aqueous humor, cornea, and conjunctiva samples were collected. Sirolimus concentration was determined by a validated liquid chromatography-tandem mass spectrometry. Results: Sirolimus was hardly detected in plasma or aqueous humor after either single or repeated dosing. The C_max of sirolimus in tears, cornea, and conjunctiva after a single instillation was 163.34 ± 69.30 μg/g, 150.56 ± 84.98 ng/g, and 113.22 ± 49.82 ng/g, respectively. As the number of instillation elevated, the C_max of sirolimus was increased to 486.18 ± 297.93 μg/g, 418.63 ± 41.07 ng/g, and 314.25 ± 63.74 ng/g, respectively. In repeat-dose administration, the steady state of sirolimus concentration was achieved on the third day. Ocular exposure to sirolimus after single and repeated dosing, based on AUC_0-t, was highest in tears, followed by cornea and conjunctiva. Compared with single administration, a significant increase in sirolimus exposure as measured by AUC_0-t was observed in tears, cornea, and conjunctiva following repeated administration. Conclusions: Topical administration of sirolimus eye drops results in extensive distribution of sirolimus in tears, cornea, and conjunctiva, while aqueous humor and systemic exposure were negligible. Repeat-dose administration increases sirolimus exposure in tears, cornea, and conjunctiva.