Kinetics of Fluorescein in Tear Film After Eye Drop Instillation in Beagle Dogs: Does Size Really Matter?

Abstract

The study aimed to determine the impact of drop size on tear film pharmacokinetics and assess important physiological parameters associated with ocular drug delivery in dogs. Two separate experiments were conducted in eight healthy Beagle dogs: (i) Instillation of one drop (35 µl) or two drops (70 µl) of 1% fluorescein solution in each eye followed by tear collections with capillary tubes from 0 to 180 min; (ii) Instillation of 10 to 100 µl of 0.1% fluorescein in each eye followed by external photography with blue excitation filter (to capture periocular spillage of fluorescein) and tear collections from 1 to 20 min (to capture tear turnover rate; TTR). Fluorescein concentrations were measured in tear samples with a fluorophotometer. The TTR was estimated based upon non-linear mixed-effects analysis of fluorescein decay curves. Tear film pharmacokinetics were not superior with instillation of two drops vs. one drop based on tear film concentrations, residual tear fluorescence, and area under the fluorescein-time curves (P ≥ 0.163). Reflex TTR varied from 20.2 to 30.5%/min and did not differ significantly (P = 0.935) among volumes instilled (10-100 µl). The volumetric capacity of the canine palpebral fissure (31.3 ± 8.9 µl) was positively correlated with the palpebral fissure length (P = 0.023). Excess solution was spilled over the periocular skin in a volume-dependent manner, predominantly in the lower eyelid, medial canthus and lateral canthus. In sum, a single drop is sufficient for topical administration in dogs. Any excess is lost predominantly by spillage over the periocular skin as well as accelerated nasolacrimal drainage.

Impact of acute conjunctivitis on ocular surface homeostasis in dogs

“…6 However, tear drainage (assessed with TTR) did not differ significantly in dogs with or without experimentally induced conjunctivitis (present study), nor did it change in a recent report of dogs receiving artificial tears at a volume of 10-100 µL. 27 Discrepancies between rabbits and dogs are likely due to notable differences in blinking rates (0.05-0.19 blinks/min vs 14.2 blinks/min, respectively), 29 an important physiologic response that promotes spillage of excessive tearing onto the periocular skin. 27,29 The quality of tears, or tear film stability, was reduced in canine eyes with a model of acute conjunctivitis.…”

“…Tear volume (TV) and tear turnover rate (TTR) were estimated based upon nonlinear mixed-effect analysis of fluorescein decay curves in tear samples (Monolix ® version 2018R2; Lixoft). 21,27,28 The biphasic decay of fluorescein allowed for calculation of reflex (first phase) and basal (second phase) values for both tear parameters. mild conjunctivitis vs severe conjunctivitis eyes were assessed with the Kruskal-Wallis test and post hoc Dunn's.…”

“…27 Discrepancies between rabbits and dogs are likely due to notable differences in blinking rates (0.05-0.19 blinks/min vs 14.2 blinks/min, respectively), 29 an important physiologic response that promotes spillage of excessive tearing onto the periocular skin. 27,29 The quality of tears, or tear film stability, was reduced in canine eyes with a model of acute conjunctivitis. Shortened TFBUT is an established feature of dogs, 11 cats, 30 and humans 9 with chronic conjunctivitis, as chronicity of inflammation reduces the density of goblet cells that secrete pre-ocular mucins, although this finding is not well documented in eyes with acute conjunctivitis.…”

Albumin Levels in Tear Film Modulate the Bioavailability of Medically-Relevant Topical Drugs

“…in the tear film) over a short duration. From a physiological standpoint, the latter method is more appropriate as the interaction time of a topical drug to albumin in tears is generally short, limited by the rapid tear turnover rate that occurs following eyedrop administration in dogs (Sebbag et al, 2019c) or other species (Shell, 1982;Wilson, 1999). Of note, tonometry was not performed in dogs receiving tropicamide (with or without albumin) in the present study given the lack of effect of tropicamide on IOP in healthy canine eyes (Jugant et al, 2019), but this parameter could be considered in future studies as IOP can vary from tropicamide in dogs receiving sedation (Jugant et al, 2019) or dogs with glaucomatous eyes.…”

“…Drug binding to proteins in tear fluid could partly explain the aforementioned variability in pupil size, a phenomenon our group investigated in the present study to help guide future diagnostic and therapeutic applications in ophthalmology. The present work was conducted in dogs, a species that represents an excellent large animal model for translational research in humans given similarities in ocular anatomy (Vézina, 2013) and physiologic parameters pertinent to topical route of drug administration (Sebbag et al, 2019b, Sebbag et al, 2019c, as well as spontaneous disease development such as glaucoma (Grozdanic et al, 2010), dry eye (Kaswan et al, 1989) and conjunctivitis (Sebbag et al, 2019a).…”