Histamine-Induced Conjunctivitis and Breakdown of Blood–Tear Barrier in Dogs: A Model for Ocular Pharmacology and Therapeutics

Abstract

Conjunctival inflammation disturbs the blood–tear barrier and thus affects the tear film stability and composition. We aimed to develop a non-invasive and reliable method to induce conjunctivitis in dogs, a large animal model for translational work on ocular surface disease in humans. Six beagle dogs underwent a randomized, vehicle-controlled, balanced crossover trial—on six separate days, one eye received topical artificial tears (vehicle), while the other eye received one of six concentrations of histamine solution (0.005–500 mg/ml). At sequential times after eyedrop administration, a conjunctivitis score was given to each eye based on the degree of palpebral and bulbar conjunctival hyperemia and chemosis, ocular pruritus, and discharge. Total protein content (TPC) and serum albumin were quantified in tear fluid at baseline and 20 min. Additionally, 13 dogs presenting for various ophthalmic diseases with associated conjunctivitis were examined. Experimentally induced conjunctivitis developed rapidly (<1 min) following topical histamine administration and lasted for 1–3 h (four lowest doses) to 6–8 h (two highest doses). The severity of conjunctivitis was dose-dependent. Histamine was overall well tolerated, although transient blepharitis, aqueous flare, and ocular hypertension occurred in a few dogs receiving histamine ≥375 mg/ml. TPC and serum albumin levels increased in tears of eyes receiving histamine ≥1.0 mg/ml, being significantly higher than vehicle and baseline in eyes receiving histamine ≥375 mg/ml. Lacrimal albumin levels were also increased in 13 dogs with naturally acquired conjunctivitis, up 2.7–14.9 fold compared to contralateral healthy eyes. Histamine-induced conjunctivitis represents a robust model for translational work on the ocular surface given the low cost, non-invasiveness, self-resolving nature, ability to adjust the duration and severity of the disease, and shared features with naturally occurring ocular diseases. Histamine solutions of 1, 10, and 375 mg/ml induce mild, moderate, and severe conjunctivitis in dogs, respectively. Leakage of serum albumin in tear fluid of eyes with conjunctivitis suggests a breakdown of the blood–tear barrier.

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

“…Two canine albumin ophthalmic solutions (0.4% and 1.5%) were formulated by mixing canine albumin lyophilized powder (Animal Blood Resource International, Stockbridge, MI) with lubricating eye drops (Artificial tears solution, Rugby, Rockville Center, NY, USA) in a sterile manner under a laminar flow hood. Of note, albumin concentrations selected herein (0.4% and 1.5%) aimed to achieve tear film albumin levels of~1 to 5 mg/ml (i.e., after dilution of the instilled drop with the canine tear film, accounting for~3-fold dilution; Sebbag et al, 2019b), thus representing a spectrum of albumin levels in tears of dogs with spontaneous or experimentally-induced conjunctivitis (Sebbag et al, 2019a). Artificial tears solution without albumin (vehicle only) was used as control for the experiment.…”

“…Such discrepancy is likely explained by two important differences in study designs. In the present experiment, the concentrations of albumin (0.4% and 1.5%) were specifically chosen based on clinically-relevant albumin levels detected in canine patients with diverse ocular diseases (Sebbag et al, 2019a), taking into account the threefold dilution effect from resident tears (Sebbag et al, 2019b) to better account for the true binding constant seen in dialysis experiments (Chrai and Robinson, 1976). In contrast, the concentrations of albumin used in rabbits were higher (1% and 3%) and may not reflect the range of biological concentrations of albumin at the ocular surface.…”

“…A recent study by Sebbag et al showed that canine eyes with diverse ocular diseases (e.g. corneal ulcer, uveitis, glaucoma) had lacrimal albumin levels that were up to 14.9-fold greater than contralateral healthy eyes (Sebbag et al, 2019a).…”

“…The impact of albumin binding on the drug's pharmacological activity is extensively studied in blood (Zhivkova, 2015), yet little is known about the physiology and function of albumin in tears or other biological fluids. In the present study, we examined the bioavailability of topically delivered drugs in the presence of clinically relevant levels of albumin in tears (Sebbag et al, 2019a). We hypothesized that the drugs' intraocular effect will be reduced by lacrimal albumin given the inability of protein-bound drugs to permeate through ocular tissue barriers.…”

“…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).…”