Dry Eye

Some estimates place the prevalence of chronic dry eye disease at approximately 5 million Americans ≥50 years of age.  Other sources indicate that the occurrence is much higher, around 25 million Americans experiencing symptoms of dry eye.  The rise in the incidence of dry eye disease is confounded by the finding that only about 50% of patients can be successfully controlled.  Therefore, it appears that the testing of new drugs to deal with the prevalence and the disparate causes of dry eye disease will only increase in importance.

Drs. Daniel M. Albert and Arthur S. Polans have developed an Atropine Model of Dry Eye in Rabbits.  Dry eye symptoms can be induced by the topical application of atropine to block sympathetic innervations of the lacrimal gland.  The atropine model in rabbits is relatively simple to initiate, it allows for rapid drug testing and is amendable to parallel pharmacokinetic studies.  Rabbits have relatively large ocular surfaces, live for long periods, and have evidence of both decreased tear secretion and ocular surface damage upon atropine treatment.  The condition in rabbits is reminiscent of the human disease and can be measured with techniques similar to those used in the clinic.

Male New Zealand white rabbits, 2.5-3.0 kg, were housed in standard cages in a light-controlled room, 220C, 50% humidity for 1 week prior to treatment.  There were no restrictions to food or water.  During the pre-treatment period, tear flow and vital dye staining were measured at least twice.  Rabbits were divided into four groups.  Following pre-treatment, eyes in all four groups were treated each day with 50 µl of 1% atropine (Bausch & Lomb atropine sulfate ophthalmic solution) at 9am, 1pm, 5pm and 9-10pm in the lower conjunctival sac.  Five minutes after each atropine addition, 50 µl of test compounds (Refresh Tears, Allergan, Inc. and Systane Balance Restorative Formula, Alcon Laboratories) were added to the eyes in two groups of rabbits.  Similarly, saline was added five minutes after atropine treatment to the eyes of one group of rabbits to control for the effects of dilution, and one group of rabbits had no additions following atropine treatment.  Animals were treated in this manner for 6 days.  At 11am each day, tear flow was determined using a Phenol Red Thread (PRT) test.  Tear flow was measured for 10 seconds following the insertion of the thread at the ventral conjunctival fornix.  At the end of the experiment, fluorescein staining was again observed with a slit-lamp biomicroscope.  Dotted staining and larger regions of staining were indicative of dry eye and scored by two independent observers as +1 (very slight fluorescein staining without any obstruction of the underlying structures; the papillary margin could be observed without associated fluorescein staining); +2 (mild fluorescent staining with the underlying structures visible, although with some loss of detail); or +3 (moderate fluorescein staining whereby the underlying structures were difficult to observe).  More extended periods of atropine treatment were required to achieve severe staining (+4).

The results indicated that atropine reduced tear flow in rabbits by the second day of treatment.  The maximum reduction in tear flow was achieved by day 4 and remained stable through day 6.  Saline drops instilled 5 minutes after each atropine treatment had no effect on tear flow compared to atropine treatment alone.  Tear flow returned to normal once atropine treatment was stopped.  This usually took 2-4 days, thus allowing the same rabbits to be used for additional testing.  OTC medications used to treat dry eye ameliorated the effects of atropine on tear flow.  Refresh Tears and Systane Balance, two major medications on the market, modulated tear flow, thus demonstrating that the rabbit model can be used to quickly test the efficacy of new, potentially therapeutic dry eye compounds.  Fluorescein staining of eyes treated either with atropine alone or with atropine followed by saline yielded +2 or greater staining in 100% of the treated eyes.  In contrast, those eyes treated with Refresh and Systane Balance following atropine, showed little if any staining with fluorescein (0 to +1).  Only 50% of the eyes had any sign of fluorescence.

Drs. Albert and Polans also measured the osmolarity of tears obtained from human subjects using the TearLab system (OcuSense, Chatsworth, CA) to verify the accuracy of the equipment and its ease of use.  They now intend to test the TearLab unit with atropine-treated rabbits since the model is more robust and reproducible.  Changes in tear osmolarity are highly quantitative and becoming an accepted measure of dry eye that can be adapted to drug testing.