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Evaluating ophthalmic ointment bases for cats

Eördögh R, Schwendenwein I, et al. Clinical effect of four different ointment bases on healthy cat eyes. Vet Ophthalmol 2016;19 (Suppl 1):4-12.

Much of ophthalmologic drug therapy involves topical medications.  Topical medications, either drops or ointments, allow treatment of  target tissues without significant systemic absorption.  For many feline patients, frequent application of topical ophthalmic therapy is not well accepted, so ointments, which tend to have longer contact time than drops, are often easier for owners to use.  On the other hand, many clinicians in private and referral practice have noted mucosal irritation with extended use of ophthalmic ointments, and find that long-term use of these products is poorly tolerated in cats.

This study of ten healthy cats was designed to evaluate the effect of various ophthalmic ointment bases in felines.  Ophthalmic ointment bases (OBs) comprise two types:  (1) a simple base, which provides a continuous ointment phase; these are usually composed of white petrolatumand liquid paraffin;  or (2) a compound (or biphasic) base, which is an emulsion of oil and water.  The liquid paraffin in the simple base is responsible for the melting of the ointment upon reaching conjunctival temperature.  The compound/biphasic bases can be divided into two subtypes:  oil in water (o/w), which is oil dispersed in water; and water in oil (w/o), wherein oil forms the continuous phase.  The w/o biphasic base is the most commonly used and is less irritating than the o/w type, but there is a higher risk of bacterial contaminaton with w/o bases.  The emulsion-type (compound) bases have better mixing properties with tear film than simple bases, which mix with tear fluid very slowly, resulting in longer contact time than with compound bases.

Cats enrolled in the study were 1.5 to 2.5 years old, healthy, and free of ocular disease.  There were 9 domestic shorthairs and one Ragdoll.  The trial of OBs involved two 28-day periods, and four different OBs were evaluated.  The OBs were randomly assigned to the cats.  In the first trial period the OB was given in the right eye, and the left eye was used as an negative/untreated control.  In the second trial period, the left eye was treated and the right eye was the negative/untreated control. The two study periods were separated by a four-month washout period.  All animals received a complete ophthalmic examination seven days prior to and immediately after each treatment period.  All eyes tested were negative for the feline herpesvirus-1 (FHV-1) and Chlamydophila felis before and after the challenge.

Four different ointment bases were tested in the subjects.  Ointment base A, the only simple base tested in the study, contained 35.17% liquid paraffin and 64.83% white petrolatum.  Ointment base B had 10.03% liquid paraffin, 84.95% white petrolatum, and 5.02% lanolin.   Ointment base C contained 18.34% liquid paraffin, 51.40% white petrolatum, 0.025% KH2PO4, 0.057%K2HPO4, 18.90% eucerinum anhydricum (a mix of mostly white petrolatum with small amounts of wool wax alcohols and cetylstearyl alcohol), and 11.28% water for injection. Ointment base D contained 20% liquid paraffin, 70% unguentum lanalcoli (a mix of mostly white petrolatum with moderate amounts of wool wax alcohols and cetystearyl alcohol), and 10% aqua conservans (water for injection with a trace amount of benzalkonium chloride as a preservative).  The lanolin, eucerinum anhydricum, unguentum lanacoli, and cetystearyl alcohol in compound bases are included to enhance water absorption.

Each of the animals received one of the OBs three times daily at specific times for 28 days. An ophthalmic examination was performed on every cat every morning of the treatment period by the same examiner prior to the first daily treatment, and a clinical score was assigned following assessment of ocular irritation signs.

With the use of all OBs, there were substantially significant differences between the clinical irritation scores of treated eyes and control eyes.  By day 10,  differences in clinical irritation scores between eyes treated with OB-D and control eyes were constantly significant; with the other bases, constantly significant differences in irritation scores between treated eyes and control eyes were apparent by day 13, day 15, and day 22 with OB-A, OB-B, and OB-C, respectively. Ointment base A, the only simple base evaluated, was found to be associated with significantly higher (worse) clinical ocular irritation scores than other bases.  Eyes treated with OB-B had the lowest (least irritated) clinical score, but the results were not statistically different from OB-C or OB-D.  There was also no significant difference in the mean clinical irritation score of OB-A compared to OB-D.  Essentially, all bases tested markedly irritated the treated eyes.

Although this study involved a small group size employing only a few types of ointment bases, it confirms the clinical impression of many practitioners that ophthalmic ointments are irritating to feline eyes and poorly tolerated when used long-term.  The animals in this study had healthy eyes and still experienced severe local reactions, so ophthalmic ointment preparations should be used with even more caution in diseased eyes. [PJS]

See also:
Baranowski P, Karolewicz B, et al. Ophthalmic drug dosage forms:  characterisation and research methods.  Scientific World Journal, 2014 Mar 18; 1-14.