Purpose: Although eye drops are the most common form of ocular drugs, they have several
limitations. Drug absorption into the eye is, in general, less than 5%, addition of preservatives
is often necessary, and many drugs cannot be formulated as eye drops. Formulating ocular
drugs as powder may solve these problems. The aim of this study was to investigate ocular
irritation in rabbits following powder administration.
Methods: Timolol maleate (TM) powder was administered to pigmented lop rabbits. Both
pure TM powder and freeze-dried with PVP-polymer (2.4% of mass) were tested in 1.0- and
0.1-mg doses. Additionally, 4 rabbits received 0.1 mg of the pure powder 3 times a day for 8
d. Redness of the bulbar conjunctiva and the amount of discharge was rated from photographs
(0–3 points, randomized and masked evaluation). The 8-d experiment additionally included
examination with a slit lamp and examination of hematoxylin-eosin stained sections of eyes
with light microscopy.
Results: No serious or irreversible signs of irritation were noted. Doses of 1.0 mg were more
irritating than 0.1-mg doses. There was no detectable difference in irritation between pure or
freeze-dried powder. Slit-lamp examination, surface photographs and histology showed a negligible
difference between drug and control eyes following the 8-d experiment.
Conclusions: The results suggest that 0.1 mg of timolol powder does not irritate the eye and
that testing topical timolol powder in humans is feasible.
