Increasing the residence time of an ophthalmic formulation on the corneal surface increases the drug bioavailability and therefore reduces frequency of administration. Although recent advances have been made in ocular drug delivery systems, eye drops are still the most commonly used formulations as they are the least expensive preparations, are easy to use, and do not interfere with vision. However, frequent administration is necessary.
The retention of a drug on the corneal surface is determined by the amount of tear flow and by the blink frequency, which can be stimulated by different factors. The most important factor influencing the retention of a drug on the corneal surface appears to be the properties of the drug itself. If a drug irritates the eye, it is difficult to obtain a long retention. If the drug is non-irritant, retention time can be increased by instillation of small drops, by adjustment of the osmolarity, tonicity, pH and by choosing the appropriate preservatives and adjuvants.
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