[Ocular drug absorption by topical route. Role of conjunctiva].

EDITORIAL The penetration of drugs in the eye after topical administration is one of the most interesting challenges the pharmaceutical science is facing. The objective of the scientist is to penetrate the protective barriers of the eye without damaging ocular structures. In contrast with other epithelia, the epithelium of the ocular surface is not prepared for absorption which means that the time of permanence of a conventional formulation is under two minutes. To this we must add the losses generated by the systemic absorption of the drug through the conjunctival and the lachrymal conduit. It is estimated that the percentage of active substance reaching the anterior segment of the eye is in the range of 1-5% of the total dosage. For this reason, even though the drug is able to reach the aqueous humor, its dissemination to the posterior segment is insignificant in most cases. When drugs are administered topically in the eye, the pathway of penetration is through the cornea (transcorneal pathway) or the conjunctiva and the sclera (conjunctival/scleral pathway). It is known that most of the active substances for topical absorption are absorbed through the transcorneal pathway. This explains why the conjunctival was considered to be purely an elimination pathway. At present we know that there are substances which can penetrate the conjunctival and reach the posterior segment on some occasions (1). In general, the corneal epithelium allows the passage of small-sized water repellent active substances through the epithelial cells by way of diffusion (trans-cellular pathway) and of hydrophilic substances between the spaces thereof in what has become known as the para-cellular pathway (2). The conjunctival is a mucous membrane which covers approximately 8% of the ocular surface. It is made up of a bulbar portion which covers the anterior part of the globe (except the cornea) and two palpebral portions in the posterior faces of the upper and lower eyelids. The conjunctival epitheli-um comprises a number of stratified epithelial cells (between 5 and 15 layers) and is covered by microvellosities. The bulbar conjunctiva is covered by the lachrymal film and contributes to the formation thereof by the secretion of electrolytes, mucus and glycoproteins. As with the cornea, the passage through the con-junctiva can be through the trans-cellular or para-cellular pathway (3). If we focus on the general properties of the active substances, the molecular size is the limiting factor in the para-cellular pathway , both for the …