All eye drops have the potential to become contaminated by microbes that could then lead to eye infections. Therefore, historically, a key component of all forms of eye drops have been preservatives to resist, or at least delay, this potential.
The two main approaches to preservatives have been in the form of detergents and oxidants. The two most widely used detergents used are benzalkonium chloride (BAK) and polyquaternium-1 (Polyquad).
BAK makes cell membranes more fragile, inducing inflammation, oxidative stress and programmed cell death (apoptosis). This is good if the cell is a harmful micro-organism, however, not so good is the fact that it also has this effect on conjunctival and corneal epithelial cells. (3) Even in very low concentrations, BAK has been shown to be toxic to the ocular surface, including causing loss of conjunctival goblet cells. (4)
Polyquad, also a detergent, makes cell membranes more fragile and has more of an attraction to bacterial cells and a repulsion to epithelial cells. These are good characteristics, however, Polyquad has also been shown to cause superficial epithelial damage, mainly by decreasing conjunctival goblet cell numbers. (5) The sensitive conjunctival goblet cells, responsible for mucin production, are not only upset by inflammation – but they are not too keen on preservatives either. An alternative approach to the detergents are oxidative or ‘vanishing’ preservatives, two of which are a stabilised oxychloro complex and sodium perborate.