# Contact Lenses as Drug Delivery System for Glaucoma: A Review
## 1 Introduction
### Pharmacological Treatment
problem of a patient's adherence to prescribed medication, improve the bioavailability of the drug, and reduce the adverse effects on the ocular surface. The ocular surface is the place of choice for the application of drugs that treat eye diseases [49]. The most common form of application of ophthalmic formulations are eye drops [49]. However, the main problem they present is the short residence time in the cornea, only 2-3 min [50], due to the dilutive effect of the tear film and its rapid clearance [51; 52] and that between 50% and 100% of the eye drop is drained through the nasolacrimal duct which can cause systemic alterations [53]. This means that the application of high doses is required for the drug to have its therapeutic effect. Another important concern is that topical eye drop therapy remains one of the toughest challenges in the management of glaucoma, as usually occurs in the treatment of all chronic relatively asymptomatic conditions. There are multiple reasons for reduced treatment adherence, including medical side effects, poor understanding of treatment aims, poor installation techniques due to dexterity issues, conditions affecting mental ability or physical barriers like arthritis and tremor, and cost [54]. In a study in which patients knew they were being monitored with an electronic device; they did not consistently take their drops in 45% of cases [55]. A major challenge when adding multiple drops is compliance. It has been demonstrated that increasing the number of drop bottles to a patient's treatment regimen negatively affects patient adherence [56]. To improve patient adherence and reduce exposure to preservatives, fixed combination therapies have been developed. Preservative-free eye drop formulations have helped patients with glaucoma, who frequently have concurrent dry eye and ocular surface disruption, as patient discomfort is reduced and their quality of life improves [57]. Within the sustained release systems, the contact lens has the essential characteristic of being placed on the ocular surface to compensate the user's anterropia during long periods of time throughout the day. This permits the contact lens to be the element that most favours the sustained release of the drug into the eye without the risk of compromising its optical function [49]. In addition to this, its hydrogel nature makes the contact lens a good candidate for drug reservoirs which are released over a longer duration from a polymeric network [49]. Drugs loaded into the contact lens diffuse into the post lens tear film in a constant and prolonged flow resulting in an increased retention of the drug on the surface of the cornea for up to 30 min [58] and providing a potential of 50% drug bioavailability [59]. Although a lot of work needs to be done in relation to the possible side effects of the residues of the remaining drug inside the contact lens, a study by [PERSON] et al. reported that the contact lenses appeared safe in cell culture and in rabbit eyes [60]. Contact lens eluted products did not demonstrate any evidence of toxicity either in the human corneal-limb epithelial cells over 24 h or in the New Zealand white rabbits. The rabbit eyes did not develop any signs of toxicity, such as conjunctival redness. Consequently, this sustained release system increases therapeutic efficacy, reduces the drug fluctuation on the corneal surface, reduces the amount of administered drug [59] and, in addition, reduces the need to add preservatives which may cause ocular irritation [61; 62]. In this review, the therapeutic benefits of contact lenses in the application of drugs for the treatment of glaucoma are analyzed.