# Contact Lenses as Drug Delivery System for Glaucoma: A Review
## 4 Conclusions  
Using contact lenses as a drug delivery mechanism is a promising system to treat some ocular pathologies like glaucoma [148]. There are some strong points to attest to that, but there are also some weak points to take into account for the future development of this type of drug delivery methods. Using contact lenses for antiglaucomatous, drug delivery increases the bioavailability due to it being able to maintain the drug for a longer time on the ocular surface, thereby enhancing the corneal epithelium passing and thus ensuring more drug concentration on the target point [50; 51; 83]. Moreover, the active agent penetration to the anterior chamber in topical instillation needs to raise the corneal epithelium permeability using various strategies, adding preservatives or other pharmacological compounds to the formula. The use of this compound increases the toxicity, and in the case of contact lenses, the symptoms would be higher due to the toxic compound remaining longer on the ocular surface. It has recently been reported that the effect of the diadenosina tetraphosphate (Ap\({}_{4}\)A), present in the tear film [159], allows the entrance of antiglaucoma agents as a consequence of a decrease in the tight junction barrier function on stratified human corneal epithelial cells [160], as this compound is non-toxic and able to be released from contact lenses [84]. This release of kinetics could probably be the strongest feature of this system. It is important to keep the contact lens transparency when it is impregnated with drugs for treatment. The combination of contact lens with the treatment compound not only permits the treatment of the disease but it may also correct the patient's anteropias [161; 100]. From a clinical point of view, drugs must present similar properties in terms of transparency and the index of refraction to commercial hydrogels or, otherwise, these contact lenses could show a certain degree of opacification affecting the visual quality of patients. This fact is also valid for other methods of loading drugs into contact lenses. Regarding the weak points, contact lenses are a potential trigger of discomfort and dryness at the end of the day. This event has been called \"contact lens discomfort\" and could be the main reason for discontinuing the contact lens wear and leading to the change in treatment to another drug delivery system [162]. Another concern is the handling of the contact lenses. Glaucoma is more prevalent in the elderly [163] and the topical treatment, with high dosing frequency, could be a concern factor leading to an increase in noncompliance and therefore, to the failure of the treatment. Contact lens drug delivery treatment would reduce this risk of dropout, however the contact lens manipulation could be more compromised than drug instillation, provoking discomfort during lens wearing, increasing the infection risk or finally, not wearing the contact lens, leading to a higher risk of treatment failure [80]. It seems evident that contact lens drug delivery is not the indicated option for all patients, especially for patients who only need to instill one drop a day. Consequently, a previous selection should be made. Finally, another concern is the contact lens regulation as a medical device [78]. There are some doubts about whether it should be regulated as a medical device, a drug, or a combination of both. The research in this field is more advanced than regulation, demonstrating a big gap in the regulations with regards to clarifying this emerging technology. Nowadays, only the soaking method, an off-label use of contact lens and drug combination is allowed. However, the drug release with this system only lasts for a short time, which is a limitation. Other methods, such as molecular imprinting, are able to maintain the supply of the drug for many days and so would be better for patients in terms of efficacy. In summary, glaucoma patients could benefit from the emerging contact lenses drug delivery systems as treatment.