The field of drug delivery come about due to the inability of current methods for drug provision such as skin patch or oral medication to target specific groups of cells or tissues deep in the body; thereby, resulting in significant side effects on other cell types (most of them healthy). Additionally, given the myriad barriers in the human body such as the blood brain barrier and inability of the gut to effectively absorb drugs across the mucous membrane, having a drug delivery vehicle capable of crossing these barriers to deliver medicine to target cells could potentially provide a quantum leap in treatment outcome.
Drug delivery seeks to solve this problem of poor specificity by targeting certain groups of cells and delivering specific drugs to the location at which the target cells reside. Doing so requires the drug delivery vehicle to have (i) recognition properties for the specific groups of cells, (ii) the ability to get to a particular locale in the body (i.e., liver), (iii) realise that it had arrived at the target cells, and (iv) initiate a mechanism to release the drug at a controlled rate (also known as drug release profile, or controlled release). The goal of the field is to tune the release profile of drugs to the needs of the treatment plan – the most common of which is a progressive surface erosion of the drug cargo for maintaining a desired local drug concentration at the target cells over a period of time.
Currently, two main types of drug delivery options involve: either (i) systemic circulation through the bloodstream of drug vehicle followed by their targeted localization at specific body sites for delivering drugs, or (ii) embedding the drug delivery carrier into a specific body site followed by tuning the drug elution rate to be in sync with maintaining a target drug concentration at the cell cluster level. The second approach is most commonly used in cases where a barrier such as the cornea or blood brain barrier need to be crossed for targeted drug delivery to cells, and the drug elution rate is usually tuned through designing the surface erosion characteristics of the system. On occasions, if a large concentration of drug is needed such as in certain forms of chemotherapy, burst release of drug is used where a mechanism induced the degradation of the drug carrier followed by massive effusion of drug to surrounding cells.
Many diseases can benefit from enhanced drug delivery options that help maintain a specified concentration of drug at a morbidity site in the body, the most important of which is cancer chemotherapy where the goal is to attack the cancer cells while reducing or preventing harm to healthy cells – the latter a key source of side effects of chemotherapy. Another disease that could potentially benefit from an embedded drug delivery option is glaucoma where the cornea hinders the efficient diffusion of drug into the inner regions of the eye.
Glaucoma is an eye disease characterized by elevated pressure in the eye which progressively results in damage to the optic nerve, leading to gradual loss of vision, starting from the central vision area. It cannot be effectively treated as vision loss is irreversible, but its progression can be controlled through medication delivered via eye drops. Usually, glaucoma requires lifelong treatment. Eye drops is a special type of drug delivery method where the drug needs to diffuse across the cornea and into the eye. Depending on the ability of the drug to cross the cornea barrier, the efficiency at which the drug is able to reach the inner regions of the eye and the drug concentration that can be maintained varied, both of which impact on treatment outcomes.
Despite the availability of eye drop based medication for controlling the disease, treatment challenges encountered in glaucoma care include the non adherence to a treatment regime where the patient frequently forgets to apply prescribed eye drops, or severe sensitivity to the medicine. A recently reported approach aimed at solving the above two problems (Today, http://www.todayonline.com/singapore/new-treatments-glaucoma-works) demonstrated the use of a nanoparticle based drug delivery device embedded via minimally invasive microsurgery technique into the eye for continuous controlled delivery of medication for treating gluacoma.
While the approach solves the administrative problem of ensuring patients follow their prescribed treatment regime and the difficulty of delivering drugs to the eye through eye drops, the need to replace the embedded drug delivery capsule every four to five months meant that significant morbidity would be inflicted on the patient despite the minimally invasive nature of the microsurgery implantation technique. Hence, the proposed treatment strategy of implanting a drug delivery device for helping patients follow on with treatment, and increasing the efficiency of drug delivery to hard to reach regions of the eye may cause more harm to the patient as the eye is a delicate organ. Future research into glaucoma treatment may look into ways for enhancing patient education on the importance of following through with glaucoma treatment, and developing combination eye drops with less side effects (such as irritation) on the eye.