| Drug delivery to the posterior segment of the eye is one of the most challenging tasks. Due to physiological constraints, topical and systemic routes are usually not satisfactory for the delivery of a therapeutic agent to the back of the eye. So far, the most promising method is intravitreal injection but this method is too invasive to be used routinely.; Subconjunctival injection is proposed as a method/location for the delivery of therapeutic agents to the back of the eye. It is hypothesized that a properly designed delivery system can achieve therapeutic concentrations in the posterior segment of the eye for an extended period of time through this route. The utilization of pharmacokinetic models is useful in characterizing drug delivery systems. In the current study, the contribution of each penetration pathway for a subconjunctivally injected compound to enter the vitreous chamber was studied and it was found that direct penetration is the only significant pathway for a subconjunctivally injected compound to enter the vitreous chamber. With this piece of information, and some data from the literature, a simple but realistic pharmacokinetic model for subconjunctival injection was developed and validated. Based on this model, the influence of various factors on the drug level in the posterior segment of the eye were studied using computer simulation with an aim to designing a drug delivery system for subconjunctival injection. It was found that a properly designed delivery system for subconjunctival injection should: (1) provide continuous drug release for an extended period of time, (2) slow down drug loss from the subconjunctival space and (3) preferable have solubilizing capacity.; Finally, Regel® was evaluated in vitro and in vivo since it has a number of unique features and it is believed that these features make it attractive as a potential system for subconjunctival injection. Although Regel® has generated some encouraging results, we still do not have sufficient evidence to accept the hypothesis that Regel ® has the requisite properties to be a properly designed drug delivery system for subconjunctival injection. Further study has to be carried out to test this hypothesis. |
