Principles of retinal drug delivery from within the vitreous

In recent years, vitreous humour, a connective tissue at the centre of the eye, emerged as a preferred reservoir for back of the eye drug delivery. Although vitreous humour is largely composed of water (>99%), its physical form can range from a firm gel in the youth to a collapsed gel in the elderly. These changes in the physical form of the vitreous, in conjunction with changes in its composition and turnover, can potentially influence drug delivery to target tissues from the vitreous. In order to enable the reader with the development of personalised medicines for the back of the eye, this chapter discusses vitreal anatomy, convective flow patterns, barriers to drug delivery, drug clearance mechanisms, and the influence of vitrectomy and vitreous substitutes on drug delivery. Further, it presents case studies on interactions of drug delivery systems with vitreous gel as well as the influence of eye movements on drug delivery from the vitreous. Wherever feasible, the above parameters were compared between normal and ageing eyes.