Posterior segment

The eye is separated into two segments by the lens the anterior segment and the posterior segment (Figs. 58-1 and 58-2) The anterior segment of the eye is separated by the iris... 

The posterior segment of the eye contains vitreous humor (a clear jelly-like substance), the retina, the retinal vasculature, and the optic nerve head. The retina transforms light energy... 

The conventional concentration of benzalkonium chloride in eyedrops is 0.01%, with a range of 0.004-0.02% [111]. While uptake of benzalkonium chloride itself into ocular tissues is limited [113], even lower concentrations of benzalkonium chloride have been reported to enhance corneal penetration of other compounds including therapeutic agents [93,112,114]. The differential effect of this preservative on the cornea compared to the conjunctiva can be exploited to target a drug for corneal absorption and delivery to the posterior segment of the eye [115]. Its use has been proposed as a means of delivering systemic doses by an ocular route of administration [116]. 

Topical application is the most common route of administration for ophthalmic drugs. Advantages include convenience, simplicity, noninvasive nature, and the ability of the patient to self-administer. Because of blood and aqueous losses of drug, topical medications typically do not penetrate in useful concentrations to posterior ocular structures and therefore are of no therapeutic benefit for diseases of the retina, optic nerve, and other posterior segment structures. 

Nevertheless, very limited research has been carried out to develop DDS for the posterior segment of the eye in the last decades. Whereas new antiangiogenic compounds and neurotrophic factors can open new therapeutic avenues in the treatment of AMD, retinal dystrophies and ocular manifestations of diabetes, the frequent intraocular injections of these compounds remain a problem and are associated with complications and discomfort for the patient. Thus, the need for efficient noninvasive DDS for the administration of drugs to the back of the eye is awaiting a ground-breaking multidiscipline advancement. 

The ciliary body is a triangular structure at the periphery of the border between the anterior and the posterior segments of the eye (Figure 24.1). Its uveal portion consists of comparatively... 

Maurice, D.M. 2002. Drug delivery to the posterior segment from drops. Surv Ophthalmol 47(Suppl 1) S41. 

Maurice, D.M. 1986. Iontophoresis of fluorescein into the posterior segment of the rabbit eye. Ophthalmology 93 128. 

FIGURE 26.11 Illustration of drug distribution to posterior segments of the eye after transscleral iontophoresis. (Adapted from Hastings, M.S. et al., Drug Deliv. Tech., 4, 53, 2004. With permission.)... 

Geroski, D.H., and H.F. Edelhauser. 2001. Transscleral drug delivery for posterior segment disease. Adv Drug Deliv Rev 52 37. 

Structure of the eye. The eye can be divided into two compartments the anterior and posterior segments. An internal cross section of an eye is shown in Fig. 2.5. 

Posterior segment. Externally, the posterior segment consists of the optic nerve and associated vasculature, and internally, it consists of the lens, vitreous, and rear ocular tissues.64 Vitreous is a colorless medium... 

To explore the utility of this approach, plasma samples obtained after injection of TACA into the posterior segment of the eye in pigs were analyzed [8], The objective of this therapeutic strategy was to achieve a local, sustained, pharmacologically active concentration in the eye, with minimal systemic concentrations. In this application, the low water solubility of TACA contributes to its prolonged duration of action at the injection site a previous human study showed that intraocular TACA maintained concentrations >100 ng/mL in the eye for at least 700 h after a single 4-mg intravitreal TACA treatment [9], However, TACA concentrations in plasma... 

Disorders of the posterior segment of the eye are particularly difficult to treat. The efficient clearance mechanisms at the front of the eye reduce the concentrations of drug able to diffuse to the back of the eye. Futhermore, many of these disorders are chronic conditions, requiring continuous therapy. The diseases of the back of the eye include Cytomeaglovirus retinits (CMVR), Proliferative vitreoretinopathy (PVR), diabetic retinopathy, age-rated macular degeneration, endophthalmitis and retinitis pigmentosa. 

As the majority of the posterior segment disorders are chronic in nature, sustained delivery of medications is highly desirable. Liposomes and microparticulates are such systems designed to release the encapsulated drag gradually and over an extended period of time. 

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