Administration, drugs retrobulbar injection

In-depth discussions of the anatomy of the eye and adnexa have been adequately covered elsewhere in the pharmaceutical literature [13-17] and in recent texts on ocular anatomy. Here a brief overview is presented of the critical anatomical features that influence the nature and administration of ophthalmic preparations. In this discussion, consideration will be given primarily to drugs applied topically, that is, onto the cornea or conjunctiva or into the palpebral fornices. Increasingly, drugs are being developed for administration by parenteral-type dosage forms subconjunctivally, into the anterior and posterior chambers, the vitreous chamber, Tenon s capsule, or by retrobulbar injection. 

Very few injectable dosage forms have been specifically developed and approved by FDA for intraocular use. However, the ophthalmologist uses available parenteral dosage forms to deliver antiinfectives, corti-costerioids, and anesthetic products to achieve higher therapeutic concentrations intraocularly than can ordinarily be achieved by topical or systemic administration. These unapproved or off-label uses have developed over time as part of the physician s practice of medicine. However, these drugs are usually administered by subconjunctival or retrobulbar injection and rarely are they injected directly in the eye [301]. 

Barza et al. (54) later reported that after subconjunctival injection of gentamicin, higher drug concentrations were found in ocular tissues from normal eyes than from inflamed, infected (Staphylococcus aureus endophthalmitis) eyes, despite the presumed reduction in blood-eye barrier in the inflamed eye. This result was not due to altered drainage into the tear film but may have been caused by increased ocular and orbital vascularity or decreased half-life within the eye (55). Similar results have been reported by Levine and Aronson (46) who found that inflammation caused a twofold decrease in ocular absorption of radiolabeled cortisol after retrobulbar injection although no such difference was seen following subconjunctival injection. Peak ocular concentrations were observed five minutes after administration. These authors also speculated that the difference in ocular absorption after retrobulbar injection was probably due to more rapid steroid removal from... 

When higher concentrations of drugs, particularly corticosteroids and antibiotics, are required in the eye than can be delivered by topical administration, local injections into the periocular tissues can be considered. Periocnlar drug delivery includes subconjunctival, snb-Tenon s, retrobulbar, and peribulbar administration. 

Classical pharmacokinetic models of systemicaUy administered drugs (see Chapter 1) do not fuUy apply to many ophthalmic drugs. Most ophthalmic medications are formulated to be apphed topically or may be injected by subconjunctival, sub-Tenon s, and retrobulbar routes (Figure 63-1 and Table 63-1). Although similar principles of absorption, distribution, metabolism, and excretion determine drug disposition in the eye, these alternative routes of drug administration introduce other variables in compartmental analysis. 

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