| Abstract (english) | In the first part of this paper is present a discussion ofthe anatomical and
physiological characteristics and limitation present in the human eye. In a
simplistic sense, the eye can be viewed as two compartment system: the anterior segment (containing the cornea, conjunctiva, sclera externally, and the
anterior chamber, iris, pupil, posterior chamber, ciliary body internally) and
the posterior segment (containing the lens, vitreous body, rear ocular tissue
layers /retina and choroid/ internally, and the optic nerve and associated vasculature externally). The anterior segment has been described as a dynamic
environment. In contrast, the posterior segment is viewed as being somewhat
static. Delivering drugs to the front of the eye is an exceedingly complicated
issue because ofthe numerous protective mechanisms that are present in the
eye to shield the visual pathway from foreign chemicals. Most conventional
ophthalmic dosage forms are simplistic. It is usual that water-soluble drugs
are delivered through topical administration in an aqueous solution, and water-insoluble drugs are administered topically as an ointment or aqueous suspension. The major deficiencies of these conventional dosage forms include
poor ocular drug bioavailability, pulse-drug entry after topical administration, systemic exposure because of nasolacrimal duet drainage, and a lack of
effective systems for drug delivery to the posterior segment of ocular tissue.
Poor ocular drug bioavailability is the result of ocular anatomical and physiological constraints, which include the relative impermeability of the corneal
epithelial membrane, tear dynamics, nasolacrimal drainage, and the high efficiency ofthe blood-ocular barrier. Pulse entry is a common pharmacokinetic
characteristic associated with eye drops. The initial high drug concentration
found in tears, followed by a rapid decline, poses a potential risk of toxicity.
Consequently, eye drops do not remain in contact with the eye for a long time,
and they must be administered at relatively frequent intervals. Suspensions
have the advantage oflonger contact time in the eye, but also the disadvantage of an irritation potential, due to the particle size of the suspended drugs.
Irritation may produce excessive tearing and, consequently, rapid drainage of
the instilled dose. Ointments have the advantages of longer contact time and
greater storage stability, but also the disadvantage of producing a film over
the eye, thereby blurring vision. The delivery of drugs to the posterior segment
of ocular tissue is prevented by the same factors that are responsible for the
poor ocular bioavailability. In addition, the blood-retinal barrier limits the effectiveness of the intravenous route in posterior drug delivery. To date, the
most acceptable method for posterior drug delivery is topical ocular injections
(intracameral, retrobulbar ). |
