Pharmaceutical Manufacturing Handbook: Production and

FORMULATION APPROACHES TO IMPROVE OCULAR BIOAVAILABILITY 737
the ocular tissues than hypertonic ones, which lead to increased lacrimation. If the
tonicity of the formulation is lower than 260 mOsm/kg or higher than 480 mOsm/kg,
the formulation becomes irritant [45] , induces refl ex tearing and blinking, and is
therefore likely to reduce the bioavailability of topically instilled drugs.
5.9.3 FORMULATION APPROACHES TO
IMPROVE OCULAR BIOAVAILABILITY
One of the major problems encountered with topical administration is the rapid
precorneal loss caused by nasolacrimal drainage and high tear fl uid turnover,
which leads to drug concentrations of typically less than 10% of the applied drug.
Approaches to improve the ocular bioavailability have been attempted in two
directions: to increase the corneal permeability by using penetrations enhancers or
prodrugs and to prolong the contact time with the ocular surface by using viscosity -
enhancing or in situ gelling polymers. Table 1 summarizes conventional and novel
ocular drug delivery approaches. Marketed ophthalmic delivery systems based on
recent formulation approaches are listed in Table 2 . An optimal ocular delivery
systems would be administered in the form of an eye drop, causing neither blurred
vision nor ocular irritancy, and would only need to be instilled once a day [1] .
5.9.3.1 Conventional Dosage Forms
Conventional dosage forms such as solutions, suspensions, and ointments account
for almost 90% of the currently accessible ophthalmic formulations on the market
[1, 169] . They offer some advantages such as their ease of administration by the
patient, ease of preparation, and the low production costs. However, there are also
signifi cant disadvantages associated with the use of conventional solutions in particular,
including the very short contact time with the ocular surface and the fast
nasolacrimal drainage, both leading to a poor bioavailability of the drug. Various
ophthalmic delivery systems have been investigated to increase the corneal permeability
and prolong the contact time with the ocular surface. However, conventional
eye drops prepared and administered as aqueous solutions remain the most commonly
used dosage form in ocular disease management.
Solutions The reasons behind choosing solutions over other dosage forms are
their favorable cost advantage, the simplicity of formulation development and production,
and the high acceptance by patients [170] . However, there are also a few
drawbacks, such as rapid and extensive precorneal loss, high absorption via the
conjunctiva and the nasolacrimal duct leading to systemic side effects, as well as the
increased installation frequency resulting in low patient compliance.
Some of these problems have been encountered by addition of viscosity -
enhancing agents such as cellulose derivates, which are believed to increase the
viscosity of the preparation and consequently reduce the drainage rate. The use of
viscosity enhancers will be discussed later in this section.
Suspensions Suspensions of the micronized drug ( < 10 μ m) in a suitable aqueous
vehicle are formulated, where the active compound is water insoluble. This is the