The conjunctiva forms an important obstacle
that does not allow drugs between 20 and 40 kDa to pass through it. The
permeability of the sclera is under debate. However, studies have shown that it
is not strictly a barrier, because molecules with a molar mass up to 150 kDa
are able to penetrate it. An important pharmacological requirement for topical
medications for glaucoma is to overcome these anatomical obstacles.
Various periocular, extraocular and
intraocular nano/micro-drug delivery systems (DDSs) have been engineered. A
number of them have been found to be safe and efficacious as anti-glaucoma
hypotensive agents. These DDSs include topical formulations, ocular inserts,
drug-eluting contact lenses, ocular ring inserts, intracameral, intravitreal,
subconjunctival and suprachoroidal injectable formulation/implants.
According to the size, drug carriers can be
classified as implants (>1 mm), microparticles (MPs, particles with a size
ranging from 1 to 1000 µm) and nanoparticles (NPs, particles with a size less
than 1000 nm). Currently, NPs remain the most intensively used form due to
their small size, easily modified surface, ability to adsorb, attach and
encapsulate various substances, and favorable biocompatibility.
Nanocarriers can be prepared in a variety
of forms, such as polymeric nanomicelles (self-assembled suspension of
amphiphilic block copolymers with hydrophobic cores and hydrophilic shells) and
nanoemulsions (a mixture of two immiscible liquids with surfactants).
The incorporation of drugs into
nanocarriers may surpass the limitations of current treatment regimens by
enhancing drug penetration, achieving targeted delivery, prolonging contact of
drugs with ocular tissues, and sustaining in vivo release.
Unlike other agents, nanocarriers are
equally effective in delivering lipophilic drugs, proteins, and even genes,
which are difficult with conventional solvents. Nanocarriers are also able to
protect the integrity of drug cargo before reaching the target sites. This
property is particularly intriguing when transporting molecules such as
neurotrophin and antibodies because these proteins easily degrade in vivo.
Based on their physical structures, drug
carriers can be divided into reservoir-type and matrix-type drug delivery
systems. In reservoir-type carriers, drug agents are trapped in an inner core,
surrounded by a polymer wall that controls the rate of drug release. In
matrix-type carriers, the agents are buried within and uniformly distributed
throughout the polymer matrix.
In order to improve biocompatibility or
optimize the drug release patterns different forms of materials or additives
are often incorporated into one hybrid system.
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