(Nanowerk Spotlight) For the treatment of eye conditions, conventional eye drops have three major disadvantages: they must be applied frequently; their ocular bioavailability is low (i.e. less than 5% of the administered active is absorbed or becomes available at the site of physiological activity); and, their use is often associated with high systemic exposure to actives. The common alternative option, ophthalmic inserts, achieve sustained drug delivery but suffer from other limitations: they are difficult to insert (especially for the elderly and others with visual impairment) and easy to misapply; they are easily expulsed from the eye; patient compliance is low (discomfort and blurring of vision, difficulty of insertion, need for removal at the end of their useful life); and, they are costly to manufacture. Researchers in the UK believe that biodegradable polymer nanoparticles show great promise as drug delivery devices for the eye. They have developed well-tolerated systems that combine the sustained release characteristics of inserts with the patient acceptability of conventional eye drops.
With the new systems, biodegradable polymers can be combined with drugs in such a way that the drug is released from the material into the eye in a predesigned manner. The release of drug can be constant or cyclic over a long period, or triggered by the environment or a chemical signal. At the end of its useful life, the drug delivering polymer can be broken down naturally by the body.
Dr. John Tsibouklis, an expert in biomaterials and drug delivery from the School of Pharmacy and Biomedical Sciences at the University of Portsmouth, UK, explained the new drug delivery systems to Nanowerk: "We have developed nanoparticle-based ocular formulations that can release actives over extended time scales (days). These are administered into the conjunctival cul-de-sac and, at the end of their useful life, disappear into the lachrymal fluid. Among other advantages, the materials are well tolerated and the rate of their bioerosion/dissolution can be controlled."
An ideal polymeric drug carrier should have a loading capacity that can ensure therapeutic doses, be able to penetrate to — and/or reside at — the desired site of action, and, release the active in a controlled manner. It should also be non-toxic, biocompatible and biodegradable. For most ophthalmic applications the carrier should not impede vision, since tolerability and acceptance by the patient are critical.
Polymeric vehicles for controlled drug release have been classified as either reservoir-type, where the polymer essentially coats the drug core, or matrix-type, where the active is homogeneously mixed with the polymer, or is bound to it through covalent or hydrogen/donor–acceptor interactions. The vehicles may assume a variety of forms, ranging from solutions or gels to
colloidal systems or solid inserts.
"The formulation of biodegradable or bioerodible polymers as water-based colloidal nano-systems holds significant promise for ophthalmic drug delivery" says Tsibouklis. "A colloidal system for poorly water-soluble drugs would allow drop-wise administration while maintaining drug activity at the site of action."
Although several synthetic methods and drug loading techniques are reported to be safe and reproducible, no procedure for the formulation of drug-loaded nanoparticles has yet been standardized. Formulation stability, particle size uniformity, control of drug release rate, and the large-scale manufacture of sterile preparations are major developmental issues that need to be addressed before commercialization.
Tsibouklis points out that the nanoparticulate drug carriers developed at the University of Portsmouth's Biomaterials & Drug Delivery Group have been formulated, and tested, with drugs for glaucoma (pilocarpine, atropine) and with antibiotics (chloramphenicol, norfloxacine).
As drugs can now be coupled to nanocarriers that are specific for cells and/or organs, nanotechnologies will play a vital role
in the future of ophthalmic medication.
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