Topical eye drops have poor bioavailability, with only 1% to 7% of the active ingredient absorbed into the eye. Corneal permeability is often hindered by solution drainage, lacrimation and tear turnover, tear evaporation, and conjunctival absorption. Nasolacrimal drainage and conjunctival absorption can result in systemic side effects from topical drops. Patient adherence is critical in chronic diseases such as glaucoma, where estimates of nonadherence range from approximately 25% to 60%. Over time, most patients will require more than a single class of topical drop to control their disease, but increased complexity and frequency of dosing regimens often result in decreased adherence. Removing the patient from the drug delivery system is a key goal in glaucoma therapeutics. Novel methods for delivering drugs to target tissues are in development, including encapsulated cell technology, gene transfer, nanoparticles, microspheres, sustained release technologies, and coated microneedles.
The strategy for several of these methods is similar. Target cells are reprogrammed to increase or decrease expression of a gene product resulting in up- or downregulation of a biochemical/physiological process.
Gene expression activation or inhibition can be manipulated by increasing the amount of the construct (e.g., introducing greater numbers of gene-expressing viral particles or siRNA), using regulation strategies like Tet off/on or inducible vectors that “turn on” in the presence of a physiological event such as elevated IOP.
Encapsulated cell therapy (ECT) is a novel sustained delivery strategy for ciliary neurotrophic factor. ECT implants are anchored to the sclera within the posterior segment. They contain cells that are genetically modified to produce the desired therapeutic factor, which is released over time. The cells are encapsulated within a semipermeable, hollow-fiber membrane, thus avoiding immune reaction. Currently, ECT is in phase II and III clinical trials for the treatment of geographic atrophy associated with dry age-related macular degeneration and retinitis pigmentosa.
RNA interference (RNAi) is a useful strategy in situations in which suppressing the expression of a single protein will address the symptoms or pathology of the disease. RNAi therapies can be effective at lower concentrations than small molecules, which could potentially mean lower doses and fewer adverse effects.
In vivo delivery is a challenging aspect of RNAi therapeutic development. Effects can be short-lived, as endo- and exonucleases and RNases that are present in many tissue microenvironments quickly degrade unmodified, naked siRNAs.
28 It is not clear whether RNases are present in human aqueous humor or that of other species. Off-target effects are difficult to predict, and careful consideration of species differences during preclinical testing is critical. It remains to be seen whether species-specific surrogate sequences will have to be designed so that preclinical toxicity testing can be performed in parallel with human-specific sequences. As with some novel strategies, RNAi may be as valuable in modeling diseases, studying the effects of silencing-specific genes in vitro and in vivo, as it is in treating them.
Both nonviral and viral gene transfer methods have proponents. Nonviral gene delivery methods (mechanical, physical, and chemical) are advantageous because of their low immunogenicity, a large capacity for DNA size, ease of manipulation, and low-cost production and production ramp-up. Generally, these methods are somewhat less efficient in gene transfer, requiring a larger amount of vector to achieve a response comparable to that achieved with viral vectors. Nonviral methods have a relatively short therapeutic duration (e.g., naked DNA), and not all ocular cell types can be easily transfected by these methods (e.g., cultured human trabecular meshwork [HTM] cells); work is ongoing to overcome these weaknesses. Viral vectors tend to have higher transfection efficiencies and smaller loading capacities, and to be more difficult to produce at a large scale. Although advances have been made to greatly reduce the risk of inflammatory and immunogenic responses and insertional mutagenesis, it cannot be said that the risks are nil. Successful viral vector–mediated gene therapy for ocular diseases such as Leber's congenital amaurosis are encouraging the development of gene therapy strategies for glaucoma, where targets include cytoskeletal-modulating proteins that enhance outflow through the TM, PG pathway elements that increase uveoscleral outflow, and neurotrophic factors (brain-derived neurotrophic factor, ciliary neurotrophic factor, glial cell–derived neurotrophic factor) that have been used in laboratory studies of neuroprotection. Recent work demonstrates that long-term (>2 years thus far) expression of reporter genes in the primate outflow pathway is possible in vivo with self-complementary AAV
29 and FIV
30 vectors, with low immunogenicity and clinically quiet anterior segments.
Viral vectors that have been investigated for ocular delivery of genes include adenovirus, herpes simplex virus, AAV, and lentivirus (FIV, EIAV). Each has strengths and weaknesses that are being addressed. The National Eye Institute's Ocular Gene Therapy unit is studying and developing the therapeutic potential of AAV vectors, which have different serotypes with different tropisms as well as novel hybrid “pseudotyped” recombinant AAV vectors.
Nanotechnology applications are being developed for several ocular diseases using a variety of nanosuspensions, liposomes, dendrimers, nanoparticles, ocular inserts, implants, and hydrogels. For glaucoma, at this stage, most are topical drop formulations that it is hoped will achieve improved corneal permeability, increased bioavailability, reduced dosages, and extended release. Surface-modified nanoparticulate carriers may be used to accommodate a wide variety of active compounds, including poorly water-soluble drugs. Several types of biodegradable polymers can be used in a single formulation to create a release profile consisting of an initial burst followed by sustained release or to facilitate penetration across different tissue layers. Drugs can also be coupled to nanocarriers that are specific for cells and/or organs. No standardized procedure for the formulation of drug-loaded nanoparticles has been developed that addresses formulation stability, particle size uniformity, consistent control of drug release rate, and large-scale manufacture of sterile preparations. As with all new treatment options, the risks and benefits and the impact of these therapies on patterns of clinical practice remain to be seen.
Delivery strategies, such as viral and nonviral gene transfer, sustained release, microneedles, canaloplasty, and others to deliver compounds directly to the relevant tissue, and encapsulated cell therapy, all offer improvements over topical drop therapeutics. However, the need for better biodelivery strategies for outflow modulations remains and includes the need for improved vectors for gene transfer in vivo. Suppression of inflow is a proven method of IOP reduction. As the molecular pathways are now known, they could be good targets for gene or siRNA approaches. More focused studies of genetic manipulation of aqueous inflow would be complementary and offer further opportunities for identification of therapeutic targets and development of research models.