11.3  Using Nanotechnology to Improve Ocular Therapeutics

This section will focus on different areas of ocular therapeutic improvement and possible solutions.

11.3.1  Improving Patient Compliance

A major concern for clinicians prescribing and administering eye injections is patient compliance due to the lack of noninvasive treatments that can deliver adequate amounts of drug to the target site. Currently, there are no treatments available that can deliver macromolecules and small molecules to the posterior segment of the eye efficiently without using invasive techniques (e.g., intravitreal injection).

As the biological basis for many ocular diseases becomes more apparent, protein, peptide, and nucleic acid drugs will be used to develop new pharmaceuticals and therefore there is a need to develop noninvasive approaches for delivering macromolecules as well. For example, the anti-VEGF antibody formulation, Lucentis® must be intravitreally injected to reach the posterior segment of the eye. Many macromolecules have poor permeability across biological barriers, which make the development of noninvasive techniques difficult. Nanotechnology approaches may be used to improve the bioavailability of many macromolecules by sequestering the drug from enzymatic degradation and by enhancing tissue uptake. For instance, surface-functionalized nanoparticle technologies were developed by Kompella et al. to enhance corneal and conjunctival uptake and transport of nanoparticles and the associated therapeutic agents (Kompella et al. 2006). These technologies entail coating of particle surfaces with a ligand capable of recognizing a cell surface receptor. By coating LHRH receptor and transferrin receptor recognizing ligands, it was shown that the corneal and conjunctival uptake as well as transport of nanoparticles can be enhanced by several fold. The functionalized nanoparticle exposure did not alter corneal epithelial cell tight junctions or paracellular permeability, indicating the safety of these nanoparticles. It is anticipated that functionalized nanoparticles will allow noninvasive delivery of poorly permeable small molecules as well as macromolecules to the back of the eye.

11.3.2  Increasing Drug Retention and Sustained Release

Many therapeutics designed to treat ocular diseases must be injected into the eye and typically they are injected multiple times to prevent relapse, e.g., Lucentis® (Valmaggia et al. 2008). It has been reported that complications related to the injection technique can occur, resulting in infection, uvetis, endophthalmitis (Ozkiris and Erkilic 2005), cataract progression (Cekic et al. 2005), and vitreous hemorrhage (Ciardella et al. 2004). The risk for these complications can be

decreased by either developing noninvasive topical modes of delivery or by injecting­ less frequently. In order for treatment injections to be less frequent, the drug must have either intrinsic sustained release properties or a controlled release mechanism may be employed by engineering a drug carrier. Nanosystems or nanoparticles can be designed to have sustained release properties. For example, Bourges et al. designed polylactic acid (PLA) nanoparticles that were injected intravitreally and were observed in RPE cells up to 4 months after injection (Bourges et al. 2003). Compared to Macugen® and Lucentis®, which are injected every 6 weeks and every 4 weeks, PLA nanoparticles are retained much longer. The use of PLA nanoparticles as a drug carrier may prove to be a successful approach to sustain the release of its contents. A reduction in dosing frequency will increase patient compliance, which reduces the risk for many complications associated with ocular injections.

11.3.3  Increasing Permeability and Tissue Partitioning

Many topical agents including steroids, antihistamines, prostaglandins, and topical anesthetics have been formulated to provide for noninvasive administration, yet these topical agents still are not able to reach the posterior segment of the eye in sufficient quantity. In eye treatments given as eye droplets such as timolol, only 1% or less of a topically applied dose is absorbed across the cornea to reach the anterior segment of the eye (Lee and Robinson 1986; Mezei and Meisner 1993; Ding 1998) and only about one-billionth of that reaches the vitreous (Maurice 2002). Ocular barriers such as the cornea and conjunctiva also create a major hurdle for topically applied agents (Kompella and Lee 1999; Kompella et al. 2010). Therefore, noninvasive formulations such as eye drops are not only are being washed away by tear drainage and blinking, but they also encounter major ocular barriers that significantly reduce the amount of drug that is able to reach the posterior segment of the eye. Therefore, the major route of administration of ocular therapeutics for the back of the eye is injection because it delivers the drug either directly to the site of action or in close proximity. With the advent of nanotechnology, noninvasive routes of administration may be finally realized for ocular treatments by overcoming the many biological barriers and providing for increased drug retention.

Surface functionalization of nanoparticles is a common approach to enhance the permeability and specific tissue levels of therapeutics. For instance, deslorelin, a luteinizing releasing hormone agonist, and transferrin functionalized polystyrene (PS) nanoparticles (approximately 100 and 85 nm, respectively) enhanced corneal epithelial uptake by 3- and 4.5-fold compared to unfunctionalized nanoparticles, respectively, at 5 min when topically applied to an ex vivo model (Kompella et al. 2006). At 1 h after a single topical application of the nanoparticle solution, the deslorelin and transferrin functionalized nanoparticles had 4.5- and 3.8-fold higher

uptake across the corneal epithelium than nonfunctionalized nanoparticles. Functionalized nanoparticles clearly are capable of improving drug transport across major ocular barriers.

11.3.4  Targeting Nanotherapies

Ocular treatments may also be improved by developing targeted nanotherapies that increase drug localization in the target tissues or reduce drug delivery to nontarget tissues associated with drug side effects. Such approaches can potentially increase drug therapeutic index by increasing drug efficacy and/or reducing drug toxicity. Macugen® (pegaptanib), a drug product approved for treating wet AMD, belongs to a class of chemicals known as aptamers (a short strand of nucleotides that recognizes a specific protein sequence) that are known to bind to their targets with affinities superior to even antibodies. Potentially, such aptamers can be used to target delivery systems following various routes of administration. Indeed, aptamers have been designed in the field of cancer therapy to target therapeutics directly to the cancer cells and similar approaches may be used for targeting specific cell types within the eye. Aptamers that specifically recognize the prostate-specific membrane antigen (PSMA) found on the surface of prostate cancer cells were ligated to PLA– PEG nanoparticles by adding the nanoparticles to 1-(3-dimethylaminopropyl)-3- ethylcarbodimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) for 15 min while stirring (Farokhzad et al. 2004). Then the NHS-activated nanoparticles were covalently linked to the PSMA aptamer. The resulting size of the nanoparticles was approximately 250 nm. An in vitro assay confirmed that nanoparticles with PSMA aptamers had 77-fold higher binding to LNCaP cells (which contain the PSMA membrane protein) than the PC3 cells (which do not contain the PSMA membrane protein).

Further, integrin-targeting peptides with RGD (arginine, glycine, and aspartic acid) sequence and transferrin functionalizations on nanoparticle surface are of potential value in increasing the delivery of nanoparticles and any associated therapeutic agents to various cell types within the eye. Using intravenously administered nanoparticles functionalized on their surface with RGD peptide or transferrin, it was demonstrated that back of the eye delivery of anti-VEGF intraceptor plasmidloaded nanoparticles can be enhanced in a choroidal neovascularization model (Singh et al. 2009). Further, these nanoparticles enhance gene expression efficiency in vascular endothelial cells, photoreceptor outer segments, and retinal pigment epithelial cells. By encapsulating the plasmid inside the nanoparticles as opposed to the anti-VEGF agent itself, this approach potentially minimizes the systemic side effects of anti-VEGF antibodies such as stroke and hypertension. Further, since the intraceptor plasmid produces an anti-VEGF protein that is selectively retained in endoplasmic reticulum, resulting in VEGF sequestration and reduced secretion (Singh et al. 2006), extracellular concentrations of this anti-VEGF protein