Nanotechnology is broadly defined as the techniques and methods used to research, design and engineer materials on a molecular scale of one to 100 nanometers.¹ Research in nanotechnology involves manipulating atoms and molecules with nanodevices in ways that allot the structures new properties and functions.¹ When described in the field of biology or medicine, nanotechnology is often referred to as nanomedicine or biomedical nanotechnology. More specifically, nanomedicine involves the application of nanotechnology to medicine where the manipulation of molecules can aid in drug delivery, monitoring devices/sensors, nanoscale prosthetics and gene therapy, with the ultimate hope of repairing damaged tissues and curing disease.^1,2 Nanotechnology is an explosive research area and has the potential to revolutionize the diagnosis, prevention and treatment of disease. Several studies from the Association for Research in Vision and Ophthalmology (ARVO) address the role of nanotechnology in drug delivery mechanisms and the new treatment possibilities using these different methods.

Discoveries in Drug Delivery
Protein-based topical eye drops is a common route of drug delivery, but they can be inefficient and cause systemic side effects secondary to absorption into the bloodstream.³ However, disposable, nanoparticle-laden contact lenses would increase efficacy and decrease systemic side effects of medications.³ To develop these contact lenses, researchers encapsulate proteins that are involved in wound healing (e.g., growth factors) in nanoparticles and then cross-link into hydrogel poly-HEMA by UV curing.³ They use both inorganic transparent nanoparticles and biopolymer nanoparticles.³ The protein releases through a diffusion-based mechanism and is monitored for 25 days.³ It was shown that hydrophilic proteins can be encapsulated in different types of nanoparticles, and it is expected that nanoparticle-laden contact lenses will be considered a new mechanism of protein (growth factor) drug delivery.³ There is potential for broad application of this technology. By using drug-eluting lenses, there is enhanced localized absorption and decreased potential for systemic toxicity.

Research is also being conducted to study the role of dendrimers as drug delivery vehicles.⁴ Dendrimers have the capability of delivering highly concentrated drugs or genetic material to specific target areas of cells in a Trojan Horse concept, where the dendrimers are used as chemical markers or for imaging.^5,6 Anionic and cationic carbosilane dendrimers were evaluated in vitro with conjunctival and corneal cells and in vivo in rabbit eyes.⁴ It was found that carbosilane dendrimers are suitable for topical ophthalmic use in both in vitro and in vivo studies.⁴ Another study also showed that dendrimers increased the topical solubility of gatifloxacin, ketorolac tromethamine and tropicamide.⁷

Ocular conditions, such as corneal graft rejection and chronic keratitis can be resilient to topical drops. Researchers studied the effect of intrastromal injection of micro and nanoparticles on sustained drug delivery using three main study groups of rabbit corneas: saline (control), microparticles or nanoparticles.⁸ Fluorescence was used to monitor the clearance of the particles over a period of two weeks.8 The control corneas showed no fluorescence at any time, and there was no significant difference between fluorescence patterns of microparticles vs. nanoparticles.8 Both micro and nanoparticles showed 21% and 23% clearance of particles after one hour, 52% and 41% after one day and 79% and 73% after 14 days, respectively.⁸ This shows that the particles were sustained through intrastromal injection for at least two weeks on rabbit corneas, and thus, offers the possibility that these particles could be loaded with medication and provide sustained drug delivery to the cornea.⁸ Researchers have also studied the nanoscale viscoelastic properties of the cornea with hopes to model the viscoelastic response of human corneal tissue and design improved bio-engineered corneas.⁹

Uveitis can be recalcitrant, chronic and difficult to treat. Tacrolimus (FK506) has shown to be a very effective immunosuppressive drug when delivered systemically in animal models and has suppressed autoimmune and uveoretinitis; however, it has poor ocular permeability, which is why is does not provide the same effect when delivered to the eye.¹⁰ Researchers have been working on developing degradable nanogels to enhance the ocular permeability of this drug.¹⁰ During the complex synthesis of the nanogels, FK506 is loaded into the nanogel and then its permeability is tested ex vivo by using diffusion cells and in vivo in rabbit eyes.¹⁰ It is administered in vivo by intravitreal injections, subconjunctival injections and topical instillation. Findings demonstate that the permeability of the drug depends on the synthesis of the nanogels; there is great potential for the sustained release of FK506 and possibly other immunosuppressive drugs to treat chronic uveitis.¹⁰ Additional studies show the ocular efficacy of other nanogels, their superior permeability and capability of sustained drug release.

Intracameral (IC) administration of anti-inflammatory and antimicrobials after intraocular surgery would decrease the need for frequent administration of topical drops and improve patient compliance. Researchers are conducting studies to develop nanoparticles that would provide sustained drug delivery to the anterior chamber. Rabbit eyes either received IC saline, IC uncoated latex particles or IC coated latex particles.¹¹ Fluorescence was used to evaluate particle retention.¹¹ Results showed that the control group exhibited no fluorescence at all times and over time, the number of uncoated and coated particles gradually declined.¹¹ After one month, 33% coated and 18% uncoated particles remained, with significantly more clearance in the uncoated particles.¹¹ This study suggests that IC administration of nanoparticles post-operatively may provide a sustained drug delivery mechanism in the anterior chamber for up to one month.¹¹

Gene Therapy Advancements
While ocular drug delivery mechanisms are a huge area of research by ARVO, so is the role of nanotechnology in gene therapy. A healthy corneal endothelium is necessary to maintain corneal clarity and normal function. Corneal endothelial disease is difficult to treat and often requires a corneal transplant. Researchers found that delivering therapeutic genes to corneal endothelial cells has potential to cure and treat endothelial disease.¹² Research methodology involved incubating human cultures of corneal endothelium with a variety of adeno-associated viruses (AAV) or gold nanoparticle stabilized in polyethyleneimine (GNP-PEI) vector under control of Rous sarcoma virus or CMV.¹² It was found that there was significant gene delivery to the corneal endothelium with both AAV and GNP-PEI vectors, with certain AAV vectors being more efficacious than others, and GNP-PEI vectors being significantly more effective than AAV vectors.12 AAV6 vector had the highest transduction rate of 4.4% vs. other AAV vectors; GNP-PEI vector showed the highest gene transduction at a rate of 19% - 45%.¹² However, both vectors showed a decrease in cellular viability: AAV vector showing 2% - 10% decrease, and GNP-PEI vector showing 29% - 41% decrease.¹² Current studies are focusing on defining dose and toxicity of certain vectors.¹²

The role of gene therapy has also been studied in different corneal diseases. These studies show that the efficacy of the GNP-PEI vector in gene therapy depends on the nature of the corneal condition.¹³ Studies comparing GNP-PEI vector transduction in normal corneas, hazy corneas and neovascularized corneas show differing degrees of gene delivery.¹³ Hazy corneas showed uptake similar to normal corneas (5% - 11% difference in gene expression), but was significantly less than neovascularized corneas, which showed up to 31% transduction.¹³ Corneal toxicity levels were not statistically significant between all three types of corneas.¹³ Although studies for corneal gene therapy show different efficacy based on corneal condition and toxicity based on vector type, they are promising.

Researchers also studied the role of an anti-VEGF-A loaded nanoparticle in reducing corneal neovascularization in mice.¹⁴ Corneal burn/injury was induced in mice, and neovascular vessels were allowed to mature for one month.¹⁴ The mice were subsequently injected with one of four groups of cocktail: pSEC.siRNA.VEGFA NR PLGA NPs (anti-VEGF-A), pSEC.siRNA.VEGFA (naked plasmid), blank nanoparticle (control) and DMSO (dimethyl sulfoxide control).14 Study findings showed that plasmid-loaded anti-VEGF-A nanoparticles significantly regressed the amount of corneal neovascularization vs. naked plasmid and control.14 Anti-VEGF-A plasmid nanoparticles showed 12.5% area of corneal neovascularization, whereas naked plasmid had 28%, and the two controls, blank and DMSO, showed 55% and 53% area, respectively.14 Based on these results, researchers concluded that gene therapy with anti-VEGF-A loaded nanoparticles is a non-toxic and effective way to treat corneal neovascularization in mice, which has futuristic implications for the treatment of neovascularization in human corneas.¹⁴ 

Disease Monitoring
Another big topic of nanotechnology research involves using nanodevices to monitor diseases. One of the main ocular nanodevices being researched is a 24-hour intraocular pressure sensing contact lens. In a study that monitored the IOP of glaucoma patients over a period of 24 hours, glaucoma management changed based on spikes and wide diurnal IOP fluctuations.¹⁵ This study, along with others, suggest that there would be great benefit to a 24-hour IOP sensor. In 2008, researchers developed a conventional contact lens fabricated with inorganic microstructures and microdevices, whose functions can include radio frequency power transmission to a contact lens and bio-sensing. Research in this area has been ongoing, and the futuristic goal of using a contact lens-based method for monitoring eye pressure was discussed at the ARVO 2010 meeting.^16,17

On the Brink of Nanotech Application
While there are a multitude of recent advances in nanotechnology applied to anterior segment, there are myriad studies relating to nanotechnology and the posterior segment as well. Nanotechnology is a fascinating research area and truly has the potential to revolutionize the diagnosis, treatment and management of disease.  Look for novel nanotech approaches via new tools, exciting methods to alter disease course by gene therapy and drug eluting devices to aid us in treating and managing anterior segment disease. 

1. Vo-Dinh T. Nanotechnology in Biology and Medicine: Methods, Devices, and Applications. CRC Press. 2007.
2. The NIH Common Fund: Nanomedicine, Overview. Available at: www.nihroadmap.nih.gov/nanomedicine/2010. 2010 Jan.(Accessed September 2010)
3. Zhang J, Thomas A, Akther KF, et al. Encapsulation and releasing of protein drugs through nanostructured contact lens. Abstract presented at ARVO 2010 439/D1143, May 2, 2010.
4. Bravo-Osuna I. In vitro and in vivo tolerance studies of carbosilane dendrimers for ophthalmic administration. Abstract presented at ARVO 2010 437/D1141, May 2, 2010
5. Holister P, Vas CH, Harper T. Dendrimers: Technology White Papers 6. Cientifica. 2003 Oct. Available at: www.sps.aero/Key_ComSpace_Articles/TSA-001_Dendrimers_White%20Paper.pdf. (Accessed September 2010).
6. Svenson S, Tomalia DA. Dendrimers in biomedical applications- reflections on the field. Adv Drug Deliv Rev. 2005 Dec;57(15):2106-29.
7. Durairaj C, Tyagi P, Chandler J, Kompella U. A novel dendrimer formulation for enhanced solubility and delivery of gatifloxacin, ketorolac tromethamine, and tropicamide. Abstract presented at ARVO 2009 2421/A509, May 4, 2009
8. Behrens A, Khan YA, Lai SK, et al. Sustained intrastromal drug delivery with micro- and nanoparticles - a potential method for treating chronic keratitis and corneal graft rejection. Abstract presented at ARVO 2010 442/D1146, May 2, 2010.
9. Lombardo M, Lombardo G, Carbone G, et al. Nanoscale viscoelastic properties of the human corneal tissue investigated with atomic force microscopy. Abstract presented at ARVO 2010 419/D1123. May 2, 2010.
10. Zhang J, Misra GP, Lowe TL. Nanogel for Delivering Fk506 Across Ocular Biological Barriers to Treat Uveitis. Abstract presented at ARVO 2010 428/D1132, May 2, 2010.
11. Khan YA. Intracameral injection of nanoparticles: evaluating a potential new approach for sustained delivery of post-operative medications to the anterior chamber. Abstract presented at ARVO 2010 435/D1139, May 2, 2010.
12. Phillips DL, Sharma A, Tovey JCK, et al. Gold nanoparticles: a potent vector for non-viral corneal endothelial gene therapy. Abstract presented at ARVO 2010 433/D1137, May 2, 2010.
13. Tovey, JCK. Does Gold Nanoparticle-Mediated Non-Viral Gene Delivery in the Cornea Depend on Disease Condition. Abstract presented at ARVO 2010 434/D1138, May 2, 2010.
14. Stagg, BC, Qazi Y, Singh S, et al. Nanoparticles delivering anti-vegf-a plasmid regress murine corneal neovascularization. Abstract presented at ARVO 2010 440/D1144, May 2, 2010.
15. Hughes E, Spry P, Diamond J. 24-Hour monitoring of intraocular pressure in glaucoma management: a retrospective review. J Glaucoma. 2003 Jun;12(3):232-6.
16. Parviz BA, Shen TT, Ho H. Functional contact lens with integrated inorganic microstructures. Abstract presented at ARVO 2008 A589/4783, April 30, 2008.
17. Parviz BA. Current and new technologies for 24-hour monitoring of intraocular pressure. Abstract presented at ARVO 2010, May 4, 2010.

Dr. Shovlin is a clinical editor for RCCL and a senior optometrist at the Northeastern Eye Institute in Scranton, Pa. He is past chair of the American Academy of Optometry Section on Cornea and Contact Lenses and of the American Optometric Association’s Contact Lens and Cornea Section.

Kristen Taddie is a fourth year optometry student at the Pennsylvania College of Optometry at Salus University. Her areas of special interest include anterior segment disease and pediatric optometry.