Harnessing Nanotechnology for Novel Ocular Drug Delivery

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You hear “nanotechnology” and think tiny, really tiny. And you’re right. But there’s more to it than size. Nanotechnology is based on the idea that matter has different properties at smaller scales. Cut so-called “bulk” matter down to nano-sized units and the very nature of that matter can shift dramatical­ly, from its magnetic properties to its chemical reactivity and conductivity.^1,2 In ophthal­mology, those attributes can be harnessed for medical use, such as ocular drug delivery.

Pharmaceutical companies have been investigating nanotech-based drugs for de­cades, and the FDA began approving some for medical use more than 20 years ago.³ Of the many types of nanocarriers, nanoparticles are the most common for ocular therapeu­tics due to their biocompatibility and their capacity to be loaded with drugs and gene therapies.⁴ Chemically attached, embedded, or encapsulated in the nanocarrier lies the actual pharmaceutical agent or gene therapy; chemical stabilizers or surfactants are add­ed to the final drug product.¹

Tejal Desai, PhD, at Brown University, said there is a convergence of technologies in the biomedical space that hold the promise to significantly impact the way diseases are treated. “For ocular drug delivery, there have been a number of advances including responsive materials that can both sense and respond to their environment, biohybrid materials that combine synthetic and biologic components, and nanostructured devices that can be finely tuned in terms of drug delivery kinetics and residence time. Addition­ally, advances in stem cell technology and gene therapy coupled with nanomaterials have great promise,” Dr. Desai said.

“Ophthalmologists are quick adopters of new technologies,” said Justin Hanes, PhD, at the Wilmer Eye Institute at Johns Hopkins. “In the future, there will be more and more nanotechnology in therapeutics,” he said.

Defining nanotechnology. Defining nanotechnology is no small feat. The boundaries—and the possibilities—of nanotechnology keep changing. The National Science Foun­dation used to describe it as working with matter smaller than 100 nanometers (nm), but the definition has been revised to include any process that manipulates individual atoms and molecules to build a material or structure with novel properties from the ground up, said Dr. Hanes.

“The pharmaceutical field usually considers things ‘nano’ if they are 1 nm to about 500 nm in size, especially if the size imparts new properties to a drug, such as an improved ability to penetrate a biological barrier like human tissues, mucus, or cells,” he said.

NANOPARTICLES. Nanotechnology-based drug delivery systems, including those that use nanoparticles and liposomes, hold promise for treating eye diseases.

The Sprawling Nano-World

Globally, it’s been a nano-world for years. Nanoparticle technology using lipids is at the heart of COVID-19 vaccine success, for example, said Dr. Hanes. And the 2022 Winter Olympics was a play­ground for nanotech, from skis built from layers of 50-nm polymer film to “quasi water,” which is a 1 nm- to 100 nm-thick layer between figure skates and ice, developed to optimize the gliding properties of ice.⁵

FDA-approved. Ophthalmic nanomedicines include the dry eye drug Restasis, a nanoemulsion FDA-approved in 2002,¹ and Luxturna, gene ther­apy for vision loss FDA-approved in 2017, made with a viral nanocarrier.⁶ And nanotech-based drug delivery platforms for anterior segment eye disease include 10 drugs FDA-approved between 2002 and 2021, targeting dry eye, post-op pain and inflammation, bacterial infection, conjunc­tivitis, and glaucoma.¹ Numerous nanocarriers are currently under investigation, and they aim to deliver drugs that can move through ocular barri­ers to reach target cells. Many drugs, for instance, don’t permeate the cornea easily and nanotech can enhance solubility. Many animal studies have been conducted over the past couple of decades that explore nanomedicines to treat glaucoma.⁴

The United States has invested more than $40 billion in the National Nanotechnology Initiative to develop nanoscale technologies in medicine, biomedical engineering, biochemistry, biophys­ics, biology, and materials science—all central to nanomedicine research in ophthalmology.⁵

Mimicking Viruses

Scientists who are developing nanotechnolo­gies for ophthalmic drugs have taken a cue from viruses, which have an uncanny ability to worm their way into cells. “I consider a virus ‘nature’s nanotechnology,’” said Dr. Hanes. “Viruses are nanoscale and package genetic material specifical­ly in ways that enable their delivery into certain cells,” he said.

Biocompatible and long-lasting. Dr. Hanes said he and colleagues are working on a lipid nanoparticle technology—fat particles with new chem­istries that can get drugs to different cell types or help them more successfully move through barri­ers such as the tear film, cornea, and conjunctiva. They have potential benefits related to safety, ease of manufacturing, size of the genetic payload, and reduced immunogenicity. “The goal is to make them as biocompatible as possible, where you don’t generate immune responses but still get efficient delivery,” he said.

Dr. Hanes also works with polymeric nanoparticles, which mimic what viruses do. Both viruses and these new nanoparticles can make genetic material more compact, enabling this material to gain access to specific cell types based on the particles’ chemistry and size, and helping it enter target cells in canny ways so the nanoparticles aren’t destroyed by the target cell—all with the aim of expressing their genetic payload within that target cell, Dr. Hanes said. A key goal for ocular gene therapy is longer-lasting therapies, such as “a gene therapy with one injection, [making] your eye produce anti-VEGF molecules for five years or longer,” he said.

Topical Nanocarriers

Topical medications lacked innovation for years, with older eyedrops in suspensions carrying drug particles too large to efficiently penetrate the tear film, said Dr. Hanes. “We’ve discovered that making suspension drugs nano-sized—and coating them with polymers for a nonadhesive, slick surface—allows the drug to reach the surface of the eye more efficiently for enhanced drug delivery. Another advantage is that these very small nanoparticles have a high surface-area-to-volume ratio,” he said, likening the way it works to cutting a bar of soap into tiny pieces, so the drug is more readily dissolved and available for penetration.

That advance led to the FDA-approved brandname drug Lotemax—made from the steroid loteprednol—for post-op pain and inflammation and used off-label for dry eye disease. “It was the first steroid that could be given twice a day versus the larger ‘micro-form’ of the drug at four times a day,” Dr. Hanes said.

Gel-based drugs. His team is also developing a hydrogel made of interlocking micelles, a type of nanocarrier made from polymers with both hydrophobic and hydrophilic parts.⁶ “By osmotic pressure, the eye absorbs the water in our low-salt formulation, bringing the polymers and the drug with it, causing the polymer to concentrate and form a very thin, transparent gel layer that coats the surface of the eye,” he said. This unique gel is so thin that the eyelid glides right over it, allowing the drug trapped in the gel to slowly release over time, leading to enhanced ocular uptake.

Dr. Hanes and colleagues have published research showing the potential to deliver drugs to the back of the eye.⁶ “We showed that, in large animals, you could deliver effective drug concentrations to the back of the eye with a topical formulation, which is one of the holy grails of the field,” he said.

FRONT OF THE EYE. An illustration shows the structure of liposomes functionalized with maleimide groups and their possible reaction with thiol groups present on the cornea and conjunctiva.

Payload-Agnostic Gene Therapy

The ability to make some types of nanotechnology “payload-agnostic” means they can be used to deliver a variety of therapies, similar to COVID-19 vaccine platforms that can carry multiple iterations of mRNA. Payload-agnostic gene therapy involves the use of biodegradable nanoparticles to safely and effectively deliver genes to the retina in a flexible way, said Jordan J. Green, PhD, at Johns Hopkins University School of Medicine.

“This technology is based on a nonviral gene therapy platform that, once validated, could be used for different ocular diseases in a widespread way,” said Dr. Green. Nonviral gene therapy has no limitations when it comes to cargo size, and particles are able to reach photoreceptors and have very widespread transfection of photoreceptors, he said. “Transfection” refers to transferring genetic material into a host cell with a nonviral vector; “transduction” uses viral vectors.

Nonviral solutions. Historically, viruses were needed to introduce DNA into a cell. “What’s emerging now is that you don’t need a virus, so you don’t have the manufacturing challenges, expense, or safety concerns with the immunogenicity of a virus,” said Dr. Green. Biodegradable polymer-based nanoparticles are nontoxic, allow for repeated administrations, and often have high durability. A single injection might last a year or longer. “In our experiments, we have not yet seen a time point where the delivered gene’s expression has stopped, which lets us look at disease in a new way,” Dr. Green said.

Dr. Green’s team successfully delivered gene therapy to photoreceptors and retinal pigment epithelium, using nonviral nanoparticles via suprachoroidal injection in a rat model.⁷

Photoreceptors are notoriously hard to transfect, said Dr. Green. But if you can ferry a nonviral plasmid (a large genetic molecule that doesn’t easily penetrate a cell’s nucleus) into photoreceptors, “the nanoparticles could degrade within days, but the plasmid could remain long-term, continuing to express its gene product, without changing the host genome,” he added.

In the back of the eye, the gene you’d want expressed might be for a rare disease like Stargardt disease or a prevalent disease like AMD, said Dr. Green, adding, “There are infinite possibilities with a nonviral method that’s nonimmunogenic, where you could potentially dose again to titrate the protein being expressed.”

Photoreceptor Tropism and mRNA

When a new type of cell in the eye—such as pho­toreceptors—can be accessed using nanotechnol­ogy, it unleashes the potential for new therapies. For more than five years, Gaurav Sahay, PhD, at Oregon State University in Corvallis, said he and colleagues have been working to deliver mRNA to the back of the eye. To date, they have successfully delivered lipid nanoparticles carrying mRNA to photoreceptors in nonhuman primates.⁸

Using a phage display library (a lab technique to study proteins and peptides), they have identi­fied novel peptides that can reach the back of the eye after an intravitreal injection, said Dr. Sahay. His research used these peptides like a “zip code” on a nanoparticle, helping direct it to photorecep­tors to see gene expression.

A first. “This is the first time a peptide-guided lipid nanoparticle was able to deliver mRNA with substantial expression to the photoreceptors in a nonhuman primate after subretinal injection,” he said. It took four years of research to reach the photoreceptors effectively, eventually using a lipid nanoparticle that can protect fragile mRNA from outside stresses while being able to penetrate pho­toreceptor cells, he said.

The main challenge is delivery, since these 100-nm lipid nanoparticles have their own tropism, said Dr. Sahay. “Most of their tropism is to epithelial cells, not neuronal cells like photoreceptors. But we identified a peptide that went into the photoreceptor region and then coated that peptide on the lipid nanoparticle to help deliver it to the photoreceptors,” he said.

Because there are roughly 300 inherited retinal-degeneration mutations, his team is researching which targets to hit first, with hopes of starting human trials in two years.

Prodrugs and Stealth Polymers

Nanotechnology helps drugs penetrate ocular barriers to access parts of the eye that historically have been difficult to reach. With conventional eyedrops, the amount of drug getting to the eye is less than 5%, said Vitaliy Khutoryanskiy, PhD, at the Reading School of Pharmacy, in Reading, United Kingdom. “My research group is developing socalled ‘sticky formulations’—sticky liposomes—as drug nanocarriers for ocular delivery that will stay longer on the ocular surface, so the drug molecules can get absorbed more efficiently for higher concentrations.”

Staying power. A polymer called polyethylene glycol (PEG) is an example of one such formulation. It creates a coating that allows nanoparticles to penetrate the mucus barrier of the eye, said Dr. Khutoryanskiy, who noted that he was inspired by Dr. Hanes’ research at Johns Hopkins developing mucus-penetrating nanoparticles. “If particles are coated with a neutral polymer, it helps them penetrate better,” he said.

In the formulation of lipidic prodrugs—where the drug is covalently bound to a lipid moiety—drug molecules are chemically modified in order to penetrate the eye’s mucus barrier. For the IOP drug latanoprost, for example, Dr. Khutoryanskiy said, “The charged groups of the drug molecules are modified chemically to make them noncharged, and once these molecules start penetrating the cornea, the enzymes in the cornea cleave that bond and the active ingredient gets released.”

Does this polymer chemist consider his work nanotechnology? “Sometimes yes, sometimes no,” said Dr. Khutoryanskiy. “Not all scientific conven­tions consider polymers nanotechnology because even though they’re nano-sized, polymers aren’t particles; they’re molecules.” Still, with nanotech definitions in flux, he said, “I see exciting promise in the capability of nanocarriers to cross the ocu­lar barriers efficiently to deliver drugs.”

Nanoparticles for Sustained Release

“We have been developing an injectable nanostuc­tured polymer-based device that can deliver both small and large molecule drugs with zero order kinetics—that means we can design the device to release approximately the same amount of drug every day for several months. This device has nanopores that finely control the passage of thera­peutics and allow for this sustained delivery,” said Dr. Desai. The device is also designed to degrade once the payload is depleted, so there is no need to retrieve the device at the end of its lifetime.

Qingguo Xu, PhD, at Virginia Commonwealth University in Richmond, is designing a long-last­ing nanoparticle to prevent corneal transplant rejection. Corneal graft rejection occurs in up to 50% of patients, even after taking steroid drops multiple times a day. Dr. Xu, collaborating with Dr. Hanes at Johns Hopkins, is exploring new ways to deliver the existing corticosteroid dexametha­sone sodium phosphate (DSP)—often used as a primary post-op injection. The challenge is to design a unique nanocarrier to address the water-solubility of the steroid and improve its efficacy.

Some ophthalmology products are already made with the polymers PLA and PLGA—aka poly(DL-lactic acid) and poly(DL-lactic-co-glycolic acid), said Dr. Xu. Both PLA and PLGA are FDA-approved, biodegradable polymers with longtime safety records, essentially the bulk-material matrix that nanoparticles are made from, he said.

“Typically, it’s very difficult to put a water-soluble drug into hydrophobic PLA or PLGA particles,” said Dr. Xu. To address the problem, he loaded the DSP steroid into a biodegradable polymer nanoparticle and designed a patentable encapsulation process for it.

Preclinical success. In preclinical animal stud­ies, a single subconjunctival injection of his PLA-DSP-loaded nanoparticles provided sustained drug concentrations in rat cornea and aqueous humor for six months without ocular toxicity. These steroid shots of PLA-DSP-loaded nanopar­ticles both prevented and reversed early signs of corneal graft rejection in rats without eyedrops or other treatment.⁹

“We can achieve very sustained drug levels, without a high spike or low trough, for better compliance and therapeutic drug levels,” he said.

From Lab Bench to Clinic

How will these nanotech breakthroughs translate into clinical care? “We’re now in an era of genetic medicine, and we’re going to see a lot of new gene-based medicines for blindness,” said Dr. Green, noting that medicines don’t have to be small-molecule drugs anymore. They can be large DNA or RNA molecules. And, Dr. Green added, “Nano­medicines in the next five to 10 years will be very durable treatments, where one injection repairs the deficit or treats disease in a long-lasting way.”

Some targets are especially promising for nanomedicine. “To me it’s in retinal diseases, with nanoparticles administered in the front of the human eye that can move to the back of the eye through different transport mechanisms, or are injected suprachoroidally,” said Dr. Green.

Nanomedicine’s holy grail remains delivering therapeutics to the back of the eye without injec­tions. Dr. Khutoryanskiy said, “Nanotechnology will be very promising if large molecules, especial­ly biological molecules like genes and monoclonal antibodies, are delivered via drops rather than injections.”

“We researchers want to introduce new parti­cles, but FDA-approval requires massive testing and funding,” said Dr. Khutoryanskiy.

Dr. Desai said it has been “a tremendous chal­lenge” to deliver safe and effective drugs to the back of the eye, but one that is poised for new solutions—and nanotechnology will be a key approach.

“One of the things that will be important is to understand the ocular microenvironment better, specifically how materials can be designed to be better tolerated in the eye and less immunogenic,” Dr. Desai said. “We also need to be cognizant of the costs and accessibility of our approaches in this space so that solutions can be widely de­ployed,” she added.

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Meet the Experts

Tejal Desai, PhD Sorensen Family Dean of Engineer­ing and Professor of Engineering at Brown University, Providence, R.I. Relevant financial disclo­sures: Oculinea: P,PS; Encellin: P,PS.

Jordan J. Green, PhD Director of the Biomaterials and Drug Delivery Laboratory and Professor of Biomedical Engi­neering, Ophthalmology, Oncol­ogy, Neurosurgery, Chemical & Biomolecular Engineering, and Materials Science & Engineering at the Johns Hopkins University School of Medicine, Baltimore. Relevant financial disclosures: Cove Therapeutics: EE,PS,S.

Justin Hanes, PhD Director of the Center for Nano­medicine at the Wilmer Eye Insti­tute of Johns Hopkins University School of Medicine. He is the Lewis J. Ort Endowed Profes­sor of Ophthalmology, and is Professor of Biomedical Engi­neering, Chemical & Biomolec­ular Engineering, Neurosurgery, Oncology, and Pharmacology & Molecular Sciences at the Johns Hopkins University, Baltimore. Relevant financial disclosures: Graybug: P; Kala: P; NovusBio: EE,P,PS; Novus Vision: EE,P,PS.

Vitaliy Khutoryans­kiy, PhD Professor of Formulation Science and Royal Society Industry Fellow at the Reading School of Pharmacy, University of Reading, United Kingdom, and a polymer scientist for pharma­ceutical applications. Relevant financial disclosures: European Union’s Horizon 2020-MSCA-

RISE-2018/823883: Soft Bio­compatible Polymeric NANO­structures: A Toolbox for Novel Generation of Nano Pharmaceu­ticals in Ophthalmology (NanoPol): S; MC2 Therapeutics: S.

Gaurav Sahay, PhD Professor of Pharmaceutical Sciences, Director of the Sahay Lab, and Codirector for the Center of Innovative Drug Delivery and Imaging at the College of Phar­macy at Oregon State Universi­ty, Corvallis. Relevant financial disclosures: EnterX Bio: EE,PS.

Qingguo Xu, PhD Associate Profes­sor of Pharmaceu­tics at the School of Pharmacy, and Associate Professor of Ophthal­mology at the School of Medi­cine at Virginia Commonwealth University in Richmond. Rele­vant financial disclosures: None.