Drug Delivery to the Retina Using Intravitreal Aerosolized Nanoparticles

TIMOTHY W. OLSEN, MD

Targeted drug delivery to the retina is an active area of research. A key goal is to provide local drug therapy to the retina while minimizing the systemic levels and systemic toxicity. The best way to understand the pharmacokinetics of drug delivery to the retina is through carefully designed studies using appropriate animal models. Even using the best possible model, human disease states, such as retinal detachments, may alter pharmacokinetics in unpredictable ways. The principles of pharmacokinetics, such as diffusion, variations in diffusional barriers, aqueous-gas diffusional kinetics, and other factors, become critical in predicting therapeutic responses to local delivery.

Recent advances in pharmacotherapeutic options for treating disorders of the posterior pole increase the need for effective delivery methods. Examples of agents that may be useful in treating retinal and optic nerve disorders include anti-vascular endothelial growth factor agents,^1,2 neuroprotectants,^3-5 antioxidants,⁶ corticosteroids,^7-9 and other specific biologic agents (eg, ciliary neurotrophic factor).^10-12

NANOTECH DELIVERY FOR PVR

Proliferative vitreoretinopathy (PVR) is the leading cause of retinal redetachment and requires aggressive surgical methods to correct the anatomic abnormalities.^13,14 Ryan postulated that mechanical intervention (surgical treatment) will be necessary until we can control the cellular processes with pharmacological intervention.¹⁴ Proliferation of dispersed retinal pigment epithelium (RPE) or glial cells from the neurosensory retina leads to preretinal fibrosis, contraction, and PVR with detachment of the retina. Current therapy to address PVR is surgical and involves the use of postoperative tamponade with agents such as perfluoropropane (C3F8), sulfur hexafluoride (SF6), and silicone oil. Investigators have demonstrated that fluorouracil (5-FU) and heparin in the infusion bottle during the fluid phase of vitrectomy in humans decreased the rate of PVR and redetachment.¹⁵ However, the technique described has not gained widespread acceptance.

Intraocular air or gas is commonly deployed in vitreoretinal surgery, due largely to its mechanical properties. Clearly, the intraocular gas bubble serves as a tamponade, holding the retina in place during reattachment surgery.¹⁶ The surface tension of the gas bubble that surrounds a retinal break prevents the fluid from disrupting the development of a strong chorioretinal adhesion.¹⁷ By generating nanoparticles and suspending these particles in the gas phase of the intraocular tamponade, a novel methodology for drug delivery is possible.¹⁸ The obvious example of a target disease is PVR; however, numerous other potential applications of this technology are possible, such as delivery of neuroprotectants during retinal detachment surgery, antiviral agents or antibiotics for infectious retinitis, and immune modulation for the postoperative treatment of uveitis.

Pharmacologic management of PVR requires a measured and reliable means of drug delivery that will allow for better local management of the healing response. If an antiproliferative agent is used during the fluid phase of drug delivery, the agent will diffuse into the space between the torn or detached retina and inhibit the healing response of the neurosensory retina and underlying RPE and choroid. However, if the drug were delivered after the retina has been reapposed to the underlying RPE/choroid, drug would naturally diffuse onto the surface of the neurosensory retina and inhibit the surface proliferation, rather than the deeper chorioretinal adhesion. This is a theoretic advantage to using gas-phase delivery over fluid-phase delivery.

METHODOLOGY AND PHARMACOKINETICS

A methodology and pharmacokinetics for the delivery of aerosolized nanoparticles in the pig model has been characterized.¹⁸ There are 2 distinct methods of aerosol drug deposition (Figure 1). First, the flow-through system utilizes an aerosolized drug passing through the eye continuously, with entry through the primary sclerotomy and exit through the secondary sclerotomy. The exit site has a filter to prevent the drug from entering the room air. This method provides maximal drug deposition because it maintains a higher concentration of intraocular drug to drive diffusion, yet with lower efficiency because of high flow (drug is lost as it exits the eye).

The second method of delivery is the single-fill method (Figure 1). This involves simply filling the eye or injecting the eye with a known concentration of aerosolized nanoparticle gas and allowing diffusion of the nanoparticle suspension into the retinal tissue. This method results in lower tissue levels because of the decreasing, equilibrating concentration of the drug, but occurring at a higher efficiency (the drug stays in the eye and is not lost via the exit sclerotomy).

FIGURE 1 REPRINTED FROM ZHANG G, FENG X, WABNER K, ETAL. INTRAOCULAR NANOPARTICLE DRUG DELIVERY: A PILOT STUDY USING AN AEROSOL DURING PARS PLANA VITRECTOMY. INVEST OPTHALMOL VIS SCI. 2007; 48: 5243-5249. COURTESY OF INVESTIGATIVE OPHTHALMOLOGY AND VISUAL SCIENCE.

Figure 1. Schematic of delivering aerosolized nanoparticles during the gas phase of vitrectomy. The flow-through method demonstrates a continuous flow of gas and drug, with the exit site filtering out drug before entering room air. The single-fill method shows the globe with aerosolized drug, and once full, the sclerotomies are closed.

Figure 2. Schematic demonstrating the formation of nanoparticles into an aerosol. The nanoparticles (*) are generated from a frequency-driven ultrasonic generator (8MHz & RF). The particles are then incorporated into the gas flow, dried, and warmed. The final particles are monitored prior to entry into the gas mixture. The final air mixture can be combined with the selected intraocular gas and infused into the eye.

Both transfer methods of aerosolized nanoparticle delivery have unique advantages. The single-fill delivery method creates a steady state in which the rate of mass deposition in the uvea becomes smaller than the rate of mass transfer from the retina and choroid to the sclera. The single-fill method would be used primarily during pneumatic retinopexy, gas-phase intravitreal injections, or at the conclusion of vitreous surgery. During surgery, the aerosolized nanoparticles can continuously circulate through the infusion, thereby driving higher tissue concentration levels. The single-fill method would be used at the end of the case, in order to supplement and sustain the higher-tissue drug levels achieved with the flow-through method.

MECHANISMS OF DEPOSIT

There are 3 main mechanisms by which aerosol particles can deposit at the retinal surface: inertial impaction, sedimentation, and diffusion.¹⁹ In experimental studies, there does not appear to be a difference in the mass of drug on either the inflow or outflow sides of the retina at any time point.¹⁸ Additionally, evidence suggests there is greater diffusion into the uveal tissues than into the lens, because the lens is less metabolically active and lacks blood flow. Drug delivery in the gas phase of vitrectomy could increase cataractogenesis, dependent upon the specific pharmacologic agent(s) delivered. Overall, the drug would be more selectively deposited into the neurosensory retina and uveal tissues. Logically, increased diffusion of drug removal from the uveal tissue also occurs, as well as diffusion into the sclera and orbital tissues. Also, the use of larger molecules or peptides will likely slow the known diffusional pharmacokinetics.

Figure 3. Experimental setup of the ultrasonic transducer generating particles into an aerosolized form. Note that the larger droplets simply fall (gravity), while the much smaller mist is the material that is sent through the drying apparatus (see Figure 2).

The following technique outlines the procedure for nanoparticle generation (Figure 2). A Pyrex (Corning Inc., Corning, NY) glass baffle is placed in a water bath directly over an ultrasonic transducer. A frequency-generator–driven ultrasonic transducer generates droplets of a particle solution (Figure 3) that are subsequently dried using a silica column. Air is directed into a baffle at a flow rate of 300 mL/min. The air entrains the droplets containing the particles into a silica drying column. The particle size distribution of the dry particles (Figure 4) is determined after isokinetic dilution of the aerosol particles with an electro-mobility analyzer coupled with a scattered light-detection system. The mass output of the aerosol may be determined by collecting the particles on a filter at reduced pressure.

Figure 4. Scanning electron photomicrograph of the approximately 400-nanometer particles that are suspended in the gas phase during drug delivery.

Calculating the pharmacokinetics for the delivery of drugs to the retina and choroid in the gas phase may be better predictive of drug levels through modeling systems. For the flow-through method, a model of aerosol particles of uniform concentration, with the assumption that the deposition occurs by diffusion through a boundary layer, the rate of mass deposition (Δm/Δt) would be given by:

Δm/Δt = DAC/h

where D is the diffusion coefficient of the aerosol particle, A is the surface area of the chamber, C is the aerosol concentration, and h is the boundary layer thickness.

For single-fill delivery, 2 models estimate the deposited mass.¹⁸ A pure diffusion model is mathematically represented. The diffusion model has an expression for the mass deposited with time that provides a solution for the diffusion equation with spherical coordinates:

Herein, M₀ is the initial mass of aerosol in a hollow sphere of radius (R), M is the mass deposited in time, t (time), and D is the molecular diffusivity.

For deposition by settling, stirred settling seems applicable, with an exponential decay in the aerosol concentration. Deposition can be represented as:

M(t) = M₀ [1 – exp(– βt)]

Herein, the deposition rate coefficient, β, is expressed as the terminal velocity of sedimentation divided by the length scale of the enclosure. For the spherical geometry, the following expression for β applies:

In this equation, υ denotes the settling velocity. The corresponding deposition represents a faster rate than that observed in animal systems.

The aerosol delivery of drug nanoparticles represents a novel method to deliver therapeutic agents to the posterior segment during an injection in the clinic or during vitrectomy surgery in the operating room. Drug deposition occurs primarily by diffusion. Careful design of the aerosol generation and delivery parameters plus proper formulation could lead to controlled and sustained release of the therapeutic agents with modification of the PVR healing response. Nanoparticle drug delivery in the gas phase could decrease the postoperative formation of epiretinal membranes and surface proliferation. Aerosolized delivery of drugs to the posterior is a novel methodology for pharmacologic management of many posterior segment disorders.

CONCLUSION

In summary, investigation of the pharmacokinetics of drug delivery to the posterior segment of the eye is an active area of ophthalmic research. Changes in our clinical interventions as we enter the pharmacologic era of treating posterior segment disease will continue to alter our clinical and surgical practices. Significant improvements in visual outcomes and lifestyles for our patients will continue to drive this active area of research. Patients are anxious for continued development of better technology, delivery systems, and therapeutic agents to treat retinal disease. Aerosolized nanoparticle drug delivery may represent a methodology to treat diseases such as PVR. RP

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