Nanotechnology: Next big thing?

Key Points

Newark, NJ-Nanotechnology is the newest and perhaps the most exciting and futuristic application in ophthalmology. Three applications of nanotechnology in ophthalmology include gene delivery, cell delivery, and nanosurgery, said Marco A. Zarbin, MD, PhD.

To put the size into perspective, he contrasted the height of an average man (2 billion nm) and the width of a red cell (7,000 nm) with the width of a strand of DNA (about 2 nm).

Polypexes are nanoparticles, according to Dr. Zarbin, that are complexes of cationic polymers and DNA, which is negatively charged.

"They can have a transfection efficiency comparable to that of viral vectors," Dr. Zarbin said. "The advantages over the traditional nonviral approaches, such as lipofection, are that polypexes are small and easy to prepare, they have a large vector capacity, are stable in nuclease-rich environments, and they deliver genes to dividing and nondividing cells. To the best of our knowledge, they are nontoxic in the eye."

However, some polypexes have low transfection efficiency, and the duration of gene expression can be short.

Xue Cai and associates exploited the use of polypexes to treat a form of retinitis pigmentosa in the retinal degeneration slow (Rds) mouse model. The mouse has a mutation in a photoreceptor protein, peripherin 2. The investigators developed a nanoparticle (~100 nm long and ~8 nm wide) to deliver normal mouse peripherin 2 DNA to the photoreceptors. Electrophysiological and histological studies performed ~120 days after treatment showed that in the animals with the nanoparticles injected, photopic b-wave amplitudes and outer nuclear layer thickness and architecture were greatly improved. The findings indicated that cone degeneration was attenuated in the treated animals.

"This very simple idea of a nanoparticle can be made more complicated and better," Dr. Zarbin said.

He described the work of James Leary, PhD, and colleagues, who developed a multilayered magnetic nanoparticle.

Using this technology, the outer layer targets the nanoparticle to the desired cell, and an inner second layer targets the nanoparticle to the desired subcellular organelle; a third layer has a biosensor that turns therapeutic gene expression on or off depending on the microenvironment that is present. The innermost layer is the superparamagnetic iron oxide core, which can be imaged.

"This particle operates like a nanomachine that only delivers a treatment if there is a problem present," Dr. Zarbin said. "This device can be both diagnostic and therapeutic."

Prow, Lutty, and coworkers have provided an example of the use of nanotechnology to create biosensors for health maintenance. This team developed a biosensor DNA tethered to a magnetic nanoparticle.

The biosensor is based on an enhanced green fluorescent protein (EGFP) reporter gene driven by an antioxidant response element (ARE), which normally is activated in the setting of oxidative stress and enhances the expression of genes downstream in its sequence. This engineered nanoparticle penetrates endothelial cells, and exposure of the cells to hyperoxia drives the expression of EGFP. After subretinal injection, these biosensor nanoparticles reported the activation of the ARE in diabetic rat retinal pigment epithelium cells.

The antioxidant biosensor could provide a means for clinicians to identify patients likely to need therapy at a time before clinical manifestations of severe disease are evident. By coupling a therapeutic gene to the ARE (instead of a reporter gene such as EGFP), one can create a combined diagnostic-therapeutic device that enables endothelial cells (or any cell that takes up the nanoparticle) to "treat themselves" in the setting of oxidative damage.