Dye light in the limelight

OKAYAMA, Japan—Patients with hereditary diseases such as retinitis pigmentosa suffer from blindness, but while the photoreceptor cells in their eyes are dead, other neurons remain alive and functional. A research team from Okayama University is exploring a new approach that could utilize those living neurons to send messages to the brain via artificial stimulation by photoelectric dyes that respond to light.

 

The researchers include Drs. Toshihiko Matsuo and Tetsuya Uchida from Okayama University, in collaboration with Kenichi Takarabe, a semiconductor scientist at Okayama University of Science. Matsuo is an associate professor in the department of ophthalmology at Okayama University Medical School, while Uchida is a polymer science engineer in the same department.

 

The trio has been pursuing this work for a number of years. In a 2009 paper of theirs that appeared in the Journal of Artificial Organs, titled “Safety, efficacy and quality control of a photoelectric dye-based retinal prosthesis (Okayama University-type retinal prosthesis) as a medical device,” they detailed the theory of their implanted prostheses approach.

 

“Patients with retinitis pigmentosa lose photoreceptor cells as a result of genetic abnormalities and hence become blind. Neurons such as bipolar cells and ganglion cells remain alive even in the retina of these patients, and ganglion cells send axons to the brain as the optic nerve. The basic concept of retinal prostheses is to replace dead photoreceptor cells with artificial devices to stimulate the remaining neurons with electric currents or potentials,” according to the paper’s abstract. “Photodiode arrays and digital camera-type electrode arrays are the two main approaches for retinal prostheses to stimulate retinal neurons, but these arrays have the problems of poor biocompatibility, low sensitivity and low output of electric currents, and hence have a requirement for external electric sources (batteries).”

 

Using “photoelectric dye-based retinal prostheses that absorb light and convert photon energy to generate electric potentials ... could induce intracellular calcium elevation in photoreceptor-lacking embryonic retinal tissues and cultured retinal neurons,” the abstract added.

 

The team’s prothesis—OUReP (Okayama University-type retinal prosthesis)—consists of two parts, the first of which is a photoelectric dye, 2-[2-[4-[dibutylamino)phenyl]ethenyl]-3-carboxymethylbenzothiazolium bromide. This has an absorption spectra that spans the visible range from 400 nm to 600 nm and is stable and readily synthesized, with a low molecular weight and no obvious toxic components. The dye is coupled to a thin polyethylene film at a concentration of around 10⁶ dye molecules per μm².

 

A thin sheet of OUReP would be implanted into the subretinal space through standard vitreous surgery, the same method used to deal with retinal detachment. The dye molecules serve as both an image (light)-receiver and a neuron-stimulator, which would result in high image resolution. In order to determine the likelihood this treatment would be a success for patients, the team plans to use optical coherence tomography to determine the level of retinal degeneration.

 

In Kelvin probe studies, the team was able to confirm the presence of electric potentials on the film surface induced in rapid response to light, and when the dye was tested in the eyes of rats at the Royal College of Surgeons, cytotoxicity analyses were promising.

 

Kelvin probe measurements of the electric potential on the prosthesis’ surface when exposed to light resulted in rapid responses across the same range of wavelengths as the dye’s known absorption spectrum, and sensitivity to different light intensities was also encouraging. In in-vitro testing of the dye on chick embryo retinal cells, with a fluorescent dye to monitor calcium ions, the researchers found that the dye-stimulated responses triggered increased calcium ion concentrations.

 

When the prosthesis was implanted into the retinas of rat models, the team observed a reduction in apoptosis in the retinal neurons that came into contact with the dye-coupled polyethylene film, OUReP, suggesting that the photoelectric dye could have a neuroprotective effect on retinal neurons. Additionally, when the rats were placed in a drum with spinning walls that were painted in black and white vertical stripes, the rats moved in the direction of the rotating stripes, suggesting some sight restoration.

 

Moving forward, Matsuo and Uchida are preparing a first-in-human clinical trial at Okayama University Hospital in consultation with the Pharmaceuticals and Medical Devices Agency.