Can we really put an implant inside the eye to deliver drugs to treat eye diseases?

TeachableMedicalNews article 06152022

Teachable moment in classrooms:

1. cellular basis of life chapter – diffusion as a means of molecular movement
2. special senses chapter – the photoreceptors rods and cones are in the retina
3. special senses chapter – liquid-filled chambers of the eye
4. special senses chapter – sclera, the white of the eye, is the toughest, outer layer

Can we put an implant inside the eye to deliver drugs to treat eye diseases?

The news item:  Recently a report appeared about a drug, Susvimo, that reverses blindness:

New technology helps Georgetown veteran restore his eyesight

If you’re living with blurry vision, there’s a chance a new device can help you get your eyesight back without frequent visits to the doctor. The newly FDA-approved Susvimo implant helped one Georgetown veteran preserve his vision after being diagnosed with wet age-related macular degeneration.

The article states that AMD (wet, age-related macular degeneration) is the leading cause of blindness over the age 60, that this disorder is caused by growth and scarring of blood vessels under the retina, and that drug-delivery through an implant has restored vision in 90% of the treated individuals.

So, Why Do I Care??  Eye diseases can rob people of their vision. Many of such diseases are treated by injection of a drug into the eye. It is estimated that there are over 10 million such injections per year in the USA. Because chronic eye disorders require several injections into the same eye, it is worth exploring alternative delivery systems for long-term administration of drugs.

Plain English, Please!!!   First, let’s review the action of Susvimo, the drug we will use as an example to learn about implants inside the eye. In a previous post of Teachable Medical News (TMN 05252022), we described the action of the drug Susvimo. Here is a brief summary:  Normally the light-sensing cell in the eye, the rods and cones, receive oxygen and nutrients from a thin sheet of cell called the retinal pigment cells. In the disorder macular degeneration the retinal pigment cells are damaged by newly formed, unwanted blood vessels, and the rods and cones in the macula die, and can not make nerve impulses from the center portion of our visual field.  The drug Susvimo reverses blindness by slowing down new blood vessel growth near the retinal pigment cells.

Second, let’s talk about Susvimo delivery into the eye. Susvimo needs to be delivered close to the blood vessels of the retinal pigment cells. During the injection the needle is pushed through the sclera, the white of the eye, and a small droplet of the drug is pushed out of the needle. The inside of the eye is like the space inside of a balloon, and the rear portion of the space is called the vitreous chamber. This chamber is filled with a jelly-like liquid called the vitreous fluid. After the injection, Susvimo is expected to spread throughout the jelly-filled space towards the blood vessels of the retinal pigment cells, because diffusion will force the drug molecules to move from the place of high concentration (the release point at the end of the needle) to the place of low concentration (throughout the jelly-like fluid). Movement of molecules by diffusion is always directed such a way that the molecules move from a crowded (high concentration) place to a less crowded (low concentration) place. Have you ever felt crowded in an elevator, in a store, or in a school hallway, when you just wanted to get some personal space? Then you felt exactly how molecules feel. Molecules, too, prefer more elbow room, so they, naturally, move to a less crowded area.

Third, let’s talk about how an implant helps to administer Susvimo for long periods of time. Susvimo needs to be injected into the eye once a month. Repeated injections through the sclera are an inconvenience for patients. To decrease the number of injections, a thin tube, an implant, is placed into the vitreous fluid through the sclera. To visualize the implant, imagine a straw with a net covering one end. The implant is so thin of a straw that it is only a little thicker than the fine needle used for the injections. By a single injection the implant is filled with enough drug to last for 6 months.  Diffusion acts identically; it moves the drug out from the net-covered end of the implant, and spreads it through the vitreous fluid towards the blood vessels of the retinal pigment cells. This type of implant may be used to administer other drugs as well for long term treatment of eye disorders.