The retina is part of the central nervous system. Delivering medicine into the central nervous system, or to the retina, by traditional means has historically been very difficult; hence the popularity of intraocular injections. By bypassing the blood-brain-barrier (or blood-retina-barrier), drugs are now effectively delivered to the “target” tissue.
The usual methods for treating a disease, be it an infection, hypertension, or even diabetes, is to take a pill. The pill is dissolved in our stomach and the medicine is absorbed into the bloodstream. The bloodstream delivers the pharmaceutical to the target tissue… and bingo! the drug does its thing.
More direct methods require intravenous (IV) delivery for the medicines. A drug is delivered directly into the bloodstream via a vein. This delivery method bypasses the digestive system and does not require absorption. This method is great for delivering higher doses of medication into the body and for delivering drugs that get altered when swallowed.
In the eye, and in the brain, drugs that have been either absorbed, or administered via an IV, get trapped within the blood vessels and are not able to diffuse into the eye or brain. This is called the “blood-brain-barrier.” Essentially, drugs do not get to the brain/retina by normal methods due to the uniqueness of the blood vessels in the central nervous system and the eye.
To beat a dead horse – This is why we are using intraocular injections to deliver steroids and anti-VEGF medications to treat macular degeneration and diabetic retinopathy. These drugs do not get into the eye via the bloodstream. By injecting directly into the eye, we bypass the blood-retina-barrier and put the medicine right where we want it.
What Does This Mean? This is one large reason why there are no pills or medicines to treat macular degeneration or diabetic retinopathy. We can’t get the medicine to the retina. The idea of intraocular injections is relatively new, but has gained wide acceptance as it is highly effective (works better than anything else), is convenient (done in the office), is safe…and is painless.
By directly injecting agents into the eye, we are able to treat the retina with “old” drugs. Steroids are certainly not new, but we have discovered many “new” uses for treating retinal disease simply because we can get the drug to the tissue.
The next generation of injections will be the sustained release drug delivery systems that I talk about once in a while. It shares the same theme as the introacular injections; it bypasses the blood-retina-barrier.