The problem with glucosamine is that it reliably increases intraocular pressure.
It is amusing you should say ways that avoid reaching the brain. The blood-brain barrier impedes a large number of therapeutics from entering the brain. It is incredibly difficult to engineer the constraints of something that: is potent enough to have an effect, can survive in the blood, cross into the brain, targets the cells of interest, has a meaningful half-life, does not decompose into something toxic, etc.
https://www.nei.nih.gov/learn-about-eye-health/eye-condition....