Can enough drug get into the brain to be effective?
In rodents the answer is yes. Penetration in humans is probably similar (less than 0.5%), but in most cases, great results in animal models of the human CNS demyelinating diseases, do not replicate in patients.
Is the optic nerve harder to reach than other nerves affected by MS?
Don't think so, perhaps the different expression of LINGO-1 plays a role.