How do you design a drug to be delivered to the CNS?
In a nutshell: in order to pass through the blood-brain barrier (BBB) the substance has to mimick soee properties of the substances that are allowed to pass through. 

There are different types of "transporters" -- integral proteins going through the cell membranes and accounting for the active transport of the substances they can actively bind to. Smaller molecules of drugs can use solute carrier transporter (SLC), whereas bigger molecules (oligo- and polymeres) would use the receptor-mediated transporter (RMT).

There is a nice article on this topic published by ScienceDaily.

Here is the quote:


  One technology for enabling active transport of small molecule drugs
  across the BBB involves targeting endogenous nutrient transporters.
  These transporters are members of the solute carrier (SLC) transporter
  superfamily. Transport of small molecules across the BBB by these
  membrane proteins is known as carrier-mediated transport (CMT).
  
  In order to design drugs that utilize CMT to cross the BBB,
  researchers modify their chemical structures so that they resemble
  nutrients that are transported across the BBB by specific SLCs. The
  prototypical drug that uses this strategy (which was developed long
  before mechanisms of CMT were known) is L-DOPA, the major current drug
  for Parkinson's disease. L-DOPA is used to replace dopamine that is
  lost due to degeneration of dopaminergic neurons in the substantia
  nigra of the brain.
  
  Another major system that is used in normal mammalian physiology to
  enable needed molecules to cross the BBB is receptor-mediated
  transport (RMT). The brain uses RMT to transport proteins, peptides,
  and lipoproteins that are needed for brain function across the BBB.
  Examples of biomolecules that are transported into the brain via RMT
  include insulin, insulin-like growth factor (IGF), leptin,
  transferrin, and low-density lipoprotein (LDL).
  
  In RMT, molecules in the circulation may bind to specific receptors on
  the luminal surface of brain capillaries (i.e., the surface that
  interfaces with the bloodstream). Upon binding, the receptor-ligand
  complex is internalized into the endothelial cell by a process called
  receptor-mediated endocytosis. The ligand may then be transported
  across the abluminal membrane of the endothelial cell (i.e., the
  membrane that interfaces with brain tissue) into the brain. This whole
  process is called receptor-mediated transcytosis.