ACCESS OF DRUGS TO ORGANS 63 



taken as indicating that there need be any hesitation in 

 applying the conclusions derived from studies of cell 

 permeability to practical problems. What is necessary in 

 such circumstances is to keep an alert eye for the com- 

 plications which may arise from failure to obey the laws 

 of thermal diffusion across membranes. 



Problems of the Access of Drugs to Organs 



The permeability problems involved in the study of drug 

 action are far from limited to those encountered in the 

 study of the permeability of cell membranes. Frequent- 

 ly, the problem of access to an organ, or of absorption 

 from the digestive tract, constitutes a more important 

 difficulty than permeation into cells of either the host 

 or the parasite. The main practical problems which tend 

 to arise are: i. inadequate adsorption from the digestive 

 tract, 2. peculiar permeability properties which prevent 

 a drug reaching a particular organ, e.g. the brain, 3. se- 

 curing an effective concentration in one organ may in- 

 volve a toxic concentration elsewhere. 



From these difficulties there are at present three gene- 

 ral procedures to which resort may be made. The first 

 of these is to modify the rate at which the drug may 

 permeate an organ by simple diffusion. Thus if a drug 

 is needed to penetrate the central nervous system, an 

 increase in its lipoid solubility should be sought, where- 

 as if permeation of the central nervous system is an 



