64 MEMBRANE PERMEABILITY AND DRUG ACTION 



undesirable feature, increasing the polar character of the 

 drug, for example by its administration as a glucoside, 

 is suggested. The second possibility is to modify the 

 structure of the drug so as to increase the probability 

 that it will, or will not, fit into the secretory pattern of 

 certain organs. The combination of a drug with cholic 

 acid may secure a heavy secretion by the hepatic cells. 

 Increasing the polar-nonpolar asymmetry of the struc- 

 ture of a drug is very likely to increase its secretion by 

 the kidney. It may also secure its penetration into the 

 central nervous system, and there are indications that 

 permeation into the mammary gland may be favoured in 

 this way. A thorough study of the so-called blood-brain 

 barrier from this point of view is likely to lead to valuable 

 results, and the economic problem of mastitis in cattle 

 might well yield to a similar study of the secretory ac- 

 tivity of the mammary gland. A third possible mechanism 

 is to administer a drug so that it shall be inactive except 

 at selected sites of action. This mechanism has been 

 used, for example, for drugs involving the grouping 



/ \as=As/ \ Compounds of this type are able to pene- 

 trate into the central nervous system, whereas the sim- 

 ple arsenoxides R^~^AsO cannot do so readily. To get 



a therapeutic concentration of arsenoxide in the central 

 nervous system is likely to involve a toxic concentration 

 elsewhere. But if the drug is given as R/ \as=As/ ^R^ 

 it penetrates into the central nervous system relatively 



