ACCESS OF DRUGS TO ORGANS 65 



readily, and is there transformed into the therapeutically 

 effective arsenoxide. There are probably many instances 

 where such procedures could be adopted intentionally, 

 instead of by chance, as was the case with the arsenic 

 compounds. Thus a drug administered as a phosphate 

 ester would be likely to be inactive until the phosphate 

 had been hydrolysed away from the rest of the drug. 

 Thus such a compound might well display a high activ- 

 ity only at sites having a relatively high concentration 

 of phosphatase, such as the kidney and bones. If it is 

 true, as has been suggested recently, that tumours have 

 a higher concentration of glucuronidase than normal 

 tissues, it is possible that a drug relatively selective in its 

 action could be obtained by administration of a toxic 

 substance as a glucuronide. In the same way, since the 

 cells of tumours of the prostate are commonly rich in 

 acid phosphatase, a phosphate ester might increase the 

 specificity with which a drug can act upon this tumour. 

 Instances of this sort could be multiplied indefinitely. 

 Optimal results are likely to be obtained in any indi- 

 vidual instance by combining several of the devices 

 mentioned above. For example, if one wishes to obtain 

 a drug which will penetrate relatively well into the central 

 nervous system, one would tend to study the effect of 

 increasing its lipoid solubility, increasing its basic char- 

 acter and increasing its polar-nonpolar asymmetry. 



