MEMBRANE PERMEABILITY AND DRUG STRUCTURE 49 



3. Can the structure of a drug be modified to give pre- 

 dictable changes in permeability ? 

 The answer to the first of these questions varies with 

 the type of a drug which is under consideration. It is 

 well known that the structure of the sulphonamides may 



TABLE III 



THE PERMEABILITY OF THE CELLS OF Chora TO DIFFERENT 



SUBSTANCES 



The figures given are the times taken for the average concentration inside 



a cell to reach 50% of that outside. For comparison similar values are 



given for dead cells, and calculated values for a water cylinder of the 



same size as a Chora cell. (After Collander) 



be altered within very wide limits without destruction of 

 activity. On the other hand among, say, the anti-malarial 

 drugs, only comparatively small changes in structure 

 are possible without loss of activity. Thus each group 

 of substances must be considered as a separate case. So 

 far as can be seen at the moment, unless the structure 

 of a drug can be modified without change of activity, 

 comparatively little can be done to make the best of the 



