MEMBRANE PERMEABILITY AND DRUG STRUCTURE 6l 



From the facts which have just been presented we 

 may draw the general conclusion that, subject to certain 

 reservations, the permeability of a cell to a drug can be 

 calculated to a first approximation. E.g. when the struc- 

 ture of a drug is changed by adding or subtracting 

 chemical groupings such as CHg, COOH, or NHg etc., 

 we can calculate the order of magnitude of the change 

 in permeability which will ensue. In theory we can cal- 

 culate the permeability of a membrane provided we 

 know the details about its structure. Thus, for a porous 

 membrane, we need to know the size of the pores, the 

 thickness of the membrane and the numbers of the 

 pores. With a lipoid membrane, we need to know the 

 thickness of the membrane, the lipoid : water partition 

 coefficients of the permeating molecule and the effective 

 viscosity of the lipoid composing the membrane. Table 

 XIV shows the permeability of several cells compared 

 with the permeabilities calculated for a lipoid layer and 

 for a water layer of the same thickness as that of the cell 

 membrane. It will be seen that whereas the permeabil- 

 ity of the lipoid membrane is of the same order of perme- 

 ability as that of the cells, a water layer is about lo^ 

 times more permeable. Since there is no known method 

 by which the viscosity of the interior of a cell membrane 

 can be determined directly, in practice one has to deter- 

 mine it indirectly by determining the permeability to 

 two or three substances. This, however, is in any case 

 necessary before it is possible to decide to which group 



