172 SECTIONAL ADDRESSES. 



Dyes. 



Many dye substances have been used in medicine : as they are readily 

 adsorbed on to cell surfaces the concentration here is always high and the 

 surface properties of such cells are often modified in consequence. One 

 general principle which follows from this is that widely different dyes 

 often possess properties in common, for example, that of antiseptic ; 

 to-day I propose to refer only to one group, the benzidine dyes. 



Benzidine 



Trypan red and trypan blue belong to this group. Trypan blue was 

 employed by my colleague, Prof. Nuttall, in piroplasma infections in 

 animals, with results that most South Africans are well acquainted with. 

 Afridol violet, a derivative of diamino diphenyl urea, and some allied dyes 

 have also a powerful action on piroplasma. 



How these substances act is not known, for like the organic arsenicals 

 itiey do not kill the parasite in vitro. They have the property, however, 

 of being adsorbed to the blepharoplast of the trypanosome ; this adsorp- 

 tion is associated with diminished virulence of the parasite in infected 

 animals, and after successive inoculations through several animals the 

 organ may disappear. This direct action of a drug on a tissue, causing 

 ultimately the complete disappearance of that tissue, is so remarkable 

 that it is worthy of notice, as it represents the first known action of the 

 kind. 



The most valuable member of the afridol-violet group so far produced 

 was first made in the Bayer laboratory, but its composition was kept 

 secret. It was, however, subsequently synthesised and its formula 

 published by Fourneau, but only after long trials and infinite patience. 

 The chemo-therapeutic index of this substance has the remarkable figure 

 of 200 to 300 ; and as little as 1/32 mgrm. will sometimes cure mice infected 

 with trypanosomes. Fourneau has made many allied substances and 

 derivatives of ' 205,' he has modified the wings of the molecule by the 

 addition of various side-chains, sometimes keeping the wings identical and 

 sometimes changing them by joining two different complexes through 

 the agency of phosgene. The number of such derivatives is obviously 

 legion, and this makes it the more remarkable that he should have 

 succeeded in synthesising ' 307,' which at the present time is superior as a 

 therapeutic agent to all other dyes in trypanosomiasis. 



NH.CO j-'^ ,/\ CO.NH 



SOgNa I I J CH3 CH3 



NH NH 



SOgNa I X J CO CO 



/ \ SO'aNa 

 NH.CO.NH I J 



Bayer ' 205 ' Fourneau ' 307 ' 



