NOTES 387 



a spirochete, its activity may remain undiminished. If, however, the parasite, 

 after treatment with the drug, be injected into a living animal, it is immediately 

 killed by the blood of the host. The same result is obtained if the drug be 

 injected into the circulation of an infected animal. To this method of treatment 

 Ehrlich has applied the term Therapia sterilans magna, and by such means it is 

 possible to sterilise the host as far as a particular parasite in question is concerned. 

 The problem of chemiotherapy therefore resolves itself into the discovery of a 

 substance which can be administered in a dose large enough to secure death of 

 the parasite as a result of the combined action of the drug and the tissues of the 

 host, without producing toxic effects upon the host. Amongst the substances 

 which appear to be particularly effective in this respect are certain organic com- 

 pounds of arsenic, notably those in which the arsenic is linked to a benzene 

 nucleus bearing an amino group. Up till quite recently, atoxyl 



NH/ >AsO 

 X — / \ONa 



was much used, and was of considerable service ; but unfortunately it is somewhat 

 too markedly " organotropic," and several cases of optic atrophy resulting in total 

 blindness have been recorded as a result of its use. After extensive researches, 

 in which 605 synthetic organic compounds containing arsenic were tested, ex- 

 cellent results were obtained with the 606th compound, dihydroxy-diamino- 

 arsenobenzene, 



As As 



NH,ly^yNH 2 

 OH OH 



now universally known as " Salvarsan " or " 606." More recently a derivative 

 of salvarsan, neo-salvarsan, has come into use, and although it is rather more 

 unstable than salvarsan, it can be administered with greater ease. 



Several interesting phenomena have been observed during researches on this 

 subject ; from the practical standpoint one of the most important is the acquisition 

 by the parasite of tolerance to the drug. If small doses, insufficient to sterilise the 

 host, are given, the parasites may become increasingly difficult to destroy by 

 subsequent injections ; hence it is important, from a therapeutical point of view, 

 to give the largest doses which can be tolerated in order to ensure immediate 

 sterilisation. For this reason it is clearly desirable to use a drug having as low 

 an "organotropic" tendency as possible. Such an acquisition of tolerance is 

 shown by many parasites. The tolerance so acquired is specific for the drug 

 employed. 



The great practical value of the new therapy has been most clearly demon- 

 strated in connection with syphilis and certain tropical diseases such as yaws 

 (frambcesia), caused by a spirochete allied to that of syphilis. The success which 

 has attended the new treatment of these diseases is remarkable. In the case of 

 soldiers treated for syphilis at the military hospital in Rochester Row, the recovery 

 has been such as to result in the annual saving to the army of a number of days 

 of sickness which is equivalent to the services of a battalion for nearly three 

 months. Even more remarkable results have been obtained in the case of yaws, 

 which can be cured with a single dose of salvarsan. As a result of this treatmen 



