The antitumor activity found in such ring systems as the purine, pyrazolo [3,4-d] pyrimidine, and the 8-azapurine prompted the preparation of the imidazo [4,5-d] pyridazine ring system. Castle and Seese were one of the first to study this ring system. 4-Aminoimidazo[4,5-d]pyridazine possessed some anitumor activity. This antitumor activity suggested the preparation of the 4,7-disubstituted-aminoimidazo{4,5-d]pyridazines reported here. The antileukemic activity reported for 6-purinethiol led to the synthesis of several 4- and/or 7-mono-and disubstitutedthioimidazo[4,5-d]pyridazines
