Studies with the experimental antitumor agent 4-aminopyrazolo[3,4-d]pyrimidine

~-A P P has been shown to inhibit bacterial growth,?. 18,19 to produce cytotoxic changes in malignant cells in tissue cultures,lo and to possess antitumor activity in several animal neoplasms, including leukemias.*. 18. 22. 23 In a 6-mercaptopurine (6-~~)-resistant line of adenocarcinoma 755, ~-A P P was the only purine antagonist reported to maintain its antitumor effect, indicating that cross resistance to ~-M P may not occur.18 Detailed animal toxicity studies of ~-A P P have been described.'? but only 3 patients have been reported to have received this compound.'* This compound is of general interest in cancer chemotherapy because of its potential antitumor activity in human neoplasms. It is of particular interest in the treatment of acute leukemia since its reported lack of ~-M P cross resistance could possibly result in another drug that would be useful in the control of this disease. It is the purpose of this paper to present data accumulated from animal and human studies of 4-APP. The animal toxicity is evaluated, and detailed information