Treatment of severe aplastic anemia with antithymocyte globulin

Eleven patients with acute-onset (less than 2 months), severe aplastic anemia were treated with antithymocyte globulin (ATG) at a total dose between 50 and 420 mg/kg. Median age was 27 (5-74) years. Two additional patients with chronic severe aplastic anemia received ATG but were excluded from analysis after development of bone marrow morphologic and cytogenetic abnormalities suggestive of acute leukemia. Of the 11 analyzed patients, 5 died within 6 months after initial ATG treatment. Six patients, or 54 percent, survived with a minimum follow-up of 24 months and the longest 48 months. Median survival is 42 months. All patients are transfusion-independent although none are completely normal, due to mild thrombocytopenia. The in vitro effect of ATG on pretreatment marrow CFUE was determined in 8 patients and concordance with clinical outcome was observed for only 3 patients. Three patients had no in vitro response and survived, and 2 patients had a positive in vitro response and died. Survival after ATG correlated with maximum percent decrease in absolute lymphocyte count during treatment. No significant correlation was determined for any other parameter. The mechanism of ATG action remains unknown but the clinical response suggests that immune dysfunction may play an important role in the development or prolongation of aplastic anemia, and that this abnormality may be reversible by ATG in some patients.