A characteristic pattern of leukemic cell differentiation without cytoreduction during remission induction in acute promyelocytic leukemia

While differentiation of leukemic cells in vitro is well established, the role of differentiation in vivo is not defined and is generally attributed to differentiation inducers. In five patients with acute promyelocytic leukemia (APL) an unusual pattern of a slow and progressive decrease of immature blasts with a concomitant increase in mature cellular elements was observed following intensive combination chemotherapy. All patients eventually achieved complete remission, four of them without an intermediate phase of marrow hypoplasia. This morphologic pattern of response suggests that leukemic cellular differentiation rather than cytotoxicity was one mechanism involved in remission induction. Cytogenetic studies of remission revealed disappearance of the cytogenetic marker, the balanced translocation between the long arms of chromosomes 15 and 17. This remission induction process appears to be due to the cellular biologic characteristic rather than the therapy used as it occurs in a substantial proportion of patients with APL. It should be considered for optimal diagnostic and therapeutic decisions.