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[Differentiation therapy of acute promyelocytic leukemia: two successful cases of remission induction by all-trans retinoic acid].

We described two pediatric patients with acute promyelocytic leukemia (APL) who were successfully induced into complete remission with all-trans retinoic acid (ATRA, 45 mg/m2 per day) after failing on conventional chemotherapy. Initial response was observed as correction of DIC within a week of treatment. Hematologically, initial increase of maturing leukocytes reached a peak peripheral WBC count on the 16th and 20th day, respectively. However, these seemingly differentiated leukocytes retained Auer body and dysplastic features and there was no concomitant recovery of erythroid and megakaryocytic lineages at this point. A sudden drop of leukocyte counts after this peak made a brief period of leukopenia before the complete remission was finally attained morphologically in 4-5 weeks. Thus, remission of APL by ATRA therapy consisted of a two-phase course. In one patient, we observed an increase of histiocytes phagocytizing leukocytes in the marrow during the recovery from leukopenia. It is, therefore, postulated that the two-phase course of recovery may reflect the differentiation of leukemic cells by ATRA and subsequent clearance of senescent cells by the reticuloendothelial system followed by regeneration and differentiation of residual normal hematopoietic stem cells.