In vivo differentiation of leukaemic blasts and effect of low dose Ara-C in a marrow grafted patient with leukaemic relapse.

The possible induction of differentiation of leukaemic cells by low dose cytosine arabinoside (Ara-C) was studied in an human chimaera with leukaemic relapse. A 35-year-old woman had been grafted with bone marrow from her HLA identical sister and developed a leukaemic relapse of the host type demonstrated by typing phosphoglucomutase 1 isoenzymes. Before treatment with low dose Ara-C only leukaemic cells and a part of the mature granulocytes were of host type. Lymphocytes, platelets and erythrocytes were still of donor type. The leukaemic cells and the mature host type granulocytes disappeared and the patient went into complete remission after treatment with low dose Ara-C. The results indicate that (a) the leukaemic blasts could differentiate in vivo into mature granulocytes, (b) leukaemic transformation was limited to granulopoiesis, and (c) low dose Ara-C acted as a cytotoxic drug instead of inducing differentiation of host type granulopoiesis.