Interleukin-3 priming in acute myeloid leukaemia patients

Several studies have demonstrated that G-CSP, GM-CSF and, in particular, IL-3 can effectively recruit acute myeloid leukaemia (AML) blasts into the cell cycle, resulting in a significant increase in cytosine-arabinoside (Ara-C) mediated cytotoxicity in vitro. Since IL-3 has shown biological and clinical activity, we investigated the cell kinetic effects of rIL-3 and high-dose Ara-C/idarubicin in three patients with refractory AML selected for the presence of chromosome 7 monosomy; this enabled differentiation between the effects of IL-3 on leukaemic and on normal cells. The in vivo administration of rhIL-3 (250 mu g/m(2) d s.c. for 6-10 d) recruited AML blasts into the cell cycle in two of the three patients, and this effect resulted in an increase in in vitro growth of clonogenic cells (CFU-L) and of their S-phase fraction. The percentage of leukaemic cells with monosomy 7 increased only in the two cases who showed a proliferative response. Normal cells were not recruited, even when rhIL-3 was administered for up to 10 d. In vitro studies showed an increased Ara-C cytotoxicity on clonogenic AML cells, in particular with IL-3 plus GM-CSF, thus confirming the priming effects of IL-3 in the two responding cases. The results of this study suggest that rhIL-3 can selectively recruit leukaemic cells into the cell cycle. Although leukaemic blasts can be sensitized to Ara-C, other mechanisms of primary blast resistance may limit the clinical benefit of kinetic-based approaches.