Combination of interferon alpha with either Ara-C or ATRA in vitro reduces the selective action of interferon against CML CFU-GM

Abstract

Although interferon (IFN)-α has no specific inhibitory effect on the plating efficiency of granulocyte-macrophage colony- forming cells (CFU-GM) from patients with chronic myeloid leukaemia (CML), it does selectively inhibit the replating ability (secondary colony formation) of CML CFU-GM. Thus, amplification of CFU-GM may be a target for IFN-α and other agents used in the treatment of CML. Here we examined whether cytarabine (Ara-C) or all-trans retinoic acid (ATRA) exert similar effects and whether they might in combination with IFN-α enhance its efficacy. We found that Ara-C preferentially inhibits the formation of CML CFU-GM compared to normal CFU-GM, but this inhibition was not increased by addition of IFN-α. When Ara-C was added to cultures containing IFN-α, the inhibition of replating by CML progenitors was abrogated. ATRA increased significantly the plating efficiency of normal CFU-GM. The addition of IFN-α to ATRA had no effect on CML or normal colony numbers. However, addition of ATRA to cultures containing IFN-α reversed the selective inhibition of CML CFU-GM replating seen in cultures containing IFN-α alone. In four IFN-α/Ara-C experiments, secondary CML patient-derived colonies were examined by fluorescence in situ hybridisation (FISH). All of them were Ph chromosome positive. No significant effects on CFU-GM production were observed when CML primitive haemopoietic progenitor cells were investigated in a delta (Δ) assay. Thus we conclude that combining IFN-α with Ara-C or ATRA neutralises the effect of IFN-α on CML CFU-GM. This observation provides a rationale for treating patients with alternating courses of IFN-α and Ara-C or ATRA, rather than giving either of these two agents in combination with IFN-α.