In vitro effects of STI571-containing drug combinations on growth of Ph-posittve myelogenous leukemia-derived cells

CML is characterized by a molecular aberration consisting of fusion bcr-abl encoding for aberrant tyrosine kinase, which is crucial for pathogenesis of CML. In vitro, specific inhibition of bcr-abl tyrosine kinase by a specially designed drug, ST1571 (COP57148B), successfully suppressed the bcr-abl clone, whereas hématologie and cytogenetic remissions were achieved in the majority of patients with chronic-phase CML. In patients with accelerated-phase and blastic-phase CML, STI571 appears less effective. Remissions are shorter and relapses are inevitable in blastic phase. In the present study, we tested combinations of STI571 and drugs with documented activity in CML: cytosine arabinoside (Ara-C), interferon (IFN)-alpha, and homoharringtonine (HHT). The single agents and their combination were studied for their in vitro effect on proliferation of bcr-abl positive cell line KBM-5 (by MTT assay) and on proliferation of primary patkent-derived bcr-abl clonogenic cells in semi-solid culture media. STI571 consistently suppressed bcr-abl positive cells in a dose-response fashion with EDW dose of 0.7 microM. The additive, synergistic, or antagonistic effect of IFN-alpha, Ara-C, and HHT on the in vitro activity of STI571 was then investigated submitting the results to a dose-effect analysis. In the model system used, IFN-alpha alone did not effect the growth of KBM-5 cells. In combination with STI, it failed to modulate its activity. STI57l/AraC and STI571/HHT combinations were more effective in inhibiting KBM-5 cell growth than either drug alone. Addictive effect of these drugs was documented by computerassisted analysis using the Calcusyn software and isobologram analysis. Analysis of HHT/STI571 combination revealed possible synergistic activity at low concentrations. These results were verified in primary CML-derived clonogenic cells in semisolid cultures. Our data documented no antagonism with STI571 plus IFN-alpha, Ara-C, or HHT and support the future use of STI571 combinations in clinical trials for patients with Phpositive leukemias.