ALL-TRANS-RETINOIC ACID POTENTIATES THE INHIBITORY EFFECTS OF INTERFERON-ALPHA ON CHRONIC MYELOID-LEUKEMIA PROGENITORS IN-VITRO

All-trans retinoic acid (ATRA) has recently been shown to synergise wi th the inhibitory effect of interferon alpha (IFN alpha) on the growth of malignant cells isolated from solid tumors. We investigated whethe r ATRA could potentiate the inhibitory effects of IFN alpha on the pro liferation of leukemic progenitors in chronic myeloid leukemia (CML). CD34(+) cells from chronic phase, newly diagnosed patients, were incub ated in short-term liquid culture with ATRA, IFN alpha or a combinatio n of both molecules and then plated on semi-solid cultures for colony- forming cell assay. IFN alpha was found to inhibit preferentially the generation of late progenitors. ATRA at a concentration of 10(-8) M wa s found strongly to inhibit CFU-M colonies. Addition of ATRA to IFN al pha dramatically potentiated the inhibitory effects of INF alpha on CF U-GM growth. In the presence of both molecules the inhibition of day 1 4 CFU-GM from CD34(+) cells was lowered to 27+/-4% of control. CFU-M c olonies were completely inhibited. RT-PCR analysis of the colonies res ulting from the action of the combination IFN alpha plus ATRA showed t he presence of an increased number of BCR-ABL-negative colonies relati vely to what was observed with IFN alpha alone. FISH analysis showed a higher percentage of Ph-negative cells in the ATRA plus IFN alpha-tre ated samples, confirming PCR experiments. These results indicate that, in vitro, the combination of IFN alpha and ATRA effectively inhibits CFU-GM colony formation in CML and suggest that it has a potential int erest for the treatment of CML.