INVITRO AND INVIVO EFFECTS OF 5-AZA-2'-DEOXYCYTIDINE (DECITABINE) ON CLONOGENIC CELLS FROM ACUTE MYELOID-LEUKEMIA PATIENTS

5-Aza-2'-deoxycytidine (Decitabine) is an analog of deoxycytidine now entering clinical trials in acute myeloid leukemia (AML) owing to a de fined antileukemic activity mediated at least in part by DNA hypomethy lation, altered gene expression, and induction of cell differentiation . In the present study, we examined the relationship between the in vi tro sensitivity to Decitabine of blast progenitors and the clinical ou tcome, in nine AML patients treated in vivo with Decitabine within a p hase II trial carried out at two different institutions. Leukemic blas t progenitors in acute myeloid leukemia (AML) undergo terminal divisio ns giving rise to colonies in methylcellulose. The self-renewal capaci ty of blast progenitors is conversely reflected by a secondary methylc ellulose assay after exponential growth of clonogenic cells in suspens ion cultures. Three out of four patients, in which clonogenic cells in methylcellulose were strongly suppressed by Decitabine and clonogenic growth of blasts cultured in suspension was only slightly affected, f ailed on Decitabine treatment in vivo. Two subjects, whose blast proge nitors in suspension culture were significantly inhibited by Decitabin e, obtained a positive hematological response (complete or partial rem ission, CR or PR) and an additional patient showing a similar in vitro pattern died in induction with an hypoplastic marrow without morpholo gical evidence of persistant leukemia. Interestingly two patients disp laying an unfavourable in vitro pattern (i.e. a minor suppression of s elf-renewal mitoses as evinced from suspension cultures) achieved a he matological response (CR and PR) upon in vivo therapy with Decitabine. The in vitro response to Decitabine of clonogenic progenitors from bo th these patients shifted to a favourable pattern (i.e. major suppress ion of self-renewal versus terminal mitoses) following manipulation of culture conditions by the addition or removal of exogenous growth fac tors. In addition, in a further patient refractory to treatment with D ecitabine in vivo, similar alterations of the culture conditions were unable to modify the unfavourable pattern of response to the drug in v itro. Our results indicate that the sensitivity of blast progenitors i n suspension cultures strongly correlates with the remission outcome o f the patients. From our data, it also appears that alterations of cul ture microenviroment are able to modify the response of AML blasts to Decitabins, unveiling the 'hidden' sensitivity of leukemic progenitors to the drug in cases characterized by a discrepancy between in vivo a nd in vitro results, i.e. apparent in vitro resistance and favourable clinical outcome.