5-Aza-2'-deoxycytidine (
Decitabine) is an analog of
deoxycytidine now entering clinical trials in
acute myeloid leukemia (AML) owing to a defined antileukemic activity mediated at least in part by
DNA hypomethylation, altered gene expression, and induction of cell differentiation. In the present study, we examined the relationship between the in vitro sensitivity to
Decitabine of blast progenitors and the clinical outcome, in nine AML patients treated in vivo with
Decitabine within a phase II trial carried out at two different institutions. Leukemic blast progenitors in
acute myeloid leukemia (AML) undergo terminal divisions giving rise to colonies in
methylcellulose. The self-renewal capacity of blast progenitors is conversely reflected by a secondary
methylcellulose assay after exponential growth of clonogenic cells in
suspension 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, failed on
Decitabine treatment in vivo. Two subjects, whose blast progenitors in
suspension culture were significantly inhibited by
Decitabine, obtained a positive hematological response (complete or partial remission, CR or PR) and an additional patient showing a similar in vitro pattern died in induction with an hypoplastic marrow without morphological evidence of persistant
leukemia. Interestingly two patients displaying an unfavourable in vitro pattern (i.e. a minor suppression of self-renewal mitoses as evinced from
suspension cultures) achieved a hematological response (CR and PR) upon in vivo
therapy with
Decitabine. The in vitro response to
Decitabine of clonogenic progenitors from both these patients shifted to a favourable pattern (i.e. major suppression of self-renewal versus terminal mitoses) following manipulation of culture conditions by the addition or removal of exogenous
growth factors. In addition, in a further patient refractory to treatment with
Decitabine in vivo, similar alterations of the culture conditions were unable to modify the unfavourable pattern of response to the
drug in vitro. Our results indicate that the sensitivity of blast progenitors in
suspension cultures strongly correlates with the remission outcome of the patients. From our data, it also appears that alterations of culture microenvironment are able to modify the response of AML blasts to
Decitabine, unveiling the 'hidden' sensitivity of leukemic progenitors to the
drug in cases characterized by a discrepancy between in vivo and in vitro results, i.e. apparent in vitro resistance and favourable clinical outcome.