Relevance of in vitro leukaemia cell survival to short- and long term clinical outcome in AML.

In ninety-three cases of newly diagnosed acute myeloid leukaemia (AML) we investigated the importance to short- and long term clinical outcome of the in vitro short term leukaemia cell survival as measured by a 4-day MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide)-assay. In 67 patients treated by intravenous remission induction therapy we found that patients who after the first induction cycle or after induction therapy overall achieved a complete remission (CR) had leukaemia cells with significantly lower in vitro cell survival ability than cells of non-responders (p = 0.02 and 0.06, respectively). These relations remained statistically significant in subsequent multivariate analyses. Likewise, a favourable effect of low in vitro leukaemia cell survival on overall survival of the patients was detected in the (largest) subgroup of adult patients treated uniformly by the same remission induction regimen as well as in all patients. However, in the 44 patients, who achieved CR, the in vitro leukaemia cell survival did not show significance to remission duration or time to first relapse. Furthermore, the leukaemia cell survival (MTT-assay) did not to correlate with the Bcl-2 expression level (quantitative flow cytometry) of the leukaemia cells (r = 0.18, n = 34, p = 0.32). In addition, in a cell line model employing the growth factor dependent MO7 human AML cell line, growth factor withdrawal was associated with rapid onset of cellular apoptosis as evaluated by morphology, occurrence of a subG1 peak in DNA histograms, and loss of cellular activity in the MTT-assay. In contrast, a more moderate decline in Bcl-2 expression and gradual loss of ability to exclude the trypan blue dye was seen in the leukaemia cells in response to growth factor withdrawal. We conclude, that the MTT-assay provides a simple and sensitive method for measuring in vitro cell survival. The differences in leukaemia cell survival seen in AML may well be clinically relevant and may help to provide a better understanding of clinical drug resistance.