IN-VITRO RESPONSE OF BLASTS TO IL-3, GM-CSF, AND G-CSF IS DIFFERENT FOR INDIVIDUAL AML PATIENTS - FACTORS THAT STIMULATE LEUKEMIC CLONOGENIC CELLS ALSO ENHANCE ARA-C CYTOTOXICITY

In vivo, growth factors are currently investigated for their capacity to trigger leukemic stem cells into cycle and thus overcome kinetic dr ug resistance. In this study, the susceptibility of leukemic clonogeni c cells to individual growth factors was related to cytosine-arabinosi de (Ara-C) sensitivity. The effects of interleukin-3 (IL-3), granulocy te-macrophage colony-stimulating factor (GM-CSF), granulocyte colony-s timulating factor (G-CSF), and combinations of these recombinant hemat opoietic factors were tested on blast cells of nine acute myeloid leuk emia (AML) patients. Growth factor responses were assessed in semi-sol id clonogenic assay and in a 10-day liquid culture followed by clonoge nic assay. Heterogeneity in growth factor response was observed in bat h test systems, resulting in a variable pattern for individual leukemi as. In the majority of cases (six of nine) the response patterns in th e semi-solid and liquid cultures were divergent. To test the Ara-C sen sitivity, leukemic blasts were exposed in liquid to various concentrat ions of Ara-C in the absence and presence of preselected growth factor s. After 10 days, the number of surviving leukemic colony-forming cell s (CFU-L) was assessed. Exposure to Ara-C in the presence of optimal s timulatory factor(s) resulted in a 3- to 1000-fold increase of the Ara -C toxicity in seven patients. The Ara-C concentrations resulting in 5 0% inhibition of clonogenicity (ID50) were 0.48-123x10(-8) M Ara-C in the absence of stimulatory growth factors, versus only 0.12-0.40 x 10( -8) M Ara-C in the presence of these factors. In two patients, additio n of one or more factors neither increased the number of CFU-L in liqu id nor enhanced the Ara-C toxicity. Even in the absence of growth fact ors the ID50 values in these cases were as low as 0.20 and 0.28 x 10(- 8) M Ara-C and in the same range as the ID50 values observed with maxi mum growth factor stimulation in the other seven patients. These resul ts indicate that Ara-C cytotoxicity can be enhanced by individually se lected, clonogenic cell growth-promoting hematopoietic factors.