B192
In vitro chemosensitivity of leukaemic stem and progenitor cells to gemtuzumab ozogamicin (Mylotarg) in acute myeloid leukaemia
Mays Jawad1, Ullas Mony1, Nigel H. Russell2, Monica Pallis2
1Division of Haematology, University of Nottingham, Nottingham, UK, 2Department of Clinical Haematology, Nottingham University Hospitals, Nottingham, UK
Background
Following remission induction, relapse occurs in 50% of acute myeloid leukaemia (AML) cases and this figure rises to 70% in patients with FLT3/ITDs. Therapeutic targeting of the leukaemic stem and progenitor cells (LSPC) responsible for relapse is essential for the complete eradication of leukaemia. Preliminary data from the MRC AML 15 trial indicated that the addition of Mylotarg to induction chemotherapy improved relapse rates. We hypothesised that this success may be underpinned by Mylotarg specifically targeting the LSPC (CD123+CD34+CD38-) subset.
Results
Using a robust and rapid flow cytometric assay, we showed that at 48hours culture with Mylotarg, a significant reduction in LSPC number (n=20; 33% cell kill; p= 0.02) was achieved and this rose to 42% cell kill (n=11; p=0.005) after 96 hours. In contrast, normal CD34+CD38- cells were insensitive to this agent. We assessed CD33 expression and found a correlation between bulk and CD34+CD38- cell fractions (p< 0.0001; n=20). 15/20 LSPC subpopulations overexpressed CD33 (compared with their normal counterparts) and these responded better to Mylotarg treatment (p=0.03). The FLT3 status of 16 AML samples was investigated for differential response to Mylotarg. Unexpectedly, we found that the LSPC fraction from FLT3/ITD samples were more sensitive to Mylotarg than samples with FLT3 WT (p=0.003).
Conclusion
In conclusion, we have developed an assay for screening drug effectiveness against LSPC and have demonstrated that Mylotarg targets this subset effectively, particularly in samples that are FLT3/ITD positive. Combination drugs with Mylotarg now need to be further investigated for the complete eradication of LSPC.
