Stem Cells in Paediatric Leukaemia Are Resistant to Parthenolide.

Abstract 1429

Poster Board I-452

The concept of cancer stem cells as developmentally early cells that are capable of continued growth and expansion in haematopoietic malignancies and solid tumours has been substantiated in recent years. Consequently these cells may be responsible for disease maintenance and relapse. Acute lymphoblastic leukaemia (ALL) is the most common paediatric cancer with survival rates around 80-85%. However, a significant proportion of patients relapse, often with disease that is highly refractory to further therapeutic intervention. Leukaemia stem cells have been described in childhood ALL that can proliferate to initiate and sustain the disease in vivo. In addition these leukaemia stem cells have also been shown to be refractory to commonly used clinical agents. Therefore it is important to investigate ALL stem cells to understand their biological properties and to identify the most appropriate agents that are capable of eradicating these cells. The sesquiterpene lactone Parthenolide (PTL) has been shown to induce apoptosis in malignant cells by inducing oxidative stress and inhibiting NF-κB activity. Importantly PTL has been shown to be effective against stem cell populations in acute myeloid leukaemia and in chronic lymphocytic leukaemia with minimal effect on normal haemopoietic cells. In this study we have attempted to assess the effects of PTL on stem cell populations in paediatric ALL. Primary cells from 20 childhood ALL cases from mixed prognostic subgroups were used in this investigation. Cells from B-ALL cases were sorted on the basis of expression of CD34/CD19, while CD34/CD7 antigens were used to sort cells from T-ALL cases. Sorted and unsorted populations were co-cultured with and without PTL at 7.5μM and 10μM for 18-24 hours. Subsequently cell viability and apoptosis were determined by flow cytometry using Annexin V and PI staining. Antibodies against phosphorylated IKKα and IKKβ were used to assess NF-κB activity in treated and untreated cells. The functional ability of the treated cells was assessed in some cases using long-term in vitro and in vivo assays. Both concentrations of PTL resulted in a significant reduction in viability in unsorted ALL cells (28±4% and 23±5% respectively). Similar results were observed with CD34+/CD19+, CD34+/CD7+ and CD34- subfractions, with viability reduced to 14-39%. In contrast the phenotypically primitive CD34+/CD19- (85±11% viable) and CD34+/CD7- (83±5% viable) populations were significantly more resistant to 10μM PTL than unsorted cells and other sorted populations (P≤0.002). FISH analyses were performed at the end of the time-course and confirmed that leukaemia cells were surviving PTL treatment. It was not possible to detect phosphorylated IKKα/β in the CD34+/CD19- and CD34+/CD7- populations, in cases examined to date, suggesting NF-kB may not be active in these subpopulations. Of note PTL treatment seemed to have minimal effect on the long-term proliferative ability of ALL cells. There were no significant differences in the absolute cell numbers generated in cultures of PTL treated CD34+/CD19- or CD34+/CD7- cells compared to untreated cells at all time points assayed up to the end of culture at week 6 (P≥0.23). Interestingly, similar results were observed with the unsorted cells and all other sorted populations. From week 3 of culture there was no difference in the absolute cell counts when growth from treated and untreated cells was compared (P>0.47), albeit they proliferated to a much lesser extent than the phenotypically primitive populations. In addition PTL treated cells were capable of engrafting NOD/SCID mice. The levels of leukaemia engraftment obtained using PTL treated unsorted (0.2-5% CD45+), CD34+/CD19- (2-10% CD45+) and CD34+/CD7- (1.5-9% CD45+) populations were similar to their respective untreated controls. These data demonstrate that while PTL showed promising effects on the bulk leukaemia cells, the effects on CD34+/CD19- B-ALL cells and CD34+/CD7- T-ALL cells were insignificant. This may be due in part to lack of NF-kB activity in leukaemia stem cells. However, the functional capacity of every ALL population evaluated in vitro was not significantly impaired by the short course of PTL treatment. These findings further highlight the importance of evaluating new therapeutic agents on leukaemia stem cell populations in addition to the bulk leukaemia and the significance of investigating the functional capacity of drug treated cells.