Progenitor cells from patients with advanced phase chronic myeloid leukaemia respond to STI571 in vitro and in vivo

The Influence of Imatinib Mesylate (STI571) used alone or in Combination with Purine Nucleoside Analogues on the Normal and Chronic Myelogenous Leukaemia Progenitor CellsIn Vitro

Imatinib mesylate (STI571, Glivec), a signal transduction inhibitor used as a single agent demonstrates significant activity in patients with chronic myelogenous leukaemia (CML). Nevertheless, the interaction between STI571 and other antileukaemic drugs such as hydroxyurea, interferon alpha or cytarabine have also been investigated in order to further improve its effectiveness. In this study we have tried to answer the question if the combination of STI571 with purine nucleoside analogues (PNAs)- cladribine (2-CdA) and fludarabine (F-ara-A) intensifies the antiproliferative effect on granulocyte-macrophage progenitor cells (CFU-GM) from patients with CML as well as from normal persons. Our studies were based on the method of semisolid CFU-GM cultures in vitro. We added STI571 or PNAs singly to the culture, each of the drugs at three concentrations, as well as in combinations of the concentrations used. We showed that STI571 (0.5, 1.0 and 2.0 microM) used alone inhibited the colony growth of CML CFU-GM, as compared to CFU-GM derived from normal donors (p = 0.03; p = 0.0004; p = 0.0001). We also observed that STI571 used together with 2-CdA (5,10 and 20 microM) or F-ara-A (0.2, 0.4 and 0.8 microM) at all the combinations significantly inhibited the colony growth of CML CFU-GM, as compared either to the control or to STI571 used alone (p < 0.05). In addition, the differences between CML and normal CFU-GM colony growth inhibition after the use of the combination of the highest concentrations of STI571 either with 2-CdA or F-ara-A were statistically significant (p = 0.03 and p = 0.01, respectively). In conclusion, STI571 used together with both the PNAs had an additive effect on CML CFU-GM cells. However, further experimental and clinical studies concerning the usefulness of these combinations in the treatment of CML patients seem warranted.

Effect of Imatinib Mesylate on Chronic Myelogenous Leukemia Hematopoietic Progenitor Cells

Treatment of chronic myelogenous leukemia (CML) has been greatly enhanced by the development of Imatinib mesylate, a specific inhibitor of the BCR/ABL tyrosine kinase. While it is clear that imatinib effectively targets BCR/ABL positive hematopoietic cells, studies examining its effect on primitive hematopoietic progenitors are much more limited. As CML arises in a primitive hematopoietic progenitor cell, it is especially important to understand the effect of imatinib on these cells. Here we review studies investigating the effect of imatinib on the proliferation and viability of primitive and committed hematopoietic progenitors in CML. We describe evidence that BCR/ABL positive progenitors may persist in patients responding to imatinib and discuss problems of resistance to imatinib. Finally we discuss studies evaluating new approaches to overcome resistance of CML progenitor cells to imatinib.

In vivo effects of imatinib mesylate on human haematopoietic progenitor cells

BACKGROUND

Imatinib mesylate has considerable antineoplastic activity in patients with chronic myeloid leukaemia (CML) and some solid tumours. Although originally regarded as nontoxic for normal haematopoiesis, mild to moderate myelosuppression is a commonly observed side-effect of this treatment. Recently, this molecule has been shown to suppress normal haematopoietic progenitor cells in vitro. This is the first study that has investigated the effect of imatinib on haematopoietic progenitor cells in vivo.

MATERIALS AND METHODS

We investigated the number of circulating haematopoietic progenitor cells in 79 patients with CML and five patients with solid tumours who were treated with imatinib for at least 3 months. Bone marrow progenitor cells were assessed in a subgroup of 18 patients with CML after 12 months of imatinib treatment. Results were compared with haematopoietic progenitor cell numbers of normal controls.

RESULTS

Circulating progenitors of all classes were significantly decreased in CML up to 24 months of imatinib therapy compared with healthy controls (median progenitor cells in CML after 12 months: CFU-GM 62, range 0-2543; BFU-E 216, range 0-3259; CFU-GEMM 0, range 0-139; versus controls: CFU-GM 208, range 50-936; BFU-E 690, range 120-1862; CFU-GEMM 20, range 4-77; P < 0.001). Similar reductions in the number of progenitor cells derived from bone marrow were found in a subgroup of 18 patients with CML. In patients with solid tumours the number of circulating progenitor cells was significantly lower under treatment with imatinib when compared with the controls. Withdrawal of imatinib in a patient with a malignant brain tumour resulted in a prompt normalization of circulating progenitors.

CONCLUSIONS

This study suggests that imatinib exerts myelosuppressive effects through inhibition of haematopoietic progenitor cells.

Why do chronic myelogenous leukemia stem cells survive allogeneic stem cell transplantation or imatinib: does it really matter?

It is generally accepted that allogeneic stem cell transplantation can 'cure' chronic myelogenous leukemia (CML), although occasional patients relapse more than 10 years after the transplant procedure. Such cures presumably result from the combined effects of leukemia stem cells (LSCs) of the conditioning regimen and the graft-vs.-leukemia (GvL) effect mediated by donor-derived T lymphocytes. The advent of imatinib has revolutionized the management of patients with CML, but much evidence suggests that it does not eradicate all LSCs, which theoretically remain a potential source of relapse to chronic phase or advanced phase disease. Moreover, sub-clones of Philadelphia-positive cells bearing mutations that code for amino-acid substitutions in the Bcr-Abl kinase domain can be identified in patients receiving treatment with imatinib and are associated with varying degrees of resistance to this agent. In the present review, we postulate that LSCs, similar to their normal counterparts, may alternate between cycling and quiescent modes. In the cycling mode, they may express Bcr-Abl protein and be susceptible to the acquisition of additional mutations, whereas, in the quiescent mode, they may express little or no Bcr-Abl oncoprotein, cannot acquire additional mutations and are unaffected by imatinib. Thus, a patient who starts treatment early in the natural history of CML, and who responds to imatinib clinically, may not have had the opportunity to acquire additional mutations in LSCs. In this case, the persistence long-term of quiescent 'non-mutated' LSCs despite imatinib treatment might be consistent with freedom from relapse to chronic or advanced phase disease, provided that they remain vulnerable to imatinib when they are recruited into cycle. Conversely, when imatinib resistant Philadelphia-positive sub-clones predominate, this is likely to be due to the recruitment to hematopoiesis of quiescent stem cells that had been in cycle before administration of imatinib and that had acquired additional mutations; in such cases, the best approach to eradication of residual LSCs might be to target expressed proteins thought to be targets for the GvL effect.

Phosphatidylinositol-3 kinase inhibitors reproduce the selective antiproliferative effects of imatinib on chronic myeloid leukaemia progenitor cells

We investigated the role of the phosphatidylinositol-3 kinase (PI-3K) pathway in regulating the proliferation of primary chronic myeloid leukaemia (CML) progenitor cells by using imatinib to inhibit the activity of p210(Bcr-Abl). The effect of imatinib on the expression of PI-3K pathway proteins was investigated by kinase assays and Western blotting; PI-3K was inhibited by wortmannin or LY294002, Jak2 by AG490 and farnesylation by FTI II; progenitor cell proliferation (self-renewal) was measured by growing myeloid colonies in vitro, then replating them to observe secondary colony formation. Suppression of p210(Bcr-Abl) with imatinib indirectly suppressed the activity of PI-3K and its downstream targets (Erk, Akt and p70S6 kinase), thereby implicating the PI-3K pathway in p210(Bcr-Abl)-mediated signalling in primary CML progenitor cells. The PI-3K inhibitors, wortmannin and LY294002 reproduced the differential effects of imatinib on normal and CML progenitor cell proliferation in vitro by increasing normal cell (P = 0.001) and reducing CML cell proliferation (P = 0.0003). This differential effect was attributable to dysregulated signalling by granulocyte colony-stimulating factor in CML. The responses of individual patient's cells to wortmannin correlated with their responses to imatinib (P = 0.004) but not their responses to AG490 (Jak2 kinase inhibitor) or FTI II (farnesyltransferase inhibitor). Individual responses to wortmannin also correlated with responses to interferon alpha (IFNalpha) (P = 0.016). Imatinib-resistant K562 cells were sensitive to LY294002. Inhibition of the PI-3K pathway may be common to imatinib and IFNalpha and reflect dysregulated cytokine signalling. As imatinib-resistant cells remained sensitive to wortmannin and LY294002, targeting the PI-3K pathway may provide an alternative therapy for imatinib-resistant patients.

Infusion of peripheral blood stem cells collected at diagnosis, with maintenance of the treatment, resulted in Ph-negative recovery in a chronic myeloid leukaemia patient in persisting aplasia on STI-571 therapy

In advanced chronic myeloid leukaemia patients, STI-571 produces complete haematological response in most cases and cytogenetic response in up to 50%. However, these patients often suffer periods of pancytopenia, which can lead to life-threatening complications, and is probably due to the small number of residual normal stem cells. We have re-infused peripheral blood stem cells collected at diagnosis, in a patient, while maintaining STI-571 treatment. The patient recovered from aplasia, with Philadelphia-negative haematopoiesis. Discontinuing an effective treatment because of persistent aplasia is a major concern; this method circumvents this problem inpatients who have undergone a stem cell harvest at diagnosis.