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Nicholas B Heaney
MBChB MRCP DipRCPath
Clinical Research Fellow (funded by Leukaemia Research UK)
Tessa L Holyoake
MB ChB FRCP MRCPath PhD
Senior Lecturer in Academic Transfusion Medicine
Honorary Consultant in Haematology, ATMU
Academic Transfusion Medical Unit: Cancer Division
Section of ï¿½Experimental ï¿½Haematology &
University of Glasgow
Chronic myeloid leukaemia (CML) is characterised by the Philadelphia chromosome (Ph+), a shortened chromosome 22, formed as a consequence of the characteristic translocation t(9:22)(q34;q11). This generates the chimeric oncogene bcr-abl with the protein product Bcr-Abl, a tyrosine kinase with constitutive activity responsible for the pathology of the disease.(1) CML is a disease of the haemopoietic stem cell (HSC). The Ph+HSC has characteristic stem cell properties of self-renewal and differentiation, forming mature blood cells of all lineages carrying the Ph+. A small proportion of Ph+HSC remain quiescent but are capable of “awakening”, causing disease recrudescence. This has been demonstrated in animal models by the development of leukaemia in mice transplanted with quiescent Ph+HSC.(2) Such dormancy is of significance for eradication of minimal residual disease. The response of patients to treatment can be monitored in a number of different ways haematological, cytogenetic and molecular response.Accepted definitions are listed in Table 1. This hierarchy of responses reflects reduction in disease burden and is used in clinical trials to measure drug efficacy.
The current recommended first-line treatment is imatinib (Glivec/Gleevec; Novartis).(3) This is a rationally designed oral tyrosine kinase inhibitor (TKI), which has transformed the outlook for patients with CML. Results generated from the IRIS study demonstrated the clear benefit this well-tolerated drug offered over the previous recommended therapy of interferon alpha (IFN-A) plus cytarabine.(4) At 60 months of treatment virtually all patients achieve a complete haematological response (CHR) and 82% of patients a complete cytogenetic response (CCyR), with 80% of these also demonstrating a major molecular response (MMolR).(5) Despite impressive rates of response to imatinib (IM), the majority of patients have persistent detectable Bcr-Abl transcripts,(5,6) also referred to as minimal residual disease.
Despite apparent adequate control, the majority of patients who interrupt IM therapy show rapid recrudescence of disease.(7-9) Our group and others have demonstrated that in responsive patients there remains a population of quiescent Ph+HSC that appears insensitive to IM.(10-12) In our view these cells act as a reservoir capable of reconstituting disease. A well-recognised phenomenon in addition to disease persistence is IM resistance. Potential mechanisms include bcr-abl gene amplification, bcr-abl mutation and altered drug efflux.(13,14) As a consequence of these problems, novel approaches are required in CML therapy.
Modification of standard IM therapy
IM has been combined with granulocyte colony-stimulating factor (G-CSF) with the aim of stimulating the quiescent Ph+HSC and enhancing susceptibility to IM. This followed promising in-vitro data and has been the subject of a recently completed phase I trial (G-CSF and IM intermittently, GIMI).(15) The initial treatment dose of IM for patients with early disease is also under review, as responses may be more rapidly gained with intensive initial therapy(16) – although those treated with standard therapy may “catch up”, as improvement in response is seen up to 60 months.(5) The roles of established therapies such as IFN-A in combination with IM are also promoted;(17) this issue forms the focus of the STI571 Prospective International Randomised Trial (SPIRIT).
New tyrosine kinase inhibitors
The development of IM has led to the synthesis of the potent TKI nilotinib (Tasigna/AMN107; Novartis) and the dual Src- and Abl-kinase inhibitor dasatinib (Sprycel/
BMS-354825; Bristol-Myers Squibb). Phase I trials of these drugs have been published, confirming their efficacy in patients intolerant or refractory to IM.(18,19) In addition, there is no evidence for cross-resistance between nilotinib and dasatinib, and there is in-vitro evidence of synergism between IM and nilotinib.(20,21) However, none of these drugs is effective in patients with the T315I bcr-abl mutation. We have also shown persistence of viable quiescent Ph+HSC despite exposure to high concentrations of either nilotinib or dasatinib. It is likely therefore that neither drug will eradicate minimal residual disease.(22,23)
Targeting IM transport
Multidrug resistance (MDR) is a well-recognised phenomenon involving members of the ATP-binding cassette (ABC) transporter family.(24) ABCB1 (MDR1) encodes Pgp, and this efflux pump may be over-expressed in some patients with IM resistance.(25-28) Inhibitors of Pgp (such as verapamil and 17-AAG) may therefore have a role. ABCG2 is also under investigation, although whether IM is a substrate or an inhibitor of this pump is not clear.(29-32)
Novel targeting of Bcr-Abl
A different approach is nuclear entrapment of Bcr-Abl. It is our belief that active Bcr-Abl is exclusively cytoplasmic. Inactivation of Bcr-Abl using IM appears to allow transport between cytoplasm and nucleus. Leptomycin B blocks the nuclear export, allowing nuclear accumulation. Reactivation of Bcr-Abl in this situation (by washing IM from the cells) induces apoptosis. This has been demonstrated in vitro, but the toxicity of leptomycin B may limit clinical application.(33,34) A further strategy is to target the stability of Bcr-Abl. Heat-shock proteins, in particular Hsp90, confer intracellular stability to Bcr-Abl. Hsp90 inhibitors (geldana-mycin and 17-AAG) target this interaction, resulting in Bcr-Abl proteasomal degradation (including T315I mutation) and apoptosis.(35,36) Histone deacetylase inhibitors may interfere with the association of Hsp90 and Bcr-Abl and confer other effects.(37) We have also shown that proliferation of Ph+ primitive cells from patients with CML can be inhibited by 17-AAG, and this effect enhanced that of IM.(38) However, the inhibition seemed reversible and may not eradicate disease in vivo.
The proteasome is an intracellular organelle providing a targeted mechanism for protein degradation, and plays a crucial role in cell cycling, adhesion and apoptosis.(39) It has been demonstrated that eukaemic cells have abnormally high levels of proteasome expression and activity.(40) It is likely that the effects of this increased activity are mediated via the transcription factor NFkB and the inhibitory molecule IkB, although other proteasomal substrates including cyclin-dependent kinase inhibitors and members of the Bcl-2 family – may be significant.(41-43) There are promising in-vitro data demonstrating the inhibitory effects of proteasome inhibitors (and related IkB kinase inhibitors) on Bcr-Abl+ cell lines and CML patient samples. This may imply a future clinical role.(44-46)
Farnesyl transferase inhibitors
Phase I clinical trial data show a limited effect in IM resistant or IM-intolerant patients with the farnesyl transferase inhibitor (FTI) lonafarnib.(47) This followed promising in-vitro work.(48) The role of a different FTI, BMS-214662, is under investigation. We have unpublished evidence that there may be significant effects on the quiescent Ph+HSC stem, although the mechanism of action remains unclear.
Other novel therapies with potential roles include: adaphostin, a member of the tyrophostin group of tyrosine kinase inhibitors;(49,50) bryostatin, a marine macrolide;(51) ciclopirox, a hypusination inhibitor;(52) and MK-0457, an aurora kinase inhibitor.(53)
The current recommended first-line treatment for patients with CML is IM. This is an effective and well-tolerated drug capable of sustained disease control. However, IM is not a cure for the majority of patients. This is due to disease persistence and IM resistance. Current strategies combating these problems are focused on different areas, although they ultimately share the same goal – namely, a cure for CML.