- Email: [email protected]
Emerging Strategies for the Treatment of Mutant Bcr-Abl T315I Myeloid Leukemia
Emerging Strategies for the Treatment of Mutant Bcr-Abl T315I Myeloid Leukemia Tariq I. Mughal, John M. Goldman
Abstract The lessons learned from the remarkably successful use of the first-generation tyrosine kinase inhibitor (TKI) imatinib in patients with chronic myeloid leukemia resulted in a major paradigm shift in the treatment of many human cancers, and now further lessons are being learned from our enhanced understanding of the molecular mechanisms of resistance to imatinib and second-generation TKIs, particularly dasatinib and nilotinib. Although diverse mechanisms seem to be involved, the principal cause appears to be the emergence of point mutations in the Abl kinase domain that affect drug affinity and some of which impair the efficacy with which the drugs bind. Currently, > 50 different mutations have been identified, and the extent to which they confer resistance varies considerably. One of the more common mutations results from the substitution of isoleucine for threonine at Abl amino acid position 351, known as the T315I mutation. It appears that the precise position of the substitution within the kinase domain dictates the degree of resistance to TKIs, and patients with the T315I mutation develop almost complete resistance to imatinib, dasatinib, and nilotinib. Herein, we discuss the emerging strategies for circumventing resistance associated with the Bcr-Abl T315I mutation.
Clinical Lymphoma & Myeloma, Vol. 7, Suppl. 2, S81-S84, 2007 Key words: bcr-abl–targeted therapies, Drug resistance, Imatinib
Introduction Chronic myeloid leukemia (CML) is a relatively uncommon form of leukemia, but much has been learned in recent years of its molecular basis. Its response to targeted agents has established a paradigm from which lessons are learned for most other human malignancies.1 Chronic myeloid leukemia is a clonal multilineage myeloproliferative disorder, which originates in a single abnormal hematopoietic stem cell that contains a bcr-abl gene. The fusion gene is usually present on the Philadelphia (Ph) chromosome, but occasionally it is localized to a normal appearing chromosome 22 and, very rarely, to a normal chromosome 9. The bcr-abl gene expresses a protein, known as p210Bcr-Abl, which is considered to be the initiating event for chronic phase CML.2 This oncoprotein has enhanced protein tyrosine kinase activity, which is presumed to be responsible for its oncogenic activity. The discovery that this dysregulated kinase activity could be inhibited in a highly specific manner has proved to be a major landmark in the treatment of patients with CML.3 Division of Hematology and Stem Cell Transplantation, University of Texas Southwestern Medical School, Dallas Hammersmith Hospital at Imperial College, London, UK Submitted: Dec 27, 2006; Accepted: Jan 25, 2007 Address for correspondence: Tariq Mughal, MD, Division of Hematology and Stem Cell Transplantation, University of Texas Southwestern Medical School, 5323 Harry Hines Blvd, Dallas, TX 75390 Fax: 214-367-3301; e-mail: [email protected]
The tyrosine kinase inhibitor (TKI) imatinib was first used in the clinic in 1998. The clinical results of using imatinib as first-line treatment for patients with chronic phase CML now show that 98% of patients exhibit a complete hematologic response by 5 years, and the cumulative incidence of complete cytogenetic response is 87%.4 The drug substantially reduces the quantity of leukemia cells in the patient’s body; approximately 80% of patients have a ≥ 3-log reduction in the number of Bcr-Abl transcripts, as measured by real time polymerase chain reaction (Figure 1).5 However, although only a minority of patients exhibit complete molecular responses on standard doses of imatinib 400 mg daily and allogeneic hematopoietic stem cell transplantation, albeit suitable for only a limited number of patients and associated with an appreciable risk of morbidity and indeed mortality, it is still the only treatment known to result in long-term leukemia-free survival.6 Moreover, up to 20% of patients in chronic phase and almost all of those in the advanced phases of CML develop resistance to imatinib, and of these, 40%-60% have Ph-positive subclones characterized by a mutation in the bcr-abl gene. The use of the new second generation Abl kinase inhibitors, dasatinib (previously BMS354825) and nilotinib (previously AMN107) has been relatively successful for patients in all phases of CML and those with Ph-positive acute lymphoblastic leukemia who are resistant to imatinib, but both agents are inactive in patients with one particular mutation that codes for the substitution of isoleucine for threonine at position 315 in the Abl kinase domain (designated T315I or T315I).7 Efforts are
Dr Mughal has no relevant relationships to disclose. Dr Goldman is a member of the Speaker’s Bureau for Novartis Oncology and Bristol-Myers Squibb. This article includes discussion of investigational and/or unlabeled uses of drugs, including the use of MK-0457, nilotinib, SGX-70430, tanespimycin, peptide vaccines targeting the b3a2 junction, and alternate schedules of imatinib in the treatment of imatinib-resistant CML.
Clinical Lymphoma & Myeloma Vol 7 Suppl 2 March 2007 • S81
Treatment of Myeloid Leukemia Figure 1 Molecular Responses: IRIS Study
Percent Reduction in Bcr-Abl Transcripts from Nadir
100% 80%
≥ 4-Log Reduction
60%
3 to < 4-Log Reduction
Figure 2 Mutations Noted in Patients on Imatinib Treatment The Map of Mutations: 4 Critical Regions Amino Acids that Controls the Kinase Activation Step
ATP-Binding Loop
2 to < 3-Log Reduction
Gate Keepers
P-Loop
40% <2 Log-Reduction M244V
20%
D276G
L248V
0%
Year 1
M343T
E255K/V
Activation Loop
E355G/D
H396R/P S417Y L387M/F
M351T/V
T277A
G250E Q252R/H
Year 4
V289A
Catalytic Domain
F311L/I F317L
Y253F/H
F359V V379I
F382L
E459K F486S
A380T
T315I
therefore being directed to developing agents that would help patients with this mutation. In this article, we review the drugs being developed and assess the future impact on the decisionmaking strategy for the newly diagnosed patient with CML.
Amino Acids Involved in Binding (Published, with permission, Goldman, ASH 2005)
Resistance Mechanisms to Imatinib Resistance to imatinib can conveniently be considered as Bcr-Abl– dependent or Bcr-Abl–independent. Bcr-Abl–independent resistance can arise if a CML cell acquires additional molecular changes that cannot be targeted by imatinib.8 Thus far, this appears to be rare, and little is known of the underlying mechanism(s). Conversely, Bcr-Abl–dependent resistance, which is usually acquired, might be caused by changes that specifically involve the Bcr-Abl oncoprotein. Such changes include amplification of the bcr-abl fusion gene with associated overexpression of the protein and overexpression of the MDR-1 gene and associated P-glycoprotein, which could lead to increased elimination of the inhibitor from the cell. Moreover, some plasma proteins, such as α-1 acid glycoprotein, or enzymes, such as P450 enzyme, might neutralize imatinib and render it ineffective.9 The principal cause of Bcr-Abl–dependent resistance appears to be expansion of Ph-positive subclones with specific point mutations in the Bcr-Abl kinase domain. Many of these mutations likely reflect selection by imatinib of mutations already present at low level before initiation of treatment rather than de novo acquisition during imatinib therapy.10 The kinase domain of the Bcr-Abl oncoprotein is identical to the kinase domain of the normal Abl protein. It can be divided into 4 component parts: an adenosine triphosphate–binding pocket or P-loop, an intervening sequence, a catalytic domain, and an activating loop component (Figure 2).11,12 At least 58 different mutations have now been identified, of which one of the more common is the T315I, referred to previously.13 It appears that the precise position of the substitution within the kinase domain dictates the degree of resistance to imatinib. For example, the methionine to threonine substitution at position 351 (M351T) is associated with a moderate degree of drug resistance, which could, to some extent, be overcome by increasing drug dosage.14,15 In contrast, the T315I mutation or a glutamic acid to lysine substitution at position 255 (E255K) is associated with resistance not only to imatinib, but also dasatinib and nilotinib.16 These mutations result in structural changes that prevent the binding of the respective drugs.
Strategies to Overcome the T315I Bcr-Abl Resistance Based on the notion that the reactivation of the Bcr-Abl signaling in CML cells that develop resistance to imatinib is a principal finding, many efforts have focused on attempts to restore inhibition of the Abl kinase.17 Dose escalation of imatinib has had some success in the treatment of many of the mutations, but not the T315I or the E255K.18 Nilotinib and dasatinib have now been tested in approximately 30 of the currently known mutations and appear to be active at different levels of sensitivity in all cases other than the T315I mutation, in which they are completely inactive.19,20 Many efforts are now focusing on developing strategies against the T315I mutant-induced resistance. One such effort has involved a drug belonging to the class of aurora kinase inhibitors, MK-0457 (previously known as VX-680).21,22 The drug was found to inhibit Bcr-Abl kinase with a considerably higher affinity compared with imatinib and uniquely appears to bind Abl without contacting the innermost cavity of the Abl kinase domain, where the T315I mutation is sited. In a recent phase I/II study, 3 patients with T315I CML resistant/refractory to imatinib and dasatinib or nilotinib were treated with continuous intravenous infusions of MK-0457; the preliminary results indicate that the drug produced good hematologic responses, but the effects on the T315I mutation itself were not reported. Another candidate drug being tested is SGX-70430, which was designed specifically to inhibit the T315I mutant, but also has activity against the wild-type and most of the other known Bcr-Abl mutants.23 A third drug, 17-allylaminogeldanamycin, which can lead to Bcr-Abl protein degradation by inhibiting the heat-shock protein–90, a molecular chaperone required for stabilization of Bcr-Abl, has just entered phase I studies.24 17allylaminogeldanamycin appears to be active in patients with the E255K and T315I mutations. It also downregulates bcr-abl messenger RNA, though the precise mechanisms remain unclear.
S82 • Clinical Lymphoma & Myeloma Vol 7 Suppl 2 March 2007
Tariq I. Mughal, John M. Goldman Other novel TKIs are active in cell lines with the H396P and M351T mutants, but not T315I, including PD166326.25 This agent also appears to be superior to imatinib in murine models. Arsenic trioxide, an agent known to downregulate Bcr-Abl, has also entered clinical trials recently and appears to be active in a number of the principal mutants, but its role in T315I remains unclear.26
Immunotherapeutic Strategies to Overcome Bcr-Abl Resistance After the recognition that the unique amino acid sequence of p210 at the fusion point was immunogenic and the notion of a graft-versus-leukemia effect was the principal reason for the qualified success of allografting patients with CML, many efforts were directed to explore the potential of developing an active specific immunotherapy strategy for patients with CML by inducing an immune response to this tumor-specific antigen.27,28 Peptides derived from the b3a2 junction avidly bind to 4 human leukocyte antigen (HLA) class I and 1 HLA class II alleles, thereafter generating peptide-specific CD8 and CD4 T cells.29 Peptide vaccines derived from the b3a2 sequence were then investigated, and the initial results, for the most part, were inconsistent.30 In a more recent small study of 15 patients who had minimal residual disease only, complete molecular responses were observed in 7 patients who had been vaccinated and treated simultaneously with granulocyte-macrophage colony-stimulating factor, an immune adjuvant.31 Notably, the study only enrolled patients with HLA alleles known to bind avidly to the fusion peptides. Nevertheless, some enthusiasm has been generated from this and 2 other studies.27,30 Further studies, particularly in the majority of imatinib-treated patients who have persistent molecular disease, are now in progress. Other potential targets for vaccine therapy include the Wilms tumor-1 protein and proteinase-3, a granule protein, both of which are overexpressed in CML cells.
Conclusions and Future Directions Imatinib has unequivocally established the value of molecularly targeted treatment in cancer medicine in general and, of course, specifically in CML and related diseases. It has also provided further evidence that the bcr-abl gene must be the initiating event for chronic phase CML. Remarkably, this gene continues to play a pivotal role even when patients in chronic phase CML develop resistance to imatinib and mutant forms of the gene are identified. The T315I mutant appears to be resistant not only to escalating doses of imatinib, but also to the second generation TKIs, nilotinib and dasatinib. Efforts to develop therapy effective against the T315I mutant have led to 3 candidate drugs that are currently being tested. The first clinical results from MK-0457 were recently published and are encouraging. Alternative strategies to overcome Bcr-Abl resistance include immunotherapy, and some evidence suggests that patients vaccinated with p210 multipeptides and other vaccines generate immune responses that can be of clinical benefit.
References 1. Fialkow PJ. Evidence for a multistep pathogenesis of chronic myeloid leukemia. Blood 1981; 58:158-163. 2. Daley G, Van Etten R, Baltimore D. Induction of chronic myelogenous leukemia in mice by the P210 bcr/abl gene of the Philadelphia chromosome. Science 1990; 247:824-830. 3. Druker BJ, Tamura S, Buchdunger E, et al. Effects of a selective inhibitor of the Abl tyrosine kinase on the growth of BCR-ABL positive cells. Nature Medicine 1996; 2:561. 4. Druker BJ, Talpaz M, Resta DJ, et al. Efficacy and safety of a specific inhibitor of the BCR-ABL tyrosine kinase in chronic myeloid leukemia. N Engl J Med 2001; 344:1031-1037. 5. Mughal TI, Goldman JM. Chronic myeloid leukaemia: STI571 magnifies the therapeutic dilemma. Europ J Cancer 2001; 37:561-568. 6. Druker BJ, Guilhot F, O’Brien S, et al. Long-term benefits of imatinib (IM) for patients newly diagnosed with chronic myeloid leukemia in chronic phase (CML-CP): the 5-year update from the IRIS Study. J Clin Oncol 2006; 24:338s (Abstract #6506). 7. Hughes TP, Kaeda J, Branford S, et al, on behalf of the IRIS study group. Frequency of major molecular responses to imatinib or interferon alfa plus cytarabine in newly diagnosed patients with chronic myeloid leukemia. N Engl J Med 2003; 349:1423-1432. 8. Griffin JD. Resistance to targeted therapy in leukaemia. Lancet 2002; 359:458-459. 9. Gorre ME, Mohammed M, Ellwood K, et al. Clinical resistance to STI-571 cancer therapy caused by BCR-ABL gene mutation or amplification. Science 2001; 293:876-880. 10. Shannon KM. Resistance in the land of molecular therapeutics. Cancer Cell 2002; 2:99-102. 11. Branford S, Rudzki Z, Walsh S, et al. Detection of BCR-ABL mutations in patients with CML treated with imatinib is virtually always accompanied by clinical resistance, and mutations in the ATP phosphate-binding loop (P-loop) are associated with a poor prognosis. Blood 2003; 102:276-283. 12. Goldman JM, Hughes T, Ridich J, et al. Continuing reduction in level of residual disease after 4 years in patients with CML in chronic phase responding to first-line imatinib (IM) in the IRIS study. Blood 2005; 106:51a (Abstract #163). 13. Deininger M, Buchdunger E, Druker BJ. The development of imatinib as a therapeutic agent for chronic myeloid leukemia. Blood 2005; 105:2640-2653. 14. Cowan-Jacob SW, Guez V, Fendrich G, et al. Imatinib (STI571) resistance in chronic myelogenous leukemia: molecular basis of the underlying mechanisms and potential strategies for treatment. Mini Rev Med Chem 2004; 4:285-299. 15. Deininger MW, Druker BJ. SRCircumventing imatinib resistance. Cancer Cell 2004; 6:108-110. 16. Gorre ME, Ellwood-Yen K, Chiosis G, et al. BCR-ABL point mutants isolated from patients with imatinib mesylate-resistant chronic myeloid leukemia remain sensitive to inhibitors of the BCR-ABL chaperone heat shock protein 90. Blood 2002; 100:3041-3044. 17. Druker BJ. Circumventing resistance to kinase-inhibitor therapy. N Engl J Med 2006; 354:2594-2596. 18. Kantarjian HM, Talpaz M, O’Brien S, et al. Dose escalation of imatinib mesylate can overcome resistance to standard dose therapy in patients with chronic myeloid leukemia. Blood 2003; 101:473-475. 19. Bradeen HA, Eide CA, O’Hare T, et al. Comparison of imatinib, dasatinib (BMS-354825) and nilotinib (AMN 107) in an Nethyl-N-nitrosourea (ENU)-based mutagenesis screen: high efficacy of drug combinations. Blood 2006; 108:2332-2338. 20. Cortes J. Overcoming drug resistance in chronic myeloid leukemia. Curr Opin Hematol 2006; 13:79-86. 21. Young MA, Shah NP, Chao, LH, et al. Structure of the kinase domain of an imatinib-resistant Abl mutant in complex with the Aurora kinase inhibitor VX-680. Cancer Res 2006; 66:1007-1014. 22. Giles FJ, Cortes JE, Jones D, et al. MK-0457, a novel kinase inhibitor, is active in patients with chronic myeloid leukemia or acute lymphoblastic leukemia with the T315I BCR-ABL mutation. Blood 2007; 109:500-502. 23. Burley SK. Application of FAST fragment-based lead discovery and structure-guided design to discovery of small molecule inhibitors of BCR-ABL tyrosine kinase active against the T315I imatinib-resistant mutant. Presented at: 97th American Association for Cancer Research Annual Meeting; April 1-5, 2006; Washington, DC. Abstract #4851.
Clinical Lymphoma & Myeloma Vol 7 Suppl 2 March 2007 • S83
Treatment of Myeloid Leukemia 24. Adrian FJ, Ding Q, Sim T, et al. Allosteric inhibitors of Bcr-Abldependent cell proliferation. Nat Chem Biol 2006; 2:95-102. 25. Azam M, Nardi V, Shakespeare WC, et al. Activity of dual SRC-ABL inhibitors highlights the role of BCR/ABL kinase dynamics in drug resistance. Proc Natl Acad Sci U S A 2006; 103:9244-9249. 26. Mauro MJ, Deininger MWN, O’Dwyer ME, et al. Phase I/II study of arsenic trioxide (trisemox) in combination with imatinib mesylate (Gleevec, STI571) in patients with Gleevec-resistant chronic myelogenous leukemia in chronic phase. Blood 2002; 100:781a (Abstract #2090). 27. Rojas JM, Knight K, Wang L, et al. The immune response to BCR-ABL peptide immunisation is variable and transient in chronic myeloid leukaemia: results from the EPIC study. Haematologica 2006;
91(1 suppl):177 (Abstract #0483). 28. Kurbegov D, Molldrem JJ. Immunity to chronic myelogenous leukemia. Hematol Oncol Clin N 2004; 18:733-752. 29. Clark RE, Dodi IA, Hill SC, et al. Direct evidence that leukemic cells present HLA-associated immunogenic peptides derived from the BCR-ABL b3a2 fusion protein. Blood 2001; 98:2887-2893. 30. Cathcart K, Pinilla-Ibarz J, Korontsvit T, et al. A multivalent bcr-abl fusion peptide vaccination trial in patients with chronic myeloid leukemia. Blood 2004; 103:1037-1042. 31. Bocchia M, Gentili S, Abruzzese E, et al. Effect of a p210 multipeptide vaccine associated with imatinib or interferon in patients with chronic myeloid leukaemia and persistent residual disease: a multicentre observational trial. Lancet 2005; 365:657-659.
S84 • Clinical Lymphoma & Myeloma Vol 7 Suppl 2 March 2007
Abstract The lessons learned from the remarkably successful use of the first-generation tyrosine kinase inhibitor (TKI) imatinib in patients with chronic myeloid leukemia resulted in a major paradigm shift in the treatment of many human cancers, and now further lessons are being learned from our enhanced understanding of the molecular mechanisms of resistance to imatinib and second-generation TKIs, particularly dasatinib and nilotinib. Although diverse mechanisms seem to be involved, the principal cause appears to be the emergence of point mutations in the Abl kinase domain that affect drug affinity and some of which impair the efficacy with which the drugs bind. Currently, > 50 different mutations have been identified, and the extent to which they confer resistance varies considerably. One of the more common mutations results from the substitution of isoleucine for threonine at Abl amino acid position 351, known as the T315I mutation. It appears that the precise position of the substitution within the kinase domain dictates the degree of resistance to TKIs, and patients with the T315I mutation develop almost complete resistance to imatinib, dasatinib, and nilotinib. Herein, we discuss the emerging strategies for circumventing resistance associated with the Bcr-Abl T315I mutation.
Clinical Lymphoma & Myeloma, Vol. 7, Suppl. 2, S81-S84, 2007 Key words: bcr-abl–targeted therapies, Drug resistance, Imatinib
Introduction Chronic myeloid leukemia (CML) is a relatively uncommon form of leukemia, but much has been learned in recent years of its molecular basis. Its response to targeted agents has established a paradigm from which lessons are learned for most other human malignancies.1 Chronic myeloid leukemia is a clonal multilineage myeloproliferative disorder, which originates in a single abnormal hematopoietic stem cell that contains a bcr-abl gene. The fusion gene is usually present on the Philadelphia (Ph) chromosome, but occasionally it is localized to a normal appearing chromosome 22 and, very rarely, to a normal chromosome 9. The bcr-abl gene expresses a protein, known as p210Bcr-Abl, which is considered to be the initiating event for chronic phase CML.2 This oncoprotein has enhanced protein tyrosine kinase activity, which is presumed to be responsible for its oncogenic activity. The discovery that this dysregulated kinase activity could be inhibited in a highly specific manner has proved to be a major landmark in the treatment of patients with CML.3 Division of Hematology and Stem Cell Transplantation, University of Texas Southwestern Medical School, Dallas Hammersmith Hospital at Imperial College, London, UK Submitted: Dec 27, 2006; Accepted: Jan 25, 2007 Address for correspondence: Tariq Mughal, MD, Division of Hematology and Stem Cell Transplantation, University of Texas Southwestern Medical School, 5323 Harry Hines Blvd, Dallas, TX 75390 Fax: 214-367-3301; e-mail: [email protected]
The tyrosine kinase inhibitor (TKI) imatinib was first used in the clinic in 1998. The clinical results of using imatinib as first-line treatment for patients with chronic phase CML now show that 98% of patients exhibit a complete hematologic response by 5 years, and the cumulative incidence of complete cytogenetic response is 87%.4 The drug substantially reduces the quantity of leukemia cells in the patient’s body; approximately 80% of patients have a ≥ 3-log reduction in the number of Bcr-Abl transcripts, as measured by real time polymerase chain reaction (Figure 1).5 However, although only a minority of patients exhibit complete molecular responses on standard doses of imatinib 400 mg daily and allogeneic hematopoietic stem cell transplantation, albeit suitable for only a limited number of patients and associated with an appreciable risk of morbidity and indeed mortality, it is still the only treatment known to result in long-term leukemia-free survival.6 Moreover, up to 20% of patients in chronic phase and almost all of those in the advanced phases of CML develop resistance to imatinib, and of these, 40%-60% have Ph-positive subclones characterized by a mutation in the bcr-abl gene. The use of the new second generation Abl kinase inhibitors, dasatinib (previously BMS354825) and nilotinib (previously AMN107) has been relatively successful for patients in all phases of CML and those with Ph-positive acute lymphoblastic leukemia who are resistant to imatinib, but both agents are inactive in patients with one particular mutation that codes for the substitution of isoleucine for threonine at position 315 in the Abl kinase domain (designated T315I or T315I).7 Efforts are
Dr Mughal has no relevant relationships to disclose. Dr Goldman is a member of the Speaker’s Bureau for Novartis Oncology and Bristol-Myers Squibb. This article includes discussion of investigational and/or unlabeled uses of drugs, including the use of MK-0457, nilotinib, SGX-70430, tanespimycin, peptide vaccines targeting the b3a2 junction, and alternate schedules of imatinib in the treatment of imatinib-resistant CML.
Clinical Lymphoma & Myeloma Vol 7 Suppl 2 March 2007 • S81
Treatment of Myeloid Leukemia Figure 1 Molecular Responses: IRIS Study
Percent Reduction in Bcr-Abl Transcripts from Nadir
100% 80%
≥ 4-Log Reduction
60%
3 to < 4-Log Reduction
Figure 2 Mutations Noted in Patients on Imatinib Treatment The Map of Mutations: 4 Critical Regions Amino Acids that Controls the Kinase Activation Step
ATP-Binding Loop
2 to < 3-Log Reduction
Gate Keepers
P-Loop
40% <2 Log-Reduction M244V
20%
D276G
L248V
0%
Year 1
M343T
E255K/V
Activation Loop
E355G/D
H396R/P S417Y L387M/F
M351T/V
T277A
G250E Q252R/H
Year 4
V289A
Catalytic Domain
F311L/I F317L
Y253F/H
F359V V379I
F382L
E459K F486S
A380T
T315I
therefore being directed to developing agents that would help patients with this mutation. In this article, we review the drugs being developed and assess the future impact on the decisionmaking strategy for the newly diagnosed patient with CML.
Amino Acids Involved in Binding (Published, with permission, Goldman, ASH 2005)
Resistance Mechanisms to Imatinib Resistance to imatinib can conveniently be considered as Bcr-Abl– dependent or Bcr-Abl–independent. Bcr-Abl–independent resistance can arise if a CML cell acquires additional molecular changes that cannot be targeted by imatinib.8 Thus far, this appears to be rare, and little is known of the underlying mechanism(s). Conversely, Bcr-Abl–dependent resistance, which is usually acquired, might be caused by changes that specifically involve the Bcr-Abl oncoprotein. Such changes include amplification of the bcr-abl fusion gene with associated overexpression of the protein and overexpression of the MDR-1 gene and associated P-glycoprotein, which could lead to increased elimination of the inhibitor from the cell. Moreover, some plasma proteins, such as α-1 acid glycoprotein, or enzymes, such as P450 enzyme, might neutralize imatinib and render it ineffective.9 The principal cause of Bcr-Abl–dependent resistance appears to be expansion of Ph-positive subclones with specific point mutations in the Bcr-Abl kinase domain. Many of these mutations likely reflect selection by imatinib of mutations already present at low level before initiation of treatment rather than de novo acquisition during imatinib therapy.10 The kinase domain of the Bcr-Abl oncoprotein is identical to the kinase domain of the normal Abl protein. It can be divided into 4 component parts: an adenosine triphosphate–binding pocket or P-loop, an intervening sequence, a catalytic domain, and an activating loop component (Figure 2).11,12 At least 58 different mutations have now been identified, of which one of the more common is the T315I, referred to previously.13 It appears that the precise position of the substitution within the kinase domain dictates the degree of resistance to imatinib. For example, the methionine to threonine substitution at position 351 (M351T) is associated with a moderate degree of drug resistance, which could, to some extent, be overcome by increasing drug dosage.14,15 In contrast, the T315I mutation or a glutamic acid to lysine substitution at position 255 (E255K) is associated with resistance not only to imatinib, but also dasatinib and nilotinib.16 These mutations result in structural changes that prevent the binding of the respective drugs.
Strategies to Overcome the T315I Bcr-Abl Resistance Based on the notion that the reactivation of the Bcr-Abl signaling in CML cells that develop resistance to imatinib is a principal finding, many efforts have focused on attempts to restore inhibition of the Abl kinase.17 Dose escalation of imatinib has had some success in the treatment of many of the mutations, but not the T315I or the E255K.18 Nilotinib and dasatinib have now been tested in approximately 30 of the currently known mutations and appear to be active at different levels of sensitivity in all cases other than the T315I mutation, in which they are completely inactive.19,20 Many efforts are now focusing on developing strategies against the T315I mutant-induced resistance. One such effort has involved a drug belonging to the class of aurora kinase inhibitors, MK-0457 (previously known as VX-680).21,22 The drug was found to inhibit Bcr-Abl kinase with a considerably higher affinity compared with imatinib and uniquely appears to bind Abl without contacting the innermost cavity of the Abl kinase domain, where the T315I mutation is sited. In a recent phase I/II study, 3 patients with T315I CML resistant/refractory to imatinib and dasatinib or nilotinib were treated with continuous intravenous infusions of MK-0457; the preliminary results indicate that the drug produced good hematologic responses, but the effects on the T315I mutation itself were not reported. Another candidate drug being tested is SGX-70430, which was designed specifically to inhibit the T315I mutant, but also has activity against the wild-type and most of the other known Bcr-Abl mutants.23 A third drug, 17-allylaminogeldanamycin, which can lead to Bcr-Abl protein degradation by inhibiting the heat-shock protein–90, a molecular chaperone required for stabilization of Bcr-Abl, has just entered phase I studies.24 17allylaminogeldanamycin appears to be active in patients with the E255K and T315I mutations. It also downregulates bcr-abl messenger RNA, though the precise mechanisms remain unclear.
S82 • Clinical Lymphoma & Myeloma Vol 7 Suppl 2 March 2007
Tariq I. Mughal, John M. Goldman Other novel TKIs are active in cell lines with the H396P and M351T mutants, but not T315I, including PD166326.25 This agent also appears to be superior to imatinib in murine models. Arsenic trioxide, an agent known to downregulate Bcr-Abl, has also entered clinical trials recently and appears to be active in a number of the principal mutants, but its role in T315I remains unclear.26
Immunotherapeutic Strategies to Overcome Bcr-Abl Resistance After the recognition that the unique amino acid sequence of p210 at the fusion point was immunogenic and the notion of a graft-versus-leukemia effect was the principal reason for the qualified success of allografting patients with CML, many efforts were directed to explore the potential of developing an active specific immunotherapy strategy for patients with CML by inducing an immune response to this tumor-specific antigen.27,28 Peptides derived from the b3a2 junction avidly bind to 4 human leukocyte antigen (HLA) class I and 1 HLA class II alleles, thereafter generating peptide-specific CD8 and CD4 T cells.29 Peptide vaccines derived from the b3a2 sequence were then investigated, and the initial results, for the most part, were inconsistent.30 In a more recent small study of 15 patients who had minimal residual disease only, complete molecular responses were observed in 7 patients who had been vaccinated and treated simultaneously with granulocyte-macrophage colony-stimulating factor, an immune adjuvant.31 Notably, the study only enrolled patients with HLA alleles known to bind avidly to the fusion peptides. Nevertheless, some enthusiasm has been generated from this and 2 other studies.27,30 Further studies, particularly in the majority of imatinib-treated patients who have persistent molecular disease, are now in progress. Other potential targets for vaccine therapy include the Wilms tumor-1 protein and proteinase-3, a granule protein, both of which are overexpressed in CML cells.
Conclusions and Future Directions Imatinib has unequivocally established the value of molecularly targeted treatment in cancer medicine in general and, of course, specifically in CML and related diseases. It has also provided further evidence that the bcr-abl gene must be the initiating event for chronic phase CML. Remarkably, this gene continues to play a pivotal role even when patients in chronic phase CML develop resistance to imatinib and mutant forms of the gene are identified. The T315I mutant appears to be resistant not only to escalating doses of imatinib, but also to the second generation TKIs, nilotinib and dasatinib. Efforts to develop therapy effective against the T315I mutant have led to 3 candidate drugs that are currently being tested. The first clinical results from MK-0457 were recently published and are encouraging. Alternative strategies to overcome Bcr-Abl resistance include immunotherapy, and some evidence suggests that patients vaccinated with p210 multipeptides and other vaccines generate immune responses that can be of clinical benefit.
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