1. Department of Clinical Haematology and Bone Marrow Transplant Service, The Royal Melbourne Hospital and The University of Melbourne, Victoria.
2. Walter and Eliza Hall Institute of Medical Research, Parkville, Victoria.
By delivering major improvements in patient outcomes, targeted therapies have revolutionised treatment paradigms for many haematological malignancies, particularly chronic myeloid leukaemia, acute promyelocytic leukaemia and diffuse large B-cell lymphoma. They promise even greater benefits in the future.
As defined by others, truly targeted therapy should attack a biologically important process, preferably one central to a hallmark of cancer.1 Such therapy involves a drug with a focused mechanism that specifically acts on a defined target or biologic pathway that, when inactivated, causes regression or destruction of the malignant process.2 The target should be measurable in the clinic and measurement of the target should correlate with clinical outcome when the targeted therapy is administered.1 The ideal target should be specific and crucial to the malignant clone and not be expressed in normal tissues to avoid toxicity seen in traditional cytotoxic chemotherapy.
Recent advances in understanding of molecular mechanisms and identification of immunophenotypic signatures specific to haematological malignancies, have led to the discovery of many novel therapeutic strategies, some of which fulfil all the above listed criteria for targeted therapy. These targeted therapies for haematological malignancies can be broadly divided into therapeutic monoclonal antibodies or small molecule inhibitors. This review will highlight some of the major advances in haematological malignancies using these smarter approaches. These drugs have already changed the landscape of care for patients.
Tyrosine kinase inhibitors for chronic myeloid leukaemia are perhaps the most convincing examples of the utility of targeted therapy. They fulfil all of the criteria that define targeted therapy and have resulted in a revolution in the management of chronic myeloid leukaemia and a fundamental beneficial change in the natural history of the disease.
Chronic myeloid leukaemia is a myeloproliferative disorder with an incidence of one to two cases per 100,000 people per year.3 Approximately 300 to 400 patients are diagnosed each year in Australia. The pathophysiology of chronic myeloid leukaemia is characterised by a clonal expansion of haematopoietic stem cells carrying the Philadelphia chromosome, a reciprocal translocation between the long arms of chromosomes 9 and 22, t(9;22)(q34;q11).4 This balanced translocation creates a unique hybrid gene known as BCR-ABL. Expression of BCR-ABL transcript is the hallmark of chronic myeloid leukaemia. It encodes the aberrant BCR-ABL protein that contains a constitutively active ABL tyrosine kinase, capable of producing a ‘switch-on’ proliferative signal affecting a number of downstream intracellular pathways. Murine models using retroviral transfection with BCR-ABL have elegantly demonstrated the ability of aberrant expression of this protein to produce a phenotype resembling that of a chronic myeloproliferative disease, providing compelling evidence that this target is essential to the development of the disease.5
The search for specific inhibitors of ABL tyrosine kinase led to the development of the first generation of tyrosine kinase inhibitor, imatinib mesylate and its phenomenal success in the treatment of chronic myeloid leukaemia in chronic phase. Prior to the imatinib era, interferon alpha plus cytarabine was the treatment of choice for most patients with chronic myeloid leukaemia who were not candidates for curative haematopoietic stem cell transplantation. This poorly tolerated therapy delivered a three year overall survival of 86% with few patients surviving greater than 10 years, due to progression to highly malignant blast phase chronic myeloid leukaemia.6
Imatinib is a potent tyrosine kinase inhibitor that binds to the inactive configuration of ABL kinase and functions as a competitive inhibitor of the ATP binding site of BCR-ABL.7 The key effect of imatinib binding is to block the autophosphorylation of the kinase, a critical event leading to downstream signal transduction.8 Inhibition of this signal transduction can be readily observed in cells from patients treated with imatinib. The cellular responses to this targeted therapy can be measured by its impact on tumour burden, using either conventional cytogenetics, or fluorescence in situ hybridisation to indicate what proportion of blood cells are from the malignant clone, and molecular techniques to quantify BCR-ABL transcripts.
The landmark International Randomised Study of IFN versus STI571 study investigated the role of imatinib in first line therapy for patients with chronic myeloid leukaemia in chronic phase. Impressive and durable responses were confirmed after a median five year follow-up; the estimated disease-specific overall and progression-free survivals of patients who received imatinib as initial therapy were 95% and 93% respectively.9 The annual rate of progression to accelerated phase or blast crisis declined remarkably to zero in the sixth year of therapy (annual rate during the first five years was 1.5, 2.8, 1.6, 0.9 and 0.6% respectively).10
400mg imatinib is now the standard of care in newly diagnosed chronic myeloid leukaemia in chronic phase. Long-term follow-up has also shown improved responses to imatinib over time: the complete cytogenetic response (CCyR) was 69% after one year and 87% after five years of therapy.9 There was a significant correlation with better overall survival for those patients obtaining a CCyR response by 12 months; furthermore, no patient progressed to the accelerated or blast crisis during a five year follow-up if a major molecular response together with a CCyR at 12 months was attained. Imatinib is generally well tolerated. Inhibition of normal ABL kinase activity does not limit the use of this drug in most patients. For the great majority of newly diagnosed patients with chronic myeloid leukaemia, this is no longer a devastating disease with a poor long-term prognosis. Rather, it is a chronic disease, more akin to chronic non-malignant disorders.
Nevertheless, resistance is one of the emerging challenges in the imatinib era. Over five years, approximately 17% of patients initially treated with imatinib will fail due to the outgrowth of a clone with a mutated BCR-ABL that has diminished binding to imatinib.9Second generation tyrosine kinase inhibitors (dasatinib and nilotinib) have different binding characteristics, are more potent than imatinib and are generally highly effective in the setting of imatinib intolerance or failure.10 However, one particular mutant form of BCR-ABL, the T351I mutation, is resistant to all approved agents and is a major hurdle yet to be overcome.
Tyrosine kinase inhibitors like imatinib are also effective in several rare myeloproliferative diseases driven by the expression of other aberrant tyrosine kinases. Serendipitously, it was empirically observed that imatinib was highly effective for patients with chronic eosinophilic leukaemia.11 Subsequent investigation revealed that the cryptic fusion oncogene, FIL1L1-PDGFR?, was not only causative of the condition, but exquisitely sensitive to inhibition by imatinib. Another imatinib-sensitive blood disease is the myelodysplastic/myeloproliferative disorder driven by constitutive activity of the PDGFR? kinase. For both conditions which generally fail to respond to conventional therapies, complete responses are now commonly seen, including complete molecular responses.
The identification of surface-specific markers on B cell malignancies has led to the development of monoclonal antibodies that target these antigens. The most widely studied therapeutic antibody, rituximab, is directed against CD20, a pan-B cell surface antigen that is also widely expressed in normal B cells. Rituximab is a chimeric monoclonal antibody of IgG1 subtype and was the first antibody approved for cancer therapy in history. Its proposed mechanism of action is mediated by antibody-dependent cell-mediated cytotoxicity and complement dependent cytotoxicity. Despite the fact that the cellular target is not specific for malignant B cells, depletion of normal B cells does not appear to be a major clinical problem with early non-malignant B cell recovery observed at 24 weeks after a single infusion of 375mg/m2 (unpublished data). Rituximab was initially approved in refractory CD20+ low grade or follicular lymphoma with an overall response rate of 50% and a median duration of response of 12 months.12 As a single agent in newly diagnosed follicular lymphoma, the overall response rate approaches 80% with median duration of responses of 18-26 months.13 Of greatest importance is the synergy observed between rituximab and cytotoxic chemotherapy. Rituximab containing chemotherapy regimens have become the standard of care in first line and relapsed follicular lymphoma.
Among the lymphoproliferative disorders, diffuse large B-cell lymphoma is the most common aggressive lymphoma despite refinements in chemotherapy. No improvement in the cure rate had been achieved in the last 20 years. In a landmark Phase III trial in elderly patients (aged between 60 to 80) with previously untreated diffuse large B-cell lymphoma, addition of rituximab to standard chemotherapy (cyclophosphamide, doxorubicin, vincristine and prednisone) ie. R-CHOP showed superior progression-free and overall survivals compared to CHOP alone. The seven year progression free survival was 52% for R-CHOP and 29% for CHOP (p<0.0001), and the overall survival was 53% for R-CHOP and 36% for CHOP (p=0.0004).14 Superiority in event free survival and overall survival of rituximab plus CHOP like regimens has also been demonstrated in younger patients with untreated diffuse large B-cell lymphoma.15 R-CHOP is now the gold standard for newly diagnosed diffuse large B-cell lymphoma.
Anti-CD20 based radioimmunoconjugate therapy is another approach to targeted therapy, enhancing cytotoxic potential of a monoclonal antibody by attaching to a radionuclide. Radioimmunoconjugate therapy targets the cells to which the antibody is bound, the surrounding lymphoma cells and the local micro-environment. Ibritumomab tiuxetan is an example of radioimmunoconjugate therapy and consists of a murine anti-CD20 antibody linked covalently to a metal chelator (MD-DTPA), permitting stable binding of 90Y to produce enhanced targeted cytotoxicity.16 It is currently indicated for relapsed, refractory or transformed low-grade lymphoma, but is not widely used in Australia. Experience with radioimmunoconjugate therapy in intermediate to high grade diffuse large B-cell lymphoma is limited to patients with refractory or relapsed disease.
Encouraged by the success of rituximab, many new agents targeting surface antigens are in development for B cell lymphoproliferative disorders. SGN-40 is a humanised antibody against CD40 (a member of the tumour necrosis factor receptor family). Epratuzumab is directed at CD22, a specific antigen expressed by pre-B cells and mature normal B cells. Apolizumab and lumiliximab target HLA-DR and CD23 respectively.17 The prospects for further major improvements in outcome for patients with lymphoproliferative disorders are excellent.
The use of all trans retinoic acid (ATRA) as part of frontline therapy for newly diagnosed patients with acute promyelocytic leukaemia, is one of the best examples of how targeting specific genetic lesions within leukaemic cells can result in a remarkable advance in cure rates. Interestingly, it is also an illuminating example of how the molecular mechanism of action was discovered only after empirical proof of its efficacy against the disease. The hallmark of classic acute promyelocytic leukaemia is t(15;17), which results in the production of a PML-RAR? aberrant fusion gene leading to a blockade in differentiation at the promyelocyte stage. Retinoic acid is a critical regulator of the balance between cellular differentiation and self-renewal, and works by binding to a retinoic acid receptor (RAR). Wild-type RAR? is a ligand-dependent transcription factor expressed primarily in haematopoietic cells and normally induces transcriptional repression in the absence of retinoic acid. Like wild-type RAR?, the PML-RAR? fusion protein is a dominant negative inhibitor of retinoid-induced transactivation.18 Treatment with ATRA reverses the inhibitory activity and induces terminal differentiation of malignant promyelocytes.19
The incorporation of ATRA in induction therapy results in a high complete remission rate, leads to rapid resolution of the characteristic life-threatening coagulopathy and most importantly, decreases the relapse rate compared with treatment with chemotherapy alone. In the Australian trial of ATRA plus anthracycline chemotherapy, followed by ATRA maintenance, an overall survival of 88% was observed after five years.20 Acute promyelocytic leukaemia is now the most curable subtype of acute myeloid leukaemia in adults. Sensitive and specific polymerase chain reaction techniques to detect PML-RAR? are available to monitor response and survey for early signs of molecular relapse.
The surface antigen CD33 has been extensively evaluated as a therapeutic target in acute myeloid leukaemia. CD33 antigen is a surface glycoprotein of unclear biological function that is expressed on leukaemic blasts in up to 90% of acute myeloid leukaemia, hence its attraction.21 The expression of this cell surface marker is normally restricted to mature myeloid cells and not in normal haematopoietic stem cells. However, expression of CD33 antigen by leukaemic stem cells capable of repopulating human acute myeloid leukaemia cells in xenograft model using immunodeficient mice has been demonstrated.22 The differential expression between acute myeloid leukaemia and normal myelopoiesis serves as the basis for the desired selective anti-leukaemic effect. Gemtuzumab ozogamicin is a novel conjugated humanised monoclonal antibody of IgG4 subtype directed against CD33 that is covalently attached to a powerful anti-tumour antibiotic, calicheamicin, which in turn is too toxic to be administered as a free drug. The binding of the anti-CD33 antibody portion of gemtuzumab ozogamicin with CD33, antigen on myeloblasts results in the formation of a complex that is rapidly internalised. After entering the leukaemic cells, the calicheamicin derivative is released from the antibody in the acidic environment of the lysosome and subsequently exerts its leukaemia killing effect. In a Phase II clinical trial involving 277 patients over the age of 60 with acute myeloid leukaemia in first relapse, gemtuzumab ozogamicin has been reported to have significant single-agent activity with an overall remission rate of 26%, including 13% complete responders and median relapse-free survival of 6.4 months.23 Gemtuzumab ozogamicin has been approved for relapsed or refractory CD33+ acute myeloid leukaemia in patients over the age of 60 years. A number of prospective trials exploring the potential benefits of gemtuzumab ozogamicin when combined with cytotoxic chemotherapy, in different settings in acute myeloid leukaemia, are underway.
Lintuzumab (SGN-33) is a humanised recombinant monoclonal antibody of IgG1 directed against CD33. Lintuzumab is thought to stimulate antibody dependant cellular cytotoxicity against leukaemic cells expressing CD33. In a dose-finding study, lintuzumab was shown to be well tolerated and have active anti-leukaemic effect with overall objective responses seen in seven of 17 elderly patients with relapsed or untreated advanced acute myeloid leukaemia, including four patients achieving complete remission.24
A number of new exciting targeted therapies are under active evaluation in various haematological malignancies.
FLT3 is a receptor tyrosine kinase that activates a number of signalling proteins involved in the regulation of growth and apoptosis. When constitutively activated by mutation or internal tandem duplication, FLT3 is an oncogene implicated in approximately 25% of acute myeloid leukaemia. The presence of an FLT3 mutation is a powerful predictor of relapse in acute myeloid leukaemia. Lestaurtinib (CEP-701) is an example of a potent inhibitor against FLT3-mutant primary acute myeloid leukaemia samples.25 In vitro studies suggest that clinical benefit may be maximised when lestaurtinib is given immediately following chemotherapy. This concept is now being investigated in randomised clinical trials of FLT3-mutated acute myeloid leukaemia patients. A plasma inhibitory activity assay has proven useful in monitoring the extent of FLT3 inhibition in these trials.
Leukaemic stem cells represent another potential target in acute myeloid leukaemia. CD123 (? sub-unit of the interleukin-3 receptor) is one of few unique markers consistently expressed in leukaemic stem cell compartment (CD34+ CD38-) capable of repopulating human acute myeloid leukaemia cells in an immunodeficient mouse model.26 A fusion protein using anti-CD123 monoclonal antibody with a diphtheria toxin (DR383 IL-3) has been shown to have modest activity in acute myeloid leukaemia in early phase trial.27 A neutralising monoclonal anti-CD123 antibody (CSL360) is currently undergoing Phase I trial in Australia (clinicaltrials.gov).
BCL-2 over-expression is the hallmark of chronic B cell lymphoproliferative disorders. While initial attempts to target BCL-2 through anti-sense therapy (oblimerson) have proved disappointing, a new class of agents, the BH3 mimetics, is emerging as a highly effective way to inhibit BCL-2 and trigger tumour cell death in these diseases. ABT-263 is a novel BH3 mimetic that binds with high affinity and inhibits multiple anti-apoptotic BCL-2 family proteins.28 A recent Phase I study reported impressive early evidence of efficacy with good partial responses observed in patients with advanced and refractory lymphoid malignancies, such as chronic lymphocytic leukaemia.
Given the wealth of new potential agents, the ongoing challenges for developing truly targeted therapies for haematological malignancies are prioritisation according to potential clinical impacts, rationalisation of combination therapies and study designs, minimisation of potential side-effects and the associated financial burden on patients and the health care system.
Targeted therapies have revolutionised care for patients with many haematological malignancies over the last 10 years. However, with the exception of tyrosine kinase inhibitors for chronic myeloid leukaemia, monotherapy with these agents is unlikely curative in the majority of patients. The most promising future approach is to combine these novel agents with conventional cytotoxic chemotherapy to improve clinical outcomes.
9. Hochhaus A, Druker BJ, Larson R, O’Brien SG, Gathmann I, Guilhot F. IRIS 6-year follow-up: sustained survival and declining annual rate of transformation in patients with newly diagnosed chronic myeloid leukemia in chronic phase (CML-CP) treated with imatinib. Blood. 2007;110:Abstract 25.
10. Hochhaus A, Baccarani M, Deininger M, Apperley JF, Lipton JH, Goldberg SL, et al. Dasatinib induces durable cytogenetic responses in patients with chronic myelogenous leukemia in chronic phase with resistance or intolerance to imatinib. Leukemia. 2008;22:1200-6.
11. Cools J, DeAngelo DJ, Gotlib J, Stover EH, Legare RD, Cortes J, et al. A tyrosine kinase created by fusion of the PDGFRA and FIP1L1 genes as a therapeutic target of imatinib in idiopathic hypereosinophilic syndrome. N Engl J Med. 2003;348:1201-14.
12. McLaughlin P, Grillo-López AJ, Link BK, Levy R, Czuczman MS, et al. Rituximab chimeric anti-CD20 monoclonal antibody therapy for relapsed indolent lymphoma: half of patients respond to a four-dose treatment program. J Clin Oncol. 1998;16(8):2825-33.
14. Coiffier B, Lepage E, Briere J, Herbrecht R, Tilly H, Bouabdallah R, et al. CHOP chemotherapy plus rituximab compared with CHOP alone in elderly patients with diffuse large-B-cell lymphoma. N Engl J Med. 2002;346:235.
15. Pfreundschuh M, Trumper L, Osterborg A, Pettengell R, Trneny M, Imrie K, et al. CHOP-like chemotherapy plus rituximab versus CHOP-like chemotherapy alone in young patients with good-prognosis diffuse large-B-cell lymphoma: a randomised controlled trial by the MabThera International Trial (MInT) Group. Lancet Oncol. 2006;7:379.
16. Gordon LI, Molina A, Witzig T, Emmanouilides C, Raubtischek A, Darif M, et al. Durable responses after ibritumomab tiuxetan radioimmunotherapy for CD20+ B-cell lymphoma: long-term follow-up of a phase 1/2 study. Blood. 2004;103:4429.
18. de Thé H, Lavau C, Marchio A, Chomienne C, Degos L, Dejean A. The PML-RAR-alpha fusion mRNA generated by the t(15;17) translocation in acute promyelocytic leukemia encodes functionally altered RAR. Cell. 1991;66:675.
20. Iland H, Bradstock K, Chong L, et al. Results of the APML3 trial of ATRA, intensive idarubicin and triple maintenance combined with molecular monitoring in acute promyelocytic leukemia(APL): a study by the Australiasian Leukaemia and Lymphoma Group (ALLG). Blood. 2003;102:141a (Abstract 484).
22. Taussig DC, Pearce DJ, Simpson C, Rohatiner AZ, Lister TA, Kelly G, et al. Hematopoietic stem cells express multiple myeloid markers: implications for the origin and targeted therapy of acute myeloid leukemia. Blood. 2005;106:4086-92.
23. Larson RA, Sievers EL, Stadtmauer EA, Löwenberg B, Estey EH, Dombret H, et al. Final Report of the Efficacy and Safety of Gemtuzumab Ozogamicin (Mylotarg) in Patients with CD33-Positive Acute Myeloid Leukemia in First Recurrence. Cancer. 2005:104:1442-52.
24. Raza A, Jurcic JG, Roboz GJ, Maris M, et al. Complete Remissions Observed in Acute Myeloid Leukemia Following Prolonged Exposure to SGN-33 (lintuzumab), a Humanized Monoclonal Antibody Targeting CD33. Blood. 2007 (ASH Annual Meeting Abstracts);110:159.
26. Jordan CT, Upchurch D, Szilvassy SJ, Guzman ML, Howard DS, Pettigrew AL, et al. The interleukin-3 receptor alpha chain is a unique marker for human acute myelogenous leukemia stem cells. Leukemia. 2000;14:1777-84.
27. Frankel AE, Weir MA, Hall PD, Holguin M, Cable C, Rizzieri DA, et al. Induction of remission in patients with acute myeloid leukemia without prolonged myelosuppression using diphtheria toxin-interleukin 3 fusion protein. J Clin Oncol. 2007;25:18S(June 20 Supplement):7068.
28. Wilson WH, Czuczman MS, LaCasce AS, Gerecitano JF, Leonard JP, Dunleavy K, et al. A phase 1 study evaluating the safety, pharmacokinetics, and efficacy of ABT-263 in subjects with refractory or relapsed lymphoid malignancies. J Clin Oncol. 2008;26:(Suppl; Abstract 8511).