Assessing the Importance of Peripheral Blast Percentage
Patients with MPNs, particularly PMF and post-ET/PV MF, are predisposed to circulating myelo-blasts in the peripheral blood (Cervantes 2007; Cervantes et al. 2007). This latter phenomenon is true even when there is not a clear increase in bone marrow blast percentage. Reasons for this phenomenon relate to the abnormal trafficking of immature myeloid cells in these patients which may originate from abnormalities of the marrow stroma and is likely responsible for the increased circulating CD34+ cells in these patients (Barosi et al. 2001). Clinically, patients have been shown to have increased peripheral blood blasts for long periods of time without evidence of BP occurring (Cervantes et al. 1997). Based upon this phenomenon, the International Working Group for Myelofibrosis Research and Treatment (IWG-MRT) has included among their clinical trial criteria that a patient on a trial must have a sustained peripheral blood blast percentage of >20% for 4 weeks sustained before acute leukemia can be declared.Until more is known regarding the biological underpinnings of a change from an MPN to MPN-BP, arbitrary clinical cutoffs will remain somewhat cumbersome. Indeed, some patients can have a clinical phenotype of MPN-BP with 15% blasts and succumb to their disease, while others with a higher blast "burden" may have a more indolent course.
Risk Factors for Blastic Transformation
The evaluation and assessment of risk factors in MPN patients is very important as they are at a higher risk over the course of their illness to transform into acute leukemia. The purpose of risk stratification is twofold. One, to identify patients at high risk of death from their disease in order to employ more aggressive therapy such as allogeneic stem cell transplantation earlier in the course of their disease (Kroger et al. 2005). Two, to avoid unnecessary introduction of therapy (in a particular subset of patients) earlier in the course of their disease which could exacerbate this underlying predisposition to acute leukemia (Finazzi et al. 2005; Wolanskyj et al. 2006). Analysis of risk factors for MPN-BP are features present (i.e., intrinsic to their MPN) at diagnosis or during the course of disease (including therapy).
Prognostication for patients with MPNs done at the time of diagnosis can look at risk of vascular events (mainly for ET and PV), death, and development of blast phase (which unfortunately usually leads to rapid death). Of these latter endpoints, we will focus on mortality and transformation. The International Working Group for Myelofibrosis Research and Treatment (IWG-MRT) recently published an International Prognostic Scoring System (IPSS-MF) to aid in assessing MF prognosis. Five features are independently associated with decreased survival age (>65), anemia (hemoglobin <10 g/dL), leukocytes >25 χ 109, constitutional symptoms (night sweats, fever, significant weight loss), and the presence of circulating blasts in the peripheral blood. The IPSS-MF defines four risk groups (0, 1, 2, or more than 2 of the 5 adverse features) with projected survival medians for each group ranging from a median of 27 months for high-risk to 135 months for low-risk disease (Cervantes et al. 2009).
Although limited data exist on predicting eventual leukemic transformation, our group has shown that (1) low JAK2 V617F allele burden,(2) peripheral blast percentage >3% (Huang et al. 2008), and (3) thrombocytopenia present at diagnosis (Huang et al. 2008) were associated with higher risk of MPN-BP in these patients.
Table 12.1 Risk factors at presentation of primary myelofibrosis, which suggest high risk of eventual transformation (MPN-BP)
|
Risk factor |
Association with developing MPN-BP |
References |
|
Demographics |
||
|
Age at diagnosis |
None |
Huang et al. 2008 |
|
Sex |
None |
Huang et al. 2008 |
|
Peripheral blood |
||
|
Hemoglobin |
Yes – univariate only |
Huang et al. 2008; Tefferi et al. 2008a |
|
Leukocyte count |
Yes (>15 χ 109/L) – univariate |
Tefferi et al. 2008a |
|
Platelet count |
<100x 109/L |
Huang et al. 2008 |
|
Presence of blasts |
Yes – univariate |
Huang et al. 2008 |
|
Blast percentage |
Yes (greatest when >3%) |
Huang et al. 2008 |
|
Physical exam features |
||
|
Splenomegaly |
None |
Huang et al. 2008 |
|
Bone marrow |
||
|
Cellularity |
None |
Mesa et al. 2007a |
|
Reticulin fibrosis |
None |
Mesa et al. 2007a |
|
Blast percentage |
Yes |
Mesa et al. 2007a |
|
Karyotypic abnormalities |
Yes |
Mesa et al. 2007a |
|
(complex >2 lesions) |
||
|
Molecular lesions |
||
|
JAK2 V617F |
Low allele burden |
Tefferi et al. 2008b; Swierczek et al. 2007 |
|
MPL mutations |
No data available |
|
|
Myelofibrosis prognostic scores |
||
|
Lille (Dupriez 1996) |
None |
Huang et al. 2008 |
|
Cervantes (Cervantes 2001) |
Yes – univariate only |
Huang et al. 2008 |
|
Mayo (Dingli 2006) |
Yes – univariate only |
Huang et al. 2008 |
Analysis of our institutional experience of eventual MPN-BP in PV and ET patients demonstrated that PV patients with baseline leukocytosis and ET patients with baseline anemia are most predisposed to development of either post-ET/PV MF or MPN-BP.Additionally, our IWG-MRT analysis would suggest patients who eventually transform are more likely to have higher LDH levels and more karyotypic abnormalities (Mesa et al. 2007a) . No uniform prognostic score for MPN-BP yet exists (Table 12.1).
Influence of Therapy up on Development of MPN-BP
No matter what type of cancer is treated, treatments such as radiation and chemotherapy have the potential to lead to the development of a second cancer in the long run. Therapy-related acute leukemia has long been a concern with chemotherapy of malignant neoplasms and is of greater concern in patients with underlying myeloid neoplasms (see Table 12.2). Also, because it can take many years for treatment-related cancers to develop, they are of greatest concern in those patients that have chronic neoplasms such as MPNs. Hence, the role of therapy to accelerate the development of MPN-BP has long been a concern in MPN patients.
Specific to MPNs, the use of myelosuppres-sive therapy with radioactive phosphorus (P-32) (Osgood 1964; Parmentier 2003) is most clearly associated with increased risk of MPN-BP along with alkylator therapy such as melphalan (Petti et al. 2002) and pipobroman (Kiladjian et al. 2006; Najean and Rain 1997). Much controversy exists regarding the agent hydroxyurea, a valid and efficacious myelosuppressive agent demonstrated to decrease risk of vascular events in patients with ET and PV.
Table 12.2 MPN therapies and their association with MPN-BP
|
Therapy |
Association |
References |
|
Medical |
||
|
Hydroxyurea |
Only as combination therapy |
Finazzi et al. 2005 |
|
Erythropoiesis stimulating |
Higher rate of blastic transformation in |
Huang et al. 2008 |
|
agents |
PMF patients treated with ESAs |
|
|
Androgens |
Higher rates of transformation in PMF, especially with danazol |
Huang et al. 2008 |
|
Melphalan |
Higher rates of transformation in trial of patients with PMF |
Petti et al. 2002 |
|
Pipobroman |
Higher leukemia rates in treated PV patients |
Najean and Rain 1997; Kiladjian et al. 2006 |
|
Phosphorus-32 |
Clear and undisputed increased risk of transformation with use |
Osgood 1964; Parmentier 2003 |
|
Thalidomide |
No increase rate seen |
Huang et al. 2008 |
|
Azacitidine |
Only therapy paper in MPN-BP. ORR 52%, CR 24%, median duration of response 9 months |
Thepot et al. 2010 |
|
Surgical |
||
|
Splenectomy |
Conflicting reports, but no clear link established |
Barosi et al. 1998; Mesa et al. 2006a |
Despite much discussion, evidence now suggests that single-agent hydroxyurea is not a significant contributor to leukemic transformation. Indeed, hydroxyurea has not been shown to be leukemogenic in the unrelated disorder of sickle cell anemia (Steinberg et al. 2003). However, there may be a synergistic leukemogenic potential role of hydroxyurea in patients who then go on to receive other treatments (Finazzi et al. 2005). In the end, there is no way to definitively negate a slight role of hydrea on the risk of leukemic transformation, and therefore, patients should be counseled accordingly. There are other agents with no suggestion of leu-kemogenicity (aspirin, anagrelide, and interferon) (Huang et al. 2008) . Our analysis suggested an independently increased risk of MPN-BP among patients exposed to erythroid stimulating agents (ESAs) and androgen (particularly danazol) (Huang et al. 2008). This risk was independent of anemia and was significant in multifactorial analysis. Whether there is a causal role remains unclear, and these single-institution observations do require further validation. Finally, patients who have been splenectomized for PMF have been reported in some series to have higher rates of transformation (Barosi et al. 1998), but it remains unclear whether this may be merely an association in patients with a more aggressive disease course (Mesa et al. 2006a) .
Clinical Course and Therapy of MPN-BP
Ph-negative MPNs usually follow an erratic but rather aggressive course once they start to progress. The absence of a uniform prognostic score to predict this progression makes it more difficult to manage these patients. Molecular events at the transformation from MPN to AML are not only complex but poorly defined. As outlined earlier in this topic, several mutations occurring in MPNs can be found at transformation to AML at the same higher or lower frequency. However, 5-10% of MPNs progress to develop acute myeloid leukemia (AML) after 10 years of onset of disease (Finazzi et al. 2005), and this number maybe even higher with longer follow-up. Once a patient progresses to MPN-BP, significant morbidity and mortality usually follow (Mesa et al. 2005). Clinically, patients will have all of the challenging peripheral blood cytopenias typical in de novo acute leukemia. Additionally, they face the significant debilitation, cachexia, and poor performance status already present from their MPN. Additionally, they will frequently have significant splenomegaly contributing both symptom-atically and to transfusion resistance (Table 12.3).
Table 12.3 Medical options for myeloproliferative neoplasm blast phase (MPN-BP) (Mesa et al. 2005)
|
Therapy |
Composition |
Median survival (months) (Mesa et al. 2005) |
|
Supportive care |
Transfusions |
2.1 (1.1-3.4) |
 |
||
 |
||
|
Noninduction chemotherapy |
Weekly vincristine |
2.9 (0.8-5.3) |
|
Oral alkylators |
||
|
Low-dose cytarabine |
||
|
Oral etoposide |
||
|
Induction chemotherapy |
Cytarabine + anthracycline (7 days) |
3.9 (1.6-8.9) |
|
High-dose cytarabine (>1,000 mg/m2/ dose) |
||
|
Mitoxantrone/VP-16/high-dose cytarabine |
||
|
Antibody therapy |
Gemtuzumab |
2.5 (0.7-3.5) |
aMedian survivals according to Mayo Clinic series
Patients with MPN-BP will frequently have multiple features which are considered characteristic of high-risk acute leukemia, specifically, advanced age, antecedent myeloid disorder, and complex and poor-risk karyotypic abnormalities (Mesa et al. 2005). Therefore, it is not surprising that therapy for these patients has been quite disappointing. We have previously demonstrated that aggressive therapy (with myelosuppressive induction intent) did not seem to offer any survival benefit over purely supportive care (transfusions +/- hydroxyurea) (Mesa et al. 2005). Patients who underwent induction therapy had a 40% chance of returning to a more chronic appearing phase of PMF, but without any clear impact on survival. Therefore, it was thought that induction therapy may well only provide a cosmetic cytoreduction in blasts without meaningful impact (Mesa et al. 2005).
In a recent study, 54 BCR-ABL1-negative MPNs who had progressed to either AML or MDS were treated with azacitidine (Thepot et al. 2010). The overall response rate was 52% (24% CR) with a median duration of response being 9 months. This is the only therapy paper that exists in the field to date; however, although the use of azacitidine is encouraging, the response duration is short, and hence other consolidation therapies need to be evaluated.
At the current time, hematopoietic stem cell transplant (HSCT) appears to the best chance for a cure in a majority of these patients. In a study by MD Anderson Cancer Center (Tam et al. 2008), resolution of blasts through induction chemotherapy was achieved in 46% of those in whom that therapy was delivered, with 8/40 patients undergoing stem cell transplant. Among these latter patients who were successfully transplanted, 73% were alive 31 months after transplant. The transplant results appear to be impacted by several features including comorbidities, conditioning regimens, and disease burden (Scott et al. 2007a). As is noted in MDS, cytoreductive therapy is associated with improved outcomes in certain patient populations (Saberwal et al. 2009; Scott and Deeg 2010) . It is unclear if that is the case with MPN-BP. In any event, an overall management plan that incorporates the possibility of HSCT should be developed for patients with MPN-BP at the time of diagnosis.
The reasons for the lack of success of therapy in MPN-BP are many and include intrinsic drug resistance, lack of tolerability, death from exacerbation of comorbidities and, most importantly, the complete understanding of the molecular patho-genesis of the role of mutations to progression to blastic phase. Given the dismal consequence of transformation and the dire outcomes with conventional therapy, the interest in novel therapeutic options is great.
Investigational Therapies in MPN-BP
In the last 6 years, several new drugs are undergoing evaluation and are in various stages of development. Though most of these drugs are being evaluated in MPNs, they may have a role in MPN-BP. These include thalidomide analogs, JAK2 inhibitor adenos-ine triphosphate (ATP) mimetics, histone deacety-lase (HDAC) inhibitors, and mammalian target of rapamycin (mTOR) inhibitors. We will discuss some of the most promising agents below.
Pomalidomide
Pomalidomide is a second-generation immuno-modulatory drug (IMiD) that appears to have activity in myelofibrosis without the severe toxicity that is seen both in thalidomide and lenali-domide.Recently, we reported the results of a phase II trial of low-dose pomalidomide (0.5 mg/day); 9 of the 10 JAK2 V617F-positive patients with anemia became transfusion independent (Bejar et al. 2011). The drug was very well tolerated, and no neuropathy or grade 4 myelosuppression was observed.
JAK2 Inhibitors
There is preliminary evidence from JAK2 inhibitor trials suggesting that leukemic transformation may be decreased when compared to historical studies (Eghtedar et al. 2010) . Although JAK2 allele burden may not play a direct role in transformation to MPN-BP, it does correlate with overall worsened disease features. This constant proliferative drive may generate cellular stress that leads to formation of reactive oxygen species (ROS) and other cellular insults with subsequent genomic damage and instability as a pathway during progression. It is therefore conceivable that JAK2 inhibitors may delay progression. However, if JAK2 inhibitors indeed can prevent or reduce transformation to AML remains to be shown. Given the overall more benign course of PV and ET, compared, for example, to myelodysplastic syndrome patients, it will require large patient populations treated with these drugs to answer those questions. Stratifying patients at higher risk for progression to AML, especially patients with PMF, may allow the reduction in sample size of patients. Conceptually, patients demonstrating increased or rising JAK2 allele burden (as a harbinger of disease progression) may be candidates for JAK2 inhibitor combination trials.
Some of the new agents targeting putative underlying molecular mechanisms are, for example, hedgehog pathway inhibitors (HPI), the first of which have completed clinical trials (Lorusso et al. 2011 ; Von Hoff et al. 2009). These agents are being tested in hematological malignancies including CML as well as studies which are planned in MPNs. Survivin inhibitors have been tested in hematological malignancies and have shown some activity in lymphomas (Tolcher et al. 2008) as well as CML and AML in an early clinical trial performed by our group (Tibes et al. 2009b) . With the increasing availability of targeted agents, many of these have been and are currently tested in various stages and phases of MPNs. For example, mTOR inhibitors have shown clinical activity in PMF and post-PV/ET MF patients with response rates for spleen size reduction of up to 46% and resolution of systemic symptoms by 52-74% (Vannucchi et al. 2010). Epigenetic modulation HDAC inhibitors seem promising and have been assessed in small studies, for example, with LBH589 (Mascarenhas et al. 2009). Overall, six patients experienced clinical improvement (Mascarenhas et al. 2009), and with another HD AC inhibitor ITF2357 (Rambaldi et al. 2008), six of eight patients had significant clinical responses. Responses were seen in ET, PV, and PMF patients. Spleen size reduction was seen in six of eight patients with splenomegaly at baseline, and pruritus was relieved in most patients. Both agents were generally well tolerated.
Other agents currently being tested include GSK3beta, TGFB, and HSP90 inhibitors. Novel agents are commonly tested in advanced stages and leukemic phase of MPNs first, and thereafter, clinical activity is assessed in chronic phase MPNs. Alternatively, agents may also be introduced earlier in the disease process which may yield better response rates.
Novel Investigational Concepts in MPN-BP
Much of the work of MPN to AML progression is derived from mouse models, and selected relevant data will be briefly summarized with a focus on therapeutic targeting of the underlying molecular mechanisms.
One proposed mechanism and gene involved include the interferon consensus sequence binding protein (ICSBP). ICSBP deficiency induces a MPN phenotype with progression to AML over time. Saberwal et al. demonstrated that in mouse models, the ICSBP has a leukemia suppressor effect by binding and regulating Fanconi F gene which is involved in DNA damage repair and maintaining genomic integrity (Saberwal et al. 2009). ICSBP deficiency leads to accumulation of chromosomal damage and aberrations over time. Interestingly, ICSBP expression is decreased in AML and CML in blast crisis (Saberwal et al. 2009). Interferon-alpha-2a yielded complete clinical as well as 33-58% molecular remissions in patients with PV and ET (Kiladjian et al. 2008; Quintas-Cardama et al. 2009). A connected candidate gene is the tyrosine phosphatase SHP2 (Konieczna et al. 2008). Constitutive activation of SHP2 in conjunction with ICSBP haploinsuffi-ciency leads to an accelerated myeloproliferative picture with apoptosis resistance and rapid progression to AML in mouse models in a cytokine-dependent manner (Konieczna et al. 2008).
Additional genes linked to progression are PU.1, an essential myeloid transcription factor, leading to accelerated MPN to AML progression in mouse models (Bejar et al. 2011). The inhibitor of apoptosis protein survivin is overexpressed in patients with chronic myelomonocytic leukemia (CMML) and other MPNs (Invernizzi et al. 2006), suggesting a role in altering the balance of proliferative, differentiation, and apoptotic signals resulting in myeloproliferation.
Further areas of research possibly connecting several underlying mechanisms focus around generation of reactive oxygen species (ROS). ROS formation leads to increased DNA damage with resulting genomic instability as a driving factor in pathogenesis of progression from MPNs to more aggressive myeloid malignancies (Sallmyr et al. 2008). It is speculated that defects in some of the major cellular repair pathways such as nonhomol-ogous end joining (NHEJ) activate compensatory repair pathways that are error prone, creating structural chromosomal abnormalities such as deletions or translocations (Sallmyr et al. 2008). This higher degree of genomic instability and karyotypic abnormalities are noted in patients with more aggressive disease, and blastic transformation has been observed in patient samples (Mesa et al. 2005, 2007b. . In MPN-BP patients, there was a threefold increase in genomic alterations when compared with samples in chronic phase. The alterations included known recurrent deletions of chromosome 7 and 5, and trisomy 8 (+8, i.e., C-MYC) among others (Thoennissen et al. 2010). Altered chromosomal regions involved in disease progression harbor established myeloid target genes (ETV6, TP53, and RUNX1). As outlined earlier in more detail, for most of the well-characterized clinical mutations encountered in chronic phase MPNs, there is insufficient evidence to suggest a contribution to leukemic transformation. However, for some mutations/deletions like TET2, IDH1 and IDH2,IKAROS family zinc finger 1 (IKZF1) (Jager et al. 2010), and possibly RUNX1 (Ding et al. 2009) mutations, higher frequency of mutations are found in the blast phase indicating their potential in the pathogenesis of progression to AML. How this increasing knowledge of involved genes and pathogenesis affects treatment decisions in MPN-BP is currently unknown. This is mainly because most of the identified genes and targets do not have candidate drugs that are available as of yet. However, it opens avenues for research and development of new clinical treatment strategies with currently available drugs or agents in development for targeting putative candidate genes. For example, TET2 mutations are involved in epige-netic regulation (Ko et al. 2010); 5-Azacytidine has shown promising results in patients with AML and MDS transformed from Ph-negative MPNs. As described previously, overall and complete response (CR) rates were 52% and 24%, respectively (Thepot et al. 2010); 5-azacytidine has been demonstrated to be less active in chronic phase MPNs (Mesa et al. 2009). There is limited but encouraging clinical experience with 5-Azacytidine in erythroid leukemias with CR rates of 58% (n=10/17 patients) accompanied by frequent cytogenetic responses (Vigil et al. 2009). With our increased molecular understanding and clinical activity data, this can be integrated into treatment strategies and, for example, to assess patients with TET2 mutations for their response to 5-Azacytidine.
In direct extension of the above described data and work, applying high-throughput RNA interference, we identified the BCL-2 family members and specifically BCL-XL as potent sensitizers to 5-Azacytidine (Bogenberger et al. 2010; Tibes et al. 2009a). BCL-XL is a lineage-specific oncogene in an erythroid and megakaryocytic lineage (Silva et al. 1998). and its targeting alone or in combination has a strong scientific rationale. A potent BCL-XL inhibitor is ABT-263 with which we have proposed a clinical trial in combination with 5-Azacytidine or interferon-alpha-2a. Altogether, there seems to be a strong component of epigenetic regulation, and 5-Azacytidine has already demonstrated encouraging clinical activity in patients with AML and MDS transformed from MPNs.
With the increasing molecular understanding of MPNs, new knowledge of genomic events occurring at transformation into leukemic phase as well as with the increasing number of targeted agents, it seems hopeful that improvement in treatment and outcome for patients with MPNs at all stages will be made over the next years.
Conclusion
Though rare, the progression of patients to MPN-BP is an extremely serious development for patients both symptomatically and prognos-tically. Salvage of patients through induction chemotherapy followed by allogeneic stem cell transplant is possible, but likely an option only in a small number of MPN-BP patients.
Although we have a partial understanding of risk factors for eventual transformation, we have an incomplete understanding as to the pathogenetic mechanisms of disease progression. Given the rapid mortality and resistance to current therapies seen in patients with MPN-BP, the need for novel and targeted therapy for these patients is great. A better understanding of mechanisms of clonal progression is required to identify valid therapeutic targets. Hopefully, blockade of JAK2 earlier in the course of an MPN will delay or inhibit disease progression, yet whether this will occur depends upon long-term follow-up on current JAK2 inhibitor trials. Given the uncertain role that the JAK-STAT pathway maintains in the process of leukemic transformation, the need for further study into mechanisms of disease progression is crucial.
