Results of a multicenter phase II trial for older patients with c-Kit-positive acute myeloid leukemia (AML) and high-risk myelodysplastic syndrome (HR-MDS) using low-dose Ara-C and Imatinib

Authors

  • Florian Heidel MD,

    1. Department Internal Medicine III, University-Hospital, Mainz, Germany
    Search for more papers by this author
    • The first 2 authors contributed equally to this work.

  • Jorge Cortes MD,

    1. Department of Leukemia, M. D. Anderson Cancer Center, Houston, Texas
    Search for more papers by this author
    • One author (L.L.) is employed by Novartis, whose product Imatinib was investigated in this study. One author (C.H.) is a consultant for Novartis. Study medication (Imatinib) was provided by Novartis.

  • Frank G. Rücker MD,

    1. Department Internal Medicine III, University-Hospital, Ulm, Germany
    Search for more papers by this author
  • Walter Aulitzky MD,

    1. Robert-Bosch-Hospital, Stuttgart, Germany
    Search for more papers by this author
  • Laurie Letvak MD,

    1. Novartis Pharmaceuticals, E. Hanover, New Jersey
    Search for more papers by this author
    • One author (L.L.) is employed by Novartis, whose product Imatinib was investigated in this study. One author (C.H.) is a consultant for Novartis. Study medication (Imatinib) was provided by Novartis.

  • Thomas Kindler MD,

    1. Department Internal Medicine III, University-Hospital, Mainz, Germany
    2. Howard Hughes Medical Institute, Boston, Massachusetts
    Search for more papers by this author
  • Christoph Huber MD,

    1. Department Internal Medicine III, University-Hospital, Mainz, Germany
    Search for more papers by this author
    • One author (L.L.) is employed by Novartis, whose product Imatinib was investigated in this study. One author (C.H.) is a consultant for Novartis. Study medication (Imatinib) was provided by Novartis.

  • Hartmut Döhner MD,

    1. Department Internal Medicine III, University-Hospital, Ulm, Germany
    Search for more papers by this author
  • Hagop Kantarjian MD,

    1. Department of Leukemia, M. D. Anderson Cancer Center, Houston, Texas
    Search for more papers by this author
  • Thomas Fischer MD

    Corresponding author
    1. Department Internal Medicine III, University-Hospital, Mainz, Germany
    • Thomas Fischer, MD, 3rd Medical Department Johannes Gutenberg University Mainz, 55101 Mainz, Germany
    Search for more papers by this author
    • Fax: (011) 49-6131-17-6678


Abstract

BACKGROUND.

Imatinib (IM) is a potent tyrosine kinase inhibitor of c-Kit. c-Kit is expressed in the majority of patients with acute myeloid leukemia (AML). Whereas clinical trials evaluating monotherapy with IM in AML revealed low response rates, Ara-C and IM showed synergistic effects in vitro. This suggested evaluation of a combination treatment.

METHODS.

Low-dose Ara-C (LDAC) combined with IM was tested to determine the efficacy and safety of this regimen. Forty patients from 4 centers with c-Kit-positive AML (n = 34) and high-risk myelodysplastic syndrome (HR-MDS) (n = 6) with a median age of 73 years were enrolled. They were either not eligible for myelosuppressive therapy and/or had recurring/refractory disease.

RESULTS.

Thirty-eight patients were evaluable for analysis. In 6 of 38 patients a blast response was observed. Eight of 38 patients showed stable disease for more than 2 months. The objective hematologic response rate was low (11%), with 2 patients showing hematologic improvement and 1 each with a partial response (PR) or complete response (CR). Median overall survival was 138 days, with 20% of patients alive after an observation period of 600 days. Study medication was applied in an ambulatory setting with minimal hospitalization time, an early mortality rate of only 18.9%, and a low toxicity rate.

CONCLUSIONS.

LDAC plus IM does not appear to be inferior in older AML patients incomparison with historic controls receiving myelosuppressive therapy. However, this trial also shows that LDAC/IM does not appear to be more effective than LDAC monotherapy in a patient population not selected for appropriate molecular markers. Cancer 2007 © 2007 American Cancer Society.

Acute myeloid leukemia (AML) shows an incidence of 17.6 per 100,000/year in patients 65 years of age or older compared with 1.8 per 100,000 per year in patients under the age of 65 years.1 More than 50% of all AML cases affect patients over age 60.2 Besides decreased tolerance to chemotherapy, different biology of the disease (eg, higher incidence of unfavorable cytogenetics and of antecedent myelodysplastic syndrome [MDS]) and increased prevalence of comorbidity contribute to a worse outcome in older patients with complete response (CR) rates between of 38% to 62% compared with a 73% CR rate in patients younger than 60 years.2–6 Additionally, long-term overall survival in adults over the age of 60 was shown to be dismal, with a range of only 5% to 15% in multicenter trials.2–4, 7

Survival advantage for older patients achieving remission upon myelosuppressive chemotherapy in comparison to best supportive care or low-dose chemotherapy regimens (21 vs 11 weeks) has been demonstrated in 1 study.8 However, median survival in that study was only 16 days longer than median time spent in hospital for chemotherapy. Patients who choose myelosuppressive chemotherapy have been shown to spend 79% of their survival time in the hospital, whereas only 14% of patients receiving low-dose regimens or best supportive care in an ambulatory setting became hospitalized.9 Thus, the role of myelosuppressive chemotherapy in this patient population is currently unclear.

c-Kit is a class III receptor tyrosine kinase (RTK) closely related to other RTKs like vascular endothelial growth factor receptor (VEGFR) or platelet-derived growth factor receptor PDGFR.10 Binding of its ligand, stem-cell factor (SCF), results in receptor tyrosine phosphorylation and activation of downstream pathways. c-Kit is expressed in hematopoietic progenitors, mast cells, germ cells, and some human neoplasias.11, 12 This RTK has a major role in maintaining normal hematopoiesis, growth, and differentiation. Additionally, mutation of c-Kit was found to cause ligand-independent receptor activation and growth of tumor cells in a variety of neoplasias (eg, gastrointestinal stroma tumors [GIST] and others).13, 14 In AML, c-Kit was proposed to play a functional role. On AML blasts c-Kit expression was detected in 63% to 80% of patients analyzed.15, 16 Recently, the efficacy of small molecule RTK inhibitors to inhibit tyrosine phosphorylation of Flk1/KDR (which has structural and sequence similarity to c-Kit) was shown in vitro. Inhibition of this RTK led to decreased proliferation and increased apoptosis. Additionally, SCF-induced phosphorylation of c-Kit was shown in AML blasts and induction of apoptosis upon c-Kit TKI therapy could be demonstrated.17 These results suggested that c-Kit inhibition might be a useful target in the treatment of AML.

Imatinib (IM; STI-571, Gleevec, Glivec) is a 2-phenylaminopyrimidine protein-tyrosine kinase inhibitor and ATP analog with strong activity in Bcr-Abl-positive chronic myeloid leukemia (CML). Additionally, IM inhibits other receptor tyrosine kinases like PDGFRalpha and beta as well as c-Kit.18, 19 Besides its significant clinical activity in CML and Bcr/Abl, a positive acute lymphoid leukemia (ALL) therapeutic benefit could be shown in GIST with activating KIT or PDGFRα mutations.20–22

Two studies investigated the efficacy and safety of imatinib as single-agent therapy in patients with c-Kit positive AML or high-risk MDS. Eighteen patients were treated in a Phase II study by Cortes et al.23 with 400 mg IM daily. Twelve of them had refractory or recurring disease, whereas 6 of them were untreated. In all, 94% showed a c-Kit positivity of >20% on AML blasts. None of these patients showed any conventionally defined hematologic response on IM monotherapy; however, 2 patients experienced reductions in spleen size.23 Of 21 patients treated with 600 mg IM daily (all patients showed c-Kit positivity >30%), 4 responded, with 2 CR, 1 partial response (PR), and 1 with no evidence of leukemia in the bone marrow. The median duration of the response was 108 days.24 However, some of the responses observed were derived from relatively small reductions in leukemic blasts.24

Low-dose Ara-C (LDAC) is suggested to induce differentiation in AML and MDS and is often used as a palliative treatment approach for these indications. In a meta-analysis a CR rate of 17% and a PR rate of 19% with a median survival of 15 months has been shown.25 However, as a single agent LDAC does not improve overall survival in these patients in comparison to the best supportive care.

IM and LDAC have demonstrated synergistic potential in patients with CML. Additionally, several groups reported in vitro synergistic effects with these agents.26, 27 For these reasons we conducted a multicenter Phase II trial using IM and LDAC as a combination therapy in a cohort of older patients who were not eligible for myelosuppressive chemotherapy and who received this study medication in an ambulatory setting.

MATERIALS AND METHODS

Target Population

Male and female AML and MDS patients older than age 18 who were not eligible for myelosuppressive chemotherapy or had recurring/refractory disease upon limited pretreatment (ie, best supportive care only, biologic/targeted agents, or <3 cycles standard chemotherapy) or were diagnosed with high-risk myelodysplastic syndrome (CMML) were included.

Patients had to have a World Health Organization (WHO) performance status between 0 and 2 and had to be recovered from prior cytotoxic chemotherapy. Treatment with hydroxyurea was allowed up to 24 hours before Day 1 of study drug administration. At least 20% CD117 positivity had to be detected in the blast population as determined by flow cytometry before administration of study medication.

Uncontrolled active infection, pregnancy, or breast-feeding were exclusion criteria. Patients had to be free of marked liver or renal disease (as indicated by levels of total bilirubin <2.0 times upper normal limit [UNL], serum alanine aminotransferase [ALT] < 3.0 times UNL, and serum creatinine <1.5 times UNL).

The study was performed in accordance with the declaration of Helsinki and was approved by the local ethics committee. All patients gave written informed consent according to the local guidelines.

Study Design

The study was designed as an international, multicenter, single-arm Phase II study. According to the treatment plan patients received oral doses of 600 mg IM daily and 10 mg of LDAC subcutaneously for 21 days every 28 days. Treatment was planned to be continued for a maximum duration of 12 months. Hematologic and nonhematologic toxicity was assessed according to the National Cancer Institute/National Institutes of Health (NCI/NIH) Common Toxicity Criteria.

Dose levels could be adapted for hematologic toxicity or non-sufficient antineoplastic effect to 6 different additional dose levels (LDAC 15 mg/IM 800 mg; LDAC 10 mg/IM 800 mg; LDAC 7.5 mg/IM 600 mg; LDAC 7.5 mg/IM 400 mg; LDAC 7.5 mg/IM 300 mg; LDAC 5 mg/IM 300 mg). Within the first 28 days of study medication no dose reductions were planned. For grade 2–4 nonhematologic toxicity therapy was withheld until resolution to grade <2. In patients with grade 3–4 liver toxicity therapy had to be withheld immediately, with retesting of liver enzymes and bilirubin within 1 week until resolution of all these parameters to grade 0–1. Treatment toxicity was evaluated by patient interview and laboratory tests at each visit.

Patients who showed no significant antitumor effect after 3 courses of therapy were taken off study. Concomitant administration of any other anticancer therapy was prohibited. For marked leukocytosis leukapheresis was possible. Subjects could receive antiinfective prophylaxis according to institutional practices. The use of colony-stimulating factors was allowed. Because of the inherent risk of either reduced activity or enhanced activity of the concomitant medication and/or IM, drugs known to interact with the same CYP450 isoenzymes (2D6 and 3A4) had to be used with caution as indicated in the appendix of the protocol.

Efficacy Assessments

Patients were evaluated for response on day 28 of treatment or before the start of cycle 2, and then every 3 months by bone marrow biopsies if necessary and peripheral blood (PB) evaluation. CR, PR, and hematologic improvement (HI) were considered as primary efficacy parameters as recommended and defined previously.2–4, 24, 28 Definition of HI was based on the affected cell lines: erythroid response (baseline hemoglobin (HGB) <11 g/dL increasing by >2 g/dL or baseline red blood count (RBC) transfusion dependent becoming independent), platelet response (baseline platelet count (PC) < 100 × 109/L increasing by >30 × 109/L or baseline platelet transfusion dependent becoming transfusion independent), and neutrophil response (baseline ANC <1.5 × 109/L increasing by at least 100% or an absolute increase >500 × 109/L, whichever is greater).

Minor responses were described as blast response (BR; reduction of blasts in peripheral blood >50% of baseline) and stable disease (SD; absence of progression for more than 2 months of therapy). Progressive disease (PD) was defined as an increase of blasts in PB or bone marrow BM >50% on therapy.

Study Dates

The final study protocol was completed in February 2004 according to recommendations of the ethics committee (Mainz, Germany) on January 29, 2004 (registration no. 837.469.03 (4131)). Start of the study was February 18 2004 (inclusion of the first patient). The last patient was included March 30 2005. Analysis of study data was started in November 2005.

c-Kit Analysis

The percentage of c-Kit-positive blasts within the blast gate was determined by FACS analysis using the anti-human CD117 (c-Kit; SCF receptor) monoclonal antibody with phycoerythrin conjugation (Beckman Coulter, Fullerton, CA).

Statistical Methods

The primary objective of this study was to assess efficacy in terms of the early mortality rate (EMR) (death within 6 weeks of treatment) to the combination of LDAC/IM in patients with AML or high-risk myelodysplastic syndrome (HR-MDS). Sample size and decision criteria were chosen to reduce the expected accrual if the treatment was ineffective.

The historical EMR for these patients is 25%.3, 4 We were interested in assessing the EMR for our new treatment. We thought that the EMR for our treatment would be 15%. We accepted EMRs similar to the historical rate because our treatment would be given on an outpatient basis, and thus have lower resource utilization and potentially higher quality of life.

The cohort analyzed in this study consisted of patients who had received at least 1 dose of LDAC/IM and for whom response status could be evaluated (intent-to-treat population). Cancer-related symptoms and their severity were documented on the basis of frequency tables on each visit. For statistical analysis GraphPad PRISM v. 4.0 (2003, GraphPad Software, San Diego, CA) was used.

RESULTS

Patients and Treatment

In total, 40 patients were treated according to the study protocol. All patients were started with 10 mg of LDAC subcutaneously and 600 mg of IM orally daily. Dose levels were adapted for hematologic toxicity as described in Materials and Methods. Baseline characteristics of patients and treatment are depicted in Table 1. Two out of the 40 patients were excluded from our analysis because of incorrect treatment schedule or inclusion criteria; 1 patient was evaluable incompletely due to loss of follow-up. This patient was censored on the last visit date.

Table 1. Baseline Characteristics of All Patients Enrolled in the Study
CharacteristicNo. of patients
  1. ECOG indicates Eastern Cooperative Oncology Group; AML, acute myeloid leukemia; HR-MDS, high-risk myelodysplastic syndrome; CMML, chronic myelomonocytic leukemia; SD, standard deviation.

Total, no.40
Age, y
 Mean± SD70.1± 7.7
 Median73
 Range42–82
 Younger than 60, %5
 At least 60, %95
Sex, no. (%)
 Men24 (60)
 Women16 (40)
Diagnosis, no. (%)
 AML34 (85)
 HR-MDS (including CMML)6 (15)
Cytogenetic risk group, no. (%)
 Favorable0
 Intermediate28 (70)
 Adverse5 (12.5)
 No data available7 (17.5)
ECOG performance status, no. (%)
 015 (38.5)
 114 (35)
 211 (27.5)
Duration of disease, days
 Mean± SD102± 226
 Median22
 Range1–1291
Previous cycles of treatment
 None20 (50)
 111 (27.5)
 29 (22.5)
c-kit positivity, %, n = 32
 Mean± SD62.1±27.2
 Median67
 Range20–100
Secondary AML, no. (%)6 (15)

After 1 month of treatment 34 (89.5%) patients, and after 3 months of treatment 24 (63.2%) patients were still evaluable for follow-up. Median follow-up was 130 days (mean follow-up, 160 days; range, 8–600 days).

Although the study was not restricted to older patients only, the inclusion criteria (‘patients not eligible for myelosuppressive therapy or HR-MDS’) led to inclusion of almost only older patients. The median age was 73 years with a range of 42 to 82 years; 95% were at least 60 years of age and 16 (40%) were women. The majority of patients (85%) had a diagnosis of AML, whereas 15% had HR-MDS. Most of the patients included were restricted in physical strength with an Eastern Cooperative Oncology Group (ECOG) score of 1–2 (62.5%). Median time from diagnosis to start of the combination therapy with LDAC/IM was 22 days. Whereas 50% of patients were previously untreated, 27.5% had received 1 line of therapy and 22.5% had received 2 lines of treatment before.

The majority of treatment time was conducted in an ambulatory setting. The mean hospitalization time, analyzed for patients treated at the University Hospital Mainz (n = 17), was 9.5 days (median, 5 days; range, 0–31 days) with a mean treatment duration of 67.5 days (median, 35 days; range, 8–426 days).

Clinical Responses and Biologic Activity

For analysis of efficacy 38 patients were evaluable. Six of 38 (16%) patients showed a blast response in peripheral blood with a reduction >50% of peripheral blasts. Eight of 38 of cases (21%) showed stable disease without progression for a minimum duration of 2 months.

The objective clinical response rate was low, 11% of patients (Table 2). One (3%) patient each reached CR and PR. These responders remained in remission for 253 days and 164 days, respectively. Two patients had hematologic improvement. Whereas 1 patient showed improvement of hematologic parameters for a period of 376 days without reaching PR, the other showed an isolated platelet response for a 195-day period. The median progression free survival (PFS) was 41 days (Fig. 1), with a maximum duration of 405. Three patients experienced long-term progression-free survival of more than 250 days.

Figure 1.

Progression-free survival (PFS) of 38 patients treated with imatinib IM and low-dose Ara-C (LDAC). The documented observation period for PFS was 405 days.

Table 2. Efficacy of Combination Therapy of Imatinib (IM) and Low-Dose Cytarabine (LDAC)
 No. (%)
  • CR indicates complete response; PR, partial response.

  • *

    Defined as absence of progression ≥2 months.

Total38 (100)
Clinical responses4 (11)
 CR1 (3)
 PR1 (3)
 HI2 (5)
Blast response6 (16)
Stable disease*8 (21)

Expression of c-Kit Receptors in BM and Cytogenetic Analysis

In all, 32 patient samples were evaluable for analysis. Median c-Kit positivity of blasts was 67% (range, 20–100%). Cytogenetic analysis was available in 33 patients with AML. Whereas the majority of analyzed samples could be classified as intermediate risk (85%), additional risk factors such as secondary AML or the presence of an FLT3 ITD mutation were seen in 6 and 1 patients, respectively.

No clear evidence for a correlation of CD117 positivity and cytogenetics with patient response was obvious. Whereas 3 of the responders showed a high percentage of c-Kit-positive blasts (75%–94%), 1 partial responder revealed c-Kit expression of only 20% of blasts.

Early and Long-Term Mortality

In this study early mortality was assessed after 6 weeks of treatment as well as after 3 months. After the first 6 weeks of treatment the mortality rate was 18.9% (7 of 37 patients). After 3 months of treatment evaluation was repeated and showed a 3-month mortality of 33.3%. At the time of this analysis all of the patients had either died of infectious complications or progressive disease. The median overall survival (OS) was 138 days, with 20% of patients alive at the end of the observation period of 600 days (Fig. 2).

Figure 2.

Overall survival of 38 patients included in this study. Within the observation period of 600 days, 20% of patients were still alive with or without ongoing treatment.

Safety Profile

As depicted in Table 3, 18 events were reported for patients who experienced grade 3–4 neutropenia, 23 events for grade 3–4 thrombocytopenia, and 21 events for grade 3–4 anemia. Most of the patients with grade 3–4 hematologic toxicity experienced aggravation of preexisting grade 2–3 pancytopenia. Pancytopenic patients were transfused with platelets (PLT < 20,000/μL) and red cells (HGB < 8.0 g/dL) in an ambulatory setting.

Table 3. Frequency and Severity of Hematologic Adverse Events
 Grade 1Grade 2Grade 3Grade 4
Neutropenia13414
Thrombocytopenia491013
Anemia213192

Mild nonhematologic adverse events (AEs) were reported in the vast majority of patients (Table 4). Grade 1–2 nausea (n = 18), vomiting (n = 7), musculoskeletal pain (n = 11), rash (n = 5), infections (n = 14), edema (n = 9), fatigue (n = 6), weight loss (n = 6), and bleeding events (n = 6) were the most common reported events. Those mild AEs were reversible in the majority of cases with supportive treatment. A severe form of nausea was observed in 1 case with preexisting liver disease and refractory ascites. Three patients developed an IM-associated grade 3 skin rash that was reversible after discontinuation of imatinib medication. Patients did not continue study medication and were taken off study due to patients' wish.

Table 4. Frequency and Severity of Nonhematologic Adverse Events Possibly, Probably, or Definitely Related to Study Drug Administration
 No. of patients (%)
Grade 1/2Grade 3/4
  1. TIA indicates transient ischemic attack.

Gastrointestinal
 Nausea18 (47)1 (3)
 Vomiting7 (18)1 (3)
 Diarrhea3 (8)0
 Constipation6 (16)0
 Mucositis2 (5)0
 Other4 (11)0
Pain
 Visceral2 (5)0
 Musculoskeletal11 (29)3 (8)
 Headache3 (8)0
 Other1 (3)0
Skin changes
 Rash5 (13)3 (8)
 Pruritus2 (5)0
Infection
 Neutropenic fever1 (3)8 (21)
 Pulmonary infection5 (13)1 (3)
 Other8 (21)1 (3)
Cardiovascular
 Edema9 (24)1 (3)
 Thrombosis/embolism02 (5)
 Cardiac arrhythmia01 (3)
 Other3 (8)0
Mental/neurology
 Depression/anxiety1 (3)0
 Agitation/confusion1 (3)1 (3)
 Syncopation/TIA02 (5)
Constitutional
 Fatigue/weakness6 (16)0
 Weight loss/anorexia6 (16)0
 Loss of appetite2 (5)0
Musculoskeletal/fracture1 (3)0
Bleeding/hemorrhage6 (16)0
Laboratory3 (8)1 (3)
 Hypokalemia3 (8)1 (3)
 Creatinine01 (3)
Allergic rhinitis1 (3)0

Other severe (grade 3–4) nonhematologic AEs (as depicted in Table 4) were not suspected to be caused by study medication. Grade 3–4 neutropenic fever and infectious complications were thought to be related to hematologic toxicity. None of the patients evaluable for safety (n = 39) experienced drug-related death. The majority of the patients died of progressive disease.

Cytarabine was administered by the patients and/or related persons who were instructed in subcutaneous injection. No adverse events were reported for drug administration and handling of the injections was reported to be safe.

DISCUSSION

This trial was conducted to assess the efficacy and safety of a combination therapy of IM and LDAC. Prior investigations using IM as a single agent in AML have shown inconsistent results,23, 24 which might be due to different dosing. Low tumor burden and start of IM in patients reconstituting with blasts after myelosuppressive chemotherapy have been associated with response.

Regarding the safety of a combination therapy with LDAC/IM, no major concerns became evident in drug administration. Oral intake of IM was generally well tolerated and subcutaneous injection of cytarabine was performed by the patients or with support of relatives. The incidence of mild nausea/vomiting as well as episodes of pancytopenia observed were comparable to symptoms of AML patients at the same age receiving hydroxyurea, cytarabine, or best supportive care.

Objective response rates (ORRs) of this combination therapy were low in comparison to a myelosuppressive regimen. Whereas induction chemotherapy achieves ORRs from 38% to 64%,2–4 even in patients older than age 60, this LDAC/IM combination showed an ORR of only 11%. In AML, response rates do not always correlate with survival or quality of life. Only 5% to 15% of older patients treated with myelosuppressive chemotherapy show long-term disease-free survival.2, 7 On 600 days of observation, 20% of patients who received LDAC/IM therapy were still alive in this trial. Interestingly, this figure for overall survival is very similar (OS less than 20% in 2 years) to results of a recent ECOG Phase III trial employing several cycles of myelosuppressive chemotherapy.29

The early mortality rate of this combination therapy at 6 weeks was 18.9%. This compares favorably to 25% in large historical control groups of patients receiving myelosuppressive chemotherapy.3, 4 Thus, the LDAC/IM regimen employed does not appear to be inferior regarding short-term survival.

From another point of view, the LDAC/IM regimen investigated here produced a slightly lower overall remission rate as an LDAC monotherapy previously described in the MRC14 trial.30 However, published follow-up information on PFS and OS from that trial is currently not available. This trial shows that LDAC in combination with IM does not appear more effective than LDAC alone. Overall, we believe this trial did not use the appropriate selection criteria for evaluation of IM therapy. Recently, it has been shown that IM has antiproliferative activity in AML blasts selected for t(8;21) and mutated/overexpressed c-KIT, whereas cells from AML cases with neither t(8;21) nor c-KIT mutation/overexpression did not respond to IM.32 In this trial, however, no selection was performed to include patients with t(8;21) and mutated/overexpressed c-KIT. Thus, this may explain why a combination of LDAC plus IM was not more effective.

Patients had to have at least 20% c-Kit-positive blasts to be eligible for this trial. However, the percentage of c-Kit-positive blasts did not correlate with clinical response. Whereas 3 of the responders had expression levels of 75% to 94% (1 CR, 2 HI), 1 patient with a partial response showed 20% c-Kit-positive AML blasts. Therefore, the mechanisms of response and prediction of response remain unclear. However, expression of c-Kit as a sole marker for potential susceptibility to IM is obviously insufficient. Only 1 of the previously performed studies using IM in the treatment of AML had controlled for c-Kit mutations within known hot spot regions (exons 2, 8, 10, 11, 12, 17).24 However, no mutations were found in this analysis, and the molecular basis for a response to IM in AML still needs to be resolved.31

Inhibition of other kinases such as PDGFR could be a possible mechanism.33 This hypothesis is supported by the efficacy of IM reported in patients with hypereosinophilic syndrome caused by FIP1L1-PDGFRα fusion.34, 35 In our study no cytogenetic translocations involving PDGF-receptor could be detected (Table 3). M-CSF-receptor (CSF-1R) is another member of the tyrosine kinase-receptor class-III family that has recently been reported to respond to treatment with IM.36 Downstream signaling and M-CSF-receptor-dependent cell growth was demonstrated to be sensitive to IM treatment. Additionally, mutant M-CSF-receptor cells (Asp-802) showed resistance to IM treatment. Point mutations at other sites have been shown to contribute to a growth advantage of clonal cells.37 This member of the RTK III-family has not been investigated for its role as a therapeutic target in AML yet. Therefore, molecular analysis of other target kinases such as PDGFR and CSF-1R is warranted in future analysis of AML patients. However, as complete clinical remission rates have also been reported upon LDAC monotherapy in 17% of cases,30 the true efficacy of IM in addition to chemotherapy as a part of combination therapy will have to be tested in a randomized, controlled Phase II trial. In this regard, in older AML patients (>60 years) the HOVON cooperative group has recently started a randomized Phase II trial employing dose-reduced induction and consolidation chemotherapy (‘5+2’) with and without IM followed by maintenance therapy with IM versus observation (protocol HOVON 67).

For the majority of older AML patients, survival advantage with myelosuppressive therapy is low.8 Selection of patients not eligible for myelosuppressive therapy is an important part of the initial assessment. AML patients over age 60 have been reported to spend 79% of their short survival time hospitalized. In contrast, in our study the vast proportion of survival time was spent in an ambulatory setting. Hospitalization rates were low, with a mean hospitalization time of 9.5 days (range, 0–31 days).

Only a small group of older AML patients may benefit from myelosuppressive induction chemotherapy by achieving CR and long-term remissions. Identification of prognostic factors for these 5% to 15% ‘responders’ could result in higher cure rates due to therapy intensification in a small group, whereas the majority of older AML patients could receive ongoing treatment in a palliative setting with the goal of high quality of life. Older AML patients who show additional risk factors for myelosuppressive therapy are ideal candidates for clinical trials employing novel agents versus best supportive care. It is reasonable to assume that relevant risk factors include disease-related characteristics such as poor risk cytogenetics, high leukocyte count at diagnosis, secondary AML, or significant comorbidities like cardiac failure or renal insufficiency. In this group the risk of early mortality and long-term hospitalization prevail over the minimal chance of cure (<5%). For these patients, novel agents including TKIs, histone-deacetylase-, or heat shock protein-inhibitors with or without concomitant dose-reduced chemotherapy are attractive candidates to be tested in future trials.

Acknowledgements

We thank Daniel B. Lipka for his contribution to the article, Ulrike Haus and Harald Gschaidmaier for support of the clinical trial, Martin Kaufmann for clinical patient care, Beate Roos for excellent study documentation, and Ilse El-Kholy for excellent study management.

Ancillary