Current Status of Therapy for CML - A Review of Drug Development published 07/08

[Guest guest]

Good Morning All from Sue (Aussie)

Steele, National Manager, Support Services, The Leukaemia

Foundation, 230 Lutwyche Rd, Windsor QLD 4030

has forwarded this information that came to him so that I can share

with some up to date info.

This was published 15th July 2008

Current Status of Therapy for Chronic Myeloid Leukemia: A Review of

Drug Development

Swami Padmanabhan; Saritha Ravella; Tyler Curiel; Francis Giles

Future Oncol. 2008;4(3):359-377. ©2008 Future Medicine Ltd.

Posted 07/15/2008

Abstract and Introduction

Abstract

Chronic myeloid leukemia (CML) has led the way for developing

rational drug development in cancer. Most cases of CML diagnosed and

treated in chronic phase are extremely well controlled with imatinib

monotherapy, and primary resistance is very uncommon. Even though the

treatment failure rate is low, the emergence of drug resistance and

the lack of eradication of the hematopoietic stem cell clone has

prompted a wave of drugs to address one or both these problems.

Several clinical trials (Phase I and II) of dasatinib or nilotinib in

the treatment of imatinib-resistant or -intolerant Ph chromosome-

positive leukemia have already reported a remarkable rate of

hematologic response greater than 90% for chronic-phase patients.

These drugs minimize the risk of acquired drug resistance that is

particularly seen within the first 24-36 months of therapy, and can

prevent early failure in these patients, Furthermore, rational,

noncross-resistant combinations that include a T315I inhibitor and

drugs that can eradicate the hematopoietic stem cell clone may extend

the coverage to virtually all patients with bcr-abl. Here we review

the 6-year impact of the 'magic pill', Gleevec®, (Glivec®), including

the emerging problems with its treatment, the efficacy data of

dasatinib and nilotinib and the very promising data of the newer

generation of drugs for CML.

Introduction

Chronic myeloid leukemia (CML) is a myeloproliferative disorder that

can occur in a bi- or tri-phasic course. CML occurs with an incidence

of approximately 1-1.5 cases per 100,000 population, and accounts for

approximately 7-15% of newly diagnosed cases of leukemia in adults.

[1] As per the NCI's Surveillance, Epidemiology, and End Results

(SEER) Cancer Statistics Review, it is estimated that 4830 men and

women (2800 men and 2030 women) will be diagnosed with CML, and 450

men and women will die from the disease in 2008. The median age at

presentation is around 66 years. In the preimatinib era, the median

survival was 4-6 years (range <1 year to >10 years). Survival after

the development of an accelerated phase is usually less than 1 year

and only a few months after blastic transformation.

The typical course of disease is characterized by an initial chronic

phase lasting for 3-6 years, followed by an accelerated, then blastic

phase usually of short duration. A total of 75-80% of patients go

through an accelerated phase before the blastic phase. The definition

for accelerated phase is not uniform, which needs to be verified when

evaluating treatments. Specific criteria associated with a survival

shorter than 18 months by multivariate analysis have been proposed,

including the presence of & #8805;15% blasts, or & #8805;30% blasts and

promyelocytes, or & #8805;20% basophils in blood or platelet count

<100. A

cytogenetic clonal evolution is also considered criteria for

acceleration. Recent analysis suggests its prognostic effect depends

on the specific abnormality, its predominance in marrow metaphases

and the time of appearance.

The cytogenetic hallmark of CML is a reciprocal t(9,22)(q34;q11)

chromosomal translocation that creates a derivative 9q+ and a small

22q-, known as the Ph chromosome. The latter harbors the bcr-abl

fusion gene encoding the chimeric bcr-abl protein with a deregulated

tyrosine kinase activity, the expression of which has been shown to

be necessary and sufficient for the transformed phenotype of CML

cells. The activation of multiple signal transduction pathways in bcr-

abl transformed cells leads to increased proliferation, reduced

growth-factor dependence and apoptosis, and perturbed interaction

with the extracellular matrix and stroma. CML is a quintessential

example in human neoplasia, wherein a single oncogenic fusion

abnormality plays a central role in its pathology.

Bcr-abl as the Target for Drug Development: Paradigm Shift With

Imatinib

This understanding of the cytogenetic and molecular pathophysiology

underlying CML has paved a way for the development of effective

targeted molecular therapies. This ultimately led to the development

of imatinib mesylate (STI-571, Gleevec®, Glivec®), an oral inhibitor

of bcr-abl kinase activity. The clinical success of imatinib mesylate

in the treatment of CML, especially the high durable response rates

in patients with chronic phase CML, has validated the therapeutic

strategy of rationally targeting the causative molecular abnormality

of CML. In the international randomized study of IFN- & #945; versus

STI571

(IRIS) study, of 343 patients in whom at least 20 cells in metaphase

had been cytogenetically analyzed in 3 months, 152 had a major

cytogenetic response (no more than 35% Ph+ cells in metaphase).[2]

Whereas CML progressed in only five of the patients with major

cytogenetic response (3.3%), disease progression was documented in 22

of the 191 patients without such a response (11.5%; p = 0.005 by the

log rank test). This is evident from the higher rates of complete

hematologic response (95 vs 56% of patients; p < 0.001) and major

cytogenetic response (85 vs 22% of patients; p < 0.001). A median

follow-up of 19 months demonstrated that imatinib mesylate was

associated with predominantly better responses than IFN- & #945; and

Ara-C

combination therapy. On the basis of these results, imatinib mesylate

was approved in 2001 by the US FDA for treatment of patients with Ph+

CML in blastic-phase, accelerated-phase and chronic-phase patients

who failed IFN- & #945; therapy. Subsequently, in 2002, imatinib

mesylate

also received accelerated approval for the treatment of newly

diagnosed Ph+ CML in chronic phase. Imatinib has changed the

management of CML and has become the current standard of treatment

for CML.

Dose & Duration of Imatinib Therapy & the Race for the Cure

Imatinib Dose Schedules

The optimal dose of imatinib is yet to be clearly defined. Although

the maximum tolerated dose was not identified in the Phase I study,

400 mg per day is the dose selected for subsequent studies, as

imatinib at 400 mg daily could achieve a blood concentration higher

than IC50 in vitro.[3,4] Moreover, reliable clinical responses were

seen at doses of 300-400 mg daily, especially in chronic-phase

patients.

In Phase II trials of accelerated- and blastic-phase CML patients,

imatinib at 600 mg/800 mg daily demonstrated greater efficacy over

400 mg.[5,6]

There is also a correlation of clinical responses with the steady-

state trough plasma concentrations (Cmin) of imatinib mesylate and

its major active metabolite, CGP74588.[7] A total of 551 patients in

the IRIS study had trough pharmacokinetic samples (24 h post dose)

obtained at day 1 and steady state (day 29). The overall mean

coefficient of variation (CV) for the steady-state trough levels

(Cmin) is a reflection of imatinib mesylate clearance and metabolism

in CML patients. Pharmacokinetic trough levels obtained for imatinib

could be divided into three groups - the lower and upper quartile

ranges (below Q1 = 25th percentile, above Q3 = 75th percentile) and

the interquartile range. Times to complete cytogenetic response (no

Ph+ metaphases) and the major molecular response within these

complete cytogenetic response patients were different in these three

groups. Mean (±SD) trough plasma imatinib concentrations were

significantly higher in the group with major molecular response (34

patients) than in the group without (1452.1 ± 649.1 ng/ml versus

869.3 ± 427.5 ng/ml, p < 0.001), whereas there was no difference in

the imatinib daily dose. For trough plasma imatinib concentrations

and their discrimination potential for major molecular response, the

area under receiver-operating characteristic curve was 0.775, with

best sensitivity (76.5%) and specificity (70.6%) at a plasma

threshold of 1002 ng/ml. By 4 years, an estimated 53% achieved major

molecular response despite low steady-state Cmin levels compared with

80% for patients with high Cmin (and 72% for patients within the

interquartile range). These results suggest that achieving and

maintaining an adequate plasma concentration (by therapeutic drug

monitoring) of imatinib mesylate is important for a good clinical

response.

High-dose imatinib mesylate (800 mg daily), as front-line treatment

has been studied in newly diagnosed chronic-phase CML patients.

Responses in 175 patients (with a median follow-up of 30 months) have

been evaluated in comparison with historical controls (n = 50)

receiving standard-dose imatinib (median follow-up of 53 months).[8]

A complete cytogenetic response with imatinib was achieved in 90% of

high-dose-treated patients, in contrast to only 78% of standard-dose-

treated patients (p = 0.03). At 12 months, the major molecular

response rates were 54% with high dose, versus 24% with standard dose

(p = 0.001), and complete molecular response rates at 24 months were

27 and 10%, respectively. Based on the pharmacokinetic data from the

IRIS studies it is very likely that due to higher Cmin patients

receiving high-dose imatinib mesylate (800 mg daily) upfront (in

newly diagnosed patients) they achieve complete cytogenetic response

at a rapid rate, but not necessarily at a significantly higher rate.

The results of these studies are somewhat difficult to compare, owing

to differences in follow-up. In addition, the reverse transcriptase

(RT)-PCR technology was not standardized. Nonetheless, the emerging

picture is that the rates of major molecular remission and complete

cytogenetic response in the combination studies are comparable with

the IRIS trial, but higher in patients treated with 800 mg imatinib

daily, while the rates of major molecular response and complete

molecular response are generally higher compared to standard-dose

imatinib. This increased efficacy comes at the cost of increased

toxicity. For example, the incidence of grade 3/4 neutropenia was 63%

in patients treated with imatinib and pegylated IFN, and 41%

experienced grade 3/4 nonhematologic toxicity. As a result, only a

fraction of the planned IFN dose was actually administered. Taken

together, these results clearly suggest that early intensification of

therapy may increase the frequency of profound remissions, although

at the price of more toxicity. Standard-dose and high-dose imatinib

are currently compared in a Phase III intergroup study taking place

in the USA, and are part of several multi-armed studies in Europe.

Initial results suggest higher rates of major molecular response and

complete molecular response, although it is being observed that the

standard-dose arm is catching up with time.

Duration of Imatinib Therapy

The optimal duration of imatinib therapy is yet to be determined. In

2006, 5-year follow-up data for imatinib mesylate from the Phase III,

multicentered, randomized, open-label, international IRIS trial of

1106 patients showed long-term survival and safety in newly diagnosed

Ph+ CML in chronic phase.[9] An estimated 89% (95% CI: 86-92%) of

patients were alive at 5 years, while the overall survival (OS) in

the IFN arm was 86%. In addition, an estimated total of 93% of

patients had not progressed to advanced phases of Ph+ CML, while only

approximately 2.4% of patients discontinued imatinib mesylate owing

to drug-related adverse events. Furthermore, the annual rates of

progression events decreased with the passing years, with 1.5% in the

first year, 2.8% in the second year and tapering down to less than 1%

in the fourth and fifth years. Approximately 382 out of 553 (69%)

patients randomized to imatinib mesylate were still receiving first-

line therapy, while only 16 out of 553 in the group given IFN plus

Ara-C continued their treatments. From the latter group, 359/553

(65%) had crossed over to imatinib mesylate. The progression-free

survival (PFS) in the intent-to-treat group was 83.2% (95% CI: 79-87)

for imatinib mesylate and 64.1% (95% CI: 59-69) in the IFN arm. In

terms of confirmed responses, the complete hematologic response rate

was 96.6%, the major cytogenetic response rate was 85.2%, and the

complete cytogenetic response rate was 73.1%. The evolving imatinib

mesylate data from the IRIS trial are summarized in Table 1 .

Given this outstanding response with imatinib mesylate, it is prudent

to continue this treatment indefinitely. Furthermore, there is

currently no evidence to indicate that imatinib mesylate can be

discontinued safely even after attaining undetectable bcr-abl

transcript levels. Most patients who have stopped imatinib mesylate

therapy have experienced molecular or cytogenetic relapse even after

achieving a sustained complete molecular response for a considerable

duration of time.[10-12] Thus, the current recommendation suggests

continuation of imatinib mesylate therapy indefinitely unless the

patient experiences unacceptable toxicity or treatment failure.

It is also not clear if imatinib mesylate can be stopped when

patients achieved major molecular response or complete molecular

response. To date, information is mostly limited to anecdotal

observations of patients who stopped therapy in complete cytogenetic

response or complete molecular response for various reasons, such as

side-effects or pregnancy.[10,11,13] Most of them had disease

recurrence, which should not be confused with relapse, since

rechallenge with imatinib mesylate usually restored response. The

only patients who maintained response were individuals who had

received imatinib mesylate for relapse after allogeneic

transplantation or who had been treated with IFN- & #945; before they

commenced imatinib. Thus, it can be surmised from all these clinical

data that imatinib alone is not capable of eradicating the leukemic

stem cell clone.[14]

Adverse Events to Imatinib

The majority of CML patients treated with imatinib mesylate

experienced adverse events at some time. Most events were of mild-to-

moderate grade, but the drug was discontinued for adverse events in

1% of patients in the chronic phase, 2% in the accelerated phase and

5% in blast crisis. The most frequently reported drug related (>25%)

adverse events were nausea, vomiting, edema and muscle cramps. Edema

was most frequently periorbital or in lower limbs, and the frequency

of severe edema was 1-5%. These events appear to be dose-related,

were more common in the blast crisis and accelerated phase studies

(where the dose was 600 mg/day), and are more common in the elderly.

The fluid retention events were usually managed by interrupting

imatinib mesylate treatments and with diuretics, or other appropriate

supportive care measures. In a recent 2006 report, imatinib was

associated with cardiotoxicity and congestive heart failure,[15]

although this toxicity is a rare event in clinical practice.[16] One

such reported serious and life-threatening event was seen in a

patient with blast crisis who subsequently died after pleural

effusion, congestive heart failure and renal failure. Grade 3-4

hematologic adverse events were infrequent, except for neutropenia

(14%) and thrombocytopenia (8%).[2]

Monitoring the Disease Responses & Measuring Minimal Residual Disease

Even though routine cytogenetic analysis is still considered the gold

standard for evaluating response in CML, the studies are often

somewhat cumbersome in practice and require analysis in metaphase. As

most patients are able to achieve complete cytogenetic responses with

tyrosine kinase inhibitors (TKIs), sensitive and accurate monitoring

of bcr-abl is required to measure residual disease. In CML patients

who achieved a complete cytogenetic response, fluorescence in situ

hybridization (FISH) is more sensitive than conventional cytogenetics

to monitor Ph negativity, and thus a biologic response to treatment.

[17] Since FISH studies typically involve looking for the bcr-abl

fusion fluorescence in at least 200 interphase cells, this precludes

the sensitivity of FISH in making judgments on the extent of residual

disease. Furthermore, since most CML studies have assessed long-term

outcomes by monitoring cytogenetics and not FISH, quantitative RT-PCR

(qRT-PCR) is currently used for assessing the depth of the molecular

response and measurement of residual disease with a sensitivity of up

to 10-8. Molecular remission can thus be defined in this fashion as a

reduction in the quantification of bcr-abl transcripts to an

undetectable level, and can be considered as a surrogate marker for

cure and/or long-term disease control. It has been shown that such

precision might help to predict disease outcome in a better way.

Major molecular response is defined as a reduction of bcr-abl

transcript levels by 3 or more logs, compared with a standardized

baseline, obtained from newly diagnosed and untreated CML patients.

So for the standardized baseline in the IRIS trial, which was the

average ratio from 30 patients and was 36%, the major molecular

response was defined as achieving levels of 0.036% or less. A

complete molecular response is defined as undetectability of bcr-abl

transcripts if confirmed on a second occasion. Given the variations

in the technical aspects of the assay, there is a need for

standardization. Therefore, to maximize the consistency and

reliability of the qRT-PCR or real-time quantitative PCR (RQ-PCR)

techniques, a recent consensus proposal suggested optimization of

several procedural aspects of the complex RQ-PCR technique used for

measuring bcr-abl transcripts (measuring the molecular response of

imatinib mesylate therapy).[18-20] An International Scale (IS) was

proposed to generate comparable values when tested in any laboratory,

and the scale is fixed to a major molecular response at a value of

0.1%. It allows for differences generated by various RQ-PCR methods

and controls. The ongoing validity of conversion is reliant on

maintaining performance of analysis within a laboratory. The speed

and amount of response are both believed to play an important role in

the determination of prognosis. In the IRIS trial, patients who

achieved a major molecular response at 18 months had 100% progression-

free survival (without progression to accelerated phase/blastic phase

at 5 years), whereas patients who failed to achieve complete

cytogenetic response had a PFS of 83% (p < 0.001).[9] Major molecular

remission rates and PFS (at 12 months, 40 and 2%, respectively) were

also found to be better with imatinib mesylate therapy. The patients

who achieved a complete cytogenetic response by 12 months had only a

3% probability of progression to acute phase or loss of complete

hematologic remission of major molecular remission over the

subsequent 12 months, compared with a 15% probability of progression

for those patients who did not achieve a major molecular remission.

This study demonstrates that achievement of major molecular

remission, complete cytogenetic response and complete molecular

response are valid efficacy end points in CML, as they correlate with

clinical benefit.

The unsolved challenges with imatinib mesylate include:

• Residual disease, even in those who have undetectable bcr-abl

transcripts, relapses with discontinuation of imatinib mesylate;

• Development of resistance, especially in the advanced stages:

CML patients in phases other than chronic-phase CML do not show a

better treatment response and survival, as is seen in chronic-phase

CML patients, despite dose increases to 800 mg daily. In addition,

since the introduction of imatinib, median survival in blast crisis

has increased from 2-3 months to only 7.5 months, with few long-term

survivors;

• Intolerance;

• Long-term effects of imatinib mesylate therapy on chromosomal

aberrations in the bone marrow[21,22] (the chromosomal changes most

commonly reported with imatinib are trisomy 8 and monosomy 7), on

bone and mineral metabolism[23] or cardiac function are unknown.[15]

We will now illustrate the first two of these issues and how further

drug development can address these.

Role of Measurement of Residual Disease in CML in the Understanding

of Relapse & Imatinib Mesylate Resistance

There is substantial variation in the responses observed among

different patients. Residual disease in imatinib-treated patients

persists because of bcr-abl kinase activity due to likely overlapping

mechanisms, explaining imatinib mesylate resistance and an

ineradicable reservoir of stem-cell activity. A recent 5-year update

of the IRIS trial provided a separate estimate of imatinib mesylate

resistance.[9] After 5 years, 31% of patients (171 of 553 patients)

who received imatinib as first-line therapy discontinued it

(including 4% for adverse events [AEs], 11% for unsatisfactory

therapeutic effect and 2.5% to cross-over to IFN- & #945; treatment).

Even

in this group, approximately 33% of imatinib-treated patients had not

achieved a complete hematologic response, and 39% had not achieved a

major cytogenetic response. Primary resistance to imatinib only

occurred occasionally in chronic-phase, showing a low and decreasing

annual rate of progression (resulting in death) after 1, 2, 3 and 4

years of therapy of 3.4, 7.5, 4.8 and 1.5%, respectively - possibly

as a result of patients with the worse prognosis progressing

relatively early.[24]

Leukemic Stem Cells

The presence of minimal residual disease in patients treated with

imatinib mesylate may be due to the reservoir of disease, the

diseased quiescent hematopoietic stem cell (HSC) subpopulation (which

is approximately 0.5% of Ph+ HSC population) present within the

cells, insensitive to imatinib mesylate therapy. The etiology of

disease 'persistence' (residual disease) at the molecular level may

be multifactorial. Primitive HSC cells are resistant to imatinib

mesylate, and exhibit drug-transport mechanisms, for example PgP, a

MDR1 resistance gene product, evolving mutations in the kinase

domain. The relative resistance of CML HSCs (lin-CD34+CD38-cells) to

imatinib mesylate may be explained at least in part by their elevated

expression of bcr-abl, and the higher tyrosine kinase activity than

is seen in the more prevalent lin-CD34+CD38+ leukemic cells.

Expression of the three transporter genes (OCT1, ABCB1 and ABCG2) was

studied in a bcr-abl-transduced BaF3 cell line, in which p210 bcr-abl

expression was modulated (by a tetracycline inducible system) and

allowed to undergo differentiation. The most primitive (lin-CD34+CD38-

) cells revealed very low expression of OCT1 (low imatinib mesylate

uptake) and highly elevated expression of ABCB1 and ABCB2 (high drug

efflux), and bcr-abl (elevated kinase activity), suggesting HSC-

mediated imatinib mesylate resistance.

There is also emerging data on some of the new drugs under

development suggesting that these recalcitrant clones can be

inhibited. BMS-214662, a cytotoxic farnesyltransferase inhibitor

(FTI), has been shown to target primitive progenitor cells (PPC) in

CML.[25] In long-term culture-initiating cell (LTC-IC) assays with

both chronic-phase CML and normal CD34+ progenitors, addition of BMS-

214662 to dasatinib in vitro has been shown to dramatically reduce

the PPC colonies and also overcome kinase domain mutation

transfectants in Baf3 cell lines.

Advanced CML & Development of Resistance Due to Bcr-abl Inhibition

In contrast to responses seen in the chronic phase, most patients

with CML in accelerated phase and blast crisis fail to achieve a

complete cytogenetic response and frequently develop resistance to

therapy and relapse. Imatinib resistance is uncommon in patients with

early chronic-phase CML, whereas its estimated 2-year incidence is 10-

20% in chronic-phase CML post-IFN- & #945; failure, 40-50% in

accelerated-

phase CML, and 70-80% in blast-phase CML or Ph+ acute lymphoblastic

leukemia (ALL). Approximately 60% of patients with advanced stage CML

(blast crisis) do respond initially; while responses to imatinib

treatment in chronic-phase CML are durable, remissions observed in

blast crisis patients are typically short-lived, with relapse

occurring within 6 months despite continued therapy. Furthermore,

even in imatinib-treated patients, subsequent failure to respond has

typically been associated with acquired resistance within the

leukemic cell to imatinib mesylate. Underlying mechanisms that

account for this include clonal evolution (as the CML patients

progress through the different phases), gene amplification or point

mutation in the bcr-abl kinase domain, and overactivity of the

networking kinases, such as the Src family kinases. Of these, point

mutations within the Abl kinase domain of the bcr-abl gene are

emerging as the most frequent mechanism for resistance to imatinib

mesylate and resultant reactivation of kinase activity. The risk of

mutation development is particularly high in patients who are beyond

the chronic phase, as well as those with a long duration of disease

prior to imatinib therapy.

ATP-competitive TKIs inhibit bcr-abl activity by blocking the ATP-

binding site on the bcr-abl kinase domain. Some ATP-competitive TKIs

bind only to the inactive conformation of bcr-abl, which blocks the

protein in its inactive conformation and prevents its activation.

This accounts for the specificity of these agents, as the inactive

conformation of bcr-abl is structurally unique, whereas the active

conformation is structurally similar to that of other kinases. Some

ATP-competitive TKIs bind to bcr-abl through an extremely complex and

energetically-inefficient 'induced fit' mechanism, so these drugs

have only a modest affinity for the target. This induced fit binding

can be impaired by the substitution of even a single amino acid in

the bcr-abl kinase domain.

Shah et al. have conducted comprehensive bcr-abl kinase domain

sequencing analysis of 45 CML patients who demonstrated imatinib

resistance.[26] Mutations were detected in over 90% of patients

(29/32) who relapsed after an initial response to imatinib, including

those with chronic phase, myeloid blast crisis and lymphoid blast

crisis CML. Mutations were also detected in 4/13 chronic phase

patients with stable disease and correlated with subsequent clinical

relapse.

In general, the most resistance-conferring mutations are distributed

throughout the Abl kinase domain. The most resistant mutations occur

in the P-loop and are close to or near residues that are in direct

contact with the drug. Thus, there is a range of resistance based on

the location, from a few-fold for some of the A-loop mutants, up to

complete resistance for the T315I mutation. To date, more than 30

mutant forms of bcr-abl have been detected in patients. Of these, the

most common arising in CML appears to be Glu255Lys/Val, Thr315Ile and

Met351Thr. The mutants possess varying degrees of imatinib

desensitization, with the most resistant mutants being Tyr253His,

Thr315Ile, Gly250Glu and Glu255Lys. Structural studies suggest that

most point mutations in the bcr-abl kinase domain cause resistance to

imatinib by impairing the flexibility of the kinase domain,

restricting its ability to adopt the inactive conformation required

for optimal imatinib binding, rather than by directly interfering

with drug contact residues. This leads to reactivation of bcr-abl

kinase activity within the leukemic cell, despite the presence of

imatinib.

The Thr315Ile (T315I) mutation is one of the most resistant mutations

in vitro. Investigators have demonstrated that the T315I mutant is

highly resistant to imatinib with a IC50 value greater than tenfold

higher than wild-type bcr-abl.[27] The IC50 values in the T315I

mutant greatly exceed the therapeutically attainable concentration of

imatinib.[28] It has been recommended that because the T315I mutation

completely prevents imatinib binding, its detection in a patient

should probably lead to cessation of imatinib, and the use of other

therapy should be considered.

This has stimulated the development of new kinase inhibitors that are

able to override resistance to imatinib. Based on data from recently

published clinical trials of dasatinib (BMS-354825)[29-43] and

nilotinib (AMN107),[44-49] it is obvious that neither of these agents

will be beneficial in patients with the T315I mutation either.

The 'Sons of Imatinib'

The new inhibitors of Abl tyrosine kinase can be distinguished by

their nature of binding to the ATP site - namely, competitive-ATP

inhibitors and noncompetitive ATP inhibitors. The drugs developed in

this class are the 2-phenylaminopyrimidin-based compounds, such as

nilotinib (AMN107), and the Src/Abl inhibitors, such as dasatinib

(formerly BMS-354825), AP23464, bosutinib and PD166326. The affinity

of the competitive ATP inhibitors is many folds higher than the first

generation, imatinib, and hence they are efficacious in most imatinib

mesylate-resistant patients. Both dasatinib and nilotinib appear to

have activity in CML patients with mutations within the abl kinase

domain, including in the P-loop, A-loop and catalytic domains.

Dasatinib and nilotinib have recently undergone testing in clinical

trials (summarized in Table 2 ), and the clinical data are discussed

below.

Dasatinib

Dasatinib (SPRYCELTM, Bristol-Myers Squibb, NJ, USA), is an orally

available novel multitargeted TKI, with approximately 325-fold higher

potency against native bcr-abl, and also blocks several other

critical oncogenic proteins, such as Src family kinases (Src, Lck,

Lyn, c-KIT, PDGFR- & #946;, and ephrin A receptor kinase) at low

nanomolar

concentrations.[50,51] Unlike imatinib mesylate, dasatinib binds both

the active and inactive conformations of the abl protein and has

demonstrated preclinical activity against 21 out of 22 imatinib-

resistant bcr-abl mutants.[26-28]

Dasatinib Development. In late 2003, dasatinib entered clinical

trials and was clinically assessed in one dose-finding study and five

subsequent studies involving more than 900 imatinib mesylate-

resistant or -intolerant patients (the START program).

In the dasatinib Phase I dose-escalation clinical trial (CA180002),

63 evaluable patients with CML and Ph+ ALL resistant or intolerant to

imatinib have been treated with dasatinib, showing both efficacy and

durability of response.[29] Nine patients have gone off study due to

progressive disease. Of these, three had T315I detected prior to

treatment, and two patients had the T315I mutation at the time of

disease progression.

The Phase II dose of dasatinib was chosen as 70 mg twice a day

(plasma half-life of 3-4 h; Cmax of 90 nM)[30-34] for the Phase II

studies. START-R is an international trial of dasatinib 70 mg twice

daily and imatinib mesylate 800 mg/day in patients with chronic-phase

CML resistant to prior imatinib mesylate 400-600 mg/day. In total,

150 patients were randomized (2:1): 101 to dasatinib, 49 to imatinib

mesylate. With a minimum follow-up of 10 months, the complete

hematologic response rate was 92% (93 dasatinib patients) versus 82%

(40 imatinib mesylate patients), and the major cytogenetic response

rate was 48% for dasatinib versus 33% for imatinib mesylate. Of

importance, the primary difference was the complete cytogenetic

response rate of 35% (35/101) for dasatinib versus 16% (8/49) for

imatinib mesylate, suggesting that dasatinib can achieve deeper

responses in this patient population. Of patients with no prior

cytogenetic response to imatinib mesylate, 44% (17/39) achieved a

major cytogenetic response with dasatinib, versus 7% (1/15) with

higher dose imatinib mesylate. Major cytogenetic response rates of

40% for dasatinib and 20% for imatinib mesylate were achieved in

patients with baseline imatinib mesylate-resistant bcr-abl mutations,

with 47% of dasatinib patients versus 0 imatinib mesylate patients

with difficult-to-treat P-loop mutations achieving a major

cytogenetic response. Patients with no prior cytogenetic response to

imatinib mesylate were able to achieve major cytogenetic response

with dasatinib, but dose escalation of imatinib mesylate was not

effective. A total of 23% of dasatinib patients versus 80% of

imatinib mesylate patients had treatment failure. Grade 3/4

nonhematologic toxicity was minimal in both arms. All grades of

superficial edema and fluid retention were more common with imatinib

mesylate than dasatinib, whereas pleural effusion was seen only with

dasatinib. Cytopenia was more frequent and severe with dasatinib.

CA180035, a randomized, global multicenter, open-label trial of

dasatinib 140 mg once a day versus 70 mg twice a day, was conducted

in patients with accelerated- or blastic-phase CML or Ph+ ALL, which

were resistant to or intolerant of imatinib mesylate.[35] Patients

were stratified by phase of disease (accelerated, myeloid blast, or

lymphoid blast/Ph+ ALL) and by prior imatinib mesylate (resistant or

intolerant). The primary objective of the study was to compare the

major hematologic response rate between the two regimens. Dose

escalation to 180 mg once a day or 90 mg twice a day was allowed for

inadequate response, and dose reduction to 100 or 80 mg once a day or

50 or 40 mg twice a day for drug toxicity. From June 2005 through

March 2006, 612 patients were randomized. Of all patients who

received prior imatinib mesylate, 42% had more than 600 mg/day, and

37% were treated for more than 3 years. Other prior therapy included

IFN in 42% of patients, chemotherapy in 57% of patients and stem-cell

transplant in 14% of patients. The major hematologic response rate

was 35%, including 21% complete hematologic response, and the major

cytogenetic response rate was 33%, including 23% complete cytogenetic

response. The most common nonhematologic drug-related toxicities

included diarrhea (24%, grade 3/4: 3%), headache (17%, grade 3/4:

1%), nausea (17%, grade 3/4: 2%), pleural effusion (15%, grade 3/4:

4%), and fatigue (12%, grade 3/4: 3%). Hematologic toxicity included

neutropenia grade 3 and grade 4 in 22 and 40% of patients,

respectively, and thrombocytopenia grade 3 and 4 in 16 and 50% of

patients, respectively.

A FDA-approved summary of interim results from four single-arm Phase

II studies showed efficacy data in 445 patients.[36] In patients with

chronic-phase CML, the major cytogenetic response rate was 45%, with

a complete cytogenetic remission rate of 33%. Major cytogenetic

response rates were 59, 32, 31 and 42% in patients with accelerated-

phase CML, myeloid CML, lymphoid-blast CML and Ph+ ALL, respectively.

Molecular Responses in Imatinib Mesylate-resistant Patients. In

addition, dasatinib was associated with molecular responses in

patients with imatinib-resistant/intolerant CML and multiple bcr-abl

kinase domain mutations.[37-41] Similar response rates were attained

irrespective of whether patients had bcr-abl mutations within the

kinase domain. In a study of imatinib mesylate-refractory patients

treated with dasatinib,[37] 46 different bcr-abl mutations involving

36 amino acids were detected in 202/394 patients (51%) prior to the

treatment. A total of 162 patients showed one mutation, 33 patients

showed two mutations, six patients showed three mutations, and one

patient showed four mutations. Mutations were observed in 84 patients

in chronic phase (42%), 47 patients in accelerated phase (60%), 23

patients in myeloid blast crisis (43%), and 48 patients in lymphoid

blast crisis and ALL (74%). In patients with mutations, hematologic

response was 91% in chronic phase, 62% in accelerated phase, 41% in

myeloid blast crisis and 34% in lymphoid blast crisis/ALL (p < 0.01).

Major and complete cytogenetic response did not differ significantly

(47 and 34% in chronic phase, 35 and 27% in accelerated phase, 33 and

28% in myeloid blast crisis, 53 and 51% in lymphoid blast crisis/ALL,

respectively). Major cytogenetic response rates were comparable in

patients bearing no mutations (44%), mutations within the P-loop

(43%), SH2 domain (47%), activation loop (56%) or other sites (49%),

but not for T315I (0%, p < 0.001). Sorting individual patients by the

underlying mutation and cellular IC50 values of dasatinib revealed

clearly higher hematologic and cytogenetic response rates in those

with lower IC50 values. In line with the virtual insensitivity to

dasatinib in vitro, none of the 17 patients carrying a T315I mutation

showed any hematologic response.

Two distinct patterns of response were observed:

• A parallel decrease of the bcr-abl load and the mutated clone

• A decrease of the bcr-abl load followed by a decrease of the

mutated clone after a delay of up to 4-6 months

Up until now, five patients developed new mutations associated with

dasatinib resistance (T315I, n = 2, accelerated phase and myeloid

blast crisis patients; F317L + F486S, n = 2, lymphoid blast crisis

and myeloid blast crisis patients; E507G, n = 1, chronic-phase

patient).

Adverse Events to Dasatinib & Dose Optimization.

Most dasatinib patients also experienced adverse drug reactions at

some time,[29-43] most of which were dose dependent. The drug was

discontinued for adverse reactions in 6% of patients in chronic-phase

CML, 5% in accelerated-phase CML and 11% in myeloid blast-phase CML.

The most frequently reported adverse events included fluid retention

events, such as pleural effusion;[42] gastrointestinal events,

including diarrhea, nausea, abdominal pain and vomiting; and bleeding

events. The most frequently reported serious AEs included pyrexia

(9%), pleural effusion (8%), febrile neutropenia (7%),

gastrointestinal bleeding (6%), pneumonia (6%), thrombocytopenia

(5%), dyspnea (4%), anemia (3%), cardiac failure (3%) and diarrhea.

In a published correspondence in the New England Journal of Medicine,

the authors report the rate of drug-related pleural effusion as 21%

in a series of five Phase II studies involving a total of 511

patients with chronic-phase, accelerated-phase, blast-crisis or Ph+

ALL.[52] Furthermore, in the Phase I study, ten patients (four in

chronic-phase CML and six in blast crisis) underwent thoracentesis

and two underwent pleurodesis.

In the dasatinib dose-optimization study of 100 mg/day versus 140

mg/day as reported by Shah et al.,[43] the hematologic and

cytogenetic response (complete hematologic response, major

cytogenetic response and complete cytogenetic response) were similar

among all arms. The PFS significantly favored 100 mg once a day as

compared with 70 mg twice a day (p = 0.032). The 100 mg/day dose arm

exhibited significantly superior toxicity profile and superior

tolerability compared with other arms. Thus the clinically optimal

dose of dasatinib without the inadvertent side effect is 100 mg/day,

and long-term follow-up of this study will help determine the

durability of this response.

Dasatinib received FDA approval in June 2006 for the treatment of

adults in all phases of CML (with chronic-phase, accelerated-phase,

or myeloid or lymphoid blastic-phase CML) with resistance or

intolerance to prior therapy including imatinib. It is also approved

for the treatment of adults with Ph+ ALL, which is resistant or

intolerant to previous therapy.

Nilotinib

Nilotinib (Tasigna®; Novartis, Basel, Switzerland) is a novel

aminopyrimidine ATP-competitive inhibitor of bcr-abl. This TKI also

binds to other kinases, such as KIT, PDGFR, ABL-related kinase ARG

and ephrin receptor EPHB4, with the exception of the Src-family of

tyrosine kinases. Nilotinib was designed to fit into the ATP-binding

site of the bcr-abl protein with higher affinity than imatinib.

Crystallographic models show that it requires less of a topographical

fit in order to inhibit bcr-abl; thus, in addition to being more

potent than imatinib (IC50 <30 nM) against wild-type bcr-abl,

nilotinib is also significantly active against 32/33 imatinib-

resistant bcr-abl mutants, with the exception of T315I.[44,45]

Because of the selectivity and broad efficacy of nilotinib in known

mutants, it has the potential to benefit patients in all stages of

Ph+ CML by achieving and maintaining the best possible response,

including one at the molecular level. Hence, the clinical trials

designed and discussed thus far include individuals likely to

respond, such as de novo patients, patients intolerant of less

selective therapy, patients who become resistant to imatinib mesylate

and patients who achieve a suboptimal molecular response on less

selective therapy.

Nilotinib Clinical Development.

The first in-human study (CAMN107A2101), is a Phase I/II

multicenter, dose escalation study of oral nilotinib on a continuous

daily dosing schedule in adult patients with imatinib-resistant/-

intolerant CML in chronic phase, accelerated phase or blastic crisis,

relapsed/refractory Ph+ ALL and other hematologic malignancies. The

Phase I portion is complete and the Phase II is ongoing. In the Phase

I portion of the (CAMN107A2101) study, 119 patients with CML-chronic

phase, accelerated phase, BC and Ph+ ALL and resistant to imatinib

were treated with nilotinib in dose cohorts from 50 to 1200 mg on a

once-daily dosing schedule.[46] No dose-limiting toxicity dose level

has been defined to date. Efficacy was assessed in 114 patients in

the Phase I portion. In this study, nilotinib was not associated with

the edema, as seen frequently with imatinib. Among patients with

chronic, accelerated and blast-phase CML, hematological/cytogenetic

responses were achieved in 92/53, 72/48 and 39/27%, respectively. The

best responses were seen at doses of 400 mg twice a day. Two of the

imatinib-resistant Ph+ ALL patients also responded. Pharmacokinetic

analysis of patients receiving 400 mg twice a day, which was the dose

selected for Phase II trials, showed mean peak and trough plasma

levels of 3.6 and 1.7 M, respectively, with an apparent half-life of

15 h. Nilotinib is now being studied in three ongoing Phase II trials

conducted in patients with imatinib-resistant or -intolerant CML.

Currently, data is available from one clinical trial cohort of 145

patients who were administered nilotinib at a dose of 400 mg twice a

day. Results from this study indicate that nilotinib achieved

complete hematologic response in 69, 16 and 4% of patients with

chronic-phase, accelerated-phase, and blastic-phase disease,

respectively, and major cytogenetic responses were seen in 46, 28 and

29% of patients. The trial design also allowed for dose escalation to

600 mg twice a day for Ph+ ALL, accelerated-phase CML, chronic-phase

CML and blast-crisis CML patients, to allow higher drug exposure for

suboptimal responders. In a recently published update of this Phase

II open-label study, nilotinib 400 mg was administered orally twice

daily to 280 patients with Ph+CML in chronic phase after imatinib

failure or intolerance.[47] Patients had at least 6 months of follow-

up and were evaluated for hematologic and cytogenetic responses, as

well as for safety and OS. At 6 months, the rate of major cytogenetic

response (Ph & #8804;35%) was 48%: complete (Ph 0%) in 31%, and

partial (Ph

1-35%) in 16%. The estimated survival at 12 months was 95%. Adverse

events were mostly mild-to-moderate, and there was minimal cross-

intolerance with imatinib. Grade 3-4 neutropenia and thrombocytopenia

were observed in 29% of patients; pleural or pericardial effusions

were observed in 1% (none severe).

Clinical data suggest that nilotinib may overcome most of the

mutation-associated resistance to imatinib mesylate (except T315I),

and may have an important therapeutic role in imatinib mesylate

resistance and in front-line CML therapy to prevent emergence of

resistant clones. Prior to nilotinib, 28 different bcr-abl mutations

involving 22 amino acids were detected in 61/101 patients (60%). Nine

patients showed two, three patients three and one patient four

mutations. Mutations were observed in 37 patients in chronic phase

(49%), 15 patients in accelerated phase (68%) and nine patients in

blast crisis (60%). In patients with mutations, the overall rate of

hematologic response was 70% (78% in chronic phase, 75% in

accelerated phase, 25% in blast crisis), compared with 88% in

patients without mutations. In chronic-phase CML, complete

cytogenetic response was achieved within 3-6 months in patients with

mutations with high in vitro sensitivity to nilotinib.[48] Response

dynamics depend on the individual type of the mutation, which may be

the basis for individualized dosage of nilotinib according to the

mutation pattern. Based on these results, nilotinib is now approved

by the FDA for the treatment of chronic-phase and accelerated-phase

Ph+ CML in adult patients resistant to or intolerant to prior therapy

that included imatinib.

Nilotinib in imatinib mesylate-resistant or intolerant accelerated

chronic myeloid leukemia. A Phase II trial was designed to

characterize the efficacy and safety of nilotinib (400 mg twice

daily) in patients with imatinib-resistant or -intolerant accelerated-

phase chronic myelogenous leukemia with hematologic response as the

primary efficacy end point.[49] A total of 119 patients were enrolled

and had a median duration of treatment of 202 days (range, 2-611

days). An hematologic response was observed in 56 patients (47%; 95%

CI: 38-56%). Major cytogenetic response was observed in 35 patients

(29%; 95% CI: 21-39%). The median duration of hematologic response

has not been reached. Overall survival rate among the 119 patients

after 12 months of follow-up was 79% (95% CI: 70-87%). Non-

hematologic adverse events were mostly mild to moderate. Severe

peripheral edema and pleural effusions were not observed. The most

common grade 3 or higher hematologic adverse events were

thrombocytopenia (35%) and neutropenia (21%). Grade 3 or higher

bilirubin and lipase elevations occurred in 9 and 18% of patients,

respectively, resulting in treatment discontinuation in one patient.

Adverse Events for Nilotinib. Safety data are available for 371

patients with CML-accelerated phase and accelerated phase enrolled in

the Phase II part of study (CAMN107A2101), and 428 patients with

chronic-phase CML, accelerated-phase CML, blast-crisis CML, Ph+ ALL,

hypereosinophilic syndrome, systemic mastocytosis and

gastrointestinal stromal tumor enrolled in other ongoing clinical

trials. The safety and tolerability profile of nilotinib is favorable

at 400 mg taken orally twice a day, with commonly reported AEs in

Phase I and II studies being myelosuppresion (grade 3-4 in 10-20% of

patients), mild-to-moderate rashes, pruritis, nausea and vomiting,

diarrhea, fatigue, constipation, arthralgia and peripheral edema.[53-

56] Severe clinical consequences, such as febrile neutropenia,

sepsis, pneumonia and bleeding associated with thrombocytopenia

occurred infrequently in both disease phases. The reported

nonlaboratory AEs were manageable with symptomatic treatment and were

reversible.

Elevations in serum lipases were commonly observed (all grades ~38%).

Patients with elevated lipase were generally asymptomatic, those with

symptoms of abdominal pain were reported in 3-5%, and both were

transient and easily managed with brief treatment interruptions. Low-

grade elevations of bilirubin and hepatic transaminases were

frequently observed; elevations of bilirubin occurred early, were

transient, and the more severe cases were managed with brief

treatment interruptions or dose reductions, rarely requiring

treatment discontinuation. Of interest, a significant increase in

relative risk of hyperbilirubinemia was seen in patients with the (TA)

7(TA)7 genotype at the (A[TA]nTAA) element of the UGT1A1, suggesting

that genetic susceptibility may contribute to the development of

hyperbilirubinemia in some patients.[57]

Nilotinib has demonstrated a modest dose-dependent potential for QT

interval prolongation as observed in both CML patients and healthy

male volunteers. In general, cardiac events occurred in patients with

other risk factors for cardiac disease, and the incidence of these

events seems to reflect the underlying cardiac risk for a population

of the same age.

Other ATP-competitive Bcr-abl Inhibitors

Bosutinib

Bosutinib (SKI-606; Wyeth, NJ, USA) is an orally available, dual

Src/Abl kinase inhibitor shown to be 200-fold more potent than

imatinib as an inhibitor of bcr-abl phosphorylation in biochemical

assays.[53,58] However, unlike dasatinib, bosutinib does not block

KIT or PDGFR.[58] The phosphorylation of the autoactivation site of

the Src-family kinases (LYN and/or HCK) is also decreased by

bosutinib therapy. It had been demonstrated that bosutinib has in

vitro activity against all imatinib-resistant mutants, except T315I.

In nude mice, bosutinib caused complete regression of large K562

(leukemia cell line) xenografts, when administered orally for 5 days

at a once-daily dose of 100 mg/kg body weight.[53] Ongoing Phase I/II

clinical trials in imatinib-resistant CML and Ph+ ALL reported

evidence of bosutinib's efficacy, safety and tolerability.[54] In the

Phase I portion of this Phase I/II study, patients in chronic phase

with imatinib relapsed or refractory disease were eligible for

treatment with bosutinib once-daily dosing. A total of 18 patients

have been enrolled in the following dose cohorts (mg/day): 400 (three

patients), 500 (three patients) and 600 (12 patients), and have been

on treatment for 30-192 days. A total of 17 out of 18 patients remain

on study; one patient discontinued with disease progression. The

following bosutinib-related AEs have been reported (n = 15, Grade

1/2): diarrhea (87%), nausea (33%), vomiting (20%), abdominal pain

(13%), rash (13%), asthenia (13%) and increased AST/ALT levels (7%).

Two patients treated at 600 mg experienced Grade 3 toxicity: rash and

thrombocytopenia. Five patients (four patients at 600 mg and one

patient at 500 mg) had dose reductions for rash, thrombocytopenia,

diarrhea, fever and increased AST/ALT levels. No pleural effusion or

pulmonary edema has been reported. Of the seven patients who entered

the study in hematologic relapse and have completed 1 month of

treatment, all have achieved complete hematologic response. Of the

seven patients on treatment for 12 weeks (time of first cytogenetic

assessment), three have achieved complete cytogenetic response and

one patient has had a minimal cytogenetic response. A total of six

out of seven patients who have achieved complete hematologic response

had pretreatment imatinib-resistant bcr-abl mutations: M351T, F359V,

T315I, and F359(V,F); and two patients had multiple mutations (L248

[L,V] and H396[H,R]; H396[H,P] and E286[E,G] and M351[T,M]). The

three patients with complete cytogenetic response had the following

mutations: M351T, M244V, H396(H,P), E286(E,G) and M351(T,M). Based on

the emergence of one dose-limiting toxicity of grade 3 rash, and

additional grade 2, grade 1 and dermatologic toxicities observed at

600 mg, 500 mg has been selected as the dose for the Phase II portion

of the study.

INNO-406

INNO-406 (Innovive, NY, USA; originally developed by Nippon Shinyaku

as NS-187) is a dual bcr-abl and LYN TKI, structurally related to

imatinib and nilotinib, and is currently being studied as a potential

treatment for CML patients in a Phase I trial. INNO-406 demonstrated

a 25- to 55-fold greater potency than imatinib against the bcr-abl-

positive leukemia cell lines K562 and KU812, and against Ba/F3 mouse

hematopoietic cells designed to express parental p210 bcr-abl.[55] As

INNO-406 is a selective inhibitor of LYN kinase and not a broad Src-

family kinase inhibitor,[55] it may be less toxic in comparison with

the broad Src-family inhibitors. However, further clinical data is

needed to prove if this can be of any clinical benefit. Once again,

INNO-406 inhibits various imatinib-resistant mutants, except T315I.

AP23464

AP23464 is a potent ABL kinase and SFK inhibitor that inhibits

proliferation and promotes apoptosis in CML cell lines.[56]

Furthermore, AP23464 has antiproliferative activity against cell

lines expressing a different imatinib-sensitive bcr-abl mutant

(Q252H, Y253F, E255K, M351I or H396P). AP23464 had no inhibitory

effect on bcr-abl T315I mutants.

Non-ATP-competitive Inhibitors of Bcr-abl

Aurora Kinase Inhibitors

Aurora kinases are a family of serine/threonine kinases that are

essential for protein phosphorylation events regulating the mitotic

progression of the cell cycle.[59] The investigation of Aurora kinase

inhibitors as potential therapeutic agents in cancer is based on the

fact that Aurora kinases are overexpressed in various human cancers.

MK-0457. MK-0457 (Merck, NJ, USA; originally developed by Vertex

Pharmaceuticals as VX-680) is an Aurora kinase inhibitor that targets

bcr-abl mutants resistant to all available TKIs, including the T315I

mutant. It is a potent inhibitor of all three Aurora kinases and FLT3

in the nanomolar range, and also a moderate-to-strong inhibitor of

other kinases, including ABL and JAK2, which are potential targets

for a variety of myeloproliferative disorders.[60,61] In Phase I

clinical trials, MK-0457 has been studied as a 5-day intravenous

infusion (20 mg/m2/h, delivering plasma levels of 1-3 µM),

administered every 2-3 weeks to patients with a wide range of

relapsed or refractory leukemias, including CML and Ph+ ALL patients.

[61] This treatment regimen is well-tolerated, with mucositis being

one of the few reported side effects, and has shown efficacy in

patients with highly refractory CML, including some who express bcr-

abl with the T315I mutation. Efficacy seems to correlate with the

level of phosphorylation of CRKL, a downstream element in the bcr-abl

signaling pathway. A Phase II study has been started to assess the

efficacy of MK-0457 in patients with CML and Ph+ ALL who carry the

T315I mutation, and in patients resistant or intolerant to second-

generation bcr-abl inhibitors.

PHA-739358. PHA-739358 (Nerviano Medical Sciences) is an orally

bioavailable inhibitor of Aurora kinases A, B and C.

It has significant inhibitory action against tumor growth in several

animal tumor models at well-tolerated doses because of its potent

antiproliferative activity on a broad range of cancer cell lines.[62]

PHA-739358 has currently entered a Phase II clinical study in CML

patients who have relapsed after treatment with imatinib.

ON012380

ON012380 (Onconova Therapeutics, PA, USA) inhibits the kinase

activity by a non-ATP competitive allosteric mechanism, which

involves bcr-abl inhibition by interacting with the substrate-binding

sites of the protein kinases, rather than involving the ATP binding

site.[63] In mice, it causes regression of leukemias induced by

injection of cells expressing the most resistant T315I mutant, by

promoting apoptosis of bcr-abl and mutant bcr-abl-expressing cells

with an IC50 in the nanomolar range. However, the drug has not yet

entered clinical trials.

Stem-cell Transplantation

The role of allogeneic stem-cell transplantation has changed from

first-line therapy to a second-line or third-line therapy, given the

success of bcr-abl inhibitors. However, allogeneic stem cell

transplant is still the only documented treatment that offers

potential cure as the graft versus leukemia does seem to eradicate

this reservoir of 'quiescent' stem cells. However, stem-cell

transplantation as a therapeutic option needs to be weighed against

the possible associated treatment-related life-threatening morbidity,

such as infections, graft versus disease, risk of secondary

malignancy and, ultimately, transplant-related mortality. Recent

estimates of current outcomes after stem-cell transplantation include

data that were analyzed from 131 chronic-phase CML patients

undergoing stem-cell transplantation (bone marrow or peripheral

blood) from related donors at a single institution in the USA between

the years 1995 and 2000.[64] The probability of disease-free survival

at 3 years was estimated to be 78%, while survival and disease

recurrence rates were estimated at 86 and 8%, respectively. Updated

data from all European patients undergoing stem-cell transplantation

for CML between 2000 and 2003 (n = 3018) was collated by The Chronic

Leukemia Working Party of the European Group for Blood and Marrow

Transplantation.[65] Analysis of this data estimated the 2-year

survival rate as 61%, the transplant-related mortality rate as 30%,

and the rate of disease recurrence as 22%.

As per the guidelines developed for the American Society of

Hematology,[66] approximately 50% of patients who underwent

allogeneic stem-cell transplantation from a matched related donor in

their first chronic phase were alive as well as leukemia-free after 5

years. Subsequent follow-up studies conformed to these data,

demonstrating survival extending to 10 and 15 years.[67,68] Survival

following stem-cell transplantation is mainly dependent on five

defined risk factors:

• Age of the patient

• Stage (phase) of CML at transplantation

• Transplantation from HLA-mismatched donor (unrelated donor)

• Male recipient/female donor

• Time from diagnosis to transplantation

All these factors can be used to evaluate the relative transplant

risk.[69] Furthermore, there are opportunities to treat the residual

or recurrent disease after transplantation, as cytogenetic and

molecular monitoring of the disease enables detection of early post-

transplant relapse. Several studies now show that relapse in this

setting responds well to therapy with donor lymphocyte infusions, IFN-

& #945; or imatinib.[70-72] In fact, CML was the first hematologic

disease

where donor lymphocyte infusion has been shown to induce durable

remissions in most patients with a relapse.[73]

A recently published study from the MD Cancer Center

evaluated outcomes of 64 CML patients with advanced-phase disease

(80% beyond first chronic phase) not eligible for myeloablative

preparative regimens owing to older age or comorbid conditions; these

patients were treated with fludarabine-based reduced-intensity

conditioning regimens.[74] The transplant characterisitics include:

donor type matched related (n = 30), one antigen-mismatched related

(n = 4), or one antigen-matched unrelated (n = 30). With median

follow-up of 7 years, OS and PFS were 33 and 20%, respectively, at 5

years. The incidence of treatment-related mortality was 33, 39 and

48% at 100 days, and 2 and 5 years after hematopoietic stem-cell

transplantation, respectively. In multivariate analysis, only disease

stage at time of hematopoietic stem cell transplantation was

significantly predictive for both OS and PFS. The authors conclude

that reduced-intensity conditioning hematopoietic stem cell

transplantation provides adequate disease control in chronic-phase

CML patients, but alternative treatment strategies need to be

explored in patients with advanced disease as treatment-related

mortality rates in this high-risk population do increase over time.

Currently, there is no evidence for increased transplant-related

toxicity either with prior imatinib mesylate use or the use of novel

TKI therapy before allogeneic stem-cell transplantation.[75,76] The

results of a recent retrospective study of 12 patients who were

treated with dasatinib or nilotinib or both for imatinib mesylate-

resistant CML before hematopoietic stem cell transplantation has not

shown an increase in transplant-related toxicity.[76]

To summarize, current guidelines recommend allogeneic stem-cell

transplantation as second-line or third-line treatment after kinase

inhibitors failure, except in those patients with high disease risk

and very low transplantation risk,[69] in those patients who prefer

an alternative treatment, or for economic reasons.

Conclusions

The current recommendation is to start imatinib as the first-line

therapy in a newly diagnosed CML patient. Despite the fact that

imatinib has revolutionized the management of CML based on the

encouraging positive results even after a follow-up of 5 years, it

still has some unresolved challenges. These include minimal residual

disease, resistant mutations, and unknown long-term effects on

chromosomes, cardiac function, and bone and mineral metabolism. Some

of these problems are answered by the newer second-generation TKIs

and other targeted therapies based on the different converging

signaling events with the bcr-abl as shown in Figure 1; however, none

so far has been proven to eradicate the HSC clone. Currently,

research work (investigation) is focused on newer ways to overcome

these pitfalls from the existing therapies.

Figure 1.

Bcr-abl, Src family and emerging cellular pathways for drug

development in chronic myelogenous leukemia. ABL = Abelson tyrosine

kinase; Akt = Protein kinase B; BCR = Breakpoint cluster region; CRKL

= V-crk sarcoma virus CT10 oncogene homolog (avian)-like; FAK = Focal

adhesion kinase; FTI = Farnesyl transferase inhibitor; Grb-2 = Growth

factor receptor-bound protein 2; Hck = Hemopoietic cell kinase; JAK =

Janus kinase; Lyn = V-yes-1 Yamaguchi sarcoma viral related oncogene

homolog; MAPK = Mitogen-activated protein kinase; P = Phosphate

group; PI3K = Phosphatidylinositol-3-kinase; SFK = Src family

kinases; SHC = Src homology 2 domain-containing; SRC = Homolog of

Rous sarcoma virus; Stat = Signal transducer and activator of

transcription.

Future Perspective

There is a growing hypothesis supported by in vitro data that a

combination of multiple Abl kinase inhibitors, such as nilotinib,

dasatinib, imatinib and T315I inhibitors, could be used to delay or

prevent the emergence of drug-resistant clones. As these optimal

sequential and/or combinatorial treatment options are evolving, there

is strong evidence to show that these treatment decisions should be

based on rational evaluation of the emerging bcr-abl mutations.

Preliminary data suggest that synergy between imatinib and nilotinib,

or dasatinib and BMS-214662, may occur at the level of the CML stem

cell owing to the ability of both imatinib and nilotinib to inhibit

or act as substrates of the multidrug efflux transporter ABCG2, which

confers resistance toward several anticancer drugs.[25,77] Thus, CML

undoubtedly can be referred to as a 'poster child' that not only

helps comprehend the underlying molecular mechanisms causing cancer,

but also paves the way for successful tailor-made drug development

and combinations in order to achieve a cure.

Table 1. Summary of Clinical Trial Data of Imatinib

Follow-up time (months) Complete hematologic response (%)

Cytogenetic Molecular response (%) Ref.

Major response (%) Complete response (%)

Estimated response rates 12 96 85 69 40%

[2]

19 95 85

Cumulative best observed response rates* 60 97 92

87 [9]

*In CML patients who remained on first-line imatinib mesylate therapy.

Imatinib survival rates: overall survival: 89%; overall survival

excluding non-CML deaths: 95%; event-free survival: 83%; survival

without progressing to AP/BP: 93%.

AP = Accelerated phase; BP = Blastic phase; CML = Chronic myeloid

leukemia.

Data taken from the imatinib IRIS trial Phase III (n = 1106).[9]

Table 2. Summary of Clinical Trial Data of the Different Bcr-abl

Inhibitors in Patients Who Received Imatinib Mesylate*

Drug Number of patients Hematologic response (%)

Cytogenetic response (%) Ref.

Partial Complete Major Complete

Dasatinib

Phase I CP (40) 92 92 45 35 [29]

AP (11) 82 45 27 18

My BP (23) 61 35 35 26

Ly BP and Ph+ ALL (10) 80 70 80 30

Phase II CP (186) Imatinib mesylate resistant (127) Imatinib

mesylate intolerant (59) – – – 90 87 97 52 39 80

39 28 64 [30]

AP (174) Imatinib mesylate resistant (161) Imatinib mesylate

intolerant (13) 64 45 39 32 [31,38]

My BP (74) 34 26 31 27 [32]

Ly BP (42) 35 26 50 43 [32]

Ph+ ALL (36) 50 33 58 58 [39]

Nilotinib

Phase I CP (17) 11/12 = 92‡ 11/12 = 92‡ 35 35 [45]

AP (56) 38/51 = 74‡ 26/51 = 51‡ 27 14

My BP (24) 42 8 21 4

Ly BP (9) 33 0 11 11

Phase II CP (279) Imatinib mesylate resistant (193) Imatinib

mesylate intolerant (86) 137/185 = 74‡ 137/185 = 74‡ 52

34 [46]

AP (64) Imatinib mesylate resistant (52) Imatinib mesylate

intolerant (12) 36 23 36 22 [48]

My BP (87) 27 21 NA NA [47]

Ly BP (27) 30 26 NA NA

Ph+ ALL active (37) 24 24 NA NA

*Careful consideration of the various differing factors, such as

patient selection criteria, response criteria and duration of

treatment and duration of follow-up between individual trials is

necessary when comparing different agents in different trials.

‡Responses are evaluated only in patients with active disease.

ALL = Acute lymphoblastic leukemia; AP = Accelerated phase; BP =

Blastic phase; CML = Chronic myeloid leukemia; CP = Chronic phase; Ly

= Lymphoid; My = Myeloid; NA = Not available; Ph+ = Philadelphia

chromosome positive.

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Sidebar: Executive Summary

• Chronic myeloid leukemia (CML) is a myeloproliferative stem-

cell disorder with a tri-phasic course, with a pathagnomonic t(9,22)

giving rise to the bcr-abl fusion protein that drives the disease

course.

• Imatinib is the first signal transduction inhibitor binding

competitively to the ATP-binding site of bcr-abl, which has shown a

remarkable clinical response including complete cytogenetic and

molecular response in over 80% of CML patients in the chronic phase.

• Stem-cell transplant still plays a potentially curative role

in a subset of CML patients; drug development, such as for BMS-

214662, is also focused on eradicating the hematopoietic stem-cell

clone.

• Imatinib treatment is a subset that is marred by the

development of over 30 mutations in the bcr-abl region reducing its

activity; second-generation bcr-abl inhibitors, such as nilotinib and

the combined src/abl inhibitor dasatinib, have already overcome

almost all these mutations, except for T315I.

• ATP-non-competitive inhibitors, such as MK-0457 and ON012380,

are able to overcome the T315I mutation.

• Future treatments for CML will be towards developing a

rational combination of these drugs that can overcome resistance and

also deplete the abnormal stem cell, paving the way for a cure.

Disclaimer

No writing assistance was utilized in the production of this

manuscript.

Reprint Address

Francis Giles, CTRC Institute for Drug Development, University of

Texas Health, Science Center at San , 7979 Wurzbach Road,

Suite 400, San , TX 78229, USA; E-mail: frankgiles@...

Swami Padmanabhan, Saritha Ravella, Tyler Curiel, Department of

Hematology/Oncology, Institute for Drug Development, Cancer Therapy

and Research Center, San , TX, USA

Francis Giles, CTRC Institute for Drug Development, University of

Texas Health, Science Center at San , 7979 Wurzbach Road,

Suite 400, San , TX 78229, USA

Disclosure: The authors have no relevant affiliations or financial

involvement with any organization or entity with a financial interest

in or financial conflict with the subject matter or materials

discussed in the manuscript. This includes employment, consultancies,

honoraria, stock ownership or options, expert testimony, grants or

patents received or pending, or royalties.