Imatinib Resistance and the Difficulty of Eradicating Leukemia Stem Cells – Dr.
Deininger
Definition of IM failure: Primary resistance is defined as not achieving
predefined level of response i.e. complete Hematologic response, or CCR.
Therefore this primary response is a time-dependent diagnosis. An expert
panel convened by the European Leukemia Net provided recommendations
based on the data from the IRIS trial. The milestones are CHR at 3 mos, some
Ph negativity at 6 mos, a MCR (major cyto response) @ 12 mos and a CCR @ 18
mos, anything less defines failure to meet the guidelines. Acquired resistance
is defined as the loss of response from a given level, i.e. CHR or CCR. The term
disease persistence refers to the low level of residual disease detectable in
almost all patients, including those who achieve CCR. Even patients who
achieve PCRU, all patients harbor residual disease, whether it is measurable or
not.
In the case of IM resistance, the topic of this presentation, BCR ABL regains
control. However, things are not so clear-cut; current thinking holds that most
acquired resistance is a BCR ABL dependent phenomenon. This has two
important implications: one, it is not easy for CML cells to completely switch
gears and use alternative transformation pathways, and 2, BCR ABL remains the
best target. Mutations in the Kinase Domain (KD) impair the ability of IM to
bind to its target. This subsequently increases BCR ABL expression allowing for
gene amplification. Other mechanisms have been implicated in reactivating
BCR ABL, but there is little evidence that these may have some clinical
relevance (drug transport proteins P-glycoprotein and increased plasma protein
binding). However, a carrier protein hOCT-1, which is responsible for
outside/inside transport of IM into cells, has been positively correlated with
response to IM.
KD mutations: 40 different residues within the KD having distinct clusters in
the ATP-binding loop, threonine 315 and methionine 351. T315I confers
complete resistance. The mutations are studied to understand the level of
activity or sensitivity to the mutations. So we are learning that the different
2nd generation TKI’s can almost be personalized depending on the mutation
found as the reason for the development of resistance and in fact, in vitro
testing allows for accurate predictions. There is the thought that KD mutations
causing resistance are present before starting therapy. This was determined by
retrospective analysis of archived material (Personal note: does this start to
make the case for upfront mutations testing i.e. at diagnosis). So, this means
that the thinking is leaning towards combining therapies at the outset to
rapidly reduce CML Leukemia cell burden in an effort to minimize the risk of
acquiring a mutation while receiving treatment.
The thought is that since not all mutant clones detected by high sensitivity
become dominant on therapy, suggesting that a mutant cell clone must fulfill
additional requirements, such as occurring in a cell with a self-renewal
capacity. Other observations may suggest that some KD mutations may alter
the biology of the disease in addition to conferring resistance by modulating
basal intrinsic Kinase activity or by altering signal transduction.
Imatinib Resistance of Undetermined Etiology
The frequency of KD mutations with resistant disease varies according to
disease phase and resistance type. Many patients have increased BCR ABL but
are mutation negative, so it is evident that for many patients, the mechanism
of resistance remains obscure. The bottom line being that although KD
mutations are intriguing but has to some extent, distracted attention from the
analysis of other resistance mechanisms. As well, all the studies have focused
on the leukemia cell; the role of the microenvironment has not been explored.
So far all that has been discussed here refers to IM resistance and mutations,
which is a weak ABL inhibitor when compared to Nilotinib and Dasatinib. When
thinking about extremely potent ABL kinase inhibitors we might learn more
about the biology of the disease. There is some thought that if leukemia
responds to potent ABL inhibitors, then it has identified itself as BCR ABL
dependent, it is addicted t this oncoprotein, so relapse will take the same
route. Additionally, failure to respond to a potent ABL inhibitor such as
Dasatinib implies that BCR ABL independence, and one could argue that these
leukemia’s are no longer to be considered CML. The only time the potent ABL
inhibitor wouldn’t work but still be defined as CML would be in the presence of
T315I, here we have seen some activity of SGX393 (personal note: there are
other molecules as well that are showing activity against T315I such as
Omacetaxine).
Disease Persistence:
CML is thought to originate in the hematopoietic stem cell, which is
transformed through the acquisition of the BCR ABL translocation. Evidence:
one, BCR ABL is present in stem cells of all hematopoietic lineages, although
not all lineages are equally subject to leukemic transformation, with
granulocytes and their precursors forming the bulk of the leukemic clones, 2 nd,
responses to multiagent chemotherapy, which is curative for many patients
with acute leukemia, are inevitably transient in CML. Patients who achieve
PCRU for quite some time and stop IM eventually relapse, the only patients who
don’t relapse are those who have received prior therapy with Interferon. This
would suggest an immunologic control. It is not clearly understood why IM does
not get to the root of the problem. Ex-vivo studies using primitive cells from
IM naïve chronic phase disease have shown that the effects of IM are primarily
cytostatic and that noncycling quiescent leukemic stem cells are insensitive to
the drug or are even enriched in its presence and to a lesser degree in
Dasatinib. It has been established that these cells are equivalent to the CML
stem cells that persist in patients. There is also some thought that CML stem
cells may rely on physiologic signals that may not be oncogene addicted,
however this hypothesis needs to be tested and would involve understanding
more about the microenvironment.
Prospects for Disease eradication:
If disease persistence is based on BCR ABL dependency then better inhibitors
will be needed, however if CML stem cells do not require BCR ABL activity,
then this would be a natural limit for ABL kinase inhibitors therapy. Again this
points to understanding the microenvironment, such as integrins or cytokines
that promote survival in the presence of ABL inhibitors. GM-CSF intercepting
with JAK2 inhibitor can be beneficial. The alternative is immunotherapy,
breakpoint peptides as vaccines is appealing. The most promising results are
seen with patients with low level of persistence disease. Nonspecific
immunotherapy such as PR-1, a peptide derived from myeloperoxidase has
been detected for patients who experience a CCR to IFN but not to IM. There
needs to be closer investigation to see if such responses are detectable from
patients who had prior exposure to IFN therapy. Of note is that
myeloperoxidase expression in CML cells is regulated by BCR ABL kinase
activity. Inhibition of BCR ABL with ABL kinase inhibitor could down regulate
the very antigen that is crucial for the immune responses implying that the
timing of IM and IFN may be crucial.