Path-breaking targeted therapies have extended the lives of people with chronic myeloid leukemia (CML) or acute lymphoblastic leukemia that shares the central CML mutation.
But what happens when Gleevec and the second-generation drugs that followed it stop working? Or patients simply cannot tolerate this class of drugs, tyrosine kinase inhibitors that stifle the aberrant BCR-ABL protein that drives these leukemias?
The U.S. Food and Drug Administration (FDA) has approved three new drugs in the past few months that provide new choices for patients and their oncologists.
Ponatinib (Iclusig), was approved last week, an effective drug for many patients with treatment-resistant disease. This comes on the heels of approvals of bosutinib (Bosulif) in September and omacetaxine (Synribo) in October.
Patients with both leukemias have enjoyed strong responses to imatinib (Gleevec) and second-generation drugs nilotinib (Tasigna) and dasatinib (Sprycel).
However, 30-40 percent of CML patients resist imatinib. Nilotinib and dasatinib work for about 40-50 percent of those patients.
"Ponatinib's availability will drastically improve the outcome of most patients with CML and PH+ALL who are resistant or intolerant to prior tyrosine kinase inhibitor therapy," said Jorge Cortes, M.D., professor and deputy chair, in The University of Texas MD Anderson Cancer Center Department of Leukemia.
Cortes led the clinical trials for all three drugs.He and other Leukemia Department faculty also led many of the clinical trials for the three previously approved CML therapies.
"It's important to have as many therapies against cancer as we can, because rarely does one drug or combination succeed for all patients," Cortes said.
"These new drugs cover different gaps in treatment, so they can serve our patients in different ways," Cortes said. "We hope to have effective treatment options for all of them."
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Ponatinib, developed by ARIAD Pharmaceuticals, was
designed to thwart treatment-resistant mutations. The most prominent is
T315I, present in up to 20 percent of patients, which blocks the
docking station where other tyrosine kinase inhibitors normally connect
to the mutant protein.
In a pivotal phase II clinical trial,
which Cortes presented in early December at the 54th American Society of
Hematology Annual Meeting and Exposition in Atlanta, ponatinib showed
responses against CML at early stage (chronic phase), accelerated
phase and blast phase, the most heavily mutated and hard-to-treat late
stage of the disease.
Ponatinib is the only tyrosine kinase inhibitor that's effective against T315I mutant disease.
Approved by the FDA in September, bosutinib
is a second-generation tyrosine kinase inhibitor that works against
many BCR-ABL mutations that cause resistance. An important exception is
the T315I mutation, which only ponatinib attacks directly
works equally as well as dasatinib and nilotinib," Cortes said. "The
significant difference is bosutinib is more specific in its activity,
inhibiting BCR-ABL and SRC, but not other tyrosine kinases. This leads
to fewer harsh side effects."
There are no issues with cardiotoxicity or pancreatitis, for example, which can arise with other tyrosine kinase inhibitors.
works in a completely different manner from the five tyrosine kinase
inhibitors. It stifles creation of the aberrant BCR-ABL protein, rather
than blocking the protein's activity.
"This is an important
option for patients who've had several tyrosine kinase inhibitors fail
and for those who cannot tolerate those drugs," Cortes said. "A small
percentage of patients just need a new approach to get a good response."
is a synthetic version of a long-time CML drug called
homoharringtonine, which is derived from an evergreen tree found in
Known commercially as Synribo, this drug is marketed by
Teva Pharmaceuticals. Clinical trials combining omacetaxine and
tyrosine kinase inhibitors are planned.
Challenges: Individualizing therapy; eradicating disease
"We need to identify which patients to treat with
each drug. Who are the ones I can treat well with imatinib, and who are
the patients who need to start with some of the newer drugs?" Cortes
said. "Right now, we start everyone with imatinib, dasatinib or
While current drugs reduce CML to extremely low,
even undetectable, levels, most patients must remain on treatment to
prevent recurrence. "The idea is to get patients to a certain point
where you can stop treatment and know the disease won't come back,"
Doing that will require development of way to conclusively show that the disease is completely gone and drugs or combinations that achieve eradication.