The investigational tyrosine kinase inhibitor quizartinib is extremely promising as a monotherapy for patients with <em>FLT3-ITD–</em>positive relapsed acute myeloid leukemia.
Mark Levis, MD, PhD
The investigational tyrosine kinase inhibitor quizartinib is extremely promising as a monotherapy for patients withFLT3-ITDpositive relapsed acute myeloid leukemia (AML), according to Mark Levis, MD, PhD.
“Quizartinib is a very different drug from [the FLT3 targeted-agent] midostaurin,” said Levis, an associate professor at Johns Hopkins Medical Center. “It is much more selective, much more potent, and you can inhibit the target as a monotherapy. It still has the single-highest response rate of any drug out there that I’ve seen.”
Levis discussed quizartinib as part of a focus group held at the European Congress on Hematology: Focus on Lymphoid Malignancies, an event hosted by Physician’ Education Resources (PER) in Paris, France, from November 4 to 5, 2016.
Quizartinib selectively inhibits FLT3-ITD, which is a growth driver of abnormal cells that contribute to the development of AML. Several trials are currently investigating quizartinib, including the ongoing phase III randomized QuANTUM-R study (NCT02039726). This trial was designed to determine whether quizartinib monotherapy prolongs overall survival compared with salvage chemotherapy for patients with relapsed/refractoryFLT3-ITDpositive AML.
The phase III randomized, double-blind, placebo-controlled QuANTUM-First trial (NCT02668653) is examining quizartinib combined with induction and consolidation chemotherapy and then as a maintenance monotherapy in newly diagnosed elderly patients withFLT3-ITDpositive AML.
“In theory, this combination will be synergistic and antileukemic,” said Levis. “But more importantly, it will need to suppress that FLT3-addicted clone from coming out.”
It is unclear how the results of QuANTUM-First will compare to the phase III RATIFY trial, said Levis, which found that the addition of midostaurinwhich also targetsFLT3to standard chemotherapy reduced the risk of death by 23% compared with chemotherapy alone in patients with AML who harbored aFLT3mutation.
It may be easier for patients to stay on quizartinib than midostaurin and it may better suppress FLT3, said Levis, but that is yet to be determined.
The effectiveness of quizartinib as a monotherapy is most likely due to its c-Kit inhibition abilities, said Levis.
“It rarely wouldn’t work in the relapsed setting,” he said. “The blasts would go down, and routinely patients get a complete response [CR] or CR with incomplete marrow recovery [CRi]. In trials, everybody cleared peripheral blasts, and then it was how much they cleared the marrow that determined partial response or CR/CRi, but most patients were CRi.”
However, c-Kit inhibition has its downsides, he added. It can cause skin nodules and the patients hair to turn white, in addition to other marrow-related side effects.
“The Kit inhibition is a good and a bad thing; I think there is an antileukemic effect there but also clearly a marrow-suppressive effect,” said Levis. “We tested a number of TKIs for their Kit inhibitory effect and it correlates with its ability to oppress a marrow progenitor, so it’s poisonous to red and white cell progenitors.”
In theory, a KIT inhibitor in a patient with a normal marrow environment would simply lower white blood cell counts slightly. However, in the marrow of a patient with AML residual disease that has further been impacted by chemotherapy, a KIT inhibitor often cases patients to require transfusions.
Differentiation syndrome can also occur when a selective FLT3 inhibitor is used. In the focus group, Levis described how he dealt with a patient with this challenge.
“I have a patient right now who just did this,” he said. “His allelic, if anything, went up after he responded. The pathologist read the marrow, no evidence of disease, morphemically normal, and obviously now the patient is transfusion dependent.”